Mechanism of hydrogen formation during the corrosion of Mg17Al12

Previous investigations (DFT calculations) showed that hydrogen atoms adsorption and H-2 desorption can occur on MgO and Al2O3 and that H atoms can diffuse on Mg and Al surfaces. However, these three simultaneous actions, i.e. H adsorption, H diffusion and H-2 desorption, have not been experimentally proved. In this paper, we propose a mechanism of formation of H-2 during the corrosion of an intermetallic compound Mg17Al12 in 3.5 wt. % NaCl aqueous solution based on in situ Raman spectroscopy analysis. We found that, through the passivation zone (e.g. E varying from the open circuit potential (OCP) to +100 mV/OCP), the oxide layer is destroyed in favor of the appearance of Mg and H atoms. Moreover, the formed H atoms are adsorbed on the oxide surface and then diffuse on either the oxide surface or the unreacted metal surface where they recombine forming H-2. In situ Raman measurements during anodic polarization experimentally prove, for the first time, the formation of a reaction intermediate which weakens the H-H bond. The obtained results explain the mechanism of hydrogen production under anodic polarization of the intermetallic compound at normal conditions of temperature and pressure.