| To safely operate radioactive tritium and optimize the tritium balance in fusion reactor, one of efficacious methods is reducing tritium permeation using coatings as tritium permeation barriers (TPB) on structural material. An aluminum rich coating consisting of an outer Al2O3 layer and an inner FeAl/Fe3Al layer is identified as a major candidate of TPB today, because of its high permeation reduction factor (PRF), good thermal property, and so on. Various preparation methods, such as hot dipping aluminum (HDA), Pack-cementation and chemical vapor deposition (PC-CVD), and vacuum plasma spray (VPS), etc., were used to prepare such aluminum rich coatings, and a pronouced permeation reduction was easily to achieve in gas phase. However, the realization of TPB on complex structures was still inaccessible. In present work, a new combined process including AlCl3/EMIC ionic liquid electroplating followed by heating treatment and oxidization is proposed to prepare such aluminum rich coatings, and the preparation technique, structure and performance of such coatings on 21-6-9 and 1Cr18Ni9Ti steel are reported.A series of aluminum electrodeposition on 21-6-9 steel from aluminum chloride (AlCl3)-1-ethyl-3-methylimidazolium chloride (EMIC) ionic liquid were carried out. The deposited Al coatings were smooth, continuous and dense micro structure assuming irregular growth feature characterized by pyramid formation. Electrochemical etching of the substrate by anodic polarization prior to electrodeposition leaded to well adherent Al coatings. With the increase of current density, particle size of Al coating increased obviously, resulting in rougher surface, whereas the coatings had similar particle size and much rougher surface as electroplating temperature and electro-plating time increased. The thickness of Al coating linearly increased with the increase of electroplating time. The coating has high quality and good adhesion under the optimum current density of 10-20 mA/cm2, electro-plating temperature of 18-30℃, electro-plating time of 30-60 minutes and in situ electrochemical etching prior to electrodeposition. Al coatings displaying attractive brightness and well adhesion to the surface of steel workspaces have also been obtained.Aluminium coated 21-6-9 steels were heat treated in air to prepare aluminized coating. With the temperature varying from 650 to 750℃, time from 1 to 30h, the aluminized coating appeared homogeneous, with thickness of 3-30μm and free of porosity, and had metallographic adherence to substrate, exhibiting a three-layer or two-layer structure. The aluminized coating growth kinetics and microstructures were determined by processing conditions, such as temperature, time, aluminum coating thickness and cooling rate after heating. The interrelation between aluminized coating thickness (hFe-Al), aluminized temperature (T), aluminized time (t) and aluminum coating thickness (hAl) could be described by hFe-Al＝2.39×106t1/2h1/2Al exp[-116900/(RT)]. The growth process of aluminized coating is consisted of threes steps:1) formation of initial aluminized layer resulting from atomic diffusion through grain boundaries and defects during heat treatment; 2) diffusion of Al and alloy elements taking place between the residual Al layer and the initial aluminized layer resulting in fast growth of aluminized layer; 3) further atomic diffusion of aluminized layer leading to slowly growth of aluminized layer.To form Al2O3 scale on surface, the aluminized coatings were oxidized in 10-2Pa O2 or Ar at 700℃with effort to artificially oxidation of Al. A compact and well adhesion Al2O3 scale, with thickness of nano-meter, formed under the optimism temperature of 700-750℃, oxidation time of 100-200 h, atmosphere of Ar or 10-2Pa O2 and composition of FeAl2/FeAl/ Fe3Al. The finally fabricated homogeneous, defects free coating showed a double-layer structure consisting of an outerγ-Al2O3 layer with thickness of 0.1-0.3μm and an inner (Fe,Cr,Mn and Ni)Al/(Fe,Cr,Mn and Ni)3Al layer of 30-50μm thickness.The optimal conditions were applied to 1Cr18Ni9Ti steel tubular specimens (one end closed) to produce such FeAl/Al2O3 coating system as TPB, and the permabilities of the coating and bare specimens were tested to determine its PRF. The coating exhibited a uniform thickness, a smooth surface and a well adhesion. The deuterium permeation rate through the coated steel was reduced by 2-3 orders of magnitude at 600-727℃. The coating preserved a perfect surface for 10 thermal cycles between 750℃and room temperature. |
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