CATHODIC DEPOSITION OF Mg(OH)2 COATINGS ON PURE Mg IN THREE Mg SALTS AQUEOUS SOLUTIONS

Film-forming effects of cathodic deposition on pure Mg substrate at constant DC in aqueous solutions of magnesium nitrate (Mg(NO3)2⋅6H2O), magnesium chloride (MgCl2⋅6H2O) and magnesium sulfate (MgSO4⋅7H2O) respectively were investigated systematically. Typical processes were studied by potentiodynamic cathodic polarization and galvanostatic polarization and typical samples were analyzed by SEM and XRD. The results indicate that the depositing efficiency is not only the highest but stablest, and deposited coatings show the best uniformity with Mg(NO3)2⋅6H2O solution employed as depositing medium and applied current density ≥1.0mA cm−2. Cathodic deposition leads to regular mass loss of Mg substrate. The cathodic polarization curve of pure Mg in magnesium nitrate solution shows more obvious pseudo-passivation, several Tafel regions with different slopes appearing before diffusion-limited current density region, and oxygen consumption is the major cathodic reduction reaction under specified current density. However, hydrogen evolution reaction is dominant in both Mg chloride and Mg sulfate solutions. The deposition coatings are all composed of continuous and uniform mesh-like “basic layer” adjacent to substrate and discrete distributed snowball-like particles on the microscopic scale. The phase compositions are all crystal Mg(OH)2, and the coatings deposited in Mg chloride solution have (011) preferred orientation.