Dealloying of Monel 400 in acetate solutions: Predicting susceptibility using Thermo-Kinetic diagrams

Monel 400 steam generator tubes in CANDU® reactors are reported to be susceptible to dealloying corrosion, but the degradation mechanism and kinetics remain unclear. We construct the Thermo-Kinetic diagrams (TK diagrams) for Monel 400, and commercially pure nickel and copper alloys in 0.1M sodium acetate solutions at 25 and 85 ℃ to predict the conditions of susceptibility to dealloying. Subsequently, coupons of Monel 400 were immersed at various pHs at both temperatures to validate the accuracy of the predictions. After 168h, the coupons showed selective dissolution of nickel at pH=6 at 85 ℃, with a dealloying depth of ~2.5µm on the polished surfaces and a maximum depth of ~60µm on unpolished edges. The dissolution of nickel in the form of acetate complexes appears to trigger dealloying, followed by the formation of intergranular cracks. Dealloying resulted in a ligament-pore structure and nickel content of the alloy decreased from 66.5 to 17.6wt% in the dealloyed layer. The results prove that TK diagrams are a powerful tool for predicting the corrosion mechanism of alloys under complex conditions.