Preparation of self-healing film on zinc-plated steel by electro-deposition of magnesium from molten salt

We tried to prepare a self-healing film on a zinc-plated steel by electrodeposition of magnesium from a molten salt. The results are as follows : (1) The phase diagram of the LiCl-KCl-MgCl2 ternary system in low MgCl2 composition was determined by the hot thermocouple method and DTA. A ternary eutectic point, E, was found at 55 mol%LiCl-40 mol%KCl-5 mol%MgCl2 and 596 K which was 97 K lower than the melting point of zinc. (2) The 55 mol%LiCl-40 mol%KCl-5 mol%MgCl2 molten salt gave a homogeneous electrodeposition of magnesium on a zinc-plated steel. The magnesium electrodeposit diffused into the zinc layer of the zinc-plated steel during electrolysis. (3) In a salt spray test at 308 K, the time of generation of red rust was 10 times longer for the maenesium-electrodepoited zinc-plated steel than for the conventional zinc-plated steel. (4) In an immersion test, it was found that a thin MgO layer was rapidly formed on the surface and acted as a protective film. It was also found that thin MgO layers were formed along the cracks in the zinc-plated layer, which blocked the cracks and prevented corrosion from proceeding through the cracks. The latter fact means that the magnersium-containing!: zinc-plated layer has a self healing ability.