Effects of Ta and Y Additions on the Oxidation and Hot Corrosion Behaviour of a γ/γ′ Alloy

Abstract

The results of investigations on the oxidation and hot corrosion of a γ/γ′ alloy identified in a NiCoCrAlYTa coating for Nickel-base superalloys, are presented. Its composition corresponds to the formula: Ni₃₇Co₂₇Cr₂₇Al₈Ta.

Yttrium-containing alloys (0.1, 0.5, and 1 at %) and a tantalum-free alloy (Ni₃₇Co₂₇Cr₂₇Al₉) were tested in isothermal and cyclic oxidation at 850°C and 1100°C in air.

The same alloys were also tested in conditions corresponding to hot corrosion: periodic contamination with Na₂SO₄, then heating in an air + SO₂ gas mixture.

At the two temperatures studied, the γ/γ′ alloys are chromiaformers. With yttrium-free alloys, at 850°C, the influence of tantalum on the oxidation kinetics is slightly beneficial while at 1100°C, the situation is inverse (tantalum additions increase the oxidation rate). In this last case, tantalum is converted into a porous and non-adherent oxide, TaCrO₄. The presence of Y at elevated temperature, 1100°C, dramatically improves the oxide scale adherence.

Yttrium and tantalum effects on the hot corrosion behaviour of γ/γ′ chromia-forming alloys are limited. Nevertheless, yttrium is subjected to acidic fluxing and this reaction has probably a deleterious influence if the alloys are alternatively exposed to hot corrosion and high temperature oxidation.