Corrosion and in vitro characteristics of cerium phosphate-based chemical conversion coating on AZ31 magnesium alloy

A two-stage chemical conversion coating procedure was proposed for cerium phosphate-based coating on Mg alloys. Initially, a layer of magnesium phosphate was obtained and the cerium phosphate-based coatings were achieved by immersing the precoated sample in a cerium nitrate solution prepared in either water or ethanol. The resultant coating obtained from a water-based nitrate bath consisted of oxides and hydroxides of cerium in addition to cerium phosphate. On the other hand, due to the lack of OH- ions in ethanol-based nitrate bath, oxides, and hydroxides were suppressed and cerium phosphate-rich coating was obtained. Scratch test and scanning vibrating electrode analysis confirmed the self-healing characteristics of the water-based coating due to the presence of cerium oxide and hydroxide in the coating. The detailed corrosion studies revealed that the ethanol-based coating exhibited superior corrosion resistance in both 1 wt% NaCl as well as in simulated body fluid due to the high stability of cerium phosphate in both test conditions. Both the coatings displayed acceptable cell viability (MG63 cell line) under physiological conditions and hence have the potential for both engineering and biomedical applications.