Formation of a corrosion-resistant coating on zinc by a duplex plasma electrolytic oxidation and conversion surface treatment

A two-step, environmental-friendly process is proposed to improve the corrosion resistance of zinc. The first step involves plasma electrolytic oxidation (PEO), to produce a uniform 6 μm thick ZnO layer in a short processing time of 30 s. This is followed by a conversion treatment by immersion in 1 M sodium bicarbonate solution for 1 h at open circuit potential. The morphology, thickness and phase composition of the surface layers were analysed using scanning electron microscopy and X-ray diffraction. The electrochemical tests (potentiodynamic polarisation and electrochemical impedance spectroscopy) showed enhanced corrosion resistance of the PEO coating after the conversion treatment. The improved corrosion resistance was attributed to the sealing of defects during the conversion treatment and the partial conversion of the ZnO layer into a protective patina. The PEO and duplex treatments also imparted a mechanically durable coating with higher scratch resistance compared to the conversion treatment.