Role of the Alloying Elements at the Steel/Rust Interface

Three low alloy steels and an ordinary carbon steel were investigated as to the distribution of the alloy elements (Cu, Cr, P, Ni) in rust coverings produced either by weathering or by slight anodic oxidation. Electron microprobe examinations of rusts showed, besides an overall enrichment in the alloy elements, the presence of sharp concentration peaks in layers some ten μm wide. The more resistant the steel to atmospheric corrosion, the higher the rust to steel concentration gradient and the closer the concentration peaks to the steel/rust interface. Corrosion potential and polarization measurements in 0.1 M Na₂SO₄ on rusted specimens evidenced a marked effect of the interfacial enrichment in increasing the anodic polarizability and the tendency to a coverage passivation. This leads to looking upon the corrosion generated steel/rust interface as directly contributing, in addition to the rust itself, to the self protection of low alloy steels during atmospheric corrosion.