Characterisation of Stress Corrosion Cracking and Internal Oxidation of Alloy 600 in High Temperature Hydrogenated Steam

In this study, the possibility of using low pressure hydrogenated steam, which is capable of reproducing similar oxidising electrochemical potentials, has been examined. The oxides formed between 350°C and 500°C in low pressure steam have been characterised and compared to oxide that formed in high pressure water in autoclave tests at 350°C. The comparison shows that the oxides formed are similar but not identical. However, constant load tests carried in hydrogenated steam at 400°C showed a similar trend to the classical dependency of PWSCC as a function of potential. The oxides formed in both environments were characterized via FIB and AEM techniques. At 500°C internal oxidation and preferential oxidation of grain boundaries was observed but only on samples that were prepared by OPS. Conversely, those samples ground to 600 grit revealed a continuous and protective oxide. At lower temperatures the internal oxidation was almost suppressed, although the effect of surface preparation was still present and had a clear influence on the SCC test carried out at 400°C. Cr-rich carbides in the alloy appeared to accelerate the oxidation in the both the bulk and along the grain boundaries. This was observed for both steam and autoclave-exposed specimens.AEM characterisation of the exposed specimens has been employed to determine the preferential oxidation rates and associated Cr depletion. The observed intergranular Cr-depletion was asymmetric and it was estimated that a two orders of magnitude difference in diffusivity were required to account for the observed difference.