The Anodic Dissolution of a Ni-Base Superalloy

A single phase nickel base superalloy, HASTELLOY⁽¹⁾ Alloy C-276, was anodically polarized in 0.5 N NaCl at 70 C, and the concentrations of metal-ion complexes in solution were determined. Relative dissolution rates of alloying elements were obtained and selectivity coefficients were calculated. The corrosion products that formed at high anodic potentials were identified as FeCl²⁺, Ni²⁺, HCrO₄⁻, and MoO₄²⁻. The pH was observed to decrease and the chloride ion concentration to increase in the electrolyte when diffusion of corrosion products out from an occluded region at the alloy surface was restricted. The increased hydrogen ion concentration in the occluded region was explained by the formation of molybdate and chromate ions as corrosion products. The high corrosion resistance of Alloy C-276 in chloride may be related to the formation of corrosion-inhibiting molybdate ion.