Effect of current frequency on coating properties formed on aluminised steel by plasma electrolytic oxidation

In the present study, plasma electrolytic oxidation process was carried out on aluminised steel with a pulsed bipolar electrical source at different frequencies, namely, 50, 300, 600 and 1500 Hz with constant 50% duty cycle for 10 min in an alkaline silicate electrolyte. Morphology, wear behaviour and corrosion resistance of the coatings were investigated by scanning electron microscopy, pin on disc and potentiodynamic polarisation experiment in 3·5 wt-%NaCl solution. The results showed that with increasing the pulse frequency, the density of the residual pores increases, but their size as well as the coating thickness decreases. The corrosion current density of the coating processed at 1500 Hz was 1·14×10⁻⁸ A cm⁻², which exhibited superior corrosion resistance compared with the bare substrate. Lower friction coefficient of the specimen processed at 600 Hz in comparison with that of the specimens prepared at lower frequencies was due to the formation of a less rough coating.