Under-film Corrosion Mechanism of Cold Rolled Steel Sheet for Automobiles

This report attempts to elucidate the under-film corrosion mechanism of cold rolled steel sheet for automobiles in cyclic wet and dry environments.It was found that an anodic site was the front of growing rust and a cathodic site was the interface between a substrate and corrosion products.The Fe (OH)2was found to be formed at the initial stage of the corrosion and it was transformed into mainly Fe3O4, α, β-FeOOH, Fe2O3by oxidation.The porosity of the corrosion products increased by the phase transformation which accompanied the change in density. The void space fraction was about 40%. This void space was a passage of Cl-, H2O and O2 from the cross scribes to the corrosion tip and to the interface between the iron substrate and corrosion products.