The initial stage of salt passivation of metals

A theoretical model of the nucleation of a passivating salt layer on the surface S of an anode-dissolving metal was developed. The layer is considered a new phase formed by the mechanism of heterogeneous nucleation, which serves as a related substrate for subsequent growth of the passivating layer. According to experiments, the surface S of real metals is inhomogeneous, that is, anode current density fluctuates on S. It is assumed that, at large current fluctuations, localized regions with volume ΔV _ϕ appear, in which the C _M and C _A concentrations of the M ^z+ and A ⁿ⁻ ions are increased. At fairly large current fluctuations, these concentrations can reach saturation. For this reason, the ΔV _ϕ regions are treated as mother phases, in which A _a M _m salt nuclei are formed with a certain probability. The formation of such ΔV _ϕ volumes and the kinetics of formation of nuclei in them, from which a passivating layer with a finite thickness is formed, are considered.