Effect of environmental sulphur on structure of alumina scales formed on Ni-base alloys

Abstract

Short term oxidation exposures of an alumina-scale forming c9-Ni 3 Al-base model alloy in air and O 2 z0?1%SO 2 at 900uC revealed that the presence of sulphur can affect the kinetic competition between the h and a structural isomorphs of Al 2 O 3 . After five hours exposure, metastable h-Al 2 O 3 growth predominated in air alone; whereas, a much larger percentage of stable a-Al 2 O 3 formed during oxidation in O 2 z0?1%SO 2 . This promotion of a-Al 2 O 3 establishment was due to sulphur enrichment on the alloy surface, which occurred even when samples were exposed to O 2 z0?1%SO 2 in a low temperature, pre-test position (~150uC), i.e. before insertion into the hot zone. It was determined from X-ray photoelectron spectroscopy measurements that the sulphur was mainly in the S 6z valence state and, correspondingly, in the form of NiSO 4 . It will be discussed that the observed results can be rationalised based on the preferential presence of sulphur at the oxidising surface hindering the h-Al 2 O 3 growth rate to the extent that a-Al 2 O 3 establishment is favoured.

A review on atom probe and correlative microscopy studies of corrosion in nickel-based superalloys

“…31,32 Hence, superalloys for high-temperature applications such as turbine blades are alumina formers. 14,34 Thermodynamically, oxidation is governed by the Gibbs free energy G of the system, where the most stable state is reached for minimized G. Considering all possible oxides, the ones leading to the strongest reduction in G are stable and will form the oxide scale. Among those, oxides that need a higher oxygen partial pressure p O are found at the oxide-gas interface, whereas others can form further below the surface.…”