Preparation And Performance Of Composite Coatings Formed By Chemical Conversion And Sol-grl Technology Of 3003 Aluminium Alloy

Microchannel heat exchangers are widely used because of their efficient heat transfer efficiency and compact design.AA3003 is the main substrate of the current aluminum alloy microchannel heat exchanger,and there are still certain corrosion problems when used in harsh environments.In order to effectively solve the corrosion failure problem and extend the service life of the microchannel heat exchanger,The research mainly focuses on the surface corrosion protection of AA3003.The main research content is to prepare a coating on AA3003 to extend the service life of the microchannel heat exchanger.In this thesis,three kinds of coatings were prepared on AA3003,which includes phosphorus-zirconium-vanadium coating prepared by chemical conversion（Zr/P/VCC）,silicon-titanium coating prepared by sol-gel（Si-Ti sol-gel）,and composite coating prepared by combining the two technologies（Zr PVCC/Si-Ti sol-gel）.The corrosion resistance of the three kinds of coatings was compared using polarization curve tests in 3.5 wt.%Na Cl solution.And the long-term corrosion resistance of the three kinds of coatings was compared using EIS tests in three corrosive environments,including neutral salt spray,immersion in 3.5 wt.%Na Cl（p H=7）solution,and immersion in 3.5 wt.%Na Cl（p H=3,HAc regulation）solution.The surface morphology of the coatings were observed by SEM/EDS,and the coating-forming mechanism was analyzed by XRD,XPS,FI-IR tests.The specific conclusions are as follows:（1）Phosphorus-zirconium-vanadium coating（Zr/P/VCC）was prepared by chemical conversion on AA3003.It was observed that the surface of Zr/P/VCC was covered with nodular particles,and the coating was uniform and dense,without obvious defects such as s holes and cracks.The coating is mainly composed of metal oxides Zr O₂,V₂O₅,and phosphide Zr（HPO₄）₂ and VOPO₄.The corrosion current density of Zr/P/VCC is reduced by about half an order of magnitude compared with AA3003.And the low-frequency impedance increases by an order of magnitude compared with AA3003.The long-term corrosion resistance test showed that the coating was resistant to neutral salt spray for 480 h,neutral solution（3.5 wt.%Na Cl,p H=7）for 240 h,and acidic solution（3.5 wt.%Na Cl,p H=3）for 96 h.（2）Silicon-titanium coating（Si-Ti sol-gel coating）was prepared by sol-gel technology on AA3003.Through the optimization of precursor solutions and preparation processes,Si-Ti sol-gel coating was obtained with smooth,dense and defect-free.The main composition of the coating is Si O₂ and Ti O₂,of which Ti O₂ esists in the form of anatase and rutile.Due to the characteristics of Ti O₂crystal structure,Si-Ti sol-gel coating had superior chemical stability.The coating and substrate are connected by covalent bonds Si-O-Al.The corrosion current density of Si-Ti sol-gel coating is reduced by about an order of magnitude compared with AA3003.And the low-frequency impedance increases by two orders of magnitude compared with AA3003.The long-term corrosion resistance test showed that the coating was resistant to neutral salt spray for 1680 h,neutral solution（3.5 wt.%Na Cl,p H=7）for 1680 h,and acidic solution（3.5 wt.%Na Cl,p H=3）for 720 h.（3）The Zr PVCC/Si-Ti sol-gel composite coating was prepared by combining chemical conversion with sol-gel technology on AA3003.By observing the micromorphology of the surface and section of the coating,it was found that the surface of the composite coating is smooth and dense,and the cross-section has an obvious double-layer structure.The existence of Si-O-Me,Si-O-Si,Ti-O-Ti and Me-O bonds proves that the upper sol-gel layer is combined with the metal oxides on the surface of AA3003 and Zr PVCC.The corrosion current density of Si-Ti sol-gel coating is reduced by about an order of magnitude compared with Zr PVCC,and the corrosion current density of composite coating is reduced by two orders of magnitude compared with Zr PVCC.The low-frequency impedance increases by two orders of magnitude compared with AA3003.The long-term corrosion resistance test showed that the coating was resistant to neutral salt spray for 1680 h,neutral solution（3.5wt.%Na Cl,p H=7）for 1680 h,and acidic solution（3.5 wt.%Na Cl,p H=3）for2160 h.Comparing the long-term corrosion resistance of Si-Ti sol-gel coating and composite coating in the three corrosion environments（Neutral salt spray,3.5 wt.%Na Cl neutral solution and 3.5 wt.%Na Cl acidic solution）,it was found that the acid resistance of the composite coating was significantly improved.