The effects of anodic amplitude and waveform of applied voltage on characterization and corrosion performance of the coatings grown by plasma electrolytic oxidation on AZ91 Mg alloy from an aluminate bath

Abstract

In this study, plasma electrolytic oxidation (PEO) was used to treat AZ91 magnesium alloy in an aluminate electrolyte to increase its corrosion resistance. The PEO process was performed at 350 and 400 V using unipolar and bipolar waveforms. Using unipolar waveform, current density reached zero after a while, but for bipolar waveforms, the current density responded during the PEO process indicating on nonstop coating growth. X-ray diffraction, scanning electron microscopy and energy-dispersive spectroscopy were applied to characterize the coatings. The coatings produced using unipolar waveform revealed a net-like surface morphology, while a crater-like morphology was observed for bipolar coatings. The presence of MgF2 in all coatings was confirmed whereas the bipolar waveform with higher cathodic duty cycle produced an increased coating thickness with higher content of MgF2. The results of corrosion tests in 3.5 wt% sodium chloride solution were in agreement with the changes observed in morphology and structure, where the samples coated at 400 V using the bipolar waveforms were able to exhibit high corrosion performance. In this way, the coating produced by the bipolar waveform with the higher cathodic duty cycle at the higher voltage (400 V) revealed the highest corrosion performance up to 28 days of immersion, where there was no sign of underlying corrosion in this case.