The Pack-Cementation Process of Iron-Aluminide Coating on China Low Activation Martensitic and 316L Austenitic Stainless Steel

In a fusion blanket design, ceramic coating such as Al₂O₃, Er₂O₃, Y₂O₃, TiC, TiN and TiC/TiN etc., has been considered as a tritium permeation barrier (TPB) on structural materials (e.g. RAFMs, 316L) by many countries in the past 20 years. The Al₂O₃ film prepared by in-situ oxidation of the iron aluminide layer is considered one of the most attractive because of the slow-growing steady protective oxide scale and its excellent self-healing ability. In order to obtain a transition aluminide layer with a certain aluminum content and thickness on two kinds of substrates such as the Reduced Activation Ferritic/Martensitic (RAFM) and 316L stainless steel, wide research efforts have been made on the effect of different pack chemistry, temperature and time on the properties and thickness of the aluminizing layers. The results indicated that a dense and uniform coating with a thickness about 20m was formed on CLAM (a Chinese RAFM steel) and 316L substrates for the pack material with low Al content (about 32wt.%). This aluminide coating had a surface aluminum content about 40-50at.% and was mainly consisted of ductile FeAl phase. For the pack material with high Al content (about 50wt.%), the thickness and the surface aluminum content of the aluminizing coating had great increases and there were mainly brittle Fe₂Al₅ phase. Especially some cracks were observed across this coating on CLAM substrate due to the mismatch in coefficient of thermal expansion (CTE) between the coating and substrate.