Influence of ultrasonic power on the structure and composition of anodizing coatings formed on Mg alloys

This paper reports the application of ultrasonic power on the anodizing of magnesium alloys. The influence of ultrasonic power at a constant frequency of 25 kHz and two various acoustic power values, 280 and 400 W, on the structure and composition of anodizing coatings formed on magnesium alloy AZ31 was investigated. Results clearly show that ultrasound enhances the growth rate of anodic coatings and plays a very important role in the formation of coating structure and the distribution of coating composition. The anodic coatings consisted of only one layer if the ultrasound field was not applied in the solution used in this work. The anodic coatings consist of two layers when the ultrasonic field was applied and the acoustic power value increases to 400 W at a constant frequency of 25 kHz. The inner layer is compact and enriched in aluminum and fluorine, moreover, its thickness is uniform. In contrast, the contents of aluminum and fluorine in the external layer are very low and its thickness is non-uniform, some areas are thick, some thin. It was noted that there are some obvious fracture sites in the inner layer and external layer although the total thickness of coating is thick. An optimal acoustic power value should exist under the ultrasound frequency of 25 kHz to avoid the layer fracture and obtain thick coatings. The anodic coatings are composed of two phases, MgO and Al2O3, no matter whether the ultrasound is applied or not.