Preparation And Investigation Of Alumina Coatings For The Application As Tritium Permeation Barrier

One of the critical issues in test blanket module (TBM) system is to reduce tritium permeation from Pb-Li into the water/helium coolant. Use of alumina coating is one of the most promising methods to solve this problem. Unfortunately, the developed methods for preparing the coatings all have several drawbacks, which limit the application. In the present study, well-adhered alumina coatings were prepared on steel substrates by the sol-gel method, self-growing process and cladding. None of the three processes has been reported for the preparation of alumina tritium permeation barriers.The parameters in sol preparation, drying, and heat treatment have been determined. X-ray diffraction (XRD), field-emission scanning electron microscope (FSEM) and energy dispersive X-ray spectroscopy (EDS) were employed to identify the phase and examine the microstructure of the coating. Atomic Force Microscope (AFM) was used to test the roughness of coating surfaces. H2O and Al(OC3H7)3 in a molar ratio of 120 to 1, and HNO3: Al(OC3H7)3 molar ratio of 0.1:1 were determined to prepare suitable sol. Dry process were chosen (90°C 3h, 70°C 10h, 120°C 5h) to avoid coatings peeling off. The coating prepared at 600-700 oC is amorphous, while that prepared at 800 oC is gamma-Al2O3 mostly. When calcination temperature is elevated to 1000 oC, alpha-Al2O3 was formed. The surface of the well-adhered coating was smooth and uniform. No spallation, cracks, or pores were observed in the coating or at the scale/alloy interface.FeCrAl stainless steel was chosen for alumina layer preparation in the self-growing method. An alumina layer grew on the surface of FeCrAl, when heated in air. The oxidation kinetics of the alloy has been studied. It was controlled by Al ions outward diffusion.Aluminum was cladded on steel to form Fe-Al sheet. The diffusion treatment of such plate at 750 oC formed Fe2Al5, while the FeAl showed at 900 oC. The alloy oxidized sequentially at 750 oC for 10 hour formed a top layer of alumina. The inner layer of Fe-Al is around 120μm, and it can provide alumina coating self-healing property.All the three methods employed can prepare coatings with excellent heat shock resistance.