The breakdown of the passive film on high yield mild steel by chlorine ions.

Passivation of steel in concrete is due to a two component system consisting of a portlandite layer and a thin pH stabilised iron oxide/hydroxide film on the surface of the metal. Chloride ions in concrete breakdown the passivity of the system by dissolution of the portlandite layer followed by debonding of the passive film. The ratio of iron to chloride across the debonded film was measured using SEM-EDS and was found to increase exponentially toward the insitu passive film. Some areas of chemically unaffected passive film were also found to be disbonded supporting the view that a physical process also operates during disruption. This has been attributed to the internal film pressure. (A)