Plasma electrolytic oxidation of zircaloy-4 alloy with DC regime and properties of coatings

The plasma electrolytic oxidation (PEO) coatings on zircaloy-4 alloy were prepared in silicate, phosphate and pyrophosphate electrolyte systems or their combination by DC current regime. The proper processing parameters were determined and the coatings were evaluated by electrochemistry technique, micro-hardness, SEM and XRD. The results show that the coating prepared in pure silicate system is uneven and after the addition of phosphate solution, the homogeneity of the coating is still poor. The coating prepared in pure pyrophosphate electrolyte system is homogeneous, but its hardness value is low. After the addition of silicate into the pyrophosphate electrolytic system, both the uniformity and hardness of the coating are improved. The XRD results show that the phase compositions are m-ZrO2 and t-ZrO2, the addition of silicate is beneficial to the formation of t-ZrO2. The results of polarization curves show that the coatings prepared in pyrophosphate and the mixture of pyrophosphate and silicate have better corrosion resistance.