Inhibition of Anodic Dissolution of Zinc-plated Steel by Electrodeposition of Magnesium from a Molten Salt

Abstract

Inhibition of anodic dissolution of zinc-plated steel by electro-deposition of magnesium from the 55 mol%LiCl-34 mol%KCl-5 mol%MgCl₂-1 mol%CsCl-5 mol%KI molten salt was investigated. On a heat treatment, the zinc layer changed to 3 layers: the outer layer; the inner layer; interface layer just on the substrate steel. Four steps of anodic dissolution were obsereved in the anodic polarization curves in 5 mass% NaCl aqueus solution. The first step was the anodic dissolution of the MgZn₂ and Mg₂Zn₁₁ 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 Γ₁ 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 suggested that the intermetallic compounds, MgZn₂ and Mg₂Zn₁₁, make a protective films on the surface, inhibit the anodic dissolution and give the high corrosion resistance to the magnesium-deposited zinc-plated steel.