Toward a Physical Description of the Role of Germanium in Moderating Cathodic Activation of Magnesium

Abstract

The role played by surface film formation in moderating cathodic activation (i.e., H2 evolution associated with anodic dissolution in NaCl [aq]) was determined for an Mg-0.3Ge (wt%) alloy and contrasted with this process in pure Mg. Cathodic activation was not detected using the scanning vibrating electrode technique (SVET) during anodic dissolution of the Mg-0.3Ge alloy under either freely corroding or anodic polarization conditions. Filament tracks that initiated under the more aggressive testing condition remained electrochemically inert. However, volumetric H2 evolution measurements revealed that Ge alloying additions “switch off” the remote cathodes observed on previously corroded pure Mg surfaces, while Ge additions did not eliminate the “local” cathode at the principal sites of anodic activity (which cannot be detected by SVET). As such, the quantity of H2 measured on the corroding Mg-0.3Ge alloy arises exclusively from cathodic H2 evolution at the anodic sites. Moderation of sustained cathodic activation by alloying with Ge was associated with the incorporation of Ge into the inner MgO/Mg(OH)2 layer during anodic dissolution of Mg. It is possible that entrapped Ge particles or GeO2 serve as an effective poison for H recombination in the overall H2 evolution reaction that would otherwise readily occur on freshly formed Mg(OH)2 at anodic dissolution sites.