DC plasma electrolytic oxidation of biodegradable cp-Mg: In-vitro corrosion studies

DC plasma electrolytic oxidation (PEO) was utilised to produce ceramic coatings on commercially pure magnesium. The coatings were produced in a base electrolyte composed of KOH and Na3PO4, which was modified either by replacing KOH with Ca (OH)2 or by adding Ca (NO3)2 to achieve the ratio of Ca/P in the solution similar to that of hydroxyapatite. Scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction were utilised to characterise the coating morphologies, chemical and phase compositions, respectively. The corrosion behaviour of the coated samples was evaluated using electrochemical methods including electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation techniques in a simulated body fluid (SBF) at 37 ± 0.5 °C. The coatings produced in the base and calcium modified electrolyte contained Mg, O and P, while the latter was not detected in the coatings produced in the nitrate modified electrolyte. The coatings were mainly composed of MgO, while Mg3(PO4)2 was only identified in the base electrolyte coatings. The corrosion process of the coated samples in the SBF was controlled by diffusion. The PEO coatings produced in the calcium modified electrolyte at 40 and 50 mA/cm2 and those fabricated in nitrate electrolyte at 70 V and 70 V/5 min + 80 V/5 min showed a superior corrosion performance in the SBF compared with those produced in the base electrolyte and the uncoated substrate. These are considered to be suitable prototypes for further development of biocompatible corrosion resistant coatings for biodegradable Mg implants.