Electrochemical and Microstructural Characterization of Alloy 600 in Low Pressure H2-steam

L. Volpe1, G. Bertali1, M. Curioni1, M. G. Burke1, F. Scenini1

AbstractLow pressure superheated H2-steam system has been extensively used in the past years to accelerate the oxidation kinetics while keeping the conditions representative to PWR primary water. One of the most important requirements of this environment is that needs to replicate the Ni/NiO transition. However, despite several studies have been carried out by different research groups in H 2-steam environment, there is still some level of uncertainty over the thermodynamic of the oxidation process. In this study, the Ni/NiO transition in hydrogenated steam was investigated via electrochemical potential measurements using a Ni/NiO solid state reference electrode. Furthermore, solution annealed Alloy 600 coupons were exposed to H2-steam at 480 °C in order to examine the effect of oxidizing conditions with respect to the Ni/NiO transition on the preferential intergranular oxidation. The effect of the redox potential on the preferential intergranular oxidation is discussed in the context of the precursor stages of stress corrosion cracking for Alloy 600.

1: Materials Performance Centre, University of Manchester, Manchester (UK)Corresponding Author: Liberato Volpe, e-mail: liberato.volpe@manchester.ac.uk

Keyword: Alloy 600; Preferential Intergranular Oxidation; Grain Boundary migration; Ni/NiO; Redox potential.

1. IntroductionNi-based alloys, such as Alloy 600 are largely used in Pressurized Water Reactors (PWRs) thanks to their mechanical properties and corrosion resistance. However, Alloy 600 is susceptible to Primary Water Stress Corrosion Cracking (PWSCC) and the Preferential Intergranular Oxidation (PIO) model, based on the internal oxidation studies of Scott and Le Calvar [1], appears to be the most comprehensive in explaining the initiation stages of PWSCC.

Oxidation and Stress Corrosion Cracking experiments are commonly conducted in high temperature hydrogenated water with the environmental conditions similar to a PWR primary water, however the kinetics can be very slow. In order to accelerate the materials/environmental interactions, high pressure hydrogenated steam can be used with the advantage of accelerating the process without changing the mechanism. In fact, Economy et al. [2] showed a monotonic dependence of crack growth rate (CGR) and SCC initiation time in both water and steam, suggesting that the SCC initiation mechanism is similar for both environments. Furthermore, Scenini et al. [3] developed a low pressure superheated H2-steam system, that thanks the higher temperature compared with the traditional experiments performed in water, enabled SCC experiments to be performed in short time with the additional benefit of not needing high pressure equipment. Several studies carried out in a similar low pressure system by Bertali et al. [4–6] and Persaud et al. [7,8] confirmed that preferential intergranular oxidation of Alloy 600 occurred under these conditions. Furthermore, using this low pressure apparatus, recent studies at the University of Manchester has unequivocally demonstrated the occurrence of marked grain boundary (GB) migration associated with preferential intergranular oxidation of Alloy 600 [4] in real plant conditions.

The crack growth rate of Alloy 600 and other Ni-based alloys [9] as a function of the dissolved hydrogen (and hence on the potential) in primary water has a bell-shape curve, where the maximum susceptibility to

PWSCC occurs in the proximity of the Ni/NiO transition. This trend was also confirmed by Lindsay et al. [10] by performing SCC experiments on Alloy 600 CW (cold worked) at 400 °C in a low pressure H2-steam environment. On the other hand, the initiation time for SCC does not appear to correlate in the same way with H2 concentration (and hence the potential). In fact, in oxidizing conditions the time to initiation is high and it decreases as the Ni/NiO transition is approached. However, even in reducing conditions where the crack growth rate is drastically decreased, the initiation time remains at very low levels [11]. The reason for this is observation is still under debate despite the influence of the dissolved hydrogen on the morphology of the Alloy 600 external oxide layer that has been extensively studied [12]. The view of the present authors is that it would be more revealing to investigate the role of redox potential on the preferential intergranular oxidation, and this was the motivation behind the current work.

Since the Ni/NiO transition represents a key point for oxidation and SCC testing, understanding the thermodynamics of this transition is of fundamental importance. While for high temperature water the required dissolved hydrogen to be at the Ni/NiO has been accurately estimated by Attanasio and Morton over a wide range of temperatures [13], the exact conditions corresponding to this transition in low pressure superheated H2-steam environment is still under debate. In fact, Capell et al. [14] suggested that the Ni/NiO transition at 400 °C occurs when the partial pressure ratio (PPR) of hydrogen and steam is equal to 9x10-2. However, analogues studies by Bertali et al. [4–6], which are based on theoretical thermodynamics, suggested that the transition occurs at a different PPR, equal to 2x10 -3. The corresponding oxygen partial pressures are different, suggesting that the Ni can be oxidized in NiO with two dissociation pressures of oxygen, placing the transition in two different regions.

The aim of this paper was to validate the low pressure H 2-steam system as possible surrogate environment to perform oxidation and SCC experiments that are relevant to a PWR primary water environment. In order to achieve this aim, the Ni/NiO transition was directly identified using an Yttria Stabilized Zirconia sensor. Furthermore, oxidation tests of Alloy 600 were performed in different oxidizing or reducing conditions with respect to the Ni/NiO in order to correlate the preferential intergranular oxidation with the redox potential.

2. Experimental procedures

2.1 H2-steam environment

The experiments were carried out in a low pressure H2-steam environment [3] at the water flow rate of 1.50mL/min and a H2 and Ar gas mixture of 50 cc/min. The Ar was used as carrier gas because it is chemically inert in a wide range of temperatures. De-ionized water was fed in a pre-heater, heated and mixed with H2/Ar gas, then injected in a stainless-steel reactor tube placed in a furnace.

The H2 flow rate was varied in order to simulate different oxidizing and reducing environments with respect to the Ni/NiO transition: the oxygen partial pressure (pO2) could be varied by changing the H2 partial pressure (e.g. by increasing the H2 flow and hence the H2 partial pressure, the redox potential decreases). The complete thermodynamics of the H2-steam environment can be described using either the parameters R (1) or PPR (2). Both R and PPR are thermodynamically correlated to each other.

R=pO2∋¿NiO

pO2(1)

PPR=p H 2

p H 2O(2)

R represents the ratio between the oxygen partial pressure at the Ni/NiO transition (pO2 Ni/NiO) and the oxygen partial pressure pO2. For values lower than 1, NiO is stable (oxidizing environment), while for values higher than 1, Ni is stable (reducing environment).

PPR is the partial pressure ratio between H2 and steam. The complete derivation of these calculations can be found in papers by Scenini et al. [3] and Persaud et al. [7].

2.2 Electrochemical measurement of the Ni/NiO transition and oxidation studies of Alloy 600

The redox potential in H2-steam was monitored using an Ivium Compactstat potentiostat, a Ni/NiO Yttria Stabilized Zirconia Solid State Reference Electrode (YSZ SSRE) and a Pt wire that was the working electrode [15]. Pt was chosen as working electrode because it is chemically inert and does not form any oxides. The reference electrode, schematic is shown in Fig. 1, is a replica (with different powders) of a Cu/Cu2O SSRE and it was initially used for feasibility studies. The Pt wire working electrode was coiled around the tip of the YSZ tube in order to minimize the electrical resistance of the H 2-steam and therefore acquire the redox potential of the dissociation of steam into H2 (g) and O2 (g).

The redox potential was monitored at 400 °C. The mixture of H2/Ar was varied in order to simulate different oxidizing and reducing environments with respect to the Ni/NiO transition (Table 2) and for each environment, the redox potential was acquired each second for 700 seconds. A final measurement was repeated with the same hydrogen partial pressure pH2 as the first measurement for reproducibility purposes. The experimental redox potential was correlated with the pO2 using the Nernst equation (3), where R is the constant gas, T the temperate in K and F the Faraday constant. The pO2 Ni/NiO represents the oxygen partial pressure at which the transition between Ni and NiO occurs and it is constant for a given temperature. It can be calculated from the Nernst equation associated with the formation of NiO in aqueous environment (Ni + H2O + ½O2 ↔ NiO + H2O)

EP¿/NiO=2.303 RT4 F

∙ log( pO2pO2∋¿NiO

) (3)

The oxidation experiments (whose details are reported in Table 3) were carried out for 210 h in the same low pressure H2-steam system but at 480 °C. The coupons were placed on a quartz boat and inserted into the tube furnace. In this study, only the samples exposed at R equal to 1/24 (oxidizing environment), 1 (Ni/NiO transition) and 24 (reducing environment) will be discussed in detail.

2.3 Sample preparationThe coupons were cut to 20 x 20 x 2 mm3 from a plate of Alloy 600 whose composition is shown in Table 1. This material was previously solution annealed at 1100 °C in air for 30 min and then water quenched. The coupons were ground with SiC papers from P320 to P4000, then metallographically polished with a diamond polishing suspension (3 µm and 1 µm). A 60 nm silica oxide polishing suspension was used for the final polishing step in order to achieve a stress-free surface, representative of the bulk material. This Alloy

600 heat has been extensively used in previous works carried out by several of the current authors [4,6]. The average grain size was around 260±20 µm and sporadically decorated by semi-coherent small intergranular M23C7 carbides, never suppressed in Alloy 600, despite the water quenching process [4].

2.4 Microstructural examinationsThe surface morphology of the coupons exposed in the H2-steam environment was characterized with a Zeiss Sigma FEG-SEM. A FEI Quanta 3D Dual Beam FIB/SEM was used to obtain a minimum of 20 cross-sections per coupon to analyze the preferential intergranular oxidation and internal oxidation behavior as function of the redox conditions. Grazing-angle XRD examinations were carried out with a Philips X’Pert MPD system equipped with Cu Kα source for a complete comprehension of the phases in the external oxide layer. The incident angle used was 3° while the detector α angle scanned from 10° to 95° with a 0.05° step size.

3. Results and Discussion

3.1 Electrochemical measurement of the Ni/NiO transition and Alloy 600 oxidation studies

The redox potential was monitored as function of the time, exposing the Ni/NiO YSZ SSRE to the H2-steam environment. A representative time evolution of the redox potential at 400°C is reported in Fig. 2, where the H2 partial pressure is progressively decreased. The curve exhibits a series of transient regions in which the PPR was kept constant (Table 2). Since the reference electrode was a Ni/NiO sensor, the measured redox potential is a potential related the Ni/NiO transition. As expected, the redox potential increases with decreasing the hydrogen partial pressure, and increases when the oxygen partial pressure in the system is increased.

All the measurements were noisy, especially in oxidizing environments, possibly associated with a non-constant local steam flow. Furthermore, at the end of the test campaign, when the initial conditions were reproduced, the potential acquired was slightly higher than the redox potential measured at the beginning but only by about 9 mV, thus showing an excellent reproducibility.

The redox potential was correlated with the theoretical pO2 and the results are shown in Fig. 3. The black line represents the theoretical redox potential at 400 °C evaluated using the Nernst equation (3). For the experimental curves, each symbol represents the mean potential value of each plateau for experiments similar to that of Fig. 2. For reproducibility purposes, the test was repeated three times and for each measurement a good linearity, reproducibility and consistency with the theoretical prediction was found. In fact, the Ni/NiO transition was evaluated to be 1.4±4.3 mV vs. Ni/NiO at the corresponding pO2 of 4.36x10-28

atm. The data show a logarithm dependence between the potential and the pO2, confirming that the Nernst equation accounts well for the data acquired in the H 2-steam environment. The reduction of the H2

in the system caused an increase of the oxygen partial pressure, increasing the redox potential. From equation (3), it can be demonstrated that the experimental redox potential is only dependent on the O2 in the environment and, even in water, any pH fluctuations would not have any effect. The oxygen partial pressure inside the YSZ sensor is kept constant by the presence of two stable phases and the redox potential was only modified by the pO2 in the system. Moreover, the experimental potential can be considered with good approximation the potential of the pH2/pH2O equilibrium, given that the Pt did not interact with the environment and it did not change the thermodynamic equilibrium of the gaseous

species. In summary, thermodynamic conditions corresponding to the Ni/NiO transition did not agree with the findings by Capell et al. [14] but agree with the previous work carried out by some of the present authors [3–6,10] as well as with the theoretical thermodynamic approach [16].

3.2 Alloy 600

3.2.1 Microstructural characterization after H2-Steam exposure

Exposure in the NiO stability region

The surface appearance of the coupon exposed for 210 h at 480°C to the24 times more oxidizing environment respect to the Ni/NiO transition is shown in Fig. 4. Scanning electron microscopy (SEM) secondary electron (SE) and backscattered electron (BSE) images showed a surface completely covered with an external oxide layer with a high number of bright nodules, located within the grain surface and principally along the residual scratches due to the previous grinding and polishing operations. A rough appearance of the oxide and a bright oxide free zone of 300-400 nm was detected along the grain boundaries (Fig. 4-b) with an evident and localized grain boundary attack, most likely due to possible thermal etching. It is worth noting that an evident undulation on a high number of grain boundaries was detected after the oxidation process (Fig. 4). This “wavy” appearance has been reported to be directly associated to grain boundary migration [17]. The corresponding FIB cross section in Fig. 8 revealed the presence of an apparent grain boundary migration without any evidence of internal or preferential intergranular oxidation processes underneath the surface, however the presence of the external oxide layer was revealed, showing a non-uniform thickness on both side of the grain.

Exposure at the Ni/NiO transition

A representative surface morphology of the coupon exposed at the Ni/NiO transition for 210 h at 480°C is showed in Fig. 5. A similar surface morphology to the sample exposed in the NiO stability region was observed, where the surface of the grains was completely oxidized and the presence of the brightly nodules (most likely Ni rich) were also visible. On the original grain boundary, a continuous brightly protrusion was noticed. High magnification SE and BSE images in Fig. 5 showed probably oxidation processes not only on the new wavy grain boundary but also on the original straight one. On contrary of the sample exposed in NiO stability region, the external oxide layer formed during exposure in the Ni/NiO transition completely covered the whole surface of the grain, without any brighter area on both sides of the original grain boundary, associated with a different oxidation process. A representative FIB cross-section (Fig. 8) showed an apparent migrated region without any evidence of continuous intergranular oxidation and a uniform external oxide layer was noticed ≈50 nm thick.

Exposure in the Ni stability region

Representative SEM SE and BSE images of the surface morphology of the coupon exposed for 210 h to the 24 times more reducing environment than the Ni/NiO transition at 480 °C is shown in Fig. 6-a and -b. Within the grain, the surface was homogeneously covered with bright and shiny nodules (most likely metallic Ni [3]). On both side of the grain boundary, a protruded region and a high density Ni nodules were detected. The high magnification SE and BSE images, showed a slightly less wavy behavior for a high number of grain boundaries compared to the coupon exposed at the Ni/NiO transition and in the NiO region. The preferential intergranular oxidation, associated with grain boundary migration was confirmed via FIB cross section GBs (Fig. 8). Analogous results were found in previous experiments [4] performed in a similar reducing H2-steam environment.

In this study, the detailed examination of Alloy 600SA exposed to three different oxygen partial pressures pO2 with respect to the pO2 Ni/NiO revealed marked differences in the surface appearance (from Fig. 4 to Fig.6), oxidation processes and grain boundary migration (Fig. 8). The surface of all samples exposed in the different H2-steam environments, presented a high number of bright nodules randomly distributed on the surface. However, the nodules detected in the NiO stability region seemed to be less bright than the nodules formed during the experiment performed in the Ni stability region. The less bright appearance of the nodules in the oxidizing environment might be due to a diffusion of different alloying elements in the nodules. The pipe-diffusion mechanism [18] could explain the formation of these bright nodules; in fact, this mechanism was already proposed in previous studies [4,7] to explain the formation of similar pure Ni nodules on an Alloy 600SA coupon exposed in a similar H2-steam environment.

The high-resolution SE and BSE images (Fig. 4 and Fig. 5) of the coupons exposed at the Ni/NiO transition and in the NiO stability region revealed the presence of a uniform external oxide layer; the thin oxide layer was confirmed and the different phases were identified via grazing-angle XRD [19] technique (Fig. 7). The XRD data from the coupon exposed to a reducing H2-steam environment (R = 24) showed only the peaks of pure Ni and Alloy 600. NiO and NiCr2O4 peaks were detected on coupons exposed at the Ni/NiO transition (R = 1) and in the NiO stability region (R = 0.042). The oxide detected on the surface might probably be an external protective oxide layer (NiCr2O4). As argued in previous studies the formation of NiO is required for the formation of an external nickel chromite oxide [20], at which was shown to occur in the proximity of the maximum PWSCC susceptibility of Alloy 600 [12].

The detailed FIB cross-sections (Fig. 8) of the coupons exposed to the H2-steam environment at different pO2 revealed marked differences in terms of oxidation processes beneath the surface. For the sample exposed in the Ni stability region, at the ratio R equal to 24 a continuous preferential intergranular oxide penetration was detected with an associated and pronounced grain boundary migration, where the maximum penetration at grain boundary observed after 210 h was 826 nm, confirming previous oxidation studies on Alloy 600 performed at the same exposure condition [4]. A similar behavior was also observed in the coupon exposed at R equal to 6. The maximum preferential intergranular oxide penetration was equal to 980 nm, showing a deeper penetration closer the Ni/NiO transition. A representative FIB cross section of the coupon exposed at the Ni/NiO transition showed a complete different internal oxide morphology in comparison to the coupons exposed in the reducing H2-steam environment. Dark particles were detected along the grain boundary, beneath an apparent migrated area. This might be a typical internal oxidation phenomena with an outward diffusion of Cr and Fe on the external oxide layer or a “selective” oxidation of M23C6 carbides, identified as reservoir of Cr during the oxidation processes [5]. For the coupons exposed in the NiO stability region (R = 1/6 and R = 1/24), the detailed FIB cross sections showed a third “oxidation” morphology; an apparent grain boundary migration without an evident oxide penetration was detected. The preferential intergranular oxidation was persistent over a wide range of potential in the Ni stability region with a sharp cut off in the proximity of the Ni/NiO transition. The dependency of the preferential intergranular oxidation with the pO2 was more consistent with the time-to-initiation trend published by Molander [11], and not with the crack growth rate trend by Morton [9]. Therefore, these results suggest that preferential intergranular oxidation is directly linked with the “precursor events” associated with stress corrosion cracking initiation.

4. ConclusionsSeveral experiments were carried out at different H2-to-steam ratios in order to identify the Ni/NiO transition and correlate the preferential intergranular oxidation trend of Alloy 600SA with the redox potential. The following findings strongly support the use the low pressure superheated H2-steam system as a possible surrogate in which perform oxidation and SCC experiments on Ni-based alloys:

The Ni/NiO transition was clearly identified in the low pressure H 2-steam environment using a Ni/NiO YSZ SSRE. At 400 °C, the transition was located at the H2-to-steam ratio equal to 2.34x10-3 at the corresponding pO2 equal to 4.36x10-28 atm.

The experimental curves showed a linear behavior of the redox potential as function of the oxygen partial pressure pO2, therefore the Nernst equation can be used to describe the electrochemical behavior of the H2-steam environment.

The coupons of Alloy 600 SA exposed to the H2-steam environment equal or more oxidizing than the Ni/NiO transition presented an external oxide layer. The oxides were identifying as NiO and NiCr2O4 by grazing angle XRD analysis. The same XRD analysis performed on the coupon exposed under reducing conditions did not show any relevant oxide on the surface.

The oxidation experiments confirmed the susceptibility of Alloy 600SA to preferential intergranular oxidation in the Ni stability region, while in the NiO stability region, Alloy 600SA seems to be immune to preferential intergranular oxidation mechanism, although grain boundary migration was detected.

The detailed FIB cross section images showed a sharp transition at Ni/NiO transition and the results are consistent with the initiation time for SCC reported by Molander [11].

5. AcknowledgmentsThe authors gratefully acknowledge the Manchester Metallurgical Society and the EPSRC through the NNUMAN programme (EP/JO21172/1) for the financial support and Dr Peter Andresen for donating the Cu/Cu2O solid state reference electrode that was initially used for preliminary electrochemical experiments.

6. References[1] P. Scott and M. Le Calvar, “Some possible mechanisms of intergranular stress corrosion cracking of Alloy 600 in

PWR primary water,” in 6th Inter. Conf. Environ. Degrad. Mats Nucl. Power Syst – water reactors, pp 657–667, San Diego, 1993.

[2] G. Economy, R. J. Jacko, and F. W. Pement, “Igscc Behavior of Alloy 600 Steam-Generator Tubing in Water or Steam Tests above 360 C,” Corrosion, vol. 43, no. 12, pp. 727–73, 1987.

[3] F. Scenini, R. C. Newman, R. A. Cottis, and R. J. Jacko, “Alloy oxidation studies related to PWSCC,” in 12th Inter. Conf. Environ. Degrad. Mats Nucl. Power Syst, 2005.

[4] G. Bertali, F. Scenini, and M. G. Burke, “Advanced microstructural characterization of the intergranular oxidation of Alloy 600,” Corros. Sci., vol. 100, pp. 474–483, 2015.

[5] G. Bertali, F. Scenini, and M. G. Burke, “The intergranular oxidation susceptibility of thermally – treated Alloy,” Corros. Sci., vol. 114, pp. 112–122, 2017.

[6] G. Bertali, F. Scenini, and M. G. G. Burke, “The effect of residual stress on the Preferential Intergranular Oxidation of Alloy 600,” Corros. Sci., vol. 111, pp. 494–507, 2016.

[7] S. Y. Persaud, A. Korinek, J. Huang, G. A. Botton, and R. C. Newman, “Internal oxidation of Alloy 600 exposed to hydrogenated steam and the beneficial effects of thermal treatment,” Corros. Sci., vol. 86, pp. 108–122, 2014.

[8] S. Y. Persaud, R. C. Newman, A. G. Carcea, G. A. Botton, J. Huang, A. Korinek, and S. Ramamurthy, “Grain Boundary Embrittlement of Alloy 600 by Internal Oxidation in Simulated Primary Water Environments,” 16th Int. Conf. Environ. Degrad. Mater. Nucl. Power Syst. React., 2013.

[9] D. S. Morton, S. A. Attanasio, and G. A. Young, “Primary Water SCC Understanding and Characterization Through Fundamental Testing in the Vicinity of the Nickel/Nickel Oxide Phase Transition,” 10th Inter. Conf. Environ. Degrad. Mats Nucl. Power Syst. - Water React., 2001.

[10] L. Lindsay, F. Scenini, X. Zhou, G. Bertali, R. A. Cottis, M. G. Burke, F. Carrette, and F. Vaillant, “Characterisation of Stress Corrosion Cracking and Internal Oxidation of Alloy 600 in High Temperature Hydrogenated Steam,” in 16th Inter. Conf. Environ. Degrad. Mats Nucl. Power Syst - Water Reactors, 2013.

[11] A. Molander, “12. On line electrochemical monitoring in light water reactor (LWR) systems,” Nucl. Corros. Sci. Eng., pp. 408–437, 2012.

[12] T. Terachi, N. Totsuka, T. Yamada, T. Nakagawa, H. Deguchi, M. Horiuchi, and M. Oshitani, “Influence of dissolved hydrogen on structure of oxide film on Alloy 600 formed in primary water of pressurized water reactors,” J. Nucl. Sci. Technol., vol. 40, no. 7, pp. 509–516, 2003.

[13] S. A. Attanasio and D. S. Morton, “Measurement of the Nickel/Nickel Oxide Transition in Ni-Cr-Fe Alloys and Updated Data and Correlations to Quantify the Effect of Aqueous Hydrogen on Primary Water SCC,” in 11th Inter. Conf. Environ. Degrad. Mats Nucl. Power Syst — Water Reactors, 2003, pp. 143–155.

[14] B. M. Capell and G. S. Was, “Selective internal oxidation as a mechanism for intergranular stress corrosion cracking of Ni-Cr-Fe alloys,” Metall. Mater. Trans. A Phys. Metall. Mater. Sci., vol. 38, no. 6, pp. 1244–1259, 2007.

[15] D. D. Macdonald, “11. Electrochemical techniques for monitoring and controling corrosion in water-cooled nuclear reactor systems,” Nucl. Corros. Sci. Eng., pp. 331–407, 2012.

[16] O. Kubaschewski and C. B. Alcock, Metallurgical Thermochemistry 5th Edition Revised and Enlarged. .[17] J. W. Cahn, J. D. Pan, and R. W. Balluffi, “Diffusion Indeced Grain Boundary Migration,” Scr. Metall., vol. 13, no.

6, pp. 503–509, 1979.[18] S. Guruswamy, S. M. Park, J. P. Hirth, and R. A. Rapp, “Internal oxidation of silver-indium alloys: stress relief

and the influence of imposed strain,” Oxid. Met., vol. 26, no. 1–2, pp. 77–103, 1986.[19] J. Panter, M. Foucault, J. M. Cloue, P. Combrade, B. Viguier, and E. Andrieu, “Surface layers on alloys 600 and

690 in PWR primary water possible influence on stress corrosion crack initiation,” in Specific Technology Group 41 - Energy Generation Corrosion; Denver, CO, USA; NACE International.

[20] D. S. Morton, N. Lewis, M. Hanson, S. Rice, and Paul Sander, “Nickel Alloy Primary Water Bulk Surface and SCC Corrosion Film Analytical Characterization and SCC Mechanistic Implications,” in 13th Inter. Conf. Environ. Degrad. Mats Nucl. Power Syst, Whistler, British Columbia, 2007, pp. 1–16.

Table 1: Chemical composition of Alloy 600 (wt.%).

Alloy 600SA C Mn S P Si Cr Ni Cu Co FeComposition 0.047 0.23 0.002 0.005 0.30 15.42 74.43 0.01 0.057 8.94

Table 2: Experiments performed in the H2-steam environment at 400°C for the evaluation of the Ni/NiO transition.

Measurement

pO2

atmpO2Ni/NiO

atmR

pO2NiNiO/pO2

H2-to-SteamRatio

Theoretical corrosion potentialmV vs. Ni/NiO

1 6.24x10-30 4.36x10-28 69.797 1.96x10-2 -61.542 1.83x10-29 4.36x10-28 23.750 1.14x10-2 -45.913 3.69x10-29 4.36x10-28 11.781 8.06x10-3 -35.754 7.77x10-29 4.36x10-28 5.606 5.56x10-3 -24.995 1.73x10-28 4.36x10-28 2.522 3.72x10-3 -13.416 4.36x10-28 4.36x10-28 1.000 2.34x10-3 0.007 1.10x10-27 4.36x10-28 0.397 1.47x10-3 13.378 2.42x10-27 4.36x10-28 0.179 9.93x10-4 24.939 1.03x10-26 4.36x10-28 0.042 4.82x10-4 45.92

10 6.24x10-30 4.36x10-28 69.797 1.96x10-2 -61.54

Table 3: Oxidation experiments performed on the Alloy 600SA coupons in the H2-steam environment at 480°C for 210h.

Test no.

H2-steam environment

pO2

atmpO2Ni/NiO

atmR

pO2NiNiO/pO2

H2-to-SteamRatio

Theoretical corrosion potentialmV vs. Ni/NiO

1 Ni stability 1.25 x10-25 2.96x10-24 24 1.47x10-2 -45.912 Ni stability 5.28x10-25 2.96x10-24 6 7.09x10-3 -24.99

3 Ni/NiO transition 2.96x10-24 2.96x10-24 1 3.00x10-3 0.00

4 NiO stability 1.66x10-23 2.96x10-24 1/6 1.27x10-3 24.935 NiO stability 7.01x10-23 2.96x10-24 1/24 6.15x10-4 45.92

Fig. 1: Ni/NiO Yttria Stabilized Zirconia SSRE used for the evaluation of the Ni/NiO transition in the H2-steam environment at 400 °C.

Fig. 2: Redox potential vs. acquisition time at 400°C. Each transient, labeled with the corresponding measurement, represents a constant H2-to-steam ratio.

Fig. 3: Redox potential vs. pO2 at 400 °C.

Fig. 4: Alloy 600SA SE (a) and BSE (b) oxidized surface morphology micrographs after H2-steam in the NiO stability region (R = 1/24).

Fig. 5: Alloy 600SA SE (a) and BSE (b) oxidized surface morphology micrographs after H2-steam at the Ni/NiO transition (R = 1).

maFig. 6: Alloy 600SA SE (a) and BSE (b) oxidized surface morphology micrographs after H2-steam in the Ni stability region (R = 24).

10 20 30 40 50 60 70 80 90

R = 1/24 R = 1 R = 24

+ Ni = Alloy 600 ± NiO and NiCr2O4

2 (Copper, Cu)

=

±

±

±

=+

=

±

=+

Fig. 7: Grazing angle XRD of the coupons exposed to H2-steam environment at different oxygen partial pressures: NiO stability region (R = 1/24), Ni/NiO transition (R = 1) and Ni stability region (R = 24).

Fig. 8: FIB cross sections of Alloy 600SA after exposure to H2-steam environment at different oxygen partial pressures pO2.