In-situ evaluation of protective ability of artificially-controlled rust layers on low-alloy steel in saltwater environment

The effect of ion species in electrolyte film covering the steel surface on the rust formation and protective ability of the rust in 3.5%NaCl solution was examined. Rust layer as mixture of beta -FeOOY, gamma - FeOOH and small amount of alpha -FeOOH hardly prevents metal dissolution. The Cr-substituted goethite layer formed under the electrolyte containing Cr3+ and SO42- strongly suppresses metal dissolution in the NaCl solution. The rust layer formed under the electrolyte containing Ni2+ and SO42- also possesses high protective ability against corrosive Cl- anion. In addition, newly developed surface treatment technique employing Cr3+ and SO42-. provides useful effect for obtaining the protective rust layer during a relatively short exposure period even in the severe environment such as coastal region. These results are attributed to difference in rust phase and structure.