Steel materials are basic structural components of social structures. An inherent property of the steel is its high reactivity with oxygen and water existing abundantly in a terrestrial environment. Thus, the steel is easily corroded with the cathodic reaction due to reduction of oxygen from the environing air causing degradation of the steel structures. To ensure long-term reliable service of the steel structures, some measures to inhibit corrosion degradation must be applied.

Rust layer comprised of iron oxides such as α-FeOOH (goethite), β-FeOOH (akaganeite), γ-FeOOH (lepidocrocite), and Fe₃O₄ (magnetite) forms on the steel surface as a result of corrosion. The rust layer is generally recognized as a surface oxide layer that accelerates the steel corrosion since the reduction of the rust itself is superimposed on the reduction of oxygen, thus accelerating the cathodic reaction. But in some cases, the rust layer is believed to work as a kind of protective coating.

The protective function of the rust layer might vary depending on the crystal structure of the rust. We have examined the influence of water-soluble metal salts, which supply ionic species to the aqueous corrosion environment, on the rust structure and corrosion protection or acceleration by the rust layer formed on the steel surface.

It is shown that some of the metal salts lead to preferential formation of a goethite structure and some form magnetite. Depending on those variety of rust phase, electrochemical reaction is affected by the rust structure. The goethite formation especially suppresses the cathodic reaction on the rusted steel surface and resultantly retards the corrosion reaction of the rusted steel in the atmospheric corrosion environment.