MODELLING THE EFFECTS OF CREVICE GEOMETRY AND OXYGEN ON CREVICE CORROSION OF PURE CHROMIUM AT CONSTANT PH AND VARIOUS CHLORIDE CONTENTS

摘要

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 precipitated chromium hydroxide which was thermodynamically calculated from the resulting crevice solution composition using the tentative ternary 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 and bulk oxygen contents diffusing into the crevices at pH = 6 and chloride contents of 7 and 13 percent. 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. It is shown that increasing bulk oxygen levels and crevice widths are retarding the breakdown times of the passivating chromium hydroxide, while, under the given conditions, increasing chloride contents mainly reduce the crevice corrosion incubation times. As a specific result it is also demonstrated that after chromium hydroxide break down, the times for consecutive repassivation by restart of the chromium hydroxide precipitation inside an active crevice are reduced at increasing bulk oxygen levels.