Corrosion of Mg alloy in the presence of ammonium ion. Evidence of hydride sub-products

Abstract Corrosion of AZ31 Mg alloy was systematically investigated by different experimental approaches in the presence and in the absence of NH 4 + . Remarkable reactivity of Mg/NH 4 + system was manifested by copious evolution of hydrogen over a dark corrosion film, associated to active deprotonation of complex Mg hydrides, more likely bridged {Mg 2+ (AlH 4 − ) 2 Mg 2+ (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 NH 4 + and SO 4 2− . Chemical recombination of hydrogen atoms of oxidic Mg hydrides is proposed as the kinetically determinant process.