The role of Al diffusion behavior in the process of forming a super-reliable Al₂O₃ protective layer during reactive air aluminization

Reactive air aluminization is attractive as a low-cost, simple method for preparing Al₂O₃ protective layer for ferritic stainless steel (FSS) components in solid oxide cell stacks. The FSS Crofer 22H is aluminized at various temperatures to obtain different levels of elements interdiffusion. The leading role of Al diffusion behavior in the process of forming the Al₂O₃ protective layer is investigated in detail. The roughening mechanism of aluminized surface is discussed. The formation process of Al₂O₃ protective layer can be divided into three stages: liquid-solid interdiffusion (<700 °C), solid-solid interdiffusion (700–900 °C), and epitaxial growth of Al₂O₃ (>900 °C). The intense interdiffusion between the Al liquid and the steel matrix leads to the formation of Al-containing intermetallic compounds (IMCs) layer in a short time. Increasing temperature promotes the diffusion of Al into the steel matrix, causing the phase transition of IMCs. The surface enrichment of Al suppresses the formation of Fe/Cr-oxides, ensuring the formation of a continuous, dense and crack free Al₂O₃ layer. The rough surface formed in the first stage basically determines the overall roughness of the sample after aluminization.