Beschreibung:
<jats:p>The presented crevice corrosion model is intended to provide basic understanding of formation and dissolution of passivating layers during time dependent metal dissolution in an already deoxygenated crevice. It operates along a constant cathodic polarization curve in subsequent time steps with time dependent anodic polarization curves and corrosion currents. The anodic polarization curves are determined by dissoluted ionic chromium and respective Nernst potentials as well as by the mass of precipitated passivating chromiumhydroxide layers. All chemical reactions including diffusion of chloride into the model crevice configuration are assumed to be at equilibrium during the respective time steps. The amounts of passivating chromiumhydroxide masses are determined by lever‐rule application at increasing chromium and chloride contents in aquaeous solution of the quaternary water‐chromium‐hydrogen‐chloride phase diagram at various Nernst potentials. For this purpose, the established water–chromium Pourbaix diagram had to be redrawn in terms of ternary water‐chromium‐hydrogen phase diagrams for constant potentials. It was tentatively extended to the quaternary chloride including diagram based on literature results. This included solubility of chromiumchloride in hydrochloric acid and most probable effects of chloride ions on chromiumhydroxide solubility at saturation with chromium metal. The results of the interactive corrosion process are based on a crevice geometry drawn from previous publications and given initial solution concentrations as well as assumed polarization curves for time steps of one second. It is shown, how the process starts with concentration changes of chromium, chloride and hydrogen in the crevice, the subsequent formation of the passive layer and the corresponding decrease in the corrosion current and increase in the mixed potentials. In the presence of the chromiumhydroxide phase, however, chromium and hydrogen remain at low levels in the equilibrium aqaeous solution while chloride is increasing. The saturated solution reaches the four phase equilibrium concentration including saturation by chromium, chromiumhydroxide and chromiumchloride at corresponding Nernst potentials. The further increase in total chromium and chloride concentrations of the crevice then leads to initiation and propagation of crevice corrosion by formation of non‐passivating chromiumchloride at the expence and finally, the total dissolution of passivating chromiumhydroxide, at decreasing mixed potentials and increasing currents. Due to the low solubility of chromiumchloride in hydrochloric acid, the latter is finally formed with the result of the well‐known increase in crevice acidity. Thus, it is demonstrated that the acidification itself is not a requirement for crevice corrosion but rather a consequence of it.</jats:p>