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

Highlights: • Remarkable reactivity of Mg/NH₄⁺ system manifested by copious evolution of hydrogen over a dark corrosion film. • Complex hydride corrosion subproducts favored by the alteration of the dissociation equilibrium of water. • Chemical recombination of hydrogen atoms composing the complex hydrides as kinetically determinant process. - Abstract: 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.