| Aluminum is the most abundant metal elements in the earth, with low density, good electrical and thermal conductivity, high specific strength, has been widely used in the aerospace, marine transportation, automotive, construction and other fields. Aluminum belongs to active metal, can generate a thin layer of Al2O3 is film in the air fast. The passivation film which occurs in the air has certain protective effects on the corrosion of aluminum metal. But Al2O3 is amphoteric oxide, both soluble in acid and alkali, which is easy to be corroded in marine and other harsh environments. So it is unable to protect the aluminum and its alloys. The research of the surface protection of aluminum and its alloys has a very important theoretical and practical significance.This paper firstly adopts the method of micro arc oxidation. The surface treatment on 2A02 aluminum alloy is carried out. By micro-arc oxidation, a layer of oxide ceramic film can form on the surface of the valve metals. The ceramic membrane has the characteristics of high hardness, corrosion resistance, high temperature resistance and connects well with matrix. It can improve the corrosion resistance, high temperature resistance and abrasion resistance of 2A02 alloy. Micro-arc oxidation ceramic membrane surface density is high, the thickness of 20μm,can well insulated aluminum alloy substrate contact with the corrosive medium.Studies have shown that compared to aluminum alloy micro-arc oxidation ceramic membrane in seawater corrosion test and salt spray test of corrosion resistance are improved, the ceramic membrane mainly pitting corrosion in the corrosive medium.This paper studies the influence of the positive electrical parameters, negative electrical parameters and time on ceramic membrane thickness, porosity and corrosion resistance. The experimental results show that positive electrical parameters can promote the growth of ceramic membrane, while the negative electrical parameters can improve the energy of the positive pulse and stablize the effect of positive pulse discharging. Negative pulse parameters should be slightly smaller than the positive pulse. The product of time and a single pulse, which determines the effect of micro arc oxidation, has great influence on performance of ceramic membrane.Solute ions have great effect on the performance of micro-arc oxidation ceramic membrane. The uniformity of ion in the solution, stability of micro-arc oxidation reaction and pulse discharging energy were increased with the addition of dispersant sodium hexametaphosphate(Na PO3)6 in the solution. Adding stabilizer glycerol C3H8O3 can make micro-arc oxidation discharge more stable. So ceramic film thickness was increased, density increases, the corrosion resistance of ceramic membrane increases, corrosion rate decreases. PO3- ion of additives reacted with Al3+ ion of electrical breakdown generation of binder in ceramic coating,which can improve the mechanical properties of ceramic membrane,increase the isolation effect of substrate and corrosion medium,therefore, the corrosion resistance of ceramic membrane to further improve the performance and the corrosion rate decreased. It is inevitable to generate crack shape of holes and the crater of the volcano in the surface of the ceramic membrane in Micro-arc oxidation. These defects result in ceramic membrane in seawater corrosion and salt spray test of pitting corrosion. So it is necessary for ceramic membrane to hole sealing. The electrophoresis of Al(NO3)3 was adopted to enclose the ceramic membrane.A layer of amorphous Al2O3 film deposited on the surface of ceramic membrane.The surface of ceramic membrane was Completely covered.And produce mechanical engagement with ceramic membrane, forming composite coating.Through orthogonal test, the best electrophoresis of sealing process parameters were obtatined: double pulse, frequency 200 Hz, plus or minus pulse ratio for 40%, closed voltage for 85 V, electrolytic liquid concentration for 20 g/L, closed time 30 min, current density for 35~40 m A/cm2. With this process the coating thickness of the composite was up 50 um, surface porosity was close to zero. After 300 h neutral salt spray test, and seawater immersion experiment and electrochemical analysis experiment analyzing, results show that the corrosion rate of the composite coating in seawater is 0.0046 g/(m2·h), which is only one-fourth of the ceramic film. The corrosion rate of neutral salt spra6 y test is 0.0026 g/(m2·h), which is one fifth of the ceramic film.Micro-arc oxidation and electrophoresis composite coating was obtained on 2A02 aluminum alloy by micro-arc oxidation with the addition of(Na PO3)6 and C3H8O3 as modification and electrophoresis sealing. The surface of the coating was stable amorphous Al2O3 with low porosity. The thickness of the coating was 60μm. The coating can protect the aluminum alloy well by isolating the corrosive medium. |
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