Impact of phase separation on corrosion resistance of AlCrFeNi thermal spraying HEA coatings

Abstract

Corrosion resistance is a critical property for materials used in harsh environments. Stainless steel 316 is known for its reliability; however, there is a continuous search for materials with better durability and lower maintenance needs. This study investigates the corrosion behavior of AlCrFeNi high-entropy alloy (HEA) and highlights its superior performance compared to SS316 in salt spray and acid immersion tests. AlCrFeNi is proposed as a protective coating to address the limitations of bulk HEAs, with its properties optimized through atmospheric plasma spraying (APS). As the properties of powders are essential for APS, their characteristics were analyzed. The gas-atomized AlCrFeNi powders exhibit an ideal high-entropy state with spherical morphology, uniform elemental distribution, and a single BCC phase due to rapid cooling and sluggish diffusion. After heat treatment above 900 °C, phase separation was observed, forming AlNi-rich phases of BCC/B2 and FeCr-rich phases of BCC/A2. Corrosion tests indicated that as-atomized powders have superior corrosion resistance compared to heat-treated powders and SS316. However, the precipitation of AlNi phases caused micro-galvanic corrosion, compromising the material's performance. APS-deposited coatings from gas-atomized powders formed a dense and uniform-layered structure, maintaining excellent corrosion resistance. Nevertheless, high-temperature exposure induced phase separation, resulting in micro-galvanic corrosion. This study demonstrated the significance of phase separation control in enhancing the corrosion resistance of AlCrFeNi HEAs, offering a promising solution for protective applications in aggressive environments.