Corrosion and Electrochemical Properties of Aluminum and Lithium Alloys

7/26/2019 Corrosion and Electrochemical Properties of Aluminum and Lithium Alloys

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FULL LENGTH ARTICLE

Corrosion behavior and electrochemical properties

of carbon steel, commercial pure titanium, copper

and copperaluminumnickel alloy in 3.5% sodium

chloride containing sulfide ions

H. Nady a,b,*, M.M. El-Rabiei a, M. Samy a

a Chemistry Department, Faculty of Science, Fayoum University, Fayoum, Egyptb Chemistry Department, Faculty of Science & Arts in Qurayat, Al-Jouf University, Saudi Arabia

Received 20 December 2015; revised 15 February 2016; accepted 28 February 2016

KEYWORDS

Corrosion resistance;

Titanium;

Copper and copper alloys;

Carbon steel;

EIS;

Passive films

Abstract In this research the electrochemical performance of Cu, Cu10Al10Ni, Cp-Ti and C-

steel in 3.5% sodium chloride containing sulfide ions was investigated. Different electrochemical

methods such as polarization technique and electrochemical impedance spectroscopy, EIS, were

used. Surface examination and morphological studies were employed. The effect of immersion time

of the different alloys was extensively studied by EIS. A comparison was made between the electro-

chemical stability of the different alloys in this media. Polarization measurements reveal that Cp-Ti

and Cu10Al10Ni alloys possess lower corrosion rates than both Cu and carbon steel. EIS mea-

surements, have shown that a more thicker and resistive passive film is formed on Cp-Ti alloy due to

the formation of TiO2film of duplex nature. The incorporation of Ni in the Cu 2O barrier film leads

to the stabilization of Cu10Al10Ni alloy and the stability is enhanced in the presence of Al. The

thickness and resistance of the barrier layer formed on the alloy surfaces increase with the increasing

the immersion time. The formation of such passive film on the surface of different alloys was dis-

cussed. An equivalent circuit model for the electrode/electrolyte interface under different conditions

was proposed. The experimental impedance data were fitted to the theoretical data according to the

proposed model.2016 Egyptian Petroleum Research Institute. Production and hosting by Elsevier B.V. This is an open

access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Hydrogen sulfide, H2S, corrosion has been a research topic for

several decades[1]. Most studies on this subject were based on

an engineering perspective because of fractures in metals

caused by H2S especially in the case of steel [24]. Also the

electrochemical analysis technology has been widely used in

* Corresponding author at: Chemistry Department, Faculty of

Science, Fayoum University, Fayoum, Egypt. Tel.: +20

01069878104, +966 535589807.

E-mail addresses: [email protected], [email protected]

(H. Nady).

Peer review under responsibility of Egyptian Petroleum Research

Institute.

Egyptian Journal of Petroleum (2016) xxx, xxxxxx

HOSTED BY

Egyptian Petroleum Research Institute

Egyptian Journal of Petroleum

www.elsevier.com/locate/egyjpwww.sciencedirect.com

http://dx.doi.org/10.1016/j.ejpe.2016.02.0081110-0621 2016 Egyptian Petroleum Research Institute. Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article in press as: H. Nady et al., Corrosion behavior and electrochemical properties of carbon steel, commercial pure titanium, copper and copperaluminumnickel alloy in 3.5% sodium chloride containing sulfide ions, Egypt. J. Petrol. (2016), http://dx.doi.org/10.1016/j.ejpe.2016.02.008
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recent decades to study the corrosion process and has been

proven to be effective[510]. Copper and copper alloys belong

to the group of alloys having sufficient resistance to corrosion

in seawater. However, they corrode under specific conditions.

Hydrogen sulfide is the most common pollutant which may

cause an intensive corrosion of copper and copper alloys in

seawater where the presence of sulfide ions lead to the modifi-

cation of the oxide layer on copper and copper based alloys.

Acceleration of corrosion is due to the enhancement of thecathodic process. One of the most important parameters play-

ing a major part in the processes of corrosion of copper alloys

in seawater polluted with sulfides is their concentration. Tra-

verso et al. [11] have found that the most intensive corrosion

is caused by sulfides at initial levels of 4 ppm. Gudas and Hack

[12]stated that wrought 90/10 CuNi is susceptible to acceler-

ated attack in seawater containing only 0.01 ppm of sulfides.

Mukhopadhyay, Baskaran [13] and Chauhan, Gadiyar [14]

determined the chemical composition of corrosion products

created on 70/30 CuNi in sulfide polluted seawater. Instead

of Cu2O and green CuCl23Cu(OH)23H2O, a gray layer ofCu2S(NiFe)3S4 and Cu2O was created. In our recent previous

paper the stability of Cu10Al05Ni alloy in chloride solutions

polluted by sulfide ions was studied[15]. Also the effect of Cland H2S on steel weld[1619]was studied. The results demon-

strated that H2S could accelerate both the anodic iron dissolu-

tion and the cathodic hydrogen evolution in most cases, but

some results showed that H2S could also inhibit the corrosion

of iron under certain special conditions [20,21]. Zimer et al.

[22] recently studied the initial stage of pitting corrosion in

AISI 1040 steel through temporal series micrographs coupled

with an open-circuit potential and polarization curves in an

alkaline sulfide solution. Due to a set of excellent properties,

chemical and mechanical properties, Titanium and titanium

based alloys are commonly employed in many functions such

as chemical, marine, space and biomedical, dental and ortho-

pedic applications [2325]. The high corrosion resistance oftitanium in aggressive environments is ensured by a compact

and chemically stable oxide film that spontaneously covers

the metal surface which is amorphous or poorly crystalline

and mainly composed of titanium oxide, TiO2 [26]. Though

much effort has been put into separate individual studies for

each alloy with the aggressive media, H2S and/or NaCl, little

research has been carried out on collection studies including

different alloys in these media. In this work, some common

alloys namely, Cu, Cu10Al10Ni, C-steel and commercially

pure titanium, Cp-Ti, are investigated in both 3.5%NaCl

and 3.5%NaCl polluted by sulfides. The mechanism of corro-

sion processes taking place at the electrode/solution interface

will be discussed, and a categorization of the different investi-

gated alloys according to their stability in this aggressive med-ium can be made.

2. Experimental details

2.1. Materials and sample preparation

The working electrodes were made from commercial grade Cu,

Cu10Al10Ni, carbon steel and commercial pure Ti. The rods

were prepared in metallurgical work shop. The prepared alloys

are cast alloys without any special treatment. The metallic rods

are mounted into glass tubes by two-component epoxy resin

leaving a surface area of 0.2 cm2 to contact the solution.

Chemical composition of Titanium in mass percent: 0.30 Fe,

0.25 O, 0.06 C, 0.03 N, 0.015 H and balance Ti. The chemical

composition of carbon steel is: 0.34 C, 0.93 Mn, 0.26 Si, 0.0.2

S, 0.04 P, 0.01 Cu, 0.01 Cr, 0.02 Ni and balance Fe. The chem-

ical composition of pure Cu and Cu10Al10Ni alloy elec-

trodes is: Cu (99.928 Cu, 0.001 Al, 0.02 Ni, 0.007 Zn, 0.003

Mn, 0.033Sn, 0.002 Fe, 0.004 Si, 0.001 Mg and 0.001 Ti) and

Cu10Al10Ni (76.00 Cu, 11.28 Al, 9.95 Ni, 0.10 Zn, 0.02Mn, 0.14 Sn, 2.26 Fe, 0.24 Si, 0.01 Mg and 0.01Ti). The elec-

trochemical cell is a three-electrode all-glass cell, with a plat-

inum spiral counter electrode and saturated calomel, SCE,

reference electrode. Before each experiment, the working elec-

trode was polished using successive grades emery papers down

to 2500 grit, washed thoroughly with bi-distilled water, then

transferred quickly to the electrolytic cell. The electrochemical

measurements were carried out in a stagnant, naturally aerated

3.5 (mass, m/volume, v)% NaCl containing 2 ppm S2 solu-

tion. The source of sulfide ions in this research is Na 2S salt.

2.2. Electrochemical measurements

The corrosion behavior of the specimens under investigation

were monitored using electrochemical impedance spec-

troscopy, EIS, and DC polarization techniques during immer-

sion in 3.5% NaCl and 3.5% NaCl containing 2 ppm S2

solution open to air and at room temperature. The polariza-

tion experiments and EIS, investigations were performed using

a Voltalab PGZ 100 All-in-one Potentiostat/Galvanostat.

The potentials were measured against and referred to the stan-

dard potential of the SCE (0.245 V vs. the standard hydrogen

electrode, SHE). To achieve quasi-stationary condition, the

polarization experiments were carried out using a scan rate

of 5 mV s1. The total impedance, Z, and phase shift, h, were

measured in the frequency range from 105 to 0.1 Hz. The

superimposed ac-signal amplitude was 10 mV peak to peakas assigned by measuring system. To achieve reproducibility,

each experiment was carried out at least twice, where a range

of 23 mV was considered to be reasonable for open-circuit

potential measurements and 5 mV for the polarization experi-

ments. The surface morphology of some electrodes was inves-

tigated by scanning electron microscopy, SEM (Model Quanta

250 FEG, FEI Company, Netherlands).

3. Results and discussion

3.1. Potentiodynamic polarization measurements

The electrochemical behavior of the different alloys was inves-tigated by the polarization technique. Tafel extrapolation mea-

surements were used. The shape of polarization curves can

obtain information on the kinetics of the corrosion reactions.

The polarization curves of the alloys were recorded after

240 min of electrode immersion in stagnant naturally aerated

3.5% NaCl containing 2 ppm S2 ions at a scan rate of

10 mV/s and 25C. Polarization curves for Cu, Cu10Al

10Ni, C-steel and Cp-Ti are presented inFig. 1. The fitted val-

ues of corresponding kinetic parameters such as anodic and

cathodic Tafel slope (ba and bcrespectively), corrosion poten-

tials, Ecorr, and corrosion current densities, icorr, are listed in

Table 1. Polarization curve for Cu shows an obvious anodic

2 H. Nady et al.

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behavior with the most shifted corrosion potential to negative

values, Ecorr 1.351 V and a high corrosion current densityapproximately of 63.6lA cm2. As the electrode potential

was increased to above Ecorr, the anodic current increased

and gradually decreased resulting in an anodic peak at 0.9 V,

While the addition of the alloying elements, Al and Ni to pure

Cu results in shifting the corrosion potential, Ecorr to more

positive values0.609 V and decreasing the corrosion currentdensity to a minimum value of 30lA cm2 just as in the case

of Cp-Ti alloy, it exhibits the smallest current density of the

same value with corrosion potential at about 0.972 V. Tiexhibited a passivation characteristic extending the widest pas-

sive region from 0.950 V to 0.35 V. Although the cor-

rosion potential of carbon steel slightly shifted from, close to,that of titanium by 37 mV, carbon steel showed an obvi-ous anodic behavior with the largest corrosion current density

of approximately 69.7lA cm2. The corrosion current density

increases gradually as the potential increases and the passive

region only extended from 0.9 to 0.6 V. Furtherincreasing the potential resulted in an abrupt current at 0.6 V indicating a passivity break down of non-protectivenature of film formed. As indicated from the polarization

parameters the corrosion rates of the different alloys obey

the sequence, C-steel > Cu > Cu10Al10Ni = Cp-Ti alloy.

The effect of the addition of sulfide ions to chloride solution

on the corrosion behavior of the different alloys is clearly pre-

sented inFig. 2. It presents the polarization curves of the four

alloys, Cu (a), Cu10Al10Ni (b), C-Steel (c) and Cp-Ti (d)

after 240 min of electrodes immersion in sulfide free chloride

solutions and containing sulfide ions. It is clear that the addi-

tion of sulfide ions to the chloride solution increases the corro-

sion current density; also, it shifts the corrosion potential to

more negative values for all alloys except in the case of tita-

nium where it shifted to more positive values. Interestingly,

the current densities for Cu, Cu10Al10Ni, and C-Steel inthe anodic branch of the polarization curves increased in the

presence of sulfide while in the case of Cp-Ti the anodic cur-

rent branches in the absence and presence of sulfide nearly

coincided with each other which may be meaning that the high

stability of Cp-Ti not much affected by the presence of sulfide

ions. For comparison, the characteristic parameter, corrosion

current densities, icorr, Tafel slops, (ba and bc), and corrosion

potentials,Ecorr of the different alloys in chloride 3.5% NaCl

solutions are presented inTable 2. The accepted cathodic reac-

tion occurring on Cu and Cu-based alloys in aqueous solutions

is always represented by the oxygen reduction [27]:

O2 2H2O 4e ! 4OH 1

The anodic dissolution and film formation on Cu surface inchloride solutions can be represented by the reactions [27,28]:

Cu 2Cl ! CuCl2 e 2

It was suggested that the presence of CuCl 2 at the metal

surface leads to hydrolysis reaction and the formation of

Cu2O[29]according to:

2CuCl2 H2O ! Cu2O 4Cl 2H 3

In Al containing alloys the surface dissolution of Al leads to

an additional passivation process and formation of Al2O3according to[29,30]:

Al 4Cl ! AlCl4 3e 4

2AlCl4 3H2O ! Al2O3 6H 8Cl 5

Accordingly, the corrosion resistance of Cu10Al10Ni

alloy increases and is enhanced in the presence of Ni. The pres-

ence of Ni with copper leads to its segregation in the Cu2O

layer resulting in a doped Cu2O layer thus corrosion resistance

is increased [3034]. Therefore, Cu10Al10Ni alloy exhibits

lower current density than pure copper, 7.4lA cm2, in chlo-

ride solutions. In sulfide containing solutions HS- will be

formed:

S2 H2O $ HS OH 6

HS will combine with the metallic copper to form an

adsorbed precursor of the oxidation reaction:

Cu HS ! CuHSads 7

The coupled anodic dissolution is then represented by Eq.

(8), which is followed by dissociation and recombination pro-

cesses [35]:

CuHS ! CuHS e 8

CuHS ! Cu HS 9

2Cu HS OH ! Cu2S H2O 10

-1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

-8

-7

-6

-5

-4

-3

-2

-1

0

1

3.5% NaCl + 2 ppm S2-

Cu

C-Steel

Cp-Ti

Cu-10Al-10Nilog(Curr

entdensity/Acm-2)

Potential vs. SCE (V)

Figure 1 Potentiodynamic polarization curves of Cu, Cu10Al

10Ni, C-Steel and Ti measured in 3.5 wt.% NaCl + 2 ppm S 2

aqueous solution at 25 C.

Table 1 Polarization parameters of the different materials

under investigation after electrode immersion in 3.5% NaCl

containing 2 ppm S2 ions at 25 C.

Alloys Ecorr/

mVSCE

icorr/

lA cm2ba/

mV dec1bc/

mV dec1

Cu 1351 63.6 122.2 67Cu10Al10Ni 609 30 146 101Cp-Ti 972 30 34.7 19.3Carbon steel 1009 69.7 125.3 86.6

Corrosion behavior and electrochemical properties 3



4/16

Therefore the overall reaction is:

4Cu 2S2 O2 2H2O $ 2Cu2S 4OH 11

However, the presence of Cu2S prevents the protective

effectiveness of Cu2O film which is always formed in aqueous

solutions, and so the passive film resistance of Cu and Cu

10Al10Ni decreases in sulfide polluted chloride solutions.

Under conditions of low (SH) and high Cl the formation

of the soluble chloride complex, CuCl2, by the reaction with

the surface intermediate, Cu(SH)ads, could compete with the

film formation reaction [36]. Dissolution as CuCl2 via Eq.

(12), when SH concentration is low at the Cu alloy surface,

could lead to the transport of Cu+ through pores to the Cu2-

S/solution interface. At a higher SH- concentration, Cu2S for-

mation would occur via Eq. (13) and this reaction will

dominate at the film/electrolyte interface:

CuHS 2Cl ! CuCl HS e 12

2CuCl2 HS ! 2Cu2S 4Cl

H 13

Such a transportdeposition process would account for the

fine particles, which nucleate and grow on the alloy surfaces at

short times.

As a result of its higher corrosion rate compared to other

different alloys, C-steel exhibits passive region, only, extends

to a small range of potential then, the current suddenly raises

steeply without any sign for oxygen evolution, denoting

the breakdown of the passive film and the initiation and

-1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6-7

-6

-5

-4

-3

-2

-1(a)

Cu

3.5% NaCl

3.5% NaCl + 2 ppm S2-

log(currentdensity/Acm-2)


-900 -800 -700 -600 -500 -400 -300 -200 -100 0

-8

-7

-6

-5

-4

-3

-2

(b)Cu-10Al-10Ni

3.5 % NaCl

3.5 % NaCl + 2 ppm S--

log(currentdensity,i/Acm-2)


-1.2 -1.0 -0.8 -0.6 -0.4-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0(c)

C-Steel

3.5% NaCl

3.5% NaCl + 2 ppm S2-

log(currentdensity/Acm-2)


-1.2 -1.0 -0.8 -0.6 -0.4

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0(d)Cp-Ti

3.5% NaCl

3.5% NaCl + 2 ppm S2-

log(currentdensity/Acm

-2)


Figure 2 Potentiodynamic polarization curves of Cu (a),Cu10Al10Ni (b), C-Steel (c) and Cp-Ti (d), alloys after 3 h immersion in

stagnant naturally aerated 3.5% NaCl and 3.5% NaCl containing 2 ppm S 2 solutions at 25 C.

Table 2 Polarization parameters of the different materials

under investigation after electrode immersion in 3.5% NaCl at

25 C.

Alloys Ecorr/

mVSCE

icorr/

lA cm2ba/

mV dec1bc/

mV dec1

Cu 601 53.5 125.3 66

Cu10Al10Ni 464 7.4 122 118Cp-Ti 1096.6 22.6 41.9 35.7Carbon steel 999.6 54.9 112.9 84.7

4 H. Nady et al.



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propagation of pitting attack (cf. Fig 2c). Consequently the

influence of Cl ions on the corrosion rate, in case of C-

steel, can be interpreted as a balance between two processes

competing on the metal surface, stabilization of the passive

film by OH adsorption and disruption of the film by Cl ions

adsorption. When the activity of chlorides overcomes that of

hydroxyls, corrosion occurs[37]. During corrosion, the rever-

sible formation of Fe(OH)ad adsorbed on the bare metal is

the first process and followed by the following sequence[38,39]:

Fe OH ! FeOHads 14

FeOHad! FeOHads e 15

FeOHads OH ! FeOH2 e

16

FeOH2 OH ! FeOOH H2O e

17

In the presence of chloride ions, the Fe(OH)ads coverage

decreases resulting in an increase in the anodic dissolution of

the metal where the following reactions occur:

Fe Cl ! FeClads e 18

FeClads ! FeClaq e

19

From the results of the potentiodynamic polarization curves

mentioned above (cf. Fig 2c) the cathodic behavior of C-steel

in the absence and presence of S2 was very similar, while

the anodic process of C-steel was accelerated in the presence

of S2. Also the passivity break down occurs at more negative

values in the presence hydrogen sulfide. This attributed to the

easy adsorption of S2 molecules on carbon steel surface and

those molecules will first occupy the active site on the steel sur-

face, thereby accelerating the steel dissolution[21]. The anodic

and cathodic processes of C-steel in H2S-containing solution

could be described by the following reactions [19,40]:Anodic

reaction:

Fe ! Fe2 2e 20

Cathodic reaction:

2H 2e ! H2 and=or 21

2H2S 2e ! 2HS H2 22

A probable formation mechanism of corrosion products

due to the easy adsorption of H2S or SH on the surface of

C-steel is as follows[20,21]:

Fe H2S H2O FeSHads H3O

and=or 23

Fe HS FeSHads 24

FeSHads ! FeSHads 2e

25

The species FeSH+ads on the electrode surface could be

incorporated directly into a layer of iron sulfide (mackinawite)

via the following reaction[41]:

FeSHads ! FeS1x xSH 1 xH 26

The amount of the corrosion products adsorbed also

increased as S2 concentration increases. However, the corro-

sion product film was loose and had some defects in its solid

structure. In addition, the protective ability of mackinawite

in the corrosion products was worse than that of troilite (iron

sulfide with the simple formula, FeS)[21]. Consequently, even

if appreciable amounts of mackinawite deposited on carbon

steel surface at higher H2S concentration, what the sulfide film

did not exhibit contribution to the protective effect. Therefore,

the corrosion rates of C-steel increased[21].

In contrast to Cp-Ti, as the potential increases, extends

over a wide range of potentials, the current density is very

low and steady in the passive regions in both free and pollutedH2S sodium chloride solutions (cf. Fig. 2d). This is attributed

to the naturally formed stable passive film, TiO2. It is generally

accepted that the mechanism of the passive film formation is

determined completely by the diffusion of metal ions to the

surface and the subsequent transfer reactions at the interface.

The formation of passive layer requires transfer of titanium

and hydroxyl ions as follows [42]:

Ti ! Ti2 2e 27

Ti2+ is unstable and once it is formed it will react with H2O

to produce Ti3+:

2Ti2 2H2O ! 2Ti3 2OH H2 28

Ti3 3OH ! TiOH3 29

Transformation of Ti(OH)3 might be taking place to

hydrated TiO2 layer in a dynamic equilibrium reaction as

follows:

2TiOH3 2TiO2 H2O H2 30

Accordingly, TiO2 formed by migration and adsorption of

the OH ions through the structure defects on the thinnest

part of the oxide film [42]. The high corrosion resistance of

Cp-Ti is due to the strong adherence of oxide layer, TiO2,

Figure 3 (a) Equivalent circuit model used in the fitting of the

impedance data at different conditions, Rs= solution resistance,

Rct= charge-transfer resistance, Cdl = double layer capacitance,

Rpf= passive film resistance and Cpf = passive film capacitance.

(b) Equivalent circuit model used in the fitting of the impedance

data for C-steel in 3.5% NaCl solution.




6/16

which exhibits a duplex structure with an inner layer (thick and

microcrystalline) and an outer layer (with grains). Substan-

tially, the formation of the adherent TiO2 oxide layer and

the incorporation of Ni2+ into Cu2O layer explains why both

Cp-Ti and Cu10Al10Ni alloys exhibit the lowest corrosion

rates than both Cu and C-steel alloys.

3.2. Electrochemical impedance spectroscopic investigations

The experimental impedance spectra obtained, at OCP, with

the different materials, Cu, Cu10Al10Ni, Cp-Ti and C-

steel exposed to 3.5% NaCl and 3.5% NaCl + 2 ppm S2 at

different intervals of electrode immersion are presented in

the form of Bode and Nyquist plots in ( Figs. 47).The impe-

dance data were analyzed using software provided with the

impedance system where the dispersion formula was used.

For a simple equivalent circuit model, consisting of a parallel

combination of a capacitor, Cdl, and a resistor, Rct, in series

with a resistor, Rs, representing the solution resistance, the

electrode impedance, Z, is represented by the mathematical

formulation:

Z Rs Rct=1 2pfRctCdla

31

where, a represents an empirical parameter (0 6 a 6 1) andfis

the frequency in Hz [43]. The dispersion formula takes into

account the deviation from the ideal capacitor (RC) behavior

in terms of a distribution of time constants due to surface inho-

mogeneity, roughness effects, and variations in properties or

composition of surface layers[44,45]. To account for the pres-

ence of a passive or barrier film, the impedance data were ana-

lyzed using the equivalent circuit model shown inFig. 3, where

another combination of Rpf Cpf representing the passive filmresistance, Rpf, and the passive film capacitance, Cpf, was

introduced.

Fig. 4 represents the Bode and Nyquist plots of Cu elec-

trode after different times of immersion in stagnant naturally

aerated 3.5% NaCl {cf. Fig. 4(a1 and b1} and 3.5% NaCl

+ 2 ppm S2 {cf. Fig. 4(a2 and b2)} solutions at 25 C. The

initial impedance (Z) recorded 15 min after electrode immer-

sion drifts continuously toward an increasing value, which

indicates a decrease in the corrosion rate of Cu with time.

From the respective phase angle Bode plots {cf. Fig. 4(a1

and a2)}, two phase maxima were present, the major one at

lower frequencies and the other as a shoulder at intermediate

frequencies, such behavior indicates the presence of two time

constants representing the electrode processes. The phase max-

-2 0 2 4 6

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Cu in 3.5% NaCl

log (f / HZ)

log(Z/cm2)

0

-20

-40

-60

-80a1

15 min.

60 min.

120 min

180 min.

240 min.

Phase/degree.

0 100 200 300 400 500

0

30

60

90

120

150

180

210

b1

Cu/ 3.5% NaCl

15 min.

60 min.

120 min

180 min.240 min.

-Zi/cm2

Zr/

cm2

-2 0 2 4 6

0.0

0.5

1.0

1.5

2.0

2.5

log (f / HZ)

log(Z/

cm2)

20

0

-20

-40

-60

-80a2

Cu / 3.5% NaCl + 2pp S--

Phase/degree.

15 min.

60 min.

120 min

180 min.

240 min.

0 40 80 120 160 200 240 280

0

20

40

60

80

100

120

b2

Cu / 3.5% NaCl + 2pp S--

15 min.

60 min.

120 min

180 min.

240 min.

-Zi/cm

2

Zr/

cm2

Figure 4 Bode (a) and Nyquist (b) plots of Cu electrode after different times of immersion in stagnant naturally aerated 3.5% NaCl (a1

and b1) and 3.5% NaCl + 2 ppm S2 (a2 and b2) solutions at 25 C.

6 H. Nady et al.



7/16

ima appearing in the low frequency range were attributed to

the protective oxide film and the phase maximum at the inter-

mediate frequency range corresponds to the electrical double

layer. The changes in the phase maximum with time indicate

the differences in the relaxation time constants at different

exposure times. From the corresponding phase angle Bode

plots, it can be seen that the angle values at the low frequency

zone increase with the immersion time, which indicates a

decrease in the corrosion rate of the alloy with time. But thehigh of phase angle decrease in presence of sulfide. The total

impedance magnitude values of the Cu increases with the

increase in exposure time implying an increase in the corrosion

resistance due to the growth of the passive film till the steady

state is achieved {cf.Fig. 4(a1 and a2)}, where the passive film

resistance,Rpf, recorded after 240 min in 3.5% NaCl + 2 ppm

S2 decreases to half its value in 3.5% NaCl. Nyquist plot of

the impedance diagrams exhibited an incomplete capacitive

reactance and a slash with a tilt angle in the low-frequency

region in 3.5% NaCl {cf. Fig. 4b1}. This behavior is typical

Warburg impedance indicating the presence of diffusion pro-

cess, diffusion of reactants and/or products to the interface

[46]. In case of Cu the appearance of the Warburg impedance

in the impedance spectrum could be attributed to the diffusion

of dissolved oxygen from the bulk solution to the Cu surface

[47]. With sulfide ion addition, the diffusion process, i.e. War-

burg impedance disappeared and a complete capacitive loop

was observed {cf.Fig. 4b2}.The same behavior was observed for Cu10Al10Ni alloy

in sulfide free and sulfide polluted 3.5% NaCl solution as

described inFig. 5. But the presence of alloying elements, Al

and Ni result in the broadness of the phase angle more than

that recorded for Cu {cf. Figs. 4(a1 and a2) and 5 (a3 and

a4)}. Also it is clear that the total impedance increases with

the increase in the immersion time, and higher than that

recoded for Cu, which indicates that the protective film formed

on the Cu10Al10Ni alloy becomes more stable with time

-2 0 2 4 60.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Cu-Al-Ni/ 3.5% NaCl

log (f / HZ)

log(Z/cm2)

0

-20

-40

-60

a3

15 min.

60 min.

120 min

180 min.

240 min.

Phase/degree.

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

0.0

0.1

0.2

0.3

0.4

0.5

0.6b3

Cu-Al-Ni 3.5% NaCl

15 min.

60 min.

120 min

180 min.

240 min.

-Zi/

cm2

Zr/ cm

2

-2 0 2 4 60.0

0.5

1.0

1.5

2.0

2.5

3.0

log (f / HZ)

log(Z

/

cm2)

20

0

-20

-40

-60

-80

a4

15 min.

60 min.

120 min

180 min.

240 min.

Cu-Al-Ni/ 3.5% NaCl+ 2ppm S--

Phase/

degree.

-100 0 100 200 300 400 500 600 700 800 900

0

100

200

300

400

500

b4Cu-Al-Ni/ 3.5% NaCl+ 2ppm S

--

15 min.

60 min.

120 min

180 min.

240 min.

-Zi/

cm2

Zr/cm

2

Figure 5 Bode (a) and Nyquist (b) plots of CuAl10Ni electrode after different times of immersion in stagnant naturally aerated 3.5%

NaCl (a3 and b3) and 3.5% NaCl +2 ppm S2 (a4 and b4) solutions at 25 C.




8/16

than that formed on Cu. This attributed to segregation of Ni,

where Ni2+ from the alloy dissolution incorporated into the

crystal lattice of Cu2O and the number of cation vacancies

decreases with increasing the nickel content[30,31], thats rep-

resents not inconsiderable ratio about 10% from the alloy con-

tent. Segregation into the Cu2O barrier layer occurs via a solid

state reaction and Ni2+ interacts with mobile cation vacancies

which leads to a decrease in the ionic conductivity and an

increase in the electronic conductivity of the barrier film lead-ing to higher corrosion resistance[33,34]. Based on these stud-

ies and EIS observations, the electrical equivalent circuit

shown in Fig. 3a was used to fit the experimental data for

Cu and Cu10Al10Ni alloys, where the calculated equivalent

circuit parameters at different immersion time in 3.5%NaCl

and 3.5% NaCl + 2 ppm S2 are presented in (Tables 3 and

4). From the equivalent circuit parameters, it is evident that

passive film resistance, Rpf, recorded in the absence of sulfide

ions is bigger than that observed in the presence of sulfide ions.

This behavior was attributed to the diffusion of sulfide ions

into the protective oxide layer, reacting with Cu ions to form

a non-adherent copper sulfide film[48].

Bode and Nyquist plots impedance diagrams of C-steel dur-

ing the early stages of corrosion for different immersion peri-ods are shown in Fig. 6. When C-steel was immersed in

aqueous 3.5% NaCl solution from 15 min to 240 min the Bode

plots exhibited one phase maxima at the intermediate fre-

quency{cf.Fig. 6(a5 and b5)}. Also Nyquist diagram approx-

imated by a single capacitive semi-circle, showing that the

corrosion process was mainly charge transfer controlled [49].

But when the electrode was immersed 3.5% NaCl was polluted

by 2 ppm sulfide ions {cf. Fig. 6(a6 and b6)}; the Bode plots

presented two phase maxima at high and low frequencies. In

addition to Nyquist plots the Nyquist presented two capacitive

reactance, the capacitive reactance in the high frequency region

corresponded to the electric double layer behavior at the inter-

face, whereas the incomplete capacitive reactance arc corre-sponded to the corrosion products in the low frequency

region. The equivalent circuit model used to fit the impedance

data when the electrode was exposed in 3.5% NaCl solution is

illustrated in Fig. 3b. In the circuit, Rs is the solution resis-

tance,Rcorr, corrosion resistance or charge transfer resistance,

while the equivalent circuit model for the C-steel immersed in

3.5% NaCl + 2 ppm S2 is illustrated inFig. 3a. In this case,

Rct, is the charge transfer resistance,Cdl, charge transfer capac-

itance,Cpf, is the corrosion product film capacitance and Rpf,

represents the corrosion product resistance. The equivalent cir-

cuit parameters obtained in the two cases are depicted in

Table 5. Its clearly shown that the corrosion resistance

increases as the immersion time increases. This result is attrib-

uted to the generation and gradual increase of corrosion prod-ucts at the electrode surface.

In case of Cp-Ti, the Nyquist plots, as seen inFig. 7(b7 and

b8) showed two relaxation time constants. This is also indi-

cated by the two peaks in the phase angle plot at the high

Table 3 Equivalent circuit parameters for the Cu electrode in 3.5% NaCl solution in the absence and presence 2 ppm of sulfide ions at

25 C.

Solutions Time/min. Rs/X Rct/Xcm2 Cdl/lF cm

2 a1 Rf/Xcm2 Cf/lF cm

2 a2

3.5% NaCl 15 1.2 202.3 24.9 0.99 281.0 89.5 0.99

60 1.0 258.7 30.8 0.99 513.5 247.9 0.99

120 1.0 342.6 37.2 0.99 482.7 263.7 0.99

180 1.1 317.0 25.1 0.99 533.7 188.4 0.99

240 1.24 350.7 22.7 0.99 532.1 148.7 0.99

3.5% NaCl + 2 ppm S2 15 1.6 111.0 35.8 0.99 140.4 283.2 0.99

60 2.4 152.4 52.2 1.0 138.2 363.8 0.99

120 2.8 158.1 201.3 1.0 156.5 406.7 0.99

180 4.8 152.3 131.6 1.0 191.0 416.5 0.99

240 4.7 216.5 146.9 1.0 218.4 581.6 0.99

Table 4 Equivalent circuit parameters for the Cu10Al10Ni in 3.5% NaCl solution in the absence and presence 2 ppm of sulfide ions

at 25 C.



2 a2

3.5% NaCl 15 2.5 260.5 12.2 0.99 973.6 326.9 0.99

60 2.5 432.0 18.4 1.0 1049.0 303.3 0.99

120 2.6 938.7 13.6 1.0 1060.0 189.1 0.99

180 2.8 734.5 5.42 1.0 1430.0 175.8 0.99

240 2.9 1060.0 4.7 1.0 1692.0 188.1 0.99

3.5% NaCl + 2 ppm S2 15 2.6 374.8 84.9 1.0 424.8 187.2 0.99

60 2.7 464.3 108.2 1.0 496.9 320.2 0.99

120 2.7 641.3 99.3 1.0 732.2 434.6 0.99

180 2.9 774.5 129.8 1.0 954.4 666.9 0.99

240 3.0 966.2 164.7 0.99 1418 897.4 0.99

8 H. Nady et al.



9/16

and low frequencies where the phase angles increase as the

immersion time increased {cf.Fig. 7(a7 and a8)}. These behav-

iors observed evinced the formation of a duplex film over Cp-

Ti. According to reports, the duplex film formed is composed

of a dense inner barrier layer and a porous outer layer [50,51].

Its clearly shown that, the phase angle at high frequency

depressed in the presence of H2S. In Bode plots, curves that

are represents the variation of frequency with total impedance,

a straight line were observed with the slope approaching 1{cf.Fig. 7(a7 and a8)}. The resistance gradually increased with

-2 0 2 4 60.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

log (f / HZ)

log(

Z/

cm2)

10

0

-10

-20

-30

-40a5 Steel/ 3.5 % NaCl

15 min.

60 min.

120 min

180 min.

240 min.

Phase

/degree.

0 5 10 15 20 25 30

0

2

4

6

8

10

12

b5

Steel/ 3.5% NaCl

15 min.

60 min.

120 min

180 min.

240 min.

-Zi/

cm2

Zr/

cm2

-2 -1 0 1 2 3 4 5 60.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6 Steel/ 3.5% NaCl + 2ppm S--

log (f / HZ)

log(Z/cm2)

10

0

-10

-20

-30

-40

-50a6

15 min.

60 min.

120 min

180 min.

240 min.

Phase/degree.

0 5 10 15 20 25 30

0

5

10

15

20

b6Steel/ 3.5% NaCl+ 2ppm S

--

15 min.

60 min.120 min

180 min.

240 min.

-Zi/

cm2

Zr/ cm

2

Figure 6 Bode (a) and Nyquist (b) plots of Carbon-steel electrode after different times of immersion in stagnant naturally aerated 3.5%NaCl (a5 and b5) and 3.5% NaCl +2 ppm S2 (a6 and b6) solutions at 25 C.

Table 5 Equivalent circuit parameters for the C-steel in 3.5% NaCl solution in the absence and presence 2 ppm of sulfide ion at 25C.



2 a2

3.5% NaCl 15 1.8 25.6 1.97 0.99

60 1.7 39.6 10.1 0.99

120 1.5 47.8 21.1 0.99

180 1.8 39.3 8.1 0.99

240 1.7 23.9 8.4 0.99

3.5% NaCl + 2 ppm S2

15 1.8 2.12 555 0.98 25.8 6.8 0.9960 1.9 1.91 995 0.93 48.3 7.5 0.99

120 1.7 1.87 1310 0.94 54.6 9.2 0.99

180 1.8 1.82 1150 0.96 60.6 13.1 0.99

240 1.9 1.67 1113 0.97 68.8 18.5 1.0




10/16

the increase in the immersion time. This constant increase in

resistance attributed to a consequence progressive thickening

of the film. The obtained spectra were interpreted with the cir-

cuit elements representing the electrochemical properties of the

alloy and its oxide film. Fig 3a shows the equivalent circuit

models used for fitting the spectra at different immersion

-2 0 2 4 60

1

2

3

4

log (f / HZ)

log(Z

/

cm2)

0

-20

-40

-60

-80a7

Ti/ 3.5% NaCl

Phase/

degree.

15 min.

60 min.

120 min

180 min.

240 min.

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5-1

0

1

2

3

45

6

7

8

9

10

11b7

b8

15 min.

60 min.

120 min

180 min.

240 min.

Ti/ 3.5% NaCl

-Zi/

cm2

Zr/ cm

2

-2 0 2 4 60

1

2

3

4Ti/ 3.5% NaCl + 2ppm S

--

log (f / HZ)

log(Z/

cm2)

0

-20

-40

-60

-80a8

Phase/degree.

15 min.

60 min.

120 min

180 min.

240 min.

0.0 0.3 0.6 0.9 1.2 1.5 1.8 2.1

0

1

2

3

4

5

6

Ti/ 3.5% NaCl+ 2ppm S--

15 min.

60 min.

120 min

180 min.

240 min.

-Zi/

cm2

Zr/cm

2

Figure 7 Bode (a) and Nyquist (b) plots of Ti electrode after different times of immersion in stagnant naturally aerated 3.5% NaCl (a7

and b7) and 3.5% NaCl + 2 ppm S2 (a8 and b8) solutions at 25 C.

Table 6 Equivalent circuit parameters for the cp-Ti in 3.5% NaCl solution in the absence and presence 2 ppm of sulfide ion at 25C.


2 a1 Rf/kXcm2 Cf/lF cm

2 a2

3.5% NaCl 15 2.5 893.7 2.2 1.0 2.3 13.6 0.99

60 1.5 1590.0 4.0 0.99 7.6 10.5 0.99

120 1.8 2002.0 9.9 1.0 9.9 8.0 0.99180 1.6 1710.0 11.7 1.0 21.2 7.5 0.99

240 2.9 3100.0 20.6 0.99 26.6 6.0 1.0

3.5% NaCl + 2 ppm S2 15 2.2 727.2 34.6 0.98 0.96 104.5 0.99

60 1.8 859.8 18.5 0.98 5.6 22.6 1.0

120 3.6 965.4 26.04 0.98 7.3 27.5 0.99

180 2.9 1002 24.6 0.98 9.3 21.6 0.99

240 2.9 930 21.9 0.99 9.18 13.9 1.0

10 H. Nady et al.



11/16

times, where, in this case,Rpfand Cpf, represent the resistance

and capacitance of barrier layer, and Rctand Cdl, present the

resistance and capacitance of the pours layer, respectively

[52]. The impedance parameters calculated for Cp-Ti are given

inTable 6.

A comparison between the impedance spectra, Bode and

Nyquist plots, for Cu, Cu10Al10Ni, C-steel and Cp-Ti alloy

after 240 min of electrode immersion in 3.5% NaCl + 2 ppm

S2

was obtained and is depicted inFig. 8(a and b). It is obvi-ous that the shape of the impedance spectra is very character-

istics of each alloy as explained previously. The inset inFig. 8b

is a close-up of Cu and C-steel data. In this figure the total

impedance magnitude for Cp-Ti is higher and Cu10Al10Ni

is much higher than that of Cu due to the duplex nature of film

and segregation of Ni2+ to Cu2O layer in both Cp-Ti and Cu

10Al10Ni alloys, respectively, while C-steel recorded the low-

est corrosion resistance in this medium which means that sul-

fide ions have a significant effect on the corrosion rate of C-

steel. Therefore, corrosion rate results obtained from both

potentiodynamic polarization and EIS methods are in good

agreement and confirm that Cp-Ti alloy is the most stable

and suitable material to be used in these media.

Fig. 9(ac) represents the variation of the corrosion resis-

tance of the barrier layer on Cu (a), CuAlNi (b), and

0 400 800

0

200

400

-Zi/cm2

Zr/cm

2

-2 0 2 4 6

0

1

2

3

44

4

4

3

log (f / HZ)

log(Z/

cm2)

20

0

-20

-40

-60

-80

3

32

2

2

1 1

1

(a)3.5% NaCl + 2 ppm S

--

Phase/degree.

Cu

Cu-10Al-10Ni

C-Steel

Cp-Ti

0 300 600 900 1200 1500 1800

0

1000

2000

3000

4000

5000

(b)

Cu

Cu-10Al-10Ni

C-Steel

Cp-Ti

-Zi/cm2

Zr/

cm2

Figure 8 Bode (a) and Nyquist (b) plots of Cu (jjj), Cu

10Al10Ni (ddd), C-Steel (NNN) and Ti (...) measured in

3.5 wt.% NaCl + 2 ppm S2 aqueous solution at 25 C.

0 50 100 150 200 250300

450

600

750

900

1050

1200

1350

1500

1650

(b)

Cu-10Al-10Ni

3.5% NaCl + 2 ppm S--

3.5% NaCl

Rf/

cm

Time /min.

0 50 100 150 200 2500

5

10

15

20

25

30

(c)

Cp- Ti

3.5% NaCl + 2 ppm S--

3.5% NaCl

Rf/

cm2

Time /min.

0 50 100 150 200 250100

150

200

250

300

350

400

450

500

550

(a)

3.5% NaCl + 2 ppm S--

3.5% NaCl

Cu

Rf/

cm2

Time /min.

Figure 9 Variation of the corrosion resistance of the barrier

layer on Cu (a), CuAlNi (b) and Cp-Ti (d) with the time of

immersion in stagnant naturally aerated 3.5% chloride solutions

of pH 7.0 in the absence and presence of sulfide ions at 25 C.




12/16

-2 0 2 4 6

0.0

0.5

1.0

1.5

2.0

2.5

3.0a1

log (f / HZ)

log(Z/cm2)

20

0

-20

-40

-60Cu

Phase/degree.

3.5% NaCl

3.5% NaCl + 2 ppm S2-

0 100 200 300 400 500

0

50

100

150

200

b1

3.5% NaCl

3.5% NaCl + 2 ppm S2-

Cu

-Zi/

cm2

Zr/ cm

2

-2 0 2 4 60.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

log (f / HZ)

log(Z/cm2)

20

0

-20

-40

-60

-80

a2Cu-Al-10Ni

3.5% NaCl

3.5% NaCl + 2 ppm S2-

Phase/degree.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

0.0

0.1

0.2

0.3

0.4

0.5

0.6b2

Cu-Al-10Ni

3.5% NaCl

3.5% NaCl + 2 ppm S2-

-Zi/

cm2

Zr/ cm

2

-2 0 2 4 60.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

log (f / HZ)

log(Z/

cm2)

10

0

-10

-20

-30

-40

-50

C-Steela3

3.5% NaCl

3.5% NaCl + 2 ppm S2-

Ph

ase/degree.

0 5 10 15 20 25 30-2

0

2

46

8

10

12

14

16

18

20

C-Steelb3

3.5% NaCl

3.5% NaCl + 2 ppm S2-

-Zi/

cm2

Zr/ cm

2

-2 0 2 4 60

1

2

3

4

a4

log (f / HZ)

log(Z/cm2)

0

-20

-40

-60

-80

Cp-Ti

3.5% NaCl

3.5% NaCl + 2 ppm S2-

Ph

ase/degree.

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0

2

4

6

8

10Cp Ti

b4

3.5% NaCl

3.5% NaCl + 2 ppm S2-

-Z

i/

cm2

Zr/

cm2

Figure 10 Bode and Nyquist plots of Cu (a1 and b1), Cu10Al10Ni (a2 and b2), C-Steel (a3 and b3) and Cp-Ti (a4 and b4), alloys after

3 h immersion in stagnant naturally aerated 3.5% NaCl and 3.5% NaCl containing 2 ppm S2 solutions at 25 C.

12 H. Nady et al.



13/16

Cp-Ti (c) with the time of immersion in stagnant naturally aer-

ated 3.5% chloride solutions in the absence and presence of

sulfide ions. The general behavior reflects the increase of bar-

rier layer with time, In case of Cu and Cu10Al10Ni alloys

the presence of S2 decreases the passive film resistance than

that recorded in chloride solution due to the diffusion of sul-

fide ions into the protective oxide layer Cu2O[48]. Also, for

Cp-Ti alloy, the barrier layer resistance decreases in the pres-

ence of H2S, this suggests that the mixed chloride-hydrogensulfide solution is relatively corrosive for Cp-Ti alloy.

Bode and Nyquist plots inFig. 10, presents a comparison of

the impedance spectra for each alloy after 3 h of exposure in

neutral 3.5% NaCl solution in the absence and presence of sul-

fide ions, Figs. (a1 and b1) for Cu, (a2 and b2) for Cu10Al

10Ni, (a3 and b3) for C-Steel and (a4 and b4) for Cp-Ti alloy.

For Cu and Cu-10Al-10Ni alloys, the magnitude of the total

impedance enhanced in the absence of sulfide ions which is

clearly shown in Nyquist plots {cf.Fig. 10(b1 and b2)}, where

passive film resistance, Rpf, recorded in the absence of sulfide

ions is bigger than that observed in the presence of sulfide ions,

therefore, the passive film resistance, Rpf, after 3 h from elec-

trodes immersion in 3.5% NaCl solution were 532.1 and

1.692 k and decreased to 218 and 1.418 k in the presence

of sulfide ions for Cu and Cu10Al10Ni alloys, respectively.

This behavior was attributed to the diffusion of sulfide ions

into the protective oxide layer reacting with Cu ions to forma non-adherent copper sulfide film[48]. Also the barrier layer

formed on Cp-Ti alloy after 3 h in 3.5% NaCl solution was

26.6 k and decreased to 9.3 k in the presence of sulfide ions.

In contrast to C-steel the magnitude of the total impedance

increased in presence of H2S {cf. Fig. 10(a3 and b3)}, where

in 3.5% NaCl solution the corrosion resistance was mainly

controlled by the charge transfer, while in the presence of sul-

fide ions another additional resistance which attributed to the

Figure 11 SEM micrograph (a) and EDX results (b) of Cu10Al10Ni alloy before electrochemical testing.




14/16

formation of some corrosion products, the mackinawite grows

and adheres, thus, the film of corrosion products increased.

3.3. Surface analyses

Scanning electron microscopy analyses, SEM, were used to

define the morphology of surface attack and the chemical com-

position of corrosion products. Surface morphology of Cu

10Al10Ni alloy was subjected to SEM/EDX analysis as an

example. The SEM/EDX investigation was carried out and

depicted in Figs. 1113 of polished and after 72 h of alloy

immersion in sulfide free 3.5% sodium chloride and 3.5%

sodium chloride solutions polluted by 2 ppm sulfide ions. In

addition to the presence of boundary phases, before immer-

sion, the electrode surface clearly seems smooth (Cf. Fig. 11).

The polished surface of Cu10Al10Ni alloy was subjected

to SEM/EDX analysis as an example after immersing in the

test solutions (Cf.Fig. 12). It is clear that the sulfide containing

solution is remarkably aggressive as can be seen on the SEMimage (Cf.Fig. 13). In the sulfide containing solution a granu-

lar layer with relatively large and irregular sized grains is

formed on the alloy surface compared to the smoother surface

recorded for sulfide free solutions, where in case of sulfide free

solution the corrosion sits present only at the boundary phases

(Cf.Fig. 12). It is also important to notice that the corrosion

products formed on the surface Cu10Al10Ni alloy in the

chloride containing 2 ppm S2 ions are relatively more than

that formed in the absence of sulfide. The results obtained

from the surface analysis are in good agreement with those

of polarization and impedance measurements.

Figure 12 SEM micrograph (a) and EDX results (b) of Cu10Al10Ni alloy after immersion in stagnant naturally aerated neutral NaCl

solution at 25 C.

14 H. Nady et al.



15/16

4. Conclusions

Investigations of the electrochemical behavior of Cu, Cu

10Al10Ni, Cp-Ti and C-steel in sodium chloride and sodium

chloride polluted by 2 ppm hydrogen sulfide ions have shown

that:

1- The presence of sulfide ions enhances the corrosion pro-cess for different alloys, where the corrosion rate of the

Cp-Ti and Cu10Al10Ni alloy is less than half of Cu,

and C-steel alloy in 3.5% NaCl containing 2 ppm S2.

2- The passive film resistance formed on Cp-Ti was larger

than those of the passive film formed on the other three

alloys due to the protective nature of the film formed on

this alloy.

3- The dissolution of copper is inhibited in the Cu-alloys;

especially thats containing Ni. This is attributed to the

incorporation of Ni2+ into the Cu2O film leading to

its stabilization, but its resistance is still less than that

of Cp-Ti due to the duplex nature of the film formed

on its surface which is composed of a dense inner barrier

layer and a porous outer layer.

4- Only, for C-steel where the total impedance magnitude

increasedin thepresence of S2 ions, where, the resistance

increases due to the formation of some corrosion prod-

ucts, and the mackinawite grows and adheres with time.

References

[1]C. Ren, D. Liu, Z. Bai, T. Li, Mater. Chem. Phys. 93 (2005) 305 .

[2] L.W. Tsay, Y.J. Lin, C. Chen, Corros. Sci. 63 (2012) 267.

[3] T.Y. Jin, Z.Y. Liu, Y.F. Cheng, Int. J. Hydrogen Energ. 35

(2010) 8014.

[4]B. Beidoknti, A. Dolati, A.H. Koukabi, Mater. Sci. Eng., A 507

(2009) 167.

[5] C. Plennevaux, J. Kittel, M. Fregonese, B. Normand, F.

Ropital, F. Grosjean, T. Cassagne, Electrochem. Commun. 26

(2013) 17.

Figure 13 SEM micrograph (a) and EDX results (b) of Cu10Al10Ni alloy after immersion in stagnant naturally aerated neutral 3.5%

NaCl containing 2 ppm S2 at 25 C.


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