Corrosion protection function and breakdown mechanism of passive films on stainless steels

Abstract

Film thinning rate as a function of potential for Fe2O 3-Cr2O3 films in 1MHCl shows the reductive dissolution region of the Fe2O3-component, no dissolution region of both the components, and the transpassive dissolution region of the Cr2O3-component. The reductive dissolution of the Fe 2O3-component of the films can be suppressed by increasing Cr2O3 content. Fe-30Cr alloy with a passive film having the threshold XCr value for suppression of the reductive dissolution of the Fe2O3-component suffers from no pitting in 1 M HCl. Fe-Cr alloys containing 14-25% Cr are susceptible to pitting in 1 M HCl and their pitting potentials exist in the reductive dissolution region of the Fe2O3-component in passive films. No initiation of pitting was observed on Fe-20Cr alloy in an acid solution containing 1 M Cl- in the no dissolution region of both the components between 0.6V and 0.7V. Potentiostatic polarization tests of Type 304 coated with a heterogeneous film with a localized small part of low Cr2O3 content in a wide matrix of high Cr2O3 content showed the initiation of pitting on the substrate Type 304 by the reductive dissolution of the small part in 1M HCl. For the above reasons, the suppression of the reductive dissolution of the Fe 2O3-component in passive films is presumed to be effective on the suppression of pitting in 1M HCl.