Preparation and Corrosion Resistance of Rare Earth Conversion Film on Hot-Dip Galvanized Steel

A nontoxic passivation process for galvanized steel,based on cerium salt solution as a replacement of highly toxic dichromate,was investigated.Thus orthogonal tests were carried out to determine the optimized parameters for the preparation of the rare earth chemical conversion film on the steel surface.The effects of bath composition,pH value,temperature and passivation time on the properties of the conversion film were discussed.Moreover,the protection of the galvanized steel by the conversion film was explored using neutral salt-spray test and polarization curve test as well.As the results,the optimum bath composition for the preparation of rare earth conversion film on galvanized steel was suggested as 20 g/L Ce(NO3)3·6H2O,30 mL/L H2O2,and 1.5 g/L H3BO3,and the optimized pH value,temperature,and duration were determined as 4,40 ℃,and 30 min.The rare earth conversion film could prevent the galvanized steel sheets from white rusting and simultaneously restrain cathodic and anodic reactions in corrosion process.At the same time,the rare earth conversion film was capable of decreasing the corrosion current density,increasing the polarization resistance,and significantly increasing the corrosion resistance of the galvanized steel.