Effect of Cr³⁺ and SO₄^2- on the Structure of Rust Layer Formed on Steels by Atmospheric Corrosion

The effect of chromium ion (Cr³⁺) and sulfate ion (SO₄^2-) on the structure of the rust layer formed by thin electrolyte-film corrosion of low-alloy steels has been examined. By using X-ray diffraction spectroscopy, it was found that coexistence of Cr³⁺ and SO₄^2- in the electrolyte film covering the steel surface quickly forms the Cr-goethite (α-(Fe _1-X, Cr_X)OOH) layer which has been known as the final protective rust layer. Scanning vibrating electrode measurements showed that the rust layer formed under the electrolyte film containing Cr³⁺ and SO₄^2- possesses higher protective ability against the aggressive chloride environment. Mössbauer spectroscopy revealed that most of the Cr-goethite formed by corrosion of the Fe-5at%Cr alloy under thin electrolyte-film containing SO₄^2- was the superparamagnetic ultra-fine Cr-goethite. It can be said that the Cr-goethites possess the high protective ability against aggressive corrosives.