Preparation and Corrosion Performance of PEO Coating With Low Porosity on Magnesium Alloy AZ91D In Acidic KF System

A plasma electrolytic oxidation (PEO) ceramic coating F-B-A with excellent compactness was prepared on magnesium alloy AZ91D in acidic KF solution with dihydric phosphate by applying a bi-polar pulse voltage. For comparison, two other coatings F-M and F-M-A were prepared in the same solution with and without addition of dihydric phosphate by applying a mono-polar pulse voltage. The composition and microstructure of the coatings were characterized by XRD, XPS and SEM. While their corrosion performance in 3.5 % NaCl solution was examined by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Results showed that the F-B-A coating was uniform and compact without fissures on surface, and its porosity was around 4.8%; while the other coatings were loose and porous with porosity around 45.6 % and 27.9%, respectively. The F-B-A coating showed a bi-layered microstructure, of which the outer consisted mainly of KMgF3 and MgF2 with seldom MgO, whereas, the inner mainly of MgF2, Mg3 (PO4)2, MgHPO4 and little MgO. The magnesium alloy with F-B-A coating exhibited corrosion resistance about three orders higher than the sample with the F-M coating. EIS tests indicated that this bi-layered coating slowed down the penetration rate of chlorine ions and postponed the initiation of pitting corrosion on substrate