High temperature oxidation of Zr-1Nb alloy with plasma electrolytic oxidation coating in 900-1200 °C steam environment

A continuous ceramic coating of 8 mu m thick on Zr-1Nb alloy was fabricated by plasma electrolytic oxidation (PEO) in alkaline phosphate electrolyte. The high-temperature steam oxidation behavior of bare and PEO coated Zr-1Nb alloys was evaluated using a thermogravimetric analyzer (TGA) in 900-1200 degrees C water vapor environment. Their morphologies, compositions, phase constituents before and after the steam oxidation were characterized by scanning electron microscopy (SEM), glow discharge optical emission spectroscopy (GDOES) and X-ray diffraction (XRD). It was found that the mass gain of PEO coated Zr alloy was about 38%, 56%, 93% and 100% of bare Zr alloy after 3600 s steam oxidation at 900 degrees C, 1000 degrees C, 1100 degrees C and 1200 degrees C, respectively. The steam oxidation resistance of Zr-1Nb alloy was obviously improved after PEO treatment below 1000 degrees C, but was hardly enhanced above 1100 degrees C. The mass gain rate for Zr-1Nb alloy samples in high-temperature steam is related to the oxygen diffusion coefficients in beta-Zr, alpha-Zr(O) and ZrO2 phases. After steam oxidation at 900-1200 degrees C, the oxide layers for bare and PEO coated Zr-1Nb alloys contain monoclinic zirconia (M-ZrO2) and a little tetragonal zirconia (T-ZrO2) phases below 1100 degrees C, but the T-ZrO2 phase disappears at 1200 degrees C.