Growth and corrosion behavior of molybdate passivation film on hot dip galvanized steel

Hot dip galvanized steel sheets were passivated by molybdate aqueous solution containing 10 g/L Na 2MoO 4·2H 2O, and the growth behavior and corrosion resistance of the passivation film were investigated. In the initial stage of passivation, the mass gain of film increases with passivation time proportionally. The film grows up more quickly and is apt to cracking at grain boundaries of zinc, then the cracks spread gradually on the whole surface of the film, and eventually the film will flake off with the increasing of film thickness. XPS results indicate that Mo compounds are present in Mo(Ⅵ) state on the surface of the film, and Mo(Ⅵ) and Mo(Ⅳ) states inside the film. NSS test shows that, the corrosion resistance of the passivation film decreases as the cracks occur, but in AASS test, the thicker the film is, the better the corrosion resistance is, the cracks of film have little effect on the corrosion resistance.