Study on corrosion behavior of nano-structured coatings developed on biodegradable as cast Mg-Zn-Ca alloy by plasma electrolyte oxidation

Despite some potentially desired properties of magnesium alloys for biomedical application, their utilizations are usually extremely restricted due to their low corrosion resistance. Therefore, the objective of this study was to apply a modified coating on a cast Mg-Zn-Ca alloy through plasma electrolytic oxidation process. This process was conducted under constant pulsed DC current conditions in a phosphate-based electrolyte and their corrosion properties were compared with the substrate. Surface characterizations and corrosion behavior by electrochemical impedance spectroscopy and potentiodynamic polarization tests in PBS solution was investigated and the results were compared with the substrate. The Kramers-Kronig relations were used to evaluate the quality of the EIS data. The nano-structured coating showed a pancake shape structure with an average pores size of ~0.7 μm which uniformly covered the ~8% of the coating area. The coating microstructure was consisted of 3-layers with complex impedance spectrum involved three times constant. Corrosion resistance of PEO coated sample was increased by about two orders of magnitude. Corrosion rate measurements demonstrated the occurrence of steeply fast corrosion reaction on Mg alloy. An increase of corrosion rate with immersion time was also observed for coated sample but with lesser extent.