Role of Ca in Modifying Corrosion Resistance and Bioactivity of Plasma Anodized AM60 Magnesium Alloys

Abstract

The effect of alloying element Ca (0, 1, and 2 wt%) on corrosion resistance and bioactivity of the as-receivedand anodized surface of rolled plate AM60 alloys was investigated. A plasma electrolytic oxidation (PEO)was carried out to form anodic oxide film in 0.5 mol dm-3 Na3PO4 solution. The corrosion behavior wasstudied by polarization measurements while the in vitro bioactivity was tested by soaking the specimensin Simulated Body Fluid (1.5xSBF). Optical micrograph and elemental analysis of the substrate surfacesindicated that the number of intermetallic particles increased with Ca content in the alloys owing to theformation of a new phase Al2Ca. The corrosion resistance of AM60 specimens improved only slightly byalloying with 2 wt% Ca which was attributed to the reticular distribution of Al2Ca phase existed in thealloy that might became barrier for corrosion propagation across grain boundaries. Corrosion resistance ofthe three alloys was significantly improved by coating the substrates with anodic oxide film formed byPEO. The film mainly composed of magnesium phosphate with thickness in the range 30 - 40 μm. Theheat resistant phase of Al2Ca was believed to retard the plasma discharge during anodization and, hence,decreased the film thickness of Ca-containing alloys. The highest apatite forming ability in 1.5xSBF wasobserved for AM60-1Ca specimens (both substrate and anodized) that exhibited more degradation than theother two alloys as indicated by surface observation. The increase of surface roughness and the degreeof supersaturation of 1.5xSBF due to dissolution of Mg ions from the substrate surface or the release offilm compounds from the anodized surface are important factors to enhance deposition of Ca-P compoundon the specimen surfaces.