Effect of chromium on corrosion resistance of Ni-Cr-Mo-Gd alloys in seawater

Neutron absorbing materials are being considered within commercial spent nuclear fuel disposal canisters to maintain nuclear subcriticality. To select candidate alloys for the canisters, both neutron absorption and corrosion resistance must be considered for this application. This work examines corrosion resistance of Ni-CrMo-Gd alloys developed specifically for neutron absorption. The addition of Gd results in a secondary gadolinide phase (Ni5Gd) that significantly changes the corrosion properties. Testing was performed primarily in seawater at 30 degrees C. Seawater was selected as the most prevalent terrestrial brine and is characterized by a high chloride concentration. Various electrochemical corrosion techniques were carried out to evaluate Ni-Cr-Mo-Gd alloys with different Cr compositions and investigate the role of Ni5Gd phase on corrosion behavior. C22 was included as a benchmark material, due to the similarity in composition and the significant corrosion data available. Test results showed a tendency to passivate over time which is attributed primarily to dissolution of surface exposed Ni5Gd phase particles leaving primary phase exposed. Cross-sectional analysis indicated that dissolution could penetrate up to hundreds of micrometers deep under aggressive conditions. It was found that higher Cr variant (21.01%) showed much shallower impact, suggesting Cr prevented primary phase corrosion and thus reduced deeper Ni5Gd phase dissolution. Acid pickling of the specimens showed much less dissolution for the higher Cr material and suggested some primary phase dissolution for the low Cr specimen. Acid pickled specimens showed positive shifts in the repassivation potential, suggesting increased surface passivation.