Mg Dissolution in Phosphate and Chloride Electrolytes: Insight into the Mechanism of the Negative Difference Effect

Magnesium (Mg) dissolution was investigated using an online measurement of dissolved Mg and simultaneous time-resolved volumetric analysis of hydrogen gas evolution. Testing was performed at open-circuit potential and under anodic polarization in neutral, unbuffered chloride and buffered phosphate solution. In the unbuffered chloride electrolyte, electrochemical dissolution occurs with a stoichiometry of n = 2, and a significant enhancement of the corrosion rate at open-circuit dissolution rate was observed following anodic polarization. The extent of the enhancement increased with anodic polarization time requiring at least 500 s to become detectable. In a phosphate buffer solution, neither the corrosion rate enhancement nor the negative difference effect (NDE) were observed. The kinetics of hydrogen evolution in nonbuffered solutions tends to support a mechanism involving the activation of the cathodic reaction related to the disruption of a protective surface film.