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 conducted by a furnace method using Na 2 SO 4 -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. 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 a-Al 2 O 3 . 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 Al 2 O 3 scale on the NiPtAl coating remained adherent and virtually no spallation was observed even after prolonged Type I hot corrosion.

Microstructure and hot corrosion behavior of hot dip siliconized coating on Ni-base superalloy IN738LC

“…Park et al [30] also reported the presence of TiO 2 after hot corrosion of nickel-based superalloys. The Cr 2 O 3 phase is corrosion resistant, in contrast to TiO 2 and NiCr 2 O 4 [31]. It should be noted that, no peaks were found in the XRD spectra representing the Cr 2 O 3 phase in A100 specimen which, in turn, denote to the weak hot corrosion resistance in the as-received IN738LC specimen.…”

“…This corresponds to chloride volatility effect. Since most metal chlorides have high vapor pressure and low melting points, chlorine tends to diffuse outward through oxide scale quickly, followed by its reaction with the alloying elements including Al and Cr at the interface of oxide and metal which leads to volatile chlorides formation [31,39]. Additionally, the presence of Cr 2 O 3 phase may lead to weight loss.…”