The study of a Phosphate conversion coating on Magnesium alloy AZ91D: IV. comparison of electrochemical behaviors in Borate buffer and Sodium Chloride solutions

Abstract

This paper involved a phosphate conversion coating on magnesium alloy AZ91D. The phosphated magnesium alloy (PMA) was obtained via the preparation of phosphate conversion coating (PCC) on the surface of uncoated magnesium alloy (UMA). The electrochemical behaviors of PMA and UMA in the borate buffer (H3BO4) solution and in the sodium chloride (NaCl) solution were characterized with the electrochemical techniques of potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), and the results were also compared. Both in the H3BO4 solution and in the NaCl solution, the corrosion current density (icorr) and the total impedance (Zall-in) of PMA were respectively smaller and larger than those of UMA, indicating the protective performance of PCC. However, the deviations of electrochemical parameters in the H3BO4 solution were significantly lower than those in the NaCl solution, which was due to the relatively stable pH value of H3BO4 solution, confirming the rationality of borate buffer solutions for the electrochemical measurements concerning magnesium alloys.