Corrosion Science 53 (2011) 3254–3261

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Corrosion Science

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Influence of Zn on oxide films on Alloy 690 in borated and lithiated hightemperature water

Junbo Huang, Xiahe Liu, En-Hou Han ⇑, Xinqiang Wu ⇑State Key Laboratory for Corrosion and Protection, Liaoning Key Laboratory for Safety and Assessment Technique of Nuclear Materials, Institute of Metal Research,Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 January 2011Accepted 2 June 2011Available online 16 June 2011

Keywords:A. AlloyA. ZnB. EISB. XPSC. High temperature corrosion

0010-938X/$ - see front matter � 2011 Elsevier Ltd.doi:10.1016/j.corsci.2011.06.001

⇑ Corresponding authors. Tel.: +86 24 2389 3841 (E(X. Wu); fax: +86 24 2389 4149.

E-mail addresses: ehhan@imr.ac.cn (E.-H. Han), xq

To clarify the corrosion control effect of Zn injection into hydrothermal environments, the oxide films onAlloy 690 in the deaerated borated and lithiated water have been investigated using potentiodynamicpolarization curves, electrochemical impedance spectra at 300 �C and ex-situ X-ray photoelectron spec-troscopy. With Zn injection in the solution, ZnCr2O4 and ZnFe2O4 were formed in the inner and outer lay-ers of the oxide films on Alloy 690, respectively, through exchange reactions between Zn2+ and Fe2+/Ni2+.A simple model for oxide film structure change and the mechanism of corrosion inhibition by Zn injectionis proposed and discussed.

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1. Introduction corrosion cracking (PWSCC) susceptibility. Liang et al. [5] believed

Nickel-based alloys such as Alloy 600 and Alloy 690 are widelyemployed as structural materials in hydrothermal environments.Zn treatment is known to cause changes in composition as wellas changes in crystalline morphology of the two layers, which isresponsible for lessening corrosion on the nickel-based alloys inthe simulated primary water. It is necessary to investigate thecomposition and structure of oxides formed on the surface of nick-el-based alloys in the Zn-containing solutions. However, the Zninfluence of the chemical composition and structure of the oxidefilms on their protective properties is still a matter of considerabledebate [1–6].

Ziemniak et al. [1–3] claimed that the two-layer corrosion oxide,M(Fe1 � xCrx)2O4, on nickel-based alloys exposed in the simulatedprimary coolants with Zn injection transformed into a chromite-rich spinel oxide phase, (Zn0.55Ni0.3Fe0.15)(Fe0.25Cr0.75)2O4, andrecrystallized nickel metal. The replacement of Ni2+ and/or Fe2+ byZn2+ results in outer smaller sized ferrite-based crystals retarded in-gress of water/oxidants. Kawamura et al. [4] considered that thethick oxide film consisting of FexNi1 � xFe2O4 and FexNi1 � xCr2O4

on Alloy 600 without Zn injection transformed into thinner andmore stable Zny + zFex � yNi1 � x � zCr2O4 spinels with Zn injectionin simulated PWR primary water. The outermost NiFe2O4 layermight be diminished or thinned, leaving the inner chromitesMCr2O4 responsible for the reduction in primary water stress

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.-H. Han), +86 24 2384 1883

wu@imr.ac.cn (X. Wu).

that Zn2+ could displace all metal ions (including Fe3+) in the spineloxide formed in the simulated BWR conditions and the ability wasin the order of Fe3+, Co2+ >> Ni2+. Beverskog [6] claimed, fromthermodynamic calculation, that with Zn injection new solid(Zn(FexCr1 � x)2O4) oxides did not form on the old oxide pre-film,consisting of outer NiFe2O4 oxide layer and inner FeCr2O4 oxidelayer. ZnCr2O4 was only formed on the bare metals in the Zn-con-taining solution.

Alloy 690 has recently been used as a steam generator tubematerial in water-cooled nuclear power reactors as an alternativeto Alloy 600. It is reasonable for us to suppose the stable spinelsNixFe1 � xFe2O4, NixFe1 � xCr2O4, ZnFe2O4 and ZnCr2O4 on Alloy690 formed in the Zn-containing solution while comparing theexperimental data with the thermodynamic calculation. In thepresent work, electrochemical properties of the oxide films formedon Alloy 690 in simulated primary coolant with Zn injection werestudied using potentiodynamic polarization and electrochemicalimpedance spectra (EIS). The chemical composition and the thick-ness of the oxide films were investigated by X-ray photoelectronspectroscopy (XPS), and the structure and stability of the oxidefilms were discussed using the relevant potential (E)-pH diagram.

2. Experimental procedures

2.1. Material and solution

The chemical composition of Alloy 690 used in the present workis given in Table 1. The alloy was solution annealed at 1060 �C for

Table 1Chemical compositions of Alloy 690 used in the present work (wt.%).

C N Cr Fe Mn P Si Al Ti Cu Nb Co Ni

0.013 0.01 29.15 9.19 0.21 0.01 0.02 0.26 0.305 0.01 0.01 0.01 Bal.

J. Huang et al. / Corrosion Science 53 (2011) 3254–3261 3255

0.5 h and isothermally treated at 715 �C for 16 h. Specimens of1 cm � 1 cm area were gradually ground with silicon papers upto #1500 grit. Then the specimens were welded to 316 stainlesssteel wires, which were subsequently shielded by heat-shrinkablepolytetrafluoroethylene tubes. The testing electrolyte was chosenas 1500 ppm (lg g�1) B as H3BO3 and different concentrations ofLi solutions in deionized water of conductivity 0.05 lm cm�1 attemperature 300 �C and pressure 10 MPa to simulate normal andabnormal water chemistry in the primary water (Table 2). The con-centration range of Zn injection was from 10 ppb (lg kg�1) to150 ppb. Before the experiments, the solution was deaerated bycontinuously bubbling with nitrogen gas (99.999%) for 2 h and anitrogen overpressure was maintained to ensure the low oxygencontent of <30 ppb. The mean heating and cooling rates for thetesting solution are 1.57 �C min�1.

2.2. Electrochemical measurements

Potentiodynamic polarization curves were measured using athree-electrode cell with a platinum counter electrode and anexternal pressure balanced Ag/AgCl reference electrode in a Ni-lined stainless autoclave having 2 l volume. The reference elec-trode was housed in separate compartment that was maintainedat ambient temperature and system pressure via a solution bridge.The reference solution was 0.1 M KCl. All electrode potentials inthe present work have been converted to standard hydrogen elec-trode (SHE) according to the following relationship [7]:

ESHE ¼ Eobs þ 0:2866� 0:001ðT � T0Þ þ 1:745� 10�7ðT � T0Þ2

� 3:03� 10�9ðT � T0Þ3 ð1Þ

where ESHE represents the electrode potential vs SHE, Eobs is themeasured electrode potential vs the Ag/AgCl external referenceelectrode, T is the experimental temperature in �C and T0 is theroom temperature (25 �C).

Then potentiodynamic polarization experiments were per-formed at a scan rate of 0.5 mv s�1 after pretreatment for 10 minat �1.2 V (vs the Ag/AgCl external reference electrode). The EISwas measured over the frequency range 100 kHz to 10 mHz atthe open circuit potential (Ecorr) with an AC amplitude of 10 mV(rms) after 24 h immersion. The potentiodynamic experimentsand EIS measurements were carried out with an EG & G Model273 and a double phase synchronous detector (Model 5210 lock-in amplifier).

Table 2Compositions of testing solutions (ppm).

Test solutions B as H3BO3 Li as LiOH�H2O Zn as ZnO pH300 �C

1 1500 2.3 0 6.82 1500 23 0 7.43 1500 230 0 8.64 1500 690 0 9.15 1500 2.3 0.01 6.86 1500 2.3 0.05 6.87 1500 2.3 0.15 6.8

Neutral solution 5.6

pH300 �C of testing solutions are calculated by a commercial software pHSC4.

3. Analytical study

The specimens for ex-situ XPS measurements were prepared bypassivating for 24 h at a constant potential in the passive potentialrange (�400 mV above Ecorr). After cooling down, the specimenswere transferred to the XPS spectrometer in the vacuum dryingcabinet immediately. The XPS measurement was performed usingan ESCALAB 250 X-ray photoelectron spectrometer. Photoelectronemission was excited by monochromatic an Al Ka source operatedat 150 W with an initial photo energy 1486.6 eV. The C 1s peakfrom contaminative carbon at 285 eV was used as a reference tocorrect the charging shifts. The depth profile information was ob-tained by sputtering the specimens with a scanning argon-iongun operating at ion energy of 2 keV. The sputtering rate was esti-mated to be about 0.2 nm s�1 (vs Ta2O5).

In present work, target factor analysis (TFA) [1,8] was appliedafter selecting a given photoelectron energy region for chemicalstate speciation, e.g., the Ni 2p region (844–887 eV), Cr 2p (568–594 eV), Fe 2p (700–740 eV), Fe 3p (50–62 eV) and Zn 2p (1015–1052 eV). The principle component analysis was performed on allspectra of a given sample taken at different sputtering time. TheTFA for the above chemical species except for Fe 3p was performedon the basis of the corresponding test components. Test compo-nents were taken as single Gaussian–Lorentzian curves obtainedby fitting the spectrum of the original sample after backgroundsubtraction. The resulting XPS Fe 3p peaks were curve-fitted usingthe standard samples of Fe2O3 and 2FeO�SiO2 and pure Fe afterbackground subtraction [9,10]. The algorithms used were pro-grammed with Matlab version 6.5.

3.1. Potentiodynamic polarization curves

Fig. 1 shows potentiodynamic polarization curves for Alloy 690at 300 �C in 1500 ppm B and different concentrations of Li solu-tions without Zn injection. Ecorr decreased sharply and the passivecurrent density declined slightly with increasing pH values. It issuggested that the Alloy 690 will form an oxide film in all pH solu-tions investigated and have a good corrosion resistance in theabove alkaline solutions. Fig. 2 shows potentiodynamic polariza-tion measured curves after 24 h immersion. The anodic currentdensity declined with immersion time and finally became stable.Furthermore, a secondary passivation was observed at a given po-tential which can be attributed to the variation of the oxide film onAlloy 690 [11].

Fig. 3 shows potentiodynamic polarization curves for Alloy 690in different Zn concentration solutions at 300 �C. The Ecorr and thepassive current density declined with increasing Zn concentration.The passive current density was in the magnitude of 10�4 A cm�2.Fig. 4 shows potentiodynamic curves measured after 24 h immer-sion in Zn injection solutions at 300 �C. Increasing the immersiontime caused a further decrease in the passive current density. Nosecondary passivation was observed on the potentiodynamicpolarization curve in the solution, suggesting that the compositionand structure of the oxide film on Alloy 690 were different with-out/with Zn injection.

3.2. Electrochemical impedance spectroscopy

Fig. 5 shows the Nyquist diagram for the oxide film on Alloy 690at Ecorr after 24 h immersion. Two capacitive arcs appeared on the

Fig. 1. Potentiodynamic curves for Alloy 690 in different pH solutions without Zn at300 �C.

Fig. 2. Potentiodynamic curves for Alloy 690 in the Zn-free solutions at 300 �C afterdifferent immersion time.

Fig. 3. Potentiodynamic curves for Alloy 690 in 1500 ppm B and 2.3 ppm Li solutionwith different Zn concentration at 300 �C.

Fig. 4. Potentiodynamic curves for Alloy 690 in the Zn-containing solutions at300 �C after 24 h immersion.

Fig. 5. Nyquist diagram of the oxide film on Alloy 690 in the Zn-free solution after24 h immersion at 300 �C.

Fig. 6. Equivalent circuit to fitting the impedance spectra: Rs: solution resistance,R1: charge transfer resistance, CPE1: duplex layer capacitance, R2: oxide resistance,CPE2: oxide capacitance.

3256 J. Huang et al. / Corrosion Science 53 (2011) 3254–3261

Nyquist diagram, indicating that the equivalent circuit for this sys-tem consisted of a series of two parallel resistance–capacitance cir-cuits [12]. A rather thick (up to a few lm) oxide layer at the hightemperature solution exists between the metal and solution. Thisoxide film can consist of an epitaxial spinel barrier layer and a por-ous deposited spinel type layer, the Helmholtz double layer on theinterface between metal and solution could be combined into theporous outmost layer. According to the shape of the above imped-ance plot, it is assumed that the Helmholtz double layer and theoutmost layer of oxide film may have a low resistance (high fre-quency arc), and the oxide film may have a relatively high resis-tance (low frequency arc). Kim et al. [12] and Park et al. [13]

reported two similar capacitive arcs from the impedance spectraobtained for anodic films on Alloy 600. Many reasons have beenproposed in the literature to explain this behavior such as surfaceroughness, frequency dispersion of time constants due to localinhomogeneities in the dielectric material, porosity mass transporteffects and relaxation effects [14]. In order to account for these ef-fects, a constant phase element (CPE) must be introduced into theequivalent circuit to reproduce the experiment results, accordingto the theory presented by Jonscher [15]. In the present work, anequivalent circuit consisting of a pure resistance R in series withparallel circuits of CPE can be proposed as shown in Fig. 6. Thechoice of this circuit was a compromise between a reasonable fit-ting of the experimental values, limited by the used software, andthe keeping of the number of circuit elements to a minimum. Forthe proposed equivalent circuits, the following transfer functionis obtained:

Zx ¼ Rs þR1

1þ ðjxR1C1Þn1þ R2

1þ ðjxR2C2Þn2ð2Þ

where j is the imaginary unit ðj ¼ffiffiffiffiffiffiffi

�1p

Þ), x is the angular fre-quency, Rs is the solution resistance, C denotes ideal capacitors, nis the dispersion parameter indicating the deviation of the modelfrom pure R–C circuits and n1, n2 correspond to the parametersfitted at high and low frequencies, respectively. The fitted values

ig. 8. Composition depth profiles obtained from XPS analysis for the oxide film onlloy 690 passivated at 300 �C for 24 h: (a) Zn-free solution and (b) 50 ppb Zn-ontaining solution.

J. Huang et al. / Corrosion Science 53 (2011) 3254–3261 3257

of parameters of the equivalent circuit are listed in Table 3. With thechoosed equivalent, the charge transfer resistance (R1) was approx-imate complex related with the Helmholtz double layer and theporous outmost layer. It was irregular with increasing Zn concen-tration. The value of R2 related with the oxide film increased sharplywith increasing Zn concentration.

3.3. XPS results

Fig. 7 shows the surface survey for specimens passivated at300 �C in 1500 ppm B + 2.3 ppm Li + 50 ppb Zn solution. The Ni2p, Cr 2p, Fe 2p, Fe 3p, Zn 2p and O 1s peaks were observed inthe oxide films formed in the Zn-containing solution. As indicatedin Fig. 8b, significant amounts of Zn and Fe ions were present al-most throughout the outer oxide film. The total percentage of Znand Fe in this layer was about 20% at the outmost surface, suggest-ing a deposit layer of mixed Zn–Fe oxides. The Cr content in the in-ner layer increased gradually to that of the base material withincreasing sputtering time and then declined. The Zn-bearing layerand Cr-rich layer were overlapped to some extent, suggesting aninner layer of Zn–Cr oxides. The Ni content at the outermost sur-face, however, was much lower than that in Zn-free solution(Fig. 8a). The oxide film in the Zn-containing solution (Fig. 8b)was much thinner than that in the Zn-free solution (Fig. 8a). TheZn injection changed the composition of the oxide films formedon Alloy 690 which would affect the stability of oxide films.

The results of TFA for the Ni 2p spectra showed the existence oftwo component, which was related to Ni0 around 852.9 ± 0.1 eVand Ni2+ around 856.3 ± 0.2 eV on Alloy 690 at 300 �C. The Ni2+

was involved in the presence of Ni(OH)2 and Ni(FexCr1 � x)2O4

[16–18]. Ni(OH)2 could be only detected in the outmost layer ofthe oxide films. The origin of Ni(OH)2 can be explained either bya surface deposition of hydrolyzed nickel. The results of TFA forCr 2p region revealed the possible presence of two components,most likely Cr0 around 574.4 ± 0.1 eV and Cr3+ around577.0 ± 0.1 eV on Alloy 690 at 300 �C. The peak position and shapeof the Cr3+ compounds namely, Cr2O3, and MCr2O4 are very close

Table 3Fitted values of parameters of the equivalent circuit in Fig. 6.

Without Zn 10 ppb Zn 50 ppb Zn 150 ppb Zn

Rs (ohm) 207.5 174.3 196.8 225.9C1 1.17 � 10�7 4.76 � 10�8 5.37 � 10�8 1.15 � 10�7

n1 0.8324 0.8773 0.8764 0.8348R1 (ohm) 524.3 446.1 495.1 476.1C2 0.002172 0.005873 0.005389 0.005496n2 0.6979 0.4508 0.3466 0.3727R2 (ohm) 1564 7271 1.29 � 104 1.66 � 108

Fig. 7. XPS survey on the specimen surface passivated for 24 h at 300 �C in the50 ppb Zn-containing solution.

FAc

[17,19,20]. For Cr3+, the exact identification of the compound wasdifficult. The obtained components as well as the reproduction ofone of the recorded spectra for Ni 2p and Cr 2p in the oxide filmson Alloy 690 at 300 �C are shown in Figs. 9 and 10, respectively.The agreement between the reproduction and the data was good,and the peak shapes or binding energies of the components werewell in accordance with those reported in the literatures for Ni0,Ni2+, Cr0 and Cr3+ [16–20]. The agreement between the reproduc-tion and the data was practically equally good for all the recordedspectra. The atomic concentration profiles of Ni and Cr with sput-tering times were summarized on Tables 4 and 5 based on the re-sults of TFA. They indicated that the oxide films were composed ofNi2+ and Cr3+ compounds in the Zn-free solution and the Zn-con-taining solution. There were an outer NixFe1 � xFe2O4 layer and aninner NixFe1 � xCr2O4 layer in the Zn-free solution in Table 4. It isconsistent with the results of Marcus et al. [21], who observed thatthe Fe-rich outer layer overlying the inner Cr-rich layer on Alloy600 in the simulated PWR coolant in the initial stages of exposure.There would be some recrystallized Ni0 and Cr0 formed within theoxide layers on Alloy 690 in the Zn-free solution in Table 4. But,there were no detectable Cr0 and few recrystallized Ni0 throughoutthe whole oxide film in the Zn-containing solution in Table 5. It isseems that more protectively compact oxides forming in the Zn-containing solution obstruct the diffusion of Cr0 and Ni0.

The results of TFA for Fe 3p region revealed the presence ofthree components, most likely Fe0 around 53 ± 0.1 eV, Fe2+ around53.7 ± 0.1 eV, and Fe3+ around 55.6 ± 0.1 eV on Alloy 690 at 300 �C[9,10]. The atomic concentration profiles of Fe with sputteringtimes were summarized on Tables 4 and 5 based on the resultsof TFA. The existence of Fe2+ and Cr3+ in Table 4 suggests the pos-sible presence of FeCr2O4 in the oxide films of Alloy 690 in the Zn-free solution. However, the analysis of MCr2O4 in the Zn-containingsolution should be combined with the results of TFA for Zn 2p re-gion on Table 5.

Fig. 9. Component spectra, Ni0 and Ni2+ obtained from the TFA and a reproductionof one of Ni 2p spectra for the oxide film on Alloy 690 at 300 �C: (a) Zn-free solution,sputtering 70 s and (b) 50 ppb Zn-containing solution, sputtering 10 s.

ig. 10. Component spectra, Cr0 and Cr3+ obtained from the TFA and a reproductionf one of Cr 2p spectra for the oxide film on Alloy 690 at 300 �C: (a) Zn-free solution,uttering 70 s and (b) 50 ppb Zn-containing solution, sputtering 10 s.

Table 4The atomic concentration profiles of chemical species with sputtering times on oxidefilms formed on Alloy 690 in Zn-free solution at 300 �C.

Sputtering time (s) Atomic percent (%)

Ni0 Ni2+ Cr0 Cr3+ Fe0 Fe2+ Fe3+

0 n.d. 77.93 n.d. 11.69 n.d. n.d. 10.3870 21.94 53.30 0.09 16.15 n.d. 3.94 4.56

310 37.94 31.09 2.40 22.15 2.43 3.53 0.46670 57.63 10.59 10.17 16.16 1.15 4.30 n.d.

1090 66.34 1.06 17.47 8.47 2.08 4.58 n.d.

n.d.: no detected.

3258 J. Huang et al. / Corrosion Science 53 (2011) 3254–3261

Fig. 11 shows the results of TFA for the Zn 2p spectra core levelspectra and their decompositions for oxide films formed on Alloy690 at 300 �C. The spectra were systematically decomposed intotwo peaks: one located at a BE of 1021.4 ± 0.1 eV and the otherone at BE of 1022.1 ± 0.1 eV. By referring to the published data[22–24], the signal at a BE of 1021.4 eV is assigned to ZnO/ZnFe2O4

[22,23] and the signal at 1022.1 eV to ZnCr2O4 [24]. The concentra-tion ratio of Zn2+⁄/Fe3+ in our experiment is more than 0.5 whilesputtering 0 s and 10 s. Besides ZnFe2O4, there should be someZnO deposit from the solutions in outmost layer of the oxide filmson Alloy 690 in Zn-containing solution. The ratio of Zn2+⁄⁄/Cr3+

remaining about 0.5 when sputtering 70 s in Table 5, suggest thepresence of ZnCr2O4 in the inner layer of oxide films on Alloy690 in the Zn-containing solution.

4. Discussion

4.1. Oxide film characteristics with Zn injection

Two capacitive arcs were observed on experimental impedancespectra in the present work, which could be related to the well-known duplex structure of oxide films formed on Cr–Fe–Ni alloysin high-temperature aqueous solutions [13,14,25]. The outer layerof the oxide film is reported to be Fe-rich NixFe1 � xFe2O4 oxideswhile the inner layer is Cr-rich NixFe1 � xCr2O4 oxides in the Zn-freesolutions. The low frequency parameters are believed to be relatedto the protective oxide film. In the present work, the oxide resis-tance R2 increased markedly with the injection of Zn, suggestinga more protective oxide film and an increased film compactness(Table 3). The thinning of the oxide film is a factor that could lead

Fosp

to an increase in film capacitance. Exposure to high temperaturewater of Alloy 690 led to the formation of oxide film whose thick-ness was typically 200–300 nm [14,25]. Larger resistance valuesmay also give rise to a decrease of the density of charge carriers.It would also lead to a decrease of the exchange current densityof the redox couple with increasing concentration of Zn injection,as indicated in Figs. 3 and 4.

Based on the EIS and XPS analysis in our experiment, the oxidelayer formed on Alloy 690 in the Zn-containing solution at 300 �Chad a duplex character, namely, an inner ZnCr2O4 layer and an out-er ZnFe2O4 layer. It is estimated that a smooth and thin oxide filmformed in the presence of Zn additions could suppress anodic dis-solution from the surface of Alloy 690 (Fig. 3). Kawamura et al. [4]observed a similar behavior on Alloy 600 in the high-temperaturehydrogenated water. The displacement of Zn2+ from the oxide filmswould be further discussed by the thermodynamic analysis later inSection 4.2.

Table 5The atomic concentration profiles of chemical species with sputtering times on oxide films formed on Alloy 690 in 50 ppb Zn-containing solution at 300 �C.

Sputtering time (s) Atomic percent (%)

Ni0 Ni2+ Cr0 Cr3+ Fe0 Fe2+ Fe3+ Zn2+* Zn2+**

0 0.45 28.00 n.d. 32.26 n.d. n.d. 19.94 19.35 n.d.10 0.53 23.65 n.d. 31.93 n.d. 10.85 10.07 11.89 11.0870 3.20 17.53 n.d. 39.73 n.d. 10.58 9.72 0.90 18.34

220 15.21 18.12 n.d. 54.44 1.38 5.88 n.d. n.d. 4.97430 66.02 n.d. 14.29 12.37 2.86 4.46 n.d. n.d. n.d.

* ZnFe2O4, the outmost porous oxide films when sputtering 0 s and 10 s were composed of some ZnO deposit from the solution [22].** ZnCr2O4.

Fig. 11. Component spectra, ZnFe2O4(ZnO) and ZnCr2O4 obtained from the TFA anda reproduction of one of Zn 2p spectra for the oxide film on Alloy 690 at 300 �C in50 ppb Zn-containing solution, sputtering 10 s.

J. Huang et al. / Corrosion Science 53 (2011) 3254–3261 3259

4.2. Thermodynamic stability of oxide film with Zn injection

It is a satisfactory approximation to neglect the pressure depen-dency and to measure the volume or density at any convenient andmoderate pressure [26]. It is reasonable to calculate the stable re-gions for Fe–Cr–Ni–Zn–H2O system with the standard Gibbs freeenergy changes DG0(T) at T(�C). The method of thermodynamicalgorithm has been discussed in Ref. [11]. Compound or speciesconsidered and their thermodynamic values are listed in Table 6[4,27–35].

Fig. 12 shows the (E)-pH diagram for Fe–Cr–Ni–Zn–H2O systemat 300 �C. The full lines on the diagram separate the stable regionsfor Fe–Cr–Ni species. The dash lines separate the stable regions forZn species. NiFe2O4, Fe3O4, NixFe1 � xCr2O4 were employed as thepresent phases in the oxide film on Ni–Cr base alloys and stainlesssteels [25,36–38]. ZnCr2O4 was employed as possible present phasein the oxide film in the Zn-containing solution [39]. ZnCr2O4 is stablewith wider (E)-pH range than NiCr2O4 and FeCr2O4 in the passive po-tential region (denoted by the vertical dot line in Fig. 12) in the pres-ent work. According to Lin et al. [40], the free energy of formation ofthe above spinel oxides is in the order of ZnCr2O4 < FeCr2O4 <NiCr2O4. Therefore, in the region where FeCr2O4, NiCr2O4 andZnCr2O4 overlap, the exchange reactions will obey reactions (3)and (4):

FeCr2O4 þ Zn2þ ! ZnCr2O4 þ Fe2þ ð3Þ

NiCr2O4 þ Zn2þ ! ZnCr2O4 þ Ni2þ ð4Þ

The free energy changes of reactions (3) and (4) at 300 �C are allnegative values (�18.41 and �49.5 kJ mol�1) [41], indicating thatthese reactions tend to proceed to the right direction. In addition,the solubility of ZnCr2O4 in the simulated PWR primary coolant is

much smaller than that of FeCr2O4 and NiCr2O4 [41]. As a result,Zn injection into the simulated primary water can retard corrosionof Ni–Cr base alloys due to the change of the structure of oxide films.

Without Zn injection, NixFe1 � xFe2O4 spinel oxide would beformed in the outer layer of the oxide film, which causes the sec-ondary passivation on the potentiodynamic polarization curves(Fig. 2) [11]. However, no secondary passivation was observed un-der the condition of Zn injection (Figs. 3 and 4). The Ni content inthe outmost surface layer was much lower while the Zn and Fecontents are relatively high, indicating that Zn–Fe-rich oxides in-stead of NixFe1 �xFe2O4 were formed in the outer layer. Combinedwith the results of TFA in Table 4 and 5, there are two possiblereactions. On one hand, the free energy of formation of the spineloxides is in the order of ZnFe2O4 < Fe3O4 < NiFe2O4 [42]. ZnFe2O4

would be formed preferably according to reaction (5) and such acompact oxide layer would block the transport of Ni and furtherslow down the growth of a Ni-rich layer.

Fe3O4 þ Zn2þ ! ZnFe2O4 þ Fe2þ ð5Þ

On the other hand, even if NiFe2O4 could be formed first, the ex-change reaction (6) can take place because ZnFe2O4 is much morestable than NiFe2O4 [41].

NiFe2O4 þ Zn2þ ! ZnFe2O4 þ Ni2þ ð6Þ

The stable range of ZnFe2O4 is out of the passive potential regionshown in Fig. 12, the substitution reactions (5) and (6) should havemixing limitation in the experimental condition. As a result, theouter NixFe1 � xFe2O4 layer in the Zn-free condition tended to be-come thin ZnFe2O4 oxides or disappear in the Zn-containing solu-tion, leaving a Cr-rich ZnCr2O4 layer in the oxide film (Fig. 8).More stable ZnCr2O4 is expected to be formed in the inner layerof the oxide film on Alloy 690 by Zn incorportion with higherconcentration of Zn2+ and after longer immersion time. As a result,the resistance of the oxide film increased with increasing Znconcentration.

4.3. Inhibitory mechanism of Zn injection

According to previous work [21,43–46], duplex-layer oxidefilms are usually formed in high-temperature aqueous environ-ments. The inner layer of the film is less porous and compact whilethe outer layer is porous and less adhesive. Fig. 13 is a schematicdiagram of an oxide film structure without and with Zn injection.In fact, Zn2+ should have been held strongly in the tetrahedral sitesin the crystal lattice [47]. Thus when Zn was injected, Zn2+ shoulddisplace the Fe2+ and Ni2+ which are held in the tetrahedral sites ofa the normal spinel, causing the formation of Zn(FexCr1 � x)2O4:

M��i !M2þ

aq þ V i ðM ¼ Fe or NiÞ ð7Þ

Vi þ Zn2þaq ! Zn��i ð8Þ

where M��i is a divalent cation in the interstice of oxide film, M2þ

aq isthe divalent cation in the solution, Vi is an empty interstice of oxide

Table 6Species considered and thermodynamic values used.

Species DG0(25 �C DS0(25 �C) DC0PðTÞ [J (mol K)�1] DG0(300 �C)

[J mol�1] [J (mol K)�1] A B � 103 C � 10�5 [J mol�1]

H2O �237191 70.08 75.44 0 0 �263975.51H2 0 130.6 27.28 3.264 0.502 �38833.13H+ 0 �22.2 �71 0 0 13175.37O2 0 205.15 29.975 4.148 �1.674 �59587.24ZnCr2O4 �1428350 127.4 173.791 9.40473 �26.8034 �1480415.82NiCr2O4 �1269137 129.7 167.2 17.87 �21.05 �1321836.54FeCr2O4 �1339400 152.2 157.019 29.2977 �17.1417 �1398043.91ZnFe2O4 �1063500 151.67 161.5 28.93 �26.53 �1122138.81NiFe2O4 �974600 125.9 77.4 235 1.42 �1030386.05Ni 0 30.12 16.99 29.46 0 �11656.36NiO �215936 37.99 �20.88 157.2 16.28 �233769.79Ni3O4 �711908 140.2 28 0 0 �753251.31Ni2O3 �469738 94.1 102 0 0 �505772.93Ni2+ �45600 �173.3 �184 0 0 �28146.97HNiO2

� �349218 62.84 �409.7 0 0 �325700.00CrO �350661 44.77 46.48 8.117 �3.682 �367953.02Cr2O3 �1049960 82.3 146.822 �3.86976 �5.23849 �1086833.75Cr2+ �166000 �126.4 �314.5 0 0 �99921.27Cr3+ �192000 �335.6 �196 0 0 �80191.81CrO2

� �535929 117.3 �385.7 0 0 �529777.49CrO4

2� �736802 80.33 �474 0 0 �711690.59HCrO4

� �773622 89.96 �282.3 0 0 �770248.83Fe 0 27.28 14.1 29.7 1.799 �10620.05FeO �256354 79.5 48.79 8.37 �2.803 �283467.89Fe3O4 �1015450 146.4 91.5 201 0 �1076294.04Fe2O3 �740986 89.96 98.28 77.82 �14.85 �779503.45Fe2+ �78900 �182.1 �188 0 0 �10100.97Fe3+ �15400 �382.5 �204 0 0 110102.35HFeO2

� �379183 41.92 �508.1 0 0 �340113.08FeO4

2- �467300 34.3 �474 0 0 �429530.34Zn 0 41.63 22.38 10.04 0 �14249.95ZnO �321900 43.93 48.99 5.104 �9.121 �338874.12Zn2+ �147100 �156.5 �164 0 0 �87730.95ZnO2

2� �389238 �56.32 �797.5 0 0 �294332.87HZnO2

� �464006 62.84 �409.6 0 0 �440497.97

In general, the values of DG0(25 �C) for each chemical species were given by [27], except for DG0(25 �C) of ZnCr2O4, NiCr2O4, FeCr2O4, Cr2O3, Fe3O4, NiFe2O4, FeO42� taken from

[28–30], Fe3+ given in [31] and ZnFe2O4 given in [4]. The data of standard entropy changes at 25 �C DS0(25 �C) given by [29,30,32], but Fe3+ given in [31]. At 300 �C, those weremodified for solids, H2O, H2 and O2 by the standard heat capacity changes DC0

PðTÞ from [28,30,33] and ions in solution estimated by the Criss–Cobble method [34,35].

Fig. 12. Potential (E)-pH diagram for the Fe–Cr–Ni–Zn–H2O system at 300 �C. Fulllines separate the stable regions for Fe–Cr–Ni species. Dash lines separate the stableregions for Zn species. The oxygen and hydrogen equilibria at 1 atm. arerepresented by dash dot line a and b, respectively. The vertical dot line revealsthe passive region in simulated PWR primary coolant in the present work.

3260 J. Huang et al. / Corrosion Science 53 (2011) 3254–3261

film, Zn2þaq is Zn2+ in the Zn-containing solution and Zn��i is Zn2+ in the

interstice of oxide film.The incorporation of Zn is assumed to proceed either via

sequential filling and emptying of available empty cation inter-stices or cation vacancies [48]. On one hand, the incorporation

of Zn in the oxide films will slow down the transport of cationvacancies in the inner layer by acting as an additional sink at theirsources. On the other hand, the incorporation of Zn in an inter-stice will slow down the transport of interstitial cations by actingas an additional sink of such cations at the inner/outer layer orouter layer/solution interfaces. The incorporation of Zn in eitherway will slow down transports of solid state point defects inthe oxides by decreasing their concentration, and in turn restrictthe hydrolysis and deposition of Ni on the alloy surface. As a re-sult, the outer layer of the oxide film becomes thinner with Zninjection and finally a thin protective oxide film is formed on Al-loy 690.

The maintenance of a surface film is important in the stress cor-rosion cracking (SCC) of Cr–Ni–Fe alloys in high-temperature aque-ous environments. According to the slip-oxidation mechanism,crack advance occurs through an iterative process of surface filmrupture during straining, which exposes the bare metal to environ-mental attack and passivation by film formation [49]. Zn injectionin the primary water improved the passivity or oxide film on Alloy690, which is believed to improve the resistance of the alloy to therelated localized corrosion such as SCC.

5. Conclusions

The effect of Zn injection on Alloy 690 was investigated in thedeaerated borated and lithiated water at 300 �C using potentiody-

Fig. 13. Oxide film characteristics and inhibition mechanism by Zn injection(M = Fe, Cr or Ni): (a) Zn-free solution and (b) Zn-containing solution.

J. Huang et al. / Corrosion Science 53 (2011) 3254–3261 3261

namic curves and EIS. Structures and chemical compositions of theoxide films formed were examined using XPS. Some conclusionscould be drawn as follows.

1. The chemical compositions and the structures of oxide filmsformed on Alloy 690 changed with Zn injection. The oxide filmsconsisted of Zn–Cr/Zn–Fe spinel oxides. The content of Ni inoxide films decreased rapidly and the oxide film became thin.With increasing Zn concentration, the resistance of the oxide filmincreased rapidly and the passive current density decreased.

2. Due to the low free energy of formation of spinel oxides, Zn2+

displaced Fe2+ and Ni2+ in the tetrahedral sites in the crystal lat-tice. The incorporation of Zn into the spinel oxides would slowdown transports of solid state point defects in the oxides bydecreasing their concentration. An oxide film composed ofZn(FexCr1 � x)2O4 was formed on Alloy 690 with Zn injection.

3. A simple model for oxide film characteristics and the inhibitionmechanism by Zn injection is proposed. It is suggested that thechange of oxide film characteristics induced by Zn injectioncause an improvement in the corrosion resistance of Alloy 690.

Acknowledgements

This study was jointly supported by the Science and TechnologyFoundation of China (50871113), the Special Funds for the MajorState Basic Research Projects (2011CB610505), the Major Scientificand Technological Special Project (2010ZX06004-010) and theInnovation Fund of Institute of Metal Research (IMR), ChineseAcademy of Sciences (CAS).

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