Evolution of Oxide Scale on Aluminide and Pt-Aluminide Coatings Exposed to Type I (870 degrees C) Hot Corrosion

摘要

Simple aluminide (NiAl) and Pt-modified aluminide (NiPtAl) coatings were produced by a low-temperature high-activity aluminizing (LTHA) technique on a Rene-80 substrate. A Pt layer (6-8 A mu m) was deposited on the substrate before the LTHA process to prepare the NiPtAl coating. Hot corrosion testing was conducted by a furnace method using Na2SO4-30 wt% NaCl molten salt at 870 A degrees C (i.e., type I hot-corrosion conditions) for 700 h. Corrosion kinetics were determined by measuring the weight change of the specimens at regular intervals of 20 h. Morphology of corroded surfaces and scale chemistry were determined as a function of exposure time. Results indicated that the addition of Pt to aluminide coatings significantly enhances the Type I hot corrosion by promoting the slow growth of a highly adherent oxide on the sample surface. In contrast to the NiAl coating, the NiPtAl coating showed no significant sample weight increase or subsequent catastrophic failure up to 700 h of the hot corrosion exposure. The improved lifetime of NiPtAl coating was attributed in large part to the excellent oxide adherence and the role of Pt in promoting the formation of slow-growing alpha-Al2O3. In addition, Pt limits diffusion of substrate refractory elements from substrate into the oxide/metal interface or the external alumina scale. As a result, the Al2O3 scale on the NiPtAl coating remained adherent and virtually no spallation was observed even after prolonged Type I hot corrosion.