Use of Sodium Bicarbonate as a Chloride-Free Aqueous Electrolyte to Explore Film Formation and the Negative Difference Effect on Pure Magnesium

Hydrogen evolution rates upon magnesium were explored in 0.1 M sodium bicarbonate (NaHCO3) solution, with pH 8.4. Although the negative difference effect (NDE) was observed on Mg in 0.1 M NaHCO3, the hydrogen evolution rates showed a peculiar behavior in this electrolyte, where the hydrogen evolution rates first increased in the proximity of the open circuit potential and then decreased, with increasing anodic potential. This transition of the slope of hydrogen evolution rates versus applied anodic potential varying from positive to negative was revealed for the first time on Mg. In NaHCO3, hydrogen evolution rates decreased with time, even in the NDE region where hydrogen evolution rates increased with anodic potential. It is posited that dissolution of the air-formed surface film and evolution of a hydroxide-free magnesium carbonate film on Mg were responsible for the observed hydrogen phenomena. These findings have significant implications in clarifying the source of superfluous hydrogen upon anodically polarized Mg. (C) The Author(s) 2018. Published by ECS.