Phosphate coatings on magnesium alloy AM60: Part 2: Electrochemical behaviour in borate buffer solution

A zinc phosphate coating was formed on the surface of magnesium alloy AM60 by immersing the specimen in a phosphating bath. Corrosion potential measurements, anodic polarisation curves and electrochemical impedance spectroscopy (EIS) were used to assess the corrosion protection of the coating. The electrochemical measurements were performed in borate solution at pH=9.2. This slightly alkaline solution was chosen because it can be used to differentiate the electrochemical behaviour of phosphated and non-phosphated magnesium alloys. The phosphate coating considerably slowed down the metal dissolution process. Measurements performed on phosphated and non-phosphated specimens in borate solution showed that the phosphate coating afforded considerable protection against corrosion. From the results obtained, it was deduced that the optimum phosphating time was 10 min. The results were fitted to an equivalent electrical circuit.