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The high temperature air oxidation of the two conventionally cast Ni-based superalloys Rene 80 containing 92 wt ppm boron and CM 247 containing 110 wt ppm of boron was studied at temperatures between 850 and 1050 °C and times ranging from 1 to 100 h. Analysis by GD-OES and SEM/WDX/EBSD revealed the surface scale on Rene 80 after exposure at 1050 °C to consist of Cr2O3 with TiO2 in the outer part and BCrO3 and CrTi2O5 in the inner part. Internal Al2O3 precipitates were present below the external scale. After 1 h exposure of CM 247 an inner dense alumina scale formed over eutectic alloy regions containing coarse γ′ precipitates. Over dendritic regions a Cr-rich oxide scale with BCrO3-precipitates formed initially and aluminum was internally oxidized. After longer oxidation times an alumina sub-scale grew from the internal precipitates, and oxidation of the initially present depleted alloy zones resulted in formation of outer Ni and Co rich oxides. In both alloys, boron depletion in the bulk alloy occurred due to formation of the mentioned boron rich oxide. The extent of depletion was substantially higher for the chromia forming Rene 80 than for CM 247. In the latter material the boron depletion was suppressed after the formation of the dense, inner alumina scale.

Modern gas turbine materials are subjected to increasing operating temperatures in service environments, which contain high concentrations of gaseous species originating from fuel impurities, e.g. sulfur. In the present study, the oxidation behavior of two commercially available nickel-base alloys, Rene 80 and PWA 1483, was investigated for up to 500 h duration in synthetic air and synthetic air containing 2% SO 2 at 1050 °C. In order to investigate the mechanisms of SO 2 attack, specimens of the two alloys after different oxidation times were characterized using a number of surface analytical techniques such as scanning electron microscopy, XRD, and glow discharge optical emission spectrometry. The corrosion reactions in the presence of SO 2 were found to be strongly alloy dependent and could not be explained simply in terms of the contents of the main scale-forming alloying elements, Cr and Al. Severe internal sulfidation was observed underneath the chromium-rich oxide scale on Rene 80, leading to breakaway oxidation after longer exposure times. For PWA 1483 sub-scale formation of a near-continuous alumina occurred, which resulted in substantial suppression of sulfidation. Consequently, no breakaway oxidation of PWA 1483 was observed up to 500 h exposure at 1050 °C in synthetic air with 2% SO 2 . The formation of a dense alumina sub-scale on PWA 1483 is believed to be correlated with the Ta-addition in this material. It is proposed thatTa can tie up Ti by forming a mixed oxide of the type TiTaO 4 . In this way, it prevents the incorporation of Ti into the chromia scale, which can lead to enhanced scale growth.