| 2024 Aluminum alloy(AA2024)is widely used in aerospace industry due to its peculiarities of high strength,high hardness and easy heat-treated.However,it is easy to appear the intergranulars corrosion due to the high content of copper in AA2024.To solve this problem,the chromate conversion film is often used as the surface treatment technology of the aluminum alloy in the aerospace industry.Recently,green environmental protection requires chromate conversion technology to be replaced by chromium free environment-friendly treatment technology.However,as we know,there is no effective environment-friendly chromium free conversion coating to meet the protection requirements of AA2024.Therefore,the development of chromium free conversion coating instead of chromate has become a technical difficulty of AA2024 surface protection.In order to solve this problem,the Ti-Zr conversion coating and silane film were prepared on the surface of aluminum alloy instead of chromate conversion coating.The formulation was adjusted and optimized by adding screened corrosion inhibitors in the conversion solution.Three high-performance chromium free conversion coatings,including Ti-Zr conversion coating,golden Ti-Zr conversion coating and silane film,were prepared on AA2024 surface.The corrosion resistance and protection mechanism of the conversion coating were also studied.The main results are summarized as following:1.The benzimidazole improved the anti-corrosion performances of AA2024 with a Ti-Zr conversion coating.When the concentration of benzimidazole is 4g/L,the Ti-Zr conversion coating had no obvious corrosion products after 168 h NSS test,showing excellent resistance to neutral salt spray.AA2024 substrate is well covered with the Ti-Zr conversion coating composed of the micron-sized particles.And the Ti-Zr conversion coating mainly consists of the metallic oxides(TiO2,ZrO2,Al2O3),metallic fluorides(ZrF4 and AIF3)and C-N complexes.More importantly,There is no apparent corrosion products or detects could be observed during the whole 168 h NSS test,indicating the significantly excellent corrosion resistance for AA2024 with Ti-Zr conversion coating compared with other chrome-free conversion coatings.The electrochemical measurements indicate that of the corrosion potential of the Ti-Zr conversion coating is reached-0.5972V,the corrosion current is reduced by three orders of magnitude,and the polarization resistance increases by two orders of magnitude.The results showed that the conversion coating has good anti-corrosion performances after 168 h NSS test.The Ti-Zr conversion coating is extended with immersion time,the charge transfer resistance first increased and then decreased,maximum at 240h.Immersion for 240h,the charge transfer resistance is more than three times that of the AA2024 substrate.Furthermore it showed that the Ti-Zr conversion coating has excellent corrosion resistance.2.The results showed that corrosion resistance of sodium benzoate and sodium hexametaphosphate remarkably improved the anti-corrosion performances of AA2024 with golden Ti-Zr conversion coating.The surface of the golden Ti-Zr conversion coating's colur is lighter,and no obvious corrosion products after 168 h NSS test.The golden Ti-Zr conversion coating composed of the micron-sized spherical particles,the film was uniform and without obvious defects,and the Ti-Zr conversion coating mainly consisted of the metallic oxides(TiO2,ZrO2,Al2O3),metallic fluorides(ZrF4 and AlF3).The electrochemical measurements indicate that the corrosion potential of the conversion film is increased by 273mV,the corrosion current is reduced by two orders of magnitude,and the polarization resistance increases by one order of magnitude after 168 h NSS test.3.A novel non-chromium silane film on AA2024 for aircraft applications was prepared and the silane hydrolysate based on A-187 was used as precursors.The results showed that the non-chromium silane film had excellent corrosion resistance and there was no obvious corrosion observed after 168h NSS test.The silane film mainly consists of C,O,and Si.The silane film corrosion potential(Ecorr)increased 154mv compared with the untreated,and the resistance of the film increased gradually with increasing immersion time.It indicated that the silane film can effectively inhibit corrosion of the AA2024 substrate,and provide good protection for AA2024.4.The combination of environmental conversion coating with the epoxy-rich aluminum coating is better than that of acrylic coating.The conversion coating with the epoxy-rich aluminum coating and acrylic coating consists of coating system after 1000h NSS test and no shedding,blister and rust,showed excellent corrosion resistance.The coating system after adhesion test showed that the Ti-Zr conversion coating and the golden Ti-Zr conversion coating with the two coatings are well matched to meet the requirements of the test standard GB9286-88.However,the silane film is inferior to the two kinds of coatings,and the degree of breaking is more than 4. |
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