Film characteristics of anodic oxidized AZ91D magnesium alloy by applied power

This study examined the film characteristics of an anodized AZ91D magnesium alloy by varying the direct current and pulse frequency conditions. In order to evaluate the effect of the pulse frequency on film formation, anodic oxidation was carried out by applying direct current and pulse current at a current density of 300 mA/cm(2). Compared with the sample groups treated with the direct current, a large number of small arcs were generated continuously on the film treated with the pulse current and the film formation rate was found to increase. Homogeneous and compact pores were formed with increasing frequency, and the rate of formation was increased rapidly to the arrival voltage. The film thickness increased and the surface roughness decreased with increasing anodic oxidation time at a fixed frequency of 125 Hz. However, treatment for more than 3 min led to decomposition of the oxidized film onto the previously formed film, which caused an increase in the number of cracks and pores within the film. The most uniform and smallest pores were acquired when the surface was anodized for 3 min at a current density and frequency of 300 mA/cm(2) and 125 Hz, respectively. X-ray diffraction revealed the formation of MgO and Mg2SiO4 peaks, with a relative decrease in peak intensity for the MgO and Mg substrate. The Vickers hardness value was higher on the treated groups than on the untreated group, and the potentiodynamic polarization test revealed satisfactory corrosion resistance through a decrease in corrosion density and a large improvement in the corrosion potential. Copyright (C) 2009 John Wiley & Sons, Ltd.