Corrosion mechanisms of Ni-base alloys in pressurized water reactor primary conditions

Oxidation mechanism of Alloy 690 has been investigated in Pressurised Water Reactor (PWR) primary coolant conditions (325 degrees C, aqueous hydrogenated media). Experiments performed with gold marker and RBS technique reveal that the passive film formation is the consequence of an anionic mechanism. This result is confirmed by experiments achieved with two sequences of corrosion in a (H2O)-O-16 media and in a mixed (H2O)-O-16/ (H2O)-O-18 media. The localisation of O-18 by SIMS analysis in the thin passive layer underlines an oxidation mechanism due to oxygen diffusion by short circuits (like grain boundaries) in the oxide scale. Moreover grain boundary diffusion coefficient in chromite like oxide was estimated to be in the range 2 10(-18) - 1 10(-17) cm(2).s(-1) and compared to values extrapolated from higher temperature.