Anodization of Mg-alloy AZ91 in NaOH solutions

Anodic oxide films were grown on the Mg–Al–Zn alloy AZ91 in NaOH-solution and examined by electron microscopy, X-ray diffraction and micro-Raman spectroscopy. The anodic film consists of an inner barrier layer and an outer porous layer. The thin barrier layer is hydrated or contains Mg(OH)2. The porous layer was identified as crystalline MgO (periclase) and contains tunnels formed by breakdown/repair events, which involve plasma-electrolytic reactions. The micro-Raman-spectrum indicates the formation of a highly disordered zone around the tunnels. Applied current density, potential limit, electrolyte concentration and alloy phase were identified as parameters which have an influence on density and diameter of the tunnels.