Corrosion behavior of AZ91 alloy with plasma electrolytic oxidation coating in simulated body fluid solution

Recent research in orthopedic implant materials has focused on the use of magnesium alloys as a base material due to its mechanical properties similar to that of human bone. Rapid corrosion of magnesium materials in aqueous environments poses a significant hurdle to their application as a biomedical implant. Plasma electrolytic oxidation (PEO) is one potential coating technology for the improvement of the corrosion resistance of magnesium and its alloys. The processing parameters of PEO, such as the applied voltages, current density, duty cycle, oxidation time, and the electrolyte concentrations, are some of the major factors which determine the quality of the coatings in terms of corrosion behaviors in simulated body fluid (SBF). In this study, the samples were treated for 20 min at 50, 100, 150 mA.cm-2 current densities. The phase composition, morphology, chemical composition, thickness, roughness and corrosion resistance were studied by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), energy-dispersive spectroscopy (EDS), thickness measurer, surface roughness tester and potentiodynamic polarization in SBF solution. It was found the thickness of the PEO coating increases with increasing current density. It is generally expected that a thicker coating gives rise to better corrosion resistance. But the variation of corrosion resistance with thickness is not linear. This indicates that the defect density is the dominating influence rather than the coating thickness. Increasing the coating thickness did not reduce the amount of open defects that provided access for the SBF to the AZ91 alloy substrate.