Sulfidation Behavior of Inconel 738 Superalloy at 500–900°C

Abstract

The high-temperature sulfidation behavior of the cast nickel-base superalloy Inconel 738 (IN-738) was studied over the temperature range 500–900°C in pure sulfur vapor over the range 102–104 Pa. The sulfidation kinetics followed the parabolic rate law in all cases. The sulfidation rates increased with increasing temperature and sulfur pressure. The scales formed were bilayered and temperature-dependent. At T≤700°C, the outer scale consisted of mostly NiS (with dissolved Co) and minor (CoS2 and NiCo2S4, while the inner layer was a heterophasic mixture of NiS, NiCo2S4, and minor amounts of Al2S3 and chromium sulfide (Cr2S3/Cr3S4). At T≥750°C, the outer scale consisted of mostly Ni3S2 (with dissolved Co) and minor amounts of Co3S4 and Cr2S3/Cr3S4, while the inner layer was a complex, heterophasic mixture of Ni3S2, Cr2S3/Cr3S4, CoCr2S4, and minor Al2S3. Platinum markers were found to be located at the interface between the inner and outer scales, suggesting that the outer scale grew by the outward transport of cations and the inner scale grew by the inward transport of sulfur. The formation of Al2S3 and Cr2S3/Cr3S4 partly blocked the transport of cations through the inner scale and consequently reduced the sulfidation rates as compared to pure nickel.