INHIBITION OF ANODIC-DISSOLUTION OF ZINC-PLATED STEEL BY ELECTRODEPOSITION OF MAGNESIUM FROM A MOLTEN-SALT

Inhibition of anodic dissolution of zinc-plated steel by electro-depos ition of magnesium from the 55mol%LiCl-34mol%KCl-5mol%MgCl2-1 mol%CsCl -5mol%KI molten salt was investigated. On a heat-treatment, the zinc l ayer changed to 3 layers: the outer layer; the inner layer; interface layer just on the substrate steel. Four steps of anodic dissolution we re obsereved in the anodic polarization curves in 5 mass% NaCl aqueus solution. The first step was the anodic dissolution of the MgZn2 and M g2Zn11 in the outer layer and the inner layer. The second one was the active dissolution of the remaining zinc in the inner layer. The third one was the active dissolution of the Gamma(1) phase in the interface layer. The fourth step was the active dissolution of the matrix steel . It should be noted that the anodic current density of the first step was very small in comparison with the others. The present results sug gested that the intermetallic compounds, MgZn2 and Mg2Zn11, make a pro tective films on the surface, inhibit the anodic dissolution and give the high corrosion resistance to the magnesium-deposited zinc-plated s teel.