Fundamental aspects of the corrosion of alloys

Phenomenological equations are given for the steady state dissolution of alloys and their composition at the surface. The slow component with the smallest rate constant essentially determines the steady state dissolution rate and is enriched in the alloy surface. The phenomenological equations are compared to experimental results both for active and passive alloys. In ideal cases, phenomenological rate constants are independent of composition. Rate constants of pure slow components being much smaller than rate constants of pure fast components are enhanced by alloying with the fast component. Thus, corrosion protection by alloying often is much less efficient than predicted by the ideal case. Deviations from the ideal case can be understood, if dissolution proceeds at kinks in steps on low index planes of a crystalline alloy, or from equivalent sites in glassy metal alloys. Thus, the dissolution rates depend on the rate constants for dissolution from a kink and on the surface concentration of kinks. The surface concentration of kinks is determined by processes parallel to the dissolution from kinks, e.g. by dissolution of atoms from straight steps or by two-dimensional nucleation in low index planes. The probability of such processes grows with the electrode potential and the concentration of a fast component. The resulting increase of the surface concentration of kinks occupied mainly by the slow component enhances its phenomenological rate constant. Steady states are approached at room temperature much slower than expected from diffusion coefficients of solid alloys extrapolated from high temperatures. Consequently, at room temperature changes of composition penetrate deeply into the bulk. The apparent increase of the interdiffusion coefficients is due to vacancy injection connected to the processes of kink generation. Selective corrosion and passivation often are accompanied by large surface stresses which may result in plastic deformation or fracture. (C) 1997 Elsevier Science Ltd.