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. (C) 2011 Elsevier Ltd. All rights reserved.