Stabilizing Process of Rust Layer Formed on Weathering Steels by Atmospheric Corrosion for a Long Period

Abstract

The rust layers formed on weathering low-alloy steels, which resist atmospheric corrosion, exposed for various periods have been examined. It is elucidated that the inner stable and protective rust layer which covers the surface of a weathering low-alloy steel exposed for 26 years mainly consists of small particles of α-FeOOH and α-(Fe_1-X, Cr_X) OOH (X=0.03) containing a considerable amount of Cr. The variation in the amount of rust constituents against the exposure period supports well a newly proposed schematic progress of long-term growth of stable and protective rust layer. Namely, the initial rust layer of γ-FeOOH is changed to a final stable rust layer of α-oxyhydroxides consisting of α-FeOOH and α-(Fe_1-X, Cr_X) OOH, probably via amorphous oxyhydroxide, during long-term exposurein an industrial atmosphere. The effect of Cr and Cu on the formation of the stable and protective rust layer is discussed on the basis of transmission electron microscopic observation of synthetic α-FeOOH particles with and without addition of Cr and Cu ions. It becomes evident that densely packed fine-particles of the α-oxyhydroxides are formed as a result of coexisting with Cr and Cu.