Study On Microbial Corrosion And Residual Strength Of Al-Li Alloy

Aluminum-lithium alloy is considered as a new material in aerospace industry due to its excellent mechanical properties.When Al-Li alloy is used to manufacture aerospace or aviation machinery structural parts,different structural parts require corresponding strength and ductility.Therefore,Al-Li alloy usually needs heat treatment to improve the strength and ductility of structural parts,but on the other hand,the heat treatment process will affect the corrosion resistance of the alloy.In addition,due to the complex structure of the aircraft,it is easy to provide conditions for the reproduction of microorganisms,so that the structural components in the aircraft are corroded and damaged,resulting in the decrease of the strength,plasticity and toughness of the structural components,resulting in the decrease of the life of the structural components,which affects the safety of the aircraft.Based on the wide application of Al-Li alloy,it is necessary to study the corrosion resistance and residual strength of Al-Li alloy,so as to provide some reference for the subsequent corrosion resistance and application research of Al-Li alloy.In this paper,the corrosion resistance of 2060 new Al-Li alloy was studied by electrochemical test,surface observation and corrosion pit measurement.The residual mechanical properties and corrosion damage of the alloy were studied by tensile test.The specific research contents and main conclusions of this paper are as follows:The polarization curves,electrochemical impedance spectra and fitting data of2060 Al-Li alloy with three different heat treatment states in sulfate-reducing bacteria（SRB）solution were obtained by electrochemical test.The current densities of 2060-T8Al-Li alloy,2060-T4 Al-Li alloy and 2060-T6 Al-Li alloy were 13.32μA/cm²,2.769μA/cm²and 9.541μA/cm²,respectively.The corrosion characteristics are 2060-T4Al-Li alloy<2060-T6 Al-Li alloy<2060-T8 Al-Li alloy.The electrochemical impedance spectroscopy（EIS）and the fitted data after soaking in SRB solution for different days in each heat treatment state were observed.Combined with the morphology of subsequent alloy corrosion,it can be seen that the corrosion can be roughly divided into three stages.In the first stage,the corrosion rate of the alloy is fast,because in this stage,the alloy is directly corroded by corrosive elements in the solution because no biofilm is formed on the surface of the alloy.In the second stage,the corrosion rate was slow,because the biofilm had a certain protective effect after the formation of biofilm.In the third stage,the corrosion rate increased compared with that in the second stage,which was due to the destruction of the biofilm on the alloy surface at this stage,resulting in the decrease of its corrosion resistance.In addition,the BCSR theory in the biofilm also caused additional metal corrosion.The copper element in the alloy will lead to the change of the aggregation trend of cyclic sulfate bacteria on the surface of the alloy.The growth rate of biofilm on the surface of Al-Li alloy with different heat treatment states is also different.The growth rate of biofilm in the study is:2060-T8 Al-Li alloy>2060-T6 Al-Li alloy>2060-T4Al-Li alloy.The inconsistent growth rate of biofilm will also lead to different subsequent corrosion degrees of different heat treatment states.The XPS analysis showed that the composition of the corrosion film on the surface of Al-Li alloy was Al（OH）₃,Al₂O₃and Al₂S₃,and the main component was Al（OH）₃.The XPS results of the three Al-Li alloys showed that different heat treatment states could only affect the amount of corrosion products,but did not affect the type of corrosion products.The tensile tests of pre-corrosion specimens of 2060 Al-Li alloy under three heat treatment states were carried out to explore the variation law of residual mechanical properties of the alloy.The results show that with the extension of immersion corrosion time,the mechanical properties of the three heat-treated Al-Li alloys decrease in varying degrees.In addition,the relationship between the residual strength of the alloy and the depth of the corrosion pit is established by fitting with the depth of the corrosion pit,and the fitting formula of the linear equation is obtained,indicating that the corrosion damage is the main factor for the change of the residual strength of the alloy.