Evidence of Two Anodic Processes in the Polarization Curves of Magnesium in Aqueous Media

The anodic branch of Mg polarization curves, in different electrolytes and for different surface conditions, systematically showed a step centered at –1.59 V vs saturated calomel electrode (SCE). This step corresponds to an anodic process starting at ≈ –1.56 V_SCE. On this basis, the following scheme for the anodic dissolution of Mg is proposed: at –2.78 V_SCE (the electrode potential usually accepted) oxidation begins by means of the monovalent process Mg → Mg⁺, and at –1.56 V_SCE the divalent process Mg → Mg²⁺ starts in parallel to the former. In normal cathodic protection conditions, a Mg anode works in the potential range in which the monovalent process prevails. This circumstance is directly related to its low electrochemical efficiency as a galvanic anode.