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 NH(4)(+ )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)center dot 0.75H(2)O and Mg-5(CO3)(4) (OH)(2)center dot 5H(2)O. When the NH 4 ' 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. (C) 2020 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.