Quantifying the Role of Transition Metal Electrodeposition in the Cathodic Activation of Corroding Magnesium

High purity (80 ppm iron) magnesium immersed in aqueous sodium chloride solution exhibits a filiform pattern of localized corrosion in which hydrogen is evolved at local (filament head) and remote (filament tail and uncorroded surface) cathode sites. Transition metal cations in solution are shown to significantly accelerate rates of corrosion, principally by activating (catalyzing) the remote cathode sites. The degree of activation is cation concentration dependent and efficiency increases in the order Mn 2+ < Fe 2+ < Zn 2+ < Cu 2+ . It is proposed that activation occurs as a result of transition metal electrodeposition through a displacement reaction. It is also shown that precipitation of insoluble transition metal (hydr)oxides through time-dependent cation hydrolysis competes with, and reduces the efficiency of, electrodeposition-induced cathodic activation.