Comparative Study of FeCrAlY and Sapphire Hot Corrosion by Mixed Oxide and Sulfate Deposits

Abstract

Alloys and oxidation-resistant coatings utilized in high-temperature applications can be degraded by aerosols that deposit onto surfaces during operation. Understanding how the deposit composition influences the hot corrosion mechanisms is essential to develop more durable materials. This work advances the understanding of the effect of complex oxide and sulfate deposits on the degradation of an alumina-forming FeCrAlY alloy in comparison to reactions with single-crystal sapphire. The deposit compositions were developed to systematically understand the effect of anion makeup (mixed oxides, oxide–sulfate, and sulfates) and the effect of adding Na and K salts. CaSO₄ was used as a control. The mixed oxide and oxide–sulfate deposits increased the frequency of thermally grown oxide (TGO) intrusions in FeCrAlY but did not produce a noticeable change in the sapphire. Pure CaSO₄ and mixed sulfate reacted with the TGO and sapphire to form calcium aluminates and led to roughening of the specimen–reaction product interface. The primary difference between the CaSO₄ and mixed sulfate deposits was the increased uniformity of the attack by the latter due to its tendency to melt and spread. Comparison between the change in the degradation features in the presence of CaSO₄ and mixed sulfate deposits on both types of specimens expands the current understanding of sulfate-based hot corrosion.