Progress in chemical conversion film treatment of aluminum and its alloys

At a certain temperature, a thin oxide film is formed on the surface of aluminum parts by chemical reaction, which is called the chemical oxidation of aluminum This method does not need current It is simpler and cheaper than electrochemical oxidation The resulting oxide film is very thin, generally about 0.5-4 μ m thick, soft and low wear resistance, so it can not be used alone The film has good physical adsorption capacity and is a good bottom layer for painting The protective layer obtained after chemical oxidation and coating can greatly improve the protective ability of parts 2 The anodizing method of aluminum alloy is to use aluminum or aluminum alloy as anode, lead, carbon or stainless steel as cathode, and electrolysis in the aqueous solution of oxalic acid, sulfuric acid, chromic acid, etc The film formation rate which is difficult to get by natural oxidation method can be obtained by anodizing with electric power In the process of oxide film formation, there are two processes at the same time: one is the formation of Al2O3 oxide film on the surface of aluminum plate; the other is the formation of oxide film accompanied by the dissolution of oxide film Only when the formation rate of the oxide film exceeds its dissolution rate can a certain thickness of oxide film be obtained The thickness of the film is generally 5-15 μ M The film is a double-layer structure composed of a dense barrier layer and a porous layer with a columnar structure The anodic oxide film can be divided into two categories: porous type - the film formed in sulfuric acid, oxalic acid and other acid electrolyzers and forming very fine pores perpendicular to the surface; barrier type - the very thin film without pores, which is usually used in electrolytic capacitors, etc generated in neutral salt electrolyzers such as ammonium borate 3 Micro arc oxidation, also known as plasma oxidation, is a kind of surface treatment technology for in-situ growth of ceramic layer on the surface of metal substrate on the basis of anodizing Its basic properties are similar to that of ceramics (corundum), with high hardness, wear resistance and high temperature resistance, as well as high insulation and acid alkali corrosion resistance 4 At present, the research of rare earth conversion film has made great progress at home and abroad, among which the rare earth passivation technology has been paid more attention because of its non-toxic, pollution-free and good anti-corrosion effect At present, the rare earth conversion membrane technology can be roughly divided into three categories: ① chemical method with strong oxidants and other film-forming promoters; ② chemical method combined with electrochemistry; ③ rare earth bohmite layer technology Adding strong oxidants such as H2O2, KMnO4, NH4) 2s2o4 and so on can greatly reduce the treatment time, and the solution treatment temperature is not high The chemical process with low temperature and short-time film-forming strong oxidants has the most development potential; the process combining chemical process and electrochemistry is cumbersome, and the solution treatment temperature is also boiling; the rare earth bohmite layer process also has relatively high treatment temperature High disadvantage 5 Organosilane treatment In the process of silylation, the aluminum alloy was immersed in this kind of dilute silylation solution for a certain time, then solidified at a certain temperature, and a coating of several hundred nanometers thick (much thinner than the traditional conversion coating) could be formed on the surface of the aluminum alloy The coating can effectively prevent various forms of corrosion of the aluminum alloy The dodecyl trimethoxysilane (DTMs) film was deposited on LY12 aluminum alloy by electrochemical technique The results show that DTMs silane reagent has chemical bonding with the surface of aluminum alloy, forming - sioal bond to realize film formation, and electrochemical impedance spectroscopy (EIS) test results show that compared with open circuit potential, the corrosion resistance of silane film obtained by cathodic potential deposition method is significantly improved 6 Phosphating primer treatment The base material of phosphating primer is composed of polyvinyl butyral resin as the main component, chromate and other antirust pigments and auxiliaries as well as phosphoric acid as the second component Before use, the two components are mixed evenly according to the specified proportion When spraying on the surface of aluminum plate, part of phosphoric acid is combined with metal aluminum, so that the metal surface and the film are integrated The film has a certain anti-corrosion ability and can be connected with the coating system The primer in the system has good binding ability, but the phosphating primer requires high construction conditions If you are not careful, the paint film will become brittle, resulting in serious consequences of large-scale paint dropping