Analyses of hydrogen release on zirlo alloy anode during plasma electrolytic oxidation

Abstract Plasma electrolytic oxidation (PEO) is a complex process, accompanied by spark discharge and intensive gas evolution around metal anode. In this paper, the gas released from Zirlo alloy electrode during PEO process in KF solution and phosphate solution was collected and its composition was analyzed by gas chromatograph (GC). The possibility of H2 evolution from Zirlo alloy anode was evaluated and its formation mechanism was discussed. It was found that the gas released from discharging anode contained H2 and O2. The H2 up to 81.6 vol% was detected under strong plasma discharge in phosphate solution. Furthermore, a little CO was also detected while the glycerol was added into electrolyte. Hence, a large amount of H2 could be evolved on Zirlo alloy anode during PEO process, which was different from the traditional electrochemical theory and common knowledge of PEO technology for O2 evolution on anode. Relatively less H2 was released in KF solution due to weak plasma discharge. The H2 concentration evolved during PEO process was related to the intensity of plasma discharge. It was believed that the H2 and O2 mainly resulted from the direct thermal decomposition of water vapour in plasma discharge zone, though the electrolyte was also decomposed to form H2, O2 and CO.