Plasma electrolytic oxidation of zinc alloy in a phosphate-aluminate electrolyte

In the frame of this work, plasma electrolytic oxidation of a zinc alloy was performed in a phosphate-aluminate electrolyte as a first approach. Phase composition and microstructure of the formed PEO coating was analysed as a function of treatment time and applied current density. The coatings are mainly composed out of ZnAl2O4 and ZnO with some smaller amounts of AlPO4 and Zn2P2O7. The coating formation starts with ZnO at lower discharge energies and changes to the reactive formation of ZnAl2O4 when the discharge energy is increasing. The phase composition is only influenced by the final voltage and not by the current density. Latter is only determining the growth rate of the coating. However, stresses in the coatings are high and the layers are cracked and seems to flake-off already during PEO processing, thus at longer treatment times the coating thickness is not increasing anymore. To prevent extensive flaking-off of the coating a constant voltage PEO “repair” mode was introduced partly successfully at the end of the treatment.