Preparation of Fe-Al alloy layer on CLAM steel

Abstract

Reduced activation ferritic/martensitic steel is one of the candidate materials for tritium breeding module in the fusion reactor. In order to control the permeability of tritium in an acceptable range, coating with low hydrogen isotope permeability, known as tritium permeation barrier, is usually prepared on the surface of such structural materials. The FeAl/Al<sub>2</sub>O<sub>3</sub> is the first choice of tritium permeation barrier for many countries, because of its fine performance of high permeation reduction factor, corrosion resistance and high-temperature resistance. The surface morphology and microstructure of Fe-Al infiltrated layer have important influence on the quality of Al<sub>2</sub>O<sub>3</sub> coating. In this study, Al coating on the surface of CLAM steel is prepared by electroplating of aluminum from AlCl<sub>3</sub>-EMIC. Then the Fe-Al infiltrated layer is obtained by diffusion between Al and substrate by annealing. The effects of annealing time and temperature on the microstructure of Fe-Al infiltrated layer are studied by X-ray diffraction, scanning electron microscope and energy dispersive spectrometer. The results show that 20-μm-thick aluminum coating is obtained on the CLAM steel surface by electroplating. The Al coating is uniform and compact, and the size of its surface columnar grain decreases with electroplating current density increasing. Annealing results show that neither hole nor gap is observed between the Fe-Al infiltrated layer and the substrate. In addition, the infiltrated layer is found to be tightly bound to the substrate with a thickness ranging from 7 μm to 45 μm, depending on the annealing parameters. At the initial stage of annealing, Cr enriched Fe-Al alloy is formed evidently. However, such a Cr enrichment disappears at higher annealing temperature or longer annealing time due to diffusion. The surface of infiltrated layer changes from aluminum-rich phase to aluminum-poor phase, and its thickness increases with annealing time or temperature rising. The temperature dependence of the growth rate of Fe-Al infiltrated layer can be described by Arrhenius equation. At this time, the Arrhenius activation energy of aluminization on CLAM steel is calculated to be 78.48 kJ/mol. At 640 ℃ and 760 ℃, the growth of Fe-Al infiltrated layer is controlled by the grain boundary as well as the volume diffusion. When the reasonable thickness and microstructure of Fe-Al alloy layer are used and annealing time or temperature keeps as low as possible, the optimal annealing temperature and time are 700 ℃/10 h, respectively.