Effect of partial oxygen pressure on the initial stages of high-temperature oxidation of γ-NiCrAl alloys

The effect of the partial oxygen pressure (pO(2)) on the initial stages of oxide-layer growth of a gamma-Ni-27Cr-9Al (at.%) alloy at 1373 K was studied using X-ray photoelectron spectroscopy, X-ray diffractrometry, scanning electron microscopy and electron probe X-ray microanalysis. The alloy was oxidised at a pO(2) of 0.1 and 0.2 x 10(5) Pa in an UHV processing chamber and a furnace, respectively. For both pressures. a double-layered oxide structure evolved upon oxidation, which consisted of a NiCr2O4/Cr2O3 layer on top of an alpha-Al2O3 layer. Two different oxidation stages were identified: an initial, very fast oxidation stage followed by a second stage of slow, parabolic growth. The initial oxidation stage was associated with the preferential oxidation of At, followed by the internal oxidation of Al and simultaneous formation of Cr2O3 (and some NiO). The onset of the second stage ran parallel with the coalescence of internal alpha-Al2O3 crystallites into a continuous closed layer. Then, the internal oxidation of At and growth of Cr2O3 ceased. A lower pO(2) reduced the activity of oxygen at the oxide/alloy interface during the initial, fast oxidation stage, thereby enhancing the rate of formation of a continuous closed alpha-Al2O3 layer and suppressing the formation of Cr2O3 (and NiO).