Natural Products as Corrosion Inhibitors of Some Metals...

Kingdom of Saudi Arabia

Ministry of High Education

Umm Al-Qura University

Faculty of Applied Science for Girls

Chemistry Department

Natural Products as Corrosion Inhibitors

of Some Metals in Aqueous Media

By

Noha Mosa Al-Qasmi

A Thesis

Submitted in Partial Fulfillment of the Requirements for the

Master Degree in Physical Chemistry

Supervisor

Prof. Dr. Seham M. Abd El-Motaal Professor of Physical Chemistry

Faculty of Applied Science for Girls

Umm Al-Qura University

Makkah Al-Mukarramah

1431-2010

My endless thank and deep respect to my dear

father, mother, brothers, sisters, relatives and

friends for their not limited love, encouragement,

patience and support which push me toward

successes.

I wish to offer all the thanks and respect for

every body who helped me directly or indirectly

throughout this study.

ACKNOWLEDGEMENT

First of all and above all I would like to thank my God the most

merciful, Lord of the world. Without Him this work could not have been

done. After that, the author wishes to express her thanks and sincere

gratitude to Prof. Dr. Seham M. Abd El-Motaal, Professor of Physical

Chemistry, Chemistry Department , Faculty of Applied Science for Girls,

Umm Al-Qura University, Makkah Al-Mukarramah for suggesting the

scope of research, continuous guidance throughout this work and

interpretation, which made the thesis possible in its present work. Then, I

wish to express my thanks to Dr. Sameha Al-Qary Dean of Faculty of

Applied Science for Girls, Umm Al-Qura University and stuff members

of Chemistry Department, at the same Faculty, Umm Al-Qura University

for help and encouragement during all stages of this study.

A special thanks and sincere gratitude to Dean of graduate studies and

Head of Chemistry Department Dr. Mohsen Al-Qhatani, Assistant

Professor of Applied Chemistry, Faculty of Science, Taif University for

his efforts and unique follow-up throughout my study. Next, I offer my

deep thanks to Dr. Saher Fadallah Assistant Professor of Physical

Chemistry, Chemistry Department, Faculty of Science, Taif University,

for her great help and encouragement. A great thanks and gratitude are

offered to Dr. Jamela Al-Malki Assistant Professor of Para Cytology,

Biology Department, Faculty of Science, Taif University, for her

encouragement and mural support.

CONTENTES

Page

Chapter I: INTRODUCTION……………………………………… 1

I.1.Corrosion inhibition…….................................................... 1

I.2. Natural product as corrosion inhibitors…………………. 7

I.3. Corrosion of zinc………………………………………. 8

I.4. Literature survey on the corrosion inhibition of zinc in

aqueous media…………………………………………..

9

I.5. Corrosion of copper and alloys…………………………. 24

I.6. Literature survey on the corrosion inhibition of copper

and its alloys aqueous media…………………………

25

Aim of the work…………………………………………….. 41

Chapter II: EXPERMINTAL……………………………………… 42

II.1. Materials…………………………………………….... 42

II.2. Chemical measurements…………………………….… 45

II.2.1. Weight loss methods……………………………….… 45

II.2.2.Thermometric measurements……………………….… 46

II.2.3.Electrochemical measurements………………………… 48

Chapter III: RESULTS AND DISCUSSION……………………… 50

III.1. Corrosion inhibition of zinc in acid media……………... 50

III.1.1. Chemical measurements……………………………… 50

III.1.1.1.Weight loss methods………………………………… 50

1- Effect of corrodent concentration……………………….. 50

2- Effect of inhibitors………………………………………. 57

3- Adsorption isotherm……………………………………... 66

4- Effect of temperature……………………………………. 72

III.1.1.2. Thermometric measurements……………………….. 81

III.1.2. Electrochemical measurements……………………... 92

III.1.2.1. Open-circuit potential measurements……………….. 92

III.1.2.2. Potentiodynamic polarization measurements……….. 102

III.2. Corrosion inhibition of α-brass in acid media…………... 113

III.2.1. Chemical measurements……………………………… 113

III.2.1.1. Weight loss methods………………………………… 113

1- Effect of corrodent concentration……………………….. 113

2- Effect of inhibitors……………………………………… 117

3- Adsorption isotherm…………………………………….. 126

4- Effect of temperature……………………………………. 131

III.2.1.2. Thermometric measurements……………………………….. 139

III.2.2. Electrochemical measurements……………………………….. 142

III.2.2.1. Open-circuit potential measurements……………………….. 142

III.2.2.2. Potentiodynamic polarization measurements……………….. 154

Summary……………………………………………………………... 166

References………………………………………………...…………… 172

Arabic Summary…………………………………………………

Caption of Figures

Page

1-Chemical structure of some compounds contained in

Ficus carica extract……………………………………………….. 43

2-Chemical structures of some compounds contained in

Olive extract………………………………………………………... 44

3- Mylius vessel for thermometric measurement……………………... 47

4- The polarization cell………………………………………………... 49

5- Weight loss-time curves for zinc in different

concentrations of HCl solution at 25ºC…………………………… 55

6- Weight loss-time curves for zinc in different

concentrations of H2SO4 solution at 25ºC………………………… 56

7- Weight loss-time curves for zinc in 2.0 M HCl

solution in absence and presence of different

concentrations of Ficus carica extract at 25ºC……………………... 59

8- Weight loss-time curves for zinc in 2.0 M HCl


concentrations of Olive extract at 25ºC…………………………… 60

9- Weight loss-time curves for zinc in 2.0 M H2SO4


concentrations of Ficus carica extract at 25ºC……………………. 61

10- Weight loss-time curves for zinc in 2.0 M H2SO4


concentrations of Olive extract at 25ºC…………………………... 62

11- Variation of inhibition efficiency with

concentration of Ficus carica and Olive extracts

for zinc in 2.0 M HCl solution at 25ºC…………………………..

63

12- Variation of inhibition efficiency with

concentration of Ficus carica and Olive extracts

for zinc in 2.0 M H2SO4 solution at 25ºC……………………….

64

13- Langmuir adsorption plots for zinc in 2.0 M HCl

with different concentrations of Ficus carica and

Olive extract at 25ºC…………………………………………….. 69

14- Langmuir adsorption plots for zinc in 2.0 M H2SO4

with different concentrations of Ficus carica and

Olive extracts at 25ºC……………………..…………………….. 70

15- Arrhenius plots of the corrosion rate for zinc in

2.0 M HCl solution in absence and presence of

plant extracts…………………………………………………….. 76

16- Arrhenius plots of the corrosion rate for zinc in

2.0 M H2SO4 solution in absence and presence of

plants extracts…………………………………………………….. 77

17- Eyring plots of the corrosion rate for zinc in 2.0 M HCl

solution in absence and presence of plant extracts……………....... 78

18- Eyring plots of the corrosion rate for zinc in 2.0 M H2SO4

solution in absence and presence of plant extracts……………… 79

19- Temperature-time curves of zinc in different

concentrations of HCl solution at Ti = 25ºC………………………

83

20- Temperature-time curves of zinc in different

concentrations of H2SO4 solution at Ti = 25ºC …………………

84

21- Temperature-time curves of zinc in 2.0 M HCl


concentrations of Ficus carica extract at Ti = 25ºC …………….

85

22- Temperature-time curves of zinc in 2.0 M HCl


concentrations of Olive extract at Ti = 25ºC …………………… 86

23- Temperature-time curves of zinc in 2.0 M H2SO4


concentrations of Ficus carica extract at Ti = 25ºC ……………… 87

24- Temperature-time curves of zinc in 2.0 M H2SO4 solution

in absence and presence of different concentrations of

Olive extract Ti =25ºC ………………………………………… 88

25- Variation between reaction number and

logarithm of the concentration of plant extracts

for zinc in 2.0 M HCl solution at 25ºC…………………………. 90

26- Variation between reaction number and

logarithm of the concentration of plant extracts

or zinc in 2.0 M H2SO4 solution at 25ºC…………………………. 91

27- The variation of the open-circuit potential of zinc

electrode in different concentrations of HCl solution

at 25ºC……………………………………………………………. 94

28- The variation of the open-circuit potential of zinc

electrode in different concentrations of H2SO4 solution

at 25ºC…………………………………………………………….

95

29- The variation of the open-circuit potential of zinc electrode

in 2.0 M HCl with different concentrations of Ficus

carica extract at 25ºC…………………………………………….

96


in 2.0 M HCl with different concentrations of

Olive extract at 25ºC……………………………………………… 97

31- The variation of the open-circuit potential of zinc electrode in

2.0 M H2SO4 with different concentrations of Ficus carica

extract at 25ºC…………………………………………………….

98


in 2.0 M H2SO4 with different concentrations of Olive

extract at 25ºC…………………………………………………….

99

33- The variation between the steady-state potentials and logarithm of

the concentration of extracts for zinc in 2.0 M HCl solution

at 25ºC……………………………………………………………. 100

34- The variation between the steady-state potentials and logarithm of

the concentration of extracts for zinc in 2.0 M H2SO4 solution

at 25ºC…………………………………………………………… 101

35- Potentiodynamic polarization curves for zinc electrode in 2.0 M

HCl in absence and presence of different concentrations

of Ficus carica extract at 25ºC…………………………………… 106


HCl in absence and presence of different concentrations

of Olive extract at 25ºC………………………………………….. 107


H2SO4 in absence and presence of different concentrations

of Ficus carica extract at 25ºC……………………………………

108


H2SO4 in absence and presence of different concentrations

of Olive extract at 25ºC…………………………………………... 109

39- Adsorption of the inhibitors by physical blocking……………… 110

40- Weight loss-time curves for α-brass in different

concentrations of HCl solution at 25ºC………………………… 114

41- Weight loss-time curves for α-brass in different

concentrations of H2SO4 solution at 25ºC……………………….

115

42- Weight loss-time curves for α-brass in 0.5 M HCl

solution in absence and presence of different 119

concentrations of Ficus carica extract at 25ºC ……………………

43- Weight loss-time curves for α-brass in 0.5 M HCl


concentrations of Olive extract at 25ºC………………………….. 120

44- Weight loss-time curves for α-brass in 0.5M

H2SO4 solution in absence and presence of

different concentrations of Ficus carica extract at 25ºC…………. 121

45- Weight loss-time curves for α-brass in 0.5 M


different concentrations of Olive extract at 25ºC ……………….. 122

46- Relationship between inhibition efficiency and the

concentration of extracts for α-brass in 0.5 M

HCl solution at 25ºC…………………………………………….. 124

47- Relationship between inhibition efficiency and the


H2SO4 solution at 25ºC……………………………………………

125

48- Linear fitting of plant extracts for α-brass HCl in

solution using Kinetic-thermodynamic model

isotherm at 25ºC…………………………………………………...

128

49- Linear fitting of plant extracts for α-brass in

H2SO4 solution using Kinetic-thermodynamic

model isotherm at 25ºC…………………………………………… 129

50- Arrhenius plots of the corrosion rate for α-brass

in 0.5 M HCl in absence and presence of plant

extracts……………………………………………………………

134

51- Arrhenius plots of the corrosion rate for α-brass

in 0.5 M H2SO4 in absence and presence of plant

extracts…………………………………………………………. 135

52- Eyring plots of the corrosion rate for α-brass in 0.5 M HCl in

absence and presence of plant extracts…………………………..

136

53- Eyring plots of the corrosion rate for α-brass in 0.5 M H2SO4 in

absence and presence of plant extracts……………… …………. 137

54-Temperature-time curves of α-brass in different

concentrations of HCl solution at Ti = 25°C……………………… 140

55- Temperature-time curves of α-brass in different

concentrations of H2SO4 solution at Ti = 25°C …………………..

141

56- The variation of the open-circuit potential for

α-brass electrode in different concentrations of

HCl solution at 25ºC……………………………………………. 146

57- The variation of the open-circuit potential for α-

brass electrode in different concentrations of

H2SO4 solution at 25ºC……………………………………………

147

58- The variation of the open- circuit potential for α-

brass electrode in 0.5 M HCl with different

concentrations of Ficus carica extract……………………………

148



concentrations of Olive extract…………………………………… 149


brass electrode in 0.5 M H2SO4 with different

concentrations of Ficus carica extract at 25ºC……………………

150




62- The variation between the steady-state potentials and

logarithm of the concentration of extracts α-brass electrode

in 0.5 M HCl solution at 25ºC……………………………………

152

63- The variation between the steady-state potentials and

logarithm of the concentration of extracts α-brass electrode

0.5 M H2SO4 solution at 25ºC………………………………… 153

64- Potentiodynamic polarization curves for α-

brass electrode in 0.5 M HCl in absence and presence

of different concentrations of Ficus carica

extract at 25ºC…………………………………………………….

159

65- Potentiodynamic polarization curves for α- brass electrode

in 0.5 M HCl in absence and presence

of different concentrations of Olive extract at 25ºC………………

160


in 0.5 M H2SO4 in absence and presence of different

concentrations of Ficus carica extract at 25ºC…………………… 162


in 0.5 M H2SO4 in absence and presence

of different concentrations of Olive extract at 25ºC………… 163

List of Tables

Page

1- Data of weight loss of zinc sample in 1.0 M HCl

solution at 25ºC…………………………………………………… 51


solution at 25ºC…………………………………………………… 51


solution at 25ºC…………………………………………………….. 51


solution at 25ºC……………………………………………………. 52


solution at 25ºC……………………………………………………. 52

6- Data of weight loss of zinc sample in 1.0 M H2SO4

solution at 25ºC…………………………………………………….. 53


solution at 25ºC…………………………………………………… 53


solution at 25ºC…………………………………………………… 53


solution at 25ºC…………………………………………………….. 54


solution at 25ºC…………………………………………………… 54

11- Effect of extract concentrations on ( corrR ), ( IE%) and (θ )

or zinc in 2.0 M HCl solution at 25ºC……………………………. 65

12- Effect of extract concentrations on ( corrR ), ( IE%) and (θ )

for zinc in 2.0 M H2SO4 solution at 25ºC………………………….

65

13- Binding constant ( K ) and standard free energy of

adsorption οads

ΔG for plant extracts in 2.0 M HCl

at 25ºC…………………………………………………………….. 71

14- Binding constant ( K ) and standard free energy of

adsorption οads

ΔG for plant extracts in 2.0 M H2SO4

at 25ºC…………………………………………………………… 71

15- Effect of temperature on the corrosion rate ( corrR ) and

efficiency( IE%) for zinc in 2.0 M HCl in absence and

presence of plant extracts…………………………………………. 75


efficiency ( IE%) for zinc in 2.0 M H2SO4 in absence and

presence of plant extracts…………………………………………. 75

17- Activation parameters of zinc in 2.0 M HCl in absence

and presence of plant extracts…………………………………….. 80

18- Activation parameters of zinc in 2.0 M H2SO4 in absence

and presence of plant extracts…………………………………….. 80

19- Reaction number (R.N.) and efficiency (IE% ) of the different

concentrations of Ficus carica and Olive extracts in 2.0 M

HCl solution at 25ºC…………………………………………….. 89

20- Reaction number (R.N.) and efficiency (IE% )of the different

concentrations of Ficus carica and Olive extracts in 2.0 M

H2SO4 solution at 25ºC………………………………………… 89

21- Electrochemical parameters and inhibition efficiency for zinc

in 2.0 M HCl in absence and presence of different

concentrations of Ficus carica extract at 25ºC…………………….

111


in 2.0 M HCl in absence and presence of different

concentrations of Olive extract at 25ºC…………………………

111






concentrations of Olive extract at 25ºC ………………………… 112

25- Corrosion rate of α-brass in different concentrations of HCl

and H2SO4 solutions at 25ºC………………………………………. 116

26- Effect of plant extract concentrations on corrosion rate ( corrR ),

inhibition efficiency ( IE%) and surface coverage (θ ) for

α-brass in 0.5 M HCl at 25ºC……………………………………. 123

27- Effect of plant extract concentrations on corrosion rate ( corrR ),


α-brass in 0.5 M H2SO4 at 25ºC………………………………….. 123

28- Linear fitting parameters and the standard free energy

of plant extracts for α-brass in 0.5 M HCl solution

at 25ºC…………………………………………………………….. 130

29- Linear fitting parameters and the standard free energy

of plant extracts for α-brass in 0.5 M H2SO4 solution

at 25ºC…………………………………………………………..


efficiency ( IE%) for α-brass in 0.5 M HCl in absence and

presence of plant extracts…………………………………………

130

133

31-Effect of temperature on the corrosion rate ( corrR ) and

efficiency ( IE%) for α-brass in 0.5 M H2SO4 in absence and

presence of plant extracts………………………………………….

133

32- Activation parameters of α-brass in 0.5 M HCl in absence

and presence of extracts…………………………………………... 138

33- Activation parameters of α-brass in 0.5 M H2SO4 in absence

and presence of extracts…………………………………………… 138

34- Electrochemical parameters and inhibition efficiency for α-

brass in 0.5 M HCl in absence and presence of different



brass in 0.5 M HCl in absence and presence of different



brass in 0.5 M H2SO4 in absence and presence of different



brass in 0.5 M H2SO4 in absence and presence of different


INTRODUCTION

I. INTRODUCTION

Corrosion(1)

is a surface phenomenon known as the attack of metals or

alloys by their environment as air, water or soil in chemical or

electrochemical reaction to form more stable compounds. It is necessary

to devote more attention to metallic corrosion nowadays than ealier due

to :

1- A more corrosive environment due to the increasing pollution of air

and water.

2- An increased use of metals within all field of technology.

3- The use for special applications as in the atomic energy field of rare

and expensive metals.

The corrosion costs(2)

in most of the countries are in the range of 2-4

% of the gross national product. So it is imperative that economically,

useful measures should be taken to minimize corrosion.

I.1. Corrosion inhibitors :

The protection of metals or alloys against corrosion can be achieved

either by special treatment of the medium to depress its aggressiveness or

by introducing into it small amounts of special substances called

corrosion inhibitors(3)

. Inhibitors are classified according to their action

(as anodic, cathodic and mixed inhibitors) and according to their

mechanism of action (as hydrogen evolution, scavengers, vapour-phase

and adsorption inhibitors).

1. Anodic passivating inhibitors:

Anodic inhibitors that cause a large shift in the corrosion potential are

called passivating inhibitors. They are also called dangerous inhibitors

because, if used in insufficient concentration, they cause pitting and

sometimes an increase in corrosion rate. There are two types of

passivating inhibitors: oxidizing anions such as chromate, nitrite, that can

passivate steel in the absence of oxygen and the non oxidizing ions, such

as phosphate, tungstate, and molybdate, which require the presence of

oxygen in order to passivate steel. However, with careful control

passivating inhibitors are frequently used because they are very effective

in sufficient quantities(4,5)

.

Passivation by inhibitors is more difficult at higher temperatures,

higher salt concentration, lower pH, and lower dissolved oxygen

concentrations.

Nonoxidizing passivators require the presence of oxygen to cause

passivation. They do not inhibit corrosion in the absence of oxygen. They

apparently function by promoting the adsorption of oxygen on the

anodes, thereby causing polarization onto the passive region.

Nonoxidizing passivators are also dangerous when used in insufficient

amount because the oxygen required for passivation is a good cathodic

depolarizer(6)

.

2. Cathodic inhibitors:

Cathodic inhibitors either slow the cathodic reaction itself, or they

selectively precipitate on cathodic areas to increase circuit resistance and

restrict diffusion of reducible species to the cathodes.

The cathodic reaction is often of reduction of hydrogen ions from

hydrogen gas. Some cathodic inhibitors make the discharge of hydrogen

gas more difficult, and they are said to increase the hydrogen

overpotential. Compounds of arsenic and antimony are example of this

type of inhibitors that are often used in acids or in systems where oxygen

is excluded. Another possible cathodic reaction is the reduction of

oxygen. The inhibitors for this cathodic reaction are different forms from

those mentioned for the more acidic systems(7)

.

Other cathodic inhibitors use the increase in alkalinity at cathodic sites

to precipitate insoluble compounds on the surface. The cathodic reaction,

hydrogen ions, and / or oxygen reduction causes the environment

immediately adjacent to the cathodes to become alkaline. Therefore, ions

such as calcium, zinc, or magnesium may be precipitated as oxides to

form a protective layer on the metal, many natural waters are self

inhibiting due to the deposition of a scale on metals by precipitation of

naturally occurring ions.

Inhibition by polarization of the cathodic reaction can be achieved in

several ways. The three main categories of inhibitors that affect cathodic

reactions are cathodic poisons, cathodic precipitates, and oxygen

scavengers(6)

.

3. Ohmic inhibitors:

Inhibitors that increase the ohmic resistance of the electrolyte circuit

have been considered to some extent in previous discussion of anodic

and cathodic filming inhibitors. Because it is usually impractical to

increased the resistance of the bulk electrolyte, increased resistance

practically achieved by the formation of a film, a micro-inch thick or

more, on the metal surface. If the film is deposited selectively on anodic

area, the corrosion potential shifts to more positive values; if it is

deposited on cathodic arease, the shift is to more negative values, and if

the film covers anodic and cathodic areas, there may be only a slight shift

in either directions(6)

.

4. Precipitation inhibitors:

Precipitate-inducing inhibitors are film forming compounds that have

a general action over the metal surface and which, therefore, interfere

with both anodes and cathodes indirectly. The most common inhibitors of

this class are the silicates and phosphates.

In waters with a pH near 7.0, a low concentration of chlorides,

silicates, and phosphates causes passivation of steel when oxygen is

present; hence, they behave as anodic inhibitors. Another anodic

characteristic is that corrosion is localized in the form of pitting when

insufficient amounts of phosphate or silicate are added to saline water.

However, both silicates and phosphates form deposits on steel which

increase cathodic polarization. Thus, their action appears to be mixed,

i.e., a combination of both anodic and cathodic effects.

Silicate is used most often in low salinity waters that contain oxygen.

It has the rare property of inhibiting corrosion of steel that is already

scaled with rust.

Phosphates, like silicates, require oxygen for effective inhibition.

5. Vapour phase inhibitors:

Vapour phase inhibitors (VPI) also called volatile corrosion inhibitors

(VCI), are compounds that are transported in a closed system to the sites

of corrosion by volatilization from a source. In boilers, volatile basic

compounds such as morpholine or octadecylamine are transported with

steam to prevent corrosion in a condenser tubes by neutralizing acidic

carbon dioxide.

Compounds of this type inhibit corrosion by making the environment

alkaline. In closed vapour spaces, such as shipping containers, volatile

solids such as the nitrite, carbonate, benzoate, salts of dicyclohexylamine,

cyclohexylamine and hexamethylenamine, are used.

The mechanism of inhibition by these compounds is not entirely clear.

On contact with metal surface, the inhibitor vapour condenses and is

hydrolyzed by moisture present to liberate, nitrite, benzoate or

bicarbonate ions. Since oxygen is present, nitrite, benzoate or bicarbonate

ions are capable of passivating steel as they do in aqueous solutions.

6. Adsorption inhibitors:

Adsorption of inhibitor compounds can be classified as anodic,

cathodic, or both. Its classification depends primarily on its reaction at the

metal surface and how the potential on the metal is affected. The

chemical structure of the inhibitor molecule plays a significant role and

often determines whether or not a compound will effectively inhibit a

specific system(7)

.

Effectiveness of inhibitors has determined in many ways and

conclusion have been drawn as to the determining factors contributing to

effectiveness. Some general concepts are:

1. The size of organic molecule.

2. The aromaticity and ⁄ or conjugated bonding.

3. Carbon chain length.

4. Bonding strength to metal substrate.

5. The type and number of bonding atoms or groups in the molecule

(can be either π or σ ).

6. Ability for layer become compact, or cross link (molecules

effectively cover extra metal area through shielding).

7. The ability to complex with the atom as a solid within the metal

lattice.

It universally accepted that the organic molecule inhibits

corrosion by adsorbing at the metal / solution interface. However, the

modes of adsorption are dependent upon:

1. The chemical structure of the molecule.

2. The chemical composition of the solution.

3. The nature of the metal surface.

4. The electrochemical potential at metal / solution interface.

There are three principle types of adsorption with organic

inhibitors:

1. π-bond orbital adsorption.

2. Electrostatic adsorption (physical).

3. Chemisorption.

1.2. Natural product as corrosion inhibitors:

A number of organic and synthetic compounds showed a good

anticorrosive activity, most of them are highly toxic to both human and

environment . These toxic affection have led to the use of natural

products as anticorrosion agents which are eco-friendly and harmless. In

1930, plant extracts(8)

dried stems, leaves, seeds and other plants were

used in H2SO4 acid pickling baths. Animal proteins found in products of

meat and milk industries were also used for retarding acid corrosion. The

additives(8)

used in acid, included flour, bran, yeast, a mixture molasses

and vegetable oil, starch and hydrocarbons (tars and oils) are used as

corrosion inhibitors. Recently started to study the application of extracts

of some common plant as a corrosion inhibitors as Onion, Garlic(9)

,

Thyme(10)

and Ginger(11)

for mild steel in acid media. The effects of

saccharides(12)

(reducing sugars – fructose and mannose) on the corrosion

of aluminium and zinc in alkaline media. These extracts contain different

hydroxy organic , and nitrogen containing compounds.

1.3. Corrosion of zinc:

Zinc is one of the most widely used in various industrial operations.

The largest use of zinc is as a protective coating for iron (galvanization).

It protects the iron by cathodic protection, since it is higher on the

electrochemical scale than iron, reducing it to the metal and eliminating

rust. Pure zinc has no reaction with water or dilute acids, because of the

formation of a thin layer of hydrogen gas on its surface. Zinc forms a

hydroxide, Zn(OH)2 that can dehydrate to form the anhydrous oxide ZnO,

and zinc salt in acid solution, or zincate in alkalin solution. Several

studies concerning the action of organic compounds on the corrosion

behaviour of zinc in acid solution . In these media the corrosion of zinc

proceeds via two partial reactions(1)

.

i. The partial cathodic reaction involves the evolution of hydrogen

gas:

2(g)(aq)

H2e2H

ii. The partial anodic reaction involves the oxidation of metal and

formation of soluble zinc ions.

2eZnZn

The study of corrosion of zinc and its inhibition is a subject of

practical significance. Many researchers in the literature studied the

corrosion inhibition of zinc in aqueous media using organic compounds.

I.4. Literature survey on the corrosion inhibition of zinc in

aqueous media:

I.4.i Organic inhibitors in aqueous media:

Agrawal et al(13)

investigated the effect of ethylenediamine N,N0-

dibenzylidene, ethylenediamine N,N0-di(p-methoxybenzylidene),

ethylenediamine N,N0-disalicylidene as corrosion inhibitors for zinc in

sulphuric acid by galvanostatic polarization technique. It was found that

the efficiency of the inhibition is characterized by a relatively greater

decrease in free energy of adsorption, relatively lower entropy of

adsorption and relatively lower heat of adsorption. Galvanostatic

polarization studies indicate that these are basically cathodic inhibitors.

Cathodic protection in the presence of these inhibitors has been studied.

With an efficient inhibitor, cathodic protection is achieved at potentials

much less negative than that required for plain acid. The difference

between protective potential and corrosion potential appears to be less for

an effective inhibitor. Mechanism of the action of inhibitors has been

provided.

Talati et al(14)

studied the corrosion inhibition of zinc in sulphuric

acid containing different m-substituted aniline-N-salicylidene using

polarization method . It appears that the salicylidene part of the inhibitor

including the iminic group in the molecule plays a dominant role in the

inhibition. As far as the effect of exposure period and temperature is

concerned, m-CNS has turned out to be the best one. For all the five

inhibitors, the heats of adsorption and free energies of adsorption are

negative, while the entropies of adsorption are positive. The free energies

of adsorption are more negative in the case of very good inhibitors like

m-TNS and m-CNS, indicating a strong interaction of the inhibitor

molecules with the metal surface. The activation energies in the presence

of these inhibitors are higher than that in plain acid. The adsorption of

these inhibitors follows Langmuir adsorption isotherm. The conjoint

effect of external cathodic current and the inhibitor is observed to be

either synergistic or additive. Galvanostatic polarization studies indicate

that these are mixed type inhibitors with predominant action on the local

cathodes. Mechanism of the action of inhibitors has been provided.

The inhibitive action of semicarbazide, thiosemicarbazide and

diphenlcarbazide towards corrosion of zinc in hydrochloric acid was

investigated by Fouda et al(15)

. They found that the rate of corrosion

depends on the nature of inhibitor and its corrosion. The values of

inhibition efficiency from, weight loss, thermometric measurements are

in good agreement with those obtained from polarization studies. From

the polarization studies, the used act as mixed adsorption type inhibitors

,increased adsorption resulting from an increase in the electron density at

the reactive C=S and C=O groups and N-atoms. The thermodynamic

parameters of adsorption obtained using Bockri-Swinkels adsorption

isotherm reveal a strong interaction of these carbazides on zinc surface.

Rajappa and Venkatesha investigated(16)

the corrosive behaviour of

zinc in HCl solution containing various concentrations of glutaraldehyde

(GTD), glycine (GLN), methionine (MTN), and their condensation

products formed between GTD + GLN(CP1) and GTD + MTN (CP2).

The corrosion inhibitive action of these compounds on zinc metal was

studied using chemical and electrochemical methods. The results showed

that the compound CP2 is the best inhibitor and that its inhibition

efficiency reaches 92.56% at 10−2

M in 0.05 M acid concentration. As an

inhibitor, CP2 was found to have a predominant cathodic effect and its

adsorption was confirmed with the Temkin isotherm. The effect of

temperature on the corrosion of zinc corroded surface was studied by

SEM technique to obtain information about the adsorption of inhibitor

molecules on the zinc surface.

El-Sherbini et al(17)

investigated the inhibitive effect of ethoxylated

fatty acids as inhibitors for the corrosion of zinc metal in 1.0 M

hydrochloric and 1.0 M sulfuric acid solutions at various temperatures

ranging from 25 to 55ºC using weight loss measurement and

electrochemical methods. The protection efficiency depends upon the

type and concentration of the inhibitor and the nature of the acid medium.

In both acid solutions the protection efficiencies of the inhibitors decrease

with the increase in temperature. The inhibition was assumed to occur via

the adsorption of the fatty acid molecules on the metal surface. The

thermodynamic functions of dissolution and adsorption processes were

calculated and discussed.

The effect of some ethoxylated fatty alcohols, with different numbers

of ethylene oxide units, on the corrosion of zinc in 0.5 M HCl was studied

by Abdallah(18)

using weight loss measurements. The inhibition efficiency

was found to increase with increasing concentration, number of ethylene

oxide units per molecule and with decreasing the temperature. Inhibition

was explained on the basis of adsorption of ethoxylated fatty alcohols

molecules on the metal surface through their ethoxy groups. The degree

of surface coverage varied linearly with logarithm of inhibitor

concentration fitting Temkin isotherm. The thermodynamic parameters

were calculated for the tested system from the data obtained at different

temperatures.

Stupnisek-Lisac

and Podbršček(19)

studied the effect of some

substituted N-arylpyrroles on the corrosion of zinc in hydrochloric acid

using electrochemical (d.c. and a.c.) and gravimetric methods. The

influence of the structure and composition of a molecule on the inhibition

characteristics was observed by investigation of the action of the

functional group located on the pyrrole ring (-CHO) and at the ortho

position of the benzene ring (-H, -C1, -CH3). The results have shown that

all the organic compounds investigated possess good inhibiting

properties. In contrast to most commercial acid corrosion inhibitors,

which are highly toxic and very hazardous products, substituted N-

arylpyrroles are nontoxic compounds with good environmental

characteristics.

The corrosion inhibition of zinc in 0.1 M HCl in presence and

absence of some hydrazide derivatives was investigated by El-Gaber et

al(20)

using mass-loss and polarization techniques. Results obtained

showed that the inhibition efficiency increased with the increase of the

concentration of the additives and decreased with the increase of

temperature. Synergism between I- , SCN

- and Br

- anions and hydrazide

derivatives was proposed. The polarization curves showed that hydrazide

derivatives act as mixed-type inhibitors, acting predominantly as cathodic

inhibitors for zinc in 0.1 M HCl . The adsorption of these hydrazide

derivatives on zinc surface follows Temkin adsorption isotherm. Some

thermodynamic parameters were calculated. The kinetic parameters of

corrosion of zinc in HCl solution have been studied.

Dobryszycki and Biallozor(21)

investigated the effect of surfactants and

polyethylene glycols (PEGs) on zinc corrosion in alkaline media using

electrochemical and non electrochemical methods. The effectiveness of

the inhibitors was compared and it was found that the PEG of average

molar weight 400 electrochemically in the first stage of exposition, but

the chemical corrosion prevails after a longer time.

The electrochemical behaviour of zinc in strong alkaline solution

containing 8.5 M of potassium hydroxide (KOH) and polymeric organic

inhibitors was evaluated by Ein-Eli(22)

. The concentration of the organic

inhibitors studies were in the range of 400-4000 ppm and included

polyethylene glycol (PEG), with a molecular weight of 600, and

polyoxyethylene alkyl phosphate ester acid form (GAFAC RA600). The

electrochemical studies included anodic, cathodic, and linear polarization

along with potentiostatic studies. It was found that the inhibition

properties of PEG, in the strong alkaline solution, are by far much more

efficient than the inhibition capability of GAFAC RA600. Surface

analysis obtained with the use of high resolution scanning electron

microscopy (HRSEM) revealed different morphology characteristic

developed at the zinc surface in the presence of the two inhibitors. A

methodology employing electrochemical tests is proposed to quickly and

conveniently evaluate inhibitors for Zn in alkaline media.

The inhibitive effect of cerium(III) chloride CeCl3 and sodium

octylthiopropionate C8H17S(CH2)2COONa (NaOTP) toward the corrosion

of zinc in 0.5 M NaCl was investigated by Aramaki(23)

using polarization

measurements after immersion of a zinc electrode in the solution for

many hours. The inhibition efficiency of 1 × 10-4

M CeCl3 plus1× 10-5

M

NaOTP mixture was high, 95.1% after both 3 and 120 h. X-ray

photoelectron spectroscopy and electron-probe microanalysis for the

inhibited electrode revealed that the zinc surface was covered with a

protective film composed of an hydrated or hydroxylated Ce-rich oxide, a

small amount of Zn(OH)2 and a trace of Zn(OTP)2 chelate. The inhibition

effect of 1 × 10-5

M NaOTP in the NaCl solution for the zinc electrode

previously treated in 1×10-3

M CeCl3 for 30 min was also examined,

indicating a higher inhibition efficiency, 96.3% after immersion of the

electrode in the solution for 120 h.

Wang et al(24)

investigated the inhibitive action of 2-mercaptobenz-

imidazole on the corrosion of zinc in phosphoric acid (H3PO4) solution

by weight loss and polarization techniques. The studies reveal that the

inhibitor is effective for the inhibition of zinc in H3PO4 solution and

retards the anodic and cathodic corrosion reactions with emphasis on the

former.

The anodic behaviour of Zn electrode in 1×10-2

M Na2B4O7 solutions

in the absence and presence of various concentrations of Na2SO4,

Na2S2O3 or Na2S as aggressive agent was studied by Abd El Aal(25)

using

galvanostatic polarization technique. In the absence of sulphur-

containing anions in solution, the polarization curves are characterized by

one distinct arrest corresponding to Zn(OH)2 and/or ZnO, after which the

potential increases linearly with time due to the formation of barrier oxide

film before reaching the oxygen evolution reaction. The duration time of

the arrest decreases with increasing current density while the rate of oxide

film formation increases. On the other hand, the duration time of the

arrest increases with the number of anodic cyclization while the rate of

oxide film formation decreases. Additions of low concentration of the

aggressive anions have no effect on the passive film formed on the metal

surface. The potential starts to oscillate within the oxygen evolution

region with increases in the concentration of these aggressive anions.

Further increases in the concentration of these aggressive anions are

associated with impaired Zn passivity that might indicate pitting attack.

The aggressiveness of the sulphur species decreases in the order:

24

SO > 232

OS > 2S . The effect of raising pH of the solution on the

anodic behaviour of Zn electrode in the presence of 24

SO anions was

also investigated. It was found that the raising the pH of the solution

affecting on the rate of oxide film formation and the breakdown potential

value.

Abd El Aal(26)

studied the open-circuit potentials of Zn electrode as a

function of time in different concentrations of Na2B4O7 solution. The

potential shifts immediately towards positive values, indicating film

thickening and repair. The rate of oxide film thickening was determined

from the linear relationship between the open-circuit potential, E, of the

Zn electrode and the logarithm of immersion time t as E = a1 + b1 log t.

The liner plots consist of two segments indicating the duplex nature of the

formed oxide film on the Zn surface. The final steady- state potential, Est.

, varied with the logarithm of molar concentration of Na2B4O7 solution

according to: Est. = a2 – b2 log C Na2B4O7. The effect of rising pH and

temperature was also studied. It was found that the rising of pH and

temperature of the solution affect on the rate of oxide film thickening and

the final steady- state potential.

Hassan(27)

investigated the corrosion behaviour of zinc in aerated

neutral perchlorate solutions using three different techniques

potentiodynamic polarization, potentiostatic current time transient, and

electrochemical impedance spectroscopy (EIS). The potentiodynamic

anodic polarization cyclic voltammetry curves exhibit an active/passive

transition followed by pitting corrosion, confirmed by (SEM), due to the

diffusion-controlled formation of a ZnO film by the dissolution–

precipitation mechanism. The cyclic voltammograms show an anodic

peak AI and two cathodic peaks CI and CII. The peaks AI and CII are

correlated to the formation and reduction of ZnO film, respectively, and

CI is attributed to the reduction of the pitting corrosion products. The

potentiostatic current time transients at different electrolyte

concentrations and applied potentials involve three stages: the first

involving ZnO layer growth, and the second and third involving pit

nucleation and growth, respectively. The nucleation rate ( 1i

t ) increases

with increasing electrolyte concentration and anodic applied potential.

(EIS) shows an increase in the charge transfer resistance with applied

potential near the anodic peak AI as a result of passive film formation. At

higher anodic potentials, the charge transfer resistance decreases as the

applied potential approaches the breakdown potential Eb. A nearly ideal

Warburg tail of a dihedral angle of 45° is obtained, suggesting that the

corrosion of Zn in NaClO4 solution is controlled by diffusion in the

passive range.

I.4.ii. Natural product inhibitors in aqueous media:

El-Hosary et al(28)

studied the effect of Hibiscus subdariffa (Karkade)

extract on the dissolution of Al and Zn in NaOH using the thermometric,

the weight-loss and the galvanostatic polarization techniques. The extent

of corrosion inhibition as measured by the three techniques is

comparable. The results indicated that the additive acts by way of

adsorption on both cathodic and anodic corrosion areas. Curves

representing the variation of the reaction number (R.N.) in thermometric

experiments, and the decrease in weight as a function of the concentration

of the additive, are invariably sigmoid in nature. When present in enough

amounts, the additive decreases the dissolution rate by as much as 85 per

cent of the value recorded in its absence. The two main constituents of the

aqueous extracts of Hibiscus subdariffa, namely the organic acids and the

colouring materials were separated and tested independently for surface

activity. Both constituents were effective in retarding the dissolution of

the two metals; but the activity of the colouring portion was considerably

higher than that of the organic acids.

Müller(12)

investigated the inhibitive effect of saccharides and

ascorbic acid on the corrosion of both aluminum and zinc pigments in

aqueous alkaline media using gasometric method. He found that the

hydrogen corrosion of aluminum pigment can be inhibited with addition

of the reducing saccharides fructose and mannose as well as with addition

of the reducing ascorbic acid whereas the non-reducing saccharose did

not inhibit this corrosion reaction. With increasing addition of fructose or

ascorbic acid the hydrogen volumes evolved increase; this observation

leads to the assumption that reaction products of aluminum and fructose

or ascorbic are the actual corrosion inhibitors. The hydrogen corrosion of

zinc pigment is inhibited by ascorbic acid only. So, the most efficient of

the examined natural corrosion inhibitors both for aluminum and zinc

pigment is ascorbic acid. But corrosion inhibition of ascorbic acid on zinc

pigment is much less effective when compared to aluminum.

The corrosion inhibition of zinc in hydrochloric acid by extract of

Nypa Fruticans Wurmb was studied by Okorosaye and Oforka(29)

using

weight loss techniques. Maximum inhibition efficiency and surface

coverage was obtained at an optimum concentration. However increase in

temperature decreased the inhibition efficiency at the temperature range

studied. The inhibition action of Nypa Fruticans Wurmb extract

compared closely to that of 1, 5 Diphenyl Carbazone (DPC). Optimum

inhibition efficiency for zinc in the presence of Nypa Fruticans Wurmb

extract was 36.43% and 40.70% with DPC. The phenomenon of physical

adsorption has been proposed from the activation energy values (-19.33

kJ mol-1

and -21.11 kJ mol-1

) with Nypa Fruticans Wurmb extract and

DPC respectively. A first order kinetics has been deduced from the

kinetic treatment of the results. The heat of adsorption, Qad range from

(-33.63 to - 58.52 kJ mol-1

) for both additives studied. Mean

adsΔG

values (-7.82 to -8.68 kJ mol-1

) are negative and suggestive of adsorption

on metal surface. The data obtained from this study fits well into the

Langmuir isotherm.

The aqueous extract of the leaves of henna (lawsonia) was tested by

El-Etre et al(30)

as corrosion inhibitor of C-steel, nickel and zinc in acidic,

neutral and alkaline solutions using the polarization technique. It was

found that the extract acts as a good corrosion inhibitor for the three

tested electrodes in all tested media. The inhibition efficiency increases as

the added concentration of extract is increased. The degree of inhibition

depends on the nature of metal and the type of the medium. For C-steel

and nickel, the inhibition efficiency increases in the order: alkaline <

neutral< acid, while in the case of zinc it increases in the order: acid <

alkaline < neutral. The extract acts as a mixed inhibitor. The inhibitive

action of the extract is discussed in view of adsorption of lawsonia

molecules on the metal surface. It was found that this adsorption follows

Langmuir adsorption isotherm in all tested systems. The formation of

complex between metal cations and lawsonia is also proposed as

additional inhibition mechanism of C-steel and nickel corrosion.

Juzeliunas and co-workers(31)

studied the effect of the

microorganisms Penicillium frequentans, Aspergillus niger and Bacillus

mycoides under laboratory conditions with controlled humidity(ca. 97%)

and temperature (ca. 26 °C) on the corrosion of Al and Zn sample.

Electrochemical impedance spectroscopy (EIS) ascertained microbially

influenced corrosion acceleration (MICA) of zinc samples. By contrast,

microbially influenced corrosion inhibition was determined for

aluminium samples. EIS data indicated a three-layer structure developed

during zinc corrosion. The reasons of (MICA) lie mainly in diminishing

of the inner layer (next to the metal), whose passivating capacity is much

higher when compared to other layers. Two-layer structure was identified

on aluminium samples: a native aluminium oxide and the layer resulted

from oxide interaction with products of metabolism of microorganisms. A

simultaneous increase in the oxide layer resistance and decrease in the

layer thickness implied that microorganisms promoted passivity at the

sites of localized corrosion.

El-Etre and El-Tantawy(32)

investigated the inhibitive action of the

Ficus nitida leaves toward general and pitting corrosion of C-steel, nickel

and zinc in different aqueous media using weight loss, potentiostatic and

potentiodynamic polarization techniques. It was found that the presence

of Ficus extract in the corrosive media (acidic, neutral or alkaline)

decreases the corrosion rates of the three tested, inhibition efficiency

increases as the extract concentration is increased. The inhibition

efficiency depends on the type of corroded metal and on the corrosive

solution. It was also found that the presence of Ficus extract in the

chloride containing solution shifts the pitting potentials of the tasted

metals toward the noble direction. The inhibitive action of Ficus extract is

discussed in view of adsorption of its components, the poly aromatic

compounds, friedelin, epifridelanol and nitidol, on the metal surface.

Also it was found that such adsorption follows Langmuir adsorption

isotherm. The calculated values of the free energy of adsorption indicated

that adsorption process is spontaneous.

Biosorption of heavy metal from aqueous solutions with tobacco dust

was examined by Qi and Aldrich(33)

. They found that the tobacco dust

exhibited a strong capacity for heavy metals, such as Pb(II), Cu(II),

Cd(II), Zn(II) and Ni(II), with respective equilibrium loadings of 39.6,

36.0, 29.6, 25.1 and 24.5 mg of metal per g of sorbent. Moreover, the

heavy metals loaded onto the biosorbent could be released easily with a

dilute HCl solution. Zeta potential and surface acidity measurements

showed that the tobacco dust was negatively charged over a wide pH

range (pH > 2), with a strong surface acidity and a high OH adsorption

capacity. Changes in the surface morphology of the tobacco dust as

visualized by atomic force microscopy suggested that the sorption of

heavy metal ions on the tobacco could be associated with changes in the

surface properties of the dust particles. These surface changes appeared to

have resulted from a loss of some of the structures on the surface of the

particles, owing to leaching in the acid metal ion solution. However,

Fourier transform infrared spectroscopy (FTIR) showed no substantial

change in the chemical structure of the tobacco dust subjected to

biosorption. The heavy metal uptake by the tobacco dust may be

interpreted as metal–H ion exchange or metal ion surface complexation

adsorption or both.

The effect of sodium eperuate prepared from Wallaba (Eperua

falcata Aubl) extract as an inhibitor on the corrosion of zinc in alkaline

solutions with chloride ions was investigated by Li et al(34)

using

electrochemical techniques. Sodium eperuate inhibits the corrosion of

zinc in 0.1 M NaCl solutions with pH 9.6. As its concentration increases

to 1.0 g/L, the inhibition efficiency reaches approximately 92%. In

alkaline solutions with pH 12.6, sodium eperuate has no adverse effect on

passivity of zinc, and retards the chloride attack. These suggest that

sodium eperuate is an effective inhibitor for the protection of zinc in

alkaline environments.

The inhibitive effect of fenugreek (Trigonell foenum graecum) seeds

extract on the corrosion of zinc in aqueous solution of 0.5 M sulphuric

acid were investigated by Abdel-Gaber(35)

at 30, 35, 40 and 45°C using

potentiodynamic polarization and electrochemical impedance

spectroscopy (EIS) techniques. Potentiodynamic polarization curves

indicated that the fenugreek seeds extract behaves as an anodic type

inhibitor. EIS measurements showed that the dissolution process occurs

under activation control. Inhibition was found to increase with increasing

concentration of the fenugreek seeds extract but decreases with increasing

temperature. The associated activation parameters were determined and

the results showed that the fenugreek seeds extract could serve as an

effective inhibitor for the corrosion of zinc in sulphuric acid media at

higher temperature. The influence of exposure time on the performance

of fenugreek seeds extract was investigated. Results showed that the

extract performed well after an induction period that is required to release

zinc ions in solution in stoichiometric ratio necessary to form stable

adsorbed complex with the chemical constituents of the extract.

The inhibitive effect of the natural juice of Onion bulb on the

corrosion of zinc in HCl solution was studied by El-Etre(36)

using

hydrogen evolution and weight loss measurements as well as

potentiodynamic polarization technique. Potentiodynamic anodic

polarization was used to determine the effect of the juice on pitting

corrosion of zinc in NaCl solution. It was found that the presence of

Onion juice reduced markedly the corrosion rate of zinc in the acid

solution. The inhibition efficiency increases as the juice concentration is

increased. Moreover, the presence of the juice in NaCl solution shifts the

pitting potential toward more positive direction. The inhibitive effect of

Onion juice is discussed on the basis of adsorption of its components on

the metal surface. Such adsorption is found to follow Langmuir

adsorption isotherm. Negative values were obtained for the energy of

adsorption indicating the spontaneity of the adsorption process. The

formation of insoluble complexes as a result of interaction between zinc

cations and S-(1-propenyl)-L-cysteine sulfoxide, which is present in the

juice, is also discussed.

The effect of the extract of Aloe vera leaves on the corrosion of zinc in

2.0 M HCl solution was studied by Abiola(37)

using weight loss technique.

Aloe vera extract inhibited the corrosion of zinc in 2.0 M HCl solution

and the inhibition efficiency increased with increasing concentration of

the extract but decreased with increasing temperature. The adsorption of

the inhibitor molecules on zinc surface was in accordance with Langmuir

adsorption isotherm. A first- order kinetics relationship with respect to

zinc was obtained with and without the extract from the kinetics

treatment of the data.

1.5. Corrosion of copper and alloys:

Copper is a valuable material especially in electronics, solar cell

fitting, household products, structural engineering, coinage, art and

decoration. Nowadays 20% of the weight of a mobile phone consists of

copper and copper alloys. Copper is easily combind with metals as Zn,

Al, Ni and Sn forming alloys. Copper with zinc forms brasses, which are

generally divided into the following categories:

1-Single phase (alpha) brasses, Zn ‹ 37%

2-Two-phase (alpha + beta) brasses, 37% ‹ Zn ‹ 46%

3-Single phase(beta) brasses , 46% ‹ Zn ‹ 50%

4-Two-phase(beta + gamma) brasses, Zn › 50%.

Alpha brasses have been widely used as tubing material for

condensers and heat exchangers in various cooling system. Their

exhibition in acid media creates problems of corrosion. When the brasses,

containing more than 15% of zinc, are exposed in corrosive environment,

they are not only causing general corrosion damage, but also causing

dezincification process involving preferential dissolution of zinc, leaving

a spongy mass of copper on the alloy surface(38)

. Recently many methods

have been developed aiming to reduce the rate of dezincification of brass.

One of these is the use of inhibitors. Several organic compounds as

heterocyclic derivatives act as inhibitors are able to form a protective film

on the surface of metal through their function groups which are adsorbed

in brasses either physically or chemically. There are a lack in the studies

concerning the use of the natural products as corrosion inhibitors of α-

brass (70% Cu- 30% Zn).

I.6. Literature survey on the corrosion inhibition of copper

and its alloys in aqueous media:

1.6.i. Organic inhibitors in aqueous media:

Barouni et al(39)

studied the inhibitive effect of five amino acids (AA)

on the corrosion of copper in molar nitric acid solution using weight loss

and electrochemical polarization measurements. He found that, Valine

(Val) and Glycine (Gly) accelerate the corrosion process; but Arginine

(Arg), Lysine (Lys) and Cysteine (Cys) inhibit the corrosion

phenomenon. Cysteine is the best inhibitor. Its efficiency increases with

the concentration to attain 61% at 10−3

M. Correlation between the

quantum chemical calculations and inhibition efficiency was discussed

using semi-empirical methods.

Quartarone and co-workers(40)

studied the corrosion inhibition of the

copper in 0.5 M H2SO4 solutions by indole-3-carboxylic acid (ICA) in the

temperature range from 25 to 55ºC using potentiodynamic polarization

and electrochemical impedance spectroscopy (EIS) techniques. The

results obtained from the both measurement techniques revealed good

inhibitor efficiency in the studied concentration range. Nyquist plots

showed depressed semicircles with their centre below real axis.

Moreover, the impedance spectra in the case of both non inhibited

solutions and inhibited ones by means of lower inhibitor concentrations

exhibited Warburg impedance. The adsorption behaviour of (ICA)

followed Langmuir isotherm.

The inhibitive action of N-(5,6-diphenyl-4,5-dihydro-[1,2,4] triazin-

3-yl)-guanidine (NTG) on the corrosion of copper in 0.5 M sulphuric acid

was investigated by Khaled(41)

. (NTG) was synthesized and studied as an

inhibitor for corrosion of copper. Results of weight loss, potentiodynamic

polarization and electrochemical impedance (EIS) measurements

consistently identify (NTG) as a good copper corrosion inhibitor.

Potentiodynamic polarization studies clearly showed that (NTG) is a

cathodic-type inhibitor for copper in 0.5 M H2SO4 solutions. Data

obtained from EIS were analyzed to model the corrosion inhibition

process through an equivalent circuit. The adsorptive behaviour of (NTG)

on copper in 0.5 M H2SO4 was also investigated. The

copper/NTG/solvent interfaces were simulated and the charges on (NTG)

molecule as well as its structural parameters were calculated in presence

of solvent effects. Adsorption of (NTG) on the surface of copper is found

to obey the Langmuir adsorption isotherm.

The corrosion inhibition of copper in aerated 0.5 M H2SO4 solution in

presence of two classes of heterocyclic compounds, namely phenylazo-

pyrazolones (PAP) or hydroxy quinoline and bromobenzyl-carboxy-1,2,3

triazole (BCT) derivatives was studied by Elmorsi and Hassanein(42)

. The

corrosion parameters were obtained at different inhibitors and the

concentration, temperature effect was also investigated. The corrosion

rate of copper was determined; using Tafel method and the polarization

resistance (Rp) technique. The electrolyte solution was analysed using

cyclic voltammetry and UV-visible spectroscopy. The results were

compared with that of some prepared Cu-complexes in order to explain

the inhibition mechanism. Moreover, the thermodynamic activation

parameters of the Cu-corrosion reaction were calculated and discussed in

relation to the stability of the protective layer. The inhibition effect was

attributed to the adsorption of the dye molecules, the precipitation of Cu-

chelates and/or formation of p-complexes at the electrode surface.

The corrosion inhibition of copper in 0.1 M HCl in the presence of

pyrazole was studied by Geler and Azambuja(43)

using potentiodynamic

techniques with rotating disc and rotating ring disc electrode (RRDE). It

was observed that the presence of pyrazole changes the mechanism of Cu

dissolution in 0.1 M HCl. RRDE measurements showed that Cu+ was

detected in the whole potential range studied, while Cu+2

was formed

only at high anodic overpotentials. The addition of pyrazole decreases

the rate of cuprous species formed and increases that of cupric species.

Pyrazole behaves like a cathodic inhibitor and the inhibition efficiency is

influenced by mass transport.

The effect of aniline derivatives, namely 2-chloroaniline, 2-

fluoroaniline, 2-aminophenetole, 2-ethylaniline, o-aminoanisole and o-

toluidine on the corrosion of copper in 0.5 M HCl was investigated by

Khaled and Hackerman(44)

. These compounds were studied in

concentrations from 10−3

to 10−4

M at temperature 298 K. Effectiveness

of these compounds was assessed through potentiodynamic polarization

and electrochemical impedance spectroscopy (EIS) measurements. These

compounds inhibit the corrosion of copper in HCl solution to some

extent. In each case, inhibition efficiencies increase with increasing

concentration. A suggested model for the interface as well as some

kinetic data is presented. These inhibitors obey the Temkin adsorption

isotherm. A correlation between structure and inhibition efficiencies is

suggested.

Zhang et al(45)

investigated the effect of a novel corrosion inhibitor,

bis-(1- benzotriazolymethylene)-(2,5-thiadiazoly)-disulfide (BBTD), on

the corrosion of copper in 3% NaCl and 0.5 M HCl. Results reveal that

(BBTD) is a good inhibitor and behaves better in 3% NaCl than in 0.5 M

HCl solution. Potentiodynamic polarization studies clearly showed that

(BBTD) is mixed-type inhibitor for copper in chloride solutions. It

decreases the anodic reaction rate more strongly than the cathodic

reaction rate and it renders the open–circuit potential of copper more

positive in either HCl or NaCl solutions. The FT-IR spectra indicated that

(BBTD) prevented copper from corrosion by adsorption on the copper

surface to form a protective complex with the Cu(I) ion.

The corrosion inhibition of copper by N-phenyl-1,4-phenylenediamine

(NPPD) in de-aerated, aerated, and oxygenated aqueous 0.5 M HCl

solutions was investigated by Sherif and Park(46)

using potentiodynamic

polarization, potentiostatic current–time, electrochemical impedance

spectroscopy and weight-loss measurements, along with scanning

electron microscopic (SEM) and energy dispersive X-ray (EDX)

experiments. Potentiodynamic polarization measurements showed that

the (NPPD) molecules significantly decrease cathodic, anodic, and

corrosion currents in all these solutions. Potentiostatic current–time

measurements as well as (SEM) and (EDX) investigations of the copper

surface revealed that (NPPD) suppresses the copper dissolution current

due to its adsorption on the copper surface as a Cu(I)–(NPPD) complex.

Impedance measurements also supported the results obtained from both

the potentiodynamic and potentiostatic experiments. The inhibition

efficiencies measured from polarization, electrochemical impedance

spectroscopy (EIS), and weight-loss experiments are all internally

consistent with each other. These results together showed that (NPPD) is

a good mixed-type inhibitor for copper corrosion in all studied solutions.

Abd El-Maksoud examined(47)

the effect of hexadecyl pyridinium

bromide (HPB) and hexadecyl trimethyl ammonium bromide (HTAB) on

the corrosion behaviour of iron and copper in hydrochloric and sulphuric

acid solutions by potentiodynamic polarization and Tafel extrapolation

methods. The polarization curves indicate that the two compounds behave

as mixed inhibitors, but the cathode is more inhibited. (HPB) is more

effective than (HTAB) in both acids; this is explained on the basis of the

charge located on the nitrogen atom on the two compounds. The

inhibition efficiency of the compounds investigated is more effective for

iron and copper metals in HCl than in H2SO4, which is explained on the

basis of the potential of zero charge of the metal surface and the

adsorption ability of both Cl¯ and -2

4SO on the metal surface.

Sherif et al(48)

investigated the corrosion inhibition of copper by 3-

amino-1,2,4-triazole-5-thiol (ATT) in aerated acidic chloride pickling 0.5

M HCl solutions, using electrochemical techniques and weight-loss

measurements, along with Raman spectroscopy. Electrochemical

measurements for copper after varied immersion periods of 0, 24, and 48

h showed that the presence of (ATT) and the increase of its concentration

significantly decrease cathodic, anodic, corrosion currents density Icorr

and corrosion rates corrK , as well as the dissolution currents at 300 mV

vs. Ag/AgCl, while increasing polarization resistance Rp, degree of

surface coverage θ and inhibition efficiency IE% to a great extent.

Weight-loss measurements after different immersion periods of 6 to 48 h

revealed that the dissolution of copper decreased to a minimum and the

corresponding IE% increased with increasing (ATT) concentration. The

detection of (ATT) molecules on the copper surface by Raman

spectroscopy indicated that inhibition of copper corrosion is achieved by

strong adsorption of (ATT) molecules onto the copper surface.

The inhibitive action of two new Schiff bases namely SB1: 2-({-1-

methyl-3-[(2-sulfanylphenyl)imino]butylidene}amino) -1-benzenethiol

and SB2:2-({-1,2-diphenyl-2-[(2-sulfanylphenyl)imino]ethylidene}

amino)- 1- benzenethiol on the corrosion of copper in hydrochloric acid

was studied by Behpour et al(49)

. The Schiff bases were synthesized and

studied as inhibitors for corrosion of copper. Results of electrochemical

impedance and Tafel polarization measurements consistently identify

both compounds as good inhibitors. Impedance spectroscopy revealed

that the corrosion of copper in hydrochloric solution was influenced to

some extent by mass transport since the Warburg impedance was

observed in some cases. Polarization curves indicate that both studied

Schiff bases act as mixed type (cathodic/anodic) inhibitors. Differences in

inhibition efficiency between SB1 and SB2 are correlated with their

chemical structures. Langmuir isotherm is found to provide an adsorption

description of Schiff bases.

Ismail(50)

investigated the efficiency of cysteine as a non-toxic

corrosion inhibitor for copper metal in 0.6 M NaCl and 1.0 M HCl by

potentiodynamic polarization measurements and electrochemical

impedance spectroscopy (EIS). Inhibition efficiency of about 84% could

be achieved in chloride solutions. The presence of Cu2+

ions increases the

inhibition efficiency to 90%. Potentiodynamic polarization measurements

showed that the presence of cysteine in acidic and neutral chloride

solutions affects mainly the cathodic process and decreases the corrosion

current to a great extent and shifts the corrosion potential towards more

negative values. The experimental impedance data were analyzed

according to a proposed equivalent circuit model for the electrode/

electrolyte interface. Results obtained from potentiodynamic polarization

and impedance measurements are in good agreement. Adsorption of

cysteine on the surface of Cu, in neutral and acidic chloride solutions,

follows the Langmuir adsorption isotherm. The adsorption free energy of

cysteine on Cu (∼−25 kJ mol−1

) reveals a strong physical adsorption of

the inhibitor on the metal surface.

The corrosion inhibition of three amino acid compounds on copper

was investigated by Zhang et al(51)

using electrochemical method. They

suppressed cathodic current densities and shifted the corrosion potential

towards more negative values. The interaction between amino acid and

copper surface was certified by reflected FT-infrared spectroscopy. The

quantum chemical parameters were obtained by PM3 semi-empirical

calculation. Glutamic acid has the smaller net positive charge of nitrogen

atom and the more net negative charge of oxygen atoms. The improved

inhibition of glutamic acid was due to the stabilization of its adsorption

on the copper surface by the oxygen atoms in its structure.

Corrosion inhibition of copper in acidic chloride pickling 0.5 M HCl

solutions by 5-(3-Aminophenyl)-tetrazole (APT) was studied by Sherif et

al(52)

using potentiodynamic polarization, chronoamperometry (CA),

electrochemical impedance spectroscopy (EIS), weight-loss and Raman

spectroscopy investigations. Electrochemical measurements showed that

the presence of (APT) and the increase of its concentration significantly

decrease the cathodic, anodic, and corrosion currents as well as corrosion

rates. This effect also decreases the dissolution currents of copper at 200

mV vs. Ag/AgCl, and greatly increases surface and polarization

resistances and inhibition efficiency as indicated by (CA) and (EIS)

measurements. Weight-loss data revealed that the corrosion rate of copper

decreases to a minimum and the inhibition efficiency increases to a

maximum in the presence of (APT) and upon increasing of its

concentration even after 72 h of copper coupons immersion. Comparing

the Raman spectrum obtained on the copper surface after its immersion in

HCl solution containing 1.0 mM APT for 72 h to the spectrum obtained

for the solid (APT) alone indicated that (APT) molecules inhibit the

corrosion of copper via their adsorption onto its surface.

Sanad et al(53)

studied the effect of benzotriazole (BTA) on the

corrosion of brass in 0.1 N HC1, 0.1 N H2SO4 and 0.1 N NH4C1 by

galvanostatic measurements. (BTA) showed a good inhibition effect in

some corrosive media. The percentage inhibition in 0.1 N HC1 solution

was found to increase with increasing (BTA) concentration. In contrast,

the percentage inhibition in 0.1 N H2SO4 was inversely proportional to the

(BTA) concentration. The highest inhibition efficiency was obtained in

0.1 N NH4C1. The role of (BTA) as an inhibitor for brass was explained

in terms of the chemisorption of (BTA) molecules at some active sites on

brass surfaces.

The dezincification of 60/40 brass in acidic chloride and sulphate

solutions under accelerated experimental conditions was studied by

Dinnappa and Mayanna(54)

using weight loss and potential measurement

techniques. A particular variation of potential with immersion time was

observed to be the characteristic feature of dezincification. The results

obtained support substantially the operation of both selective dissolution

of zinc and simultaneous dissolution of copper and zinc followed by

redeposition of copper, alternatively with time, during dezincification.

The effect of surface active compounds such as thiourea, thioglycolic

acid, thioglycol and halo-acetic acids on dezincification of brass has been

discussed on the basis of models proposed for synergistic effects of

organic molecules and anions involved in the medium.

The effect of the addition of some tetrazolic type organic compounds:

1-phenyl-5-mercapto-1,2,3,4-tetrazole (PMT), 1,2,3,4- tetrazole (TTZ), 5-

amino-1,2,3,4-tetrazole (AT) and 1-phenyl-1,2,3,4-tetrazole (PT) on the

corrosion of brass in nitric acid was studied by Mihit et al

(55) using

weight loss, polarization and electrochemical impedance spectroscopy

(EIS) measurements. The explored methods gave almost similar results.

Results obtained reveal that (PMT) is the best inhibitor and the inhibition

efficiency IE% follows the sequence: (PMT) > (PT) > (AT) > (TTZ).

Polarization measurements also indicated that tetrazoles acted as mixed-

type inhibitors without changing the mechanism of the hydrogen

evolution reaction. Partial p-charge on atoms has been calculated.

Correlation between the highest occupied molecular orbital energy

EHOMO and inhibition efficiencies was sought. The adsorption of (PMT)

on the brass surface followed the Langmuir isotherm. Effect of

temperature is also studied in the (25–50ºC) range.

The comparative study of corrosion behaviour of brasses 70Cu3 0Zn

and 60Cu40Zn in HNO3 solution in absence and in presence of 1-phenyl-

5-mercapto-1,2,3,4-tetrazole (PMT) was studied by Mihit et al(56)

using

gravimetric and electrochemical methods. Results obtained are in good

agreement and reveal that the corrosion rate depends on immersion time

and zinc content in the alloy. Copper and zinc losses from each specimen

studied, at various immersion times, were estimated by Atomic

Adsorption Spectroscopic analysis. This shows that the inhibition

efficiency of the inhibitor towards copper is more significant than zinc.

PMT was adsorbed preferentially on the copper surface and inhibits the

process of corrosion of brasses in the nitric acid medium.

Ravichandran et al(57)

investigated the effect of N-[1-(benzotriazol-1-

yl) methyl]aniline (BTMA) and 1-hydroxy methyl benzotriazole (HBTA)

on corrosion of brass in neutral aqueous NaCl solution using weight-loss

measurements, potentiodynamic polarization and electrochemical

impedance spectroscopy (EIS). Polarization studies showed that these

inhibitors were found to act as mixed type for brass in chloride solution.

It suppresses the cathodic and anodic reactions rates and it renders the

open-circuit potential to more noble directions. Solution analysis was

used to calculate the dezincification factor. The Fourier transform

infrared spectroscopy (FTIR) was used to characterize the surface film.

Abd El Meguid and Awad(58)

studied the effect of benzotriazole

(BTAH) on the dissolution of α-brass (70% Cu–30% Zn) in 1.0 M LiBr

using potentiodynamic polarization. Polarization curves showed that an

initial active region of the alloy dissolution followed by two well defined

anodic current peaks then a narrow passivation region before the pitting

potential (Epit) is reached. The initial active anodic region exhibited Tafel

slope with 90 mV dec-1

attributed to the formation of 2

CuBr complexes.

The anodic current peaks were attributed to the formation of CuBr and

Cu2+

ions, respectively. The change of pH values of LiBr solution did not

affect the anodic polarization curves of α-brass. Increasing the solution

temperature from 30 to 90ºC changed the corrosion type from pitting to

general one. The addition of 10-2

M benzotriazole (BTAH) to 1.0 M LiBr

solution is completely inhibited the pitting corrosion at 30ºC while it did

not inhibit the pitting at 90ºC. The inhibition effect was attributed to the

adsorption of BTAH molecules on the alloy surface, which obeys

Langmuir isotherm. The presence or absence of pitting corrosion was

confirmed by using (SEM).

The corrosion behaviour of brass (67% Cu, 33% Zn) in chloride

solutions and in the presence of acetate ions was investigated by

Kilinççeker(59)

using electrochemical impedance spectroscopy (EIS) and

potentiodynamic polarization measurements. The pH of solution was

adjusted to 8.5. It was shown that, CH3COO− ions could inhibit anodic

dissolution of brass in aggressive chloride media. The effect of

temperature was also studied in a range between 298 and 328 K. The

inhibition effect of acetate ions was resulted from complex formations

(between acetate ions and corrosion products) which physically adsorbed

on the surface. The inhibition efficiency decreased with the temperature.

The results obtained at different temperatures were used to evaluate

various thermodynamical properties.

Kosec and co-workers(60)

investigated the inhibitive effect of

benzotriazole (BTAH) on the corrosion of copper, zinc and copper-zinc

(Cu–10Zn and Cu–40Zn) alloys in chloride solution using

electrochemical techniques, atomic force microscopy (AFM) and X-ray

photoelectron spectroscopy (XPS). Electrochemical reactions and surface

products formed at the open-circuit potential and as a function of the

potential range are discussed. The addition of benzotriazole to aerated,

near neutral 0.5 M NaCl solution affects the dissolution of copper, zinc,

Cu–10Zn and Cu–40Zn alloys. The research also compares the inhibition

efficiency and Gibbs adsorption energies of the investigated process.

Benzotriazole, generally known as an inhibitor of copper corrosion is also

shown to be an efficient inhibitor for copper–zinc alloys and zinc metal.

The surface layer formed on alloys in BTAH-inhibited solution

comprised both oxide and polymer components, namely Cu2O and ZnO

oxides, and Cu(I)-BTA and Zn(II)-BTA polymers. The formation of this

mixed copper–zinc oxide polymer surface film provides an effective

barrier against corrosion of both metal components in chloride solution.

El Warraky(61)

studied the effect of 2-methyl benzimidazole (MBIA)

and Armohib 28 inhibitors on corrosion behaviour of brass 70Cu/30Zn in

deaerated solutions of pure dilute HCl and acidified 4% NaCl of pH 1.8-2

at different temperatures between 25 and 60°C by the weight loss

method. The curves representing the variation of loss in weight with time

indicate that there is an induction period followed by a linear relation.

Analysis of the solution by the atomic absorption technique and

investigation of the surface by scanning electron microscopy indicate that

the linear relation begins at a critical value of copper-ion concentration of

the corrodent due to autocatalysis. The addition of different

concentrations of CuCl2 eliminates the induction period and increases the

corrosion rate. The effect of the addition of different concentrations of 2 -

methyl benzimidazole (MBIA) and Armohib 28 inhibitors on the

dissolution of the alloy in both media was tested at 60~ MBIA has no

effect in the case of acidified 4% NaCl, but with pure dilute HCl the

inhibition efficiency in the presence of 300 ppm (MBIA) amounted to

84.6%. On the other hand, the inhibition efficiency reached 93.4% and

94.6% in the presence of 10 ppm Armohib 28 inhibitor in pure dilute HCl

and acidified 4% NaCl, respectively.

Abdallah et al(62)

investigated the effect of phenylhydrazone

derivatives on the corrosion of 70Cu-30Zn brass in 2.0 M HCl solution

using weight loss and galvanostatic polarization techniques. The

percentage inhibition efficiency was found to increase with increasing

concentration of inhibitor and with decreasing temperature. The addition

of KI to phenylhydrazone derivatives enhanced the inhibition efficiency

due to synergistic effect. The adsorption of these inhibitors on the brass

surface obeys Temkin isotherm. Some activated thermodynamic

parameters were calculated. The addition of these compounds to the

potentiodynamic anodic polarization curves of α-brass electrode in

chloride solutions shift the pitting potential to more positive values,

indicating an increased resistance to pitting attack.

I.6.ii. Natural product inhibitors in aqueous media:

The inhibitive effect of Azadirachta indica leaves extract (AI) on the

corrosion of copper in 0.5 M sulphuric acid was investigated by Valek

and Martinez(63)

using electrochemical polarization and weight loss

techniques. The inhibition efficiency of AI was compared to that of the

already proven good inhibitors 2-acetamino-5- mercapto-1,3,4-

thiadiazole (AAMTDA) and 1,2,3-benzotriazole (BTAH). In the region

of active copper dissolution, the highest inhibition efficiency was

exhibited by AAMTDA (92.7%). AI exhibited somewhat higher

efficiency (86.4%) than the widely used BTAH (85.5%), showing that the

extract could serve as a effective substitute for currently preferred copper

corrosion inhibitors in sulphuric acid. The weight loss results were

interpreted by means of the Frumkin isotherm of adsorption on the metal

surface. The values of οads

ΔG equal to -41.96 kJ mol−1

for AAMTDA and

-35.22 kJ mol−1

for BTAH indicate strong spontaneous adsorption while

the surface coverage dependence on the log C following the Frumkin

isotherm is suggestive of chemisorptions in case of all three tested

inhibitors.

The inhibition action of natural honey on the corrosion of copper in a

0.5 M sodium chloride solution was evaluated by El-Etre(64)

using weight

loss measurements and cathodic polarization technique. A good inhibition

efficiency is observed which increases with an increase in inhibitor

concentration. After some time, the inhibition efficiency decreased due to

the growth of fungi in the medium. The Tafel slope is changed markedly

in the presence of natural honey. The adsorption of natural honey on the

copper/chloride interface is found to follow the Langmuir isotherm.

The effect of Boc–Phe–Met–OCH3 as corrosion inhibitor for 60Cu–

40Zn in nitric acid solution was performed by Abed et al(65)

using weight

loss and electrochemical polarization measurements. The corrosion rate is

highly reduced in the inhibited nitric acid. The inhibition efficiency IE%

reached a maximum value of 100% for Boc–Phe–Met–OCH3.

Polarization measurements indicated that Boc–Phe–Met–OCH3 acted as

cathodic inhibitors without changing the mechanism of the hydrogen

evolution reaction. The results showed that the inhibition occurred via

chemisorption of the inhibitor molecules on the corroding metal

following Frumkin’s isotherm. The effect of the temperature in the range

303–353 K on the corrosion of Cu–Zn alloy in 0.5 M nitric acid with and

without addition of Boc–Phe–Met–OCH3 has been studied. The

associated activation energy has been determined.

Mahmoud (66)

studied the corrosion inhibition of muntz alloy (63%

Cu, ≈37% Zn) in 1.0 M HCl by water extracts of some naturally

occurring plants, outer brown skin of Onion (A), Onion bulb (B), the

cloves of Garlic bulb (C), Orange peels (D), and Henna leaves (E) using

weight- loss, galvanostatic polarization, linear polarization and atomic

absorption spectroscopy. From these measurements the values of surface

coverage, θ, and inhibition efficiency were calculated. It was found that

the investigated extracts have high inhibition efficiency on the corrosion

of muntz alloy in 1.0 M HCl. Their inhibition efficiency decreases

according to the order: C› D› E› B› A. These extracts behave as mixed

inhibitors, i.e., they affect both the cathodic and anodic processes. The

activation energy of corrosion was calculated in absence and in presence

of extracts. It was found that the presence of extract in 1.0 M HCl

solutions increases the values of activation energy of corrosion in that

order of their inhibition efficiency. The inhibiting effect of these extracts

results from their adsorption on the electrode surface via the adsorption

centers of the compounds present in the extracts. The adsorption of these

extract onto the surface of munts follows Frumkin isotherm. The atomic

absorption spectroscopic measurements showed that the presence of these

extracts greatly inhibits the preferential dissolution of zinc from the alloy

and the occurrence of simultaneous dissolution of both zinc and copper.

AIM OF THE WORK

Corrosion inhibitors are widely used in industry to reduce the

corrosion rate of different metals and alloys which are present in contact

with aggressive environments. Many studies have been carried out to find

suitable compounds to use as corrosion inhibitors. Most of these

compounds are synthetic chemicals which may be very expensive and

hazardous to living creatures and environments. It is very important to

choose cheep and safety handled compounds to be used as corrosion

inhibitors.

The object of the work is to direct attention to highly inhibitive

properties of some natural compounds as the extracts of some common

plants as Ficus carica and Olive leaves to be used as corrosion inhibitors

for zinc and α-brass in aerated solutions of hydrochloric and sulphuric

acid media. There is no reports in literature on the use of these plants as

corrosion inhibitors. The study was also extended to investigate the effect

of temperature on the inhibition efficiency and evaluation of some

thermodynamic parameters using the chemical and electrochemical

techniques.

EXPERIMENTAL

II.EXPERIMENTAL

II.1. Materials

Working electrode:

Experiments were performed with zinc and α-brass. Pure zinc

(99.999%) from Sigma-Aldrich. A cylindrical samples of zinc with

exposed area of 3.43 cm2 were used for weight-loss and thermometric

measurements. Also the coupons of α-brass of the chemical compositions

Cu 70% and 30% Zn from Egyptian Copper Works with dimensions

2.4×1.0×0.3 cm were used for weight-loss methods.

For open-circuit and potentiodynamic measurements, a cylindrical

rods of zinc or α-brass mounted into glass tube by epoxy resin. The

exposed surface area was 0.5024 cm2. Before each experiment, the

electrode surface was mechanically polished using successive grades of

emery papers down 4/0. Then, degreased with acetone and then washed

several times with bidistilled water and dried. In this way, the electrode

acquired a reproducible bright surface.

Electrolytes :

The chemical used to prepare all solutions studied were of Analar

grade reagents. All solutions were freshly prepared by bidistilled water.

Different concentrations of HCl and H2SO4 solutions were prepare by

analytical dilution from stock solution. Each solution was titrated in the

usual manner against standard base using appropriate indicator.

Natural compounds :

Two natural compounds are used in this study. The main chemical

composition(67,68)

as in Figures (1 and 2) for Ficus carica and Olive leaves

extract.

NH

Indol

CHO

Cinnamaldehyde

COOH

Cinnamic acid

CH2OH

Benzyl alcohol

O O

Coumarin

Figure(1):Chemical structures of some compounds contained in Ficus

carica extract.

OH

OH

CH2CH2OH

Hydroxy tyrosol

OH

OH

H2CH2C

O

C

O

CH2

O

COOCH3

CHCH3

O

Glucose

Oleuropein

Figure(2):Chemical structures of some compounds contained in Olive

extract.

Preparation of plant extract :

The tested extracts were prepared by boiling weighed amounts of the

dried and ground leaves of Ficus carica or Olive plants for one hour in

one liter bidistilled water and left overnight. Then, the solution was

filtered and stored. The concentration of the extracts are expressed as g/l.

II.2. Chemical measurements :

II.2.1. Weight loss methods :

The samples of zinc or α-brass were suspended by suitable glass

hooks and kept under the test solution (50 ml) by about 1cm at

temperature range 25-55ºC in thermostated water bath. After specified

period of time each sample was taken out of the test solution, rinsed with

bidistilled water, dried between two filter papers and weighed again using

a Metler balance. Triplicate samples were used to check reproducibility

of results. The corrosion rate was determined using the relation:

tA

WR

corr

(1)

Where ΔW is weight loss, A is the area and t is the immersion time, and it

was expressed in g/cm2.min. . The efficiency percentage of inhibitors is

taken as (69)

:

100R

R1IE%

free

inh

(2)

Where free

R and inh

R are corrosion rate of samples zinc or α-brass in

free and inhibited solutions, respectively.

II.2.2. Thermometric measurements :

The reaction vessel used in thermometric experiments was described

by Mylius(70)

. The Mylius vessel Figure (3) was kept in thermos to be

thermally isolated from the surrounding during the whole experiment

time. Each experiment was carried out with (15 ml) of test solution and

with a fresh test sample of zinc. The initial temperature in all experiments

was 25ºC. The reaction number ( .R.N ) was used by Mylius to represent

the rate of corrosion as(70)

:

t

TTR.N. im

(°C/min) (3)

Where mT and iT are the maximum and initial temperatures, respectively

and t is the time in minutes elapsed to reach mT . The inhibition

efficiency was evaluated from the corrosion number ( .R.N ) as:

100R.N.

R.N.1IE%

free

inh

(4)

Where free

R.N. and inh

R.N. are reaction number of zinc in free and

inhibited solution, respectively.

Figure(3): Mylius vessel for thermometric measurement.

Specimen

Solution under test

testunder

Thermometer

II.2.3. Electrochemical measurements :

Open-circuit potential measurements were carried out by using

electronic multimeter (Type GDM-8145). The working electrode, zinc or

α-brass was immersed in (100 ml) of test solution and potential was

recorded with respect to saturated calomel electrode as a reference

electrode . The potentials were recorded as a function of time till steady

state potential values established. All measurements were carried out at

room temperature (25 1ºC).

Potentiodynamic measurements were performed using three

compartment cell, Figure (4) with a cylindrical rod of zinc or α-brass as

working electrode, a platinum wire as counter electrode and an Ag/AgCl

as reference electrode. The working electrodes were polished and cleaned

as mentioned before.

Potentiodynamic measurements were achieved using

Autolab/PGSTAT 30. Before each experiment the working electrode was

introduced into the test solution and left for 60 min at the open -circuit

potential. Polarization curves were obtained at scan rate 5 mV/s starting

from cathodic potential going to anodic direction. All the measurements

were performed in freshly aerated solutions at room temperature, (25 1

ºC) and under unstirred conditions.

Figure(4):The polarization cell.

RESULTS AND

DISCUSSION

III. Results and Discussion

III.1. Corrosion inhibition of zinc in acidic media:

III.1.1 Chemical measurements:

III.1.1.1 Weight loss methods:

III.1.1.1.1- Effect of corrodent concentration:

In this part the rate of weight loss-time of zinc sample in aerated

solutions of hydrochloric and sulphuric acid of different concentrations

range (1.0-3.0 M) was constructed under weight-loss method. The

corrosion rate corrR was calculated from the slope of the straight line

obtained. The data of weight-loss are shown in Tables (1-10) and

represented graphically in Figures (5,6). Inspection of these curves, the

weight loss (g/cm2) increases with time, along a period of 120 minute

(immersion time), as well as upon increasing the concentration of all

solutions under test. This attributed to presence of water, air and H+

which accelerate the corrosion process. This indicates that the corrosion

rate of zinc is a function of the concentration of acids solution. This

observation agrees with the fact that the rate of a chemical reaction

increases with increasing concentration and probably due to the increase

in the rate of diffusion and ionization of active species in the corrosion

reaction. This conforms to reports by Omodudu and Oforka(71)

. The

results of data in Tables (1-10) show that the corrosion rate of zinc

sample in HCl higher than in H2SO4 solution.

Table(1):Data of weight loss of zinc sample in 1.0 M HCl solution at

25ºC.

Time

(min)

Wt. of sample

(g)

Wt.loss

(g/cm2)

corrR

(g/cm2.min)

0 3.5015 0 0

30 3.4905 0.0110 1.069×10-4

60 3.4789 0.0226 1.098×10-4

90 3.4637 0.0378 1.224×10-4

120 2.4500 0.0515 1.251×10-4


25ºC.

Time

(min)

Wt. of sample

(g)

Wt.loss

(g/cm2)

corrR

(g/cm2.min)

0 3.5178 0 0

30 3.4904 0.0274 2.663×10-4

60 3.4553 0.0625 3.037×10-4

90 3.4232 0.0946 3.064×10-4

120 3.3903 0.1275 3.098×10-4


25ºC.

Time

(min)

Wt. of sample

(g)

Wt.loss

(g/cm2)

corrR

(g/cm2.min)

0 3.5178 0 0

30 3.4560 0.0618 6.006×10-4

60 3.3941 0.1237 6.016×10-4

90 3.3318 0.1860 6.025×10-4

120 3.2693 0.2485 6.037×10-4


25ºC.

Time

(min)

Wt. of sample

(g)

Wt.loss

(g/cm2)

corrR

(g/cm2.min)

0 3.5411 0 0

30 3.4663 0.0748 7.269×10-4

60 3.3907 0.1504 7.308×10-4

90 3.3151 0.2260 7.321×10-3

120 3.2391 0.3020 7.337×10-3


25ºC.

Time

(min)

Wt. of sample

(g)

Wt.loss

(g/cm2)

corrR

(g/cm2.min)

0 3.4487 0 0

30 3.3617 0.0870 8.455×10-4

60 3.2735 0.1752 8.513×10-4

90 3.1840 0.2647 8.575×10-3

120 3.0964 0.3523 8.588×10-3

Table(6):Data of weight loss of zinc sample in 1.0 M H2SO4solution at

25ºC.

Time

(min)

Wt. of sample

(g)

Wt.loss

(g/cm2)

corrR

(g/cm2.min)

0 2.9129 0 0

30 2.9059 0.0065 6.317×10-5

60 2.8939 0.0185 8.989×10-5

90 2.8850 0.0279 9.038×10-5

120 2.8754 0.0375 9.111×10-5

Table(7):Data of weight loss of zinc sample in 1.5 M H2SO4 solution at

25ºC.

Time

(min)

Wt. of sample

(g)

Wt.loss

(g/cm2)

corrR

(g/cm2.min)

0 2.8534 0 0

30 2.8414 0.0120 1.166×10-4

60 2.8279 0.0255 1.239×10-4

90 2.8146 0.0388 1.257×10-4

120 2.8009 0.0525 1.275×10-4


25ºC.

Time

(min)

Wt. of sample

(g)

Wt.loss

(g/cm2)

corrR

(g/cm2.min)

0 2.4636 0 0

30 2.4410 0.0226 2.196×10-4

60 2.4105 0.0531 2.580×10-4

90 2.3831 0.0805 2.607×10-4

120 2.3541 0.1095 2.660×10-4


25ºC.

Time

(min)

Wt. of sample

(g)

Wt.loss

(g/cm2)

corrR

(g/cm2.min)

0 3.1928 0 0

30 3.1595 0.0333 3.236×10-4

60 3.1186 0.0742 3.065×10-4

90 3.0803 0.1125 3.644×10-4

120 3.0431 0.1515 3.681×10-4


25ºC.

Time

(min)

Wt. of sample

(g)

Wt.loss

(g/cm2)

corrR

(g/cm2.min)

0 3.2100 0 0

30 3.1569 0.0525 5.102×10-4

60 3.1040 0.1060 5.151×10-4

90 3.0330 0.1770 5.734×10-4

120 2.9124 0.2976 7.230×10-4

Time (min)

0 20 40 60 80 100 120 140

Wt.

loss

(g/c

m2)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

1.0 M

1.5 M

2.0 M

2.5 M

3.0 M

Figure(5):Weight loss-time curves for zinc in different

concentrations of HCl solution at 25ºC.

Time (min)

0 20 40 60 80 100 120 140

Wt.

loss

(g/c

m2)

0.00

0.02

0.04

0.06

0.08

1.0 M

1.5 M

2.0 M

2.5 M

3.0 M

Figure(6):Weight loss-time curves for zinc in different

concentrations of H2SO4 solution at 25ºC.

III.1.1.1.2- Effect of inhibitors:

The effect of adding the plant extracts under test, Ficus carica and

Olive leaves on the weight-loss of zinc samples in 2.0 M of both HCl and

H2SO4 as corrosive media was studied at 25ºC. Figures (7-10) represent

the weight loss–time curves for zinc sample in 2.0 M both HCl and

H2SO4 in the absence and presence of concentration range (8.0 -20.0 g/l)

of Ficus carica and Olive extracts.

As can be seen from the Figures the weight loss of zinc in presence of

plant extracts varies linearly with time, and it is lower than obtained in

the blank solution. The linearly indicates that the absence of insoluble

surface film during corrosion, and that the inhibitors were first adsorbed

onto the metal surface and thereafter, inhibits the corrosion process(72)

.

Also it is obvious that the corrosion rate decreases with increasing the

concentration of the inhibitors. This indicates that the degree of

inhibition depends upon the nature and the concentration of the inhibitor

and the nature of the corrosive medium. The inhibition efficiency percent,

IE%of the inhibitors in HCl and H2SO4 solutions was calculated using

the equation (2). The variation between the concentrations of Ficus carica

and Olive extracts in both acids and inhibition efficiency percent are

shown in Figures (11,12). These curves (S-shape) are characterized by

initial rising IE% with concentration indicating a formation of a mono-

layer adsorbate film on zinc surface, until reached to 20.0 g/l. At the same

time, the increase of extracts concentration above this value the inhibition

efficiency remained more or less constant suggesting a complete surface

coverage by the extracts molecules. The protective action of inhibitor

during metal corrosion is based on the adsorbability of their molecules,

where the resulting adsorption film isolates the metal surface from the

corrosive medium. The degree of the surface coverage θ which represents

the part of the metal surface covered by inhibitor molecules was

calculated using the following equation (73)

:

free

inh

R

R1θ (5)

Where free

R and inh

R are the corrosion rates of zinc in HCl and H2SO4

in absence and presence of different concentrations of inhibitors.

The values of corrR , IE%and θ for different concentrations of Ficus

carica and Olive extracts in both acids are listed in Tables (11,12).

Inspection of these Tables, the data revealed that as the plant extract

concentration is increased, the corrosion rate decreases, while the

inhibition efficiency percent IE% , and surface coverage θ , increase. This

behaviour may be attributed to be the increased the surface coverage θ

due to increase of the number of adsorbed molecules on zinc surface. A

good efficiency is observed at constant concentration of plant extracts

(20.0 g/l), with IE% of Ficus carica and Olive extracts in HCl are

99.67% and 97.67%, and in H2SO4 are 96.61% and 93.06%,

respectively. It is concluded that the Ficus carica and Olive extracts act as

excellent inhibitors for corrosion of zinc in both acids under study .

Time (min)

0 20 40 60 80 100 120 140

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

8.0 g/l

10.0 g/l

12.0 g/l

15.0 g/l

20.0 g/l

0 20 40 60 80 100 120 140

0.00

0.01

0.02

0.03

0.04

free acid

Figure(7):Weight loss-time curves for zinc in 2.0 M HCl


concentrations of Ficus carica extract at 25ºC.

Wt.

loss

(g/c

m2)

Time(min)

0 20 40 60 80 100 120 140

Wt.

loss

(g/c

m2)

0.000

0.001

0.002

0.003

0.004

8.0 g/l

10.0 g/l

12.0 g/l

15.0 g/l

20.0 g/l

0 20 40 60 80 100 120 140

0.00

0.02

0.04

0.06

0.08

free acid

Figure(8):Weight loss-time curves for zinc in 2.0 M HCl


concentrations of Olive extract at 25ºC.

Time(min)

0 20 40 60 80 100 120 140

Wt.

loss

(g/c

m2)

0.000

0.005

0.010

0.015

0.020

0.025

0.030

free acid

8.0 g/l

10.0 g/l

12.0 g/l

15.0 g/l

20.0 g/l

Figure(9):Weight loss-time curves for zinc in 2.0 M H2SO4



Time (min)

0 20 40 60 80 100 120 140

Wt.

loss

(g/c

m2)

0.000

0.005

0.010

0.015

0.020

0.025

0.030

free acid

8.0 g/l

10.0 g/l

12.0 g/l

15.0 g/l

20.0 g/l

Figure(10):Weight loss-time curves for zinc in 2.0 M H2SO4



Cinh

(g/l)

5 10 15 20 25 30 35

IE%

94

95

96

97

98

99

100

ficus carica ext.

olive ext.

Figure(11):Variation of inhibition efficiency with

concentration of Ficus carica and Olive

extracts for zinc in 2.0 M HCl solution at 25ºC.

Cinh

(g/l)

5 10 15 20 25 30 35

IE%

78

80

82

84

86

88

90

92

94

96

98

ficus carica ext.

olive ext.

Figure(12):Variation of inhibition efficiency with

concentration of Ficus carica and Olive

extracts for zinc in 2.0 M H2SO4 solution at 25ºC.

Table(11):Effect of extract concentrations on ( corrR ), ( IE%) and (θ )

for zinc in 2.0 M HCl solution at 25ºC.

Olive extract Ficus carica extract

θ IE% corrR

(g/cm2.min)

θ IE% corrR

(g/cm2.min)

Conc.

(g/l)

0.00 0.00 6.01×10-4

0.00 0.00 6.01×10-4

0.00

0.9466 94.66 3.20×10-5

0.9685 96.85 1.89×10-5

8.0

0.9540 95.40 2.76×10-5

0.9733 97.33 1.60×10-5

10.0

0.9597 95.97 2.42×10-5

0.9799 97.99 1.21×10-5

12.0

0.9668 96.68 1.98×10-5

0.9846 98.46 9.21×10-6

15.0

0.9767 97.67 1.40×10-5

0.9967 99.67 1.94×10-6

20.0

Table(12):Effect of extract concentrations on ( corrR ), ( IE%) and (θ )

for zinc in 2.0 M H2SO4 solution at 25ºC.


θ IE% corrR

(g/cm2.min)

θ IE% corrR

(g/cm2.min)

Conc.

(g/l)

0.00 0.00 2.58×10-4

0.00 0.00 2.58×10-4

0.00

0.8085 80.85 4.94×10-5

0.8333 83.33 4.30×10-5

8.0

0.8461 84.61 3.97×10-5

0.8779 87.79 3.15×10-5

10.0

0.8837 88.37 3.00×10-5

0.9174 91.74 2.13×10-5

12.0

0.9081 90.81 2.37×10-5

0.9569 95.69 1.11×10-5

15.0

0.9306 93.06 1.79×10-5

0.9661 96.61 8.73×10-6

20.0

III.1.1.1.3- Adsorption isotherm:

The nature of corrosion inhibition has been deduced in terms of the

adsorption characteristics of the inhibitors. The adsorption of an organic

adsorbate on a metal surface is regarded as a substitution adsorption

process between the organic molecule in the aqueous solution and water

molecules adsorbed on the metallic surface(74)

.

(sol)2(ads)(ads)2(sol)

OHx OrgOHx Org (6)

Where x is the size ratio representing the number of water molecules

replaced by one molecule of organic adsorbate. The adsorption of organic

compounds can be described by two main types of interaction; physical

adsorption and chemisorption. These are influenced by the nature and

charge of the metal, the chemical structure of the inhibitor and the type of

electrolyte. The interaction between the inhibitor and the metal surface

can be provided by the adsorption isotherm. The dependence of θC on

C was used to obtained the best fitting adsorption isotherm where θ is

the surface coverage which represents the part of the metal surface

covered by inhibitor molecules. The values of θ were evaluated from

the corrosion rate using equation (5). For the extracts under test it was

found that the Langmuir adsorption isotherm(74)

fits the experimental

data, is given by:

CK

1

θ

C (7)

Where K is binding constant of adsorption reaction and can be calculated

from the reciprocal of the intercept. The relation between θC and C at

25°C in 2.0 M of both HCl and H2SO4 with various concentrations of

Ficus carica and Olive extracts is shown in Figures (13,14). Inspection of

these Figures reveals that a straight line was obtained with slops close to

unity indicating that the adsorption of plant extracts obey Langmuir

adsorption isotherm. These involves the assumption of no interaction

between the adsorbed inhibitor constituents on the zinc surface(75)

. The

Langmuir isotherm is based on the assumption that each site of metal

surface holds one adsorbed specie as in equation (3). Therefore, one

adsorbed H2O molecule is replaced by one molecule of the inhibitor

adsorbate (plant extracts) on the zinc surface. The relation between

binding constant, K and the standard free energy change of adsorption

οads

ΔG is given by the following equation(74)

:

RT

ΔGexp

55.5

1K

οads (8)

Where 55.5 is the molar concentration of water in solution , R is the gas

constant (8.314 J/mol/degree) and T is the absolute temperature. The

calculated values of standard free energy change οads

ΔG and binding

constant K , for the plant extracts in 2.0 M HCl and H2SO4 are shown in

Tables (13,14). The values of K are relatively small indicating that the

interaction between the adsorbed extract molecules and zinc metal

surface is physically adsorbed. Generally, values of οads

ΔG until -20 KJ

mol-1

are consistent with the electrostatic interaction between the charged

molecules and the charged metal surface (physical adsorption)(76)

. The

values obtained of οads

ΔG in Tables (13,14) suggests physical adsorption

of Ficus carica and Olive extracts on the zinc surface in both acids. The

negative sign indicates that the adsorption of Ficus carica and Olive

extract molecules on the zinc surface is a spontaneous process.

Cinh

(g/l)

6 8 10 12 14 16 18 20 22

6

8

10

12

14

16

18

20

22

ficus carica ext.

olive ext.

Figure(13):Langmuir adsorption plots for zinc in 2.0 M HCl

with different concentrations of plant extracts at

25ºC.

C/θ

6 8 10 12 14 16 18 20 22

6

8

10

12

14

16

18

20

22

24

ficus carica ext.

olive ext.

Figure(14):Langmuir adsorption plots for zinc in 2.0 M H2SO4

with different concentrations of plant extracts at

25ºC.

C/θ

Cinh (g/l)

Table(13): Binding constant ( K ) and standard free energy of adsorption

οads

ΔG for plant extracts in 2.0 M HCl at 25ºC.

Plant extracts

K

(g-1

l)

- οads

ΔG

KJ mol-1

Ficus carica 2.3479 12.07

Olive 2.1570 11.86

Table(14): Binding constant ( K ) and standard free energy of adsorption

οads

ΔG for plant extracts in 2.0 M H2SO4 at 25ºC.

Plant extracts

K

(g-1

l)

- οads

ΔG

KJ mol-1

Ficus carica 0.5057 8.26

Olive 0.4647 8.05

III.1.1.1.4- Effect of temperature:

To again insight into the nature of inhibitor adsorption, the effect of

temperature range (25-55ºC) on the corrosion behaviour and the

inhibition efficiency of zinc in both HCl and H2SO4 solutions in presence

of 15.0 g/l of Ficus carica and Olive extracts, was studied by weight loss

method. The data in Tables (15,16) indicate that the corrosion rate of zinc

in absence and presence of the plant extracts increased with rise in

temperature. This is because an increase in temperature usually

accelerates corrosion processes, particularly in acid media in which H2

gas evolution accompanies corrosion, giving rise to higher dissolution

rates of the metal. The values of IE% of the extracts decrease with

increasing temperature, and the high inhibition efficiency at 25 ºC. This

result suggests a physical adsorption of Ficus carica and Olive extracts

on the corroding zinc surface. To elucidate the mechanism of inhibitor

adsorption(3)

, it is necessary to establish the adsorption modes of the

inhibiting species, whether, molecular or ionic. The predominant

adsorption mode will be dependent on factors such as the extract

composition, type of acid anion as well as chemical changes to the

extract. The physical adsorption mechanism, obtained from electrostatic

attractive forces between inhibiting organic ions or dipoles and the

electrically charged surface of the metal due to the electric field existing

at the metal / solution interface. A negative surface charge will favour the

adsorption of cations whereas, anion adsorption is favoured by positive

surface charge. A corroding zinc specimen under test carries a negative

surface charge in both hydrochloric and sulphuric acid solutions, since

the Ficus carica and Olive extracts are a polar molecules, physisorpotion

could occur with the negatively zinc surface. On the other hand, the

ability of Cl ions in hydrochloric acid solution, pick up the cations

appear in the solution and facilitate the physical of inhibitor cations on

the negatively charged surface by electrostatic interaction.

The apparent activation energies ( *aE ) for the corrosion process in

absence and presence of plant extracts were evaluated from Arrhenius

equation(77)

:

2.303RT

EA logR log

*a

corr (9)

Where corrR is the corrosion rate (g/cm2.min) , A is the constant

frequency factor and *aE is the apparent activation energy, R is the gas

constant (8.314 J/mol/degree) and T is the absolute temperature. By

plotting of logarithm of the corrosion rate of zinc in both acids in

absence and presence of 15.0 g/l of extracts, versus the reciprocal of

absolute temperature range (298-328 ºK), give straight lines with slope

equal to - 2.303RE*a

is represented in Figures (15-18).

The activation parameters of zinc corrosion in both acids media in

absence and presence of extracts under test are given by Eyring

equation(77)

:

RT

ΔH-exp

R

ΔSexp

Nh

RTR

**

corr (10)

Where h is Plank's constant, N is Avogadro's number, *ΔS and *ΔH

are the activation entropy and enthalpy change, respectively. A plot of

TRlog corr versus T1 gives straight line with slope of [(- *ΔH /R ] and

an intercept of 2.303RΔSNhRlog * , represented in Figures

(17,18). The values of activation parameters are listed in Tables (17,18).

Inspection of Tables (17,18) demonstrate that, the presence of Ficus

carica and Olive extracts in both acids increase the values of *aE

comparing to its unhibited, these attributed to an appreciable decrease in

the adsorption process of the inhibitors on the metal surface with

increasing of temperature and a corresponding increase in the reaction

rate because of the greater area of the metal that is exposed to the acids

and corrosion inhibition occurred through physical adsorption(3)

. The

higher *ΔH values in presence of the extracts indicate that the degree of

surface coverage decreased with rise in temperature, supporting the

proposed physisorption mechanism. Also the values of *ΔS in the

absence and presence the extracts is negative meaning that the increasing

in ordering accompanied the dissolution process.

Table(15):Effect of temperature on the corrosion rate ( corrR ) and

efficiency ( IE%) for zinc in 2.0 M HCl in absence and

presence of plant extracts.

Olive extract Ficus carica extract 2.0 M HCl

Temp.

(ºC) IE% corrR

(g/cm2.min)

IE% corrR

(g/cm2.min)

corrR

(g/cm2.min)

96.68 1.99×10-5

98.46 9.23×10-6

6.01×10-4

25

95.47 2.92×10-5

96.54 2.23×10-5

6.45×10-4

35

93.63 4.81×10-5

95.75 3.25×10-5

7.56×10-4

45

92.59 7.48×10-5

94.47 5.58×10-5

1.01×10-3

55

Table(16):Effect of temperature on the corrosion rate ( corrR ) and

efficiency ( IE%) for zinc in 2.0 M H2SO4 in absence and

presence of plant extracts.

Olive extract Ficus carica extract 2.0 M H2SO4

Temp.

(ºC) IE% corrR

(g/cm2.min)

IE% corrR

(g/cm2.min)

corrR

(g/cm2.min)

90.77 2.38×10-5

95.65 1.12×10-5

2.58×10-4

25

88.68 4.13×10-5

91.89 2.96×10-5

3.65×10-4

35

84.79 7.33×10-5

89.23 5.19×10-5

4.82×10-4

45

81.97 1.17×10-4

85.54 9.38×10-5

6.49×10-4

55

3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

free acid

ficus carica ext.

olive ext.

Figure(15):Arrhenius plots of the corrosion rate for zinc in

2.0 M HCl solution in absence and presence of

plant extracts.

103/T(

°K

-1)

Log R

corr (

Rcorr, g c

m-2

.min

-1)

3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40

-5.2

-5.0

-4.8

-4.6

-4.4

-4.2

-4.0

-3.8

-3.6

-3.4

-3.2

-3.0

free acid

ficus carica ext.

olive ext.

Figure(16):Arrhenius plots of the corrosion rate for zinc in

2.0 M H2SO4 solution in absence and presence of

plants extracts.

103/T(

°K

-1)

Log R

corr (

Rcorr, g c

m-2

.min

-1)

3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

free acid

ficus carica ext.

olive ext.

Figure(17):Eyring plots of the corrosion rate for zinc in 2.0 M

HCl solution in absence and presence of plant

extracts.

103/T(

°K

-1)

Log R

corr (

Rcorr /

T, g c

m-2

. m

in-1

)

3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40

-2.6

-2.4

-2.2

-2.0

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

free acid

ficus carica ext.

olive ext.

Figure(18):Eyring plots of the corrosion rate for zinc in 2.0 M

H2SO4 solution in absence and presence of plant

extracts.

103/T(

°K

-1)

Log R

corr (

Rcorr /

T, g c

m-2

.m

in-1

)

Table(17):Activation parameters of zinc in 2.0 M HCl in absence

and presence of plant extracts.

*ΔS- J mol

-1 ºK

-1

*ΔH KJ mol

-1

*aE

KJ mol-1

Plant extract

157.57 16.33 13.677 Free acid

79.09 49.68 47.10 Ficus carica

110.02 38.90 36.08 Olive

Table(18):Activation parameters of zinc in 2.0 M H2SO4 in absence

and presence of plant extracts.

*ΔS- J mol

-1 ºK

-1

*ΔH KJ mol

-1

*aE

KJ mol-1

Plant extract

127.04 27.36 24.76 Free acid

54.66 51.85 59.19 Ficus carica

84.08 43.49 46.10 Olive

III.1.1.2. Thermometric measurements:

Figures (19,20) show the dissolution of zinc in HCl and H2SO4

solutions with different concentrations range (1.0-3.0 M) using

thermometric technique. The curves represented by an initial slow rise of

temperature with time from the moment of zinc sample introduced into

acids solution, followed by a sharp rise and finally a decrease after

attaining a maximum temperature, mT . The slow rise of the first part of

the curve due to the oxide film on the metal surface. The maximum

temperature increases with increasing concentration. This indicates that

the dissolution rate of zinc in both acids increase with increasing

concentration. The maximum temperature, mT measured corresponds to

a reaction number (R.N.), which is evaluated using the relation (3).The

maximum temperature, mT measured in 2.0 M of HCl and H2SO4 are

39ºC and 35ºC are attained after 150, 180 minutes corresponds to reaction

number (R.N.) of 0.0933 and 0.0555 °C/min, respectively. These indicate

that the dissolution rate of zinc in HCl is higher than in H2SO4 solutions.

Temperature change of system involving zinc in 2.0 M of both acids in

absence and presence of different concentrations (8.0–20.0 g/l ) of Ficus

carica and Olive extracts are represented in Figures (21-24). It is evident

that the curves of the extracts containing system falls below that of the

free acid. On increasing the concentration of the extracts the mT is

decrease and, the time required to reach mT increase, these factors cause

decrease in (R.N.) of system. This is indicates that the plant extracts

behaves as inhibitor over the concentration range studied, on anodic and

cathodic sites of the metal surface and provide the differentiating between

strong and weak adsorption(78)

. The inhibition efficiency of the extracts is

calculated by using equation (4). The values of ( .R.N ) and IE% of

different concentrations of extracts in 2.0 M of HCl and H2SO4 were

given in Tables (19,20). The values denoted that the reaction number

( .R.N ) decreases with increasing the concentration of the extracts and

consequently the IE% increases. The variation between reaction number

( .R.N ) and the logarithm of concentration ( C log ) of the extracts in 2.0

M of both acids gives a straight lines, were represented in Figures

(25,26), denoting that the ( .R.N ) decreases on increasing the extract

concentration. The results of the inhibition action is in a good agreement

to that obtained from weight loss method.

Time (min)

0 50 100 150 200 250 300

25

30

35

40

45

1.0 M

1.5 M

2.0 M

2.5 M

3.0 M

Figure(19):Temperature-time curves for zinc in different

concentrations of HCl solution at Ti = 25ºC.

Tem

per

atu

re(°

C)

Tem

per

atu

re(°

C)

Tem

per

atu

re(°

C)

Time(min)

0 50 100 150 200 250 300

25

30

35

40

45

1.0 M

1.5 M

2.0 M

2.5 M

3.0 M

Figure(20):Temperature-time curves for zinc in different

concentrations of H2SO4 solution at Ti = 25ºC.

Tem

per

atu

re(°

C)

Time (min)

0 100 200 300 400 500 600 700

25

30

35

40

free acid

8.0 g/l

10.0 g/l

12.0 g/l

15.0 g/l

20.0 g/l

Figure(21):Temperature-time curves for zinc in 2.0 M HCl


concentrations of Ficus carica extract at Ti = 25ºC.

Tem

per

atu

re(°

C)

Time (min)

0 100 200 300 400 500 600 700

25

30

35

40

free acid

8.0 g/l

10.0 g/l

12.0 g/l

15.0 g/l

20.0 g/l

Figure(22):Temperature-time curves for zinc in 2.0 M HCl


concentrations of Olive extract at Ti = 25ºC.

Tem

per

atu

re(º

C)

Time (min)

0 100 200 300 400 500 600 700

25

30

35

free acid

8.0 g/l

10.0 g/l

12.0 g/l

15.0 g/l

20.0 g/l

Figure(23):Temperature-time curves for zinc in 2.0 M H2SO4


concentrations of Ficus carica extract at Ti = 25ºC.

Tem

per

atu

re(°

C)

Time (min)

0 100 200 300 400 500 600 700

25

30

35

free acid

8.0 g/l

10.0 g/l

12.0 g/l

15.0 g/l

20.0 g/l

Figure(24):Temperature-time curves for zinc in 2.0 M H2SO4


concentrations of Olive extract at Ti = 25ºC.

Tem

per

atu

re(°

C)

Table(19):Reaction number (R.N.) and efficiency (IE%) of the different

concentrations of Ficus carica and Olive extracts for zinc

in 2.0 M HCl solution at 25°C.


IE% R.N. ×103 IE% R.N. ×10

3 Conc.

(g/l)

-93.30 - 93.30 0.00

86.60 12.50 87.24 11.90 8.0

89.28 10.00 90.24 9.090 10.0

91.75 7.792 93.00 6.522 12.0

94.04 5.555 95.53 4.166 15.0

96.10 3.636 97.85 2.000 20.0

Table(20):Reaction number (R.N.) and efficiency (IE%) of the different

concentrations of Ficus carica and Olive extracts for zinc

in 2.0 M H2SO4 solution at 25°C.


IE% R.N. ×103 IE% R.N. ×10

3 Conc.

(g/l)

-55.50 - 55.50 0.00

78.37 12.00 80.41 10.80 8.0

82.32 9.804 84.66 8.510 10.0

86.14 7.692 88.66 6.122 12.0

89.89 5.555 92.93 3.922 15.0

93.44 3.636 96.60 1.886 20.0

log Cinh

(g/l)

0.8 0.9 1.0 1.1 1.2 1.3 1.4

R.N

.

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

ficus carica ext.

olive ext.

Figure(25):Variation between reaction number and

logarithm of concentration of plant extracts

for zinc in 2.0 M HCl solution at 25ºC.

log Cinh

(g/l)

0.8 0.9 1.0 1.1 1.2 1.3 1.4

R.N

.

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

ficus carica ext.

olive ext.

Figure(26):Variation between reaction number and

logarithm of concentration of plant extracts

for zinc in 2.0 M H2SO4 solution at 25ºC.

III.1.2. Electrochemical measurements:

III.1.2.1- Open-circuit potential measurements:

The open-circuit potential measurements are a very suitable method to

investigate the metal corrosion rates. The use of this method is based on

the difference between the corrosion potential of metal under test and the

open-circuit potential of the substrate material. The open-circuit potential

,with reference to a saturated calomel electrode, of zinc electrode as a

function of time were constructed under aerated solutions of HCl and

H2SO4 of different concentrations range (1.0-3.0 M) till reached to steady

state potential, represented in Figures (27,28). Inspection of these curves

the behaviour can be classified into two parts, the first one exhibit a

general tendency for the immersion open-circuit potential to shift from

less negative to more negative values (corrosion process), this may be

attributed to the destruction of pre-immersion oxide film present on the

surface of zinc electrode in both acids. The potential of zinc becomes

negative as concentration of both acids increased. The second part of the

curves, the potential shifts gradually from the more negative towards to

the positive direction and thereafter it remains almost constant for a long

time, the steady-state potential both of HCl and H2SO4 solutions tends

towards to positive direction as the concentrations are increased.

According to Evan's(7)

, the shift of electrode potential towards nobler

values signify the repair and further thickening of the air-formed pre-

immersion oxide film on the surface of the metal. Figures (29-32)

represent the variation of potential of zinc electrode with time in aerated

solutions of 2.0 M HCl and H2SO4 in absence and presence of different

concentrations range (8.0- 20.0 g/l) of Ficus carica and Olive extracts.

Inspection of the curves, there is a general tendency for the open-circuit

potential to drift with time towards a positive potential and the time for

which the steady state values were reached depends upon the nature of

extract. Also it shows that Ficus carica and Olive extracts in high

concentration (20.0 g/l), were effective in retarding or preventing the

corrosion process because this additive shifts the potential towards more

noble values. This may be due to the adsorption of the inhibitors on an

oxide-free metal surface and prevention the hydrogen evolution reaction

at the cathodic sites. The inhibition increases the protection qualities of

the oxide surface or other surface layer from the aggressive solutions. The

fundamental step involves displacement of pre-adsorbed water molecules

by the extract inhibitors followed by electrostatic force reaction at the

surface(3)

.

The variation of the steady-state potential with the logarithm of

concentration of extracts is given in Figures (33,34). It shows that the

steady state potential, Ess is shifted to more positive direction as the

concentration of extract solutions is increased, according to the linear

relation(3)

.

C log b aEss

(11)

Where a, b are constant.

This indicates that the potential of zinc electrode depends on the type

and concentration of extracts under test. Thus, at high concentration (20.0

g/l) of Ficus carica and Olive extracts, the potential of zinc electrode

becomes less negative, indicating that the retardation of the corrosion of

zinc metal and the extracts behave as inhibitors.

Time (min)

0 20 40 60 80 100 120 140 160

E,

mV

(SC

E)

-1100

-1080

-1060

-1040

-1020

-1000

-980

-960

1.0 M

1.5 M

2.0 M

2.5 M

3.0 M

Figure(27):The variation of the open-circuit potential for

zinc electrode in different concentrations of

HCl solution at 25ºC.

Time (min)

0 20 40 60 80 100 120 140 160 180

E,

mV

(SC

E)

-1100

-1080

-1060

-1040

-1020

-1000

-980

-960

1.0 M

1.5 M

2.0 M

2.5 M

3.0 M


zinc electrode in different concentrations of

H2SO4 solution at 25ºC.

Time (min)

0 20 40 60 80 100 120 140 160 180

E,

mV

(SC

E)

-1080

-1060

-1040

-1020

-1000

-980

-960

-940

-920

free acid

8.0 g/l

10.0 g/l

12.0 g/l

15.0 g/l

20.0 g/l


zinc electrode in 2.0 M HCl with different


Time (min)

0 20 40 60 80 100 120 140 160 180

E,

mV

(SC

E)

-1080

-1060

-1040

-1020

-1000

-980

-960

-940

free acid

8.0 g/l

10.0 g/l

12.0 g/l

15.0 g/l

20.0 g/l


zinc electrode in 2.0 M HCl with different


Time (min)

0 20 40 60 80 100 120 140 160 180

E,

mV

(SC

E)

-1080

-1060

-1040

-1020

-1000

-980

-960

-940

-920

-900

-880

free acid

8.0 g/l

10.0 g/l

12.0 g/l

15.0 g/l

20.0 g/l


zinc electrode in 2.0 M H2SO4 with different


Time (min)

0 20 40 60 80 100 120 140 160 180

E,

mV

(SC

E)

-1080

-1060

-1040

-1020

-1000

-980

-960

-940

-920

free acid

8.0 g/l

10.0 g/l

12.0 g/l

15.0 g/l

20.0 g/l


zinc electrode in 2.0 M H2SO4 with different


log Cinh

(g/l)

0.8 0.9 1.0 1.1 1.2 1.3 1.4

Ess

,mV

(SC

E)

-990

-980

-970

-960

-950

-940

-930

ficus carica ext.

olive ext.

Figure(33):The variation between the steady-state potential

and logarithm of the concentration of extracts

for zinc electrode in 2.0 M HCl solution at 25ºC.

log Cinh

(g/l)

0.8 0.9 1.0 1.1 1.2 1.3 1.4

ES

S,

mV

(SC

E)

-970

-960

-950

-940

-930

-920

-910

-900

ficus carica ext.

olive ext.

Figure(34):The variation between the steady-state potential

and logarithm of the concentration of extracts

for zinc electrode in 2.0 M H2SO4 solution at

25ºC.

III.1.2.2- Potentiodynamic polarization measurements:

Typical potentiodynamic polarization curves for the zinc electrode in

both aerated 2.0 M HCl and H2SO4 acid solutions in absence and

presence of the Ficus carica and Olive extracts inhibitors at different

concentrations range (8.0-20.0 g/l) are given in Figures (35-38). It is

evident that the extracts inhibit both cathodic and anodic parts of the

polarization curves and this acting as mixed-type inhibitors, the shape of

the curves are not modified on adding the plant extracts or by increasing

the concentration, but the more pronounced behaviour that the curves

produces a little shift on the cathodic region and a great shift on anodic

region. The anodic and cathodic reactions involved the corrosion of metal

electrode in acidic solution are:

Anodic nenMM (12)

Cathodic HeH (13)

2

HHH (14)

The adsorbed inhibitor(3)

blocks either the anodic or cathodic reaction

or both. The effect of the inhibitor may be due to charges in the electric

double layer, by reducing the metal reactivity, or by the inhibitor

participation in partial electrode reaction and by formation of physical

barrier. The adsorbed inhibitor may not cover the entire metal surface, but

occupies sites which are electrochemically active and thereby reduced the

extent of anodic or cathodic reactions or both. The corrosion rate will be

decreased in proportion to the extent to which the electrochemically

active sites are blocked by the adsorbed inhibitor, was represented in

Figure (39).

Tafel equation arises from the energy required to change a species

from one state to another at an electrode surface at specific rate giving by

the following equation(79)

:

I log baE (15)

Where E is referred to as activation polarization for electrode reactions

involving oxidation or reduction a is constant and b is Tafel slopes, c

b

and ab for cathodic and anodic, respectively and I is the current density.

The electrochemical parameters corrE , corrI , cb , ab and IE% are

given by equation:

100I

I1IE%

freecorr

inhcorr

(16)

Where freecorrI and inhcorrI refer to the corrosion current densities in

absence and presence of inhibitor respectively, determined by

extrapolation of cathodic and anodic Tafel lines to corrosion potential.

Electrochemical parameters are collected in Tables (21-24). Inspection of

this Tables, it is observed that the corrosion potentials corrE shifts to less

negative values (noble direction) with increasing the extracts

concentration. This indicates that the addition of Ficus carica and Olive

extracts primarily affect the anodic process, and acts predominately as

anodic-type inhibitors. Consequently, the adsorption of the extracts in

HCl and H2SO4 solutions are more likely at anodic sites. Also the values

of the corrosion current density (corr

I ) decreased with increasing the

concentration of the extracts, indicating that the inhibitive property of

these extracts on the corrosion of zinc in acid solutions. Cathodic Tafel

slope, cb and anodic Tafel slope, a

b are more or less constant upon

increasing the extract concentrations. These results indicate that the

extracts act by simple blocking the available cathodic and anodic sites on

zinc surface. In the other word, these inhibitors decreases the surface area

for corrosion without affecting the mechanism of corrosion process and

only cause inactivation of a part of the metal surface with respect to

corrosive medium. Also the inhibition efficiency, IE% for extracts in

both acid solutions increase with increasing concentration, and at certain

concentration (20.0 g/l), IE% reaches 98.79%, 93.91% and 94.72%,

90.64% for Ficus carica and Olive extracts in 2.0 M of HCl and H2SO4

solutions, respectively. The results of electrochemical measurements are

in good agreement with the chemical methods.

Conclusions:

The main conclusions of first part in the present study can be

summarized as follows:

1- The corrosion rate of zinc increases with increasing the

concentrations of HCl and H2SO4 solutions.

2- Ficus carica and Olive are act as inhibitors, and the highest

efficiency was observed at 20.0 g/l in both acid solutions.

3- The corrosion process is inhibited by adsorption of the extracts on

zinc surface follows Langmuir isotherm, in which one molecule of

inhibitor occupying one active site.

4- The inhibition efficiency of the extracts under test decreases with

the rise in temperature and the apparent activation energy increases

in presence of inhibitors.

5- The results of thermometric measurements obtained show that the

reaction number ( .R.N ) decreases with increasing the extracts

concentration indicating that the extracts act as inhibitors and

adsorbed on both anodic and cathodic sites of zinc surface.

6- Open-circuit and polarization measurements indicate that the plant

extracts in both acids act as anodic- type inhibitors.

7- The inhibition efficiency obtained from polarization measurements

show a good agreement with those obtained from comparative

weight loss and thermometric determinations.

Figure(35):Potentiodynamic polarization curves for zinc

electrode in 2.0 M HCl in absence and presence

of different concentrations of Ficus carica extract

at 25ºC.

log I (A/cm2)

E,V

(A

g/A

gC

l)


electrode in 2.0 M HCl in absence and presence

of different concentrations of Olive extract

at 25ºC.

log I (A/cm2)

E,V

(A

g/A

gC

l)


electrode in 2.0 M H2SO4 in absence and presence

of different concentrations of Ficus carica extract

at 25ºC.

log I (A/cm2)

E,V

(A

g/A

gC

l)


electrode in 2.0 M H2SO4 in absence and presence

of different concentrations of Olive extract at

25ºC.

log I (A/cm

2)

E,V

(A

g/A

gC

l)

Figure(39): Adsorption of the inhibitor by physical blocking

Table(21):Electrochemical parameters and inhibition efficiency for zinc

electrode in 2.0 M HCl in absence and presence of different

concentrations of Ficus carica extract at 25ºC .

corrR

(mpy) IE%

Rp

(Ω.cm2)

ab

(V/dec)

– cb

(V/dec)

corrI

)A/cm2)

corrE

(V)

Conc.

(g/l)

4.677×10-4

0.00 2.309 0.10 0.107 3.977 ×10-3

1.06 0.00

1.061×10-4

97.73 30.827 0.021 0.187 9.025 ×10-5

0.961 8.0

8.793×10-5

98.11 40.257 0.023 0.165 7.477 ×10-5

0.954 10.0

7.791×10-5

98.33 40.287 0.023 0.205 6.625 ×10-5

0.941 12.0

7.182×10-5

98.46 60.07 0.019 0.162 6.107 ×10-5

0.935 15.0

5.642×10-5

98.79 100.106 0.031 0.199 4.798×10-5

0.913 20.0

Table(22):Electrochemical parameters and inhibition efficiency for zinc

electrode in 2.0 M HCl in absence and presence of different


corrR

(mpy) IE%

Rp

(Ω.cm2)

ab

(V/dec)

– cb

(V/dec)

corrI

)A/cm2)

corrE

(V)

Conc.

(g/l)

4.677×10-3

0.00 2.309 0.10 0.107 3.977×10-3

1.06 0.00

7.944×10-4

83.01 10.354 0.074 0.214 6.73×10-4

0.966 8.0

7.297×10-4

84.39 10.468 0.073 0.210 6.206×10-4

0.958 10.0

6.0126×10-4

85.13 20.039 0.044 0.211 5.113 ×10-4

0.957 12.0

3.624×10-4

92.25 20.126 0.030 0.169 3.082×10-4

0.954 15.0

2.847×10-4

93.91 30.342 0.025 0.135 2.421×10-4

0.949 20.0

Table(23): Electrochemical parameters and inhibition efficiency for zinc

electrode in 2.0 M H2SO4 in absence and presence of different


corrR

(mpy) IE%

Rp

(Ω.cm2)

ab

(V/dec)

– cb

(V/dec)

corrI

)A/cm2)

corrE

(V)

Conc.

(g/l)

1.115×10-3

0.00 5.544

0.057 0.109 9.74×10-4

0.977 0.00

2.0555×10-4

82.05 20.548 0.031 0.224 1.748×10-4

0.896 8.0

1.0711×10-4

90.64 20.661 0.015 0.178 9.109 ×10-5

0.881 10.0

1.005×10-4

91.22 30.405 0.02 0.219 8.548 ×10-5

0.876 12.0

9.349×10-5

91.83 40.48 0.012 0.204 7.95 ×10-5

0.874 15.0

6.036×10-5

94.72 70.751 0.02 0.224 5.133 ×10-5

0.867 20.0

Table(24): Electrochemical parameters and inhibition efficiency for zinc

electrode in 2.0 M H2SO4 in absence and presence of different


corrR

(mpy) IE%

Rp

(Ω.cm2)

ab

(V/dec)

– cb

(V/dec)

corrI

)A/cm2)

corrE

(V)

Conc.

(g/l)

1.115×10-3

0.00 5.544 0.057 0.109 9.74×10-4

0.977 0.00

4.795×10-4

78.98 7.966 0.024 0.158 4.078×10-4

0.905 8.0

2.055×10-4

82.05 8.707 0.031 0.224 1.748×10-4

0.903 10.0

1.742×10-4

84.78 10.011 0.028 0.125 1.482×10-4

0.902 12.0

1.691×10-4

85.30 20.548 0.013 0.109 1.429×10-4

0.900 15.0

1.071×10-4

90.64 30.405 0.015 0.178 9.109×10-5

0.881 20.0

III.2. Corrosion inhibition of α-brass in acidic media:

III.2.1. Chemical measurements:

III.2.1.1.Weight loss methods:

III.2.1.1.1- Effect of corrodent concentration:

Figures (40,41) represent the variation of weight loss (g/cm2) with time

of α-brass immersed in aerated solutions of HCl and H2SO4 of different

concentrations range (0.01-1.0 M). Inspection of these Figures reveal

that, the curves are characterized by gradual rise in weight loss with time

along a period of 96 hours as well as upon increasing the concentrations

of HCl or H2SO4 acid solution. The corrosion rate corrR (g/cm2.h) was

calculated from the slopes of the straight line obtained, which increase

with the increasing of concentration of HCl and H2SO4 solutions. This

indicates that the corrosion rate of α-brass depends on the concentration,

this is probably due to the increase in the rate of diffusion and ionization

of active species in the corrosion reaction. The results of weight loss

measurements are listed in Table (25). It is clear from these results that

the corrosion rate of α-brass in HCl higher than in H2SO4 solution.

Time(h)

0 20 40 60 80 100 120

Wt.

loss

(g/c

m2)

0.000

0.005

0.010

0.015

0.020

0.025

0.01 M

0.1 M

0.3 M

0.5 M

1.0 M

Figure(40):Weight loss-time curves for α-brass in different

concentrations of HCl solution at 25ºC.

Time(h)

0 20 40 60 80 100 120

Wt.

loss

(g/c

m2)

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.020

0.01 M

0.1 M

0.3 M

0.5 M

1.0 M

Figure(41):Weight loss-time curves for α-brass in different

concentrations of H2SO4 solution at 25ºC.

Table(25):Corrosion rate of α-brass in different concentrations of HCl

and H2SO4 solutions at 25ºC.

Corrosion rate ( corrR )

(g/cm2.h)

Conc. of acid

(M)

H2SO4 HCl

2.893×10-6

2.025×10-5

0.01

3.472×10-5

6.366×10-5

0.1

5.497×10-5

8.681×10-5

0.3

6.944×10-5

1.534×10-4

0.5

1.302×10-4

2.054×10-4

1.0

III.2.1.1. 2- Effect of plant extracts as corrosion inhibitors:

Figures (42-45) represent the relation between weight loss (g/cm2) and

the immersion time for α-brass in 0.5 M of both HCl and H2SO4 solutions

in absence and presence of different concentrations of Ficus carica and

Olive extracts at 25ºC. Inspection of these Figures reveal that, all the

curves are characterized by gradual rise in weight loss with time along

a period of 120 h. The addition of different concentrations of the extracts

inhibits the corrosion of α-brass electrode in both media, and the rate of

inhibition increases with increasing the concentration of extracts. The

linearity indicates that the absence of insoluble surface film during

corrosion, and that the inhibitors were first adsorbed onto the alloy

surface and thereafter, inhibits the corrosion process

. In acidic

solutions(66)

zinc dissolves preferentially first from the surface of the

alloy. The rate of dezincification decreases with time and at a later stage

simultaneous dissolution of both copper and zinc takes place. These

results indicate that the free acids without extracts zinc preferentially

dissolves from the alloy ,as indicated from the presence of zinc in

solution after the exposure time, . The presence of the investigated

extracts in acidic solution greatly inhibits the process of preferential

dissolution of zinc from the alloy and the simultaneous dissolution of

both zinc and copper takes place instead. The inhibition efficiency

percent, IE% in acid solutions was calculated using the equation (2). The

calculated values of inhibition efficiency and corrosion rate obtained

from the weight loss are listed in Tables (26,27). It is obvious that the

IE% increases with increasing the concentration of inhibitors, whereas

corrR decreases. The relationship between inhibition efficiency percent

and the concentration of extracts are represent in Figures (46,47). The

curves show that, the inhibition efficiency percent increase with

increasing the concentration of the extracts, and above a certain

concentration, 20.0 g/l has no effect on the inhibition efficiency. This is

confirmed by the fact that the inhibition efficiency does not increase

linearly with extract concentrations, (sigmoidal shape) thus, the physical

adsorption is takes place on α-brass surface.

The protective action of inhibitor during metal corrosion is based on

the adsorbability of their molecules, where the adsorption film isolates

the metal surface from the corrosive media. The degree of the surface

coverage θ which represents the part of the metal surface covered by the

inhibitor molecules was calculated using the equation (5).

The values of corrR , IE% and θ for different concentrations of Ficus

carica and Olive extracts in both acids are listed in Tables (26,27).

Inspection of these Tables, the data revealed that as the extract

concentrations are increased, the corrosion rate decreased while the

inhibition efficiency percent and surface coverage increases. This

behaviour may be attributed to increase the number of adsorbed

molecules at α-brass surface. A good efficiency is observed at 20.0 g/l , in

HCl are 84.91%, 86.86% and in H2SO4 83.33%, 85.58% for Ficus carica

and Olive extracts, respectively.

Time(h)

0 20 40 60 80 100 120

Wt.

loss

(g/c

m2)

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

free acid

8.0 g/l

10.0 g/l

12.0 g/l

15.0 g/l

20.0 g/l

Figure(42):Weight loss-time for α-brass in 0.5 M HCl



Time(h)

0 20 40 60 80 100 120

Wt.

loss

(g/c

m2)

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

free acid

8.0 g/l

10.0 g/l

12.0 g/l

15.0 g/l

20.0 g/l

Figure(43):Weight loss-time curves for α-brass in 0.5 M

HCl solution in absence and presence of

different concentrations of Olive extract at

25ºC.

Time(h)

0 20 40 60 80 100 120

Wt.

loss

(g/c

m2)

0.000

0.002

0.004

0.006

0.008

free acid

8.0 g/l

10.0 g/l

12.0 g/l

15.0 g/l

20.0 g/l



different concentrations of Ficus carica

extract at 25ºC.

Time(h)

0 20 40 60 80 100 120

Wt.

loss

(g/c

m2)

0.000

0.002

0.004

0.006

0.008

free acid

8.0 g/l

10.0 g/l

12.0 g/l

15.0 g/l

20.0 g/l



different concentrations of Olive extract at

25ºC.

Table(26):Effect of plant extract concentrations on corrosion rate( corrR ),


α-brass in 0.5 M HCl at 25ºC.


θ IE% corrR

(g/cm2.h)

θ IE% corrR

(g/cm2.h)

inhC

(g/l)

- - 1.534×10-4

- - 1.534×10-4

0.0

0.5850 58.50 6.365×10-5

0.4531 45.31 10-5

×8.388 8.0

0.6430 64.30 5.476×10-5

0.5474 54.74 6.942×10-5

10.0

0.7170 71.70 4.340×10-5

0.6793 67.93 4.919×10-5

12.0

0.8113 81.13 2.894×10-5

0.7736 77.36 3.472×10-5

15.0

0.8686 86.86 2.014×10-5

0.8491 84.91 2.315×10-5

20.0

Table(27):Effect of plant extract concentrations on corrosion rate( corrR ),


α-brass in 0.5 M H2SO4 at 25ºC.


θ IE%

corrR

(g/cm2.h)

θ IE% corrR

(g/cm2.h)

inhC

(g/l)

- -6.944×10-5

- - 6.944×10-5

0.0

0.6172 61.17 2.658×10-5

0.5417 54.17 3.183×10-5

8.0

0.6666 66.66 2.315×10-5

0.6250 62.50 2.604×10-5

10.0

0.7500 75.00 1.736×10-5

0.7033 70.33 2.025×10-5

12.0

0.8333 83.33 1.157×10-5

0.7873 78.73 1.447×10-5

15.0

0.8558 85.58 1.001×10-5

0.8333 83.33 1.157×10-5

20.0

Cinh

(g/l)

5 10 15 20 25 30 35

IE%

40

50

60

70

80

90

ficus carica ext.

olive ext.

Figure(46):Relationship between inhibition efficiency and the



Cinh

(g/l)

5 10 15 20 25 30 35

IE%

50

55

60

65

70

75

80

85

90

ficus carica ext.

olive ext.

Figure(47):Relationship between inhibition efficiency and the



III.2.1.1.3- Adsorption isotherm:

Molecules of organic compounds(80)

(adsorbate) can attach to the

surface of metal in two ways; physical adsorption or chemisorption. In

physisorption where Vander Waals interaction between adsorbate and

substrate. This sort of interaction has a long range but is weak, with

typical enthalpy of condensation energy values in the region of -20

KJ/mol. Physisorbed molecules retain their identity, while the energy is

insufficient to break bonds. By contrast, the chemisorption process

includes the formation of covalent bonds. The molecules find sites that

maximize their coordination number with the substrate. The enthalpy of

chemisorption is an order of magnitude greater than of physisorption, as

the distance between surface molecules and adsorbate molecules are

shorter. Basic information on the interaction between the inhibitor and the

metal surface can be provided by the adsorption isotherm. In this study

the adsorption of Ficus carica and Olive extracts on the α-brass surface

was fit by the kinetic thermodynamic model ( El-Awady isotherm)(81)

:

(17)

C log YK logθ1

θlog /

Where Y is the number of inhibitor molecules occupying a given active

site. The binding constant K is given by:

)

Y1/(

KK (18)

Where K is a constant related to the binding constant of the adsorption

process, and Y1 represents the number of active sites of the surface

occupied by one molecule of an inhibitor. Values of Y1 greater than

unity imply the formation of multilayers of the inhibitor on the metal

surface. Values of Y1 less than unity, however, mean that a given

inhibitor will occupy more than one active site(81)

. The relation between

the binding constant of adsorption ( K ) and the standard free energy of

adsorption

adsΔG was given by the equation (8). Figures (48,49) show

the plot of θ1θ log versus C log for of different concentrations, (8.0-

20.0 g/l) of Ficus carica and Olive extracts, yield straight lines, with

intercept log K⁄ , proving that the adsorption of extracts on the α-brass

surface obey the kinetic-thermodynamic model (El-Awady isotherm).

The parameters of K, Y, Y1 and

adsΔG are given in Tables (28,29).

Inspection of these Tables, the values of K are relatively small

indicating that the interaction between the adsorbed extract molecules and

α-brass surface is physical adsorbed. The values of Y1 is less than one,

which means that each molecule of Ficus carica and Olive extracts

occupied more than one active site.

The low values of standard free energy change of adsorption,

adsΔG

indicates that the physical adsorption, probably electrostatic nature and

that no covalent bond between inhibitor molecule and α-brass surface.

The negative sing meaning spontaneous interaction of inhibitor molecules

with corroding metal surface(83)

.

0.8 0.9 1.0 1.1 1.2 1.3 1.4

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

ficus carica ext.

olive ext.

Figure(48):Linear fitting of plant extracts for α-brass in 0.5 M

HCl solution using Kinetic-thermodynamic model

isotherm at 25ºC.

log Cinh(g/l)

log θ

/1-θ

0.8 0.9 1.0 1.1 1.2 1.3 1.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

ficus carica ext.

olive ext.

Figure(49):Linear fitting of plant extracts for α-brass in 0.5 M

H2SO4 solution using Kinetic-thermodynamic

model isotherm at 25ºC.

log Cinh(g/l)

log θ

/1-θ

Table(28):Linear fitting parameters and the standard free energy of plant

extracts for α-brass in 0.5 M HCl solution at 25ºC.

-

adsΔG

KJ mol-1

Y

1 Y

K

(g-1

l)

Plant extract

10.77 0.4625 2.162 1.390 Ficus carica

10.50 0.5655 1.768 1.247 Olive

Table(29):Linear fitting parameters and the standard free energy of plant

extracts for α-brass in 0.5 M H2SO4 solution at 25ºC.

-

adsΔG

KJ mol-1

Y

1 Y

K

(g-1

l)

Plant extract

9.95 0.6127 1.632 1.226 Ficus carica

10.13 0.6856 1.458 1.077 Olive

III.2.1.1.4- Effect of temperature:

The effect of temperature on the corrosion rate of α-brass in 0.5 M of

HCl and H2SO4 in presence of 15.0 g/l of Ficus carica and Olive extracts

was studied in the temperature range (25 -55ºC) using weight loss

measurements. The calculated values of corrosion rate corrR and

inhibition efficiency IE% are listed in Tables (30,31). It is obvious that

the corrosion rate of α-brass in absence and presence of 15.0 g/l of plant

extracts increase with increasing temperature, indicated that the extracts

was adsorbed on α-brass surface at all temperatures. The inhibition

efficiency of the extracts decrease markedly with an increasing in

temperature. Thus, as the temperature increases the number of adsorbed

molecule decreases leading to decrease inhibition efficiency. This

behaviour proves that the adsorption of Ficus carica and Olive extracts on

α-brass surface occurs through physical adsorption. Here again, the

apparent activation energy *aE , the enthalpy change of activation *ΔH

and entropy change of activation *ΔS for the corrosion of α-brass in

0.5 M of both HCl and H2SO4 solutions in absence and presence 15.0 g/l

of plant extracts were calculated from Arrhenius and Eyring equations

(9,10).

Figures (50,51) represent Arrhenius plot of log corrR against T1 for

α-brass in 0.5 M of HCl and H2SO4 solutions in absence and presence of

15.0 g/l of extracts. A straight lines were obtained with a slope equal to

2.303R*E . The values of *aE for corrosion reaction in absence and

presence of the extracts under test were calculated and given in Tables

(32,33). The values of *aE were found to be 28.79 and 16.82 KJmol

-1 in

0.5 M of both HCl and H2SO4 , respectively. The increase of the

activation energies in the presence of the extracts is attributed to an

appreciable decreasing the adsorption process of the extracts on the α-

brass surface with increase in temperature and a corresponding rise in the

reaction rate occurs owing to the greater area of the α-brass that is

exposed to the acidic solution. Two types of adsorption process had

been distinguished(84)

physisorption in which the activation energy is less

than 40 KJ/mol and chemisorption where activation energy is greater than

80 KJ/mol. On the basis of the experimentally determined activation

energy values, the extracts are physically adsorbed on the α-brass surface.

The activation thermodynamic parameters *ΔH and *ΔS were obtained

from the Eyring equation (10). By plotting of TRlogcorr

against T1 for

α-brass in 0.5 M of both HCl and H2SO4 solutions in absence and

presence of 15.0 g/l of extracts as represented in Figures (52,53). The

relation gave straight lines with slope equals to - 2.303RΔH* and the

intercept is 2.303RΔSNhRlog * . The obtained values of *ΔH and

*ΔS are given in Tables (32,33). Inspection of these results reveals that

the activation enthalpy *ΔH in inhibited solutions higher than in free

acids solution. This indicates that the addition of the extracts under test to

the acid solutions increase the height of the energy barrier of the

corrosion reaction to an extent depends on the type and concentration of

the extracts under test. The negative values of *ΔS in the absence and

presence of the inhibitors implies that the activated complex is the rate

determining step, and represents association rather than dissociation. It

also, reveals that an increase in the order takes place in going from

reactants to the activated complex. The obtained results suggested that

such types of inhibitors give a good inhibition at ordinary temperature

with considerable loss in inhibition efficiency at elevated temperature.

Table(30):Effect of temperature on ( corrR ) and ( IE%) for α-brass in

0.5 M HCl in absence and presence of plant extracts.

Olive extract Ficus carica extract 0.5 M HCl

Temp.

°C IE% corrR

(g/cm2.h)

IE% corrR

(g/cm2.h)

corrR

(g/cm2.h)

81.13 2.894×10-5

77.36 3.472×10-5

1.534×10-4

25

76.56 4.339×10-5

60.92 7.236×10-5

1.852×10-4

35

64.69 8.682×10-5

50.59 1.215×10-4

2.459×10-4

45

55.47 1.881×10-4

38.36 2.604×10-4

4.225×10-4

55

Table(31):Effect of temperature on ( corrR ) and ( IE%) for α-brass in

0.5 M H2SO4 in absence and presence of plant extracts.

Olive extract Ficus carica

extract 0.5 M H2SO4

Temp.

°C IE% corrR

(g/cm2.h)

IE% corrR

(g/cm2.h)

corrR

(g/cm2.h)

83.33 1.157×10-5

79.17 1.446×10-5

6.944×10-5

25

79.30 1.736×10-5

75.83 2.027×10-5

8.388×10-5

35

71.44 2.893×10-5

65.72 3.472×10-5

1.013×10-4

45

60.00 5.208×10-5

55.55 5.787×10-5

1.302×10-4

55

3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35

-4.8

-4.6

-4.4

-4.2

-4.0

-3.8

-3.6

-3.4

-3.2

free acid

ficus carica ext.

olive ext.

Figure(50):Arrhenius plots of the corrosion rate for α-brass

in 0.5 M HCl in absence and presence of plant

extracts.

103/T(

°K

-1)

Log R

corr(R

corr, g.c

m-2

.h-1

)

3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40

-5.2

-5.0

-4.8

-4.6

-4.4

-4.2

-4.0

-3.8

free acid

ficus carica ext.

olive ext.

Figure(51):Arrhenius plots of the corrosion rate for α-brass

in 0.5 M H2SO4 in absence and presence of plant

extracts.

103/T(

°K

-1)

Log R

corr(R

corr, g c

m-2

.h-1

)

3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35

-5.2

-5.0

-4.8

-4.6

-4.4

-4.2

-4.0

-3.8

-3.6

free acid

ficus carica ext.

olive ext.

Figure(52):Eyring plots of the corrosion rate for brass in

0.5 M HCl in absence and presence of plant

extracts.

103/T(

°K

-1)

Log R

corr/T

(Rco

rr, g c

m-2

.h-1

)

3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40

-5.6

-5.4

-5.2

-5.0

-4.8

-4.6

-4.4

-4.2

free acid

ficus carica ext.

olive ext.

Figure(53):Eyring plots of the corrosion rate for brass in

0.5 M H2SO4 in absence and presence of plant

extracts.

103/T(

°K

-1)

Log R

corr/T

(Rco

rr, g c

m-2

.h-1

)

Table (32):Activation parameters of α-brass in 0.5 M HCl in absence

and presence of extracts.

Plant extracts

*aE

KJ mol-1

*ΔH

KJ mol-1

- *ΔS

J mol-1

ºK

-1

Free acid 28.79 33.12 171.18

Ficus carica 54.64 57.25 104.29

Olive 56.61 59.23 95.17

Table (33):Activation parameters of α-brass in 0.5 M H2SO4 in absence

and presence of extracts.

Plant extracts

*aE

KJ mol-1

*ΔH

KJ mol-1

- *ΔS

J mol-1

ºK

-1

Free acid 16.82 19.42 222.26

Ficus carica 38.07 40.67 162.43

Olive 40.71 43.31 157.37

III.2. Thermometric measurements:

Figures (54,55) show the rate of dissolution of α-brass in different

concentrations range (0.01-1.0 M) of HCl and H2SO4 solutions are too

low to be detected by thermometric technique(85)

. Because there is no

contribute significantly to the heat evolved during the dissolution of α-

brass.

Time(min)

0 50 100 150 200 250 300 350

24.0

24.5

25.0

25.5

26.0

26.5

27.0

0.01 M

0.1 M

0.3 M

0.5 M

1.0 M

Figure (54):Temperature-time curves of α-brass in different

concentrations of HCl solution at Ti = 25°C.

Tem

per

atu

re (

°C)

Time(min)

0 50 100 150 200 250 300 350

24.0

24.5

25.0

25.5

26.0

26.5

27.0

0.01 M

0.1 M

0.3 M

0.5 M

1.0 M

Figure (55):Temperature-time curves of α-brass in different

concentrations of H2SO4 solution at Ti = 25°C.

Tem

per

atu

re (

°C)

III.2.2. Electrochemical measurements:

III.2.2.1- Open-circuit potential measurements:

The open-circuit potential measurements are a very suitable method

to investigate the corrosion rates of metal or alloy. The use of this method

is based on the difference between the corrosion potential of the metal

under investigation and the open-circuit measurements of the inhibitors.

In this part the potential-time curves of α-brass electrode was

constructed, under open-circuit conditions, in aerated solutions of HCl

and H2SO4 using different concentrations (0.01-1.0 M). Also the effect of

Ficus carica and Olive extracts with concentration range (8.0-20.0 g/l).

The potentials of α-brass electrode were measured till the attainment of

constant potentials. Figures (56,57) show the open-circuit potential

behaviour of α-brass electrode with time in different concentrations range

(0.01-1.0 M) of HCl and H2SO4 solutions. The potential measured

directly after immersion, the steady-state potential vary with the time and

concentrations of the test solutions.

Accordingly, the behaviour can be classified into two different

behaviours depending upon the mode of variation of the steady-state

potentials with time and the concentration of the test solutions.

In the first behaviour which represented by the potential-time curves of

α-brass electrode in HCl solutions. Figure (56), these curves exhibit a

general tendency for immersion potential to shift from negative to the

positive direction. This indicates formation of passive film, then the

potential drops to negative value until the steady state potential is

reached.

On the other hand, the second behaviour to which α-brass electrode in

H2SO4 solutions is recorded in Figure (57), these curves exhibit a general

tendency for the open-circuit potential to shift from positive to more

negative values than the initial potential denoting the destruction of the

pre-immersion oxide film present on the surface of the electrode. The

steady-state potential of α-brass electrode becomes more negative as the

concentration of both acidic solutions are increased. Comparing the

potentials for α-brass electrode in both HCl and H2SO4 solutions by

using the same experimental conditions, it was concluded that α-brass is

more corrosive in HCl than H2SO4 solutions.

The corrosion of copper is usually an electrochemical reaction

involving dissolution of the metal as ions at anodic areas and deposition

of hydrogen from the electrolyte at cathodic area. Copper being a noble

metal, does not normally displace hydrogen from a solution containing

hydrogen ions.

In fact, the corrosion of copper in aerated acidic solution is a result of

two partial reactions(86)

. The successive anodic reaction can be written as:

2e2Cu2Cu (19)

2e2Cu2Cu 2 at anode (20)

with copper and its alloys the predominant cathodic reaction is the

reduction of oxygen to form hydroxide ions. Therefore, the presence of

oxygen or other oxidizing agent is essential for corrosion to takes place.

The cathodic reaction is :

O2H4eO4H22

at cathode (21)

If copper is placed in hydrochloric acid, cuprous chloride (CuCl) formed,

with copper locked up mainly as cuprous chloride complex[CuCl2](87)

.

On the other hand, the reaction between copper and sulphuric acid is not

thermodynamic possible; but corrosion proceeds in the presence of

oxygen, and the products are copper sulphate and water. The first

oxidation products film to form should be a hydrated sulphate, not Cu2O,

and the film should be CuSO4.5H2O in lower and moderate

concentrations and CuSO4. H2O in higher concentrations(88)

.

The open-circuit potentials of α-brass electrode were followed over

three hours from the electrode immersion in HCl or H2SO4 solutions, in

absence and presence of Ficus carica and Olive extracts.

Figures (58-61) represent the variation of open-circuit potential with

time of α-brass in aerated 0.5 M of both acids in presence of different

concentrations (8.0-20.0 g/l) of extracts under test. Figures (58-61) show

that, the steady- state potential, Ess was shifted towards less negative

(noble) values than free acids. This can be attributed to the adsorption of

extracts molecules on the active corrosion sites on the α-brass surface

(anodic process). This potential shift towards the positive direction

indicates that the extracts acts as anodic inhibitor(89)

. Moreover, the

addition of the extracts has also a significant influence on the initial

potential value. The initial potential value was shifted towards less

negative value as extracts concentration increases. This behaviour can be

attributed to the influence of the inhibitors on the cathodic process.

Figures(62,63) represent the relationship between Ess and logarithm of

the concentration of extracts under test. It is clear from this Figures, that

as the concentration of inhibitors increase, the steady-state potential

shifted to less negative values (towards the positive direction) in

accordance with the equation (11). It is concluded that the positive shift

of steady-state potential indicated that the increase of the resistance to

corrosion attack, and the extracts act as anodic-type inhibitors.

Time (min)

0 20 40 60 80 100 120 140 160 180

E,m

V(S

CE

)

-400

-380

-360

-340

-320

-300

-280

-260

-240

-220

0.01 M

0.1 M

0.3 M

0.5 M

1.0 M


α-brass electrode in different concentrations of


Time (min)

0 20 40 60 80 100 120 140 160 180 200

E,m

V(S

CE

)

-300

-280

-260

-240

-220

-200

-180

-160

-140

-120

0.01 M

0.1 M

0.3 M

0.5 M

1.0 M

Figure(57):The variation of the open-circuit potential for α-

brass electrode in different concentrations of


Time (min)

0 20 40 60 80 100 120 140 160 180 200

E,m

V(S

CE

)

-400

-380

-360

-340

-320

-300

-280

-260

-240

-220

-200

free acid

8.0 g/l

10.0 g/l

12.0 g/l

15.0 g/l

20.0 g/l

Figure(58):The variation of the open- circuit potential for α-



Time (min)

0 20 40 60 80 100 120 140 160 180 200

E,m

V(S

CE

)

-400

-380

-360

-340

-320

-300

-280

-260

-240

-220

-200free acid

8.0 g/l

10.0 g/l

12.0 g/l

15.0 g/l

20.0 g/l




Time (min)

0 20 40 60 80 100 120 140 160 180 200

E,m

V(S

CE

)

-300

-280

-260

-240

-220

-200

-180

-160

-140

-120

-100

free acid

8.0 g/l

10.0 g/l

12.0 g/l

15.0 g/l

20.0 g/l




Time (min)

0 20 40 60 80 100 120 140 160 180 200

E,m

V(S

CE

)

-300

-280

-260

-240

-220

-200

-180

-160

-140

-120

free acid

8.0 g/l

10.0 g/l

12.0 g/l

15.0 g/l

20.0 g/l




log Cinh

(g/l)

0.8 0.9 1.0 1.1 1.2 1.3 1.4

Ess

, m

V(S

CE

)

-340

-330

-320

-310

-300

-290

-280

-270

-260

ficus carica ext.

olive ext.

Figure(62):The variation between the steady-state

potentials and logarithm of the concentration of

extracts for α-brass electrode in 0.5 M HCl

solution at 25ºC.

log Cinh

(g/l)

0.8 0.9 1.0 1.1 1.2 1.3 1.4

Ess

, m

V(S

CE

)

-255

-250

-245

-240

-235

-230

-225

-220

ficus carica ext.

olive ext.

Figure(63):The variation between the steady-state

potentials and logarithm of the concentration of

extracts for α-brass electrode in 0.5 M H2SO4

solution at 25ºC.

III.2.2.2- Potentiodynamic polarization measurements:

Anodic dissolution of copper in acidic chloride media has been studied

extensively(90 , 91)

. The accepted anodic reaction at 0.5 M HCl solution is

the dissolution of copper through oxidation of Cu to Cu .

eCuCu (22)

Then Cu reacts with chloride ion from the solution to form CuCl

CuClClCu - (23)

Insoluble CuCl precipitates on the copper surface, has poor adhesion, is

unable to produce enough protective for the copper surface, and

transforms to the sparingly soluble cuprous chloride complex, 2

CuCl(92)

.

2

- CuClClCuCl (24)

It has also been reported that the 2

CuCl adsorbed on the surface

dissolves by further oxidation(93)

:

e2ClCuCuCl 2ads2

(25)

The anodic and cathodic polarization curves for α-brass electrode in

aerated 0.5 M HCl solution at 25ºC in absence and presence of different

concentrations (8.0-20.0 g/l) of Ficus carica and Olive extracts after one

hour immersion are shown in Figures (64,65). The anodic curve for α-

brass electrode in 0.5 M HCl, blank solution exhibits three distinct

regions, which are the active dissolution (apparent Tafel region), the

active passive transition region, and the limiting current region. The

limiting current plateau indicates a role of a diffusion limiting rate,

probably both the transport of chloride ion ( Cl ) to the surface and the

diffusion of ( 2

CuCl ) in the solution. It is reported that the anodic

dissolution of copper in acidic chloride solution is controlled by both

electrodissolution of copper and diffusion of 2

CuCl to the bulk

solution(92, 93)

. The cathodic curve for HCl blank can be described to the

reduction of dissolved oxygen present in the test solution. The cathodic

reaction in an aerated acidic chloride solution is :

O2H4eO4H22

(26)

and the total corrosion reaction of copper in HCl solutions is as follows:

O2H4Cl2CuO4Cl4H2Cu2

22

(27)

In presence of extracts the shape of both anodic and cathodic curves

are not modified on adding the extracts or by increasing the

concentration, but the more pronounced behaviour that, the extracts had a

greater influence on the anodic than cathodic, curves.

The corrosion parameters[corrosion potentials ( corrE ), corrosion

current ( corrI ), cathodic Tafel slope ( cb ), anodic Tafel slope ( ab ) and

corrosion rate ( corrR )] are listed in Tables (34,35). The data in this

Tables showed that the decreases in current density ( corrI ) with

increasing the extracts concentration, this may be due to the decrease in

chloride ions attack on the α-brass surface due to the adsorption of the

inhibitor molecules. Tafel slopes cb , ab are more or less constant

suggesting that, the inhibitive action of the extract does not affect on the

mechanism of the corrosion process. Furthermore, the addition of extracts

shifts the corrosion potential ( corrE ) to positive direction indicate that

the Ficus carica and Olive extracts have effect on anodic dissolution of α-

brass and acts as anodic-type inhibitors. The values of inhibition

efficiency IE% , were calculated from the corresponding corrosion current

densities, corrI using the equation (15).

The calculated values of IE% are listed in Tables (34,35). The values

of IE% show that, the Ficus carica and Olive extracts inhibits the

corrosion process in HCl solution. The inhibition efficiencies increase as

the inhibitors concentration increases and reach maximum values of

82.28% and 84.66%, at 20.0 g/l for Ficus carica and Olive, respectively.

In general, the extracts act as anodic-type inhibitors.

Anodic and cathodic potentiodynamic polarization curves of α-brass in

aerated solution of 0.5 M H2SO4 in absence and presence of various

concentrations of (8.0-20.0 g/l) of Ficus carica and Olive extracts at 25ºC,

were obtained in Figures (66,67). The anodic curves consist of three

regions were, region(I) the active dissolution (apparent Tafel region),

region(II) the transition region and the limiting current region (III). The

anodic dissolution of copper in acidic media can be described as:

2e2HOCuOH2Cu22

(28)

2eOH2Cu2HOCu2

22

at anode (29)

The cathodic corrosion reaction in an aerated acidic solution is oxygen

reduction expressed in the following:

O2H4eO4H22

at cathode (30)

From the Figures (66,67) it is evident that in the presence of the

extracts the anodic and cathodic curves were shifted to less negative

potential region and the shift was dependent on the concentration of the

extracts. Also the anodic reaction is inhibited to large extent than the

cathodic reaction. The electrochemical parameters, corrosion potential

( corrE ), corrosion current density ( corrI ), cathodic Tafel slope ( cb ) ,

anodic Tafel slope ( ab ) and inhibition efficiency ( IE%), were listed in

Tables (36,37). It is observed that the corrE values were shifted towards

less negative (noble direction), this observation clearly shows that the

extracts acts as mixed type inhibitor but the greater effect on the anodic

reaction so it acts predominantly as anodic-type inhibitors.

The corrosion current density ( corrI ) decreased with increasing

concentration of extracts. This indicates the inhibiting effect of Ficus

carica and Olive extracts on the α-brass corrosion. Tafel slopes, cb and

ab , are approximately constant upon addition of inhibitors suggesting

that the inhibiting action occurred by a simple blocking of the available

cathodic and anodic sites on the metal surface, without affecting the

corrosion mechanism. The values of IE% increase with increasing the

extracts concentration, and the higher efficiency at 20.0 g/l are 78.49%

and 83.58% for Ficus carica and Olive extracts in 0.5 M H 2SO4 ,

respectively. Thus, the extracts under test act as anodic-type inhibitors in

both HCl and H2SO4 solutions. This results in a good agreement with the

results obtained from the open-circuit potential.

Conclusions :

From the results of this study, it can be concluded that:

1- Ficus carica and Olive extracts are found to perform well as

corrosion inhibitors for α-brass in both HCl and H2SO4 solutions.

And the maximum IE% was achieved at 20.0 g/l.

2- The inhibitive active of these extracts take place through the

adsorption of their molecules on the α-brass surface.

3- The inhibition mechanism of the extracts for α-brass in both acids

is agreement with the kinetic-thermodynamic model, in which, one

molecule of inhibitor occupying more than one active site.

4- Temperature studies revealed that, a decrease in efficiency with

rise in temperature and corrosion activation energies were higher in

the presence of the extracts, leading to a physical adsorption.

5- The open-circuit potential measurements indicated that the extracts

in both acids are affected on anodic and cathodic processes. And

the steady-state potential indicated that, the extracts act as anodic-

type inhibitors.

6- Potentiodynamic polarization measurements showed that the

extracts were adsorbed mainly at anodic sites, and they act as

anodic-type inhibitor.

7- The electrochemical measurements are in a good agreement with

the weight loss methods. This improves the validity of the results

obtained.

Figure(64):Potentiodynamic polarization curves for α-

brass electrode in 0.5 M HCl in absence and

presence of different concentrations of Ficus

carica extract at 25ºC.

log I(A/cm2)

E,V

(Ag/A

gC

l)


brass electrode in 0.5 M HCl in absence and

presence of different concentrations of Olive

extract at 25ºC.

log I (A/cm2)

E,V

(Ag/A

gC

l)

Table(34):Electrochemical parameters and inhibition efficiency for α-

brass electrode in 0.5 M HCl in absence and presence of

different concentrations of Ficus carica extract at 25ºC.

corrR

(mpy) IE%

Rp

(Ω.cm2)

ab

(V/dec)

– cb

(V/dec)

corrI

)A/cm2)

corrE

(V)

Conc.

(g/l)

7.614×10-5

0.00 10.249 0.149 0.09 8.363 ×10-5

0.491 0.0

3.849×10-5

49.44 10.810 0.148 0.096 4.228 ×10-5

0.499 8.0

3.713×10-5

51.23 13.765 0.148 0.091 4.078 ×10-5

0.490 10.0

3.534×10-5

53.58 28.543 0.140 0.060 3.882 ×10-5

0.486 12.0

1.544×10-5

79.72 46.528 0.136 0.055 1.696 ×10-5

0.482 15.0

1.350×10-5

82.28 46.600 0.108 0.029 1.483 ×10-5

0.455 20.0


brass electrode in 0.5 M HCl in absence and presence of

different concentrations of Olive extract at 25ºC.

corrR

(mpy) IE%

Rp

(Ω.cm2)

ab

(V/dec)

– cb

(V/dec) corrI

)A/cm2)

corrE

(V)

Conc.

(g/l)

7.614×10-5

0.00 10.249 0.149 0.09 8.363 ×10-5

0.491 0.0

3.747×10-5

50.78 17.600 0.145 0.062 4.116 ×10-5

0.506 8.0

1.853×10-5

75.65 24.620 0.117 0.055 2.036 ×10-5

0.505 10.0

1.607×10-5

78.89 34.122 0.155 0.062 1.765 ×10-5

0.499 12.0

1.350×10-5

82.28 47.600 0.138 0.038 1.483 ×10-5

0.495 15.0

1.212×10-5

84.66 66.483 0.096 0.028 1.331×10-5

0.466 20.0


brass electrode in 0.5 M H2SO4 in absence and

presence of different concentrations of Ficus

carica extract at 25ºC.

log I (A/cm2)

E,V

(Ag/A

gC

l)


brass electrode in 0.5 M H2SO4 in absence and

presence of different concentrations of Olive

extract at 25ºC.

log I(A/cm2)

E,V

(Ag/A

gC

l)


brass electrode in 0.5 M H2SO4 in absence and presence

different concentrations of Ficus carica extract at 25ºC.

corrR

(mpy) IE%

Rp

(Ω.cm2)

ab

(V/dec)

– cb

(V/dec)

corrI

)A/cm2)

corrE

(V)

Conc.

(g/l)

4.147×10-5

0.00 22.82 0.151 0.108 4.555 ×10-5

0.479 0.0

1.963×10-5

52.64 25.669 0.145 0.074 2.157 ×10-5

0.502 8.0

1.361×10-5

67.17 26.955 0.144 0.060 1.495 ×10-5

0.479 10.0

1.163×10-5

71.94 27.472 0.142 0.052 1.278 ×10-5

0.466 12.0

1.122×10-5

72.93 44.511 0.127 0.048 1.233 ×10-5

0.448 15.0

8.916×10-6

78.49 61.122 0.115 0.024 9.794 ×10-6

0.444 20.0


brass electrode in 0.5 M H2SO4 in absence and presence

different concentrations of Olive extract at 25ºC.

corrR

(mpy) IE%

Rp

(Ω.cm2)

ab

(V/dec)

– cb

(V/dec)

corrI

)A/cm2)

corrE

(V)

Conc.

(g/l)

4.147×10-5

0.00 22.82 0.151 0.108 4.555 ×10-5

0.479 0.0

1.586×10-5

61.73 33.080 0.149 0.108 1.743 ×10-5

0.484 8.0

1.499×10-5

63.84 33.295 0.145 0.084 1.647 ×10-5

0.471 10.0

9.759×10-6

76.46 44.350 0.148 0.074 1.072 ×10-5

0.463 12.0

9.141×10-6

77.95 46.950 0.148 0.066 1.004 ×10-5

0.455 15.0

6.808×10-6

83.58 68.341 0.105 0.044 7.478×10-6

0.440 20.0

Inhibition mechanism of plant extracts:

The obtained results indicated that Ficus carica and Olive extracts

performs a good inhibition for the corrosion of zinc and α-brass in both

HCl and H2SO4 solutions. The inhibition efficiency depends on many

factors including, number of adsorption sites, functional groups,

molecular size and mode of interaction(3)

. Ficus carica under test contains

mixture of benzyl alcohol and aromatic compounds (Cinnamic aldehyde,

Cinnamic alcohol) and a nitrogen containing compound (indol). The

aqueous extracts of Olive contains polyphenolic compounds, the major

constituents are oleuropein (C25H32O13) and hydroxytyrosol (3,4-

dihydroxphenylethanol). Oleuropein is readily hydrolyzed into elenolic

and hydroxytyrosol(68)

, these latter may be play the major role in the

inhibition process. The presence of oxygen atoms in both extracts as in

phenolic, aldehyde and alcoholic may be responsible for adsorption on

zinc or α-brass surface through the lone pair of electrons present on the

oxygen atoms which decrease the electronic density at an adsorption

centers of plant extracts molecules on a negative charged zinc or α-brass

surface, the physical adsorption determined by electrostatic attraction and

forming a covering film. Also attraction lateral(3)

interaction between the

inhibitor molecules containing long hydrocarbon chains due to Vander

Waals forces, give rise to stronger adsorption and higher inhibition

efficiency. The aromatic cycles as well as the hydrocarbon chains repel

the aqueous solution out of the electrode surface leading to inhibition of

the attack of the aggressive solution on the metal. Therefore, it could be

concluded that the Ficus carica and Olive extracts act as excellent

corrosion inhibitors for zinc and α-brass in aerated solutions of HCl and

H2SO4 by using chemical and electrochemical techniques.

SUMMARY

SUMMARY

The main objective of the present work involved the study of the

inhibitive properties of some natural products as Ficus carica and Olive

plant extracts as a safety and an environmentally friendly corrosion

inhibitors for zinc metal and α-brass (70%Cu -30% Zn) in acidic media.

This work comprises three main chapters:

Chapter I includes a definition of corrosion of metals and its inhibition,

types of inhibitors, and a notes about the use of the natural products as

corrosion inhibitors for protection of metals and its alloys. Also it

includes a literature survey on the corrosion inhibition of zinc and α-brass

in aqueous media. Finally the aim of the work.

Chapter II describes the experimental techniques, including the

material, description of apparatus, preparation of electrolytes,

measurements by chemical (weight-loss and thermometric) and

electrochemical (open-circuit and potentiodynamic) measurements.

Chapter III deals with the results and discussion of the data obtained

from the different techniques. It is divided into two main parts:

Part 1 includes the study of the corrosion inhibition for zinc sample in

acidic media. This part involves two subdivided parts, the first one deals

with the effect of different concentrations of HCl and H2SO4 solutions, as

corrodent media on zinc sample using weight loss and thermometric

methods. The results of the weight loss indicated that the corrosion rate of

zinc increases with increasing the concentrations of corrosion media, and

the corrosion rate ( corrR ) in HCl is higher than H2SO4 .

In the presence of different concentrations (8.0-20.0 g/l) of plant

extracts, (Ficus carica and Olive), the corrosion rate was found to

decrease and consequently the inhibition efficiency percent increases with

increasing extracts concentration. The highest efficiency was observed at

(20.0 g/l) in both acid solutions.

Theoretical fitting of Langmuir isotherm was investigated to clarify

the nature of adsorption. The results indicated that one adsorbed H2O

molecule is replaced by one molecule of inhibitor, and there is no

interaction between the adsorbed molecules. The values of standard free

energy change

adsΔG are low suggesting that the adsorption is

physisorption and spontaneous interaction of the inhibitor molecule with

corroding zinc surface.

The effect of temperature range (25-55ºC) on the corrosion rate of zinc

in 2.0 M of both acids was studied using the weight loss method. It was

found that the corrosion rate increases with increasing temperature in

absence and in presence of 15.0 g/l of Ficus carica and Olive extracts.

The values of IE% decreases with increasing temperature, leading to a

physical adsorption. The activation thermodynamic parameters, *aE ,

*ΔH and *ΔS were calculated from Arrhenius and Eyring equations. The

results indicated that, *aE and *ΔH increases in presence of the plant

extracts in both acid solutions, suggested that the corrosion inhibition

occurred through physical adsorption. The negative sign of *ΔS

indicating a highly ordered extract molecules in the solution under test .

The results of the thermometric measurements obtained showed that

the reaction number (R.N.) decreases with increasing the extracts

concentration indicating that Ficus carica and Olive extracts act as

inhibitor and adsorb on both anodic and cathodic sites of zinc surface.

Inhibition efficiency is in a good agreement with the weight loss, data.

The second subdivided part involves the investigation of the

electrochemical behaviour of zinc in aerated solutions of 2.0 M HCl and

H2SO4 in absence and presence of different concentrations of the extracts

using open-circuit potential and potentiodynamic polarization

measurements. The results obtained from the open-circuit potential

indicated that the addition of different concentrations of the extract,

affected on anodic and cathodic processes. Also the steady-state

potentials shifted towards noble direction indicated that the extracts act

as anodic-type inhibitors. In potentiodynamic measurements the

electrochemical parameters, Ecorr and Icorr indicated that the corrosion

inhibition process under anodic control and the extracts behave as anodic-

type inhibitors. Also Tafel slopes bc and ba are more or less constant

suggesting that the inhibitive action of the extracts do not effect on the

mechanism of corrosion process. The IE% value attains a maximum at

20.0 g/l of Ficus carica and Olive extracts in both acidic solutions.

The electrochemical measurements are in a good agreement with the

chemical methods, this improves the validity of the obtained results.

Part II includes the study of the corrosion inhibition for α-brass in acidic

media. This part involves two subdivided parts.

The first subdivided part includes that the study of the corrosion and

corrosion inhibition of α-brass in aerated solutions of HCl and H2SO4 by

weight loss method. The obtained results denoted that the corrosion rate

increases with increasing the acid concentration, this indicates that the

corrosion rate of α-brass depends on the concentration, and the corrosion

rate ( corrR ) in HCl is higher than H2SO4. In the presence of different

concentrations (8.0 20.0 g/l) of the extracts under test, the corrosion rate

was found to decreased and IE% increase with increasing the extracts

concentration, and the maximum IE% was achieved at 20.0 g/l.

The results of the adsorption of the extracts molecule on α-brass

surface fits well with the kinetic-thermodynamic model (El-Awady

isotherm) in which one molecule of inhibitor occupying more than one

active site. The values of the standard free energy change

adsΔG and

binging constant K, are relatively small indicating that the interaction

between the adsorbed molecules and α-brass surface is physically

adsorbed.

The results obtained from the study of the temperature range (25-

55ºC) on α-brass in both acids in presence of 15.0 g/l of the extracts

under test, indicating that the inhibition efficiency decrease with

increasing temperature, leading to a physical adsorption. The activation

thermodynamic parameters, *aE , *ΔH and *ΔS indicated that the increase

of *aE in presence of the extracts is attributed to an appreciable decreases

in adsorption process of the extracts on the α-brass surface with

increasing in temperature, leading to the greater area of α-brass is

exposed to the acidic solution.

Also it was found that the values of *ΔH in presence of the extracts

are higher than in free acids indicate that the addition of the extracts

increases the energy barrier of the corrosion reaction to an extent depends

on the type and concentration of the extracts under test. The negative

value of *ΔS indicated that the activated complex is the rate determining

step. Also the increasing in order take place in going from reactants to the

activated complex.

There is no results obtained from the study of the corrosion rate of α-

brass in both acids, using thermometric technique. Because there is no

contribute significantly to the heat evolved during the dissolution of α-

brass in HCl or H2SO4 solutions.

The second subdivided parts involve the study of the electrochemical

behaviour of α-brass in aerated solutions of 0.5 M HCl and H2SO4 in

absence and presence of different concentrations of the extracts using

open-circuit potential and potentiodynamic polarization measurements.

The results obtained from the open-circuit potential indicated that the

behaviour of α-brass in different concentrations of both acids can be

classified into two different behaviours depending upon the mode of

variation of the steady-state potentials with time and the concentration of

the test solutions. The first one, for α-brass in HCl solutions, in which the

immersion potential shifted from negative to positive direction indicated

that the formation of passive film. While, the second for α-brass in H2SO4

solutions, in which open-circuit potential shifted from positive to more

negative values, donated that the destruction of the pre-immersion oxide

film on α-brass surface. Addition of different concentrations of the

extracts, the steady-state potentials shifted towards noble direction

indicated that the extracts act as anodic-type inhibitors.

Also this part includes that the study of electrochemical behaviour for

α-brass in 0.5 M of both acids in absence and presence of different

concentrations of the plant extract using potentiodynamic measurements.

The behaviour of the anodic curves in absence and presence the extracts

were exhibit three distinct regions, the active dissolution region, the

active passive transition region, and the limiting current region. Also the

shape of both anodic and cathodic curves are not modified on adding the

extracts or increasing the concentration, but had a great influence on the

anodic curve than the cathodic. Electrochemical parameters corrE , corrI

indicated that the extracts effect on anodic dissolution of α-brass and act

as anodic-type inhibitors. Tafel slopes cb , ab are more or less constant

suggesting that the inhibitive action of extracts has no effect on the

mechanism of the corrosion process. Also, the maximum IE% was

achieved at 20.0 g/l.

The results obtained in the present study showed that the inhibitive

effect is explained on the basis of the number of adsorption sites,

functional groups, molecular size and the mode of interaction . Therefore

Ficus carica and Olive plant extracts are used as an excellent inhibitors

for zinc and α-brass in both HCl and H2SO4 solutions at room

temperature.

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ARABIC

SUMMARY

الولخص العربي

ف األسبؽ ( brass-α)رؼذ اشسبخ دساسخ رآو و ؼذ اضه احبط األطفش

ثبسزخذا ضجـبد ؿج١ؼ١ ٬( حغ ا١ذسوس٠ه حغ اىجش٠ز١ه)ابئ١خ احبؼ١خ

ح١ش ٬( Olive)اض٠ز (Ficus carica)از١ طذ٠م ج١ئخ وبسزخظبد اجبر١خ ى

. أ اسزخذا ز اضجـبد سزحمك أذافب الزظبد٠ ث١ئ١ ؼب

: رزى اشسبخ صالصخ أثاة سئ١س١ وب ٠

: مذ رزؼ ااػ١غ ازب١خ: الباب األول

رؼش٠ف و ازآو اضجـبد -

أاع اضجـبد -

دا ازجبد اـج١ؼ١خ وضجـبد حب٠خ اؼبد ازآوجز خزظش ػ اسزخ -

سح شجؼ ػ اذساسبد اسبثمخ از ربذ رضج١ؾ رآو اضه احبط سجبئى ف -

.األسبؽ ابئ١خ

ازجبسة اؼ١خ ٠زؼ طفب شبال أللـبة احب١ األجضح اسزخذخ : الباب الثاني

: وزه ؿشق ام١بط

اـش٠مخ اضشزش٠خ (Weight loss) ؿش٠م افمذ ف اص : ام١بسبد اى١بئ١خ -١

((Thermometric

ؿش٠مخ ل١بط رغ١ش اجذ غ اض ف ػذ جد ر١بس وشث : ام١بسبد اىشو١بئ١خ -٢

ؿش٠م رحش٠ه اجذ ( Open-circuit)ثبسزخذا اذائشح افزحخ

(Potentiodynamic .)

: ٠زؼ ازبئج ابلشخ ٠شز ػ جضئ١١ سئ١س١ ب: ثالباب الثال

زا اجضء ٠٬زؼ دساسخ رضج١ؾ رآو ؼذ اضه ف األسبؽ احبؼ١خ :الجزء األول

: ٠زؼ لس١ ب

حغ ( HCl)رذ دساسخ رؤص١ش رشاو١ض و حغ ا١ذسوس٠ه :القسن األول

ػ رآو ؼذ اضه ف غ١بة جد رشاو١ض خزفخ سزخظبد ( H2SO4)اىجش٠ز١ه

أساق جبر از١ اض٠ز ثبسزخذا ؿش٠مز افمذ ف اص ؿش٠م ام١بط اضشزش٠خ لذ

أػحذ زبئج افمذ ف اص أ ؼذي رآو ؼذ اضه رحذ اذساسخ ٠ضداد ثض٠بدح رشو١ض

رآو اؼذ ف حغ ا١ذسوس٠ه أػ حغ اىجش٠ز١ه أ ؼذي ٬سؾ ازآو

ثإػبفخ رشو١ضاد خزفخ اسزخظبد اجبر١خ ى از١ اض٠ز ػ رآو اؼذ ف

فمذ أػحذ ازبئج اخفبػب ٬H2SO4 HClى M 2.0حب١ راد رشو١ض ؼ١

فؼب١ ازضج١ؾ ره ثض٠بدح رشو١ض اسزخظبد حظب ف ؼذي ازآو ٠ظبحج ص٠بدح ف

g/l 20.0ػذ رشو١ض %93.03 97.80 ث١ ( %IE)اجبر١خ ح١ش حظ ػ أػ فؼب١

ولذ ح حطبك ىرس ٬ػ اخىا H2SO4و HClى اخ واضخى ف و حط

واضخى ػ سطح اؼذ ف الضش وره ؼشفه غبؼت إخضاص ىىاث سخخصاث اخ

وا حسبج ل اخغش ف اطالت احشة الخضاص حذ حذي األخشة ػ ٬األوساغ احاعت

وأعا ح دساست حخبغ ؼذالث ٬حذود إخضاص حمائ فضائ ىىاث ازبػ ػ سطح اؼذ

g/l 15.0ووصىد ف غاب H2SO4و HClى M 2.0 حآو ؼذ اضه ف حا

ولذ ٬(C°55-25)ى اسخخصاث اباحت لذ اذساست ػذ دسصاث حشاسة خخفت اذي

ثض٠بدح دسج احشاسح ب ٠ذي ػ أ إزضاص ( %IE)ىحع اخفاض له فؼاه اخزبػ

٠شاد وب ر حسبة ازغ ٬ىبد سزخظبد اذساسخ ػ سـح اؼذ اع اف١ض٠بئ

*) اذ٠ب١ى١خ a

E ٬*H ٬*ΔS ) ؼه روبا ؼذ اضه ف حا اذساست ف غاب

٬ووصىد سخخصاث باح اخ واضخى وره بخطبك و ؼاده أسهىط وؼاده اشش

* وأوظحج اخائش أ له و a

E *H احسىبت ف حاه األظت ازبطت أػ

و احت ٬حه احسىبت ف األظت ااظشة غش ازبطت ا ذي ػ حذود إخضاص فضائ

. ه ازوبا صاحبها حاه اخظااسابت ف غاب ووصىد ازبػ أ ػ ΔS*أخشي حذي ل

ى M 2.0ح حخبغ دساست حؤرش اسخخصاث اباحت ػ حآو ؼذ اضه ف حىي لذ

ولذ بج اخائش اخفاظا . وره باسخخذا غشمه اماط ازشىخشت H2SO4و HClحط

دي ػ فؼاه سخخصاث اخ بضادة حشوض و اسخخص ا R.N) )ف ػذد اخفاػ

ولذ ىحع حىافما صذا ب اخائش . واضخى وأها حخض ػ و اىالغ اصؼذه واهبطه

. اخ حص ػها وحه احسىبت سابما غشمه افمذ ف اىص

ا اطشق وخع دساست حزبػ حآو ؼذ اضه ف األوساغ احاعت باسخخذ: القسم الثاني

فمذ دج دساست إظافت حشاوض خخفت اسخخصاث اباحت لذ اذساست ػ ٬اىهشووائت

وره باسخخذا غشمه H2SO4و HClحط ى M 2.0 حآو ؼذ اضه ف حا

ولذ ٬حخ اىصىي إ حاه رباث اضهذ( Open-circuit)حغش اضهذ غ اض بذو حاس

أوظحج اخائش أه ف وصىد حشاوض خخفت اسخخصاث اباحت لذ اذساست أها حؤرش ػ

ووصذ أعا أ ل اضهذ ازابج حخضه حى ل أوزش إضابه ٬و اىالغ اصؼذه واهبطه

هشووائ ولذ ح أعا حخبغ اسىن اه ٬وهزا ذي ػ أ زبطاث اذساست اىع اىد

Potentiodynamic)( )خحشن)باسخخذا لاساث االسخمطاب ػذ صهذ داى

)حذ ػج خغشاث االسخمطاب ٬corr

E ٬corrI ٬ ab٬ c

b ) أوظحج ل ولذ ٬اخخفه

و corr

E٬ corrI ي أ حزبػ حآو ؼذ اضه خ ححج ححى آىد وبزه حصف ػ

g/l 20.0ولذ حص ػ أػ فؼاه ػذ حشوض . زبطاث اذساست وزبطاث اىع اىد

. H2SO4و HClى اخ واضخى ف و حط

. وخعح اخائش اسابمت حىافك خائش اطشق اىائت غ اطشق اىهشووائت

. ف األسبؽ احبؼ١خ( brass-α)وخع دساست حزبػ حآو احاط األصفش :الجزء الثاني

: زا اجضء ٠زؼ لس١ ب

حط اهذسووىسه M 1.0-0.01) )حج دساست حؤرش حشاوض خخفت :القسم األول

ودج ٬وحط اىبشخه ػ حآو احاط األصفش وره باسخخذا غشمت افمذ ف اىص

وأ ؼذي اخآو ف حط . اخائش ػ أ ؼذي اخآو ضداد بضادة حشاوض وسػ اخآو

وػذ دساست إظافت حشوض خخفت . حط اىبشخه اهذسووىسه أػ

(8.0-20.0 g/l ) اسخخصاث اباحت ححج اذساست ػ حآو احاط األصفش ف حىي

0.5 M ى HCl وH2SO4 فمذ أظشد ازبئج أ ؼذي ازآو ٠م ثض٠بدح رشو١ض ٬

ى از١ اض٠ز ف و g/l 20.0 ح١ش حظ ػ أػ فؼب١خ ػذ رشو١ض ٬ اضجؾ

فمذ دذ ازبئج ػ أ ازضج١ؾ وبخطبك ىرس اضورشو اؼىظ. H2SO4و HCl حغ

ثحسبة ل١ ازغ١ش . ٠ز خالي ادظبص جض٠ئبد اسزخض ػ سـح احبط األطفش

ف اـبلخ احشح ads

G الزضاص رج١ حذس إزضاص رمبئ ف١ض٠بئ ىبد اضجؾ ػ

ػ ؼذي رآو احبط ( C°55-25)ثذساسخ رؤص١ش دسجبد احشاسح . سـح احبط األطفش

ى g/l 15.0ف غ١بة جد H2SO4و HClى M 0.5األطفش ف حب١

فمذ حظ اخفبع ل١خ فؼب١خ ازضج١ؾ ثض٠بدح دسجخ احشاسح ب ٬اسزخظبد رحذ اذساسخ

وب ر حسبة ل١ . بد سزخض از١ اض٠ز اع اف١ض٠بئ٠ذي ػ أ إزضاص ه

أػحذ ٬ازغ١شاد اذ٠ب١ى١خ زش١ؾ ره ثزـج١ك و ؼبدخ أس١ط ؼبدخ ا٠شج

*ازبئج أ ل١خ و a

E ٬*H احسثخ ف ف حبخ األظخ اضجـخ أػ ره

. األظخ غ١ش اضجـخ ب ٠ذي ػ حذس إزضاص ف١ض٠بئ

M HCl 1.0-0.01))ولذ ح أعا حخبغ ؼذالث اخآو ف حا راث حشاوض خخفت

لذ أظشد ازبئج أ ٠ى بن رؤص١ش ٠زوش . ثـش٠مخ ام١بط اضشزش٠خ H2SO4و

. حبط األطفشالـالق احشاسح أصبء رثب ا

٠زؼ دساسخ رضج١ؾ رآو احبط األطفش ف األسبؽ احبؼ١خ ثبسزخذا : القسن الثاني

. اـشق اىشو١بئ١

لذ ر دساسخ رؤص١ش رشاو١ض خزفخ حغ ا١ذسوس٠ه حغ اىجش٠ز١ه ػ احبط

فمذ أػحذ ازبئج أ ٬اساألطفش ل١ذ اذساسخ ثبسزخذا ؿش٠م رغ١ش اجذ غ اض ثذ ر

لذ رج١ أ٠ؼب جد . و ل١ اجذ اضبثذ جذ اغش ٠ؼزذ ػ ؿج١ؼخ رشو١ض احي

األي ف حغ ا١ذسوس٠ه فف و احب١ ٠زج اجذ ام١ : سو١١ خزف١

س أب اسن اضب اسبجخ إ اججخ زا د١ جد ؿجمخ خبخ ػ سـح احبط األطف

فمذ جذ أ ل١ اجذ رزج ام١ اججخ إ اسبجخ زا ٬فإ ٠ظش ف حغ اىجش٠ز١ه

وب أػحذ ازبئج أ ف جد . د١ ػ حذس رثب ـجمخ األوس١ذ ازىخ ػ اسـح

رج ح ل١ جج زا ٠ذي رشاو١ض خزفخ اسزخظبد اجبر١خ ل١ذ اذساسخ أ ل١ اجذ د

. ػ أ سزخظبد از١ اض٠ز ضجـبد اع ا٢د

لذ ر رزجغ اسن اىشو١بئ حبط األطفش ف جد غ١بة رشاو١ض خزفخ

ره ثبسزخذا H2SO4 ٬ HCl حغ M 0.5سزخظبد از١ اض٠ز ف حب١

لذ أظشد ح١بد االسزمـبة ا٢د (. زحشن)د جذ د٠ب١ى ل١بسبد االسزمـبة ػ

ـمخ فق ٬ Tafel)) ـمخ ازثب ب رس ثـمخ : ظس صالس بؿك

لذ ػ١ذ (. Limiting current)ص رز ثـمخ از١بس احذد ( Transpassive)اخي

و دذ ل ٬زغ١شاد االسزمـبة اخزفخcorr

E٬corr

I ػ أ رضج١ؾ ازآو ٠ز رحذ

. رحى آد ثزه رظف اضجـبد رحذ اذساسخ اع ا٢د

ازبئج اسبثمخ ٠ى اشثؾ ث١ رشو١ت اىبد افؼبخ ى سزخض ث١ خظبئض

ثبء ػ ره جذ أ ٬إلزضاص اجب١غ افؼبخازضج١ؾ ره ح١ش ػذد شاوض ا

سزخض از١ اض٠ز ب لذسح ػب١خ ػ رضج١ؾ رآو ؼذ اضه احبط األطفش ف

. حب١ حغ ا١ذسوس٠ه حغ اىجش٠ز١ه ػذ دسجخ حشاسح اغشفخ

بسم اهلل الرمحن الرحيم

باهلل عليه توكلت وما توفيقي إال"

"وإليه أنيب

صدق اهلل العظيم

اىخ اؼشث١خ اسؼد٠خ

صاس ازؼ١ اؼب

جبؼ أ امش

فشع اـبجبد -و١خ اؼ ازـج١م١خ

لس اى١١بء

ـج١ؼ١خ وضجـبد زآو ثؼغ اسزخذا ازجبد ا

اؼبد ف األسبؽ ابئ١خ

سسبخ مذ

نها هىسى القاسوي

وجضء ازـجبد حظي ػ دسج ابجسز١ش ف اى١١بء اف١ض٠بئ١خ

اإلششاف

سهام هحوذ عبذ الوتعال .د.أ

أسزبر اى١١بء اف١ض٠بئ١خ

فشع اـبجبد -و١ اؼ ازـج١م١خ

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Natural Products as Corrosion Inhibitors of Some Metals...

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