Anodic and cathodic polarisation of magnesium in borate buffer solutions with and without addition of chloride

This report summarises the results of polarisation tests on high-purity (99.95 %) magnesium electrodes in borate buffer solutions at pH 9.2, pure or with additions of NaCl (0.1 M to 3.0 M). The investigation included both potentiostatic tests with mass loss determinations and potentiodynamic tests with H ₂ gas evolution measurements. The corrosion was uniform in the absence of chloride ions, and the hydrogen evolution rate decreased steadily towards zero upon anodic polarisation above the open-circuit potential. Addition of chloride caused localised corrosion at anodic polarisation together with anomalous H ₂ evolution from the localised attacks. The anomalous H ₂ evolution rate started decreasing when a certain applied polarisation was reached. The potential at which this drop initiated decreased as the NaCl concentration increased. This observation may be interpreted as an effect of increasing solution conductivity lowering a high ohmic resistance in the localised attacks.