Effect of the chromium content on the corrosion of nickel based alloys in primary water of pressurised nuclear reactors

Stress corrosion cracking (SCC) is a damaging mode of alloys used in pressurised water reactors. Those damages led to the replacement of Alloy 600 (15% Cr) by alloy 690 (30% Cr) but the mechanism responsible for the SCC and the reason for the positive effect of chromium are not yet very well understood. In this paper, we studied the corrosion of synthetic alloys - with controlled chromium content varying from 5 to 30 wt%. Characterisation was done using SEM and TEM observations together with chemical analysis and mapping using EDX, EFTEM and SIMS. The outer oxide scale is composed of crystallites, beneath it the presence of a continuous chromium oxide is accompanied with a Cr depleted zone for alloys that contain more than 10% Cr. The penetration of oxygen over very large distances (several microns) on triple junctions is demonstrated, as well as the role of plastic deformation that modifies strongly the overall structure of the oxide scale.