Jordan Journal of Chemistry Vol. 5 No.4, 2010, pp. 371-388

 

371  

JJC

Chemical and Electrochemical Techniques for Evaluation of Performance of some Amino Acids as Corrosion Inhibitors for Low

Alloy ASTM A213 Grade T22 Boiler Steel in HCl Solutions

Mohammed A. Amin*

Chemistry Department, Faculty of Science, Ain Shams University, 11566 Abbassia, Cairo, Egypt

Received on April 22, 2010 Accepted on Aug. 22, 2010

Abstract In this work, chemical (weight loss) and electrochemical (Tafel extrapolation and EFM)

methods were employed for investigation of the corrosion inhibition of low alloy ASTM A213

grade T22 boiler steel in aerated stagnant 0.50 M HCl solutions at different temperatures (283-

333 K) by some amino acids. The investigated amino acids were serine (Ser), therionine (Thr)

and glutamine (Glu). Tafel plots showed that the three tested amino acids function mainly as

cathodic-type inhibitors. The inhibition efficiency increased with increase in inhibitor

concentration but decreased with temperature, suggesting the occurrence of physical

adsorption. Among the tested inhibitors, Glu presented the highest inhibition performance. The

inhibition process was attributed to the formation of an adsorbed film on the metal surface that

protects the metal against corrosive agents. XPS examinations of the electrode surface

confirmed the existence of such adsorbed film. Activation energies (Eoa) have been calculated in

the absence and presence of various concentrations of these amino acids by measuring the

temperature dependence of the corrosion rate obtained from weight-loss and Tafel extrapolation

methods. It was found that the activation energy in the presence of the tested inhibitor is higher

than that in bare HCl solution. The adsorptive behaviour of the three tested amino acids followed

Temkin-type isotherm. Values of the standard free energy of adsorption (∆Goads) of the tested

inhibitors were found to be lower than -40 kJ mol-1. These results confirmed the occurrence of

physical adsorption.

Keywords: Corrosion inhibition; Low alloy boiler steel; Amino acids; Tafel extrapolation method; EFM; XPS.

Introduction Acid solutions are widely used in industry, with important fields of application

including acid pickling of iron and steel, chemical cleaning and processing, ore

production and oil well acidification. Among acidic media, the hydrochloric acid used in

the pickling of metals, the acidification of oil wells and the cleaning of scales is more

economical, efficient and straightforward compared to other mineral acids [1].

The use of inhibitors is one of the most practical methods for protection against

corrosion and prevention of unexpected metal dissolution and acid consumption,

especially in acid solutions. Different organic and inorganic compounds have been                                                             * Corresponding author: e-mail: maaismail@yahoo.com

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studied as inhibitors to protect metals from corrosive attack. The efficiency of these

organic corrosion inhibitors is related to the presence of polar functional groups with S,

O or N atoms in the molecule [2]. Such compounds can adsorb onto the metal surface

and block the active surface sites, thus reducing the corrosion rate. Although many

synthetic compounds show good anticorrosive activity, most of them are highly toxic to

both human beings and the environment [3], and they are often expensive and non-

biodegradable.

The strict environmental regulations and the increasing of ecological awareness

have resulted in the use of substitute nontoxic compounds, acceptable from the

environmental point of view ‘‘green inhibitors’’ [4,5]. Amino acids have been found to

satisfy the strict environmental regulations (safe-inhibitors). They are from a class of

organic compounds, completely soluble in aqueous media. These properties would

justify their use as corrosion inhibitors. Some studies involving amino acids on the

corrosion of iron, steel and stainless steel were reported [6–8]. In our laboratory we

have reported on the use of some amino acids as safe corrosion inhibitors for

aluminium, iron and steel [9-13].

In a continuation of our previous study [9,13], the objective of the present work is

to inhibit corrosion of low chromium alloy steel SA213-T22 in 0.50 M HCl solution. To

achieve this goal, some amino acids, namely serine (Ser), therionine (Thr) and

glutamine (Glu) are applied as corrosion safe inhibitors. Measurements were

conducted using chemical (weight loss) and electrochemical (Tafel polarization and

EFM) methods. Rates of corrosion measured by Tafel extrapolation method were

compared with those recorded from the weight loss and EFM methods. The aim is to

confirm validation of corrosion rates measured by Tafel extrapolation method. XPS

examinations were also carried out. It was also the purpose of the present work to test

the experimental data with several adsorption isotherms at different temperatures, in

order to determine the standard free energies of the adsorption process and gain more

information on the mode of adsorption of the inhibitor on the electrode surface.

Experimental The working electrodes employed in the present work were made of low alloy

steel (ASTM A213 grade T22), this type of alloy is widely used as a super-heater pipes

at steam power plants. The composition (wt%) of the low alloy steel was determined

using Energy Dispersive X-ray spectroscopy (EDX); Si(0.53%); Mn(0.64%); Cr(2.30%);

Mo(0.86%); Fe(95.85%). Low alloy steel sheets were used for weight loss

measurements and surface analysis. On the other hand, cylindrical rods were used for

electrochemical measurements.

For weight loss measurements, corrosion inhibition tests were performed using

rectangular coupons measuring 1cm × 2cm × 0.1cm prepared from the steel alloy

sheets. These coupons were first briefly ground with no. 600 emery paper,

subsequently polished with no. 2000 emery paper, washed with deionized water,

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degreased with absolute ethanol, dried, and then rinsed with deionized water rapidly,

followed by immediate rinsing with absolute ethanol. The weight loss (in mg cm-2) was

determined at different immersion times (up to 8 h) at the required temperature (±0.2 oC) by weighing the cleaned samples before and after hanging the coupon into 100

cm3 of the corrosive solution, namely 0.50 M HCl (in open air) in the absence and

presence of various concentrations of the tested inhibitors. After the time elapsed the

cleaning procedure consisted of wiping the coupons with a paper tissue and washing

with distilled water and acetone, finally dried and stored in a desciccator. The weight

loss was determined on an analytical balance with a precision of 0.1 mg. The

measures of weigh loss were obtained according to ASTM G31-72 [14], which

standardizes the methodology for the use of this technique in the laboratory.

For electrochemical measurements, the cylindrical rods were welded with Cu-

wire for electrical connection and mounted into glass tubes of appropriate diameter

using Araldite to offer an active flat disc shaped surface to contact the test solution.

Prior to each experiment, the surface pre-treatment of the working electrode

mentioned in weight loss measurements was performed. A conventional

electrochemical cell of capacity 100 cm3 was used containing three compartments for

working, platinum spiral counter and reference electrodes. A Luggin–Haber capillary

was also included in the design. The reference electrode was a saturated calomel

(SCE) one used directly in contact with the working solution. Some experiments were

performed at different temperatures (10-60 oC ± 0.2 oC) using a temperature control

water bath.

Polarization measurements were carried out starting from a cathodic potential of

-0.6 V to an anodic potential of 0.0 V at a sweep rate of 0.50 mV s-1. The linear Tafel

segments of the cathodic curves and the calculated anodic Tafel lines were

extrapolated to corrosion potential to obtain the corrosion current densities (jcorr) [9,13].

The inhibition efficiency, ITafel(%), was evaluated from the measured jcorr values using

the relationship:

ITafel(%) = 100 x [(jocorr - jcorr) / jocorr] ................................................................... (1)

where jocorr and jcorr are the corrosion current densities in the absence and presence of

the inhibitor, respectively. All the recorded jcorr values were converted into the corrosion

rate (υTafel) in µm y-1 (Micrometer per year; the penetration rate of corrosion through a

metal) using the expression [15]:

υTafel = 3280 x jcorr x (M / nd) .............................................................................. (2)

where M is the atomic weight of Fe (55.85 g/mol), n the number of electrons

transferred in the corrosion reaction (n = 2) and d the density of Fe (7.88 g cm-3).

EFM measurements were performed with applying potential perturbation signal

with amplitude of 10 mV with two sine waves of 2 and 5 Hz. The Intermodulation

spectra contain current responses assigned for harmonical and intermodulation current

374  

peaks. The larger peaks were used to calculate the corrosion current density (jcorr), the

Tafel slopes (βc and βa) and the causality factors CF2 & CF3 [16,17].

All Electrochemical experiments were carried out using Gamry PCI300/4

Potentiostat/Galvanostat/Zra analyzer, DC105 Corrosion software, EFM140

Electrochemical Frequency Modulation software and Echem Analyst 5.21 for results

plotting, graphing, data fitting & calculating. A corroded sample, immersed for 24 h in

0.5 M HCl solution containing 10 mM Ser, was subjected to XPS analysis using a

Leybold Heraeus spectrometer with a hemispherical energy analyzer, working with a

Mg anode and the MgKα line.

Results and discussion Electrochemical measurements

Polarization measurements

Figure 1a-c shows the cathodic and anodic potentiodynamic polarization curves

of steel in 0.5 M HCl solutions without and with various concentrations of Ser, Thr or

Glu at a scan rate of 0.50 mV s-1 at 30 oC. In all cases, the cathodic current-potential

curves giving rise to parallel Tafel lines. These results indicate that the addition of

these amino acids to the 0.5 M HCl solution does not modify the hydrogen evolution

mechanism and the reduction of H+ ions at the steel surface takes place mainly

through a charge transfer mechanism [18-20]. However, the anodic polarization curve

does not display an extensive Tafel region, may be due to passivation and pitting (see

the inflection in the anodic branch).

Corrosion current densities (jcorr), Tafel slopes (βa and βc) and the corrosion

potential (Ecorr) were calculated via extrapolation of the cathodic Tafel lines and the

calculated anodic Tafel lines. Their intersection determines the corrosion potential.

More details are presented in our previous studies [9,13]. These parameters are

collected in table 1 as a function of each inhibitor concentration at 30 oC.

It can be clearly seen from figure 1 that both anodic dissolution of steel and

cathodic hydrogen evolution reactions were inhibited after the addition of the three

tested amino acids to the aggressive solution. This result is indicative of the adsorption

of inhibitor molecules on the active sites of steel surface. The inhibition of both anodic

and cathodic reactions is more pronounced with the increasing inhibitor concentration.

In addition, the increase in inhibitor concentration shifts the corrosion potential (Ecorr) in

the negative direction. However, these inhibitors suppressed the cathodic reaction to

much greater extents than the anodic one. Based on these results, the tested inhibitors

act mainly as cathodic-type inhibitors.

It is well known that amino acids are molecules containing both aminic and

carboxylic functional groups and act as inhibitors through adsorption on the metal

surface [21,22]. These amino acids can be protonated at the aminic group in acidic

solution, and can be adsorbed to cathodic sites of the electrode surface. Thus, they

hinder the cathodic reaction and acts as cathodic inhibitors.

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-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0-4

-3

-2

-1

0

1

2

3

4

5(a)

log

(j / m

A c

m-2

)

E / V (SCE)

blank 1 mM Ser 2 mM 5 mM 10 mM 20 mM 50 mM

-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0-4

-3

-2

-1

0

1

2

3

4

5(b)

log

(j / m

A c

m-2

)

E / V (SCE)

blank 1 mM Thr 2 mM 5 mM 10 mM 20 mM 50 mM

-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0-4

-3

-2

-1

0

1

2

3

4

5(c)

log

(j / m

A c

m-2

)

E / V (SCE)

blank 1 mM Glu 2 mM 5 mM 10 mM 20 mM 50 mM

Figure 1: Polarization curves recorded for steel in aerated 0.50 M HCl solutions without and with various concentrations (1.0 – 50 mM) of (a) Ser, (b) Thr or (c) Glu at a scan rate of 0.50 mV s-1 at 30 oC.

376  

Table 1: Electrochemical parameters, inhibition efficiencies (I) and corrosion rates (υ) associated with Tafel polarization, EFM and weight-loss measurements for steel in 0.5 M HCl solutions without and with various

concentrations of Ser, Thr or Glu at 30 oC.

[Ser] / mM

Tafel polarization EFM Weight-loss (WL)

Ecorr / mV

(SCE)

jcorr / mA cm-2

βa / mV

dec-1

-βc / mV

dec-1

υTafel

(µm y-1)

ITafel(%) υEFM

(µm y-1)

IEFM(%) jcorr / mA cm-2

υWL

(µm y-1)

IWL(%)

Blank -459 0.20 86 55 2.32 ------ 2.54 ----- 0.219 2.27 -----

for Ser

1 -461 0.163 88 54 1.86 19.25 1.98 21.00 0.171 1.82 19.71

2 -465 0.109 87 56 1.28 45.73 1.39 43.88 0.122 1.26 44.54

5 -468 0.093 85 55 1.05 55 1.16 58.00 0.103 1.00 55.81

10 -471 0.074 87 55 0.81 67 0.70 69.54 0.064 0.75 67.07

20 -475 0.043 86 57 0.46 82 0.41 83.88 0.035 0.43 81.83

50 -478 0.022 85 58 0.23 91 0.17 93.05 0.015 0.19 91.77

for Thr

1 -465 0.153 87 58 1.778 23.56 1.93 24.11 0.166 1.707 24.81

2 -471 0.104 89 57 1.209 48.00 1.29 49.23 0.111 1.163 48.77

5 -474 0.078 87 59 0.907 61.00 1.023 59.97 0.088 0.903 60.24

10 -478 0.055 88 58 0.639 72.77 0.686 73.05 0.059 0.615 72.91

20 -483 0.03 85 61 0.349 85.00 0.36 86.00 0.031 0.321 85.88

50 -491 0.013 88 63 0.151 93.75 0.139 94.55 0.012 0.116 94.91

for Glu

0.25 -469 0.128 89 59 1.488 35.91 1.697 33.22 0.146 1.493 34.21

0.50 -477 0.064 88 56 0.744 67.88 0.849 66.81 0.073 0.772 65.98

1 -480 0.042 88 58 0.488 79.00 0.50 80.23 0.043 0.429 81.08

2 -485 0.018 87 59 0.209 91.00 0.198 92.05 0.017 0.187 91.77

5 -491 0.009 88 60 0.105 95.67 0.128 94.97 0.011 0.088 96.12

10 -503 0.004 89 61 0.046 98.05 0.058 97.93 0.005 0.033 98.55

It is obvious that the shapes of the polarization plots for inhibited electrodes are

not substantially different from those of uninhibited electrodes. The presence of the

inhibitor decreases the corrosion rate but does not change other aspects of the

behaviour. This means that the inhibitor does not alter the electrochemical reactions

responsible for corrosion. In addition, the absence of significant changes in the

cathodic Tafel slope in the presence of inhibitor indicates that the hydrogen evolution is

slowed down by the surface blocking effect of the inhibitor. This indicates that the

inhibitive action of the inhibitor may be related to its adsorption. The adsorption

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process is followed by the formation of a barrier film of the inhibitor on the electrode

surface. XPS examinations of the electrode surface confirmed the existence of such

adsorbed film (see later).

The calculated jcorr values were introduced in equations 1 and 2 to get the

inhibition efficiency, ITafel(%), and the corrosion rate, υTafel, values as a function of each

inhibitor concentration (see also table 1). It is apparent that the inhibition efficiencies of

the three tested inhibitors enhance with increase in their concentration. A maximum

inhibition efficiency of about 98% was recorded for Glu even at a very low

concentration (10 mM Glu). These results reflect the highest inhibition performance of

Glu compared with Ser and Thr under these conditions. According to the number of the

methylenic groups in the main chain, amino acids were categorized into two groups:

short chain and long chain amino acids. Glu is a long chain amino acid (chain length of

the tested amino acids increases in the order: Ser < Thr < Glu). The long chain and the

presence of two aminic groups in the Glu molecule may be the reasons for the higher

inhibition performance record for Glu compared with Ser and Thr (see more details in

XPS examinations section).

Electrochemical frequency modulation method

Corrosion measurement employs a variety of techniques to determine how corrosive the environment is and at what rate metal loss is being experienced. Corrosion measurement is the quantitative method by which the effectiveness of corrosion control and prevention techniques can be evaluated and provides the feedback to enable corrosion control and prevention methods to be optimized. Several electrochemical techniques are available to determine corrosion rate, such as the linear polarization resistance (LPR) technique, Tafel-extrapolation, and electrochemical impedance spectroscopy (EIS).

The LPR and EIS techniques require B value to calculate corrosion rate from polarization resistance Rp according to the equation jcorr = B/Rp. In addition, Tafel-extrapolation method, despite its application here to evaluate corrosion rates, is time consuming and will damage the electrode surface due to the polarization over a wide potential range. Compared with these three electrochemical techniques, EFM technique can quickly determine the corrosion current value without prior knowledge of Tafel slopes, and with only a small polarizing signal. These advantages of EFM technique make it an ideal candidate for online corrosion monitoring [23].

The main objective of performing EFM measurements is to confirm validation of corrosion rates measured by Tafel extrapolation method, one of the most popular DC techniques for estimation of corrosion rate. The EFM intermodulation spectra (spectra of current response as a function of frequency) of steel in 0.50 M HCl solutions containing various concentrations of the three tested amino acids have been studied (data not presented here). The values of causality factors obtained under different experimental conditions were found approximately equal the theoretical values (2 and 3) indicating that the measured data are of high quality [16].

378  

Corrosion currents, (jcorr)EFM, were calculated as a function of each inhibitor

concentration (see also table 1) using equations presented elsewhere [24]. These

(jcorr)EFM values were inserted in equations 1 and 2 to calculate inhibition efficiency

IEFM(%) and corrosion rates (υEFM), respectively. The recorded values of IEFM(%) and

υEFM are also collected in table 1. The corrosion current density decreases, while the

inhibition efficiency increases with increase in additive concentration. Here also Glu

recorded the highest inhibition efficiency among the tested inhibitors. These findings

agree well with polarization measurements.

Chemical methods

Weight loss measurements

In polarization experiments, scanning is started from a relatively large negative

cathodic potential. This negative potential may cause polymerization or potentiostatic

adsorption of the inhibitor that may not occur in absence of this negative potential. If

this were true then polarization measurement technique will alter the chemistry of the

species on the surface of the metal which is not the case in absence of the applied

potential. Weight-loss method data for determination of corrosion rate will be very

supportive for the conclusions in the present work and provide clear evidence that the

protective film is not produced by electrochemical perturbation. In addition, weight-loss

measurements provide the most reliable results concerning the efficiency of a given

inhibitor compound, so that the corresponding corrosion data obtained from them

approach service conditions more accurately than the data obtained with any other

test. It is also the purpose from the weight-loss measurements to confirm validation of

corrosion rates measured by the Tafel extrapolation method.

In weight-loss experiments, to avoid the accumulation of corrosion products, the

solution volume (mL) was at least 15 times greater than the surface area of the sample

(cm2) [14]. Three measurements for each inhibitor and the uninhibited solution were

performed and the average mass-loss was then calculated (outliers were discarded by

the Grubbs test). In the course of weight- experiments, localized corrosion should not

take place. Otherwise, the results cannot be compared to those obtained by the Tafel

extrapolation method, which measures only the uniform corrosion rate. In fact, after 7.0

days of immersion in the aggressive solution, no pitting was observed under the optical

microscope (100× magnification), thus no extensive localized attack was present.

The variation of the weight loss (in mg cm-2) of steel with the immersion time has

been studied in 0.50 M HCl solutions in the absence and presence of various

concentrations of Ser, Thr or Glu at 30 ◦C, figure 2a-c. The slope of each line (weight

loss per unit time; mg cm-2 h-1) represents the corrosion rate of steel at the specified

conditions. The linear increase in the weight loss with time in the absence and

presence of the tested inhibitor indicates that insoluble surface films do not form on the

electrode surface during steel corrosion in HCl solutions.

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0 1 2 3 4 5 6 7 8 90

10

20

30

40

(a)

WL

/ mg

cm-2

Time / h

blank 1.0 mM Ser 2.0 mM 5.0 mM 10 mM 20 mM 50 mM

0 1 2 3 4 5 6 7 8 90

10

20

30

40

(b)

WL

/ mg

cm-2

Time / h

blank 1.0 mM Thr 2.0 mM 5.0 mM 10 mM 20 mM 50 mM

0 1 2 3 4 5 6 7 8 90

10

20

30

40

(c)

WL

/ mg

cm-2

Time / h

blank 0.25 mM Glu 0.50 mM 1.0 mM 2.0 mM 5.0 mM 10 mM

Figure 2: Weight-loss vs. immersion time plots recorded for steel in aerated 0.50 M HCl solutions without and with various concentrations (1.0 - 50 mM) of (a) Ser, (b) Thr or (c) Glu at a scan rate of 0.50 mV s-1 at 30 oC.

380  

This means that the three tested inhibitors are first adsorbed on the electrode

surface impeding corrosion either by merely blocking the reaction sites (anodic and

cathodic) or by altering the mechanism of the anodic and cathodic processes [25]. It

follows from the data of figure 2 that, in all cases, the weight loss decreased (i.e.,

corrosion rate is suppressed), and therefore the corrosion inhibition strengthened, with

increase in inhibitor concentration. This trend may result from the fact that adsorption

and surface coverage increases with the increase in inhibitor concentration.

The numerical values of the slopes of figure 2 were converted into the

corresponding corrosion current density (jcorr)WL values using Faraday,s law. The

recorded (jcorr)WL values were introduced in Eq. (1) to obtain the inhibition efficiency, IWL

(%), and subsequently converted into corrosion rates, υWL (in µm y-1) using Eq. (2).

Table 1 collects all these parameters as a function of the concentration of the three

tested inhibitors at 30 oC. It is obvious that the data obtained from weight-loss

measurements agree with the results recorded using the Tafel extrapolation and EFM

methods (inspect again table 1). In all methods employed, Glu proved to be the most

effective inhibitor among the three tested amino acids.

Effect of temperature

The temperature increases the rate of all electrochemical processes and

influences adsorption equilibria and kinetics as well. Temperature investigations allow

the determination of the apparent activation energy of the corrosion process in

absence and in presence of inhibitor. The obtained results can elucidate the

mechanism of corrosion inhibition (see later).

The effect of temperature on the rate of corrosion of steel in 0.50 M HCl

solutions without and with various concentrations of Ser, Thr or Glu, was also studied

based on weight-loss and Tafel polarization measurements. Figure 3 presents, as an

example, the data obtained for Ser using Tafel polarization measurements. Similar

results were obtained for Thr and Glu (data not shown here). The corresponding

corrosion parameters are depicted in table 2.

It is obvious from figure 3 that increasing the solution temperature, irrespective

of the absence or presence of the inhibitor, leads to increasing the current density

values of the two branches of the polarization curves and consequently the values of

jcorr. Moreover, anodic shifts were observed in the corrosion potential (Ecorr) values at

higher temperatures. This result reflects the enhancement of both the cathodic

hydrogen evolution reaction and anodic corrosion reaction with temperature.

Table 2 reveals that each inhibitor has inhibiting properties at all the studied

temperatures and the values of the inhibition efficiency decrease with temperature

increase. This shows that each inhibitor has experienced a significant decrease in its

protective properties with increase in temperature. This decrease in the protective

properties of the inhibitor with increase in temperature may be connected with two

effects; a certain drawing of the adsorption-desorption equilibrium towards desorption

381  

(meaning that the strength of adsorption process decreases at higher temperatures)

and roughening of the metal surface which results from enhanced corrosion. These

results suggest that physical adsorption may be the type of adsorption of the tested

inhibitors on the steel surface.

Table 2: Electrochemical parameters, inhibition efficiencies (I) and corrosion rates (υ) associated with Tafel polarization and weight-loss measurements, for steel in 0.5 M

HCl solution containing 10 mM Ser, Thr or Glu at different temperatures.

T / K Tafel polarization Weight loss (WL)

Ecorr / mV (SCE)

jcorr / mA cm-2

βa / mV dec-1

-βc / mV dec-1

υTafel (µm y-1)

ITafel(%) υWL (µm y-1)

IWL(%)

for Ser

283 -454 0.02 87 57 0.23 82 0.25 80.77

293 -465 0.05 86 55 0.58 75 0.59 76.05

303 -471 0.07 87 55 0.81 67 0.78 65.91

313 -477 0.12 91 65 1.39 61 1.31 60.23

323 -482 0.22 102 78 2.56 52 2.67 53.77

333 -495 0.43 118 91 5.00 38 4.91 39.82

for Thr

283 -461 0.016 86 59 0.186 82.91 0.178 83.12

293 -467 0.03 85 57 0.349 78.00 0.336 79.13

303 -478 0.055 88 58 0.639 72.77 0.615 72.91

313 -481 0.091 89 63 1.058 65.72 1.066 66.23

323 -488 0.165 96 75 1.918 58.00 1.81 59.22

333 -500 0.353 103 84 4.103 47.05 4.212 48.00

for Glu

283 -481 0.002 85 61 0.023 99.91 0.031 99.95

293 -489 0.003 87 59 0.035 99.72 0.039 99.65

303 -503 0.004 89 61 0.046 98.05 0.033 98.55

313 -508 0.015 91 68 0.174 93.00 0.182 92.12

323 -512 0.113 95 81 1.313 88.91 1.321 87.97

333 -518 0.205 98 89 2.383 83.03 2.371 84.71

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-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0-4

-3

-2

-1

0

1

2

3

4

5

log

(j / A

cm

-2)

E / V(SCE)

10 oC 20 oC 30 oC 40 oC 50 oC 60 oC

Figure 3: Polarization curves recorded for steel in aerated 0.50 M HCl solution containing 10 mM Ser at a scan rate of 0.50 mV s-1 at different temperatures.

Basic information on the interaction between the inhibitor and the steel surface

can be provided by the adsorption isotherm. In order to obtain the isotherm, the linear

relation between the values of the surface coverage (θ), θ = (I%)/100, and the inhibitor

concentration (Cinhib) must be found. Attempts were made to fit the θ values to various

isotherms including Langmiur, Hill de Boer, Parsons, Temkin, Flory-Huggins, Dahar-

Flory-Huggins and Bockris-Swinkel. By far the best fit is obtained with Temkin

isotherm. According to this isotherm, θ is related to Cinhib by:

θ = (1/2a) lnK + (1/2a) lnCinhib ............................................................................ (3)

where a the lateral interaction term describing the molecular interactions in the

adsorption layer and the heterogeneity of the surface and is a measure for the

steepness of the adsorption isotherm. K the adsorption-desorption equilibrium

constant.

The proposed relation is plotted in figure 4 which represents fitting of the data

obtained from the Tafel extrapolation and weight-loss methods with Temkin isotherm

for Ser at 303 K (as a representative example). Similar relation was obtained using

weight-loss and EFM methods at the other studied temperatures. The strong

correlations (R2 = 0.998) confirm the validity of this approach. The values of K obtained

from the Temkin isotherm are listed in Table 3 for the three inhibitors as a function of

temperature. The values of the free energy of adsorption (∆Goads) (Eq. 4) are also

presented in table 3.

∆Goads = RT ln (55.5 x K) ................................................................................... (4)

It follows from the data of table 3 that the values of K are relatively high and

decrease with temperature. It is well-known that large values of K mean better

383  

inhibition efficiency of the inhibitor, i.e., strong electrical interaction between the double

layer existing at the phase boundary and the adsorbing inhibitor molecules. On the

other hand, small values of K, however, compromise such interactions between

adsorbing inhibitor molecules and the metal surface so that the inhibitor molecules are

easily removable by the solvent molecules from the surface. These results confirm the

suggestion that this inhibitor is physically adsorbed and the strength of adsorption

decreases with temperature. Positive values of “a” may indicate the existence of lateral

forces of attraction between adsorbate molecules in the adsorption layer [26]. The high

values of K recorded for Glu indicate stronger adsorption of this compound more than

Ser and Thr.

Table 3: Adsorption parameters (K and a) and the free energy of adsorption (∆Goads) recorded for steel in 0.5 M HCl solution containing 10 mM Ser, Thr or Glu. Data

obtained from the Tafel extrapolation and the weight-loss methods after fitting the

results with the Temkin isotherm at different temperatures.

T / K Tafel polarization Weight-loss

K a -∆Goads / kJ mol-1 K a -∆Goads / kJ mol-1

for Ser

283 27500 3.22 33.44 26833 3.11 33.39

293 11250 2.98 32.45 10591 2.78 32.31

303 4469 2.85 31.24 4298 2.73 31.14

313 2237 2.22 30.47 2181 2.15 30.40

323 1050 1.95 29.42 982 1.88 29.24

333 320 1.87 27.04 272 1.75 26.59

for Thr

283 28561 3.25 33.53 28872 3.31 33.56

293 11880 2.93 32.59 12100 2.83 32.63

303 5594 2.91 31.80 5775 2.93 31.88

313 2750 2.28 31.01 2561 2.32 30.82

323 1220 1.99 28.91 1329 2.11 30.05

333 630 1.85 28.90 743 1.92 29.37

for Glu

283 38620 3.88 34.24 38223 3.92 34.22

293 21000 2.98 33.97 20523 2.95 33.91

303 12200 2.96 33.76 11020 2.91 33.51

313 7228 2.55 33.52 7022 2.66 33.44

323 4050 2.32 33.04 4223 2.44 33.15

333 1988 2.11 32.09 1455 2.33 31.23

384  

-7 -6 -5 -4 -30.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

WL method

Tafel extrapolationmethod

Surf

ace

cove

rage

ln (CSer / M)

Figure 4 – Fitting of the surface coverage, derived from Tafel extrapolation and weight loss methods, for steel in 0.50 M HCl solution containing 10 mM Ser to Temkin

adsorption isotherm at 303 K.

The negative value of ∆Goads ensures the spontaneity of the adsorption process

and stability of the adsorbed layer on the electrode surface. This stability of the

adsorbed layer decreases with increase in temperature. This is clearly seen from the

decrease in the absolute value of ∆Goads with the rise in temperature (see again table

3). This also refers to physical adsorption. Generally, values of ∆Goads less negative

than -40 kJ mol-1 are consistent with the electrostatic interaction between the charged

molecules and the charged metal (physi-sorption); those more negative than -40

kJ/mol involve charge sharing or charge transfer from organic molecules to the metal

surface to form a coordinate type of bond (chemi-sorption) [27,28].

Based on the data presented in table 3, the calculated value of ∆Goads, obtained

from methods employed, was found to be less negative than -40 kJ/mol-1. This

confirms the occurrence of physical adsorption. Thus, the most probable adsorption

mechanism of the tested inhibitors is the physical type (see XPS examinations

section). In general, the proceeding of physical adsorption requires the presence of

both electrically charged metal surface and charged species in the bulk of the solution.

Further insight into the adsorption mechanism is offered by considering the

apparent activation energies (Eao) for the steel dissolution in HCl solutions in the

absence and presence of different concentrations of the tested inhibitor. The apparent

activation energies were obtained by applying the well-known Arrhenius equations [29]:

ln (rate) = - (Eao / RT) + lnA ................................................................................ (5)

Arrhenius plots obtained from the Tafel extrapolation and EFM methods were

constructed for steel in 0.50 M HCl solutions containing various concentrations of Ser,

Thr or Glu in the range of temperature (283-333 K). Figure 5 represents the Arrhenius

385  

plots, obtained from the Tafel extrapolation and weight-loss methods, of steel in 0.50 M

HCl solution containing 10 mM Ser (as an example). Similar trend was obtained for the

other tested concentrations of the three inhibitors. The values of Eao were calculated

from these plots and listed in table 4 as a function of the tested inhibitor concentration.

The value of 57 kJ mol-1 obtained for the activation energy Eao of the corrosion process

in HCl lies in the range of the most frequently cited values, the majority of which are

grouped around 60.7 kJ mol-1 [30]. The higher values of Eao in presence of the inhibitor

than in its absence can be interpreted as an indication of physical adsorption. Values

of Eao are always greater at any given concentration in presence of Glu, reflecting its

higher inhibition performance as compared with Ser and Thr.

Table 4 – Values of the apparent activation energies (Eoa) recorded for steel in 0.5 M HCl solutions without and with various concentrations of Ser. Data obtained from the

Tafel extrapolation and the weight-loss methods after applying Arrhenius equation.

[Inhib.] / mM

Ser Thr Glu

Tafel extrapolation

method

Weight-loss

method

Tafel extrapolation

method

Weight-loss

method

Tafel extrapolation

method

Weight-loss

method

blank 56.91 57.88 56.91 57.88 56.91 57.88

1 62.88 63.12 63.91 63.72 75.23 76.56

2 65.91 66.41 67.87 66.96 79.98 82.00

5 71.00 71.21 74.71 75.13 88.93 90.04

10 75.45 75.90 79.22 81.00 97.97 99.07

20 82.54 83.00 84.82 85.05 109.07 112.08

50 92.05 91.55 95.25 96.61 118.73 122.00

0.0030 0.0031 0.0032 0.0033 0.0034 0.0035 0.0036

-3

-2

-1

0

1

2

3

WL (10 mM Ser)

Tafel (10 mM Ser)

blank (calculated from Tafel)

ln (r

ate

/ mm

y-1)

T-1 / K-1

Figure 5 – Arrhenius plot of the corrosion rate, obtained from the Tafel extrapolation and weight-loss methods, recorded for steel in 0.50 M HCl without and with 10 mM

Ser.

386  

The increase in Eao is proportional to the inhibitor concentration, indicating that

the energy barrier for the corrosion interaction is also increased [31-33]. This means that

the corrosion reaction will be further pushed to the surface sights that are

characterised by progressively higher values of Eao as the concentration of the inhibitor

in the solution becomes larger. In other words, the adsorption of the inhibitor on the

electrode surface leads to the formation of a physical barrier that reduces the metal

reactivity in the electrochemical reactions of corrosion [34].

XPS examinations

The formation of a protective surface film of inhibitor at the electrode surface

was further confirmed by XPS examinations of the electrode surface. Figure 6 presents

the XPS recorded for steel immersed in 0.5 M HCl solution containing 10 mM Ser (as

an example) for 24 h at Ecorr. XPS spectra of the N(1s) confirmed the protonation of

nitrogen in the Ser molecule. The N(1s) spectra presented in figure 6 can be

deconvoluted into component bands at 399.9 ± 0.2 eV (-NH-), 401 ± 0.2 eV (-NH2+)

and 402.9 ± 0.2 eV (quaternary N+), respectively. Two chemically distinct adsorbed Ser

species were detected in the experiment, namely 399.9 eV for the unprotonated N (N:),

and 401.3 eV and 402.8 eV for the protonated N (-NH2+ and quaternary N+).

Two possible mechanisms of inhibitor adsorption can be presented, see the

different modes of adsorption in the insert of figure 6. The first may be direct

physisorption (electrostatic in nature), the most probable mechanism, of the protonated

inhibitor species on the electrode surface already charged with a negative layer of

chemisorbed Cl- ions [35]. The second may be chemical adsorption owing to the

coordinate bonds formed between the lone electron pairs of the unprotonated N-atoms

and the empty orbits of Fe atoms.

The two aminic groups in the Glu structure are two centers of electrostatic

adsorption (both Ser and Thr include only one aminic functional group). It is probable

that, in case of Glu, the two adsorbed nitrogen atoms make the molecule adsorb

parallel to the metal surface. This mode of adsorption suggests that the adsorbed Glu

molecule covers a large area, thereby inhibiting steel corrosion effectively more than

Ser and Thr.

387  

396 398 400 402 404 406 408

Surface chargeat Ecorr

ChemisorbedCl- ion

+Physicaladsorption

Chemicaladsorption

+N

Cl-

Fe

..N

Fe

Binding Energy / eV

(quaternary N+)

(-NH2+-)

(-NH-)N(1s)

Figure 6- The N(1s) photoelectron spectra recorded for steel immersed for 12 h in 0.5 M HCl solution containing 10 mM Ser at Ecorr at 30 oC.

Conclusion In this work, weight-loss, Tafel extrapolation and EFM method were applied to

monitor corrosion rates of a low alloy boiler steel in 0.5 M HCl solutions without and

with various concentrations of three selected amino acids, namely serine (Ser),

therionine (Thr) and glutamine (Glu) at different temperatures. The principle

conclusions are:

The three selected amino acids exhibited inhibiting properties for low alloy steel

corrosion in 0.50 M HCl solutions. Among the tested amino acids, Glu was the most

effective one in inhibition the acid corrosion of the boiler steel alloy.

The inhibition efficiency increases with the increase in inhibitor concentration,

while it decreases with temperature, suggesting the occurrence of physical adsorption.

Tafel polarization plots of these compounds indicated that they act mainly as

cathodic-type inhibitors.

XPS examinations of the steel surface confirm the existence of a protective

adsorbed film of the inhibitor on the low alloy steel surface.

Adsorption of the three tested amino acids was found to follow Temkin isotherm.

The investigated inhibitors were suggested to be physi-sorbed on the electrode

surface on the basis of the low values of the free energy of adsorption (∆Goads < 40 kJ

mol-1).

Apparent activation energies in the presence of any of the tested inhibitors are

higher than that in bare HCl solution, confirming the occurrence of physical adsorption.

Results obtained from the three employed methods were in good agreement.

388  

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