Structure and 900 °C hot corrosion behavior of chromium-modified platinum aluminide coatings

Abstract

It is well known that the addition of platinum to diffusion aluminide coatings has demonstrated great success in producing protective coatings which delay the onset of high temperature hot corrosion attack in the range 800 – 1000 °C. Recently, it has been found that the addition of chromium to diffusion aluminide and platinum aluminide coatings improves low temperature (700 °C) hot corrosion resistance on IN738 nickel-base superalloy substrates. Therefore, a program was initiated to study the effect of the combined addition of chromium and platinum with varying processing sequences on aluminides in accelerated hot corrosion environments at 900 °C. In this investigation, ten coating types on the nickel-base superalloy substrate IN738 were tested at 900 °C for 200 h. Of these ten coatings, two coatings showed excellent high temperature (900 °C) hot corrosion resistance: (i) Pt-Al (produced by an intermediate temperature, intermediate activity (ITIA) process) and (ii) Cr-Pt-Al (process D). The average measured depth of attack rate was about 0.025 μm h -1 which is about one order of magnitude slower than that of published low temperature (700 °C) hot corrosion data. Scanning electron microscopy was used to examine the microstructures of the coatings. The ITIA Pt-Al coating has a thin (3 μm) surface layer of PtAl 2, with an outer intermediate zone of profuse PtAl 2 particles dispersed in an NiAl matrix and an inner intermediate zone of substrate-element-rich precipitates and carbides in a β-NiAl matrix over an interdiffusion zone. The Cr-Pt-Al (process D) coating has a thick (18 μm) continuous surface layer of PtAl 2, with an outer intermediate zone of a small volume fraction of α-Cr and other substrate-element-rich precipitates dispersed in a β-NiAl(Pt, Cr) matrix and an inner intermediate zone of α-Cr-rich precipitates, refractory metal carbides and NiAl over a typical interdiffusion zone. It appears that an outer dense layer of the PtAl 2 phase backed up with a fairly high level of chromium and platinum near the surface is beneficial in protecting the superalloy substrate against high temperature corrosion attack.