| Al-Li alloy has high specific strength and stiffness,and can be used as the best structural material in aerospace field.However,due to the presence of alloying elements and impurities,Al-Li alloys will produce non-uniform structure during as-cast solidification and thermomechanical processing,and micro-electrical coupling between different phases will easily form,which will lead to local corrosion of the alloys.In order to improve the corrosion of Al-Li alloy in practical application,this paper combines anodic oxidation technology with silanization technology to prepare anodic oxidation/silane composite film on its surface.SEM,AFM,FTIR and XPS were used to characterize the structure,morphology and chemical composition of the film.The corrosion resistance and corrosion mechanism of the film were analyzed by electrochemical testing and etching experiments.Firstly,tartaric acid-sulphuric acid environmentally friendly solution system was used to treat 2198-T851 Al-Li alloy by anodizing.The effects of anodizing voltage on the growth behavior,morphology and corrosion resistance of anodized film were studied.The results show that the anodic oxide film is porous structure?pore size is about 10 nm?,and there were some defects on the surface.The main components of the film were Al2O3,AlOOH and Al2?SO4?3.The pitting potential of 2198-T851 Al-Li alloy was-0.68V,and there were no pitting potential in TSA samples;the corrosion potential of TSA samples moved in the positive direction as that of the matrix,and the corrosion current density decreased by one order of magnitude;the low frequency impedance of the 2198 matrix was 103.84?·cm2,and the low frequency impedance of the alloy increased by two orders of magnitude after TSA treatment.The corrosion resistance of the anodic oxide film is related to the thickness of the barrier layer and the surface defects,and the existence of surface defects can significantly reduce the corrosion resistance of the oxide film.After 4 days of immersion,the low frequency impedance of the unsealed anodic oxide film rapidly decreased to the same order of magnitude as that of the substrate,and the barrier layer no longer had the ability of corrosion protection.Secondly,the porous structure and crack defects of the anodic oxide film were repaired by using organic-inorganic mixed silane?TEOS-GPTMS?.The results show that a discontinuous silane film was formed when the anodized film was directly subjected to silanization treatment.The effect of silane film formation is determined by the structure and morphology of the anodic oxide film.The smaller the thickness and the less surface defects,the better the silylation sealing effect.Compared with TSA samples,the corrosion potential of TSAS samples shifted positive direction and the corrosion current density decreased.The low frequency impedance and high frequency impedance of TSAS samples were higher than those of TSA samples.The corrosion protection ability of the porous layer of TSAS sample was invalid after 3 days immersion,and the corrosion protection ability of the barrier layer of TSAS sample was invalid after 14 days immersion.After 21 days immersion in 0.1M NaCl solution,the low frequency impedance value of TSAS sample was one order of magnitude higher than that of TSA sample.The TSA samples had been completely corroded after 21 days immersion,while the TSAS samples only had local corrosion after 21 days immersion.Finally,the content of hydroxyl groups on the surface of anodic oxide film was increased by hydrothermal method,and then silanized and sealed.The results show that after hydrothermal pretreatment,the porous structure of the anodic oxide film changed into petal structure,and the proportion of AlOOH/Al2O3 increased.After hydrothermal hydroxylation pretreatment and silylation treatment,a smooth and compact silane film was deposited on the surface of the anodic oxide film to completely coat the porous structure.The corrosion potential of TSAHS sample was 136.9 mV higher than that of2198 Al-Li alloy matrix,and the corrosion current density decreased by three orders of magnitude.The porous layer of TSAH sample has failed after 12 hours of immersion,the barrier layer has failed after 7 days of immersion,and comprehensive corrosion has occurred after 21 days of immersion.After 14 days of immersion,the porous layer of TSAHS gradually failed,and the corrosion protection ability of the film to the substrate surface remained unchanged after 21 days of immersion.After 21 days of immersion,the low frequency impedance of TSAHS sample remained 105.70?·cm2,with only one pitting hole on the surface.This fully shows that the silane/anodic oxidation composite film prepared by hydrothermal pretreatment has good long-term corrosion resistance. |
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