Protectiveness of oxide films in simulated BWR crack conditions, SO42- enrichment and cracking susceptibility

Sulphate ions in the BWR coolant dissolved e.g. from ion exchange resins increase the risk of environmentally assisted cracking (EAC). Influence of sulphate is likely to be related to the properties and behaviour of oxide films formed on material surfaces within a crack. For this purpose, the influence of sulphate ion content on the protectiveness of oxide films forming on AISI 316NG stainless steel, Inconel alloys 182 and 82 was characterised in simulated BWR crack chemistry conditions. The crack propagation rates for Inconel 182 and AISI 316NG have been found to depend on the sulphate content, being higher at higher sulphate contents. The results of the present work show that the electrochemical protectiveness of the crack wall oxide film decreases with increasing sulphate content. Thus a correlation between the protectiveness of the crack wall oxide film and susceptibility to environmentally assisted cracking is indicated. This correlation can be used to assess the susceptibility of a given material to EAC in different environments. However, a comparison of the EAC susceptibility of different materials only by comparing the protectiveness of the crack wall oxides is not recommended