Effects of steam concentration and flow rate on the high temperature oxidation of PEO-coated zirconium alloy at 1000 °C and 1200 °C

A compact oxide coating of 7 μm thick on ZIRLO zirconium alloy was fabricated by plasma electrolytic oxidation (PEO) in phosphate electrolyte. The effects of steam concentration and flow rate on the steam oxidation behavior of PEO-coated ZIRLO alloy at 1000 °C and 1200 °C were evaluated by thermogravimetric analysis (TGA). The morphologies, compositions, phase constituents before and after the steam oxidation were characterized. It is found that the critical steam concentration without steam starvation for PEO-coated ZIRLO alloy at 50 ml/min flow rate is 35 % at 1000 °C. Meanwhile, the critical flow rate at 50 % steam concentration is 35 ml/min at 1000 °C and 150 ml/min at 1200 °C. A dimensionless number (P = 39) is suggested corresponding to the critical steam concentration and flow rate. It means that the minimum mass of steam injection is about 39 per unit mass gain of sample, in order to avoid the steam starvation for Zr alloys after 3600 s steam exposure. The steam oxide layers consist of monoclinic zirconia (M-ZrO2) and a little tetragonal zirconia (T-ZrO2) phase at different temperatures, steam concentrations and flow rates. The hydrogen has not diffused into the steam oxide layer and ZIRLO substrate due to the hindrance of ZrO2 coating to the hydrogen diffusion.