| 2024 aluminum alloys are widely used in the aerospace and automotive transportation industries because of their light weight,high strength and resistance to damage.The addition of other elements such as copper and magnesium gives 2024 aluminum alloys good mechanical properties,but they are prone to galvanic corrosion due to the difference in potential between them and with the substrate.In order to improve the corrosion resistance of 2024 aluminum alloy and to maintain a good adhesion state between the substrate and the coating during subsequent coating,the industry usually performs a series of surface treatment processes on 2024 aluminum alloy.Among a series of chemical passivation methods for metals,hexavalent chromium passivation has good corrosion resistance,but is highly carcinogenic,a highly toxic pollutant,and does not meet the requirements of environmental protection,while different chromium-free passivation methods have varying corrosion resistance.The common trivalent chromium passivate solution is mainly used for 6 and 7 series aluminum alloys,while the protection effect for 2 series aluminum alloys is general.In view of the above problems,it is very important to develop a green,efficient and corrosion-resistant trivalent chromium passivate solution.In this thesis,under the premise of Cr-Zr-F passivation system,the main components and concentrations of the passivation solution were selected and optimized by singlefactor test and Box-Behnken method,and the results were as follows:0.15 mol/L Cr(OH)SO4,0.03 mol/L H2ZrF6,0.01 mol/L NH4HF2,and the order of the effect of each component concentration on the passivation film The order of the effect of the concentration of each component on the salt spray resistance time was:Zr(Ⅳ)concentration>F-concentration>Cr(Ⅲ)concentration.After the main components were determined,different types and concentrations of corrosion inhibitors were added to them,and the best corrosion resistance of the film was determined when 1 g/L of HS-DT was added.The addition of corrosion inhibitor can improve the denseness and uniformity of the film to a certain extent,as shown by the white light confocal 3D morphometer and SEM.The surface compositions of the passivated film obtained by XPS are Cr2O3,Al2O3,ZrO2,etc.,and the presence of ZrN on the surface of the specimen with HS-DT may prove the presence of HS-DT.Single-factor test and Box-Behnken method were used to optimize the process parameters to obtain the process:passivation temperature of 38℃,pH of 3.80,passivation time of 6 min,and the order of the influence of each parameter on the salt spray resistance time of the passivation film was obtained:pH>temperature>passivation time.Finally,the passivation film was tested and the structure of the film was investigated by ToF-SIMS,and the thickness of the film was about 90 nm.The outer part of the film mainly consisted of Cr2O3 and ZrO2,and the inner part mainly consisted of Al2O3 and AlO-F compounds.After spraying TB06-9 primer on the surface of the passivated specimen,the impact resistance and coating adhesion were tested,and the impact resistance of the coating was 50 cm,and the scratching and wet adhesion met the requirements.The corrosion resistance of the trivalent chromium passivation film developed in the thesis was found to be up to 168 h without pitting when compared with a typical commercially available passivation solution using a 100 mm×50 mm×l mm passivation specimen.In electrochemical tests in 3.5%NaCl solution and 0.1 mol/L Na2SO4 solution electrolyte,the corrosion current density of the developed passivate film was lower and the corrosion potential was higher. |
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