Dependent-Variable Transformation for the Treatment of Diffusion, Migration, and Homogeneous Reactions - Application to A Corroding Pit

Abstract

The anodic dissolution of nickel in chloride medium is treated for a stagnant solution and the hemispherical cavity geometry common to the dissolution of passivating, corrosion-resistant metals. A new coordinate system developed recently by the authors again proves to be useful in dealing with the problem numerically, particularly with regard to the singularity at the pit lip. The quasipotential transformation allows us to reduce the coupled, nonlinear partial differential equations governing the concentrations and the potential to a single linear equation, Laplace's equation, with a great reduction in computation time ard an increase in the accuracy attainable. The method thus treats coupled diffusion and migration with an arbitrary number of facile homogeneous reactions and the participation of an arbitrary number of species in the heterogeneous electrode reaction. Numerical calculations for the dissolution of nickel in chloride solutions are compared with published experimental results