Effects of Cu on the Passive Film Stability of Fe-20Cr-x Cu (x = 0, 2, 4 wt.%) Alloys in H2SO4...

Accepted Manuscript

Title: Effects of Cu on the Passive Film Stability ofFe-20Cr-xCu (x = 0, 2, 4 wt.%) Alloys in H2SO4 Solution

Authors: KkochNim Oh, Ihsan-ul-Haq Toor, SooHoon Ahn,HyukSang Kwon

PII: S0013-4686(12)01672-6DOI: doi:10.1016/j.electacta.2012.10.058Reference: EA 19418

To appear in: Electrochimica Acta

Received date: 17-7-2012Revised date: 16-10-2012Accepted date: 16-10-2012

Please cite this article as: K.N. Oh, I.-u.-H. Toor, S.H. Ahn, H.S. Kwon, Effects of Cu onthe Passive Film Stability of Fe-20Cr-xCu (x<ce:hsp sp=0̈.25/̈>=<ce:hsp sp=0̈.25/̈>0,2, 4<ce:hsp sp=0̈.25/̈>wt.%) Alloys in H2SO4 Solution, Electrochimica Acta (2010),doi:10.1016/j.electacta.2012.10.058

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dx.doi.org/doi:10.1016/j.electacta.2012.10.058

dx.doi.org/10.1016/j.electacta.2012.10.058

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Effects of Cu on the Passive Film Stability of Fe-20Cr-xCu1

(x=0, 2, 4 wt.%) Alloys in H2SO4 Solution2

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KkochNim Oha, Ihsan-ul-Haq Toorb, SooHoon Ahna, and HyukSang Kwona*4

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a Dept. of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1, 6GuSeongDong, YuSeongGu, DaeJeon, 305-701, Republic of Korea7

8b Dept. of Mechanical Engineering, King Fahd University of Petroleum and Minerals (KFUPM),9

P. O. Box 1308, Dhahran 31261, Kingdom of Saudi Arabia1011

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* Corresponding author. Tel.: +82-42-350-3326; fax: +82-42-350-3310.22

E-mail address: [email protected]

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Abstract1

Effects of Cu on the stability of passive film formed on Fe-20Cr-xCu (x=0, 2, 4 wt.%) 2

alloys in deaerated 0.1 M H2SO4 solution were examined by potential decay test, X-ray 3

photoelectron spectroscopy, and the Mott-Schottky analysis. The potential decay curves of 4

Fe-20Cr-xCu (x=0, 2, 4 wt.%) alloys passivated for 1 h at 400 mVSCE exhibited a 1st stage-5

potential decay to a potential of approximately -300 mVSCE. Especially, the stability of6

passive film decreased with an increase in Cu content, suggesting that the film formed on the7

Cu-containing alloys was less stable than that of Fe-20Cr alloy. The rapid potential decrease 8

in the Cu-containing alloys was primarily due to the Cu-dissolution from the film, which 9

increased the defect density of the passive film and hence decreased its stability. After the 1st10

stage-potential decay, Fe-20Cr alloy exhibited the 2nd stage-potential decay to -654 mVSCE or 11

to an active state in 9000 s. However, the 2nd stage-potential decay did not occur in the Cu-12

containing alloys because of the Cu enrichment on the surface of the alloy. The Cu 13

enrichment in the Cu containing alloys decreased the defect density in the film, and hence14

increased the stability of the passive film.15

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Keywords: Copper; Passive film; Stability; XPS; Mott-Schottky.20

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1. Introduction1

Stainless steels (SSs) have been widely used as structural materials in corrosive 2

environments because of their excellent corrosion resistance and good mechanical properties.3

The excellent corrosion resistance of SSs is primarily due to the formation of thin protective 4

passive film on the surface of SSs. The passive films formed on SSs have been reported to be 5

Cr-enriched (Fe, Cr) oxide and/or hydroxide [1-5], although there is still some controversy 6

about the detailed structure and composition of the passive films. In particular about the7

minor alloying elements such as Mo, Cu, Si etc., which are added to SSs to improve their 8

corrosion behaviors in various corrosive environments, and which significantly affect the 9

stability of passive film [6-8].10

Among the minor alloying elements, 0.5~4 wt.% Cu is often added to various 11

commercial austenitic and duplex SSs to enhance the corrosion resistance in corrosive 12

environments [6, 7, 9-13]. Seo et al. [6] pointed out that the presence of 0.4 wt.% Cu 13

decreased the anodic dissolution rate of ferritic SS in sulfuric acid solution. Ogura et al. [11] 14

reported that the addition of 0.1~1.0 wt.% Cu to an austenitic SS cause a decrease in its 15

corrosion rate in 5 N H2SO4/NaCl solution. Streicher [12] claimed that the addition of 3.5 16

wt.% Cu to 20Cr-33Ni-2.5Mo alloy greatly enhanced the resistance to corrosion in sulfuric 17

and phosphoric acid environments. Moreover, Cu excels among alloying elements in 18

decreasing the critical anodic current density (IC), the maximum current density in the active 19

state in an active-passive polarization behavior, which is a criterion to assess the passivation 20

tendency of the alloys [14]. Cu is enriched on the surface of Fe-26Cr-0.4Cu alloy and Fe-21

18Cr-8Ni-xCu (x=0~3 wt.%) alloys respectively, during the anodic dissolution of the alloys in 22

deaerated sulfuric acid solution [6, 7]. Hence, the enrichment of Cu was mainly associated 23

with the reduction in corrosion rate of the alloys in active region. On the other hand, at 24

passive state, Cu showed detrimental effects on the stability of passive films on Fe-26Cr-25

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0.4Cu alloy and Fe-18Cr-8Ni-xCu (x=0~3 wt.%) alloys [6, 7]. Bautista et al. [13] have 1

reported that the passive layer of 204Cu type (Fe-16Cr-2Ni-8Mn-2.7Cu-0.13N) austenitic 2

stainless steel, which is used as concrete reinforcements in low alkaline media, is attacked 3

more significantly than that of commercial AISI 304 grade and duplex SAF 2205 grade, and 4

hence 204Cu stainless steel has less protective passive film. Moreover, Oguzi et al. [15] found 5

that Cu has negative effects on the passive film stability of austenitic, ferritic, and martensitic 6

stainless steel in deaerated sulfuric acid solution due to the presence of Cu in the passive film. 7

The reason for the detrimental effects of Cu on the stability of passive film of SSs has not yet 8

been fully understood and hence needs to be clarified. Most of SSs that have been used for the 9

investigation on the Cu effects contain small amounts of Cu with other alloying elements such 10

as Ni, Si and Mn etc., and hence it is very difficult to clarify the independent effects of Cu on 11

the stability of passive film on SSs. Hence, it is necessary to investigate the independent 12

effects of Cu on the passive film stability of SSs containing relatively large amounts (0~4 13

wt. %) of Cu, while excluding the effects of other alloying elements on the passivity.14

Therefore, the research objective of the present work is to examine the independent effects of 15

Cu on the stability of passive film on Fe-20Cr-xCu (x=0, 2, 4 wt.%) alloys in an acidic 16

solution.17

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2. Experimental19

Fe-20Cr-xCu (x=0, 2, 4 wt.%) alloys were prepared by vacuum arc melting, and then cast 20

in the form of button. The cast was homogenized for 120 min at 1200 oC and then hot rolled 21

into 3 mm thick plate. The specimens were prepared by cold rolling the hot-rolled plates to 22

1.6 mm thick sheet, and then solution annealed for 30 min at 1050 oC, followed by water 23

quenching. Chemical composition of the alloys is given in Table 1. For electrochemical tests, 24

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the specimens were prepared using 1500-grit SiC paper, subsequently polished to 3 µm 1

alumina paste and then ultrasonically cleaned with ethanol.2

The potential decay test was conducted in deaerated 0.1 M H2SO4 solution at 30 oC to 3

examine the effects of Cu on the passive film stability of Fe-20Cr-xCu (x=0, 2, 4 wt.%) alloys. 4

The electrochemical cell for the test was equipped with a platinum counter electrode, a 5

saturated calomel reference electrode (SCE), and a test specimen as a working electrode. 6

After cathodic cleaning at -800 mVSCE for 100 s, the alloys were passivated at 400 mVSCE for 7

1 h and then, open circuit potentials were recorded with time for days.8

For the chemical analysis for passive film by X-ray photoelectron spectroscopy (XPS), 9

passive films were formed on Fe-20Cr-xCu (x=0, 4 wt.%) alloys potentiostatically for 1 h at 10

400 mVSCE, for 1 h at 400 mVSCE + 1 h at open circuit potential (OCP), and for 1 h at 400 11

mVSCE + 24 h OCP, respectively, in deaerated 0.1 M H2SO4 solution at 30 oC. XPS 12

measurement was performed using Al-Kα X-ray source (15 kV, 20 mA, 300 W), and a pass 13

energy of 50 eV. The reference energies were the C1s signal at 284.5 eV and the O1s signal at 14

531.0 eV.15

For the Mott-Schottky analysis on the passive films formed similarly as those for the XPS 16

test, the capacitance on the film was measured by sweeping the applied potential in the active 17

direction from the passivation potential at a scan rate of 25 mV/s. The excitation voltage was 18

10 mV (peak-to-peak) and the frequency was 1 kHz [16].19

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3. Results and discussion21

Fig. 1 shows effects of Cu on the potential decay curve of Fe-20Cr-xCu (x=0, 2, 4 wt.%) 22

alloys when the alloy was at open circuit after passivation for 1 h at 400 mVSCE in deaerated 23

0.1 M H2SO4 solution at 30 oC. The polarization responses of Fe-20Cr-xCu (x=0, 2, 4 wt.%) 24

alloys shown in the inset of Fig. 1 revealed that passive current density of the alloys increased 25

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with Cu content at potentials noble to -100 mVSCE due primarily to the Cu dissolution1

phenomenon [6, 17]. The potential decay curve of Fe-20Cr alloy exhibited a 1st stage-rapid 2

potential decay to a potential of approximately -300 mVSCE and then a 2nd stage-potential 3

decay to a potential of -654 mVSCE at 7800 s after the 1st potential decay, which is considered 4

a typical behavior of such alloys [8]. As a result of the 2nd stage-potential decay, the OCP of 5

Fe-20Cr alloy was lowered to its corrosion potential (-624 mVSCE), which was also6

determined by the potentiodynamic polarization test shown in the inset of Fig. 1. On the other 7

hand, the Cu containing alloys, Fe-20Cr-xCu (x=2, 4 wt.%) alloys did not show the 2nd stage-8

potential decay for 5 days. The OCP values and Ecorr measured from the polarization 9

responses shown in Fig.1 are summarized in Table 2.10

3.1. 1st stage of potential decay curves and XPS analysis11

The 1st stage-potential decay occurred much earlier in the alloys with higher Cu content;12

after 287 s for Fe-20Cr-2Cu alloy and after 174 s for Fe-20Cr-4Cu alloy. That is, the 13

activation time (τ) of the alloys or a measure of the passive film stability decreased with an 14

increase in the Cu content. The results suggest that the film formed on the alloy with higher 15

Cu content at 400 mVSCE for 1 h in deaerated 0.1 M H2SO4 solution at 30 ºC was less stable 16

than that of Fe-20Cr alloy. This phenomenon was also observed by Seo et al. [6] and Lin et al.17

[7], but the reason for that has not yet been investigated. According to Lizloves, the rate of the 18

potential decay is dependent on the chemical composition of the passive film [8], which 19

confirms that presence of Cu in the passive film of these alloys may have detrimental effects 20

on its stability. To confirm it further, XPS investigation was carried out on the passive films 21

formed on these alloys at 400 mVSCE for 1 h in deaerated 0.1 M H2SO4 solution at 30 ºC, i.e 22

Fe-20Cr-xCu (x=0, 4 wt.%).23

Fig. 2 presents XPS depth concentration profiles of the passive film formed on Fe-20Cr-24

xCu (x=0, 4 wt.%) alloys at 400 mVSCE for 1 h in deaerated 0.1 M H2SO4 solution at 30 oC. 25

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The passive film of each alloy is enriched with Cr, the concentration of which is more than 2 1

times of the actual alloy composition, exhibiting the maximum Cr % at the surface of the 2

passive film. The Cr ratio in the passive film was 45~50 % in depth of the film corresponding 3

to etch time of 150 s and then decreased gradually with depth of passive film as shown in Fig. 4

2(a) and 2(b). For Fe-20Cr-4Cu alloy, the Cu content in the passive film is as high as its alloy 5

content. Thus, the passive films on Fe-20Cr alloy and also on Fe-20Cr-4Cu alloy can be 6

regarded as a Cr-enriched (Fe, Cr) oxide and Cr-enriched (Fe, Cr, Cu) oxide, respectively. 7

However, interestingly, the Cu content was not observed on the surface of the passive film on 8

Fe-20Cr-4Cu alloy as shown in Fig. 2(b). The absence of Cu on the surface of the film was 9

confirmed further by the binding energy curves in Fig. 2(c), in which the Cu2p peak was not 10

detected on the surface or at etch time 0 s. Similar results have been reported by Lin et al. for 11

Cu-containing steels (Fe-18Cr-8Ni-xCu (x=0~3 wt.%)) passivated at 400 mVSCE in deaerated 12

sulfuric acid solution [7].13

According to the Pourbaix diagram for the Cu-H2O system [18], Cu is dissolved as Cu2+14

ion in an acidic solution at 400 mVSCE. We have found in another study (results to be 15

published in a separate paper) that Cu-dissolution (Cu+ Cu2+ + e-) occurs in the passive 16

region of Fe-20Cr-xCu (x=0, 2, 4 wt.%) alloys at potentials noble to -100 mVSCE [17]. 17

Therefore, the absence of Cu content on the surface of the passive film on Fe-20Cr-4Cu alloy 18

in Fig. 2(b) and 2(c) was due primarily to the Cu-dissolution phenomenon. The rapid potential 19

decrease observed during the 1st stage-potential decay in Fe-20Cr-xCu (x=2, 4 wt.%) alloys 20

was closely related to the Cu-dissolution phenomenon in the alloys.21

3.1.1. Mott-Schottky analysis22

Passive films on stainless steels have the features of highly doped semiconductor. The 23

Mott–Schottky analysis for a passive film provides precise information on the electronic 24

properties of the passive film. It has been reported elsewhere [19, 20] that the passive film on 25

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Cu is a p-type semiconductor, and the majority of defects are metal vacancies (VMχ-). 1

According to the point defect model (PDM), which provides an analytical description of the 2

growth and breakdown of passive films on the atomic (subnanometer) scale [21], cation 3

vacancy (VMχ-) is generated at the interface between passive film and solution as shown in Eq.4

(1);5

eVMM MM )( (1)6

where MM and VMχ- is cation on cation sublattice and cation vacancy on cation sublattice, 7

respectively, in the barrier layer. Eq. (1) indicates that VMχ- is generated by metal dissolution at 8

the interface between passive film and solution. Therefore, the cation vacancy generation may 9

occur due to the Cu-dissolution at the passive state. Hence, the Mott-Schottky analysis is very 10

useful in revealing the effects of the Cu-dissolution on the cation vacancy generation in the 11

passive film on Fe-20Cr-xCu (x=0, 2, 4 wt.%) alloys in deaerated 0.1 M H2SO4 solution.12

Semiconducting properties of passive film can be explored by measuring the capacitance 13

of the space charge layer (CSC) as a function of the electrode potential (U). The Mott-Schottky 14

relationship expresses the potential dependence of CSC of a semiconductor electrode under the 15

depletion condition [22];16

))(2

(1

02 e

kTUU

NC FBDSC

for n-type (2)17

))(2

(1

02 e

kTUU

eNC FBASC

for p-type (3)18

where ND and NA are donor density and acceptor density respectively, e is electron charge, ε is 19

the dielectric constant of the passive film (ε = 15.6 [23, 24]), ε0 is the permittivity of free 20

space (ε0 = 8.854 x 10-14 Fcm-1), U is the applied potential, UFB is the flat band potential, k is 21

the Boltzman’s constant, and T is the absolute temperature. Thus, for n-type semiconductor, 22

CSC-2 vs U should be linear with a positive slope that is inversely proportional to the donor 23

density, whereas for a p-type semiconductor it exhibits a linear region with a negative slope 24

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that is inversely proportional to the acceptor density. The Mott-Schottky relationship is based 1

on the assumption that the capacitance of the space charge layer is much less than that of the 2

Helmholtz layer. Normally Helmholtz capacitance reported to be 25~50 μFcm-2 satisfied the3

assumption [25-28].4

Fig. 3 shows the Mott-Schottky analysis for the passive film formed on Fe-20Cr-xCu 5

(x=0, 2, 4 wt.%) alloys at 400 mVSCE for 1 h in deaerated 0.1 M H2SO4 solution at 30 oC. The 6

passive films of Fe-20Cr-xCu (x=0, 2, 4 wt.%) alloys showed p-type semiconducting behavior 7

at potentials below -200 mVSCE, and above the potential, it exhibited n-type behavior. NA and 8

ND (shown in Figs. 3 (b) and 3(c)) were calculated from the slope in the linear region in Fig. 3 9

(a) based on Eqs. (2) and (3). The addition of Cu to Fe-20Cr alloy increased the acceptor 10

density or cation vacancy concentration ([VMχ-]) of the passive film [29]; 2.44ⅹ1021 cm-3 for 11

Fe-20Cr alloy and 3.94ⅹ1021 cm-3 for Fe-20Cr-4Cu alloy. As mentioned earlier, such an 12

increase in [VMχ-] results from the Cu-dissolution phenomenon of Fe-20Cr-xCu (x=0, 2, 4 13

wt.%) alloys passivated at 400 mVSCE for 1 h in deaerated 0.1 M H2SO4 solution at 30 oC. The 14

donor density, ND or the oxygen vacancy concentration ([VO··]) [29] was also increased from 15

2.19ⅹ1021 to 2.71ⅹ1021 cm-3 to satisfy the charge neutrality condition of the passive film. 16

Based on the results mentioned above, it was confirmed that the initial rapid potential decay 17

of Fe-20Cr-xCu (x=2, 4 wt.%) alloys, was primarily due to the Cu-dissolution phenomenon, 18

leading to an increase in the defect density of the passive film and a decrease in the stability 19

of the passive film.20

3.2. 2nd stage of potential decay curves and XPS analysis21

As discussed in the previous section, for the Cu-containing alloys, the 1st stage-potential 22

decay occurred much earlier than the alloy without Cu (as shown in Fig. 1). However, the 2nd23

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stage-potential decay did not occur until 5 days in the alloys with Cu. On the contrary, the 1

OCP slightly increased with time. On the other hand, Fe-20Cr alloy exhibited the 2nd stage-2

potential decay. The potential, -200 to -300 mVSCE at which OCP is almost constant after the 3

1st stage-potential decay in Fig. 1 is called an “arrest potential”, and the length of time at the 4

potential depends on the chemical composition of the passive film [8]. Therefore, XPS 5

analysis was conducted to observe the change in the chemical composition of the passive 6

films of Fe-20Cr-xCu (x=0, 2, 4 wt.%) alloys at the 2nd stage-potential decay.7

Figs. 4 (a) and (b) present the XPS depth concentration profiles of the passive film 8

formed on Fe-20Cr-xCu (x=0, 4 wt.%) alloys that were passivated for 1 h at 400 mVSCE and 9

then held for 1 h at OCP in deaerated 0.1 M H2SO4 solution at 30 oC. The passive films on Fe-10

20Cr and Fe-20Cr-4Cu alloys can be regarded as a Cr-enriched (Fe, Cr) oxide and Cr enriched 11

(Fe, Cr, Cu) oxide, respectively. Results showed that the surface of the film formed on Fe-12

20Cr-4Cu alloy was more enriched with Cu, when compared with that on the surface of the 13

passive film formed at 400 mVSCE for 1 h, shown in Fig. 2 (b) and (c). Also, it is significant in 14

Fig. 4 (c) that Cu concentration was increased with an increase in time holding at OCP (from 15

0 to 24 hours), while Fe concentration in the film was decreased with time. The results 16

suggest that the Cu-enrichment was intensified with time after the first potential decay, and 17

were responsible for the difference in the 2nd stage-potential decay behavior between Fe-20Cr 18

alloys and Fe-20Cr-xCu (x=2, 4 wt.%) alloys.19

Fig. 5 presents the Cu-XPS spectra at etch time 0 s for the passive film described in Fig. 420

(b) and (c), and the data was analyzed by XPSPEAK41 software to investigate the effects of 21

Cu on the passive film with increase in time holding at OCP. Cu-XPS spectra of the passive22

film passivated for 1 h at 400 mVSCE + 1 h OCP (Fig. 5(a)) showed a Cu chemical bond for 23

Cu2O (932.5 eV) [30]. However, CuO (933.4 eV) [31], in addition to Cu2O, was also formed 24

when Fe-20Cr-4Cu alloy was passivated for 1 h at 400 mVSCE and then held for 24 h at OCP 25

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(Fig. 5(b)).This confirms that CuO was formed with an increase in time holding at open 1

circuit. CuO can be formed from either solid substances such as metallic Cu and Cu2O or 2

dissolved substance (Cu+) by electrochemical reaction [18]. However, the standard electrode 3

potentials (ECu/CuO0 = 329 mVSCE, ECu2O/CuO

0 = 428 mVSCE, ECu+/CuO0 = 379 mVSCE) are so high 4

that CuO cannot be formed at quite low-potential such as -200 mVSCE by an electrochemical 5

reaction. However, CuO is formed via a chemical reaction when Cu2+ accumulated on Cu 6

surface reacts with water, as represented in Eqs. (4) and (5) [18]. 7

Cu2+ + H2O = CuO + 2H+ (4)8

log [Cu+] = 7.89 – 2pH (5)9

However, the OCP of approximately -200 mVSCE is close to the standard electrode 10

potential between Cu+ and Cu2+ species (ECu+/Cu2+0 = -88 mVSCE) [18, 32]. The equilibrium 11

and corresponding Nernst equations for Cu+ and Cu2+ are shown in Eqs (6) and (7) 12

respectively [18].13

Cu2+ + e- = Cu+ (6)14

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ECu+/Cu2+ = -88 + 59.1 log ([Cu2+]/[Cu+]), (mVSCE) (7)1

where the saturated calomel electrode potential (VSCE) in the Nernst equation was changed to 2

standard hydrogen electrode potential (VSHE). According to Eq. (6), the equilibrium potential 3

depends on both [Cu2+] and [Cu+]. Especially, Cu+ is formed easily from Cu2O at the interface 4

between the oxide film and the solution by chemical reaction shown in Eqs (8) and (9). The 5

concentration of [Cu+] at the surface of the alloy is much higher than [Cu2+], so it decreases6

ECu+/Cu2+ easily under the OCP (-200 mVSCE) of the alloy. Therefore, reaction of Eq. (6) takes 7

place when ECu+/Cu2+ is lower than the OCP of the alloy, and Cu2+ accumulates at the surface,8

thereby forming CuO. According to Eq. (6), the electron might be consumed in the reduction 9

reaction of H+ (2H+ + 2e- = H2) on the surface of the alloy.10

Cu2O + 2H+ = 2Cu+ + H2O (8)11

log [Cu+] = -0.84 – pH (9)12

3.2.1. Mott-Schottky analysis13

Fig. 6 presents the Mott-Schottky analysis for Fe-20Cr alloy passivated for 1 h at 400 14

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mVSCE + 1 h OCP, Fe-20Cr-4Cu alloy passivated for 1 h at 400 mVSCE + 1 h OCP and also 1

Fe-20Cr-4Cu alloy passivated for 1 h at 400 mVSCE + 24 h OCP, respectively, in deaerated 0.1 2

M H2SO4 solution at 30 oC. The starting sweep potential was not 400 mVSCE but -200 mVSCE3

because the Cu-dissolution phenomenon occurred at potential noble to -100 mVSCE, which 4

affects the generation of defects such as cation (VMχ-) and oxygen vacancies (VO

··). To avoid 5

this unexpected defects generation, we selected the starting sweep potential to be -200 mVSCE. 6

The passive film on these alloys showed p-type semiconducting behavior at potentials below7

-200 mVSCE. NA was calculated from the slope in the linear region in the Mott-Schottky 8

relationship based on Eq. (3) and results are presented in Table 3. The addition of Cu to Fe-9

20Cr alloy decreased NA or the concentration of cation vacancy ([VMχ-]) in the passive film 10

from 4.43ⅹ1021 cm-3 for Fe-20Cr to 3.51ⅹ1021 cm-3 for Fe-20Cr-4Cu. For Fe-20Cr-4Cu 11

alloy with an increase in the OCP time of film formation, the cation vacancy concentration 12

([VMχ-]) in the passive film was decreased from 3.51ⅹ1021 cm-3 for 1 h at OCP to 2.82ⅹ102113

cm-3for 24 h at OCP.14

Based on the above results, it can be concluded that the Cu enrichment in the passive film 15

of Cu containing alloys at OCP decreased the defect density in the film, and hence makes the 16

passive film more stable in the acidic solution. Therefore, the Cu enrichment was responsible 17

for the absence of the 2nd stage potential decay in the Cu containing alloys.18

19

4. Conclusions20

The effects of Cu on the passive film stability of Fe-20Cr-xCu (x=0, 2, 4 wt.%) alloys 21

in deaerated 0.1 M H2SO4 solution at 30 oC were examined by the potential decay behavior, 22

XPS studies and Mott-Schottky analysis for the film, and the conclusions can be drawn as 23

follows;24

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1) Potential decay curve of Fe-20Cr alloy passivated for 1 h at 400 mVSCE1

exhibited the 1st stage of rapid potential decay to a potential of approximately -300 2

mVSCE and then a much delayed potential decay to about -654 mVSCE at the 2nd stage. 3

However, the Cu containing alloys or Fe-20Cr-xCu (x=2, 4 wt.%) did not show the 2nd4

stage-potential decay.5

2) At the 1st stage-potential decay behavior, the activation time (τ) of the alloys 6

or a measure of the passive film stability was decreased with an increase in the Cu 7

content. The result suggests that the stability of the passive film decreased with the Cu 8

content in the alloys.9

3) It was found from XPS analysis that Cu was not detected on the surface of 10

the passive film formed on Fe-20Cr-4Cu alloy at 400 mVSCE for 1 h, which was due 11

primarily to the Cu-dissolution phenomenon at that potential.12

4) It was demonstrated from the Mott-Schottky analysis for the passive film 13

formed on Fe-20Cr-xCu (x=0, 2, 4 wt.%) alloys that the cation vacancy concentration 14

([VMχ-]) in the passive film of the alloys significantly increased with the Cu content;15

2.44ⅹ1021 cm-3 for Fe-20Cr alloy and 3.94ⅹ1021 cm-3 for Fe-20Cr-4Cu alloy. The 16

oxygen vacancy concentration ([VO··]) in the film was also increased with the Cu 17

content due to the charge neutrality condition in the passive film; 2.19ⅹ1021 cm-3 for 18

Fe-20Cr and 2.71ⅹ1021 cm-3 for Fe20Cr-4Cu. Therefore, the decrease in the 19

activation time on the 1st stage-potential decay in the Cu-containing alloy is associated 20

with the increase in the defect density of the passive film that results from the Cu 21

dissolution in the passive film. 22

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5) The Cu-containing alloys did not show the 2nd stage-potential decay of until 5 1

days, while Fe-20Cr alloy exhibited the 2nd stage-potential decay in 9000 s. The 2

absence of the 2nd stage-potential decay in Cu containing alloys results from the Cu 3

enrichment in the passive film. The Cu enrichment in the passive film is more 4

intensified either with an increase in Cu content or with an increase in holding time at 5

OCP. The Cu enrichment in the passive film was also associated with the decrease in 6

the vacancy concentration ([VMχ-]) in the passive film, as confirmed by the Mott-7

Schottky analysis.8

9

Acknowledgment10

The authors gratefully acknowledge the financial support of the work by POSCO and 11

Brain Korea (BK 21) project.12

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Figure Captions:1

Fig. 1 Potential decay curves of Fe-20Cr-xCu (x=0, 2, 4 wt.%) alloys, measured after 2

passivation for 1 h at 400 mVSCE in deaerated 0.1 M H2SO4 solution at 30 oC. The 3

polarization responses of Fe-20Cr-xCu (x=0, 2, 4 wt.%) alloys in the same solution is 4

shown in the inset of Fig.1 (scan rate = 1 mV/s).5

Fig. 2 XPS depth concentration profiles of the passive film formed on Fe-20Cr-xCu (x=0, 4 6

wt.%) alloys for 1 h at 400 mVSCE in deaerated 0.1 M H2SO4 solution at 30 oC. (a) 7

Fe-20Cr, (b) Fe-20Cr-4Cu, (c) binding energy of Cu through the etch time from (b).8

Fig. 3 Mott-Schottky analysis of Fe-20Cr-xCu (x=0, 2, 4 wt.%) alloys passivated for 1 h at 9

400 mVSCE in deaerated 0.1 M H2SO4 solution at 30 oC. (a) Mott-Schottky plots, (b) 10

acceptor density, (c) donor density.11


wt.%) alloys (a) Fe-20Cr; passivated for (1 h at 400 mVSCE + 1 h OCP), (b) Fe-20Cr-13

4Cu; passivated for (1 h at 400 mVSCE + 1 h OCP), (c) Fe-20Cr-4Cu; passivated for 14

(1 h at 400 mVSCE + 24 h OCP).15

Fig. 5 Cu-XPS spectra of on the surface of Fe-20Cr-4Cu alloy. (a) passivated for 1 h at 400 16

mVSCE + 1 h OCP (etch time 0 at Fig. 4(b)), (b) passivated for 1 h at 400 mVSCE + 24 17

h OCP (etch time 0 at Fig. 4(c)).18

Fig. 6 Mott-Schottky analysis of Fe-20Cr alloy passivated for (1 h at 400 mVSCE + 1 h OCP) 19

and Fe-20Cr-4Cu alloy passivated for (1 h at 400 mVSCE + 1 h OCP) and for (1 h at 20

400 mVSCE + 24 h OCP) in deaerated 0.1 M H2SO4 solution at 30 oC.21

22

Table Caption23

Table 1 Chemical composition (wt.%) of Fe-20Cr-xCu (x=0, 2, 4 wt.%) alloys. Analyzed by 24

EDS.25

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Table 2. OCP and Ecorr values that are measured from the potential decay and polarization 1

responses of Fe-20Cr-xCu (x=0, 2, 4 wt.%) alloys shown in Fig.1.2

Table 3 Acceptor density of Fe-20Cr alloy passivated for (1 h at 400 mVSCE + 1 h OCP) and 3

Fe-20Cr-4Cu alloy passivated for (1 h at 400 mVSCE + 1 h OCP) and (1 h at 400 4

mVSCE + 24 h OCP) in deaerated 0.1 M H2SO4 solution at 30 oC.5

6

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21

1

Table 1. Chemical compositions (wt.%) of Fe-20Cr-xCu (x=0, 2, 4 wt.%) alloys. Analyzed by 2

EDS 3

Alloys Fe Cr Cu

Fe-20Cr 79.35 20.65 -

Fe-20Cr-2Cu 77.21 20.70 2.09

Fe-20Cr-4Cu 75.01 21.05 3.94

4

5

Table 2. OCP and Ecorr values that are measured from the potential decay and polarization 6

responses of Fe-20Cr-xCu (x=0, 2, 4 wt.%) alloys shown in Fig.1. 7

Type of Alloy OCP after potential decay Ecorr in polarization curve

Fe-20Cr 1

st stage at 3000 s -288 ± 12 mVSCE

-624 ± 4 mVSCE 2

nd stage at 10000 s -654 ± 5 mVSCE

Fe-20Cr-2Cu 1


-583 ± 3 mVSCE 2

nd stage none

Fe-20Cr-4Cu 1


-580 ± 3 mVSCE 2

nd stage none

8

9

10

Table 3. Acceptor density of Fe-20Cr alloy passivated for (1 h at 400 mVSCE + 1 h OCP) and 11

Fe-20Cr-4Cu alloy passivated for (1 h at 400 mVSCE + 1 h OCP) and (1 h at 400 mVSCE + 24 12

h OCP) in deaerated 0.1 M H2SO4 solution at 30 oC. 13

Fe-20Cr passivated for (1 h at 400 mVSCE + 1 h

OCP)

Fe-20Cr-4Cu

passivated for (1 h at

400 mVSCE + 1 h OCP)

Fe-20Cr-4Cu

passivated for (1 h at

400 mVSCE + 24 h OCP)

Acceptor density

(NA, VMχ-

) 4.43 x 10

21 3.51 x 10

21 2.82 x 10

21

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1

Fig. 1 Potential decay curves of Fe-20Cr-xCu (x=0, 2, 4 wt.%) alloys, measured after 2

passivation for 1 h at 400 mVSCE in deaerated 0.1 M H2SO4 solution at 30 oC. The polarization 3

responses of Fe-20Cr-xCu (x=0, 2, 4 wt.%) alloys in the same solution is shown in the inset of 4

Fig.1 (scan rate = 1 mV/s). 5

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1

2

3


wt.%) alloys for 1 h at 400 mVSCE in deaerated 0.1 M H2SO4 solution at 30 oC. (a) Fe-20Cr, 5

(b) Fe-20Cr-4Cu, (c) binding energy of Cu through the etch time from (b). 6

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1

2

3 Fig. 3 Mott-Schottky analysis of Fe-20Cr-xCu (x=0, 2, 4 wt.%) alloys passivated for 1 h at 4

400 mVSCE in deaerated 0.1 M H2SO4 solution at 30 oC. (a) Mott-Schottky plots, (b) acceptor 5

density, (c) donor density. 6

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3 Fig. 4 XPS depth concentration profiles of the passive film formed on Fe-20Cr-xCu (x=0, 4 4

wt.%) alloys (a) Fe-20Cr; passivated for (1 h at 400 mVSCE + 1 h OCP), (b) Fe-20Cr-4Cu; 5

passivated for (1 h at 400 mVSCE + 1 h OCP), (c) Fe-20Cr-4Cu; passivated for (1 h at 400 6

mVSCE + 24 h OCP). 7

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Fig. 5 Cu-XPS spectra of on the surface of Fe-20Cr-4Cu alloy. (a) passivated for 1 h at 400 3

mVSCE + 1 h OCP (etch time 0 at Fig. 4(b)), (b) passivated for 1 h at 400 mVSCE + 24 h OCP 4

(etch time 0 at Fig. 4(c)). 5

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Fig. 6 Mott-Schottky analysis of Fe-20Cr alloy passivated for (1 h at 400 mVSCE + 1 h OCP) 2

and Fe-20Cr-4Cu alloy passivated for (1 h at 400 mVSCE + 1 h OCP) and for (1 h at 400 3

mVSCE + 24 h OCP) in deaerated 0.1 M H2SO4 solution at 30 oC. 4

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Effects of Cu on the Passive Film Stability of Fe-20Cr-x Cu (x = 0, 2, 4 wt.%) Alloys in H2SO4...

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Transcript of Effects of Cu on the Passive Film Stability of Fe-20Cr-x Cu (x = 0, 2, 4 wt.%) Alloys in H2SO4...