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Electrochimica Acta 52 (2007) 70687074

Evaluation of corrosion and erosioncorrosion resistances of mildsteel in sulfide-containing NaCl aerated solutions

Abdel Salam Hamdy a,b,, A.G. Saeh c,M.A. Shoeib b, Y. Barakat d

a Department of Materials Science and Engineering, Boise State University, Boise, ID, USAbDepartment of Surface Technology and C orrosion Protection, Central Metallurgical Research and Development Institute,

CMRDI, P.O. Box 87, Helwan, Cairo, Egyptc Faculty of Engineering, University of Tripoli, Libya

d Tabbin Institute for Metallurgical Studies, Helwan, Cairo, Egypt

Received 8 March 2007; received in revised form 14 May 2007; accepted 16 May 2007

Available online 21 May 2007

Abstract

This paper reports results of potentiodynamic polarization, electrochemical impedance measurements and erosioncorrosion of mild steel in

aerated sulfide containing 3.5% NaCl solutions at room temperature. The pitting corrosion behavior was studied in NaCl solution containing 0.001,

0.005 and 0.010M Na2S, using potentiodynamic polarization and electrochemical impedance spectroscopy. The erosioncorrosion resistance

was evaluated after rotating the samples in sulfide polluted NaCl solution for 24 h at a velocity of 300, 600 and 900 ppm using a rotating disc

electrode. Results showed that the presence of sulfide ions in NaCl solution resulted in a significant increase in the corrosion attack due to the

local acidification caused by iron sulfide formation. The localized replacement of the protective Fe-oxide film by a non-protective iron sulfide film

is responsible for the pitting and erosioncorrosion attack. The study concluded that the higher the concentration of sulfide in NaCl solution, the

lower the resistance to pitting and erosioncorrosion. Moreover, increasing the solution rotating velocity affects negatively the erosioncorrosion

resistance.

2007 Elsevier Ltd. All rights reserved.

Keywords: Pitting corrosion; Sulfide; Erosioncorrosion; Mild steel; NaCl solution; EIS; Polarization

1. Introduction

The corrosion of steel in sulfide-containing solutions has

received considerable attention for many years due to its

importance in several industrial processes such as oil and gas

production and transport. The evaluation of steel corrosion in

sulfide environments is important in the petrochemical indus-

try, as this phenomenon is responsible for costly economic and

human loss [1].

Mild steel is used to manufacture essential parts and com-

ponents used in the petroleum industry. Because these oil

environments contain sulfides, numerous studies have been

Corresponding author at: Department of Surface Technology and Corrosion

Protection, Central Metallurgical Research and Development Institute, CMRDI,

P.O. Box 87, Helwan, Cairo, Egypt. Tel.: +20 2 501 0640; fax: +20 2 510 0639.

E-mail address: [email protected] (A.S. Hamdy).

made concerning iron sulfide corrosion products formed on

steels under a variety of reaction conditions [13].

In addition, sulfide pollution of seawater at the coastal

areas can occur from industrial waste discharge, biologi-

cal and bacteriological process in seawater (seaweed, marine

organisms or microorganisms, sulfide reducing bacteria) which

promotes aqueous corrosion of different materials includ-

ing steels. It has been reported that the corrosion rates of

materials, such as copper alloys, increases by a factor of

1030 when seawater contains sulfur compounds as impuri-

ties [4]. Corrosion problems can occur when an oil processing

plant is left idle for long periods, with stagnant seawater

(after testing or during downtime). In this case, corrosion

can be particularly aggressive as a result of sulfide pollution

[2].

Therefore, the objective of this work is to evaluate the cor-

rosion and erosioncorrosion resistance of mild steel in 3.5%

NaCl as a function of sulfide content.

0013-4686/$ see front matter 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.electacta.2007.05.034
mailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_4/dx.doi.org/10.1016/j.electacta.2007.05.034http://localhost/var/www/apps/conversion/tmp/scratch_4/dx.doi.org/10.1016/j.electacta.2007.05.034mailto:[email protected]


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A.S. Hamdy et al. / Electrochimica Acta 52 (2007) 70687074 7069

Table 1

Chemical composition of mild steel (wt.%)

Si Mn Ti V Al Cr Ni Mo Nb Cu Co W Pb C Fe

0.087 0.44 0.013 0.026 0.005 0.002 0.0005 0.0009 0.0006 0.003 0.029 0.011 0.011 0.05 Bal.

2. Experimental

2.1. Materials

The electrodes used for these corrosion studies were cut from

a mild steel sheet in the form of 6 cm 3 cm 0.3 cm pieces.

The specimens for erosioncorrosion studies were cut from a

mild steel rod and prepared in cylindrical form with diameter of

0.85 cm and height of 3 cm. The chemical composition of the

mild steel under investigation is shown in Table 1.

2.2. Surface preparation and solutions

Each steel specimen was abraded to a 800 grit finish withSiC paper, degreased in acetone, washed with distilled water,

and dried in dry air. For the erosioncorrosion experiments, the

samples were weighed before immersion in the corrosive solu-

tions. A series of mild steel samples were subjected to corrosion

under the following conditions.

All the experiments were performed at room temperature

under stagnant conditions, except that for erosioncorrosion

tests. Specimens for erosioncorrosion studies were immersed

in the corrosive solutions (listed in Table 2) for 24 h under differ-

ent rotating speeds: 0, 300, 600, and 900 rpm, respectively. After

24 h of rotation, the samples were rinsed with distilled water and

acetone, dried with hotair,and weighed again forweight loss cal-culations. Laboratory grade chemicals and distilled water were

used to prepare all solutions.

3. Methods

3.1. Electrochemical impedance tests

The corrosion behavior of the previous specimens was moni-

tored using electrochemical impedance spectroscopy (EIS) and

DC polarization techniques during immersion in the corrosive

solutions (Table 2). The solutions were open to air and kept at

room temperature for up to one week.

A three-electrode set-up was used with impedance spectrabeing recorded at the corrosion potential ECorr. A saturated

calomel electrode (SCE) was used as the reference electrode.

It was coupled capacitively to a Pt wire to reduce the phase

Table 2

Experimental parameters for corrosion of mild steel

Symbol Electrolyte composition pH Rotating speed (rpm)

Blank Pure 3.5% NaCl 6.5 0, 300, 600, 900

S1 3.5% NaCl + 0.001 M Na2S 8.13 0, 300, 600, 900

S2 3.5% NaCl + 0.005 M Na2S 8.60 0, 300, 600, 900

S3 3.5% NaCl + 0.01 M Na2S 10.88 0, 300, 600, 900

shift at high frequencies. EIS was performed between 0.01 Hz

and 65 kHz frequency range using a frequency response ana-

lyzer (Autolab PGSTAT 30, Eco-Chemie). The amplitude of the

sinusoidal voltage signal was 10 mV.

3.2. Polarization tests

Liner polarization measurements of specimens, previously

immersed for 30 min in the solutions (Table 2), were made

using a scan rate of 0.07 mV/s in the applied potential range

from 0.15 to 0.7 VSCE with respect to ECorr using an Autolab

PGSTAT 30 galvanostat/potentiostat. The corrosion potential

(Ecorr) and corrosion current density (icorr), calculated using the

Tafel extrapolation method [5]. The exposed surface area was25.4 mm2. All curves were normalized to 1 cm2.

3.3. Energy-dispersive spectrometry

After immersing the samples in NaCl solutions for one week,

the samples were washed with deionized water and then dried.

SEM images of the cleaned samples were obtained using a

digital scanning electron microscope (Model JEOL JSM 5410,

Oxford Instruments). Microprobe analysis was performed using

energy dispersive spectrometry (EDS) (Model 6587, Pentafet

Link, Oxford Microanalysis Group).

3.4. X-ray diffraction

After drying the corroded specimen (one week in corrosive

solutions) with methanol, the solid surface corrosion products

were characterized by an X-ray diffractometry (XRD). XRD

analysis was performed using Bruker AXS, Model D8, 40 kV,

40 mA, Cu K ADVANCE.

3.5. Surface morphology

Corrosion morphology was examined with a metallographic

microscope (LEICA DMR) with a Quips programming window,

LEICA Imaging Systems Ltd.

4. Results and discussions

4.1. Effect of sulfide concentrations

The effect of sulfide concentrations on the corrosion of mild

steel in 3.5% NaCl solution was studied using electrochemi-

cal impedance spectroscopy (EIS), polarization resistance and

cyclic potentiodynamic techniques. EIS techniques have been

applied to study pitting corrosion and other localized corrosion

and has been shown to have marked advantages in the study of

interfacial reactions and other interfacial phenomena [512]. In


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7070 A.S. Hamdy et al. / Electrochimica Acta 52 (2007) 70687074

particular, the information obtained from EIS measurements has

time-resolved and surface-averaged characteristics. Moreover,

EIS is a non-destructive technique.

4.1.1. Electrochemical impedance spectroscopy

EIS has been successfully applied to the study of corrosion

systems for 30 years and been proven to be a powerful andaccurate method for measuring corrosion rates especially for

coatings and thin films. An important advantage of EIS over

other laboratory techniques is the possibility of using very small

amplitude signals without significantly disturbing the proper-

ties being measured. To make an EIS measurement, a small

amplitude signal is applied to a specimen over a range of fre-

quencies.

The expression for impedance is composed of a real and an

imaginary part. If the real part is plotted on the z-axis and the

imaginary part on the y-axis of a chart, we get a Nyquist plot.

However, Nyquist plots have one major shortcoming. When you

look at any data point on the plot, you cannot tell what frequency

was used to record that point. Therefore, other impedance plots

such as Bode plots are importantto make a correct interpretation.

In Bode plots, the impedance is plotted with log frequency on the

x-axis and both the absolute value of the impedance (|Z|=Z0)

and phase-shift on the y-axis. Unlike the Nyquist plot, the Bode

plot explicitly shows frequency information.

The impedance measurement technique has been applied

to the study of pitting corrosion and other localized corro-

sion [69]. The impedance technique has marked advantages in

the study of interfacial reactions and other interfacial phenom-

ena. The impedance information obtained has time-resolved and

surface-averaged characteristics [9]. In this work, the corrosion

behavior of mild steel specimens, immersed in NaCl solutioncontaining different sulfide ratios was investigated. Nyquist

plots (Fig. 1a) show the surface resistances after one week of

immersion in sulfide polluted NaCl solutions. The corrosion

resistances decreased with increasing sulfide concentrations.

The surface resistance of mild steel in pure NaCl solution

(Blank) is 2.8 103 cm2. The resistance sharply decreased

to be 0.1 103, 0.4 103, and 0.7 103 cm2 after addition

of 0.001, 0.005, and 0.010 M Na2S, respectively.

Bode plots (Fig. 1b) showed that the surface resistance of the

mild steels was generally decreased by increasing the sulfide

ratio. The samples that immersed in 3.5% NaCl + 0.001 M Na2S

(S1), 3.5% NaCl + 0.005 M Na2S (S2)and3.5% NaCl+ 0.010 MNa2S (S3) showed dramatic decrease of the impedance in the

capacitive region, which is characteristic for the pitting process

on steel. In addition, the phase angle () tended toward zero at

low frequencies, indicating that theresistance of the barrier layer

Fig. 1. (a) Nyquist plots and (b) Bode plots of mild steel after one week of

immersion in 3.5% NaCl containing varying concentrations of Na2S. (Blank)

0 M Na2S; (S1) 0.001 M Na2S; (S2) 0.005 M Na2S; (S3) 0.010 M Na2S.

was being approached. The changes of the spectra at very low

frequencies indicate the occurrence of pitting and were in agree-

ment with visual and SEM inspection. On the other hand, the

samples tested in pure NaCl solution, without sulfide (blank),

showed very stable impedance behavior, even after one week

of immersion, indicating that the presence of sulfide ions in thesolution negatively affects the protectionperformance of the pas-

sive oxide films. This would indicate that sulfide ions interfere

with the protective iron oxide layer to form less protective layers

of iron oxide plus ion sulfide.

4.1.2. Linear polarization measurements

Linear polarization measurements of mild steel in 3.5% NaCl

solution containing 0, 0.001, 0.005, and 0.010 M Na2S, respec-

tively, is shown in Table 3. The corrosion potential (Ecorr)

and corrosion current density (icorr), calculated using the Tafel

extrapolation method [5] are also given in Table 3. It is evident

from Table 3 that the addition of sulfide increases the icorr andcorrosion rates and decreases the polarization resistances (Rp).

The extent of increase in icorr and corrosion rates is found to be

a function of the concentration of Na2S, higher the concentra-

tion of Na2S, higher the values of corrosion rate and larger the

Table 3

Effect of sulfide concentrations on the corrosion behavior of mild steel in 3.5% NaCl solution

Sample Ecorr (mV) vs. SCE icorr (A/cm2) Rp () Corrosion rate (MPY) ba (mV/de) bc (mV/de)

Blank 690 1.144 9.811 8.56103 26 25

S1 724 2.019 5.788 15.11103 28 24

S2 709 2.065 5.127 15.45103 22 28

S3 699 3.790 3.801 28.36103 29 29


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increase in icorr values. The corrosion rate increases more than

three times the value of blank samples when the sulfide con-

centration is increased from 0 to 0.010 M. These results are in

agreement with the previous work of Gudas andHack[13]. They

suggested that soluble sulfide concentrations as low as 7 g/l

could cause significant adverse effects to brass. Soluble sulfides

could increase the overall corrosion rates through acceleration

of anodic, cathodic, or both reactions. The observed negative

shift in Ecorr and the increase in icorr in the presence of sulfide

ions (Table 3) are considered to be due to the change in nature

of surface films on steel and suggest that the corrosion is under

anodic control. It is suggested that the iron oxide layer becomes

defective in the presence of sulfide ions in the medium. This

defective surface layer consists of iron oxide and iron sulfide

that permit rapid ionic and electronic transport through it and

causes an increase in the corrosion rate. The iron sulfide seems

to be non-protective compared with the iron oxide. The presence

of sulfide ions in the electrolyte solution catalyzes the corro-

sion reaction by preventing formation of protective iron oxide

[1420]. Hence, increasing the corrosion rate of mild steel in3.5% NaCl solution containing sulfide (Fig. 2) is due to the for-

mation of non-protective iron sulfide film at the steel surface.

Moreover, the presence of iron sulfide promotes the anodic dis-

solution and accelerates local corrosion under the precipitate

film [19,20].

4.1.3. Potentiodynamic polarization measurements

Localized corrosion is one of the most dangerous corrosion

species for degradation of pipes or containers intended for long-

term use in industry. Pitting is of particular concern because

it may lead to premature breaching of the container materials

through electrochemical dissolution processes that can acceler-ate with time [1520].

The effect of sulfide ion concentrations on the corrosion

behavior of mild steel was studied in NaCl solution. Potentio-

dynamic polarization technique (Fig. 3) was used to evaluate

the pitting corrosion resistance for different samples in NaCl

solution containing sulfide. In this technique, the potential was

recorded starting from a cathodic potential(about1.75 V/SCE)

and be allowed to sweep to the anodic potential direction till it

reaches the pitting potential. At that potential, a sudden shift in

Fig. 2. Effect of sulfide concentrations on the corrosion rate of mild steel after

one week in 3.5% NaCl solution.

Fig. 3. Potentiodynamic polarization curves of mild steel after one week of

immersion in different corrosive solutions.

the current to the active direction will be observed. The pitting

tendency of the mild steel can be estimated by the determi-

nation of pitting potential, Epit. The Epit is always defined as

the potential below which the metal surface remains passive

and above which pits can nucleate and propagate. The differ-

ence between Epit and the corrosion potential ECorr representsthe passive domain for any materials or alloys. In this work,

the difference between (Epit ECorr) was taken as a relative

measure of the passivation ability. The greater the difference

between Epit ECorr is, the greater the passive range and hence,

the materials will be safe from corrosion (Fig. 3).

According to the potentiodynamic data in Table 4 and Fig. 3,

the pitting corrosion resistances decreased with increasing sul-

fide concentrations. The passivity domain of mild steel in pure

NaCl solution (blank) is 221 mV. The passivity domain grad-

ually decreased to be 117, 24, and 4 mV after one week

of immersion in NaCl solution containing 0.001, 0.005, and

0.010 M Na2S, respectively. These results are in agreement withthe EIS results, visual inspection and microscopic examinations.

4.1.4. Surface examinations

SEM micrographs (Fig. 4), optical microscope and visual

inspection of S1, S2 and S3 samples showed severe pitting as

well as crevice corrosion after one week of immersion. The

number of pits exceeded 15 pit/cm2 for S3. Pitting corrosion

was also observed in S1 and S2 samples with limited areas

of crevice corrosion. However, the number of pits was less

than 15 pit/cm2. The best localized corrosion resistance was

observed from the blank samples (without sulfide). Few zones of

crevice corrosion were observed in addition to general corrosion

Fig. 5.

The chemical composition of corrosion products was inves-

tigated by XRD. Generally, iron sulfide and iron oxide (Fe 2O3)

Table 4

Potentiodynamic polarization data for mild steel after one week of immersion

in different corrosive solutions

Sample Epit (mV) ECorr (mV) Epit ECorr (mV)

Blank 523 744 221

S1 696 813 117

S2 745 769 24

S3 776 780 4


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Fig. 4. SEM micrograph of mild steel after one week of immersion in 0, 0.001, 0.005, and 0.010 M sulfide containing NaCl solutions, respectively.

were detected in the surface layers of samples S1, S2 and S3.

The blank samples revealed formation of both ferric and ferrous

oxides. Microprobe analysis using EDS (Table 5), revealed pres-

ence of sulfur for S1 and S3 samples. However, the amount of

sulfur detected in S3 samples is three times higher than S1. The

lowest sulfur content was detected from S2 samples. However,

about 1% of chlorine with the highest oxygen content (66.8)

was also observed in S2. These results may indicate formation

of Fe2O3 and iron oxychloride compound. Although such com-

pounds have an inhibitive action on the corrosion of iron, the

presence of sulfide mixed with such compound increases the

Fig. 5. EDX analysis for mild steel surface layer after one week in 3.5% NaCl

solution + 0.001 M Na2S (S1).

competitive action between the different forms of oxide and

sulfide and hence, decreases the localized corrosion resistances

after one week of immersion. It seems that the presence of sul-

fur inside the outer iron oxide layer decreases the protective

power of such layer to corrosion attack due to the local acidifica-

tion caused by iron sulfide formation. The localized replacement

of the protective Fe-oxide film by a non-protective iron sulfide

(FeS1x) film is responsible for the pitting corrosion. Therefore,

blank samples showed the highest resistance to pitting corro-

sion. Further details about the chemical composition of the outer

surface layers will be discussed in a futurepaper for better under-standing the nature of oxide and sulfide formed due to corrosion

attack especially for S2.

4.2. Effect of solution speed (erosioncorrosion)

In sulfide free 3.5% NaCl solution, the weight loss of mild

steel slowly increases with increasing the rotation speed from 0

to 600 rpm due to the corrosion reaction between the protective

iron oxide layers with chloride ions from the NaCl solution.

Increasing the speed up to 900 rpm will increase the possibility

to remove such protective layer and hence the corrosion rate will

increase dramatically.On the other hand, the corrosion rates increased dramatically

with the increase of rotation speed from 0 to 300 rpm in pres-

ence of Na2S in NaCl solution. The presence of sulfide ions in

NaCl solution resulted in a significant increase in the corrosion

attack due to the local acidification caused by iron sulfide forma-

tion. The localized replacement of the protective Fe-oxide film

by a non-protective iron sulfide film is responsible for increas-

ing the pitting and erosioncorrosion attack. The higher the

concentration of sulfide in NaCl solution is, the lower the resis-

tance to erosioncorrosion. Increasing the rotation speed more

than 300 rpm increases the corrosion rate slowly. This can be

attributed to the removal of sulfide ions from the surface at high


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Table 5

EDS of mild steel after one week of corrosion

Sample Fe, atomic ratio (%) S, atomic ratio (%) O, atomic ratio (%) Cl, atomic ratio (%)

Blank 31.49 68.46

S1 41.13 1.93 55.24

2S 30.62 0.66 66.80 0.92

3S 34.27 5.13 60.43

speed preventing the local acidification which is the main source

of corrosion.

4.3. Effect of immersion time

The immersion time in sulfide containing NaCl solution

was found to have a marked effect on the initiation and for-

mation of pitting corrosion. Nyquist plots (Fig. 6a) show the

surface resistances after one day of immersion in sulfide con-

taining NaCl solutions. Taking the surface resistance of theblank sample as a base, the corrosion resistances of sulfide-

containing samples decreased, as expected, by increasing the

sulfide concentrations as shown from the spectra of S1 and

S3 samples. The surface resistance of the blank sample is

3.3103 cm2. The surface resistance of S1 and S3 decreased

to be 1.6 103 and 1.2 103 cm2, respectively. However,

S2 showed a distinct improve in the corrosion resistance to be

4.6103 cm2.

Accordingto Bode plots (Fig. 6b), S1 and S3 samples showed

a dramatic decrease in the impedance in the capacitive region,

Fig. 6. (a) Nyquistplots and (b)Bode plots ofmildsteel after oneday of immer-

sion in 3.5% NaCl containing varying concentrations of Na2S. (Blank) 0 M

Na2S; (S1) 0.001 M Na2S; (S2) 0.005 M Na2S; (S3) 0.010 M Na2S.

which is characteristic for the pitting process on steel. In addi-

tion, the phase angle () tended toward zero at low frequencies,

indicating that the resistance of the barrier layer was being

approached. In addition, maximum of was observed for the

S1 samples, as evident in Fig. 3b. These changes of the spec-

tra at very low frequencies indicated the occurrence of pitting

and were in agreement with visual and SEM inspection. On the

other hand, S2 samples showed very stable impedance behavior

(Fig. 7a and b), indicating that sulfide ions, especially in the first

hours of immersion, may be temporary converted to intermedi-

ate chemical species such as oxychloride or sulfite and sulfate

compounds which have an inhibitive action on steel [19,20].

Therefore, the pitting resistance increased compared with the

blank samples (Fig. 7a and b) after one day of immersion. After

one week of immersion, the corrosion resistances decreased

sharply for all samples. The surface resistance of mild steel

in pure NaCl solution (Blank) is 2.8 103 cm2. The surface

resistance of S1, S2 and S3 sharply decreased to be 0.1 103,

0.4103, and 0.7 103 cm2, respectively.

Fig. 7. (a)Nyquistplots and(b) Bode plotsof mild steelafterdifferentimmersion

times in 3.5% NaCl containing varying concentrations of Na2S. (Blank) 0 M

Na2S; (S1) 0.001 M Na2S; (S2) 0.005 M Na2S; (S3) 0.010 M Na2S.


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5. Conclusion

1. The presence of sulfide ions in 3.5% NaCl solution results in

a significant increase in the corrosion and erosioncorrosion

attack on mild steel.

2. Formation of a non-protective iron sulfide film instead of the

protective oxide film in sulfide containing solution is due

to the catalyzing ability of the sulfide ions to prevent oxide

formation and hence, increase corrosion attack.

3. Increasing the sulfide concentration affects negatively the

corrosion resistance of mild steel.

4. The solution speed was found also to have a marked effect

on the corrosion resistance. In pure NaCl solution, increasing

the speed increases the corrosion rates. In sulfide containing

NaCl, the corrosion rates increase rapidly with increasing

the solution speed. However, at very high speed the corrosion

rates increase gradually. This can be explained by continuous

removal of the local acidic zones caused by sulfide ions at the

mild steel surface. Hence, preventing corrosion to initiate.

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corrosion cv steel2

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