Effect of composition on the hot corrosion resistance of NiAI and (Ni, Pt)Al

Cast nickel aluminides, alloyed with combinations of Cr, Pt, and Hf were tested in 1-h cycles under hot corrosion conditions at 950°C, and in oxygen at 1150°C. In addition to these model alloys, a cast alloy resembling the composition typically found in commercial platinum aluminide coatings was tested. Hot corrosion tests clearly demonstrated that Cr rather than Pt was the alloying addition needed to decidedly improve hot corrosion resistance of NiAI. Pt-containing alloys showed no improvement in hot corrosion resistance as compared to rapidly attacked Hf-doped NiAI. However, as little as 2 at.% Cr retarded the rate of hot corrosion attack significantly. Hot corrosion resistance improved with increasing Cr content, with preoxidation prior to hot corrosion testing resulting in further improvement for low Cr additions only. At 1150°C, Cr degraded oxidation resistance; both oxide scale formation rate and susceptibility to scale spallation of Cr-containing NiAl+Hf were increased relative to simple Hf-doped NiAI. The addition of Pt to undoped NiAI improved oxidation resistance by improving spallation resistance, however, Pt was less effective than reactive element doping with Hf. The cast platinum aluminide coating composition exhibited poor hot corrosion and oxidation resistance relative to Cr-containing and Hf-doped NiAI, respectively. In essence, the present results on cast aluminides indicated that hot corrosion resistance and exceptional scale spallation resistance appears to be mutually incompatible goals for coating performance.