Alumina- zirconia coatings produced by Plasma Electrolytic Oxidation on Al alloy for corrosion resistance improvement

Compact alumina-zirconia nano composites with corrosion protection potential were coated on 7075 Al alloy through the Plasma Electrolytic Oxidation (PEO) method in DC galvanostatic mode. The layers were coated at constant current density of 20 A/dm2 and 100-350 s growth time in an alkaline K2ZrF6 containing electrolyte. The characteristics of the coatings were investigated as a function of PEO processing time. Electrochemical properties of the layers were studied by conducting potentiodynamic polarization experiments in 3.5% NaCl solution. The results showed that under the present PEO experimental conditions, alumina-zirconia nanostructured coatings can be produced with 10-30 μm thickness and 0.4-2.35 μm roughness depending on the processing time. Phase analysis showed that the nanostructured coatings contained alumina and zirconia high temperature phases (tetragonal zirconia and α-alumina). Processing for 300 s was found to produce the most compact layer with low surface porosity (0.69%) and 26 μm thickness. This particular PEO treatment was found to reduce the corrosion rate by 2.5 orders of magnitude compared to the uncoated substrate. This significant improvement in corrosion resistance is attributed to the barrier effect of the dense layer and the presence of tetragonal zirconia.