An understanding of the anomalously enhanced hydrogen evolution reaction (HER) of magnesium (Mg) under
anodic polarisation in aqueous corrosion is paramount for a predictive theory of its corrosion and metal
electrocatalysis. Previous theoretical and experimental studies have proposed that sub-surface hydride phases
play a role in this behaviour but the underlying atomic mechanisms remain unclear. By constructing theoretical
surface Pourbaix diagrams, based on density functional theory (DFT) calculations, we have identified the
atomic structure of a sub-surface hydride phase on the Mg (0001) surface that remains electrochemically
stable under significant anodic overpotentials across a wide pH range. Specifically, this stability persists up
to 0.38 VSHE under mildly alkaline conditions (e.g., pH = 8), thus providing thermodynamic support for
the proposed hydride-enhanced HER under anodic conditions. Reaction barrier analysis establishes that the
proposed sub-surface hydride phase could promote anodic HER via a Heyrovsky pathway, based on hydrogen
outward diffusion, with an energy barrier of 1.54 eV as the rate-limiting step, showing an anodic characteristic
and significantly favouring external anodic polarisation. Furthermore, we have established that the surface
adsorption condition, contingent on both the pH and potential, significantly influences the mechanism and
kinetics of the initial corrosion of Mg.