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 basicrnunderstanding of the local corrosion mechanism in an already deoxygenated crevice with a givenrngeometry. The time stepwise calculation of chromium dissolution was based on arbitrarily selectedrnlevels of initial corrosion potential and slopes of the cathodic polarization curve at the openrnsurface in contact with the bulk liquid. As a basic feature, the anodic polarization slopes inside therncrevice were controlled by the mass of precipitated chromium hydroxide which wasrnthermodynamically calculated from the resulting crevice solution composition using the tentativernternary phase diagram water -chromium- chromium hydroxide - chromium chloride. The presentrnpaper applies additional oxygen diffusion calculation steps and demonstrates the effects of variousrncrevice widths and bulk oxygen contents diffusing into the crevices at pH = 6 and chloride contentsrnof 7 and 13 %. The calculations are based on the assumption of chemical equilibrium conditions asrnwell as on rather short electromigration times of the chromium controlled chloride ionsrncomplementary to the OH~- ions resulting from oxygen diffusion and reduction during the individualrntime steps. It is shown that increasing bulk oxygen levels and crevice widths are retarding thernbreakdown times of the passivating chromium hydroxide, while, under the given conditions,rnincreasing chloride contents mainly reduce the crevice corrosion incubation times. As a specificrnresult it is also demonstrated that after chromium hydroxide break down, the times for consecutivernrepassivation by restart of the chromium hydroxide precipitation inside an active crevice arernreduced at increasing bulk oxygen levels.