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

Abstract

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 lm) was deposited on the substrate before the LTHA process to prepare the NiPtAl coating. Hot corrosion testing was con- ducted by a furnace method using Na2SO4-30 wt% NaCl molten salt at 870 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. Mor- phology of corroded surfaces and scale chemistry were determined as a function of exposure time. Results indicated that the addition of Pt to aluminide coatings sig- nificantly 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 life- time 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 a-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.