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

Abstract

a b s t r a c tShort-term oxidation exposures of an alumina-scale forming c 0 -Ni 3 Al-based model alloy in air and O 2 + 0.1%SO 2 at 900°C revealed that the presence of sulfur can affect the kinetic competition between the h and a structural isomorphs of Al 2 O 3 . After 2 h 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 + 0.1%SO 2 . This promotion of a-Al 2 O 3 establishment was due to sulfur enrichment on the alloy surface, which occurred even when samples were exposed to O 2 + 0.1%SO 2 in a low-temperature, pre-test position ($150°C), i.e., prior to insertion into the hot zone. It was determined from XPS measurements that the sulfur was mainly in the S 6+ valence state and, correspondingly, in the form of NiSO 4 . Cross-sectional scanning transmission electron microscopy (STEM) using energy-dispersive X-ray spectroscopy (EDS) corroborated the XPS results by detecting that a $20 nm zone of sulfur enrichment within the surface region of a $90 nm oxygen-rich layer formed during the pre-test exposure. A systematic explanation for this intriguing observation of sulfur promoting a-Al 2 O 3 establishment is provided from the perspective of kinetics competition between h and a. This explanation was supported by kinetic calculations and complementary tests in a low p O 2 atmosphere.

Corrosion behaviour of Ni and nickel aluminide coatings exposed in a biomass fired power plant for two years

“…After 1 year of exposure at Randers, there was minimal attack of the nickel layer, and there was no clearly visible oxide [24], but after two years a distinct NiO layer could be observed. It would be expected that there was a NiO layer of a few microns after 7100 hours exposure based on measurements at 500 ºC, where a 1 µm layer was formed after 136 hours in O 2 [27] [32]. The stability of NiO, NiSO 4 , Ni and NiS in SO 2 gas has been investigated with respect to time at 540 o C, which showed that NiS was the stable compound while the other corrosion products decreased over time [33].…”