Effect of current mode on PEO treatment of magnesium in Ca- and P-containing electrolyte and resulting coatings

Plasma electrolytic oxidation (PEO) coatings were produced on commercially pure magnesium in a biologically friendly electrolyte composed of 2 g L−1 Ca(OH)2 and 12 g L−1 Na3PO4·12H2O using pulsed unipolar and bipolar current regimes with negative biasing varying from 0 to 20 mA cm−2. Analysis of voltage transients was performed to characterise the PEO processes. The coating morphology and phase composition were studied by scanning electron microscopy and X-ray diffraction technique, respectively. In vitro corrosion performance of the coatings was evaluated in a simulated body fluid at 37 ± 1 °C, using electrochemical techniques including open circuit potential monitoring, electrochemical impedance spectroscopy and potentiodynamic polarisation scans. The influence of the negative biasing on the PEO process and resulting coating characteristics is discussed. Unlike generally recognised beneficial effects of the negative biasing in PEO treatments of some other metal-electrolyte systems, it was found that detrimental effects are induced to the coatings on cp-Mg produced in the studied electrolyte when the negative current amplitude increases, which may be attributed to hydrogen liberation at the coating/substrate interface during the negative biasing cycles. As a result, a deterioration of vitro corrosion performance was observed for the pulsed bipolar PEO coatings compared to those produced using the pulsed unipolar regime which provides better quality coatings.