| Aluminum and its alloys are widely used in aerospaces,automobiles,buildings,appliances,electronic equipments and so on due to its excellent properties.However,aluminum has a low reduction potential and is readily oxidized and corroded in the surrounding environments,which greatly limits the applications of aluminum and its alloys.Therefore,the surface protective treatment of aluminum alloys is crucial before their extensive applications.The most effective method for surface protective treatment of aluminum alloys is chromate conversion treatment in advance and then coated by electrostatic powder in the industrial production.The chromate conversion coatings(CCCs)have excellent corrosion resistance and adhesion to organic coatings.However,the chromate solution contains one or more sources of hexavalent chromium [Cr(VI)],which is both toxic and carcinogenic.This has led to several environmental and health-realted problems during the production,storage and application of chromate solutions.Thus,its application in electronic equipment has been banned by European Union.As a consequence,the non-chromate conversion treatments are necessary to be developed as alternative options of the chromate conversion process.The conversion treatment technology of Ti/Zr conversion coating(TZCC)is similar to that of CCC.Therefore,the TZCC has a great industrial prospect.However,the TZCC can’t replace the CCC at present mainly due to the corrosion resistance and adhesion to organic coating of the TZCC is inferior to those of the CCC.The studies of the formation mechanism and self-healing properties of the TZCC are still lacking.The research on the corrosion and adhesion mechanism of epoxy coating on the TZCC is reraly reported.In view of the above problems,this thesis prepared a golden,environmentally friendly TZCC on the surface of aluminum alloy.The growth process,formation mechanism and selfhealing property of the TZCC were systematically studied.Also,the effects of the TZCC on the adhesion and anti-corrosion properties of epoxy coated aluminum alloy had been investigated.The corresponding mechanisms were also discussed.The detailed research contents were shown as follows:1)The growth process and formation mechanism of TZCC: the growth process,chemical compsotion and microstructure of the TZCC were investigated by SEM,X-ray photoelectron spectroscopy(XPS),fourier translation infrared spectrum(FTIR)and transmission electron microscopy(TEM).The results showed that the optimal conversion treatment time was 50 s,when the TZCC was very compact and smooth according to the scanning electron microscope(SEM)observation.The TZCC was preferentially formed on the second phase particles and then spread out across the particles and surrounding areas,eventually covered the surface of aluminum alloy.The TZCC displayed an amorphous structure from TEM obsvervasion and was mainly composed of metal oxide,metal fluoride and metal organic complex based on the XPS analysis results.2)The corrosion resistance property of the TZCC: the corrosion resistance property of the TZCC was studied by electrochemical technique,neutral salt spray and scanning electrochemical microscope(SECM).The corresponding corrosion mechanism was also discussed.The results demonstrated that the electrochemical corrosion and long-time salt spray corrosion resistance of the aluminum alloy were improved by the TZCC due to the factor that the TZCC had an obvious inhibiting effect on the cathodic and anode reaction.The localized corrosion of the aluminum alloy was also inhibited by the TZCC.Furthermore,The TZCC showed better corrosion resistance than the CCC.3)The self-healing property of the TZCC: the self-healing ability of the TZCC with defects(drilling or/and scratch)was investigated by SECM and SEM/EDS.The results showed that the scratched TZCC was gradually ameliorated from the initial cracked configuration to more fillers with the increase of the immersion time in 3.5% NaCl solution.The self-healing ability could be chiefly related to the dissolving and redepositing of the metal organic complex.4)The effect of the TZCC on the adhesion performance of epoxy coating on aluminum alloys: the effects of the conversion time and drying temperature of the TZCC on the adhesive strength of the epoxy coating on TZCC treated aluminum alloy were investigated by pull-off tests and the adhesion mechanism of epoxy coating on TZCC was studied by TEM.The results showed that the surface microroughness and surface free energy of the aluminum alloy were significantly increased by the TZCC.The best adhesion property could be obtained when the TZCC conversion time was 50 s and the corresponding drying temperature was 80?C.The adhesion of epoxy coating on TZCC was mainly associated with mechanical interlocking,molecular bonding and diffusion theory.5)The effect of the TZCC on the corrosion resistance of the epoxy coating on aluminum alloy: the effects of the immersion time,chloride ion concentration and pH value on the corrosion resistance of the epoxy coated aluminum alloy treated in Ti/Zr based solution were studied by electrochemical workstation and compared with epoxy coating/CCC/aluminum alloy and epoxy coating/aluminum alloy.The results showed that the electrochemical corrosion resistance of the epoxy coating/TZCC/aluminum alloy was decreased with the increase of the immersion time and chloride ion concentration,while increased with the increase of the pH value of the corrosive solution.In addition,the electrochemical corrosion resistance of the epoxy coating/TZCC/aluminum alloy was better than epoxy coating/CCC/aluminum alloy and epoxy coating/aluminum alloy. |
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