Preparation Of Al₂O₃ Coating On Steel Substrate

Nowadays, with the shortage of energy resources, people pay more attention to the thermonuclear reactor. And China, Europe, the United States et al launch the plan named International Thermonuclear Experimental Reactor(ITER)to solve energy crisis. Tritium is one of the most important fuels in this reactor. But it has a high permeability in the structural steel. The permeation of tritium not only causes nuclear pollution, but also wastes large sums of energy. The way to solve this problem is to obtain an anti-permeation coating on the surface of structural steel. Al₂O₃ coatings possess a high permeation reduction factor and the ability of self-repairment. It is a reasonable anti-permeation coating.In this research, we combined electroplating and hot dip aluminizing to obtain aluminum coating on the surface of structural steel. Then, the methods of high temperature oxidation and electrophoretic deposition(EPD) were used to form Al₂O₃ coating. The surface and interface structure of coating were investigated by optical microscope, XRD, SEM, EDS, respectively. Surface hardness was tested by HV-1000 micro hardness tester. Corrosion resistance was measured by SI1287 electrochemical workstation. The influences of AlCl3 content, electroplating time, pre-aluminizing on the surface morphology and micro structure were studied. Morphology, thickness, density of Al₂O₃ film under different oxidation temperature and time were investigated. The mechanism of oxidation process was introduced. To study the principles of electrophoretic deposition, the influences of suspension dispersing system, current/voltage, and deposition time were analyzed. The main results are as follows:(1) A smooth, dense aluminum coating on structural steel was prepared by electroplating under AlCl3-NaCl-KCl molten salts system, on condition of 80 % AlCl3 content, 5.35 A/dm2 current density, 6 h electroplating time. The thickness of this coating is 75μm, which is consisted of densely-arranged spherical particles.(2) After electroplating, a high quality aluminum coating was obtained by hot dip aluminizing. The interface contains three phases, which are aluminum layer, diffusion layer and substrate. The thickness of aluminum layers is about 35μm and the one of diffusion layers is 50μm. The interface of aluminum coatings and substrate changes from mechanical activation bonding to metallurgical bonding during hot dip aluminizing.(3) A continuous, dense Al₂O₃ film was obtained after oxidation at 900℃for 20 h. This film is made up by two dimensional nano flake crystal grains. Surface hardness is raised from 182 HV to 948 HV. Corrosion resistance is also improved.(4) With electrophoretic deposition (EPD), a pureα-Al₂O₃ coating is obtained on the surface of aluminum coating. Under mode of constant voltage, a uniform, dense coating can be prepared when the voltage is 20³0 V. With the increasing of deposition time, the slope of coating thickness-electro-deposition (EPD) time decreases. Different thickness ofα-Al₂O₃ coating nearly has the same equilibrium electrode potential and self-corrosion current, which indicates that theα-Al₂O₃ coating has certain porosity ratio.