New technologies and applications for Mg require further improvements of its corrosion resistance. Fundamental comprehension of the Mg corrosion mechanisms is necessary to address the issue, as the Mg corrosion behavior is unexpected from the theory of electrochemical kinetics. The anomalous hydrogen evolution phenomenon (anomalous HE) exhibit by Mg, also known as the negative difference effect (NDE), is still an unresolved issue. Recent research suggests that the anomalous HE is a consequence of a surface change during dissolution. The proposed mechanisms debate the location of the phenomenon on the surface. The phenomenon is proposed to happen at the corroded areas behind the corrosion front, in the dark corrosion products left behind the corrosion front and/or at the active dissolving sites. New techniques and experimental data are needed to further address the discussion.

In this work, the 2D pit growth method is used to study the anomalous HE phenomenon on Mg with the aim to provide further experimental evidence towards an explanation. The method is called 2D pitting because the sample is a thin metallic film deposited on an inert substrate, so the pit depth is constant and limited to the film thickness. By analyzing 2D pit growth, it is possible to accurately track the active dissolving surface and to measure the anodic, net and cathodic current densities. The hydrogen evolution current density was shown to increase with increasing applied potentials in the ohmic/activation control region, which is direct evidence for the anomalous HE behavior. In addition, the results clearly demonstrated that the anomalous HE phenomenon happens at the active dissolving site, which in this case is the 2D pit wall. Anomalous HE did not occur at the corroded areas behind the corrosion front, which in the case of a 2D pit is the inert substrate. It also did not occur in the corrosion product, which in this case was broken and lifted out by the hydrogen bubbles being electrically disconnected from the Mg matrix. The 2D pits were also evaluated at high applied potentials where a salt film precipitated on the active dissolving surface. The results showed that the anomalous HE rate decreased in the transport control region approaching zero at the limiting current density. In this case, the rate of hydrogen evolution was limited by water diffusion through the salt film to reach the active surface. It was also shown by TEM/EDX analysis that impurities do not accumulate at the pit wall and cannot account for anomalous HE. The results support the idea that the actively dissolving Mg surface is the site of anomalous HE, and that, in the absence of a salt film, the dissolving surface is extremely catalytic to the HE process.