Analysis of Chemical State for Alloying Elements in Iron Rust

Abstract

A binary-phase potential-pH diagram has been investigated to evaluate the chemical stability of various kinds of complex iron rust (Fe-X), that is a corrosion-resistance principle of low alloy steel. It was found that there are the following types of corrosion-resisting elements in the rust: (1) iron substitution type (Ni), (2) oxide formation type (Al), (3) metallic type (Ru), and (4) oxygen-acid salt type (WO₄). XPS and TEM analysis have been conducted on the rust formed on the low alloy steel in saline environment. The physical analysis agrees with potential-pH diagrams. The iron substitution type and the oxide formation type elements make complex spinel oxides with iron. In the corrosion tests, steels added with Ni or Al had high corrosion resistance. Thus it is possible to obtain high corrosion resistance by the creation of complex spinel such as Fe₂NiO₄ and FeAl₂O₄ in an inner layer.
On the other hand it was found that the metallic type and the oxygen-acid salt type elements were not absorbed into the rust. In particular the oxygen-acid salt elements were excluded from the iron rust and concentrated into the defects of the rust. It is suggested that insoluble salts like FeWO₄ are formed on the base metal in the defects to act as an anodic inhibitor. Thus the addition of a small quantity of W gives high corrosion resistance.
In this way the penetration of Cl ions can be prevented by the complex spinel in an inner layer and the oxygen-acid salt in the defects. Thus the high corrosion resistance by the addition of these elements can be understood from the potential-pH diagram and the physical analysis.