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

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 H-2 evolution from Zirlo alloy anode was evaluated and its formation mechanism was discussed. It was found that the gas released from discharging anode contained H-2 and O-2. The H-2 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 H-2 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 O-2 evolution on anode. Relatively less H-2 was released in KF solution due to weak plasma discharge. The H-2 concentration evolved during PEO process was related to the intensity of plasma discharge. It was believed that the H-2 and O-2 mainly resulted from the direct thermal decomposition of water vapour in plasma discharge zone, though the electrolyte was also decomposed to form H-2, O-2 and CO.