Corrosion current distributions in a low-alloy steel covered with a layer of rust formed under electrolyte thin films in a NaCl solution

The effects of ion species in electrolyte films covering a weathering steel surface on the rust formation and protective ability of the rust in a 3.5%NaCl solution have been examined. Scanning vibrating electrode measurements of corrosion current distributions at the steel surfaces, which are covered with various rust layers formed under the electrolyte thin layers mentioned above, in the NaCl solution, and the structural analyses of the rust layers show the following results. (1) Rust layer as mixture of beta-FeOOH, gamma-FeOOH and a small amount of alpha-FeOOH does not prevent metal dissolution. A large number of anode and cathode current peaks are observed. (2) The Cr-substituted goethite layer formed under the electrolyte containing Cr3+ and SO42- strongly suppresses metal dissolution in the NaCl solution, maybe due to the densely aggregated structure of fine particles of the goethite. (3) The rust layer formed under the electrolyte containing Ni2+ and SO42- also possesses high protective ability against corrosive Cl- anion, probably due to an effect inactivating Cl- in the rust layer as well as its aggregated structure of rust particles. These results indicate that the corrosion rate of steel is closely related to the rust phase and structure controlled by ion species in an electrolyte thin film.