Effects of pulsed current on plasma electrolytic oxidation

In this paper, the effects of different current pulse parameters on the resulting PEO layers are described. The properties of the layers were assessed optically, with a scanning electron microscope, and the corrosion resistance has been measured. It is shown that the polarity and the frequency of the applied current has a high influence on the properties of the resulting layers. Unipolar pulses with low frequencies produce rougher layers with reduced corrosion protection. Different additives were added to the electrolyte in order to improve corrosion resistance of the resulting layers. Some of the additives, especially if based on tungstate and molybdate resulted in a colouring effect of the layer. This colouring effect is most pronounced when low frequencies and unipolar pulses are used. Higher frequencies and bipolar pulses create smoother and denser layers with higher corrosion resistance. The highest corrosion resistance was obtained by combining addition of tungstate and high frequency bipolar pulses.