Effect of [Ca2+] and [PO43-] levels on the formation of calcium phosphate conversion coatings on die-cast magnesium alloy AZ91D

The effect of varying [Ca2+]total and [PO43-]total concentrations (nine conditions in total) on the formation of calcium phosphate conversion coatings on magnesium alloy AZ91D was investigated. Corrosion performance was evaluated by measurement of polarisation behaviour and also by salt spray testing. A consistent trend was observed of improved corrosion performance with reduced ionic concentrations. Low [PO43-]total concentration was found to be especially important for the attainment of good corrosion resistance. Both direct measurement using X-ray photoelectron spectroscopy and chemical modelling using MEDUSA software indicated an increase in hydroxyapatite content of the coating accompanying the improved corrosion resistance.

► A simple calcium phosphate conversion-coating route was exploited on magnesium alloy AZ91D. ► Concentration of Ca2+ and PO43- ions in the solution determined the coating composition. ► Low [PO43-] coating bath produced a coating containing high hydroxyapatite content. ► While high [PO43-] solution was beneficial for high proportion of DCPD deposition. ► Results revealed that the hydroxyapatite coating offered better corrosion resistance than DCPD.