| The application of vacuum die casting AM50 magnesium alloy in automobile and aviation industry becomes a research hotspots because of its excellent machinability,high specific strength and specific stiffness,good damping performance and liquid forming ability.However,the application of magnesium and magnesium alloy has been limited due to its poor corrosion resistance.In order to study the corrosion behavior of vacuum die casting AM50 magnesium alloy,as-cast state and two kinds of heat treatment conditions including T4 state and T6 state were chosen as research object.The phase composition,microstructure and element constitution were studied by using XRD,scanning electron microscopy and energy spectrometer.The corrosion resistance was tested by measuring corrosion rate,polarization curve and electrochemical impedance spectroscopy of alloy through static weight loss and electrochemical experiments.Finally,the kinetics of corrosion process and the corresponding corrosion mechanism were systematically analyzed.Experimental results show that second phase existed on grain boundary of vacuum die casting AM50 magnesium alloy dissolves into the matrix after solution treatment,which results in a slight increase in grain size and a clear grain boundary.Second phase is precipitated from the grain boundary of ?-Mg phase under different aging time at 170°C,and its number and size are related to the length of aging time.But with the increase of aging temperature,second phase can precipitate faster and finally forms a thick network structure under different aging time at 230°C.The corrosion rate of vacuum die casting AM50 magnesium alloy in NaCl solution with mass fraction of 3.5% is 1.6252mm/a,and the main corrosion products are Mg(OH)2,Mg6Al2(OH)18·5H2O and a small amount of Mg17Al12.The corrosion rate increases to 2.5349mm/a after T4 heat treatment,which is 56.0% higher than that of as-cast alloy.The corrosion rate of alloy is related to the aging temperature and holding time under T6 heat treatment.The corrosion rate of alloy reaches its lowest after aging at 170°C for 12 h.The corrosion rate of alloy is more than 10.0mm/a after aging at 230°C for 4h,the corrosion resistance of alloy sharply decreases.The electrochemical corrosion behavior of alloy under different states is similar.There is no obvious change rule of corrosion potential,but the change trend of corrosion current density is almost consistent with corrosion rate.A capacitive arc exists in intermediate and high frequency region of electrochemical impedance spectroscopy,respectively.The arc in low frequency region is related to inductive behavior.The corrosion rate of alloy obtained by piecewise power function fitting is coincide with the measured values.Piecewise function can better reveal the corrosion process. |
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