Simple passivation and depassivation model for pitting corrosion.

Abstract

A simple two-dimensional passivation and depassivation model for pitting corrosion has been developed. This model describes corrosion processes that are limited by the diffusion of either a corrosive agent or the diffusion of corrosion products that inhibit the corrosion process. Despite its simplicity, this model exhibits both stable and unstable pit growth, along with distinctive current fluctuations. For the case of a single corrosion pit, the corrosion current I(t) has the form I(t)=f(t/${\mathit{t}}^{\mathrm{*}}$)/ln(t) if there is no depassivation. The function f(x) has the form f(x)=const for x\ensuremath{\ll}1 and f(x) decays faster than any power of x for x\ensuremath{\gg}1. The characteristic time ${\mathit{t}}^{\mathrm{*}}$ is given by ${\mathit{t}}^{\mathrm{*}}$ln(${\mathit{t}}^{\mathrm{*}}$)\ensuremath{\simeq}${\mathit{k}}_{\mathit{p}}^{\mathrm{\ensuremath{-}}2}$, where ${\mathit{k}}_{\mathit{p}}$ is the passivation rate constant.