Thick oxide coatings formed by spark anodizing of Mg-Al alloy in alkaline phosphate-silicate electrolytes

Anodic coatings formed by spark anodizing (SA) with different electrolyte recipes often have distinct composition and properties. In this paper we report on the study of correlations between electrolytes and the coating properties of a thick SA layer (190 μm) formed on an AZ91D magnesium alloy. The voltage transient during this SA process in phosphate-silicate electrolyte solution is characterized by an abrupt anodic voltage drop after initial voltage increase, followed by a stabilized voltage plateau during which the microdischarge noise is completely suppressed. Microstructure investigations show that the cross-section of such SA thick coating contains three distinctive regions. In terms of composition, the outer layer contains uniform distribution of P and Si, and the middle and inner layers are dominated by Mg and O. Correspondingly, microstructures of these regions are also different, the inner layer with a cellular band and barrier structure is much more compact comparing to the middle layer. The individual thickness ranges for the cellular band and barrier regions are 500nm-2μm and 100-600 nm and their individual micro-hardness reaches about 3 GPa and 8 GPa respectively.