The Impact of Ni- and Cr-Containing Thermally Grown Oxides on the Intensity of Oxide and Sulfate Induced Hot Corrosion of an Alumina-Forming Alloy

Aerosols deposited on the surfaces of alloys and coatings used in high-temperature applications react with the protective thermally grown oxide (TGO) and accelerate component degradation. Understanding how the TGO composition effects these reactions is essential to develop corrosion-resistant materials. This work studies the corrosion of Haynes-214 alloy, which forms a bilayer Ni(Cr,Al)₂O₄ (spinel) + Al₂O₃ TGO, exposed to complex oxide and sulfate deposits to understand how the Cr and Ni oxides affect the corrosion mechanisms. The deposit compositions were systematically varied to understand the effect of anion makeup (mixed oxides, oxide-sulfate, and sulfates). The results are compared to a FeCrAlY alloy that forms a simple Al₂O₃ TGO. On the Haynes-214 alloy, deposits containing mixed oxides (with or without sulfates) have limited corrosive effect. In contrast, a pure CaSO₄ deposit reacts aggressively with the TGO to form calcium chromate and aluminate reaction products (RP) that substantially increase the TGO + RP thickness compared to that on the FeCrAlY alloy. Surprisingly, a deposit composed of a mixture of sulfates caused less severe corrosion than the CaSO₄ alone since magnesium chromate formation appears to limit aggressive reactions involving Ca. The results are discussed in the context of opportunities for corrosion-resistant alloy design.