| Aluminum alloy microchannel heat exchangers,as a kind of heat transfer equipment with low energy consumption and high efficiency,have been widely used in industrial refrigeration,automobile air-conditioning,household air-conditioner and other fields.Being subjected to condensate water and refrigerants which contain chloride ions and other impurities,Microchannel heat exchangers suffer serious corrosion problems.Cerium conversion coating is a kind of green and environment-friendly surface protection coating.In this thesis,cerium conversion coating was applied to the surface protection of aluminum alloy microchannel heat exchanger.The optimum cerium conversion coating formula and the processing conditions were determined by Orthogonal experiment on the surface of 3003 aluminum alloy,and the conversion coating was post-treated with phosphate.The surface morphology and composition of cerium conversion coating was observed and analyzed by SEM/EDS.The adhesion,hydrophilicity and crystal water content of cerium conversion coating was tested and studied.3.5%NaCl solution and 0.03 g/L(NH4)2SO4 solution were used to simulate two common corrosive environments for microchannel heat exchangers.The corrosion resistance properties of the prepared conversion coatings were studied in two solutions by electrochemical methods such as polarization curves and electrochemical impedance spectroscopy.Service life of cerium conversion coating was investigated by immersion test.The main conclusions of the thesis are as follows:(1)The cerium conversion coating was prepared by chemical immersion method on 3003 aluminum alloy used in microchannel heat exchanger.The content of oxidant,regulator and coupling agent in the coating formula were determined by Orthogonal experiment.The thickness of the coating was about 1.5 ?m.The conversion coating was post-treated in acid phosphate solution to inhibit the surface cracks of in coating and improve the film performance furtherly.(2)The composition of cerium conversion coating is mainly cerium hydroxide and oxide,in which the proportion of Ce(?)is about 72%.After phosphate post-treatment,the CeO2·2H2O in the coating is transformed into a stable CePO4,in which process part of the crystalline water in the coating is removed and the cracking of the coating is inhibited.After phosphate post-treatment the Ce(?)/Ce(?)content ratio in the coating,is increased,the electronic structure and surface polarity of the film is changed,and the hydrophilicity of the coating is improved.(3)The cerium conversion coating in Neutral Salt Spray test maintains integrity about 336 h.After phosphate post-treatment,the coating shows better corrosion resistance in the salt spray test.Based on the results of the corrosion immersion test,the low-frequency impedance variation,the equivalent circuit fitting parameters,the coating surface morphology and microstructure with scanning electron microscope,it can be inferred that,the lifetime of the conversion coating in 3.5%NaCl solution is about 18 d,the coating with phosphate post-treatment maintains a more complete structure and better shielding performance till 35 d.(4)In the immersion test of 0.03 g/L(NH4)2SO4 solution,the cerium conversion coating presents good performance with no corrosion products on the surface more than 90 d,the low frequency impedance and coating resistance keep invariable status.The results showed that the performance of the cerium conversion coatings in 0.03 g/L(NH4)2SO4 solution are much better than in the 3.5%NaCl solution.A dense hydrated alumina layer formed on the surface of the matrix in 0.03 g/L(NH4)2SO4 solution has corrosion inhibition effect. |
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