Potentiostatic Oxide Growth Kinetics on Ni-Cr and Co-Cr Alloys: Potential and pH Dependences

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Electrochimica Acta xxx (2015) xxx–xxx

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Potentiostatic Oxide Growth Kinetics on Ni-Cr and Co-Cr Alloys: Potential and pH Dependences Ahmed Y. Musa, Mehran Behazin, Jungsook Clara Wren * Department of Chemistry, the University of Western Ontario, London, Ontario N6A 5B7, Canada

A R T I C L E I N F O

A B S T R A C T

Article history: Received 14 August 2014 Received in revised form 19 February 2015 Accepted 20 February 2015 Available online xxx

Oxide growth kinetics on the Ni-Cr-Fe alloy Inconel 600 and the Co-Cr alloy Stellite 6 under potentiostatic polarization have been investigated by current measurements augmented by ex-situ surface analyses. The results reveal a mechanism for metal oxidation and oxide formation that is common to both alloys. The reaction thermodynamics for the oxidation of a metal determine whether a certain metal oxidation can or cannot occur. However, the metal oxidation proceeds via two competing pathways, oxide formation and metal ion dissolution. At pH 10.6 where the solubilities of FeII, NiII or CoII species are near their minima, oxide formation is favoured over metal ion dissolution. As the oxide grows, the rate of metal oxidation decreases with time due to an increase in the electrochemical potential barrier. The oxide formation occurs sequentially; the conversion of the preformed Cr2O3 film to chromite (FeCr2O4 or CoCr2O4) proceeds before the next layers of Fe3O4/NiFe2O4 and NiO/Ni(OH)2 grow on Inconel 600, or CoO/ Co(OH)2 grows on Stellite 6. The effect of a different EAPP is to limit the oxidation sequence. The pH does not directly affect the driving force for metal oxidation but it strongly influences the relative rates of oxide formation and metal dissolution, thereby affecting metal oxidation kinetics. ã 2015 Elsevier Ltd. All rights reserved.

Keywords: Ni-Cr-Fe alloy Co-Cr alloy Metal oxidation kinetics Oxide growth kinetics Potentiostatic polarization

1. Introduction Nickel-chromium-iron alloys such as Inconel 600 and Co-Cr alloys such as Stellite 6 are used in many applications that require high mechanical strength and corrosion resistance. These alloys are used for some applications in nuclear power plants where they may be exposed to ionizing radiation. Ionizing radiation decomposes water and produces redox active species (such as H2O2) [1]. Knowledge of both the uniform and localized corrosion kinetics of the alloys in the presence of radiation is required to assess their integrity over prolonged service lifetimes. Key to this assessment is an understanding of the relative rates of metal dissolution and oxide growth as a function of the water redox environment. Aqueous corrosion is an electrochemical process involving interfacial redox reactions, the oxidation of solid metal species coupled with the reduction of aqueous species accompanied by solid-liquid interfacial transfer and transport in the bulk phases of charged species [2]. The driving force for corrosion (the free energy of reaction) is the electrochemical potential difference between the reacting system (metal, metal oxide or aqueous species) as it exists and that of the system at equilibrium. It is related to the

* Corresponding author. Tel.: +1 519 661 2111; fax: +1 519 661 3022. E-mail address: [email protected] (J.C. Wren).

difference between the equilibrium potentials of the two coupled half-redox reactions involved in corrosion. Water radiolysis produces highly oxidizing species (
OH, H2O2, O2) [1,3,4] and a radiation field will affect the driving force for the interfacial redox reaction, and hence the rate of corrosion. In an environment where there is a continuous ionizing radiation field, water radiolysis will lead to the formation of steady-state concentrations of these oxidizing species. This effect of water radiolysis has been observed as an increase in the corrosion potential (Ecorr) on various alloy surfaces [4–6]. The rate of corrosion is controlled by a number of additional factors. Particularly important are the type and the thickness of the oxide film present on an alloy surface. Due to its semiconducting nature, an oxide film acts as a Coulombic potential barrier to charge transport as well as a chemical potential barrier to oxide formation [7–10]. Since an oxide film plays an important role in determining the corrosion resistance, the chemical composition and phase structure of the oxide film that can be formed, and the rate of its growth are factors in determining the rate of corrosion. Numerous studies have shown that the phase structure and the electrochemical properties of the oxide film play a key role in localized as well as general corrosion of Co-Cr and Ni-Fe-Cr alloys [9–12]. Nevertheless, there still exists some controversy over the type of oxide that can be formed, the mechanism of oxide formation and growth, and the effect of the oxide film on corrosion kinetics of

http://dx.doi.org/10.1016/j.electacta.2015.02.176 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: A.Y. Musa, et al., Potentiostatic Oxide Growth Kinetics on Ni-Cr and Co-Cr Alloys: Potential and pH Dependences, Electrochim. Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.02.176

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these alloys. This may have arisen due to the role of the different aqueous redox environments and mass transport conditions in those studies [13–21]. The challenge in fully understanding corrosion kinetics is linked to understanding the roles that elements of the solution environment (pH, temperature, ionic strength and the chemical and redox activities of aqueous species) play in the corrosion process for a given alloy. We have previously reported on detailed kinetic studies of the corrosion of an Fe-Cr alloy (stainless steel type-316L) [4] and a CoCr alloy (Stellite 6, at 25  C) [5,22]. In those studies the corrosion kinetics were investigated as a function of potential, pH and g-radiation exposure using a combination of corrosion tests and ‘in-situ’ kinetic measurements using electrochemical techniques. These measurements were augmented by oxide surface and depth analyses using various spectroscopic and imaging techniques, and by post-test solution analyses for the dissolved metal content in the solutions. Since then we have extended our work to include a wider range of aqueous environments, higher temperatures, and a number of Ni-Fe-Cr alloys. We have seen common features in the oxide growth kinetics of different of Cr-containing alloys. In this paper, we present new results from potentiostatic polarization studies on a Ni-Cr-Fe alloy (Inconel 600) and compare the results with those previously reported for Stellite 6 [22] identifying common features of oxide growth on the two alloys. As far as we are aware this is the first study to report on the combined effect of potential and pH on the corrosion behaviour of Inconel 600. 2. Experimental The elemental compositions of the Inconel 600 alloy used in this study and that of Stellite 6 are listed in Table 1. The polarization experiments were performed in a standard electrochemical cell consisting of a working electrode (WE) made of the alloy of interest, a counter electrode (CE) made of a Pt mesh and a saturated calomel reference electrode (RE). Details of the electrochemical cell, electrode and solution preparation, and the polarization experimental procedure can be found in references [5,22,23]. Only a brief description of the experimental procedures is given here. The WE was subjected to cathodic cleaning at 1.1 VSCE for about 10 min to remove easily reducible surface films before applying a potential, EAPP. The metal oxidation kinetics were followed by monitoring the current on the WE during potentiostatic polarization over 5 h. After the polarization, the oxide on an electrode surface was analyzed using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) with Ar+ sputtering. Detailed descriptions of the instruments and the spectral and depth analyses of elemental and oxide compositions can be found in references [4,5,22,23]. The range of potentials studied was from 0.8 VSCE to +0.2 VSCE. This spans potentials for reducing deaerated water to highly oxidizing water (e.g., that containing H2O2 or water exposed to ionizing radiation), while remaining within the water stability range [4,5,24]. The polarization experiments were performed at three different pH25  C values (6.0, 8.4 and 10.6) and at two different temperatures (25 and 80  C).

Table 1 Alloy compositions (wt.%). Alloy

Fe

Co

Cr

Ni

Minor elements

Inconel 600 Stellite-6

8 2.9

– Balance

15 27.6

Balance 2.6

Mn (1) Mo, C, Mn, Si, W (sum <6.3)

3. Results and discussion 3.1. Effect of Potential on Oxide Film Growth at pH25  C 10.6 Reaction thermodynamics determine whether a certain metal oxidation can or cannot occur, but the metal oxidation proceeds via two competing pathways, oxide film formation and metal ion dissolution. At pH25  C 10.6 where the solubilities of FeII, NiII or CoII species are near their minima, oxide film formation is favoured over metal ion dissolution. Hence, the type of oxide that can be formed and the effect of oxide growth on metal oxidation kinetics are most clearly observed at pH25  C 10.6. This section hence focuses on the oxide formation as a function of polarization potential (EAPP) at this pH. The effect of changing pH is discussed in Section 3.2. 3.1.1. Oxide film growth on Inconel 600 at pH25  C 10.6 3.1.1.1. Electrochemical analysis results. The current density, j, which is a direct measure of the net rate of oxidation on the working electrode, was monitored as a function of time during 5-h potentiostatic polarization. For a given pH and temperature, the polarization was performed at increments of 0.05 or 0.1 VSCE in the range from 0.8 VSCE to +0.2 VSCE. At potentials below 0.8 VSCE no anodic current was observed at any time. The data obtained for Inconel 600 at pH25  C 10.6 and 80  C are shown in Fig. 1. Multiple graphs are used to display the behaviour at different applied potentials more clearly. As can be seen from the log |j(t)| vs. log t plots, the current density decreases several orders of magnitude in less than an hour after initiation of the polarization. Due to the large span of the current densities that we measured, the effect of EAPP on the long-term kinetic behaviour is difficult to observe from the log |j(t)| vs. log t plots. The current density at long times is also small (< 1 mA  cm2) and fluctuating with time. Hence, the same data are presented as accumulated charge Q(t) vs. t plots in the right side panel of Fig. 1 to show the long-term reaction kinetics more clearly. The slope of an accumulated charge Q(t) vs. t plot provides a more accurate  current value  at times when the system is . near or at steady state jðtÞ ¼ dQðtÞ dt Before we discuss the oxidation kinetics, it should be noted that the net current is initially positive but switches to negative at long times during polarization at EAPP < 0.2 VSCE. This behaviour has also been observed on other alloys including Stellite 6, carbon steel and stainless steel type 316 L [4,6,22]. It indicates that the water reduction rate dominates the net current as corrosion progresses. The presence of a cathodic current at long times on the WE suggests that the oxide formed on the electrode surfaces may passivate ionic conductivity, but does not completely suppress electronic conductivity. Due to the contribution of water reduction to the net current, the rate of any metal oxidation that may also occur on the WE at long times cannot be directly extracted from the measured current. Nevertheless, both metal oxidation and water reduction on the WE require charge transport through the oxide layer in order to complete the electric circuit in the cell. Hence, the cathodic current at long times still provides useful information on the nature of the oxide that has grown on the surface. The results presented in Fig. 1 show that the time-dependent behaviour of the current density varies with EAPP. At a potential above 0.8 VSCE, the current is initially positive and its value is generally higher at a higher EAPP. However, the slope of log jjðtÞj vs. log t is negative, showing that the metal oxidation rate decreases as corrosion progresses even at a constant potential. In addition, the log jjðtÞj vs. log t plot at a given EAPP shows distinct time (or kinetic) stages having different slopes, and the number of such stages seen over the 5-h polarization period depends on EAPP. For example,

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Fig. 1. Current density as a function of time (left panel) and total accumulated charge as a function of time (right panel) observed during potentiostatic polarization on Inconel 600 at pH25  C 10.6 and 80  C. The results are separated based on the characteristic potential regions discussed in the text.

during polarization at +0.1 VSCE, the slope is initially about 2/3 (in Stage I), but it switches to 1.0 at 50 s and holds this value for 20 min (Stage II). This is followed by a progressively slower decrease in the magnitude of the slope with time (Stage III) until it eventually approaches zero, i.e., a constant current. The slope, p, of a log jjðtÞj vs. log t plot represents the timedependence of the oxidation rate (or current), jðtÞ a tp . Thus, a change in the slope indicates a change in the oxidation kinetics as corrosion progresses. In presenting the log jjðtÞj vs. log t plots in Fig. 1, the data are grouped according to the potential regions, with each region having the same number of distinct stages and the same values of p in the individual stages. These regions are designated using Roman numerals in Fig. 1. The potential regions are better recognized from the Q(t) vs. t plots. The time-dependent behaviour of Q(t) varies for the characteristic potential regions. In Region I (0.8 VSCE and 0.6 VSCE) the negative slope of Q(t) vs. t at long times decreases with EAPP while the maximum Q(t) increases with EAPP. Upon an increase in EAPP from 0.6 VSCE to 0.55 VSCE, the cathodic current value does not change while the maximum Q(t) decreases. Within Region II (0.55 VSCE and 0.5 VSCE), the cathodic current value does not change with EAPP while the maximum Q(t) increases with EAPP. In Region III (0.4 VSCE and 0.3 VSCE) the current at long times is still cathodic and its value decreases with increasing EAPP while the maximum Q(t) increases. In Region IV (0.2 VSCE and

+0.2 VSCE) the current at long times is anodic and its value increases with increasing EAPP. The potential regions identified from the long-term behaviour of Q(t) match very well with those identified from the short-term behaviour of log |j(t)|. We have observed similar potential dependent behaviour of metal oxidation kinetics on Co-Cr alloy Stellite 6 [5,22], see also Section 3.1.2. 3.1.1.2. Comparison of the electrochemical data with reaction thermodynamics. The potential regions determined from the potentiostatic polarization studies can be related to the
thermodynamic electrochemical equilibrium potentials Eeq for rdx the redox reactions that can occur during corrosion of Inconel 600 (see Fig. 2). The equilibrium potentials shown in the figure were calculated for pH25  C 10.6 from the standard reduction potentials
of the redox pairs reported in literature [25–30]. The Eeq values rdx are shown by vertical bars and the redox couple involved in each equilibrium reaction is given beside each bar. Two potential scales are used depending on pH, since the equilibrium potential for a redox couple changes with pH (59 mV shift with one pH unit increase for a redox process involving a 1:1 ratio of e/H+) [31]. The diagram also shows dissolution pathways of the more soluble metal ions, Fe2+(aq) and Ni2+(aq), from the oxides/hydroxides of the metal ions. The potential ranges where the formation of different oxides is thermodynamically possible are indicated at the top of the diagram (Eeq Regions) in Fig. 2. Also shown in bars above

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Fig. 2. Electrochemical equilibrium potentials for the redox reactions that can occur on Inconel 600. The vertical bars show the equilibrium potentials and the corresponding redox pairs are listed on both sides of these bars. The potential ranges where the formations of different oxides are thermodynamically possible are indicated at the top of the potential diagram (Eeq Regions). These regions are compared with the potential regions determined from the polarization experiments (PP Regions) at pH25  C 10.6 and pH25  C 6.0 at 80  C.

the Eeq diagram are the potential regions determined from the potentiostatic polarization study at two different pHs at 80  C (see pH25  C 6.0 results in Section 3.2). Comparison of the potential regions shows that the final set of oxides that form during polarization at pH25  C 10.6 is determined by the redox reaction thermodynamics. However, the potential regions as determined from the polarization experiments are shifted to more positive values compared to those defined by the
values. These shifts are attributed to the additional potential Eeq rdx drop that arises across the oxide layer that grows on the polarized electrodes [32]. At pH25  C 10.6 where the solubilities of FeII, NiII or CoII species are near their minima, most of the oxidized metals would be used to grow oxides rather than dissolve into solution. The comparison suggests that the type(s) of oxide formed on Inconel 600 during potentiostatic polarization would depend on the potential region in which the EAPP lies. The oxides that form would be mostly Fe spinel oxide (Fe chromite, FeCr2O4) at an EAPP in Region I, Fe spinel oxides (FeCr2O4/Fe3O4) at an EAPPin Region II, Fe and Ni spinel oxides (FeCr2O4/NiFe2O4) at an EAPP in Region III, and an underlying spinel oxide layer and an outer layer of mostly NiO/Ni(OH)2 at an EAPP in Region IV. 3.1.1.3. Characterization of surfaces. The surfaces of the polarized electrodes were analyzed by SEM, XPS and AES with Ar+ sputtering [22,23]. The SEM images of surface of the electrodes polarized at four potentials, 0.7 VSCE (in Region I), 0.4 VSCE (in Region III), 0.2 VSCE (at the onset of Region IV), and +0.1 VSCE (well into Region IV), are shown in Fig. 3. Compared to the freshly polished electrode, the corroded surfaces show filament-like morphology. This filament-like network is denser and thicker on the electrodes polarized at a higher potential. This morphology is common to hydroxides of transition metals and is frequently observed on the surfaces of transition metals corroded at a high pH [30,33]. The observed change in morphology is consistent with the AES depth profiles and the XPS analyses discussed below that show a thicker layer of Ni(OH)2 on a surface polarized at a higher potential.

Fig. 3. SEM micrographs of the Inconel 600 electrode surfaces following 5-h polarization at potentials in different regions at 80  C. Also shown is the SEM of a freshly prepared surface.

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Variations in the elemental composition of the surface oxides with depth were investigated by AES depth profiling, Fig. 4. The atomic percentages (at.%) in the AES depth profiles are the ratios of individual elements to the sum of all of the elements analyzed at a given depth. The depth profile of the main alloy element Ni shows at least two regions, a region of linear increase in Ni at.% with depth followed by a region of near constant Ni at.% with depth. The depth range of the linear increase in the Ni at.% increases with EAPP. The at.% of O decreases with depth in the range where the Ni at.% increases with depth. However, the O at.% continues to decrease further with depth before it reaches a constant background level (<4 at.%). In this region the atomic percentages of the other main metal elements, Fe and Cr, also continue to increase with depth before they level off. The depth at which the O at.% reaches the background level and all of the different ratios become nearly constant is where the pure metallic phase begins. This depth is about 25 nm on the surface polarized at EAPP= +0.1 VSCE and is found at a progressively shallower depth on a surface polarized at a lower potential. As discussed in more detail below the region above the pure metallic phase is not a pure oxide phase, but a mixed oxidemetallic phase. However, for simplicity, the layer above the pure metallic phase will be referred to as the oxide layer and the boundary between these two layers will be referred to as the metal/oxide (m|ox) interface hereafter. The changes in the atomic percentages of individual elements can obscure changes in the nature of the oxides of the three main elements, Ni, Fe and Cr as a function of depth. To identify more clearly the degree of oxidation of the different elements and their relative abundances in the surface layer as a function of depth, the AES data were further analyzed as the ratio of O atom to the three main metal atoms (O/(Ni + Fe + 1.5Cr)), the ratios of two main alloy elements to the sum of the three main alloy elements (Ni/ (Ni + Cr + Fe) and Cr/(Ni + Cr + Fe)), and the ratios of Fe to the two main elements (Fe/Ni and Fe/Cr). An O/(Ni + Fe + 1.5Cr) ratio of 1.0 represents a layer in which all of the main metal elements, Ni, Fe

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and Cr, are oxidized to their lowest stable oxidation states, NiII, FeII and CrIII. A ratio less than 1.0 represents a layer in which a fraction of the metals has not been oxidized. An example of this AES depth profile analysis is shown for a surface polarized at 0.1 VSCE in Fig. 5. The depth profile of the O at.% in Fig. 4 shows a change in slope at 7 nm. This depth coincides with the depth where changes in the depth dependences of Cr/ (Ni + Cr + Fe) and Fe/Cr occur. The depth profile of Ni/(Ni + Cr + Fe) also shows a maximum at this depth although this ratio does not vary much with depth near 7 nm, except for the first data point. The ratio of O/(Ni + Fe + 1.5Cr) at 7 nm is less than one (0.6) and all of the metal components present at this depth have not been oxidized. The observed ratios of O/(Ni + Fe + 1.5Cr) and M/(Ni + Cr + Fe) (where M = Ni, Cr or Fe) at a few selected depths are listed in Table 2. The observed atomic ratios of M/(Ni + Cr + Fe) at a given depth were used to determine the type of oxide that may be present at that depth. Also listed in the table are the ratios of O/ (Ni + Fe + 1.5Cr) that would occur assuming that different metal oxides are present. For example, if all of the chromium detected by the AES at a given depth were present as Cr2O3 while the rest of metals (Fe and Ni) detected by the AES at that depth were present in their metallic states, the ratio of O/(Ni + Fe + 1.5Cr) would be (1.5  Cr)/(Ni + Fe + 1.5Cr). The observed M fractions at 7 nm are 0.82, 0.09 and 0.09 for Ni, Fe and Cr, and these fractions result in 0.13 for (1.5  Cr)/(Ni + Fe + 1.5Cr). The value of (1.5  Cr)/(Ni + Fe + 1.5Cr) is 0.18 at 13 nm based on the observed atomic fractions of Ni, Fe and Cr at that depth. These calculated ratios are presented under the Cr2O3 column in Table 2. If the oxide were present only as FeCr2O4 and the excess Fe or Cr and all Ni were present in their metallic states, the ratio of O/(Ni + Fe + 1.5Cr) would be (2  Cr)/ (Ni + Fe + 1.5Cr) when Fe at.% > 1/2 Cr at.%, because the FeII at.% is half the CrIII at.% in FeCr2O4 and the O content in FeCr2O4 is twice the Cr content. The ratio of O/(Ni + Fe + 1.5Cr) would be (4.0  Fe)/ (Ni + Fe + 1.5Cr) when Fe at.% < 1/2 Cr at.%. The observed fractions of

Fig. 4. Depth profiles of the atomic percentages of all of the elements determined by AES following 5-h polarization at potentials from different regions at 80  C.

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Ni, Fe and Cr at 7 nm result in a value of 0.17 for (2  Cr)/ (Ni + Fe + 1.5Cr). At 13 nm the value of (2  Cr)/(Ni + Fe + 1.5Cr) is 0.24. The calculated ratios are listed under the FeCr2O4 column in Table 2. The column (NiFe2O4 + FeCr2O4) represents the case where it is assumed that all of the Fe and Cr are present as NiFe2O4 and FeCr2O4 and any excess Ni is in the metallic state. In this case, the ratio of O/(Ni + Fe + 1.5Cr) would be ((2  Cr)/(Ni + Fe + 1.5Cr) + 2  ( Fe  1/2 Cr)/(Ni + Fe + 1.5Cr) where the first term accounts for the O content in FeCr2O4 (which is twice the Cr content) and the second

term accounts for the O content in NiFe2O4 (which is twice the Fe content not used in FeCr2O4). The columns (NiO + NiFe2O4 + FeCr2O4) and (Ni(OH)2 + NiFe2O4 + FeCr2O4) represent the cases where it is assumed that all of the Fe and Cr are present as the oxides NiFe2O4 and FeCr2O4, and any excess Ni is present as in NiO and Ni(OH)2, respectively. For the (NiO + NiFe2O4 + FeCr2O4) layer, the ratio of O/(Ni + Fe + 1.5Cr) would be ((2  Cr)/(Ni + Fe + 1.5Cr) + 2  (Fe  1/2 Cr)/(Ni + Fe + 1.5Cr) + (Ni  1/2 (Fe  1/2 Cr))/(Ni + Fe + 1.5Cr)), where the first term accounts for the O content in FeCr2O4, the second term accounts for the O content in NiFe2O4, and the third term accounts for the O content in NiO which equals the remaining Ni content not used in NiFe2O4. For the (Ni(OH)2 + NiFe2O4 + FeCr2O4) layer, the third term is replaced with (2  (Ni  1/2 (Fe  1/2 Cr))/(Ni + Fe + 1.5Cr) since the O content in Ni(OH)2 is twice the remaining Ni content not used in NiFe2O4. We can use the observed oxygen ratio, O/(Ni + Fe + 1.5Cr), to determine the likely mixture of oxides present as a function of depth by matching the ratio to the calculated oxygen ratio that would arise for those oxides. For example, for a surface polarized at 0.1 VSCE at 2 nm a ratio of 1.1 can be obtained if all of Cr and Fe are present as FeCr2O4/NiFe2O4 and the rest of Ni (not present as NiFe2O4) is present as NiO (with a minor fraction as Ni(OH)2). At 7 nm and 9 nm, the observed ratios are 0.6 and 0.4, respectively. These ratios could arise if the oxide at these depths consists of Fe and Cr in FeCr2O4 and NiFe2O4 while the rest of the Ni is present as a mixture of NiO and Ni0, with a fraction of NiO at 7 nm than at 9 nm. At a depth greater than 11 nm the oxygen ratio can be obtained if all of the Ni present at those depths is in the metallic form and only Cr and Fe are present as oxides (Cr2O3 and/or FeCr2O4). The amount of FeII present in the Cr2O3/ FeCr2O4 mix is higher at 11 nm than at 13 nm. Below 13 nm only small fractions of Fe and Cr are oxidized. The combinations of the oxide ratios that are calculated to match the observed oxygen ratios are highlighted in Table 2. Based on the AES data and the above analysis the surface layer on the Inconel 600 electrode polarized for 5 h at +0.1 VSCE at pH25  C 10.6 consists of a graded oxide layer structure from the m|ox interface to the surface: Cr2O3/FeCr2O4/NiFe2O4/NiO/Ni(OH)2

Fig. 5. Depth profiles observed on Inconel 600 following 5-h polarization at 0.1 VSCE at 80  C: from the top, the ratios Ni/(Ni + Cr + Fe) and Cr/(Ni + Cr + Fe); and the ratios Fe/Ni and Fe/Cr. The drawing below the graphs is a schematic representation of the oxide layer structure determined from the different ratios.

This layer structure does not contain pure oxide phases. It also contains unoxidized metal atoms whose fraction decreases with decreasing depth. The surface layer structure determined from this analysis is shown at the bottom in Fig. 5. This structure is consistent with the current measurements during polarization at 0.1 VSCE discussed above. At 0.1 VSCE in Region IV, oxidation of Fe and Ni occurs sequentially: first oxidation of Fe to FeII leading to conversion the preformed Cr2O3 to FeCr2O4, followed by the oxidation of Fe to FeII/III forming Fe3O4, oxidation of Ni to NiII converting Fe3O4 to NiFe2O4 and then formation of NiO/Ni(OH)2. We have performed similar calculations to match the observed O/(Ni + Fe + 1.5Cr) ratios for surfaces polarized at different EAPP. For example, the O/(Ni + Fe + 1.5Cr) ratio at 2 nm depth is 0.4 at 0.2 VSCE. Because the metal atom ratios are now different as a function of depth at this applied potential, this oxygen ratio still requires that the layer at 2 nm consists of mostly FeCr2O4/NiFe2O4 with the remaining Ni as NiO and Ni0. This structure is the same as the one that we see for the surface polarized at 0.1 VSCE at a depth of 9 nm. The oxide layer structures determined from the AES depth profiles on the surfaces polarized at different potentials are compared in Fig. 6. The depth profiles of the ratio of Ni/(Ni + Cr + Fe) are also compared in the figure. Two different depth scales are used in presenting the oxide layer structures and the depth profiles: Fig. 6a presents them on a scale with reference to the outermost surface while Fig. 6b presents them on a scale with reference to the depth of the m|ox interface. When the data are plotted on the latter

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Table 2 Observed versus calculated atomic ratios of O/(Ni + Fe + 1.5Cr) as a function of depth on the surface polarized at +0.1 VSCE at pH25 oC 10.6 and 80  C. Depth (nm)

Calculated O/(Ni + Fe + 1.5Cr)a

Observed M/(Ni + Fe + Cr)

2 7 9 11 13 18 25

Ni

Fe

Cr

0.81 0.82 0.82 0.79 0.80 0.74 0.70

0.08 0.09 0.08 0.08 0.08 0.08 0.09

0.11 0.09 0.10 0.13 0.13 0.18 0.21

O/ (Ni + Fe + 1.5Cr)

Cr2O3

FeCr2O4

NiFe2O4/FeCr2O4

NiO /NiFe2O4/FeCr2O4

Ni(OH)2 /NiFe2O4 /FeCr2O4

1.1 0.55 0.40 0.20 0.12 0.08 0.06

0.16 0.13 0.15 0.18 0.18 0.24 0.28

0.21 0.17 0.20 0.24 0.24 0.33 0.33

0.27 0.26 0.25 0.27 0.27 0.33

1.03 1.04 1.03 1.01 1.02 1.00

1.79 1.82 1.80 1.76 1.76 1.67

a

Calculated values assuming different oxides present as a function of depth.

scale it is clear that all of the data agree very consistently. Fig. 6 shows only the Ni/(Ni + Cr + Fe) results, but all of the other ratios behave similarly. Fig. 6b shows clearly that the oxides are formed sequentially with additional oxides being added at progressively higher potentials. For example, the oxide layer formed after 5-h polarization at 80  C is mostly FeCr2O4/NiFe2O4 at 0.4 VSCE (Region III), and FeCr2O4/NiFe2O4/NiO/Ni(OH)2 at higher potentials (0.2 VSCE and +0.1 VSCE in Region IV). The main difference between the oxide structures formed at the two potentials in Region IV is the thickness of the outer NiO/Ni(OH)2 layer. Distinct identification of spinel oxides, FeCr2O4/Fe3O4 and NiFe2O4, is not possible from our AES data, but the reaction thermodynamics (Fig. 2) suggests that the formation of NiFe2O4 would be very slow at 0.7 VSCE (Region I) and hence the spinel oxide layer is mostly FeCr2O4 at this low potential.

Survey and high resolution XPS were also used to determine the oxidation states of the metals in the surface layers (up to the analysis depth of the XPS instrument, 9 nm). The survey spectra show that the fractional Cr content decreases with increasing EAPP because the Fe and Ni content in the outer surface is enhanced, Fig. 7a. This is consistent with the oxide layer structures determined by AES analysis. High-resolution spectra of the O-1s, Cr-2p and Ni-2p bands were deconvoluted using reference spectra of single-phase metals and metal oxides, and a deconvolution technique developed by Biesinger et al. [4,5,22,23]. The high resolution XPS analysis results are summarized in Fig. 7. The Fe content in the alloy is small (8 at.%) and the Fe-2p band in the high resolution XPS partially overlaps with the Ni Auger electron spectrum. Consequently, it is difficult to obtain Fe speciation from the deconvolution of the Fe-2p band and this was not performed. Note that the oxide layer contains metals in metallic states. In addition, the outer Ni oxide layer grows in a filament-like

Fig. 6. Depth profiles of the ratio Ni/(Ni + Cr + Fe) on Inconel 600 following 5-h polarization at potentials in different regions at 80  C: (a) on a depth scale with the solution/ oxide interface as the reference point (0), and (b) on an oxide height scale with the oxide/metal interface as the reference point (0). The drawings below the graphs indicate the positions of the different oxide layers that are formed as a function of EAPP.

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Fig. 7. XPS analysis results of the surfaces of Inconel 600 polarized at different EAPP at 80  C: (a) relative ratios of Ni, Cr and Fe present in the surface layer determined by the survey spectra, (b) relative contributions of CrIII and Cr0 to the Cr 2p band, (c) relative contributions of NiII and Ni0 to the Ni 2p band, and (d) the fraction of OH in the oxide O determined from the O 1 s band.

structure and the depth of this oxide layer varies somewhat in a microscopic scale. At an EAPP below 0.4 VSCE the oxide fraction of Ni (i.e., NiII) in the Ni-2p band and the oxide fraction of Cr (i.e., CrIII) in the Cr-2p band are nearly independent of EAPP whereas the OH fraction in the O-1s band decreases slightly with increasing EAPP at these potentials However, at EAPP > 0.4 VSCE the NiII fraction in the Ni-2p band and the OH fraction in the O-1s band increase rapidly with increasing EAPP, while the CrIII fraction in the Cr-2p band is nearly independent of EAPP. These results are also consistent with the SEM and AES depth profile analyses. At an EAPP below 0.4 VSCE the oxide layer consists of mostly a mixed spinel oxide, FeCr2O4 and Fe3O4 or NiFe2O4. At a higher EAPP than 0.4 VSCE (i.e. in Region IV) an outer layer of NiO/Ni(OH)2 grows on the top of the mixed spinel oxide layer, and the outer layer grows faster at a higher EAPP in Region IV. This is consistent with the AES analysis which shows a thicker NiO/Ni(OH)2 on the surface polarized at +0.1 VSCE than on a surface polarized at 0.2 VSCE (Fig. 6). The constant ratio of CrIII and Cr0, independent of the EAPP, is due to the location of the Cr in the surface layer. The CrIII is in a chromite that is the first mixed oxide that is formed and it is located just above the m|ox interface so that XPS interrogation of this region sees both the oxidized Cr and the underlying metallic Cr. For the surfaces oxidized at higher applied voltages, the m|ox interface lies at an oxide depth that is greater than the nominal depth for reliable XPS measurements with the instrument that we used. However, in the case of our samples, the XPS was clearly able to detect the presence of the Cr down to the m|ox interface. The combined results of the electrochemical and surface analyses show that a characteristic set of oxides are formed on Inconel 600 during potentiostatic polarization and this set of oxides is dependent upon the potential region in which the EAPP lies. At pH25  C 10.6 the oxides that form during corrosion on Inconel 600 are:


Fe spinel oxide (Fe chromite, FeCr2O4) at an EAPP in Region I,
Fe spinel oxides (FeCr2O4/Fe3O4) at an EAPP in Region II,
Fe and Ni spinel oxides (FeCr2O4/NiFe2O4) at an EAPP in Region III,

and
an underlying Fe and Ni spinel oxide layer and an outer layer of

mostly NiO/Ni(OH)2 at an EAPP in Region IV. The layers where these oxides are present also contain a significant fraction of Ni in metallic state except for the outermost layer on surfaces polarized at a potential in Region IV. The combined results further show that at a given EAPP the oxide formation occurs in stages, even when a positive potential is applied to a cathodically cleaned surface in a step function. The role of EAPP is to limit the extent of this sequence and we can understand this limitation in terms of the thermodynamics of the oxides that are formed in different potential regions. 3.1.2. Oxide growth on Inconel 600 versus Stellite 6 at pH25  C 10.6 We have performed the same potentiostatic polarization experiments and surface analyses on a Co-Cr alloy Stellite 6 [5,22]. The details of that study have been reported previously [5,22] and only the key findings are summarized here for comparison with the Inconel 600 results. The log |j(t)| vs. log t and the Q(t) vs. t plots obtained for Stellite 6 show the same potential dependent behaviour for cobalt oxidation as that observed for Fe and Ni in Inconel 600. At a given potential the cobalt oxidation occurs in stages and there are distinct potential regions with a specific number of stages. The potential regions determined for Stellite 6 oxidation also correlate well with potential regions based on the thermodynamic electrochemical
for the redox reactions that can occur equilibrium potentials Eeq rdx during corrosion of Stellite 6 (see Fig. 8). The post-test surface analyses also showed that the oxides formed on Stellite 6 are [5,22]:

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formation progresses through local minima on the reaction potential surface. Whether a certain metal oxidation reaction can occur at a substantial rate or not depends on the redox reaction thermodynamics. An increase in EAPP within a specific potential region increases the rates of individual reactions, but even under potentiostatic conditions, the rate of metal oxidation decreases with time due to oxide growth as corrosion progresses. This increases the potential drop across the oxide layer and decreases the effective overpotential for the interfacial redox reaction. At some point the oxide stops growing, either because the rate of metal oxidation has approached zero because the oxide layer has grown too thick, or because the rate of metal dissolution from the oxide matches the rate of metal oxidation. 3.2. Combined Effect of pH and Potential on Oxide Growth

Fig. 8. Electrochemical equilibrium potentials for the redox reactions that can occur on Stellite6. The vertical bars show the equilibrium potentials and the corresponding redox pairs are listed on both sides of these bars. The potential ranges where the formations of different oxides are thermodynamically possible are indicated at the top of the potential diagram (Eeq Regions). These regions are compared with the potential regions determined from the polarization experiments (PP Regions) at pH25  C 10.6 and pH25  C 6.0 at 25  C.


CoCr2O4 at an EAPP in Region I,
CoCr2O4/Co(OH)2 at an EAPP in Region II, and
CoCr2O4/Co(OH)2/Co3O4/CoOOH at an EAPP in Region III.

(Note that in our previous publication on Stellite 6 [22] we defined these three regions as Regions (I + II), III and IV, respectively. The potential region where CoII oxidation leads to mostly Co2+(aq) dissolution (due to the slow growth of CoCr2O4 at pH 6.0) was considered to be separate from the potential region where CoII oxidation leads to the significant growth of CoCr2O4.) The extent of the sequence (the nature of the final oxide formed) is limited by the potential region in which the EAPP lies. At a potential in Region I oxidation is limited to the oxidative conversion of the preexisting defective Cr2O3 to chromite, FeCr2O4 on Inconel 600 and CoCr2O4 on Stellite 6, and, hence, there is only one oxidation stage (Stage I). In Region I no other oxide can grow (dissolution of FeII or CoII occurs in parallel) but water reduction can occur on a chromite covered surface (and hence the steady decrease in the Q(t) with time at later times at EAPP in Region I in Fig. 1). At a higher potential Fe and Ni on Inconel 600 or Co on Stellite 6 can oxidize to form other oxides/hydroxides. Nevertheless, these oxides are thermodynamically less stable than chromite and are formed only when most of the Cr2O3 initially present has been converted to chromite. At a sufficiently high potential the conversion to chromite is completed in about 20 min at 80  C, which coincides with the time for the Q(t) to reach a maximum in Region I (see Fig. 1). Since the metal oxidation progresses sequentially due to kinetic constraints in addition to thermodynamic constraints, the oxide has a layered structure on both alloys. On Stellite 6 the next facile oxidation path is the oxidation of Co metal to Co(OH)2 through the CoCr2O4 layer. On Inconel 600 it is the oxidation of Fe metal to Fe3O4 through the FeCr2O4 layer, followed by the oxidation of Ni metal to NiII to convert Fe3O4 to NiFe2O4 and then to Ni(OH)2, all through the FeCr2O4 layer. The oxides that form, even at long times, are not necessarily the most thermodynamically stable oxides that can exist under a given set of aqueous conditions, see further discussion in Section 3.2. The oxide formed in the outer layer is influenced by the kinetic path by which the underlying oxide is formed. Nevertheless, the oxide

Since metal cation dissolution is strongly affected by pH the effect of potential on the corrosion behaviour can vary significantly with pH. At pH25  C 10.6 the solubilities of FeII, NiII or CoII species are near their minima and oxide film formation is favoured over metal ion dissolution. Metal dissolution becomes more important at lower pHs. We have performed the potentiostatic polarization experiments on Inconel 600 as a function of pH25  C (6.0, 8.4 and 10.6) at 25  C and 80  C. Only key results obtained at pH25  C 6.0 at 80  C are reported here for comparison with the results obtained at pH25  C 10.6 at 80  C. 3.2.1. Oxide growth on Inconel 600 at pH25  C 6.0 The potentiostatic polarization at pH25  C 6.0 was studied as a function of EAPP in range of 0.8 VSCE to +0.3 VSCE. Analysis of the entire set of data for log |j(t)| vs. log t as a function of EAPPconcluded that there are only two discernible potential regions over the range from 0.6 VSCE to +0.3 VSCE at pH25  C 6.0: Region I (0.6 VSCE to 0.3 VSCE) and Region IV (0.2 VSCE to +0.3 VSCE). At pH25  C 6.0 a positive current was not observed even when the potential is first applied, when the EAPP was below 0.6 VSCE. The results obtained at a few selected potentials obtained at pH25  C 6.0 and pH25  C 10.6 are compared in Fig. 9a. The potential regions determined from the potentiostatic polarization results at pH25  C 6.0 are summarized and compared with the potential regions determined from the results found at pH25  C 10.6 in Fig. 2. Note that the potential scale for the pH25  C 6.0 regions is shifted by 0.28 V with respect to that for pH25  C 10.6 regions to address the dependence of the equilibrium potentials of the solid-state (metal and metal oxides) redox couples on pH (59 mV shift per pH unit increase). As for the potential regions seen at pH25  C 10.6, the potential regions determined from the potentiostatic polarization results at pH25  C 6.0 correlate well with regions defined by thermodynamic electrochemical equilibrium potentials. The potential regions at pH 6.0 are again shifted to more values. positive values compared to those determined from the Eeq rdx The shift for the onset of Region I is nearly the same at both pHs, indicating the shift is due to the potential drop across the same oxide layer, the air-formed Cr2O3 that is always present on alloys containing Cr. The different metal oxidation kinetics at the two pHs can be seen from a comparison of the log |j(t)| vs. log t plots shown in Fig. 9a. At all potentials the current observed in the first 20 min at pH25  C 6.0 shows very different time-dependent behaviour from that observed at pH25  C 10.6. The current at pH25  C 6.0 is initially lower but stays nearly constant over a long period before decreasing at a near linear rate. However, at a potential in the same potential region the current at longer times shows similar time dependent behaviour for both pHs. At potentials in Region I, the current switches from anodic to cathodic after which it

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Fig. 9. Metal oxidation on Inconel 600 at 80  C and at pH25  C 10.6 versus pH25  C 10.6: (a) Current density as a function of time observed during 5-h polarization, and (b) post polarization depth profiles of the ratios of the oxygen and the main metal element to the sum of the metal elements determined by the AES analysis.

remains cathodic for the remaining test period. At potentials in Region IV, the current at times > 20 min decreases at a near linear rate for a while but reaches  steady state at longer times, and the steady state current is higher at a higher potential. The higher initial current at a higher pH can be attributed to a higher rate of surface hydration and hydrolysis of metal ions at the higher pH: MII in the oxide layer + n H2O ? M2+(H2O)n M2+(H2O)n ? (M(OH))+(H2O)n-1 + H+ ? (M(OH)2)(H2O)n-2 + 2H+ ? (M(OH)3)(H2O)n-3 + 3H+ Hydrolyzed metal species can quickly precipitate as solid metal hydroxide at a pH where the solubility of the metal cation is low: (M(OH)2)(H2O)n-2 ? M(OH)2(s) + (n-2) H2O M(OH)2(s) ? MO(s) + H2O

The hydrolysis and phase equilibria enable the oxidized metal, MII, to participate in two competing processes, dissolution into solution and formation of a solid oxide/hydroxide. The rate of metal dissolution is initially higher at our higher pH because the rate of surface hydration and hydrolysis is higher at the higher pH. However, the current at pH25  C 10.6 starts decreasing immediately because some metal oxide/hydroxide forms very quickly on the surface and this starts increasing the potential barrier for further electrochemical oxidation. At pH25  C 6.0 the Fe and Ni oxides grow more slowly and this allows the metal oxidation to continue at a near constant rate over a longer period. However, even with a high metal hydroxide solubility at pH25  C 6.0, an oxide layer slowly forms and the current eventually decreases. The surfaces polarized at different potentials at pH25  C 6.0 were also analyzed using SEM, XPS and AES. A few examples of the AES depth analysis results are shown in Fig. 9. Consistent with the current behaviour, the Inconel 600 electrodes polarized at pH25  C 6.0 have very different AES depth profiles from those polarized at pH25  C 10.6. The depth profiles of O/(Ni + Fe + 1.5Cr) show that the oxide layer present on electrodes polarized at pH25  C 6.0 is much thinner (1.5 nm to 3.0 nm) than the oxide layer formed at pH25  C

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Table 3 Observed versus calculated atomic ratios of O/(Ni + Fe + 1.5Cr) as a function of depth on the surface polarized at +0.1 VSCE at pH25 oC6.0 and 80  C. Depth (nm)

Calculated O/(Ni + Fe + 1.5Cr)a

Observed M/(Ni + Fe + Cr)

0 0.5 1 2 2.3 5 a

Ni

Fe

Cr

0.58 0.52 0.53 0.65 0.70 0.72

0.15 0.20 0.11 0.10 0.10 0.08

0.27 0.28 0.36 0.26 0.20 0.20

O/ (Ni + Fe + 1.5Cr)

Cr2O3

NiFe2O4

NiCr2O4

NiCr2O4 /NiFe2O4

NiO /NiCr2O4 /NiFe2O4

Ni(OH)2 /NiCr2O4 /NiFe2O4

1.3 1.0 0.7 0.25 0.20 0.08

0.36 0.37 0.46 0.35 0.27 0.28

0.26 0.35 0.19 0.18 0.18 0.15

0.48 0.49 0.61 0.46 0.36

0.74 0.84 0.80 0.64 0.55

1.07 1.09 1.05 1.04 1.05

1.39 1.33 1.30 1.45 1.55

Calculated values assuming different oxides present as a function of depth.

10.6 (15 nm to 25 nm) (Fig. 4). More interestingly, the depth profiles of Ni/(Ni + Fe + Cr) in the oxide layers formed at the two different pHs show the opposite depth dependences. At pH25  C 10.6 the ratio is high near the surface and decreases steadily with depth before it reaches a constant value. On the other hand, at pH25  C 6.0 the ratio is significantly smaller near the surface and increases with depth, reaching a maximum before decreasing to a constant value. The atomic ratios of O/(Ni + Fe + 1.5Cr) and M/(Ni + Fe + Cr) obtained at various depths on the surface polarized at +0.1 VSCE at pH25  C 6.0 are listed in Table 3. The surface polarized at pH25  C 6.0 has a higher Cr ratio and a lower Ni ratio than those observed at pH25  C 10.6 (Table 2). The depth dependences of these ratios are also different. At pH25  C 10.6 the ratio for Ni decreases, the ratio for Fe is nearly constant and the ratio for Cr increases with increasing depth (Table 2). At pH25  C 6.0 the ratio for Ni increases, the ratio of Fe decreases and the ratio for Cr increases and then decreases with increasing depth (Table 3). These differences can arise if the rate of

metal dissolution, and particularly Ni dissolution, is significantly enhanced at pH25  C 6.0 compared to that at pH25  C 10.6. We performed similar analysis of the values of oxygen ratio O/ (Ni + Fe + 1.5Cr) for different combinations of oxides on the surfaces polarized at pH25  C 6.0 as we performed for the measurements obtained pH25  C 10.6. The results are summarized in Table 3. As an example, this analysis results is consistent with a layer on a surface polarized at 0.1 VSCE at pH25  C 6.0 that consists of the following oxides, in order of decreasing depth from the m|ox interface (5 nm) to the surface: Cr2O3/NiFe2O4/NiCr2O4/NiO/Ni(OH)2 As for the electrodes oxidized at pH25  C 10.6 the surface layer is not of a pure oxide phase and it still contains some atoms in metallic states with their fraction decreasing with decreasing depth. No protective oxide layer is formed until Ni can oxidize to NiII and this requires a potential in Region IV. Since the oxide formation

Fig. 10. Comparison of the XPS analysis results of the surfaces of Inconel 600 at 80  C and at pH25  C 10.6 versus pH25  C 6.0: (upper panel) relative contributions of NiII and Ni0 to the Ni 2p band and (lower panel) relative contributions of CrIII and Cr0 to the Cr 2p band.

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occurs sequentially the fast dissolution of Fe2+(aq) prevents from forming substantial amounts of FeII or FeII/III oxides (Fe3O4 or FeCr2O4) and the oxides that form are those of Ni oxides. The XPS results for the surfaces polarized at pH25  C 6.0 are compared with those obtained for surfaced polarized at pH25  C 10.6 in Fig. 10. At both pHs, the NiII fraction in the Ni-2p band increases with increasing EAPP while the CrIII fraction in the Cr-2p band is essentially constant with EAPP . The Ni0 fraction is much higher at pH25  C 6.0 because the oxide layer is thinner and the XPS is seeing more of the underlying alloy. 3.2.2. Effect of pH on oxide growth on Inconel 600 The combined results of the electrochemical and surface analyses show that the competition between dissolution and oxide growth is a factor in determining the type of oxide that form on a corroding surface. This factor, in turn, influences the rate of metal oxidation. The pH affects corrosion kinetics by influencing the rates of the metal dissolution and hydrolysis. The effect of pH on the kinetics of metal oxidation on Inconel 600 is presented schematically in Fig. 11. To summarize, at a high pH (ca 10.6) where the FeII and NiII solubilities are low the behaviour is as follows: In Regions I & II: Fe is oxidized to FeII and this combines with preformed Cr2O3 to form FeCr2O4. The Fe is further oxidized to form a mixed FeII/FeIII oxide (Fe3O4). Dissolution of Fe2+(aq) occurs at a slow rate, and a significant fraction of the oxidized metal FeII is used to grow spinel oxides, FeCr2O4/Fe3O4. In Region III: Both Fe and Ni can be oxidized. However, Fe is oxidized faster and hence FeCr2O4/Fe3O4 is formed first. Potentials in this region are not high enough to form NiO or Ni(OH)2, but are high enough to form NiFe2O4 in the presence of Fe3O4. Dissolution of Fe2+(aq) and Ni2+(aq) occurs but at a very slow rate, and significant fractions of the oxidized metals FeII and NiII are used to grow spinel oxides, FeCr2O4/Fe3O4/NiFe2O4. In Region IV: In addition to the oxidation processes that occur at a potential in Region III, the oxidation of Ni to NiII to form NiO/Ni (OH)2 occurs. The rate of Ni oxidation to NiII increases with potential, increasing the rates of both NiII oxide growth and Ni2

+

(aq) dissolution initially. At pH25  C 10.6 oxide growth occurs faster than dissolution, and most of NiII that is formed is used to grow an outer layer of NiO/Ni(OH)2 on the inner spinel oxides. At a lower pH (ca 6.0) where the FeII and NiII solubilities are high, metal dissolution suppresses the formation of certain oxides on the surface oxide formation proceeds as follows: In Region I & II: Fe is oxidized to FeII which then dissolves. The reaction of FeII with Cr2O3 to form FeCr2O4 (and Fe3O4) cannot compete with the FeII dissolution rate. In Region III: Oxidation of Ni to NiII and conversion of Fe3O4 to NiFe2O4 cannot occur because of the lack of FeCr2O4 and Fe3O4. Consequently Region III is absent at pH25  C 6.0. In Region IV: Both Fe and Ni are oxidized and dissolve into the solution. At these potentials the oxidation of Ni to NiII occurs at an appreciable rate. Because of fast metal dissolution the oxide growth is slow. The oxides that are formed fastest are Ni oxides, NiCr2O4/NiO/Ni(OH)2. The low content of Fe in the alloy and the high fraction of FeII that dissolves leave the surface layer depleted of FeII that can react with Cr2O3. Until the outer layer of NiO/Ni (OH)2 is formed Ni oxidation continues to occur at a fast rate. 3.2.3. Effect of pH on the oxide growth on Inconel 600 versus Stellite 6 Our previous work on Stellite 6 [5,22] provided findings similar to those for Inconel 600; there are characteristic potential regions and reaction thermodynamic constraints governed by the solution pH apply. The regions identified at pH25  C 6.0 are also compared with those identified at pH25  C 10.6 in Figure 8. The oxidation pathways on Stellite 6 are simpler than those on Inconel 600 because Stellite has a smaller number of reactive metal constituents and consequential oxides. (However, Stellite 6 has two distinct alloy phases, Cr-rich and Co-rich, on which the metal oxidation progresses at different rates.) At pH25  C 6.0 where the solubility of CoII is high, metal oxidation leads to mainly to metal dissolution and growth of an oxide film is slow. At pH25  C 10.6 where the CoII solubility is very much lower, metal oxidation leads mainly to oxide growth. As for Inconel 600 the net effect of increasing EAPP at pH25  C 6.0 is to increase metal dissolution while at pH25  C 10.6 it increases oxide growth. 4. Conclusions

Fig. 11. Schematic illustration of corrosion reaction paths on Inconel 600 at 80  C, at pH25  C and at pH25  C 6.0, under potentiostatic polarization.

Comparison of the potentiostatic polarization results obtained for Inconel 600 and Stellite 6 has shown that metal oxidation and oxide film formation on the two alloys follow similar kinetics. The reaction thermodynamics of metal oxidation determine whether a certain metal oxide can form at a given applied potential. Metal oxidation on these alloys proceeds via two competing pathways, oxide formation and metal ion dissolution. The pH of the corroding system does not affect directly the reaction thermodynamics and the nature of the oxide that can form, but it strongly influences the relative rates of the two competing reactions. At pH25  C 10.6 where the solubilities of FeII, NiII or CoII species are near minimum, oxide film formation is favoured over metal ion dissolution. At pH25  C 6.0 where those metal ions are much more soluble, metal dissolution is favoured over oxide film formation. Consequently an increase in EAPP results in an increase in the rate of metal ion dissolution at pH25  C 6.0, but it mostly results in an increase in oxide film growth at pH25  C 10.6. At a given EAPPoxide formation occurs sequentially; the conversion of the preformed Cr2O3 film to chromite, FeCr2O4 or CoCr2O4, proceeds before formation of the next layers of Fe3O4/ NiFe2O4 and NiO/Ni(OH)2 on Inconel 600, or CoO/Co(OH)2 on Stellite 6. The effect of a different EAPP is to limit the oxidation sequence, the lower the EAPP the shorter the sequence. In the presence of an oxide film, the driving force for the oxide formation depends not only on the magnitude of EAPP with respect to the

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Please cite this article in press as: A.Y. Musa, et al., Potentiostatic Oxide Growth Kinetics on Ni-Cr and Co-Cr Alloys: Potential and pH Dependences, Electrochim. Acta (2015), http://dx.doi.org/10.1016/j.electacta.2015.02.176