INFLUENCE OF VOLTAGE ON ANODE GAS COMPOSITIONS DURING PLASMA ELECTROLYTIC OXIDATION OF 60 VOL.% SiCp/2009 ALUMINUM MATRIX COMPOSITE

Plasma electrolytic oxidation (PEO) was performed on 60 vol.% SiCp/2009 aluminum matrix composite under different voltages in silicate solution and NaF solution, respectively. Anode gas evolutions under different conditions during the PEO process were evaluated by the gas chromatograph, optical emission spectroscopy, scanning electron microscope, X-ray diffraction and inductively coupled plasma atomic emission spectroscopy. The anode gas released at different voltages in both solutions contained H2, O2 and trace CO. The H2 concentration increased with the increase of voltage in the silicate solution, and then it was kept at 75–80 vol.% in the range from +480 V/–0 V to +520 V/–0 V, which was related to the appearance of strong discharge sparks. The H2 concentration of the anode gas in NaF solution was much lower than that in the silicate solution due to the weak discharge. The excessive H2 was caused by nonfaradaic reactions in the plasma discharge region such as thermal decomposition of water. Meanwhile, the O2 in anode gas mainly came from thermal decomposition of water in plasma discharge region and traditional electrochemical reactions in nondischarge region. In addition, SiC reinforcement particles were oxidized in plasma discharge channels with the temperature up to 5000 K, accompanied by the release of trace CO. Furthermore, CO2 was hardly detected due to the effect of H2 reduction in the discharge channels.