IN-SITU EVALUATION OF PROTECTIVE ABILITY OF ARTIFICIALLY-CONTROLLED RUST LAYERS ON LOW-ALLOY STEEL IN SALTWATER ENVIRONMENT

摘要

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 β -FeOOH, γ - FeOOH and small amount of α -FeOOH hardly prevents metal dissolution. The Cr-substituted goethite layer formed under the electrolyte containing Cr~(3+) and SO_4~(2-) strongly suppresses metal dissolution in the NaCl solution. The rust layer formed under the electrolyte containing Ni~(2+) and SO_4~(2-) also possesses high protective ability against corrosive Cl~- anion. In addition, newly developed surface treatment technique employing Cr~(3+) and SO_4~(2-) 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.