Study On Surface Ceramization Of 2024 Aluminum Alloy At Low Temperature And Its Mechanism

2024 aluminum alloy with low density,high strength,good plasticity,heat treatment and other advantages,is widely used in aerospace,engineering machinery,national defense and other fields.At present,copper or steel shell casings are mainly used in China,and the lightweight weapon is one of the methods to improve the combat ability of individual soldiers.The military shell casings made of 2024 aluminum alloy with high strength and toughness can increase the ammunition carrying capacity of individual soldiers.However,the low wear and corrosion resistance of2024 aluminum alloy restricts the application of high strength and toughness 2024 aluminum alloy.In order to improve the wear resistance and corrosion resistance of 2024 aluminum alloy,In this paper,a dense and uniformγ-Al₂O₃ ceramic film was prepared on the surface of 2024aluminum alloy,and the preparation process,microstructure,mechanical properties and corrosion properties ofγ-Al₂O₃ ceramic film were studied.The following studies were carried out:（1）Different sulfuric acid concentration（100 g/L,200 g/L,300g/L）,different voltage（20 V,24 V,26 V,30 V,34 V）,different time（30min,40 min,50 min,60 min,70 min）effect on microstructure and properties of ceramic coatings on 2024 aluminum alloy.The results show that the 2024 aluminum alloy sample prepared at 32 V,60 min,200 g/L H2SO4 has a more smooth surface and high density.The maximum thickness and hardness of ceramic film are 62.14μm and 273.14 HV,respectively.The ceramic film has the best corrosion performance,the maximum self-corrosion potential is-0.60 V,the minimum self-corrosion current density is 1×10^-6 A/cm².The microstructure and properties of ceramic film prepared under these conditions are the best.（2）The effect of step voltage process on the microstructure and properties of 2024 aluminum alloy surface ceramic film was studied.The results show that the ceramic membrane with constant voltage has smooth surface and large pore size,while the ceramic membrane with stepped voltage has fish-scale surface and small pore size.The maximum hardness of the ceramic film is 289 HV,the maximum self-corrosion potential is-0.54 V,and the minimum self-corrosion current density is 1×10^-7 A/cm²The electrochemical impedance spectroscopy（EIS）analysis shows that the ceramic films have the same corrosion interface structure.The capacitive circle radius of the ceramic films prepared at the stepped voltage of 30 V and 60 min is the largest,and the maximum thickness of the ceramic film barrier layer is 132.4 nm.The results show that the ceramic film has the best corrosion resistance when the step voltage is 30 V and 60 min.（3）The effects of three additives（Al₂（SO₄）₃,Ce（SO₄）₂,Ce（SO₄）₂and citric acid C₆H₈O₇）on the microstructure and properties of 2024aluminum alloy surface ceramic film were investigated.The results show that the three additives can improve the properties of ceramic membrane.The hardness of ceramic film prepared by aluminum salt additive and cerium salt additive reaches 338 HV,which is 60 HV higher than that of ceramic film prepared by sulfuric acid electrolyte.The hardness of cerium salt and citric acid composite additive reaches 372.7 HV,which is 100 HV higher.Adding organic acid（C₆H₈O₇）in electrolyte containing cerium salt can form cerium organic citric acid complex ceramic film during the ceramization process of 2024 aluminum alloy.The cerium organic citric acid complex ceramic film has the highest self-corrosion potential and the lowest self-corrosion current density,which are-0.18 V and 5.4×10^-7A/cm²,respectively,and the best corrosion performance.The Nyquist diagram of cerium-organic citric acid complex ceramic film shows the largest capacitance-reactance circle radius and the maximum barrier thickness of ceramic film is 369 nm,which indicates that the ceramic film prepared with cerium salt and citric acid composite additives has the best corrosion resistance.