Effect of frequency on black coating formation on AZ31 magnesium alloy by plasma electrolytic oxidation in aluminate-tungstate electrolyte

Plasma electrolytic oxidation (PEO) has been carried out under unipolar and bipolar pulsed current regimes to investigate the formation of black ceramic coatings on AZ31 magnesium alloy in an aluminate-tungstate electrolyte at frequencies of 100, 1000 and 2000 Hz. The coatings were characterized by scanning electron microscopy (SEM) assisted by energy dispersive X-ray spectroscopy (EDS), and micro-discharges were investigated by real-time imaging and photomultiplier methods. Coating color was quantified by the CIELAB method. Blackness of coatings increased with frequency and tungsten incorporation, with blackest coatings obtained under unipolar conditions. Blacker coatings also revealed increased roughness and large internal pores, which were attributed to stronger plasma discharges and greater gas evolution. Continuing light emission after termination of each anodic current pulse suggested that sites of high coating temperature may facilitate discharge initiation in the subsequent anodic pulse when the cycle time is short. Large porosity and increased voltage indicated cathodic discharges also occur at high frequency when the coating is sufficiently thick. However, it seems that the cathodic discharges have no effect on the blackness of the coating.