Influence of ammonium sulfate on the corrosion behavior of AZ31 magnesium alloy in chloride environment

Electrochemical corrosion of AZ31 magnesium alloy in the NH4+-SO42−-Cl− environment is studied. Effect of NH4+ overshadows that of Cl− as the (NH4)2SO4 concentration is 0.005 M or higher, yielding an evolution from localized corrosion to uniform corrosion. Acceleration effect of NH4+ can be attributed to that (i) NH4+ dissolves the inner MgO and hinders the precipitation of Mg(OH)2 and (ii) the buffering ability of NH4+ provides H+, enhances the hydrogen evolution, and expedites the corrosion process. The latter is demonstrated as the dominant factor with the results in unbuffered and buffered environments. The severe corrosion and hydrogen process in NH4+-containing solution results in a high Hads coverage and yields an inductive loop within the low frequency. Meanwhile, SO42− is helpful in generating cracked but partially protective corrosion products, while Cl− could broaden the corrosion area beneath the corrosion product.