Study On Corrosion Resistance Of Al-7.0Zn-2.9Mg Alloy And Polyurea Coating Protection Of Aluminum Alloy

The Al-Zn-Mgalloy（7xxx）is an important series of the high strength aluminum alloy.The research on the composition,structure,heat treatments and properties of the high-strength aluminum alloys has always been an important work.In this dissertation,single-stage aging（T6）,two-stage aging（T73）and the retrogression and re-aging（RRA）were carried out for 7A52（752）and Al-7.0Zn-2.9Mg（762）,which are the two components of a new kind of high-strength aluminum alloy laminates.The corrosion resistance of these two alloys was studied.The corrosion resistance of the modified polyurea coating was studied for the background of polyurea coating protection of aluminum alloy in the fields of ground armor and transportation.Pitting,intergranular corrosion and exfoliation corrosion experiments were carried out on the two alloys of different aging systems.However,it is difficult to distinguish the corrosion resistance by the observation of the corrosion depth in intergranular corrosion test and the surface morphology in exfoliation corrosion test.Combined with the data of pitting potential（E_b）or corrosion current density(I_corr)from the polarization curve,the comprehensive analysis of local corrosion resistance can be obtained:762>752,T73>RRA>T6.By comparing the stress corrosion resistance coefficient(I_ssrt)of the 762 alloy with different aging systems,the relationship between stress corrosion resistance is T73>RRA>T6.For the 762 alloy,combined with the TEM images,the relationship of corrosion resistance between the aging system and the organizational structure can be obtained.When the grain boundary precipitation phase（MgZn₂）is continuously distributed in chain shape and the precipitation phase size is small,the corrosion sensitivity is high;when the grain boundary precipitation phase（MgZn₂）is distributed in granular shape and the precipitation phase size is large,the corrosion sensitivity is weak.By combining the macroscopic morphology and EIS data of the 762 alloy after soaking in the EXCO solution,the following conclusions can be drawn:1)The changes in the characteristics of EIS reveal that the local corrosion behavior develops from pitting to intergranular corrosion and exfoliation corrosion.The interface of corrosion undergoes the dissolution of oxide film,formation and exfoliation of corrosion product;2)By comparing the appearing time of the minimum value of the charge transfer resistance（R_t）and the polarization resistance（R_p）in the Bode diagram,the relationship of the aging system and corrosion resistance is T73（12 h）>RRA（8 h）>T6（2 h）.After doping the Al₂O₃ powder,the hardness of the polyurea coating gradually increases,the contact angle gradually decreases meaning that the wetting property gradually increases.The results of the EIS during soaking in 0.1 mol/LNaOH solution showed that the doped Al₂O₃ powder can improve the corrosion resistance of polyurea coating,but the excessive doping of Al₂O₃ powder may reduce the protective ability.The relationship between the doping amount and the corrosion resistance of the coating is 5 wt.%>10 wt.%>15 wt.%>0 wt.%.After doping the carbon nanotubes,the surface’s hardness of the polyurea coating gradually increases,and the adhesion on the aluminum alloy substrate is slightly enhanced.The corrosion resistance of the coating after doping with carbon nanotubes was significantly improved,and the relationship between corrosion resistance and doping amount is 1.0 wt.%>0.5 wt.%>0 wt.%.