New insights on the influence of aluminum on the anomalous hydrogen evolution of anodized magnesium using scanning electrochemical microscopy

•Anomalous hydrogen evolution from anodically-polarized Mg was imaged by SECM.•The HER occurred separately at the anodic and cathodic sites on a model Mg-Al pair.•HER occurred on Mg regardless the electrical condition of the two metals.•HER solely occurred over Al when galvanically-coupled to Mg.•The dark oxy-hydroxide layer formed on Mg shows catalytic efficiency toward HER. Scanning electrochemical microscopy (SECM) in the sample generation-tip collection mode was employed to gain new insights on the effect of aluminum on the anomalous hydrogen evolution from anodically-polarized magnesium exposed to an aqueous saline solution. Various experiments were designed to investigate the processes occurring on a model Mg-Al galvanic pair in regards to the hydrogen evolution reaction occurring separately at the anodic and cathodic sites. The data obtained in this work do not support the so-called noble impurity theory for HER on anodically-polarized magnesium, whereas evidences were obtained that the dark oxy-hydroxide layer formed on this metal may exert a combined catalytic efficiency toward HER whereas imperfectly passivating the metal surface. [Display omitted]