| Al-Zn-Mg alloy has been widely employed in transportation and aerospace industries for its high strength,good extrusion performance and excellent welding performance.However,the application of Al-Zn-Mg alloy has been limited due to its poor corrosion resistance.The material of this study is hot rolled of Al-4.2Zn-1.3Mg alloy.The microstructure of this alloy after different solution treatments were studied by using optical microscopy and scanning electron microscopy.The physical and mechanical properties of the alloy were tested by conductivity,hardness and tensile properties experiments.The corrosion resistance were tested by measuring corrosion rate,polarization curve,electrochemical impedance spectroscopy and intergranular corrosion depth of alloys through immersion corrosion experiment,intergranular corrosion experiment and electrochemical experiments.Finally,the kinetics of corrosion process and the behavior of intergranular corrosion were systematically analyzed in order to determine the best solution treatment.Experimental results show that the best single-stage solution treatment of Al-4.2Zn-1.3Mg alloy is 470?×2h.After the single-stage solution treatment and aging treatments,the hardness of the alloy reaches 105.5HV while the tensile strength and the yield strength are 354MPa and 312MPa respectively.In addition,the self corrosion potential of the alloy is-0.88V and the corrosion current density is 8.9×10-6A·cm-2.Compared with single-stage solution treatment,multi-stage solution treatment can decrease the fraction of the second phase and the extent of the recrystallization of the alloy.The tensile strength and the yield strength of the alloy increases by about 40MPa after 450?×1h+480?×0.5h and aging treatments.After different solution treatments,the corrosion resistance of the alloy increases as following order:single-stage solution treatment,double-stage solution treatment,three-stage solution treatment.The best corrosion resistance of this alloy was obtained through three-stage solution treatment.The intergranular corrosion depth of the alloy decreases from 118?m to50?m.Combined with the mechanical properties and corrosion resistance,the best solution treatment of Al-4.2Zn-1.3Mg alloy is 420?×2h+450?×1h+480?×0.5h+120?×24h.The alloy was immerged in 3.5%NaCl solution after this solution treatment.With the time prolonging,the corrosion rate decreases firstly and then maintains a stable value.After using D?t?=0.1543t0.5040.504 power function fitting,the theoretical corrosion rate is in line with the value above.The intergranular corrosion process of Al-4.2Zn-1.3Mg alloy starts from pitting initiation,and gets into the stable period,with the prolongation of corrosion time,corrosion products begin to accumulate at pitting holes,and the corrosion rate decreases gradually,then the pitting transforms into the intergranular corrosion. |
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