Hideaki Miyuki

The long term growth of the protective rust layer formed on weathering steel by atmospheric corrosion during a quarter of a century

Abstract The rust layers formed on weathering and mild steels by atmospheric corrosion in an industrial region for a quarter of a century have been characterized using various analytical techniques. In particular, analysis of a local portion of the rust layers by means of Raman spectroscopy gives important information on the structure of the layers. It is elucidated that the inner stable and protective rust layer which covered the surface of weathering steel mainly consists of nano-particles of α-FeOOH containing a considerable amount of Cr. The relative change of the amount of rust constituents for low alloy steels supports well a newly proposed schematic progress of long term alteration in stable and protective rust layer formed on a weathering steel in an industrial environment, i.e. the γ-FeOOH, as an initial rust layer, is transformed into a final stable rust layer of α-FeOOH, probably via an amorphous oxyhydroxide substance, during the long term atmospheric corrosion of a weathering steel.

Corrosion behavior of steel under wet and dry cycles containing Cr3+ ion

Abstract The corrosion behavior of mild steel has been investigated during the wet and dry cyclic transitions containing Cr 3+ ion added as sulfate in order to gain a better understanding of the influence of Cr on the atmospheric corrosion of steels. The corrosion rate during drying is greatly suppressed by the existence of Cr 3+ ion in the electrolyte covered with the surface. Lower corrosion rates are observed during drying even if the surface have been polarized to negative potentials below −200 mV SHE during the wet corrosion conditions in which the surface-covered electrolyte contains Cr 3+ ion. This corrosion behavior is identical to the case of Cr-containing steel for the wet and dry cyclic transitions without the addition of Cr 3+ ion. The composition of rust layer after the wet and dry cyclic transitions is composed of α-FeOOH, γ-FeOOH and Fe 3− δ O 4 for both cases of non-Cr 3+ and Cr 3+ -containing condition, and no significant difference in the mass fraction of the above rust substances between two conditions is observed by means of Mossbauer spectroscopy. The only difference in the rust layer is that the rust formed under the wet and dry cyclic transitions containing Cr 3+ ion contains a certain amount of Cr near the steel/rust interface. Those results suggest that the role of Cr during the wet and dry cyclic transitions is the inhibition of the rust reduction and the formation of Fe 2+ -state intermediate by the existence of Cr in the rust layer. This can lead to the inhibition of the oxygen reduction during drying.