Mechanistic study of ammonium-induced corrosion of AZ31 magnesium alloy in sulfate solution

The influence of NH4+ ions on the corrosion behavior of AZ31 magnesium alloy was investigated by immersion test, hydrogen evolution, electrochemical methods and morphology observation. The results demonstrate the acceleration effect of NH4+ on corrosion of AZ31 magnesium alloy due to the disruption of protective MgO film in NH4+-containing solution. The loose and cracked corrosion products of AZ31 magnesium alloy in NH4+-containing solutions are mainly composed of (Mg0.833Al0.167)(OH)2(CO3)0.083·0.75H2O and Mg5(CO3)4(OH)2·5H2O. When the NH4+ concentration is lower than 0.01 M, knife-cut like corrosion occurs in some active area of the surface due to the partial dissolution of MgO layer. As the NH4+ concentration is increased to 0.1 M, the MgO layer is completely disrupted, resulting in the occurrence of uniform corrosion. Both cathodic and anodic reactions are accelerated by NH4+ ions, while the effect of original pH values on the reaction kinetics can be neglected in NH4+-containing solutions.