The effect of chloride ions on passive layers on stainless steels

Abstract

The effect of chromium on the formation and stability of the passive film formed on ferritic stainless steels (5.9 – 29.9% Cr) in 3% NaCl was studied by electron spectroscopy for chemical analysis. The film was formed by selective dissolution of iron with concomitant enrichment of chromium on the surface. The chromium enrichment depends on the alloy concentration and is proportional to the logarithm of the exposure time. In chloride-free solutions the same mechanism occurs but with a lower dissolution rate. The stability of the film depends on the chromium content in the steel and on the chloride activity. Low chromium steel does passivate in pure water but the film will break down in the presence of chloride ions. Even with high chromium steels there is a reversal of the chromium build-up after several hundreds of hours in salt water, which must be assumed to lead to eventual breakdown. When steels containing 11% chromium or more are pre-passivated in pure water and are then immersed in salt water, the enrichment of chromium proceeds at the rate originally established in pure water, i.e. much more slowly than when fresh surfaces are exposed to salt water. Pre-passivation of low chromium alloys (5.8% and 7.8% Cr) in pure water causes films to be formed which are soon destroyed in salt water.