Stability of oxide-sulfate mixtures and implications for deposit-induced degradation of advanced alloys and coatings

Abstract

Turbine engines ingest atmospheric aerosols that include multi-cation oxide-sulfate mixtures, which deposit onto surfaces within the engine. These deposits are often implicated in hot corrosion of alloys and bond coats and can accelerate thermomechanical failure of ceramic coatings through reaction and infiltration mechanisms. Understanding the intrinsic stability of oxide-sulfate deposits, including the tendency for sulfate decomposition, is important to identify efficient testing protocols. This work addresses that need by integrating experiments and computational thermodynamics models to systematically analyze the effect of deposit composition and temperature on the sulfate decomposition and melting behavior of five model mixed-anion deposits. Mass loss analysis and energy dispersive x-ray spectroscopy show that sulfate decomposition occurs much faster in the mixtures compared to pure calcium sulfate. The thermodynamic computations show that reaction pathways forming ternary and quaternary silicates accelerate sulfate decomposition faster than those forming binary reaction products. The results also show that multi-cation mixtures can suppress the evaporation of sodium and potassium sulfates, retaining the sulfate-bearing liquid to higher temperatures. The results can be used to differentiate the behavior of sulfate-, sulfate-oxide-, and oxide-based deposits, and to guide the discovery of new high temperature alloys and coating materials.