The Sealing Of Cerium Salt On Anodic Film Of Aluminum Under Pulse Electric Filed

In order to improve the corrosion resistance of aluminum, it is necessaryto take the anodizing and subsequent sealing processing on aluminum. Thetraditional sealing such as potassium dichromate sealing and nickel fluoridesealing are harmful to the environment and the healthy, and the boilingwater sealing is high energy consumption.Rare earth sealing isenvironmentally-friendly, and there are many scholars at home and abroadtaking on the research.In this paper, the mechanism of the sealing of ceriumsalt on anodic film of aluminum under pulse electric filed was Put forward.The parameters which effected the sealing were discussed. The aging ofanodic film under rare earth sealing and some other sealings was alsoconsidered. The main research conclusions are as follows:(1) The sealing quality of cerium salt bi-directional pulse sealing is betterthan that of soaking sealing, anaerobic pulse sealing and single negative pulsesealing. The anodic film is almost sealed after the cerium salt pulse sealing. Itwas found that there were particles sealing the hole. The sealing quality ofcerium salt bi-directional pulse sealing is closely related to the impressed current, oxygen and the positive current. We assume the mechanism is that theproviding of the negative current not only promotes the accumulation ofnegative charge on the boundary of the oxidation film which creates OH-, butalso promotes the migration of Ce3+to the surface of the oxidation film. Thepresence of oxygen is the root cause of producing OH-. The providing of thepositive current promotes the migration of OH-to the pore and the suface ofthe oxidation film.(2) It is obvious that there is cerium existed on the surface of anodic filmwhich is sealed by bi-directional pulse power. The cerium that existes on theoxidation film is mainly in trivalence and the rest is in quadrivalence. Theoxide and hydrogen chloride were found on the surface of the sealed film. Thecerium is existed in the form of oxide and hydrogen chloride. The cerium oftrivalence is in oxide and hydrogen chloride, and that of quadrivalence is inoxide which is the oxidization product of trivalence cerium compound due tothe exposure to the air.(3) It was proposed some influencing factors that effected the sealingquality, They were sealing time, sealing temperature, valtage, pulse frequency,negative duty cycle, positive duty cycle, the amout that cerium added, theadding of the fluorin ion and so on. All of them effect the corrosion resistanceof the anodic film under the sealing more or less. The corrosion resistance isaffected mostly by sealing time, pulse frequency and negative duty cycle.Fluorine ion was first introduced to rare earth sealing, and it is beneficial to speed up the closed process.(4) Compared to the unsealed anodic film, the corrosion resistance of theanodic film under cerium salt sealing largely increased. The results show thatrare earth sealing for anodized film is comparable to that of chromate sealing,and is better than boiling water sealing. The aluminum sheets are put in thesolution including NaCl of30g/L for180days, the corrosion resistance of theanodic film under cerium salt sealing is hither than that of potassiumdichromate sealing and boiling water sealing.