Influence of aerated transients on the oxidation and stress corrosion cracking of a cold-worked 316L austenitic stainless steel in PWR primary water

Operational feedback on the primary circuit of Pressurized Water Reactors (PWR) shows cases of stress corrosion cracking (SCC) affecting cold-worked stainless steel components. Some working steps require water from auxiliary reservoirs to be added to the primary water. Depending on the operator, this water can be either aerated, or deaerated and monitored. These aerated transients may have a detrimental effect on the SCC susceptibility of stainless steels. In this context, the aim of this work is to study the influence of dissolved oxygen on the oxidation and SCC susceptibility of a cold-worked 316L stainless steel in PWR primary water. For this purpose, oxidation and SCC tests were performed in PWR primary water with nominal (hydrogenated and deaerated), fully aerated, and aerated transients conditions. Dissolved oxygen was found to have a significant impact on the oxidation processes of stainless steels. In particular, the inner surface oxide layer is chromium-rich and dense in nominal conditions, while it is chromium depleted and porous in the two other environments, with the exception of a thin oxide film (10 to 30 nm thick) located at the oxide/alloy interface. Moreover, intergranular oxide penetrations, which can be considered as precursors for SCC initiation, were observed in nominal and aerated transients conditions, but not in fully-aerated ones. Intergranular and micrometric SCC cracks were observed after the SCC tests, in all three environments. A detailed quantification showed that they are more numerous and significantly longer and deeper in nominal conditions, and with aerated transients, compared to the fully-aerated ones. This difference seems coherent with the absence of intergranular oxide penetrations in fully-aerated conditions. Moreover, with aerated transients, oxides at the crack flanks and tip can either have a different morphology than in nominal conditions, suggesting that aerated transients may have an effect on SCC susceptibility. Based on these results, scenarios were proposed for intergranular SCC cracks initiation, in the three environments of this study. (author)