A Review of Oxidation Phenomena in Ni Alloys Exposed to Hydrogenated Steam Below 500°C

The following is a review of the accumulated literature over the past 20 years for oxidation in Ni-Fe-Cr alloys exposed to 400°C to 500°C hydrogenated steam. Hydrogenated steam at temperatures of 500°C and below is considered to simulate 300°C to 360°C deaerated water often used in the primary circuit of nuclear power plants. Alloy 600 (Ni-16Cr-9Fe) is highly susceptible to stress corrosion cracking (SCC) in primary water and the peak crack growth rate is observed near the Ni/NiO equilibrium potential. It is believed that the oxidation tendency, internal and/or external, is directly related to the mechanism of SCC in Alloy 600. An initial review will be conducted exploring inter- and intragranular oxidation in Alloy 600 and the relationship to embrittlement and SCC. Following this, investigations into oxidation phenomena, in particular the possibility of internal oxidation, in alloys with high reactive element contents, such as Alloy 690 and Alloy 800, will be analyzed. Finally, the practical applicability and validity of a high-temperature hydrogenated steam environment as a simulant for accelerated primary water conditions will be reviewed through comparisons to oxidation phenomena commonly reported in 300°C to 360°C deaerated water. In general, evaluation of the literature suggests that intergranular oxidation phenomena and the mechanism of SCC/embrittlement in Alloy 600 are likely to be similar, ranging from 300°C deaerated water to 500°C hydrogenated steam.