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

摘要

The effect of the partial oxygen pressure (pO_2) on the initial stages of oxide-layer growth of a gamma-Ni-27Cr-9Al (at. percent) 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 NiCr_2O_4/Cr_2O_3 layer on top of an alpha-Al_2O_3 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 Al, followed by the internal oxidation of Al and simultaneous formation of Cr_2O_3 (and some NiO). The onset of the second stage ran parallel with the coalescence of internal alpha-Al_2O_3 crystallites into a continuous closed layer. Then, the internal oxidation of Al and growth of Cr_2O_3 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-Al_2O_3 layer and suppressing the formation of Cr_2O_3 (and NiO).