  The electrochemical corrosion rate of Magnesium (Mg) and Mg alloys depends on
the stability of the formed surface layer. Based on the Mg substrate, the oxide
structure comprises a dense MgO/Mg(OH)2 mixture underneath a porous plate-like
Mg(OH)2 layer. While the kinetics of the anodic partial reaction has been
mainly attributed to MgO, recent studies showed an effect of the Mg(OH)2 layer
thickness on the cathodic partial reaction. A thinner Mg(OH)2 layer has been
associated with a higher kinetics of the oxygen reduction rate. In the present
study, the proposed mechanism has been further investigated via in situ
respirometric measurements with Mg-Al-Ca solid solution in electrolytes with
different pH values (pH = 8-13). The results indicate an additional effect
based on the structure of the surface layer in the passive state of Mg
corrosion. Furthermore, two different Al enriched interlayers at the Mg/MgO-
and MgO/Mg(OH)2 interfaces were observed and discussed in terms of their
thermodynamic stability under alkaline immersion conditions.
