A fundamental study of hot corrosion and interdiffusion of chromium, aluminum, and silicon coatings on a nickel-201 substrate

Abstract

Modern turbine engine systems require increased efficiency and durability. To achieve these goals, high-temperature materials with high-strength, low-cost and non-strategic compositions are needed. In advanced turbine applications, combustor liners, blades and vanes are exposed to corrosive combustion byproducts, such as alkali salts, at temperatures up to ~1700°C with high gas velocities, entrained particulates, and other foreign objects at pressures of up to 3 MPa (30 atm). These extreme conditions can drive a dangerous phenomenon known as "hot corrosion", an accelerated form of oxidation that occurs when metals and metal alloys are heated in the temperature range 700-900°C in the presence of alkali salts. An increased understanding of the fundamental behaviors of common high temperature alloys and their degradation mechanisms is therefore critical for the production of reliable components. In this study a model substrate, Nickel 201, was coated on one side with Cr, Al, or Si thin films (~1 micron) via magnetron sputtering physical vapor deposition (PVD). Uncoated and PVD coated samples were then exposed to laboratory air at 700°C and 900°C and to an environment similar in composition to atmospheres found in post combustion turbine systems, comprised of air/SO 2 gas mixture, at 700°C. The exposures were conducted over time intervals observing coating-substrate interactions and surface oxide development. Identical samples were subjected to the same exposures with addition of a deposit of sodium sulfate (Na 2SO 4), a model alkali salt. Sample mass gains were recorded and resulting oxide compositions assessed as a function of exposure time using microscopy techniques on sample surfaces and cross sections. The development of intermetallic species was determined by X-ray diffraction. At 700°C, coated and uncoated samples displayed different oxidation behaviors. Under laboratory air, no hot-corrosion occurred. While at 700°C in air/SO 2 exposures, evidence for hot corrosion on deposited samples was observed. When sodium sulfate was introduced at 900°C, coated and uncoated samples displayed rapid corrosion consistent with hot corrosion. The oxidation processes and coating/substrate inter-diffusion phenomena are presented and discussed in the context of establishing basic approaches to improve the fundamental understanding of hot corrosion, and the protection mechanisms of high temperature materials.