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Plasma electrolytic oxidation (PEO) was carried out to form black ceramiccoatings on AZ31 magnesium alloy in aluminate-tungstate electrolyte. Influencesof current frequency (such as 100, 1000 and 2000 Hz) on properties of the PEOcoatings have been explored. It was found that increment of frequency led tocoatings with higher thickness, higher roughness and darker appearance. Inorder to explore the coating formation mechanism at different frequencies, themicro-discharges at different frequencies were investigated by real-timeimaging method. Microstructure of coatings was characterized by scanningelectron microscopy (SEM) assisted with energy dispersive X-ray spectroscopy(EDS). Coatings formed at 100 Hz show the most compact microstructure whileporosity of the coatings, both in the pore number and pore size, is greatlyincreased at higher frequencies. The coatings formed at higher frequencies showan increased atomic ratio of W/Al. The cancel of the negative pulse from thepulsed waveform can also lead to a higher W/Al ratio, and hence a darkercoating. The higher porosity of the coatings was attributed to the strongerplasma discharges and gas evolution at higher frequencies. The stronger plasmadischarge is induced by the fact that there is not sufficient time for thecooling down of discharge channels at higher frequencies.

A disclosure of the role of frequency on the plasma electrolytic oxidation of AZ31 magnesium alloy in aluminate-tungstate electrolyte