Evaluation of Corrosion Performance of AZ31 Mg Alloy in Physiological and Highly Corrosive Solutions

Resorbable Mg and Mg alloys have gained significant interest as promising biomedical materials. However, corrosion of these alloys can lead to premature reduction in their mechanical properties, and therefore their corrosion rate needs to be controlled. The aim of this study is to select an appropriate environment where the effects of coatings on the corrosion rate of the underlying Mg alloy can be discerned and measured in a relatively short time period. The corrosion resistance of uncoated AZ31 alloy in different solutions [Hank's Balanced Salt Solution, 1× phosphate buffered solution (PBS), 4× PBS, 0.9%, 3.5%, and 5 M sodium chloride (NaCl)] was determined by measuring the weight loss over a 2 week period. Upon exposure to physiological solutions, the uncoated AZ31 alloys exhibited a variable weight increase of 0.4 ± 0.4%. 3.5% and 5 M NaCl solutions led to 0.27 and 9.7 mm/year corrosion rates, respectively, where the compositions of corrosion products from AZ31 in all saline solutions were similar. However, the corrosion of the AZ31 alloy when coated by electrochemical oxidation with two phosphate coatings, one containing fluorine (PF) and another containing both fluorine and silica (PFS), showed 0.3 and 0.25 mm/year corrosion rates, respectively. This is more than 30 times lower than that of the uncoated alloy (7.8 mm/year), making them promising candidates for corrosion protection in severe corrosive environments. Cross-sections of the samples showed that the coatings protected the alloy from corrosion by preventing access of saline to the alloy surface, and this was further reinforced by corrosion products from both the alloy and the coatings forming an additional barrier. The information in this paper provides a methodology for evaluating the effects of coatings on the rate of corrosion of magnesium alloys.