Hot Corrosion and Mechanical Performance of Aluminide Coated Austenitic Steel

Abstract

The present study aims at improving coatings to allow higher operating temperatures – up to 700 °C – while resisting hot corrosion in future heat transfer fluids and storage systems for CSP technology. An aluminide coating was produced onto a 347H austenitic alloy to resist Li-Na-K molten carbonate corrosion at 700 °C. Multiple surface preparations were investigated and, after optimization, a new diffusion heat treatment was selected. A homogeneous 3-layer coating was formed with Al content reaching up to 69 at.% at the top surface. Corrosion was significantly decreased by the coating compared to the uncoated material. After 500h-exposure, a 100 µm-thick multi-layered and non-adhesive alkali elements rich oxide formed at the surface of uncoated 347H alloy. On the contrary, the coated material preserved its morphology and a mix of α-LiAlO2 and γ-LiAlO2 formed on the top coating. From the mechanical perspective, local hardness measurements highlighted a broad variation along the different layers of the coating. 3-point bending tests showed that deformation in the plastic domain was required to cause crack formation. Cracks remained in the two outermost layers of the coating and did not reach the underneath substrate.