Application of 2D Pit Growth Method to Mg Thin Films: Part II. Salt Film and Hydrogen Evolution

Abstract

In this work, 2D pit growth in Mg thin films is used to provide further understanding of the anomalous hydrogen evolution phenomenon. The 2D pits were evaluated at high applied potentials where a salt film is expected to precipitate at the active dissolving surfaces, or in this case, at the pit wall. The pit wall was also evaluated for the accumulation of impurities after dissolution during high rates of anomalous hydrogen evolution. The results showed that, in the transport control region where a salt film covers the active dissolving surfaces, the anomalous hydrogen evolution rate decreases with increasing salt film thickness, approaching zero at the limiting current density. It is assumed that water needs to be in direct contact with the active surfaces for anomalous hydrogen evolution to happen. In this case, the rate of hydrogen evolution is limited by water diffusion through the salt film to reach the active surface. It is also shown that impurities do not accumulate at the pit wall and cannot account for anomalous hydrogen evolution.