Anodic Kinetics of NiCr[Mo] Alloys During Localized Corrosion: I. Diffusion-Controlled Dissolution

Artificial pit electrodes have been used to measure the pitting dissolution kinetics in the anodic limiting current region for a range of Ni-22Cr-xMo (wt %) alloys with x=0, 3, 6, 9 and 13. Most of the data were obtained at 25―90°C using I M NaCl solution. Type 316 stainless steel and pure nickel were used for comparison. At high temperatures the anodic limiting current density for the NiCr(Mo) alloys (x = 0, 3, 6, 9) is nearly the same as that of stainless steel, other things being equal; at intermediate temperatures the NiCr(Mo) alloys give higher i lim values. Changing the NaCl concentration between 0.1 and 1 M causes a 20% change in i lim at a given pit depth, and the reasons for this are discussed. Comparison of pure Ni and Ni-22Cr shows a 35―40% higher i lim for the latter at a given pit depth, and the reasons for this are discussed. Literature values of solubility and diffusivity, and a 1-D finite difference model, are used to rationalize ― in part ― the limiting current densities and associated average diffusivities in terms of concentration-dependent diffusivities.