Comparative analysis of unipolar and bipolar plasma electrolytic oxidation coatings on Al-Mg laminated macro composites

Abstract

Plasma electrolytic oxidation (PEO) coatings were prepared on Al-Mg laminated macro composites (LMCs) using both unipolar and bipolar waveforms in an appropriate electrolyte for both aluminum and magnesium alloys. The techniques of FESEM/EDS, grazing incident beam X-ray diffraction (GIXRD), and electrochemical methods of potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used to characterize the coatings. The results revealed that the coatings produced using the bipolar waveform exhibited lower porosity and higher thickness than those produced using the unipolar one. The corrosion performance of the specimens' cut edge was investigated using EIS after 1, 8, and 12 h of immersion in a 3.5 wt. NaCl solution. It was observed that the coating produced using the bipolar waveform demonstrated the highest corrosion resistance after 12 h of immersion, with an estimated corrosion resistance of 5.64 k ohm<middle dot>cm2, which was approximately 3 times higher than that of the unipolar coating. Notably, no signs of galvanic corrosion were observed in the LMCs, and only minor corrosion attacks were observed on the magnesium layer in some areas.