Structure of Rust Layer Formed on Atmospheric Corrosion Resistant Steels

Abstract

The distribution of elements in rust layer formed on low alloy steels after various periods of exposures in industrial and rural atmospheres was investigated by means of microscopic examination and electron probe microanalysis. The rust layer in large pits contains Cu, P, and Cr in a localized state. Cr and P concentrate around such defects in the rust as voids and cracks. The local concentration of Cu seems to diminish as the exposure period becomes longer. The rust layer on the flat surface surrounding large pits and also the surface layer above large pits have shown no evidence of localized concentration of Cu and P and are characterized by the fact that little Cr content is found in the rust. Si, main constituent of dusts, has been found in these parts of the rust, but none in the pits. It is supposed that Cr, P, and Cu produced by corrosion reaction presumably form far less soluble compounds than ferrous ion and are likely to precipitate in the pits while ferrousion diffuses out from the pits and precipitates on outer surface.