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.
Plasma electrolytic oxidation of a cast A356 aluminum alloy was carried out in aluminate electrolytes to develop wear and corrosion resistant coatings. Different concentrations of 2, 16 and 24 g/L NaAlO2 solutions and a silicate electrolyte （for comparison） were employed for the investigation. Wear performance and corrosion resistance of the coatings were evaluated by WC （tungsten carbide） ball-on-flat dry sliding tests and electrochemical methods, respectively. The results show that the coating formed for a short duration of 480 s in 24 g/L NaAlO2 solution generated the best protection. The coating sustained 30 N load for sliding time of 1800 s, showing very low wear rate of -4.5×10^-7 mm3/（N· m）. A low corrosion current density of -8.81×10^-9 A/cm2 was also recorded. Despite low α-Al2O3 content of the coating, the compact and nearly single layer nature of the coating guaranteed the excellent performances.
