Corrosion of Mg alloy in the presence of ammonium ion : evidence of hydride sub-products

Corrosion of AZ31 Mg alloy was systematically investigated by different experimental approaches in the presence and in the absence of NH4+. Remarkable reactivity of Mg/NH4+ system was manifested by copious evolution of hydrogen over a dark corrosion film, associated to active deprotonation of complex Mg hydrides, more likely bridged Mg2+(AlH4-)2Mg2+(H-)2 complexes covered by a skin of MgO. Metastability of hydride phases is favored in the presence of species that alter the equilibrium dissociation of water, such as NH4+ and SO42-. Chemical recombination of hydrogen atoms of oxidic Mg hydrides is proposed as the kinetically determinant process.