Preparation Process Optimization for Lanthanum Conversion Coating on Galvanized Steel

The influencing factors of film formation on the corrosion resistance of lanthanum conversion film on hot dip galvanized steel were studied by the orthogonal experiment. The influences of bath composition, treatment temperature and time on the properties of conversion film were discussed. The optimum parameters gained from orthogonal tests were: La(NO3)3·6H2O 20 g/L, H2O2 8～12 ml/L, T 65～80 ℃, t 10～30 min, pH 4.0. The corrosion resistance of the lanthanum conversion film was evaluated by salt spray test. The results showed that the lanthanum conversion film was silvery gray, uniform and its corrosion rate decreased significantly compared with that of the hot dip galvanized steel.