The mechanism of atmospheric rusting and the protective amorphous rust on low alloy steel

Atmospheric rusting of mild and low alloy steels was studied by means of infra-red and far infra-red spectrophotometries, X-ray and electron diffraction methods and scanning electron microscopy. The rusting process can be interpreted on the basis of a previously reported diagram for rust formation in aqueous solution.A large amount of amorphous matter in rust formed in semi-rural atmosphere was identified by infra-red and far infra-red spectra as amorphous ferric oxyhydroxide, FeO_x (OH)_8–2x. The amorphous ferric oxyhydroxide rust on low alloy steel was dense and uniform, and contained a considerable amount of bound water. From these results it can be concluded that the amorphous ferric oxyhydroxide rust acts as a protective barrier against atmospheric rusting of the steels. Cu, P and Cr in low-alloy steels are inferred to favour the formation of crack-free, uniform rust layer and help to produce uniform amorphous ferric oxyhydroxide.