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Venkatesh Coimbatore

School of Mechanical Engineering

The University of Western Australia

2008

This thesis is submitted as fulfilment

of the requirements for the degree of Master of Engineering Science by Research in

the Faculty of Engineering, Computing and Mathematics of

The University of Western Australia

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ABSTRACT

Corrosion of steel structures caused by sulphide is a common engineering

problem encountered by many industries, such as the petroleum, chemical processing,

mining and mineral processing industries. The control of sulphide corrosion is still a

controversial topic among corrosion engineers. There is an absence of guideline for a

reliable acceptable limit of sulphide level in service and each processing industry has

its own empirical values. Selection of inhibitors in the sulphide environment depends

on laboratory testing before its actual application in pipelines and reaction vessels.

Many investigators have postulated the corrosion mechanisms due to sulphide based

on operating envelopes such as pH, chloride, manganese, hydrogen sulphide, sulphate

reducing bacteria levels and inhibitor concentration. It is recommended in the

literature that the batch dosing of inhibitor and biocide needs to be evaluated in

regards to sulphide reducing bacteria (SRB) level, which may produce sulphide

concentrations up to 2000 ppm. Although sulphide scale formation may protect the

base metal by providing a physical barrier, the detrimental effects of sulphide are

often inevitable, such as stress corrosion cracking, hydrogen embrittlement, etc.

Currently, there are many chemicals that are used as inhibitors to prevent corrosion

by scavenging the sulphide from the environment. Cerium, a rare-earth element, is

not used as inhibitor in the sulphide environment. Also, there are no previous

research findings on the effects of compounds of rare-earth metals, such as cerium

chloride (CeCl3), in sulphide environment.

This research examines the corrosion behaviour of 0.4Mo-0.8Cr steel, a High

Strength Low Alloy (HSLA) steel, in sulphide-polluted artificial seawater with the

addition of CeCl3 and glutaraldehyde. CeCl3 is studied as a potential corrosion

inhibitor whereas glutaraldehyde is added as a non-oxidising biocide for sulphide-

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reducing bacteria. The corrosion behaviour of the HSLA steel is determined by

means of electrochemical methods, including open-circuit potential, linear

polarisation, and potentiodynamic polarisation studies. In addition, long-term

immersion studies were also carried out. In this study, it was found that the sulphide

pollution in deaerated seawater generally decreases the open-circuit potential of the

steel, implying enhanced corrosion tendency, and increases the anodic Tafel slope,

implying increased resistance to further anodic polarisation. The decrease in the open

circuit potential is an indication of the corrosiveness of the pollutant to the steel

whereas the increase in the anodic Tafel slope is attributed to the formation of

corrosion products on the steel surface, which hinder the further corrosion. However,

no passivation is observed, implying that the corrosion products merely provide a

physical barrier to the corrosion, instead of a passivation layer. Addition of 400 ppm

CeCl3 in deaerated seawater was found to be effective in increasing the open-circuit

potential of the steel, implying a suppression of corrosion tendency, and on the other

hand decreasing the anodic Tafel slope, implying a reduced resistance to anodic

polarisation, or reduced corrosion resistance. However, the overall anodic

polarisation current density for a given cathodic reaction is lower for the solution

inhibited with 400 ppm CeCl3 as compared to that in the uninhibited solution,

indicating that lowering of the open circuit potential is the dominant effect of CeCl3

for corrosion inhibition.

In comparison with uninhibited seawater, CeCl3 addition caused suppression

to the sulphide activity on the steel. The inhibiting efficiency, as defined on the basis

of polarization resistance, of 400 ppm CeCl3 is 62% in the solution polluted with 2

ppm sulphide, 52% in the solution containing 10 ppm sulphide, and 19% in the

solution containing 100 ppm sulphide. The inhibiting efficiency of CeCl3 decreased

with increasing sulphide concentration in the solution.

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It is postulated that the moderate inhibiting effect of CeCl3 is due to the

scavenging effect thereby forming Ce2S3 complex. Further reaction of sulphide with

steel resulted in ferrous sulphide, leading to an increased corrosion rate. It is also

concluded that the CeCl3 interferes with both anodic and cathodic reactions in

deaerated conditions.

Addition of glutaraldehyde in the sulphide-polluted seawater was found to

decrease the corrosion rate. According to the electrochemical measurements

conducted, the concurrent addition of glutaraldehyde and CeCl3 appeared to have an

added effect on reducing the corrosion of the steel, as evidenced by the increase of the

open circuit potential during the short-term testing. From the weight loss

measurements after 60 days, sulphide pollution in deaerated seawater was found to

increase corrosion rate. This is attributed to the increase of sulphide activity whereby

continual dissolution of steel was encountered.

From the weight loss tests, it was found that the addition of CeCl3 and

glutaraldehyde reduced the corrosion rate of the steel in the solutions containing 0-10

ppm sulphide. There is no noticeable corrosion rate decrease for the solution

containing 100 ppm sulphide. The added effect of CeCl3 and glutaraldehyde to the

SRB medium has resulted in lower corrosion rates. Further detailed experimentation

is required to elucidate the corrosion reduction mechanism in glutaraldehyde-

containing environments.

.

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ACKNOWLEDGEMENT

I would like to thank Associate Professor Yinong Liu for supervising this

project. I would like to take the opportunity to thank Professor Brett Kirk, Head of

the School of Mechanical Engineering, and all the staff of the School for their kind

support and encouragement.

The author expresses his thanks to Mr. William Hamilton, Librarian,

Mathematical and Physical Science Library, and Mr. Dennis Brown, Technician,

Mechanical Workshop, University of Western Australia for their exuberant help in

this research.

Assistance of Mr. Douglas Raymond of Alcoa Alumina for providing the

metal samples and that of Mr. Barry Price of Chemistry Centre of Western Australia

for conducting the chemical composition analysis is also appreciated.

Special thanks are also due to examiners Associate Professor Daniel

Blackwood, and Dr David Druskovich for their valued inputs for the revision of the

thesis.

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TABLE OF CONTENTS

1. INTRODUCTION .................................................................................................. 8

2. CORROSION INHIBITION .............................................................................. 13

2.1 CORROSION INHIBITORS ............................................................................... 14

2.1.1 Anodic Inhibitors ...................................................................................... 14

2.1.2 Cathodic Inhibitors ................................................................................... 16

2.1.3 Mixed Inhibitors ....................................................................................... 18

2.1.4 Passivating Inhibitors ................................................................................ 20

2.1.5 Organic Inhibitors ..................................................................................... 20

2.1.6 Precipitation Inhibitors ............................................................................ 20

2.1.7 Volatile Inhibitors ..................................................................................... 21

2.2 CERIUM FOR CORROSION PREVENTION ................................................... 22

2.2.1 Properties of cerium .................................................................................. 22

2.2.2 Cerium as a corrosion preventive agent .................................................... 23

2.3 EFFECTS OF SULPHIDE IN AQUEOUS CORROSION ................................. 25

2.4 MICROBIOLOGICALLY INFLUENCED CORROSION ................................. 27

2.5 ROLE OF SULPHATE REDUCING BACTERIA IN CORROSION ................ 29

2.6 BIOCIDES ........................................................................................................... 30

2.7 OBJECTIVES OF THE CURRENT STUDY ..................................................... 30

3. EXPERIMENTAL PRINCIPLES AND TECHNIQUES ................................ 32

3.1 ELECTROCHEMICAL CORROSION PRINCIPLES ....................................... 32

3.1.1 Electrochemical Corrosion Cell and Reactions ........................................ 32

3.1.2 Electrochemical Corrosion Parameters ..................................................... 35

3.2 MIXED ELECTRODE CORROSION CELLS ................................................... 40

3.3 CORROSION EXPERIMENTAL TECHNIQUES ............................................. 41

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3.3.1 Electrochemical test cell and Instrumentation .......................................... 41

3.3.2 ASTM IMMERSION TEST ............................................................................. 44

3.3.2.1 Immersion test sample preparation ........................................................ 44

3.3.3 SRB CORROSION TESTING ......................................................................... 44

3.3.3.1 Preparation of SRB Medium ................................................................. 44

3.4 TEST SOLUTIONS ............................................................................................. 45

4. EXPERIMENTAL RESULTS ............................................................................ 47

4.1 SHORT TERM ELECTROCHEMICAL TESTS ................................................ 47

4.1.1 Electrochemical measurements in unpolluted seawater ........................... 47

4.1.2 Effect of sulphide pollution on the electrochemical tests ......................... 50

4.1.3 Effect of sulphide-reducing bacteria ......................................................... 61

4.2 LONG-TERM ELECTROCHEMICAL TESTS ................................................. 62

4.2.1 Open-Circuit Potential Measurement ....................................................... 62

4.2.2 Linear Polarisation Measurement ............................................................. 66

4.2.3. Potentiodynamic Polarisation Measurements .......................................... 69

4.3 WEIGHT LOSS EXPERIMENT – 60 DAYS EXPOSURE ............................... 76

4.4 MICROSCOPY AND MICROANALYSIS ........................................................ 82

5. CORROSION MECHANISMS AND ANALYSIS ........................................... 85

6. CONCLUSIONS .................................................................................................. 88

6.1 EFFECT OF SULPHIDE POLLUTION ............................................................. 88

6.2 EFFECTS OF CERIUM CHLORIDE ................................................................. 88

6.3 EFFECT OF GLUTARALDEHYDE .................................................................. 89

7. REFERENCES ..................................................................................................... 90

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1. INTRODUCTION

Corrosion is a challenging engineering problem to the industry and the

society. It spreads across the entire spectrum of industry, such as oil and gas,

chemical processing, mining and mineral processing, construction and infrastructure,

transportation, maritime, food packaging, agriculture, manufacturing, and household

industries [1]. A recent survey conducted by the National Association of Corrosion

Engineers (NACE) estimated that in the United States, the direct costs of corrosion is

$276 billion/year, which accounts to 3.1% GDP [2]. In the United Kingdom, the cost

of corrosion is estimated to be 3.5% GNP. In Australia, the cost of corrosion is

estimated to be 5% of GDP, i.e. $1-5 billion/year. This equates to 4-5% tax on every

thing we earn or purchase on a weekly basis for all citizens for Australia [3,4].

In Australia, due to its large-scale open-environmental operations, corrosion is

particularly a challenging problem in the mining, offshore oil and gas, and petroleum

industries. The cost of failures caused by corrosion in these industries is often far

beyond the immediate cost of the failed component and structure and the lost of

production. Corrosion failures are also often catastrophic. Figure 1 shows the

explosion and fire of the ethylene reactors of the Pasadena Refinery in USA that

occurred in 1989 [5]. The cause of the disaster corrosion failure, due to poor

maintenance and inadequate corrosion control strategies of the plant.

Control of corrosion is a multi-fold task and a costly exercise for the industry.

At the same time, proper management of corrosion and appropriate engineering of

corrosion prevention lead to paramount savings to the industry. The added value of

corrosion prevention includes cost reductions in corrosion management and spare

parts stockpiling. Corrosion protection also facilitates against costs associated with

production loss, product contamination, personnel training, environment

rehabilitation, society safety protection, and insurance liabilities. Today, with the

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ever-mounting demand for longer operation intervals, fewer shutdowns, longer

service lifetimes, and more reliable system performances, the need for corrosion

control is increasing [6]. Therefore, protection of metal structures and components

against corrosion has always been a major engineering concern for the engineers.

Figure 1 - Explosion and fire from ethylene reactor of Pasadena Refinery (USA) in 1989.

Corrosion is generally described as irreversible interfacial reactions of a

material with its environment that result in consumption of the material or destruction

of its functionality. Corrosion may occur to all major types of engineering materials,

including metals, ceramics and polymers. The corrosion of metals, largely owing to

their inherent chemical instability, is the most commonly encountered engineering

problem. Figure 2 shows some common examples of corrosion of engineering

components in various industries. Photograph (a) shows the corrosion on a welded

joint of a pipe showing fluid leak. Photograph (b) shows extensive corrosion damage

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on a steel pipe work in a mining industry. Photograph (c) shows the failure of a valve

due to severe uniform corrosion on the bolts. Photograph (d) shows an underground

gas pipe that had been attacked by microbial activities resulting in corrosion.

Figure 2 - Common examples of corroded metal components. (a) Corrosion of welded joints of water pipe; (b) corrosion in the mining industry; (c)

corrosion of a valve; (d) corrosion of gas pipe caused by microbial action.

Figure 3 shows corrosion caused by microbial activities [7]. Photograph (a)

shows corrosion of heat exchanger tubes caused by sulphate reducing bacteria (SRB).

Photograph (b) shows corrosion caused by the iron bacteria resulting in tubercles in

the cooling water system. It has been reported that microbiologically influenced

corrosion has caused lifetime reductions of process pipelines in Western Australia

from the designed 20 years to less than 3 years [8]. Therefore, the need to understand

the influences of these corrosion forms and the ability to control them are vital for the

healthy operation of the industries.

(a) (b)

(c)

(d)

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Figure 3 - Common examples of microbial induced corrosion. (a) Corrosion caused by sulphate reducing bacteria. The shiny area shows the metal

surface after removal of corrosion tubercles; (b) corrosion tubercles caused by iron bacteria.

The state of Western Australia is the largest mining and natural gas producing

state in Australia. Despite the general perception that offshore oil and gas production

platform facilities serve in sulphide-free environments, there have been reports in

recent times that the onshore and offshore structures are detected with sulphide

production and encounter the threat of sulphide-induced corrosion [9]. The sulphide

is mostly generated from decaying flora, industrial waste and the activity of sulphate

reducing bacteria [10]. Micro-organisms have long been known to influence

corrosion on piping and heat exchanger tubes 10-100 times faster than normal [11].

Sulphide production has long been associated with the cause of corrosion damage and

sulphide stress cracking (SSC) in high strength steels used in oil and gas production,

petroleum refining, and petrochemical and chemical processing industries [12].

High strength low alloy (HSLA) steels are a common type of structural

materials favoured for the construction of large welded structures, such as power

plants, petroleum industries especially in oil platforms, rigs and heavy machineries.

In such applications, they experience moderate temperature ranges and exposure to

extraneous ions like sulphide, nitride and chloride. Seawater containing sulphide can

be very corrosive to steels, resulting in high metal loss, blistering and hydrogen

(a)

(b)

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embrittlement. Ferrous sulphide, a corrosion product, is cathodic to iron and is

known to cause severe localised corrosion after precipitating onto the metal surface

[13]. The mitigation of sulphide corrosion in the seawater system and the selection of

environmental friendly water-soluble inhibitors invigorate further research to the

corrosion engineers. It was reported that cerium can be used as inhibitor to mitigate

corrosion [14].

Cerium, a rare earth element that is found abundant, has not been not trialled

as inhibitor in the sulphide environment. Hence, this research investigates the

corrosion and possible inhibition behaviour of HSLA steels in the presence of

sulphide using cerium and non-oxidising biocide like glutaraldehyde in polluted

seawater. The study was conducted using short-term and long-term electrochemical

techniques, such as open circuit potential measurement, steady-state potential

measurement, DC polarisation measurement and controlled potential-step

measurement, and long-term immersion tests.

Effects of pre-exposure of HSLA steels to cerium-containing solutions in

sulphide environment and also under micro bio-fouling conditions are also studied.

Surface morphology of HSLA steel is studied using optical and scanning electron

microscopy to elucidate the corrosion reaction mechanisms.

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2. CORROSION INHIBITION

There are many different techniques to prevent corrosion. Common industrial

practices include the following.

Electrochemical protection: impressed current cathodic protection using

an electrical power system; cathodic protection using sacrificial anodes;

anodic protection. Cathodic protection methods use electrochemical

principles to offset the electrical potential of the corroding metal to be more

noble or to suppress the corroding current. Common metals for sacrificial

anodes include Al, Zn and Mg alloys. Anodic polarisation uses the principle

of passivation and is applied to metals that exhibit active-passive transitions.

Surface covering: coating, plating, surface anodisation treatment, cladding,

etc.

Selection of more corrosion resistant materials: for example,

replacement of soft materials with harder and more wear resistant materials

in the case of erosive conditions; avoid dangerous material-environmental

combinations for stress-corrosion cracking; replace more active materials

with more noble materials, avoid active galvanic corrosion coupling. These

measures often imply increased initial cost for the metals.

Structural design and maintenance: proper design to eliminate or

minimise the occurrence of corrosion; proper design for easy replacement of

corroded components to minimise system shutdown time and production

loss; proper design for early warning and monitoring for corrosion failure to

avoid unexpected, catastrophic failures.

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Corrosion inhibition: addition of special substances into the corroding

materials or the environments to interfere with the corrosion process to

hinder it. This method may be used in industrial boiler operation, cooling

systems, distillation columns, stockpile storages etc. Many inhibitors have

been developed for various specific corrosion systems and most of these

inhibitors are proprietary.

2.1 CORROSION INHIBITORS

According to International Standards Organisation (ISO), an inhibitor is

defined as a “chemical substance which decreases the corrosion rate when present in

the corrosion system at a suitable concentration without significantly changing the

concentration of any other corrosive agent” [15]. This definition is limited to the

chemicals that can reduce the corrosion rate by affecting the composition of the

environment, e.g., oxygen scavengers, chemicals that alter the pH, and chemicals that

change the hardness or scale forming properties. The performance of an inhibitor is

usually assessed by its effectiveness in reducing the corrosion rate. This is generally

expressed, in engineering practice, as a rate of thinning or penetration in millimetres

per year. Based on the working mechanism, there exist three major types of

inhibitors: anodic, cathodic and mixed inhibitors [15,16].

Inhibitor Classification by Working Principles

2.1.1 Anodic Inhibitors

Anodic inhibitors suppress anodic reactions, i.e. the reaction by which metal

atoms being ionised and transferred into the environment. Anodic inhibition may be

achieved either by adsorption of positively charged particles onto the surface of the

corroding metal, thus off-setting the electrical potential of the corroding metal, or by

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a reduction of the active surface area of the corroding metal electrode due to

passivation.

Anodic inhibition principle is schematically illustrated in Figure 4. Anodic

inhibitors work on the mechanism through which the rate of anodic reaction is

reduced and thereby resulting in lower corrosion rates. With the addition of an

anodic inhibitor, the initial corrosion potential, Ecorr' and the corrosion current, icorr' at

point A' is shifted to the final Ecorr'' and icorr'' values at point A''. It can be seen from

Figure 4 that the icorr'' is less than the icorr'. Hence lower corrosion rate is obtained as

a result of the inhibition.

Anode

(Without Inhibitor)

Anode

(With Inhibitor)

Cathode

Log (i)

Po

ten

tia

l (E

)

A'

A"

icorr" icorr'

Ecorr"

Ecorr'

Figure 4 – Schematic illustration of the principle of anodic inhibition.

Anodic inhibitors promote formation of protective films through oxidation

reactions. Typical anodic inhibitors include chromates, nitrites, molybdates and

phosphates. For example, when chromium is added as an inhibitor into the

environment for ferrous alloys, a layer of mixed oxides (Cr2O2 + Fe2O2) is formed. It

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was reported that this oxide layer is dense, continuous and impermeable to water, thus

provides a physical barrier between the medium and the metal, preventing further

corrosion [17]. Molybdates are another common type of inhibitors. They are non-

oxidising and they require suitable oxidising agents, such as sodium nitrites, to

enhance the inhibition effect and to impart protective films. Molybdates can be used

in closed recirculating water systems. The mechanism by which molybdates inhibit

the corrosion of ferrous metals is complex. It appears that when iron corrodes,

molybdate ions (MoO42+

) in conjunction with other anions adsorb to form a non-

protective complex layer with ferrous (Fe2+

) ions. Because of dissolved oxygen or

other oxidizers in the water, some of the Fe2+

ions are oxidised to ferric (Fe3+

) state,

and the ferrous-molybdate is transformed to ferric-molybdate, which is insoluble and

protective in nature and alkaline waters [18].

However, there exists a potential risk for the use of these inhibitors. If an

anodic inhibitor is not present at a concentration level sufficient to block off all the

anodic sites, accelerated localised corrosion, such as pitting, may occur due to the

large cathode-to-anode area ratio. In this regard, anodic inhibitors are often

considered “dangerous inhibitors”.

2.1.2 Cathodic Inhibitors

Cathodic inhibitors increase cathodic polarisation resistance. Addition of a

cathodic inhibitor into the system has the effect of shifting the corrosion potential of

the metal to more negative values. Their working principle is schematically

illustrated in Figure 5. Cathodic inhibitors work on the mechanism through which the

rate of cathodic reaction is reduced and thereby resulting in lower corrosion rates.

With the addition of cathodic inhibitor, the initial corrosion potential, Ecorr' and the

corrosion current, icorr' at point C' are shifted to Ecorr'' and icorr'' values at point C''. It

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can be seen in Figure 5 that icorr'' is less than the icorr'. Hence lower corrosion rate is

obtained at point icorr''.

Cathodic inhibitors function by either slowing the electrochemical kinetics of

the cathodic reactions or by causing selective precipitation of cations in cathodic

areas to increase circuit resistance and to restrict diffusion of reducible species to

cathodic areas. Precipitating inhibitors fall in the category of cathodic inhibitors.

They produce insoluble films on the cathode and isolate the cathode from the

solution. For example, calcium bicarbonate (Ca(HCO3)2) may react with alkaline

media at the cathode to form CaCO3. A local alkaline condition may be created due

to oxygen reduction process, generating OH- ions. These OH

- ions react with

Ca(HCO3)2 to form insoluble CaCO3:

Ca(HCO3)2 + OH- → CaCO3 + HCO3

- + H2O (1)

At the appropriate pH level, CaCO3 will precipitate to form a hard, smooth

deposit that prevents oxygen from diffusing onto the metal surface. Similarly, zinc

ions are used for general reduction in corrosion by precipitating Zn(OH)2 at the

cathode due to locally elevated pH levels. Zinc chromate (ZnCrO4) is one of the most

effective multi-component treatment additive. It inhibits corrosion of steels by

suppressing the oxygen reduction reaction and is therefore classified as a cathodic

inhibitor.

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Cathode

(Without Inhibitor)

Cathode

(With Inhibitor)

Cathode

Log, Current Density

Po

ten

tia

lC'

C"

icorr" icorr'

Ecorr"

Ecorr'

Anode

Figure 5 - Schematic illustration of the principle of cathodic inhibitors.

2.1.3 Mixed Inhibitors

Based on the above two fundamental principles, there exists a third type of

inhibitors, known as the mixed inhibitors. As explained above, using anodic

inhibitors alone has the risk of inducing pitting to the metal. To eradicate this

problem, it becomes common practice to incorporate a cathodic inhibitor whenever an

anodic inhibitor is used. Mixed inhibitors contain effectively both an anodic and a

cathodic inhibitor and thus interfere with both anodic and cathodic reactions. Figure

6 illustrates schematically the working principle of mixed inhibitors. Mixed

inhibitors work on the mechanism through which the rates of both anodic cathodic

reactions are reduced and thereby resulting in lower corrosion rates. With the

addition of mixed inhibitor, the initial corrosion potential, Ecorr' and the corrosion

current, icorr' at point M' are shifted to Ecorr'' and icorr'' at point M''. It can be seen from

Figure 6 that the icorr'' is less than the icorr'. Hence lower corrosion rate is obtained at

point icorr''.

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Po

ten

tia

l

Anode

(with Inhibitor)

Cathode

(without Inhibitor)

Cathode

(With Inhibitor)

Ecorr"

Ecorr'

icorr" icorr'

M'

M''

Anode

(without Inhibitor)

Figure 6 – Schematic illustration of the principle of mixed inhibitors.

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Inhibitor Classification by Physical Forms

2.1.4 Passivating Inhibitors

Passivating inhibitors, also known “passivators”, are a special type of anodic

inhibitor. They may cause not only an increase of the anodic Tafel slope, but also

encourage a large anodic shift of the corrosion potential, forcing the metallic surface

to form a barrier surface, resulting in passivation [19].

2.1.5 Organic Inhibitors

Organic compounds constitute a broad class of corrosion inhibitors. They

include all types of anodic and cathodic. Anodic or cathodic inhibition effects are

sometimes observed in the presence of organic inhibitors. Both anodic and cathodic

areas are probably inhibited, but to varying degrees depending on the potential of the

metal, chemical structure, and size of the inhibitor molecule. The film formed by

adsorption of soluble organic inhibitors is only a few molecules thick and is invisible.

Organic inhibitors are adsorbed usually by the principle of ionic charge polarisation

of the inhibitor and the metal surface. For example, amines are positively charged

cationic inhibitors whereas sulphonates are negatively charged anionic inhibitors.

They are adsorbed onto metal surfaces preferentially depending on whether the metal

is charged negatively or positively.

2.1.6 Precipitation Inhibitors

Precipitation inhibitors are film-forming compounds that have a general action

over the metal surface, thereby providing a protective film to block both anodic and

cathodic sites indirectly. Common precipitation inhibitors include silicates and

phosphates. In waters with 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 localised in the

form of pitting when insufficient concentration of phosphate or silicate are added to

saline water. However, both silicates and phosphates form deposits on steel, which

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increase cathodic polarisation. Thus their functions appear to be mixed, i.e. a

combination of both anodic and cathodic effects.

2.1.7 Volatile Inhibitors

Volatile corrosion inhibitors (VCI), also known as vapour phase inhibitors

(VPI), are compounds such as sodium chromate and potassium chromate. With an

appropriate vapour pressure usually about 10-2

to 10-7

mm Hg, these inhibitors

volatilise and condense on all surfaces at ambient conditions. They are used

exclusively in closed conditions [20]. The inhibiting compounds effectively increase

the atmospheric corrosion resistance of the exposed surfaces of metals, usually steels.

A compound with a vapour pressure too low does not release sufficient vapour to

effectively inhibit corrosion. These inhibitors are usually in solid forms, and are

applied in closed environments, such as spare parts storage houses, packaging for

long-term storage, museum cabinets to protect antiques against corrosion where

humidity control is difficult. The solid inhibitors release vapours by volatilisation on

contact with the metal surface. The vapour of these substances condenses and is then

hydrolysed by absorbing moisture in the environment to release protective ions.

VCIs may contain organic or inorganic compounds. Typical products, such as

dicyclohexyl amine mitrite (DAN), have been found effective as VPIs for protection

of steel surfaces, but not for copper alloys. VPIs based on nitobenzoate organic

compounds are found to be suitable for protecting ferrous, copper and other alloy

systems.

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2.2 CERIUM FOR CORROSION PREVENTION

2.2.1 Properties of cerium

Cerium is the most abundant of the rare-earth metals. First isolated as an

impure oxide in 1803, the element was named after the earliest recognized asteroid

Ceres. Cerium is a grey, ductile, highly reactive metal. It is easily attacked by dilute

or concentrated mineral acids and alkalies and readily oxidises in moist air at room

temperature. It is in fact the second most reactive among the rare-earth metals. It has

four allotropic forms. Cerium forms alloys with other lanthanides. It also reacts with

hydrogen and forms carbides and intermetallic compounds with other elements. It is

non-toxic and may ignite on heating to 149oC. Cerium is characterised chemically by

having two stable valence states, ceric (Ce4+

) and cerous (Ce3+

). This property

underlies several technological uses of the element. The ceric ion is a powerful

oxidizing agent, but when associated with strongly co-ordinating ligands like oxygen

it is completely stabilised. Metallic cerium is prepared by metallothermic reduction

techniques, such as by reducing cerous fluoride with calcium, or by electrolysis of

molten cerous chloride.

In fact, cerium oxide, CeO2, also known as ceria, is the form of cerium most

widely used. The pure metal is likely to ignite if scratched with a knife. Ceric salts

are orange red or yellowish; cerous salts are usually white. Cerium is a component of

misch metal, which is extensively used in the manufacture of pyrophoric alloys for

cigarette lighters and as inoculant in the production of nodular cast irons. While

cerium is not radioactive, the impure commercial grades may contain traces of

thorium, which is radioactive [21,22]. Table 1 highlights some of the physical and

chemical properties of cerium.

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Symbol Ce

Atomic Number 58

Atomic Weight 140.12

Density 6.657 g/cm3

Melting Point 799oC

Boiling Point 3426oC

Thermal Conductivity 0.113/cmK at 298.2 K

Electrical Resistivity 75.0 microhm-cm at 25oC

Specific Heat 0.049 Cal/gK at 25oC

Magnetic Moment 2.4 Bohr Magnetons

Temperature Coefficient of Resistance 7 x 10-5

at 0oC to Room Temperature

Electrode Potential Ce (s) Ce3+

+ 3e¯ [2.34 V]

Table 1 - Physical and chemical properties of cerium.

2.2.2 Cerium as a corrosion preventive agent

Common corrosion inhibitors for steels in cooling water systems are mostly

hexavalent chromium compounds. The inherent toxicity and carcinogenicity of these

compounds impose health and ecological problems [23]. In the mid-1980’s Hinton et

al found that salts of rare-earth elements, such as cerium, possess the same corrosion-

inhibiting properties as hexavalent chromium compounds but without the associated

health problems [24]. Hinton and co-workers investigated the effects of non-toxic

cerium conversion coatings for aluminium alloys and reported their better corrosion

inhibiting performances in comparison with chromate compounds in various aqueous

environments [25]. They found that immersion in aqueous solutions containing 1000

ppm CeCl3 resulted in the formation of a hydrated Ce2O3 coating on an aluminium

alloy. As a result, pitting potential of the alloy was increased and the corrosion

current was reduced by an order of magnitude on subsequent exposure to NaCl

solution. Mansfeld et al studied the effects of immersion treatment of aluminium

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alloys in boiling CeCl3 solutions and found that this surface treatment drastically

enhanced corrosion resistance of the alloys. Based on this finding they recommended

the replacement of chromate conversion coatings by this surface treatment [23]. Lai

et al reported that salts of rare-earth elements were effective corrosion inhibitors for a

variety of metals and alloys [26]. With respect to CeCl3, they found that the degree of

protection strongly depended on the time of immersion in the CeCl3 solution. Hinton

reported that addition of CeCl3 in tap water suppressed the cathodic oxygen reduction

reaction on mild steels, leading to corrosion inhibition [27]. However, it was reported

that an anodic inhibition mechanism was responsible for the reduction of corrosion of

mild steels in saline solutions containing CeCl3 [28]. Subsequently, Lai and Hinton

conducted a more detailed electrochemical investigation of corrosion inhibition on

mild steels with CeCl3 in saline solutions [29]. They showed that CeCl3 acted as both

anodic and cathodic inhibitor for mild steels in 0.1 M NaCl solution.

Corrosion tests have shown that chloride salts of the rare-earth metals cerium

(Ce), lanthanum (La), neodymium (Nd) and praseodymium (Pr) inhibit the corrosion

of various metals, including aluminium alloys, zinc and steels, in aqueous

environments [22]. It was reported that the rate of corrosion of high strength

aluminium alloy 7075 in 0.1 M NaCl is decreased by at least an order of magnitude

when as little as 100ppm of rare-earth chloride is present in the solution [23]. Surface

spectroscopic studies of films formed on 7075 exposed to 0.1 M NaCl containing

CeCl3 identified a hydrated cerium oxide over the specimen surface [25]. Similar

hydrated rare-earth oxide films are produced on carbon steels [26] and pure zinc [27].

Electrochemical studies indicate that the presence of rare-earth salts in aerated

solutions decreases the rate of the cathodic half of the corrosion reaction like oxygen

reduction and barrier against oxygen diffusion to the metal surface [28,29].

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2.3 EFFECTS OF SULPHIDE IN AQUEOUS CORROSION

Sulphide is a common species present in polluted seawater. Sulphide in

seawater may derive from many sources, including rotting vegetation, industrial

effluent discharge [12,30] and the activity of sulphate reducing bacteria [10,31]. A

deleterious effect of sulphide on the corrosion of many alloys in seawater has been

reported by many investigators [32]. The important parameters affecting the

corrosion behaviour of alloys in seawater are:

(i) dissolved sulphide [33]

(ii) hydrodynamic conditions [34]

(iii) oxidation products of dissolved sulphide [35]

(iv) exposure time [36]

(v) pre-exposure to low concentrations of extraneous ions [37]

The effect of sulphide ions on corrosion of steels in seawater is a complex

phenomenon. The sulphide anion further acts as a “poison”, which retards

recombination of nascent H-atoms on corroding surfaces, increases the residence time

of nascent H, and enhances hydrogen penetration into the metal lattice. Thus all

modes of hydrogen damage including hydrogen induced cracking, hydrogen

blistering, hydrogen attack, and hydride formation are accelerated in susceptible

alloys by the presence of H2S [38]. Furthermore, sulphide (S2-

) is reactive in itself,

and H2S solutions will corrode carbon steels and copper alloys, forming soluble

sulphide corrosion products and insoluble sulphide deposits. Sulphide ion is found to

accelerate the corrosion of steels, when the concentration is greater than 2 ppm.

Therefore, sulphide corrosion is a major concern for corrosion engineers [39].

Previous studies have shown that sulphides can strongly accelerate corrosion

of iron, chromium and nickel in acidic solutions. Sulphide ions in electrolyte greatly

accelerate both the anodic dissolution of iron and the production of hydrogen ions on

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iron surface by shifting the corrosion potential strongly towards the active direction

by forming ferrous sulphide [FeS] compound [40, 41].

It is also known that ferrous sulphide is a porous product, through which iron

and sulphur ions can maintain high diffusion rates [42]. The corrosion caused by

sulphide in this case has two detrimental effects: (1) it stimulates anodic dissolution

of iron and (2) it liberates hydrogen into gaseous molecules causing hydrogen

embrittlement to the steel.

Lee et al studied the corrosion of copper alloys in sulphide polluted seawater

using a once though cooling water system with continuous addition of up to 0.1 ppm

sulphide, and concluded that sulphide is highly detrimental [43]. He found that

sulphide can react with copper more preferentially than nickel in the copper alloys.

He also pointed out that the formation of cuprous sulphide increased the overall

corrosion rate of the copper/nickel alloys. On the other hand, Woods et al confirmed

that alloys exposed in aerated seawater containing 1-8 ppm sulphide formed a thick

brown adherent layer of Cu2S containing pockets of CaCO3 crystals that are rich in

iron [44]. Such adherent layers act as protective coatings in the environment, which

lead to reduction of corrosion rate. The protectiveness of the layers is determined by

the solution chemistry. Previous work has shown that a combination of sulphide and

complexing agents like Ethylene Diamine Tetra Acetic acid (EDTA) can be more

deleterious than sulphide alone for ferrous alloys [45-49]. Thus, it can be concluded

that the effect of sulphide on corrosion rate is still controversial and requires further

investigation. An objective of this study is to determine the corrosion rate of HSLA

steels in sulphide-containing solutions under stagnant conditions. Sulphide

concentrations up to 100 ppm were studied in order to encompass the full range of

possible chemical concentrations that may exist underneath active biofilms caused by

microbes in the seawater.

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2.4 MICROBIOLOGICALLY INFLUENCED CORROSION

Microbiologically Influenced Corrosion (MIC), or bio-corrosion, is an

electrochemical process where participation of micro-organisms is able to initiate,

facilitate and/or accelerate corrosion reactions without changing the electrochemical

nature [50]. MIC is initiated by the fouling of surfaces, a process which occurs

naturally in the environment. Bio-fouling is the undesirable formation of deposits on

a surface. It comprises both organic and inorganic components, which include micro-

organisms, precipitated materials, particulates and corrosion products. MIC is a

danger when nearly neutral waters (pH 4 to 9, 10oC to 50

oC) are in constant contact,

especially under stagnant conditions, with carbon steels, stainless steels and alloys of

aluminium and copper. The first sign of MIC is often unexpected and severe

corrosion of any of the metals noted above in nearly neutral ambient temperature

water or dilute solutions where corrosion rates are normally low. MIC is

characterised by excessive deposits or tubercles. Breaking the deposits often reveals

biologic slimes with black magnetite and iron sulphide deposits having the

characteristic rotten egg odour of H2S. Treatment of the deposits with dilute

hydrochloric acid will also generate H2S smell. Pit surfaces beneath the deposits are

usually bright but rust rapidly on exposure to ambient air. Micro-organisms can

influence a corrosion process by one or a combination of the following phenomena.

Patchy microbial corrosion products in the form of deposits, colonies, or tubercles can

form discontinuous biofilm which create conditions for formation of new galvanic

cells and/or alter the conditions in the existing galvanic cells [51], as shown

schematically in Figure 7. SRB’s can oxidise hydrogen through hydrogenase

enzymes, and drives sulphate reduction [52]. The marine environment is generally

toxic for many bacteria. However, some micro-organisms (e.g. vibrio bacillus

species) not only survive in the marine environments, they can in fact also live on

chloride ions [53]. Also, the slime formed by bacteria can create sites for initiation of

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pitting corrosion by creating differential oxygenation near the pitted area. The

exposed metal surface acts as cathode and the inside of pit acts as anode. Thus

smaller anode and larger cathode leads to area effect and hence propagation of

corrosion is accelerated.

2H2O + O2 + 4e 4OH-

4OH-

2H2O + O2 + 4e

CATHODE CATHODE

ANODE

TubercleO2 O2

4e4e 4Fe

4Fe++

4Fe(OH)2

Solution

Figure 7 – Corrosion cell created by biofilm.

Biofilms of SRB generally develop in anaerobic regions. However, there are

instances of biofilm formation in aerobic bulk water [54]. Some metals, such as

stainless steels, derive corrosion resistance from the formation of passive oxide films,

which are stable in oxidising environment. Metabolic processes of micro-organisms

can destroy the existing protective films of some corrosion resistant alloys [55].

Steels derive corrosion resistance from the formation of oxide films.

Localised damage in the passive films can result in formation of pits and crevices.

However, in some instances, steels are put in use where there is lack of oxygen, such

as deaerators for multi stage flash evaporators [56]. This environment helps the

anaerobic bacteria to proliferate and result in pits and crevices. In summary,

microbial action is influenced by three factors such as metal, solution and micro-

organisms [57].

Prevention of MIC requires frequent mechanical surface cleaning and

treatment with biocides to control population of bacteria. Biocide treatments without

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cleaning may not be effective because organisms sheltered beneath deposits may not

be reached by the injected chemicals.

2.5 ROLE OF SULPHATE REDUCING BACTERIA IN CORROSION

Cast irons and carbon steels usually have very low corrosion rates in deaerated

neutral water and dilute salt solutions because the cathodic oxygen/hydrogen

reduction reactions occur very slowly. However, water saturated soils and deaerated

cooling waters may support relatively high corrosion rates attributed to anaerobic

bacteria that do not require oxygen for growth. SRBs are anaerobic bacteria and are

responsible for forming corrosive hydrogen sulphide with water. Although SRB

require anaerobic conditions for growth, many strains can stay alive for long periods

in aerobic conditions, ready for activation when conditions become favourable. The

biological influences can be divided into three general categories [58]: (1) production

of differential aeration or chemical concentration cells; (2) production of organic and

inorganic acids as metabolic by-products; and (3) production of sulphides under

oxygen free (anaerobic) conditions. Among these variables biogenic sulphides

produced by SRB can alter the iron oxide layer on steels, thereby increasing

corrosion.

Hamilton explained that SRB and many other types of bacteria often occur in

polymeric layers (Biofilm) that form on the metal surfaces [57]. The mechanism by

which anaerobic SRB accomplish an increase in corrosion of iron and carbon steel is

uncertain, but a recent review highlights the cathodic reduction reaction of steels

induced by sulphide reducing bacteria, i.e. metal becomes polarised in water by loss

of positive metal ions through anodic reactions [59]. In a normal corrosion process,

the electrons generated by the ionisation of the metal reduce hydrogen ions, which are

sourced from dissociation of water, to atomic hydrogen. The hydrogen molecules

remain on the metal surface where dynamic equilibrium is established. When SRB

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are present, they may function to remove H2 permanently from the metal surface by

oxidation with sulphate as the electron acceptor, thus facilitate the complete reaction

for metal corrosion. Some of the metal ions react with sulphide to form metal

sulphide (e.g. FeS) whereas others may form ferrous hydroxide[60].

2.6 BIOCIDES

Substantial improvement in establishing an understanding of the factors

influencing MIC has drawn much attention of corrosion researchers. This has led to

the development of numerous biocides for combating MIC problems in cooling water

systems employing seawater as coolant. It has been reported that the

biocides/antimicrobials control the bacteria consortia by minimising conditions

conducive to the survival and proliferation of micro-organisms [59]. It is also

documented that the restriction of bacterial population stagnation on the metal

surfaces may control the MIC [61]. Chemical treatment involves removing the

corrosion products, debris and biofilm from a system. Biocides should be selected to

suit the environment and the micro-organisms targeted [62]. The biocide must also

be compatible with the system, i.e. not to cause corrosion of materials itself.

Examples of micro-biocides to control bacteria, fungi and algae are chlorine,

bromine, quaternary ammonium salts, isothiazolinone and methylene bisthiocynate.

2.7 OBJECTIVES OF THE CURRENT STUDY

Even though CeCl3 as an inhibitor has been studied to a greater extent in

oxidising environments, its effects in reducing environments and on sulphide have not

been studied. Its positive performances in oxidising environments warrant a promise

of possible favourable effects in reducing environments, thus demand for further

investigation. In this study, effects of CeCl3 on the corrosion behaviour of HSLA

steels in sulphide-polluted synthetic seawaters are investigated. The effect of non-

oxidising biocide like glutaraldehyde is studied on the corrosion of HSLA steel in

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sulphide-polluted seawaters. DC electrochemical techniques were used to study the

effect of corrosion of HSLA steel in the polluted seawater. Corrosion rates are

determined from long-term immersion exposures in the test solutions.

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3. EXPERIMENTAL PRINCIPLES AND TECHNIQUES

Corrosion may occur in many different forms and in many different media,

such as treated and untreated waters, oil and other organic liquids, chemical solutions,

gaseous conditions, high temperatures, slurry pipelines, and dry sandy environments.

According to the type of physical and chemical mechanisms, corrosion is

usually classified into three basic types: chemical corrosion, electrochemical

corrosion and erosion corrosion. This study uses electrochemical analytical

techniques to study chemical corrosion of HSLA steel in sulphide-polluted synthetic

seawater solutions.

3.1 ELECTROCHEMICAL CORROSION PRINCIPLES

3.1.1 Electrochemical Corrosion Cell and Reactions

Corrosion occurs by an electrochemical process. The phenomenon is similar

to that which takes place when a carbon-zinc “dry” cell generates a direct current.

Basically, an anode (negative electrode), a cathode (positive electrode), an electrolyte

(ionic conductor), and a circuit (electrical conductor) connecting the anode and the

cathode are required for corrosion to occur. Dissolution of metal occurs at the anode

where the corrosion current enters the electrolyte and flows to the cathode. The

general reaction (reactions, if an alloy is involved) that occurs at the anode is the

dissolution of metal as ions.

An electrochemical corrosion process is characterised by concurrent anodic

and cathodic reactions at usually different sites. The separation of the anodic and

cathodic reactions creates a closed electrical circuit, known as the electrochemical

cell. Corrosion cell can be present when a metal is immersed in an electrolyte.

However, for an explanation, two different electrodes immersed in two different

electrolytes are shown in Figure 8. A corrosion cell can be established only when

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connected by a salt bridge. To measure the corrosion current a digital multimeter is

included in the circuit.

Digital Multimeter

Salt Bridge

Solution A Solution B

Electrode

AElectrode

B

– +

Figure 8 – Schematic drawing of electrochemical corrosion cell.

In order for the corrosion cell to establish the following five conditions should

be present. If anyone of the component is removed, corrosion reaction will not

proceed [63].

1) Presence of an electrolyte

An electrolyte is a medium that contains electric current by ionic movement.

The speed with which the corrosion reaction proceeds is influenced by the number of

ions present, and by the specific chemical nature of the electrolyte. There will be

positively and negatively charged ions present in the electrolyte. When a current is

generated, anions are attracted to the anode and cations are attracted to the cathode. It

is noted that in acid solutions (pH less than 7), there is a high number of available

hydrogen (H+) ions. In basic solutions (pH greater than 7), there is a high number of

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available hydroxyl (OH-) ions. In neutral solutions (pH =7), there are roughly equal

numbers of hydrogen and hydroxyl ions.

2) Presence of primary corrodents, which usually provide the cathodic reaction

The cathode is the site where electrons produced by the anode arrive via the

external circuit to react with and to reduce cations in the electrolyte.

3) Presence of the corroding metal, which is the anode

The anode is the portion of the metal where an electron flow is discharged,

and corrosion occurs. Current leaves the anode in two ways:

Positively charged metal ions go into solution in the electrolyte and

combine with negatively charged ions present in the electrolyte.

Electrons leave the corroding metal, flow from the anode to the cathode

through the external circuit.

4) Difference in electrochemical potential between the anode and cathode

The difference in electrochemical potential between the anode and the cathode

determines the corrosion tendency of the cell. With respect to a reference electrode,

if a metal is more electronegative, then the tendency to lose electrons proceeds at a

higher rate and hence the metal is termed “active”. If a metal is less electronegative,

then the tendency to lose electrons proceeds at a lower rate and hence the metal is

termed “noble”. When two metals are connected, the active metal corrodes and the

noble metal does not corrode.

5) Electrical connection between the anode and cathode

This allows electricity to flow from the anode to the cathode in the form of

electrons. While current flow in the electrolyte is called “ionic” flow, current flow in

the electronic path is called “electronic” flow.

6) Anodic and cathodic reactions

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In the case of corrosion of metals, various anodic reactions occur at the site of

the lower reduction potential. They may be expressed in generalised form as:

M → Mn+

+ ne- (2)

where M stands for the corroding metal. In aqueous environments, possible cathodic

reactions that occur at the site of the higher reduction potential include:

O2 + 4H+ + 4e

- → 2H2O

(reduction of oxygen in acidic solutions) (3)

O2 + 2H2O + 4e- → 4OH

- (reduction of oxygen in alkaline solutions) (4)

2H+ + 2e

- → H2↑ (hydrogen reduction) (5)

Cu2+

+ 2e- → Cu↓ (example of metal deposition) (6)

Fe3+

+ e- → Fe

2+ (example of reduction of metal ions) (7)

3.1.2 Electrochemical Corrosion Parameters

3.1.2.1 Open circuit potential and exchange current density

Electrochemical techniques of corrosion measurement are currently

experiencing increased popularity among corrosion engineers, primarily due to the

rapidity and versatility in determining various corrosion parameters from these

measurements. Long-term corrosion studies, such as weight loss measurements, take

days or weeks to complete, whereas electrochemical experiments will require, at

most, several hours [64]. At the same time, it has long been established that many

practical corrosion phenomena can be explained in terms of electrochemical reactions

and parameters.

When an electrode is immersed in an electrolyte, the potential of the corroding

surface relative to a reference electrode under open-circuit conditions, i.e. in the

absence of a current, is referred to as the open circuit potential, Eo, which is also

known as rest potential, or freely corroding potential. Under this condition, there is

no net current flowing into or from the electrode, and the corrosion and reduction of

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the metal reach dynamic equilibrium in the electrolyte. This local current is known as

the exchange current, io.

3.1.2.2 Polarisation

When an electrical current is generated, the potential of an electrode will shift.

This is known as polarisation. The principle of this phenomenon is explained in

Figure 9, which shows schematically the situation of dissolution of metal of M.

When metal M is immersed in an electrolyte, electrochemical reactions

proceed at finite rates. The corrosion of metal occurs with the release of metal ions in

the solution. The electrons produced by the oxidation of electrode are consumed by a

cathodic reaction, for example reduction of hydrogen ions in the electrolyte to form

hydrogen gas.

M2+

H+

H+

H2

H+

H+

H+

H+

H+

Metal

M

Electrolyte

e-

e-

Figure 9 – Schematic drawing of corrosion phenomenon.

The dynamic process of metal ionisation and dissolution into the solution

results in a build-up of positive charges at the metal dissolution site, shifting its

potential to the positive direction, and of negative charges at the hydrogen reduction

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site, shifting its potential to the negative direction. This phenomenon, that the cathode

potential shifts towards that of the anode and the anode potential shifts towards that of

the cathode, is known as polarisation [65, 66]. Obviously, the magnitude of the

polarisation of the electrodes is dependent on the kinetics of the cathodic and anodic

reactions and the rates of migration of anions and cations in the solution.

Polarisations corresponding to these two mechanisms are referred to as kinetic

polarisation and concentration polarisation, respectively.

3.1.2.3 Linear polarisation resistance

When polarisation current is very small, the current density and the over

potential are linearly related, as shown in Figure 10. This condition is referred to as

linear polarisation. The intercept point of the linear polarisation curves defines

numerically the open circuit potential and the exchange current density of the

electrode.

Cathodic curve

Current Density

Po

ten

tia

l

io

Open Circuit Potential

Anodic curve

Eo

Exchange Current Density

Cu

rre

nt

exit

Cu

rre

nt

en

try

Figure 10 – Schematic drawing of polarisation curve.

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The slope of the polarisation curve in this linear range is known as linear

polarisation resistance. A high linear polarisation resistance implies a low corrosion

current density for a given over potential, thus a low corrosion rate. Therefore, linear

polarisation resistance is a helpful parameter in determining the corrosion tendency of

an electrode [67]. In mathematical form, linear polarisation resistance, Rp, is

expressed as:

p

ai

dER

dI

at E Eo (8)

This parameter may be experimentally determined within a narrow over

potential range at the vicinity of the open circuit potential using low scan rates [68],

as shown in Figure 11.

Current Density

Po

ten

tia

l

Rp = dE/dI

Figure 11 – Schematic illustration of linear polarisation.

The linear polarisation method is used to measure the resistance of the

electrode against corrosion. The main advantage of the linear polarisation is that it

uses the smallest potential spectrum of all the DC corrosion measurement methods

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and that it is essentially a non-destructive test. Consequently, linear polarisation

measurements can be repeatedly made on the same test electrode, allowing it to be

used for applications like: (a) long–term corrosion monitoring and (b) determining

when a test electrode is at its steady state corrosion rate.

3.1.2.4 Tafel slope and active-passive transition

When over potential is large, electrodes exhibit more complex polarisation

behaviour. Figure 12 shows schematically the polarisation behaviour of a

hypothetical metal electrode (304 stainless steel), where the over potential is plotted

against log(i). This electrode exhibits an initial linear increase of E with log(i). The

slope of this linear relationship is known as the Tafel slope. With further increase in

over potential to beyond this stage at above a critical value, the corrosion current

density experiences a drastic reduction. This phenomenon is known as passivation.

Passivation is generally associated with the formation of protective surface film of the

corrosion product. The critical potential at which protective film is formed is denoted

Eprot. The electrode remains in passivated state (low current density) within a window

of potential above Eprot until Epit, at which the protective film is destroyed [69, 70]

and the electrode becomes active again with further increase of the potential. Epit is

referred to as the pitting potential.

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Po

ten

tia

l

Active

Passive

Pitting

Epit

Eprot

Figure 12 - Schematic drawing of Type 304 Stainless Steel polarisation

curve.

Measurement of current-potential relations under proper conditions can yield

information on corrosion rate [66], conditions of surface coatings and films [67],

passivity and passivation behaviour of metals [68] and pitting tendencies [69].

3.2 MIXED ELECTRODE CORROSION CELLS

When two electrodes are connected in the same electrochemical cell, each

serves as the driving force for the polarisation of the other, and a net current is

generated between the electrodes. In such case, the noble electrode (cathode) is

polarised towards the active electrode (anode) and the active electrode is polarised

towards the noble electrode, until the two meet at a common potential. This common

potential, Ecorr, and the particular current density at this moment, icorr, define the

corrosion condition of the anode electrode. This concept is schematically shown in

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Figure 13. Obviously, the values of Ecorr and icorr are specific to the electrochemical

cell, i.e. the two electrodes involved, the temperature and the electrolyte chemistry. It

is also apparent that the corrosion current density of the anode is higher in the

presence of the cathode (icorr) than when it is alone in the same medium (io).

icorr


Po

ten

tia

l

H 2

2H+ +

2e-

2H + + 2e -

H2

Ecorr

Zn/Zn2+

Zn2+

+ 2e -

Zn

H2/H+

Zn

Zn

2+ + 2e-

ioH+/H2

ioZn2+

/Zn

(corrosion state)

(open circuit state)

Eo

Figure 13 – Schematic drawing of mixed electrochemical corrosion cell.

3.3 CORROSION EXPERIMENTAL TECHNIQUES

3.3.1 Electrochemical test cell and Instrumentation

Figure 14 shows the test cell used in this study. The cell includes the working

electrode, which is the steel tested, a pair of graphite counter electrodes and a

saturated calomel reference electrode (SCE) in seawater solution. The reference

electrode contacts the solution via the bridge tube, a compartment filled with test

solution, which provides optimum positioning of the reference electrode. Two

counter electrodes are used to obtain uniform current distribution during testing [71].

To maintain uniform concentration of the electrolyte around the electrode low stirring

(100-150 rpm) is maintained during testing. The electrochemical cell is controlled

using an EG&G 263 Potentiostat/Galvanostat (Figure 15), which performs two main

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functions: (a) It controls the potential difference between the reference electrode and

the working electrode, i.e. it imposes an applied potential (b) It measures the current

flow between the working electrode and the counter electrode. The scan rate in the

electrochemical reaction is 0.166mV/sec. Its working principle is shown in the

Figure 16.

Graphite Counter

Electrode (1) Graphite Counter

Electrode (2)

Working Electrode

Reference Electrode

Saturated Calomel Electrode (SCE)

Electrolyte

Magnetic Stirrer

Resin

Working

Electrode

Reference

Electrode

Counter

Electrode

Potentiostat

Computer

Data Acquistion

Figure 14 – Schematic drawing of electrochemical test cell.

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Figure 15 - Model 263 Potentiostat/Galvanostat.

PotentiostatMeasuring Resultant

CurrentApply Potential

Working

Electrode

Electrolyte

Reference Electrode

Counter

Electrode(s)

Figure 16 – Schematic drawing of Potentiostat operation.

In this research, electrochemical corrosion measurements were performed

using Model 263 Potentiostat/Galvanostat with M352 corrosion measurement

software supplied by EG&G Princeton Applied Research, USA [72]. Experiments

were conducted at 25oC under deaerated condition by purging the solution with

Nitrogen (N2) through a closed system. Samples were stabilized for three hours in the

solution to ensure a steady state before linear polarisation resistance and

potentiodynamic measurements were made. A fresh sample was used for each

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measurement condition and the experiments were repeated to obtain reproducibility.

In this research, the following electrochemical parameters were monitored:

Open-circuit potential

Linear polarisation resistance

Potentiodynamic polarisation curves (Tafel Plots)

3.3.2 ASTM IMMERSION TEST

To test the corrodibility of the HSLA coupons, weight loss method were

carried out according to ASTM G31-90 [71].

3.3.2.1 Immersion test sample preparation

Steel coupons of 75 x 8 mm were made from 4 mm thickness sheet. The

coupons were polished with 500 grit silicon carbide paper and degreased with

acetone. Then the coupons were suspended using nylon thread through a hole and

immersed in the solution. The coupons were removed from the solution after 60 days

for corrosion rate determination [73]. The corroded coupons were cleaned in 5%HCl

solution for 2 minutes in accordance to the ASTM testing procedure [66]. The hard

deposits were removed by soft wire brush. The coupons were rinsed with distilled

water and dried. The corrosion rate is determined as following:

R (mm/year) =

exposed) in)(days sq.in agm/cc)(arein (density

0.0254)65)(1000)(3.061)(0( grams)in loss wt.( (9)

where 0.061 is a unit conversion factor.

3.3.3 SRB CORROSION TESTING

SRB corrosion testing is carried out in Postgate Medium C for Sulphate

Reducers. Main chemicals required for SRB culture medium is shown in Table 2.

3.3.3.1 Preparation of SRB Medium

SRB medium was prepared in accordance to the NACE TM 0194-94

procedure and is shown below [74]:

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1. Chemicals listed in Table 2 are added to synthetic seawater and mixed

thoroughly. The volume was brought to 1.0 litre.

2. The pH was adjusted to 7.5, by adding NaOH pellets (pH 7.5 ± 0.2 at 25o

C)

3. The prepared solution was distributed to hungate tubes or vials degassed

with nitrogen

4. The hungate tubes were autoclaved for 45 minutes at a pressure of 15 psi

at 121oC and inoculated with SRBs.

5. Incubation for 28 days in the incubator was maintained at 25oC ± 3

oC.

6. The inoculated Postgate C media were checked periodically for possible

SRB growth. If the solution and the sample blacken then it indicates the

presence of SRBs.

Compound Weight (g)

Sodium Lactate 6.0

Sodium Sulphate 4.5

Ammonium Chloride 1.0

Yeast Extract 1.0

Potassium Hydrogen Phosphate 0.5

Sodium Citrate 0.3

Calcium Chloride 0.06

Magnesium Sulphate 0.06

Iron Sulphate 0.004

Table 2 – Postgate Medium C for Sulphate Reducers.

3.4 TEST SOLUTIONS

The test solutions used for all electrochemical measurements are artificial

seawater. Some solutions were polluted with sulphide and added with CeCl3 and

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glutaraldehyde. CeCl3 is studied as a potential corrosion inhibitor whereas

glutaraldehyde is added as a non-oxidising biocide for sulphide-reducing bacteria.

Sulphide was added in the form of pre-prepared Na2S solution pipetted into the

seawater solution. The addition was controlled by titration analysis. Frequent

iodometric tests [75] were carried out to monitor the sulphide level.

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4. EXPERIMENTAL RESULTS

4.1 SHORT TERM ELECTROCHEMICAL TESTS

4.1.1 Electrochemical measurements in unpolluted seawater

When a metal sample is first placed in a test solution, its open circuit potential

(OCP) drifts to establish an equilibrium steady state. During this initial period, OCP

is recorded to monitor the process. Figure 17 shows the initial stabilisation curves of

the 0.4Mo-0.8Cr steel in three solution conditions. It is seen that in all three cases,

the open circuit potential of the electrode shifted rapidly towards the more active

direction during the first 2000 seconds and gradually proceeded to a more stabilised

level after 10800 seconds (3 hours). The initial activation appears to be much more

rapid and pronounced for the uninhibited solution compared to the solutions added

with CeCl3 and glutaraldehyde. The stabilised OCP of the steel after 3 hours of

immersion was –714 mV in the uninhibited seawater, -635 mV in the seawater

inhibited with 400 ppm CeCl3 and –624 mV in the seawater added with 400 ppm

CeCl3 and 25 ppm glutaraldehyde. It is obvious that the OCP of the steel does not

stabilise to a steady state with its environment within the testing period. The shift of

the OCP in the anodic direction is attributed to the breakdown of air-formed oxide in

seawater [76]. On the other hand, the observations also seem to suggest that

activation of the 0.4Mo-0.8Cr steel in the seawater is hindered by the addition of 400

ppm CeCl3 and of 25 ppm glutaraldehyde.

Figure 18 shows the polarisation curves of the steel under unpolluted

conditions. The measurements were carried out on the steel electrode after the initial

stabilisation. All three samples show practically identical pattern. The open circuit

potential at zero polarisation is lower for the uninhibited solution than for the

solutions added with CeCl3 and glutaraldehyde, consistent with the measurements

shown in Figure 17. The polarisation curves are highly asymmetric. The Tafel slopes

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for cathodic polarisation are much greater than those for anodic polarisation for all

the three samples.

-750

-700

-650

-600

-550

-500

0 2 4 6 8 10 12

Time 103 (Sec)

0 ppm Sulphide

400 ppm CeCl3 + 25 ppm Glutaraldehyde

Seawater Only

400 ppm CeCl3

E v

s. S

CE

(m

V)

Figure 17 – Initial stabilisation of open circuit potential of 0.4Mo-0.8Cr steel in synthetic seawaters.

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-1000

-900

-800

-700

-600

-500

-400

-300

-9 -8 -7 -6 -5 -4 -3 -2

E v

s. S

CE

(m

V)

log (i) A/cm2

0 ppm Sulphide

Seawater Only

400 ppm CeCl3


B

A

Figure 18 - Potentiodynamic polarisation of 0.4Mo-0.8Cr steel in

synthetic seawaters. The anodic polarisation appears to exhibit a two-stage activation,

approximately in the current density ranges of 1x10-6

~ 1x10-5

A/cm2 and 1x10

-4 ~

1x10-3

A/cm2. The small Tafel slopes for the anodic polarisation imply that a small

increment in over potential is able to cause a large increase in corrosion current, an

unfavourable condition for corrosion prevention. The electrochemical parameters,

including open circuit potential (Eo), exchange current density (io), linear polarisation

resistance (Rp), and anodic and cathodic Tafel slopes (a and c), for the seawater

system are summarised in Table 3. It is measured that for the uninhibited plain

seawater system, Eo is lower (more negative), io is higher and Rp is lower as

compared to those of the inhibited systems. Lower Eo and higher io values imply

more active electrode reaction kinetics. This is consistent with the lower polarisation

resistance of the uninhibited system. The values of these parameters imply that the

steel in the uninhibited system has higher tendency for corrosion. In contrast, the

anodic Tafel slope (a) for the uninhibited seawater system is higher than those of the

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two inhibited systems, implying a lower corrosion rate when polarised to a given over

potential.

Environment

Eo

(mV)

io

(A/cm2)

Rp

(K cm2)

a

(mV/d)

c

(mV/d)

Seawater -734 1.99 3.6 73 235

400ppm CeCl3 -646 1.58 8.0 55 135

25ppm Glutaraldehyde +

400ppm CeCl3

-629 0.79 13.5 30 200

Table 3 - Electrochemical parameters and corrosion rates of 0.4Mo-0.8Cr

steel in the synthetic seawaters.

However, this measurement needs to be taken in perspective. Despite the easy

polarisation of the inhibited systems (lower values of a) at a given over potential, the

actual over potential achieved for a cathodic reaction is much lower for the inhibited

system, as schematically indicated by the dashed line in Figure 18, resulting in a

lower corrosion current density (at point B) compared to the uninhibited system (at

point A). Obviously, the main cause for the reduction of corrosion rate for the

inhibited system is the reduction of Eo.

4.1.2 Effect of sulphide pollution on the electrochemical tests

Figure 19 shows short-term open circuit potential measurements of the steel in

seawater polluted with 2 ppm sulphide. The open circuit potentials of the samples

exhibit similar trend of stabilisation behaviour as those shown in Figure 17. The

potential generally shifted gradually towards the more anodically active direction

with increasing exposure time. After 10800 seconds (3 hours) of immersion in the

solutions, the stabilised open circuit potentials are –737 mV for the uninhibited

solution, -687 mV for the solution inhibited with 400 ppm CeCl3, and –643 mV for

the solution added with 400 ppm CeCl3 and 25 ppm glutaraldehyde. It is noted that

the open circuit potential of the sample in the solution added with 400 ppm CeCl3 and

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25 ppm glutaraldehyde is less stabilised after 3 hours of exposure compared to the

other two.

-750

-700

-650

-600

-550

-500

0 2 4 6 8 10 12

Time 103 (Sec)

2 ppm Sulphide


2 ppm Sulphide

400 ppm CeCl3

E v

s. S

CE

(m

V)

Figure 19 - Short-term open circuit potential measurements in solutions

containing 2 ppm sulphide.

Figure 20 shows the polarisation curves of the steel in seawater solutions

polluted with 2 ppm sulphide. It is seen that the polarisation behaviour of the steel in

the uninhibited solution is more symmetric between the anodic and cathodic

directions than in the inhibited solutions. Similar to the case of unpolluted seawater

solutions, the anodic polarisation of the uninhibited sample occurs in two stages and

the separation of the two stages is more apparent. At low over potential levels, the

current density increases with relatively low increase in over potential. At current

densities in the range between 1x10-6

~ 1x10-5

A/cm2, the increase of current density

experienced increased resistance. At above 1x10-5

A/cm2, electrode is activated

again. On the other hand, the cathodic polarisation slope appears to have been

reduced relative to that in the unpolluted seawater. For the samples in the inhibited

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solutions, the polarisation behaviour is highly asymmetric. The two-stage anodic

activation for the inhibited samples seems to have been suppressed. The anodic

polarisation exhibited low Tafel slopes for the two inhibited solutions. It is also

evident that there is a clear reduction of the open circuit potential as a result of

inhibition with CeCl3 and glutaraldehyde, by a margin of >160 mV. The

electrochemical parameters determined form the polarisation measurements for the

seawater systems polluted with 2 ppm sulphide are summarised in Table 4. It is noted

that for the uninhibited system, Eo is lower and io is higher than those of the inhibited

systems. This is consistent with the observation shown in Table 3 for the unpolluted

systems. The polarisation resistance is also somewhat lower compared to that of the

inhibited solution. These parameters imply higher electrode activity for the

uninhibited systems. The anodic Tafel slope of the uninhibited seawater system is

high compared to the other two systems. This suggests that addition of CeCl3 reduces

the resistance of the steel to over potential (i.e. corrosion). It may also be noted that

the open circuit potentials of the steel in the solutions polluted with 2 ppm sulphide

are lower (more negative) compared to those shown in Table 3 for the unpolluted

systems. This indicates the detrimental effect of sulphide on the steel.

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-1100

-1000

-900

-800

-700

-600

-500

-400

-300

-9 -8 -7 -6 -5 -4 -3 -2

E v

s. S

CE

(m

V)

log (i) A/cm2

2 ppm Sulphide

2 ppm Sulphide only

400 ppm CeCl3


Figure 20 – Effects of CeCl3 and glutaraldehyde addition on polarisation behaviour of 0.4Mo-0.8Cr steel in solutions containing 2ppm sulphide.

Environment

(2 ppm Sulphide)

Eo

(mV)

io

(A/cm2)

Rp

(K cm2)

a

(mV/d)

c

(mV/d)

Seawater -840 0.0158 5.80 112 185

400ppm CeCl3 -673 1.41 4.25 50 223


400ppm CeCl3

-641 1.25 11.0 45 245

Table 4 - Electrochemical parameters and corrosion rates of steel in

2 ppm sulphide polluted seawater.

Figure 21 shows the open circuit potential measurements in seawater solutions

polluted with 10 ppm sulphide. The open circuit potential showed similar activation

behaviour to those shown above in Figures 17 and 19. For the electrode exposed in

the uninhibited solution, the open circuit potential decreased rapidly during the initial

period and then increased slightly by ~20 mV with prolonged exposure after 6000

seconds. For the electrodes in the solutions inhibited with CeCl3 and glutaraldehyde,

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the activation of the electrode occurs at a decreasing rate, but there is no sign of

stabilisation after 10800 seconds. At the end of the exposure, the open circuit

potential is –733 mV for the uninhibited seawater, -687 mV for the solution inhibited

with 400 ppm CeCl3. Little change is observed for the solution further added with 25

ppm glutaraldehyde.

-750

-700

-650

-600

-550

-500

0 2 4 6 8 10 12

Time 103 (Sec)

10 ppm Sulphide


10 ppm Sulphide

400 ppm CeCl3

E v

s. S

CE

(m

V)

Figure 21 - Short-term open circuit potential measurements in solutions containing 10 ppm sulphide.

Figure 22 shows the polarisation curves of the steel in the solutions polluted

with 10 ppm sulphide. The polarisation behaviour of the electrode is similar to those

shown in Figure 20. The anodic polarisation of the electrode in the uninhibited

solution showed clearly two-stage activation. The separation of the two-stage

activation is more pronounced than in the previous two cases. In contrast, the two-

stage anodic activation of the inhibited samples is almost completely suppressed. It is

interesting to note that the anodic polarisation curves of all three samples practically

overlap at i > 1x10-5

A/cm2. This implies that in this current density range, the

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advantage of the lower open circuit potential caused by the addition of the 400 ppm

CeCl3 has been lost.

-1000

-900

-800

-700

-600

-500

-400

-300

-9 -8 -7 -6 -5 -4 -3 -2

E v

s. S

CE

(m

V)

log (i) A/cm2

10 ppm Sulphide

10 ppm Sulphide only

400 ppm CeCl3


Figure 22 – Effects of CeCl3 and glutaraldehyde addition on polarisation behaviour of 0.4Mo-0.8Cr steel in solutions containing 10 ppm sulphide.

The electrochemical parameters of the electrode behaviour determined from

these measurements are summarised in Table 5. The open circuit potentials of the

inhibited systems are higher than that of the uninhibited system, consistent with the

observations in the previous two cases. The difference in the open circuit potential

between the inhibited and the uninhibited systems, however, is ~80 mV, much

smaller than in the previous cases. This signifies a reduced inhibiting effect of CeCl3

in the solutions polluted with 10 ppm sulphide. The measured io values are 1.58x10-6

A/cm2 and 1.25x10

-6 A/cm

2 for the solutions added with 400 ppm CeCl3 and with 400

ppm CeCl3 + 25 ppm glutaraldehyde, respectively. However, for the uninhibited

solution, the io value is much lower, 0.63x10-6

A/cm2. This is uncharacteristic. The

linear polarisation resistance of the uninhibited system is lower than those of the

of 93

inhibited systems. The anodic polarisation Tafel is greater for the uninhibited system.

These observations are consistent with the previous two cases.

Environment

(10 ppm Sulphide)

Eo

(mV)

io

(A/cm2)

Rp

(K cm2)

a

(mV/d)

c

(mV/d)

Seawater -764 0.63 6.40 85 135

400ppm CeCl3 -676 1.58 7.90 65 145


400ppm CeCl3

-676 1.25 11.6 40 96

Table 5 - Electrochemical parameters and corrosion rates of steel in 10

ppm sulphide polluted seawater.

Figure 23 shows the open circuit potential measurements in seawater solutions

polluted with 100 ppm sulphide. The open circuit potential in all the three solutions

generally shift anodically to the more active direction, as in the previous cases, but

the trend is less stable. At the end of the exposure period of 10800 seconds, the three

samples show similar open circuit potential values, at between –710 and –727 mV.

Figure 24 shows the polarisation behaviour of the steel in the solutions

polluted with 100 ppm sulphide. The three samples show more symmetrical

polarisation behaviour between the anodic and cathodic directions as compared to the

previous cases. The two-stage anodic polarisation phenomenon is more pronounced

for all three samples. The increase in open circuit potential measurement caused by

the addition of 400 ppm CeCl3 is < 80 mV, even lower than that for the case of 10

ppm sulphide pollution. It is also noted that at higher current density levels, the

anodic polarisation curve of the inhibited systems appear at below that of the

uninhibited system, implying higher corrosion rate for the inhibited system. This

trend of losing inhibition effect with increasing sulphide concentration is consistent

among the four solution conditions presented above.

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-750

-700

-650

-600

-550

-500

0 2 4 6 8 10 12

Time 103 (Sec)

100 ppm Sulphide


100 ppm Sulphide

400 ppm CeCl3

E v

s. S

CE

(m

V)

Figure 23 - Open circuit potential measurements in solutions containing 100 ppm sulphide.

-1000

-900

-800

-700

-600

-500

-400

-300

-9 -8 -7 -6 -5 -4 -3 -2

E v

s. S

CE

(m

V)

log (i) A/cm2

100 ppm Sulphide

100 ppm Sulphide

400 ppm CeCl3


Figure 24 - Effects of CeCl3 and glutaraldehyde addition on polarisation behaviour of 0.4Mo-0.8Cr steel in solutions containing 100 ppm sulphide.

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The electrochemical parameters of the electrode behaviour determined from

these measurements are summarised in Table 6. It is evident that the uninhibited

system is still more active compared to the inhibited systems, with lower Eo and

higher io. The polarisation resistances of the three samples are similar. The

differences in anodic Tafel slope (a) are also much less compared to the previous

cases, with those of the inhibited systems exhibiting much increased values.

Environment

(100 ppm Sulphide)

Eo

(mV)

io

(A/cm2)

Rp

(K cm2)

a

(mV/d)

c

(mV/d)

Seawater -841 1.99 5.80 140 120

400ppm CeCl3 -769 1.58 6.21 108 130


400ppm CeCl3

-764 1.25 5.78 114 95

Table 6 - Electrochemical parameters and corrosion rates of steel in 100

ppm sulphide polluted seawater.

Figure 25 shows the open circuit potential values after 3 hours of exposure for

the sulphide-free and sulphide-polluted solutions. It is evident that in all three cases

the open circuit potential of the steel decrease continuously to lower values with

increasing sulphide concentration. This clearly demonstrates the corrosive effect of

sulphide on the steel in seawater environment. At the same time, it is also evident

that CeCl3 addition increases the open circuit potential of the steel, implying

inhibiting effect of the additive. The exceptionally low value of OCP for the

uninhibited solution containing 2 ppm sulphide appears to be an experimental error.

of 93

-900

-850

-800

-750

-700

-650

-600

0 20 40 60 80 100

Sulphide Concentration (ppm)


Sulphide Only

400 ppm CeCl3

E v

s. S

CE

(m

V)

Figure 25 – Effect of sulphide concentration on the open circuit potential of the steel under various environments

Figure 26 shows the effect of sulphide concentration on the polarisation

resistance (Rp) of the steel. It is seen that at low sulphide concentrations, the

inhibited systems show much higher Rp than the uninhibited systems, implying

decreased corrosion tendency for the inhibited systems. With increasing sulphide

concentration, the difference diminishes, much by the decrease of Rp of the inhibited

systems, implying the loss of the inhibiting effect. It is also evident that the duel

addition of 400 ppm CeCl3 and 25 ppm glutaraldehyde results in much increased Rp

compared to the single addition of 400 ppm CeCl3. This suggests the added effect of

CeCl3 and glutaraldehyde for corrosion inhibition in sulphide polluted seawater.

of 93

2000

4000

6000

8000

1 104

1.2 104

1.4 104

-20 0 20 40 60 80 100 120

Po

lari

sati

on R

esi

stance

(Oh

ms

cm

2)


Sulphide Only

400 ppm CeCl3

400 ppm CeCl3 + 25ppm Glutaraldehyde

Figure 26 – Effect of sulphide concentration on the linear polarisation resistance of the steel under various environments.

Figure 27 shows the effect of sulphide concentration on the anodic Tafel

slopes of the steel. It is seen that the anodic Tafel slope increases with increasing

sulphide concentration in both uninhibited and inhibited solutions. This is believed to

be due to the build-up of corrosion products on then sample surface, which impose

hindrance to further corrosion. In this regard, the corrosion product may be regarded

of certain protective effect. However, it is noted that systems quickly reactivate at

higher anodic polarisation conditions, implying the break down of the surface films.

This implies that the protection is temporary only at the formation of very thin films

during the early stages of corrosion. It is also clear that the Tafel slopes of the

samples in the inhibited solutions are consistently lower than that in the uninhibited

solutions. This is probably due to the preventive effect of the inhibitor on the

formation of such product.

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20

40

60

80

100

120

140

0 20 40 60 80 100 120


a (

mV

/dec

ade)

400 ppm CeCl3

Sulphide only


Figure 27 – Effect of sulphide concentration on the anodic Tafel slopes of the steel under various environments

4.1.3 Effect of sulphide-reducing bacteria

Table 7 shows the weight loss measurement of coupon samples of the steel

exposed in SRB Postgate C media for 17 days. The media contained CeCl3 as the

corrosion inhibitor and glutaraldehyde as the biocide. Concurrent addition of 400

ppm CeCl3 and 25 ppm glutaraldehyde show clear reduction in corrosion rate of the

steel compared to the case when only 400 ppm CeCl3 is added, demonstrating added

effect of the two additives on the corrosion of the steel in seawater.

Environment Corrosion Rate

(mm/year)

Seawater 0.0953

400 ppm CeCl3 0.0258

400 ppm CeCl3 25 ppm Glutaraldehyde 0.0141

Table 7 - Corrosion rates of 0.4Mo-0.8Cr steel coupons in SRB media

after 17 days.

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4.2 LONG-TERM ELECTROCHEMICAL TESTS

Electrochemical reactions require a finite amount of time to reach steady state

conditions. Electrochemical corrosion measurements can be made at any point after

an electrode is submerged in an electrolyte. It may be necessary to measure corrosion

under steady state conditions when corrosion data are used to estimate long-term

service lifetime. Mottern and Myers found that the initial corrosion rate for steel in

tap water is approximately 0.1524 mm/year whereas the actual rate in service over

long period is approximately 0.01524 mm/year, an order of magnitude lower [77].

Other researchers also observed that corrosion rates require a finite amount of time to

reach steady state values, and that initial corrosion rates are typically higher than that

in the steady state [78, 79]. In this study, effects of long-term exposure in the CeCl3

inhibited seawater solutions are carried out using open circuit potential, linear

polarisation resistance and dynamic polarisation behaviour measurements.

4.2.1 Open-Circuit Potential Measurement

Figure 28 shows the open-circuit potential of 0.4Mo -0.8Cr steel over a period

between 4 and 24 hours of exposure in the test solutions. Generally the samples show

reasonably steady open circuit potentials. For the uninhibited seawater system

(sample (a)), the open-circuit potential showed slight fluctuation, with a small

decrease up to 10 hours, reaching a minimum of -788 mV, and then increased slightly

to -778 mV after 24 hours of testing. The saturated potential of –778 mV is

comparatively low for the deaerated condition, but consistent with previously

reported values [49]. Under this condition, the corrosion activity of the steel is low

due to the absence of oxygen, despite the low open-circuit potential.

In contrast, the open-circuit potential of the sample in the solution inhibited

with 400 ppm CeCl3 (sample (b)) was measured to be -662 mV, approximately 116

mV above that of the sample in the uninhibited solution, demonstrating the inhibiting

effect of CeCl3. The open circuit potential of this sample was found to decrease

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slightly over the duration of testing from -662 mV to -682 mV, indicating the

progressive loss of the inhibition. At the end of the 24 hour immersion test, the

sample surface was found to be partially covered with a grey-yellow deposit,

consistent with a previous study [26]. It has also been reported in the literature that a

mixed cerium/iron oxide layer under light optical microscope showed thick nodules

and needle-like particles believed to be cerium compounds [27].

-800

-750

-700

-650

-600

5 10 15 20 25

Time (hour)

(a) Seawater

(b) Seawater +

400 ppm CeCl3

(c) Seawater + 33 hours

pre-exposure to

400 ppm CeCl3

E v

s. S

CE

(m

V)

Figure 28 - Open circuit potential of 0.4Mo-0.8Cr steel pre-exposed in seawater solutions inhibited with 400ppm CeCl3

Sample (c) had been pre-exposed to a solution inhibited with 400 ppm CeCl3

for 33 hours prior to the measurement in uninhibited plain seawater. It is seen that

the open circuit potential of this specimen was comparable to that of the sample in the

inhibited solution. This suggests the protective surface film formed on the steel

during the pre-exposure in the inhibited solution is stable during subsequent exposure

in uninhibited seawater.

of 93

Figure 29 shows the effect of sulphide pollution on the open circuit potential

of 0.4Mo-0.8Cr steel in seawater. There are some variations in the open-circuit

potential during the time of testing for all the four conditions; but in general the open-

circuit potential was found to increase with increasing sulphide concentration in this

long-term immersion test. This contradicts with the previous findings for the short-

term exposure testing, where the samples showed decreased open-circuit potential

with increasing sulphide concentration. These long-term results seem to suggest

some apparent “passivation” effect of the sulphide in seawater for the steel. This is

possibly related to the building up of corrosion products on the surfaces of the

samples for the long-term testing, whereas in the short term testing the effect of

sulphide is mostly on the reaction kinetics of the electrode. However, the current

study does not permit conclusive identification of the reasons and the clarification

requires further dedicated experimentation.

-800

-780

-760

-740

-720

-700

-680

-660

-640

5 10 15 20 25

Time (Hrs)

0 ppm CeCl3 10 ppm Sulphide

100 ppm Sulphide

2 ppm Sulphide

0 ppm Sulphide

E v

s. S

CE

(m

V)

Figure 29 - Open circuit potential of 0.4Mo-0.8Cr steel in sulphide-

polluted seawater.

of 93

Figure 30 shows the effect of inhibition with 400 ppm CeCl3 on the open

circuit potential of the steel in sulphide-polluted seawater. In comparison with the

evidences shown in Figure 29, it is clear that there is a significant increase in the open

circuit potential for the unpolluted solution and the solution containing 2 ppm

sulphide, demonstrating inhibition effect of CeCl3. The net increments of the open

circuit potential is ~100 mV for the uninhibited solution and 70 mV for the solution

containing 2 ppm sulphide.

-800

-780

-760

-740

-720

-700

-680

-660

-640

5 10 15 20 25

Time (Hrs)

100 ppm Sulphide10 ppm Sulphide

2 ppm Sulphide

0 ppm Sulphide400 ppm CeCl

3

E v

s. S

CE

(m

V)

Figure 30 – Effect of CeCl3 on open circuit potential of 0.4Mo-0.8Cr steel

in sulphide-polluted seawater.

For the solution containing 10 ppm sulphide, the open circuit potential is

comparable to the uninhibited solution shown in Figure 29, implying dismissed

inhibiting power for the 400 ppm CeCl3. For the solution containing 100 ppm

sulphide, the open circuit potential in the inhibited solution (-690 mV) is lower than

that in the uninhibited solution (-670 mV). This appears to imply that addition of 400

CeCl3 has the effect of cancelling the apparent “passivation” effect of the sulphide, a

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testimony of the sure interaction between the two additives. In general it is evident

that CeCl3 has inhibiting effect on sulphide, and the inhibiting power diminishes with

increasing sulphide concentration for a given amount of CeCl3.

4.2.2 Linear Polarisation Measurement

Figure 31 shows the effect of sulphide concentration in seawater on the

polarisation resistance (Rp) of the steel. The polarisation resistance is measured as the

slope on the linear polarisation at 20 mV from the open circuit potential of the

electrodes. The polarisation resistance is plotted as the inverse value (1/Rp), which is

a measure of the tendency of corrosion. It is observed that sulphide pollution

generally increases the 1/Rp value (decreases polarisation resistance) of the steel in

seawater. For the unpolluted seawater, the 1/Rp value was 0.026 kOhm-1

after 24

hours of exposure. For the solution polluted with 2 ppm sulphide, the 1/Rp value was

0.065 kOhm-1

at the end of the 24 hours of exposure. For both these two solutions,

the 1/Rp value decreased with time during the measurements. The 1/Rp value for the

solutions containing 10 ppm and 100 ppm sulphide showed less stable behaviour, but

generally appeared at higher levels than that for the lightly polluted and unpolluted

solutions.

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0

0.05

0.1

0.15

0.2

0.25

5 10 15 20 25

1/R

p (K

oh

m-1

)

Time (Hrs)

100 ppm Sulphide

10 ppm Sulphide

2 ppm Sulphide 0 ppm Sulphide

0 ppm CeCl3

Figure 31 - Linear polarisation measurements of 0.4Mo-0.8Cr steel in

sulphide-polluted seawater.

Figure 32 shows the effect of addition of 400 ppm CeCl3 on the polarisation

resistance of the steel in sulphide-polluted seawater. It is evident that the 1/Rp value

generally increases with increasing sulphide concentration, consistent with the

observations shown in Figure 31. In comparison with the results shown in Figure 31,

there is a clear reduction in 1/Rp for the CeCl3 inhibited systems. For the unpolluted

seawater, the polarisation resistance is 0.026 kOhm-1

after 24 hours of exposure for

both the inhibited (Figure 32) and the uninhibited (Figure 31) solutions. For the

solutions containing 2 ppm sulphide, the polarisation resistance is 0.029 kOhm-1

under the inhibited condition compared to 0.065 kOhm-1

for the uninhibited

condition. For the solutions polluted with 10 ppm and 100 ppm sulphide, the 1/Rp

was measured higher in then inhibited systems than inn the uninhibited systems.

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0

0.05

0.1

0.15

0.2

0.25

5 10 15 20 25

1/R

p (k

Oh

m-1

)

Time (Hrs)

100 ppm Sulphide

10 ppm Sulphide2 ppm Sulphide

0 ppm Sulphide

400 ppm CeCl3

Figure 32 - Linear polarisation measurements of 0.4Mo-0.8Cr steel in

CeCl3 added sulphide polluted seawater.

The inhibiting effect of CeCl3 towards the sulphide activity can be evaluated

using the polarisation resistance parameters. As the polarisation resistance is

inversely proportional to corrosion rate, the inhibiting efficiency may be defined as

[78, 80]:

100xR

R1

i,p

p

(9)

where Rp and Rp,i are the polarisation resistance of the HSLA steel in plain and in

CeCl3 injected seawater solutions, respectively. The inhibitor efficiency of the 400

ppm CeCl3 after 24 hours of immersion in the sulphide-polluted seawater solutions is

summarised in Table 8.

Sulphide (ppm) 0 2 10 100

(%) 26% 62% 52% 19%

Table 8 – Inhibition efficiency based on 400ppm CeCl3 for the various

sulphide levels.

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4.2.3. Potentiodynamic Polarisation Measurements

Figure 33 shows the polarisation behaviour of the steel in sulphide-polluted

seawater solutions. The polarisation behaviour of the steel is found practically

identical in the unpolluted seawater and the solution polluted with 2 ppm sulphide.

The open circuit potential of the steel is –836 mV and the exchange current density is

0.35 x 10-6

A/cm2. The anodic polarisation exhibits two-stage activation, at below i

=10-6

A/cm2 and at between 10

-5~10

-4 A/cm

2. This is consistent with the short-term

test observations.

Addition of 400 ppm CeCl3 is seen to raise the equilibrium potential by ~150

mV, demonstrating the inhibiting effect of CeCl3. The equilibrium potential of the

sample in the unpolluted solution is –645 mV, an increase of 191 mV compared to the

uninhibited solution. The open circuit potential in the solution containing 2 ppm

sulphide and 400 ppm CeCl3 is –673 mV, an increase of 163 mV compared to the

uninhibited solution. It is also seen that the anodic polarisation Tafel slope is

decreased as a result of the addition of CeCl3 whereas the resistance to cathodic

polarisation (the cathodic Tafel slope) is increased. Consequently, the polarisation

curves become more asymmetric between the anodic and cathodic branches in the

inhibited solutions compared to the cases without the inhibitor. At the meantime, the

two-stage anodic activation is much subdued, too. These observations are consistent

with those made in the short-term tests.

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-1100

-1000

-900

-800

-700

-600

-500

-400

-300

-8 -7 -6 -5 -4 -3 -2

E v

s. S

CE

(m

V)

log (i) A/cm2

0 ppm Sulphide

2 ppm Sulphide

0 ppm Sulphide+

400 ppm CeCl3

2 ppm Sulphide+

400 ppm CeCl3

Figure 33 - Potentiodynamic Polarisation measurements of 0.4Mo-0.8Cr

steel in plane seawater and seawater polluted with 2 ppm sulphide.

Figure 34 shows the effect of addition of 400 ppm CeCl3 on the polarisation

behaviour of the steel in seawater solutions polluted with 10 ppm sulphide. The open

circuit potential of the steel in the uninhibited solution is -760 mV and the exchange

current density is 0.501 x 10-6

A/cm2. The anodic polarisation of the steel exhibited

the typical two-stage activation, at below i=10-6

A/cm2 and at between 10

-5~10

-4

A/cm2. Addition of 400 ppm CeCl3 raised the equilibrium potential of the steel to

676 mV, an increase of 84 mV compared to that of the uninhibited solution. There is

a decrease in the resistance to anodic polarisation (the anodic Tafel slope) as a result

of the addition of CeCl3 whereas the resistance to cathodic polarisation (the cathodic

Tafel slope) increased. As a result of the reduced increment of the open circuit

potential and the decrease of the anodic Tafel slope, the anodic polarisation curves of

the uninhibited and then inhibited systems virtually overlap current densities greater

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than 1x10-5

A/cm2, demonstrating the diminish of the inhibition effect of CeCl3 at this

sulphide concentration level.

Figure 35 shows similar comparison between uninhibited and inhibited

systems for the seawater polluted with 100 ppm sulphide. The open circuit potential

of the steel is –843 mV in the uninhibited solution and –772 mV in the inhibited

system, showing an increase of 71 mV due to the addition of 400 ppm CeCl3. In

comparison with solutions containing less sulphide, as shown above in Figures 33 and

34, it is clear that all the features associated with the addition of 400 ppm CeCl3 are

weakened, such as the decrease of the anodic Tafel slope, the increase of the cathodic

Tafel slope, and the suppression of the two-stage anodic activation. Consequently,

the anodic polarisation branch of the uninhibited system cross to be above that of the

inhibited system at high current densities. This demonstrates that the system added

with 400 ppm CeCl3 has higher corrosion rate in solutions containing 100 ppm

sulphide. This implies that concurrent presence of sulphide and CeCl3 at high

concentrations (referring to the specific situation presented here) is detrimental to the

steel.

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-1000

-900

-800

-700

-600

-500

-400

-300

-8 -7 -6 -5 -4 -3 -2

E v

s. S

CE

(m

V)

log (i) A/cm2

10 ppm Sulphide

10 ppm Sulphide +

400 ppm CeCl3

Figure 34 - Potentiodynamic polarisation measurements of 0.4Mo-0.8Cr

steel in seawater polluted with 10 ppm sulphide.

-1000

-900

-800

-700

-600

-500

-400

-300

-8 -7 -6 -5 -4 -3 -2

E v

s. S

CE

(m

V)

log (i) A/cm2

100 ppm Sulphide

100 ppm Sulphide +

400 ppm CeCl3

Figure 35 - Potentiodynamic polarisation measurements of 0.4Mo-0.8Cr

steel in seawater polluted with 100 ppm sulphide.

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A comparison between the exchange current density (io) values in Table 9 and

10 shows the higher values with CeCl3. This indicates that the CeCl3 has increased

the steels corrosion rate. It is noted that the io increases with sulphide concentration.

Electrochemical parameters determined from the above measurements in the

sulphide-polluted solutions are summarised in Table 9. The a values are found to be

increasing with sulphide concentration. No clear trend of effect of sulphide pollution

on the open circuit potential of the steel is identified.

Electrochemical parameters determined for the inhibited systems in the

sulphide-polluted solutions and are summarised in Table 10. There are mixed

tendencies in the open circuit potential Eo, exchange current density io and anodic

Tafel slope a with respect to sulphide concentration. It appears that the inhibiting

effect of CeCl3 decreases with sulphide concentration.

Sulphide

(ppm)

Eo

mV

io

(10-6

Amp/cm2)

βa

(mV/decade)

βc

(mV/decade)

0 778 0.320 55 190

2 760 0.398 65 90

10 674 0.501 115 120

100 760 0.794 102 97

Table 9 - Electrochemical parameters of 0.4Mo - 0.8Cr steel in sulphide-

polluted seawater.

Sulphide (ppm)

+ 400ppm CeCl3

Eo

mV

io

(10-6

A/cm2)

βa

(mV/decade)

βc

(mV/decade)

0 -682 2.510 74 178

2 -694 0.398 90 85

10 -681 3.162 70 180

100 -689 1.259 50 70

Table 10 - Electrochemical parameters of 0.4Mo - 0.8Cr steel in

sulphide-polluted seawater injected with 400 ppm CeCl3.

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Figure 36 shows the effect of sulphide concentration and CeCl3 inhibition on

the open circuit potential of the steel in long-term exposure. It is evident that the

open circuit potential vary only slightly among different sulphide concentrations, but

the addition of 400 ppm CeCl3 results in a significant increase in the nobility of the

steel, demonstrating clear inhibiting effect. The margin of increase in open circuit

potential is ~70 mV.

-780

-760

-740

-720

-700

-680

-660

0 20 40 60 80 100 120


E v

s. S

CE

(m

V)

400 ppm CeCl3

Sulphide only

Figure 36 – Effect of sulphide concentration and CeCl3 inhibitor on the

open circuit potential of 0.4Mo-0.8Cr steel.

Figures 37 and 38 shows the effect of sulphide concentration and CeCl3

inhibition on the Tafel slopes for anodic and cathodic polarisation of the steel in long-

term exposure. Addition of up to 10 ppm sulphide is found to have a significant

effect on increasing the anodic Tafel slope, signifying a retardation of corrosion.

Higher sulphide concentration appears to decrease βa slightly, implying an increased

corrosion tendency. For the solutions added with 400 ppm CeCl3, βa appears to

decrease with increasing sulphide concentration. It is also evident that the anodic

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Tafel slope in the inhibited solutions is lower than that in the uninhibited solutions,

particularly at higher sulphide concentration levels. This diminishes the inhibiting

effect of CeCl3 achieved by raising the open circuit potential of the steel. For

cathodic polarisation, increasing sulphide concentration leads to a general decrease of

βc for both the uninhibited and the inhibited systems.

40

50

60

70

80

90

100

110

120

0 20 40 60 80 100 120


a (

mV

/dec

ade)

400 ppm CeCl3

Sulphide only

Figure 37 – Effect of sulphide concentration and CeCl3 addition on the

anodic polarisation Tafel slope of 0.4Mo-0.8Cr steel.

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60

80

100

120

140

160

180

200

0 20 40 60 80 100 120


c (

mV

/dec

ade)

400 ppm CeCl3

Sulphide only

Figure 38 – Effect of sulphide concentration and CeCl3 addition on the

cathodic polarisation Tafel slope of 0.4Mo-0.8Cr steel.

4.3 WEIGHT LOSS EXPERIMENT – 60 DAYS EXPOSURE

Electrochemical measurements are useful in determining corrosion kinetics,

for example, the activity of the electrolyte and the rate at which the reactions occur on

the electrode surfaces. On the other hand, weight loss measurement via long-term

immersion tests is useful in directly determining corrosion rates that take place in real

time [81]. Some researchers tried to compare the corrosion rates determined in

electrochemical measurement and in weight loss tests and failed to obtain consistent

correlation [82]. It was reported that it is very difficult to directly correlate results of

electrochemical tests and those of immersion tests because of the differences in the

nature of the corrosion processes. It was also suggested that in spite of the correlation

difficulties between the two techniques, useful parameters such as corrosion tendency

of the electrolyte from electrochemical tests and corrosion rates from immersion tests

can be obtained [83].

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The immersion tests were conducted in accordance with the ASTM G31-90

procedure [71]. Tables 11-13 show the corrosion rates of the 0.4Mo-0.8Cr steel

determined after 60 days of immersion in seawater solutions added with sulphide, 400

ppm CeCl3 and 25 ppm glutaraldehyde. The corrosion rates determined are plotted in

Figure 39 against sulphide concentration. It is clear that for all the uninhibited and

inhibited systems, the corrosion rate of the steel increases with sulphide concentration

and that inhibition with CeCl3 and glutaraldehyde is effective in reducing the

corrosion rate. The further reduction of corrosion rate for the system inhibited with

400 ppm CeCl3 and 25 ppm glutaraldehyde compared to the system inhibited with

400 ppm CeCl3 suggests an added effect of the two additives in the seawater

environment.

Figure 40 shows corrosion rate reduction efficiency of 400 ppm CeCl3 and the

double addition of 400 ppm CeCl3 and 25 ppm glutaraldehyde. It is seen that at low

sulphide concentration levels, corrosion rate may be reduced by 50% with a single

addition of 400 ppm CeCl3 and 80% with a double addition of 400 ppm CeCl3 and 25

ppm glutaraldehyde. The corrosion inhibition efficiency decreases to ~20% with 100

ppm sulphide.

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Sulphide

(ppm)

Initial Weight

(g)

Final Weight

(g)

Weight Loss

(g)

Corrosion Rate

(mm/year)

0 18.8830 18.7531 0.1299 0.086

2 18.6100 18.4886 0.1214 0.081

10 18.3532 18.2528 0.1004 0.067

100 18.5290 18.3876 0.1414 0.094

Table 11 - Corrosion rates measured after 60 days of immersion in

sulphide-polluted seawater.

Sulphide

(ppm)

Initial Weight

(g)

Final Weight

(g)

Weight Loss

(g)

Corrosion Rate

(mm/year)

0 18.6670 18.5986 0.0684 0.046

2 18.8520 18.7848 0.0672 0.045

10 18.6450 18.5703 0.0747 0.049

100 18.5320 18.4059 0.1261 0.084

Table 12 - Corrosion rates measured after 60 days of immersion in sulphide-polluted seawater solutions inhibited with 400 ppm CeCl3.

Sulphide

(ppm)

Initial Weight

(g)

Final Weight

(g)

Weight Loss

(g)

Corrosion Rate

(mm/year)

0 18.6838 18.6503 0.0335 0.022

2 18.6749 18.6400 0.0349 0.023

10 18.7088 18.6644 0.0444 0.029

100 18.6057 18.4850 0.1207 0.080

Table 13 - Corrosion rates measured after 60 days of immersion in

sulphide-polluted seawater solutions inhibited with 400 ppm CeCl3 and 25 ppm glutaraldehyde.

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0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0 20 40 60 80 100 120

Co

rro

sio

n R

ate

(mm

/yea

r)


Sulphide only

400 ppm CeCl3

400 ppm CeCl3 + 25 ppm glutaraldehyde

Figure 39 – Effects of sulphide pollution and CeCl3 and glutaraldehyde addition on corrosion rate of 0.4Mo - 0.8Cr steel in seawater.

0

20

40

60

80

100

0 20 40 60 80 100 120

Co

rro

sio

n R

ate

Red

uct

ion

(%

)


400 ppm CeCl3

400 ppm CeCl3 + 25 ppm glutaraldehyde

Figure 40 – Corrosion rate reduction efficiency of CeCl3 and glutaraldehyde addition to 0.4Mo - 0.8Cr steel in seawater.

Figure 41 shows photographs of two corrosion coupons tested in seawater

solutions for 60 days under deaerated conditions. Sample (a) was immersed in a

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solution polluted with 100 ppm sulphide. Sample (b) was immersed in a solution

polluted with 100 ppm sulphide and inhibited with 400 ppm CeCl3. Sample (a)

developed a thick black loose scale on its surface, which is characteristic of sulphide

attack. A pungent odour was also noted at the time of experimentation. It has been

reported that reaction of sulphur gas with magnetite (iron oxide) results in formation

of thick black layer and release of rotten-egg odour of hydrogen sulphide [74]. In

comparison, sample (b) experienced much reduced corrosion. It developed a

relatively thinner surface layer of orange scale, which is a distinctive feature of iron

oxide. The reduction in corrosion in sample (b) is ascribed to the scavenging effect of

CeCl3 on sulphide. The fresh sulphide-polluted seawater solution is light grey in

colour and the cerium chloride solution is clear and transparent. When the two

solutions are mixed, after the reaction, dark precipitates are formed deposited at the

cell bottom. During the experiment light black precipitates were found at the bottom

of the test cell around the working electrode. These black precipitates are believed to

be cerium-sulphide compound. In addition, the samples are observed to have

developed a light yellow film surface with traces of grey colour, which corroborates

the findings reported earlier [29]. The samples are also observed with reddish brown

precipitates, which are characteristic of ferrous/ferric oxides and chlorides (rust).

Figure 41 - Photographs of coupons of 0.4Mo-0.8Cr steel after 60 days of immersion in seawater with: (a) 100 ppm sulphide; (b) 100 ppm sulphide

and 400 ppm CeCl3.

(b)

(a)

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In order to ascertain the scavenging effect of CeCl3 on sulphide, sulphide

concentration in solutions was analysed over time. The sulphide is added to the

inhibited seawater solution in the titrimetric flask and sealed to prevent sulphide

desertion to the surrounding. The pH levels were in the range of 8.2-8.6 for the 2-100

ppm sulphide range. Sulphide level is determined by Iodometric analytical method

[75].

Figure 42 shows the evolution of sulphide concentration in four solutions. It

is seen that sulphide concentration decreases continuously over time. It cannot be

ruled out that minor loss of sulphide to its surrounding by evaporation because of its

instability. For the solutions polluted with 2 ppm sulphide, the effect of the CeCl3

addition is insignificant, obviously due to the low concentration of the sulphide. For

the solutions polluted with 10 ppm sulphide, the sulphide concentration deteriorated

more rapidly in the solution added with 400 ppm CeCl3. In the case of sulphide

solution, when added with CeCl3 solution, black precipitates are noted on the cell

bottom, which is believed to be Ce3S complex. This implies that CeCl3 has a

scavenging effect on sulphide in seawater solutions.

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0

2

4

6

8

10

12

0 2 4 6 8 10 12

Su

lph

ide

Co

ncen

trat

ion (

pp

m)

Time (Hours)

2 ppm Sulphide

10 ppm Sulphide

2 ppm Sulphide +

400 ppm CeCl3

10 ppm Sulphide +

400 ppm CeCl3

Figure 42 – Scavenging effect of CeCl3 to sulphide in seawater.

4.4 MICROSCOPY AND MICROANALYSIS

The surface morphology and the corrosion products of 0.4Mo-0.8Cr steel

sample exposed to the seawater solution containing 10 ppm sulphide and 400 ppm

CeCl3 were studied using JEOL JSM 4300 scanning electron microscope operating at

an accelerating voltage of 15 kV. The specimen had been polarised to 400 mV from

its equilibrium potential after 24 hours stabilisation in the solution prior to the

examination. After experimentation, the sample was carefully dried in hot air. Figure

43 shows a collection of SEM images of the surface of the sample. The surface

exhibits variations in terms of the degree of corrosion damage. Micrograph (a) shows

the surface of a relatively lightly corroded area. The surface is covered with a

continuous smooth surface layer, as evidenced by some local cracked areas, and

granules scattered on top of the film. The smooth film is believed to be the iron oxide

formed on the original surface of the steel. This implies that this area has not been

severely corroded by the seawater solution. Energy Dispersive X-ray Spectroscopy

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(EDS) analysis revealed that the granules contain both iron and cerium, with trace

amount of sulphur. The Fe content is believed to be from the surrounding area and

the granules are attributed to cerium compounds precipitated on the surface.

Micrograph (b) was taken at another location on the surface, where the surface

layer is thicker. The thick scale is highly uneven and severely cracked. It is also

noted that on the surface of the thick scale there are some obvious crystal formations.

Micrographs (c) and (d) show detailed views of the crystalline particles at higher

magnifications. It is seen that the particles are composed of a large number of crystal

platelets. Figure 44 shows an EDS spectrum collected from the surface of these

platelets. The spectrum consists of Ce, S and Fe.

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Figure 43 - SEM micrographs showing the surface morphology of 0.4Mo-0.8Cr steel after exposure for three hours in seawater containing

10 ppm sulphide and 400 ppm CeCl3.

Figure 44 - X-ray energy dispersive spectrum of the crystal formation on the 0.4Mo-0.8Cr steel immersed in seawater solution containing 10 ppm

sulphide and 400 ppm CeCl3.

(a) (b)

(c)

(d)

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5. CORROSION MECHANISMS AND ANALYSIS

Corrosion of steels in deaerated seawater may proceed via the reduction

reaction of H+ ions as:

Fe + 2H2O Fe(OH)2 + H2 (10)

In the case where sulphide is present in the solution, the corrosion may take the

following path, as suggested by Szklarska-Smialowska [84]:

Anodic reaction: Fe Fe2+

+ 2e (11)

Cathodic reactions: H2S + 2e- S

2 + H2 (12)

Net reaction: Fe + H2S FeS + H2 (13)

This reaction produces ferrous sulphide over the steel. Ferrous sulphide is a black

deposit, which is porous and non-protective. In this study, black deposit at the

bottom of the container was found. It is attributed to the iron-cerium-sulphide

complex compound. Black, loose and non-protective scale was also found formed on

the surface of the samples, consistent with the expectation for the formation of FeS.

This agrees with the findings of Rozenfeld [85], who concluded that the ferrous oxide

films formed during the initial exposure to unpolluted seawater were relatively

smooth and non-porous; but the films formed in sulphide-polluted solutions,

presumed to be stoichiometric and sub-stoichiometric forms of ferrous sulphide, were

relatively thick and porous. He further stated that the presence of dissolved sulphide

or sulphide oxidation products does not lead directly to accelerated corrosion but

rather that the porous ferrous sulphide corrosion product formed in the polluted water

interferes with the normal growth of the protective oxide films on subsequent

exposure to unpolluted seawater. In this study, addition of sulphide, in the form of

Na2S, was found to decrease the open-circuit potential of the HSLA steel electrode,

implying an increased corrosion tendency in the long-term electrochemical test. This

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is consistent with the common understanding that sulphide encourages the corrosion

of steels in seawater.

Ce3+

is known to cause inhibition to steels and other alloys in aerated aqueous

solutions [86]. This is generally attributed to the formation of protective surface

films. It has been reported that additions of small concentrations of CeCl3 in the

range of 100-1000 ppm to 0.1 Molar NaCl solution reduces the corrosion rate of 7075

aluminium alloy by a factor of 10 [26]. Pre-exposure of the same alloy to CeCl3 also

imparts corrosion resistance in 0.1 Molar NaCl solutions [29]. It was reported that

the increased corrosion resistance is due to the residual oxygen in the solution, which

resulted in the precipitation of non-porous cerium oxide layer [86].

In this study the electrochemical measurements were conducted in deaerated

stagnant solutions, thus the formation of compound oxide of iron and cerium was

restricted. On the other hand, it was noted that a fine greyish deposit was formed

when CeCl3 was injected into the sulphide-polluted solutions as opposed to the black

colour of FeS formed when iron reacts with sulphide alone. The greyish deposit

adsorbed on the steel surface is attributed to an intermediate form of cerium-iron

sulphide Fe[Ce3S]ads. In this regard, Ce3+

addition to the sulphide-polluted seawater

solutions has duel effects: (1) scavenging effect to remove sulphide form the solution

and (2) formation of protective films on the surfaces of the steel. Examination of the

results seems to suggest that the scavenging effect on sulphide is the dominant

contribution to corrosion inhibition of HSLA steels in sulphide-polluted seawater. In

addition, the presence of dissolved sulphide or sulphide oxidation products in the

solution accelerates corrosion through porous ferrous sulphide layer. This study

proposes that the reduced inhibition efficiency of the cerium is due to the formation

of ferrous sulphide layer, which increased the activity of sulphide and its derivatives.

This effect can be observed as the instability in the Ecorr and 1/Rp values at high

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sulphide concentration levels. Some of the sulphide ions in the solution reacts with

cerium to form Ce2S3 and results in the formation of intermediate complex Fe(Ce2S3).

Hence, stable protective film is not formed by cerium in the deaerated solution and

resulted in poor inhibiting efficiency.

Fe

Fe2+

+ 2e (14)

2Ce + 3S

Ce2S3 (15)

Fe + Ce2S3 Fe(Ce2S3) (16)

Fe2+

+ S2

FeS (17)

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6. CONCLUSIONS

6.1 EFFECT OF SULPHIDE POLLUTION

Sulphide pollution in deaerated seawater was found to generally decrease the

open-circuit potential of the 0.4Mo-0.8Cr steel in the short-term test, as evident in

Figure 25. Over long period, however, the open-circuit potential of the 0.4Mo-0.8Cr

steel appears not to be influenced by sulphide concentration, as shown in Figure 36.

This is attributed to the enclosed system used, in which sulphide concentration is

consumed over time with the progress of corrosion reactions.

Sulphide pollution in deaerated seawater was found to increase anodic Tafel

slope of the 0.4Mo-0.8Cr steel in the short-term test, as evident in Figure 27,

implying increased resistance to corrosion. In contrast, anodic Tafel slope of the steel

in the long-term test was found to decrease with increasing sulphide concentration,

implying increased tendency for corrosion. This suggest that in the early period of

immersion sulphide content in the solution has the effect of forming passivating

films, but for longer immersion times the films are damaged and the corrosion

tendency is increased.

Sulphide pollution in deaerated seawater was found to increase linear

polarisation resistance of the 0.4Mo-0.8Cr steel, as evident in Figures 26 and 37,

implying decreased corrosion susceptibility for the steel.

Weight loss measurements revealed that at low sulphide concentration levels

corrosion rate of the 0.4Mo-0.8Cr steel in deaerated seawater is decreased and at high

sulphide concentration levels the rate is increased, as evident in Figure 39.

6.2 EFFECTS OF CERIUM CHLORIDE

Addition of 400 ppm CeCl3 in deaerated seawater was found to be effective in

increasing the open-circuit potential of the 0.4Mo-0.8Cr steel, as shown in Figures 25

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and 36, indicating corrosion prohibition effect. The corrosion prohibition power

appears to be diminished with increasing concentration of sulphide in the solution.

Addition of 400 ppm CeCl3 in deaerated seawater was found to increase the

linear polarisation resistance of the 0.4Mo-0.8Cr steel, as shown in Figure 26.

Addition of 400 ppm CeCl3 in deaerated seawater was found to increase the

anodic polarisation Tafel slope of the 0.4Mo-0.8Cr steel, as evident in Figure 27 and

37. This implies a relaxation of the resistance to corrosion.

It is also found that CeCl3 has a scavenging effect to sulphide in deaerated

seawater, as evident in Figure 42. It is postulated that the moderate inhibiting effect

of CeCl3 is contributed by the scavenging effect, by which Ce2S3 complex is formed.

It is also concluded that CeCl3 appeared to interfere with both anodic and cathodic

reactions in deaerated conditions.

Addition of 400 ppm CeCl3 in deaerated seawater was found to reduce the

corrosion rate of the 0.4Mo-0.8Cr steel, as evident in Figure 40. The corrosion rate

reduction efficiency diminishes with increasing sulphide concentration, as shown in

Figure 41.

6.3 EFFECT OF GLUTARALDEHYDE

The additional injection of glutaraldehyde with CeCl3 in sulphide-polluted

seawater was found to further increase linear polarisation resistance but decrease

anodic polarisation Tafel slope of the 0.4Mo-0.8Cr steel, as shown in Figures 26 and

27. It is also found to further decrease corrosion rate and further increase corrosion

rate reduction efficiency, as shown in Figures 39 and 40. These observations suggest

that there exists an added effect between CeCl3 and glutaraldehyde on corrosion

inhibition for the steel in sulphide-polluted seawater. The effects of glutaraldehyde

diminish with increasing sulphide concentration.

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