| AA2024-T3 aluminum alloys are widely used in aircraft industry due to their high strength-to-weight ratio.However,the presence of certain second phase particles in its matrix reduces the corrosion resistance of the aluminum alloys.One of the effective methods in corrosion prevention on aluminum alloys is applying anticorrosive organic coatings.The most commonly used coatingsarechromate-based coatings.But because of the high toxicity of chromates,the development of chromium-free coatings for corrosion protection of aircraft aluminum alloys has become a popularresearch direction.Rare earth organic inhibitors which are the combination of rare earth cations and organic ligands has shown broad prospects among chromate substitutes.In this thesis,two kinds of new rare earth organic inhibitors(cerium tartrate and cerium cinnamate)and rare earth inorganic inhibitors(cerium nitrate)were added into the epoxy coating,respectively.The protectiveproperty of the coatings containing different cerium salts on AA2024 alumium alloys and the inhibition mechanismof rare earth organic inhibitors in epoxy coating were studied.Then,based on the above study,AA2024 aluminum alloys were pretreated by silane to improve the whole protective performance of the coating system.The electrochemical behaviors of coating/aluminum alloy samples in 3.5 wt.%NaCl solution were studied byelectrochemical impedance spectroscopy technology.The influence of cerium salts on physical and chemical properties of epoxy coatings was researched by weight gain,fourier transform infrared spectroscopy,differential scanning calorimetry test methods.The corrosion inhibition process of cerium salts in epoxy coatings was studied by X-ray photoelectron spectroscopy and inductively coupled plasma mass spectrometry.The main conclusions are as follows.1.The epoxy coatings containing cerium tartrate have excellent protection property for aluminum alloy substrate compared to the epoxy varnish coatings.With the content of cerium tartrate(0 wt%,1.5 wt%,3 wt%,5 wt%)increasing,the low-frequency impedance of the coating is increased,and the protective performance of the coating is improved.The reason probably is that during the process of the corrosive solution penetrating through the coating,the Ce3+ releasing from the coating migrates to the surface of the substrate together with the water transfer.When corrosion occurs on the surface of the substrate,the pH of the micro cathode area increases,some Ce3+is oxidized to Ce4+,and a film of cerium oxide(Ce2O3 and Ce02)and cerium hydroxide(Ce(OH)3 and Ce(OH)4)is formed on the substrate.This can inhibit the further development of the substrate corrosion and the appearance of new corrosion sites.2.Epoxy coatings containing ceriumcinnamate(5 wt%)and cerium tartrate(5 wt%)provide better protection for aluminum alloy substrates compared with coatings containing cerium nitrate(5 wt%).Because the solubility of cerium cinnamate and cerium tartrateismuch lower than that of cerium nitrate in water,these two rare earth organic inhibitors can act as fillers to block the defects in the coating,which may decreasecoating porosityand enhance the barrier performance of the coating.When the epoxy coating with cerium tartrate and cerium cinnamate absorbs water,the cerium ionsslowly release from the coating,which could inhibit the corrosion of the aluminum alloy substrate for a long time.Cerium tartrate and cerium cinnamate have no significant difference in improving the protective performance of epoxy coatings.3.Ce3+ in the rare earth can affect the cross-linking degree and network structure of the polymer during curing of the epoxy resin.Glass transition temperature of the coating increase with cerium salts adding into it,and Ce3+ canpromoting the organicpolymerization during the ring opening of the epoxy rings.As a result,the crosslinking degree of the polymer in the coatings increases,and the porosity decreases,so as to improve the overall protective performance of the coating.4.Cerium tartrate was added into the silane pretreatment solution,and silane pretreatment film was prepared on the aluminium alloy,then the epoxy coatings containing cerium tartrate(5wt%)were coated.A strong Al-Si-O bond is formed between the aluminium alloy substrate and the silane film,which significantly improves the adhesion between the coating and the substrate.Compared with thesamplewithout silane pretreatment,the adhesion between pretreated substrate and coating is improved by nearly 6 Mpa,and the low impedance |Z|0.01Hz of the sample isincreased by one order of magnitude,and the protective performance of the coating systemis obviously improved. |
