Modeling Crevice Corrosion of Pure Chromium by Coupling of Phase and Polarization Behavior Available to Purchase

In a recent paper, a crevice corrosion model for pure chromium was presented aiming at basic understanding of the local corrosion mechanism in an already deoxygenated crevice with a given geometry. The time stepwise calculation of chromium dissolution was based on arbitrarily selected levels of initial corrosion potential and slopes of the cathodic polarization curve at the open surface in contact with the bulk liquid. As a basic feature, the anodic polarization slopes inside the crevice were controlled by the mass of the precipitated chromium hydroxide phase, which was thermodynamically calculated from the resulting crevice solution composition using the tentative quaternary phase diagram: water—chromium—chromium hydroxide—chromium chloride. The present paper applies additional oxygen diffusion calculation steps and demonstrates the effects of various crevice widths, chloride contents, and bulk oxygen contents diffusing into the crevices at pH = 6. The calculations are based on the assumption of chemical equilibrium conditions as well as on rather short electromigration times of the chromium-controlled chloride ions complementary to the OH⁻ ions resulting from oxygen diffusion and reduction during the individual time steps. Considering total oxygen consumption and its respective corrosion currents inside the crevice, it is shown that increasing bulk oxygen levels and decreasing crevice widths are reducing the start times for dissolution as well as the final breakdown times of the passivating chromium hydroxide. Increasing chloride ion contents also reduce the hydroxide dissolution start times but increase the total breakdown times. The total corrosion currents are increased by oxygen and higher crevice widths but are reduced slightly by chloride contents. The model demonstrates reasonable corrosion rates and the accumulation of hydrogen ions’ respective severe pH decrease in the crevice following the total hydroxide breakdown.