Initial Stages of Na2SO4-Induced Degradation of β-Ni–36Al at 700 °C: I-Intrinsic Behavior

Abstract

The early-stage scaling behavior of a β-Ni–36Al alloy undergoing Na2SO4-deposit-induced degradation at 700 °C was systematically studied using SEM and TEM. After 20 h of exposure in an O2–1000 ppm SO2 ambient, the deposit-coated alloy formed a dense but thin Al2O3 scale on most areas of the surface; however, large nodules formed locally. Nodule formation occurred where the scale had lost its protective character, with rapid internal oxidation ensuing. The presence of sulfur both in the environment and in the salt played a key role in nodule formation. Removal of SO2/SO3 from the gas mixture, or of the Na2SO4 deposit from the surface, prevented nodule formation, while removing the sulfur source after nodule formation prevented further nodule growth. The degradation could be linked to the dissolution of reaction products in the Na2SO4 deposit and the formation of a low-temperature eutectic liquid. Further, when an Na2SO4–48% MgSO4 deposit was used, the nodule density increased.