Performance study of slurry aluminide coated 347H immersed in Li-Na-K carbonate molten salt for 5000-h at 700 °C

Abstract

Concentrated Solar Power (CSP) systems coupled with thermal energy storage (TES) are increasingly considered to provide dispatchable, low-carbon energy. To further improve their performance, next-generation CSP plants are designed to operate under more severe conditions, which raises concerns regarding high-temperature corrosion of structural materials by molten salts. This study examines the corrosion behaviour of bare and slurry aluminide-coated 347H stainless steel after exposure to a Li–Na–K carbonate eutectic at 700 °C for up to 5000 h. The uncoated alloy showed rapid degradation, characterized by a brittle, multi-layered oxide scale, extensive internal oxidation, and carburization. The outer oxide layer consisted mainly of LiFeO₂ and LiMnO₂, while the internal oxidation zone contained (Fe,Cr)₃O₄ spinels, LiCrO₂, and Ni-rich metallic islands. In contrast, the aluminide-coated samples exhibited excellent corrosion resistance without spallation or mechanical failure. The coating transformed into a compact FeAl layer, overlaid by a dense oxide composed of α- and γ-LiAlO₂. The α-LiAlO₂ phase, forming at the oxide–coating interface via the reaction of Al₂O₃ with lithium oxide, acted as a continuous and chemically stable barrier. Its persistence during exposure was key to prevent molten salt ingress and ensure long-term protection. These findings demonstrate that slurry aluminide coatings effectively increase the durability of structural alloys in molten carbonate environments relevant for advanced CSP-TES applications.