Stress corrosion cracking of alloy 600 in PWR primary water : influence of chromium, hydrogen and oxygen diffusion

Abstract : Alloy 600, a nickel base alloy containing 15 % chromium, is used in primary circuit of Pressurized Water Reactor (PWR). This alloy is well known to be susceptible to Stress Corrosion Cracking (SCC) in PWR primary water. Despite the fact that many laboratory studies have been performed and that many models are proposed in the literature, the mechanisms involved are still not well known. In the proposed model, the transport of species (oxygen, hydrogen and chromium) is considered as playing a key role. Therefore, experiments and calculations are performed in order to study the transport of chromium, hydrogen and oxygen in Alloy 600 and in model alloys. The results lead to the conclusion that the transport of oxygen and hydrogen cannot be considered as the rate-controlling steps. The dissymmetric aspect of the crack tip and of the chromium depletion ahead of the crack lead to the conclusion that chromium diffusion could play a significant role in the mechanism.