Study On The Corrosion Mechanism Of Two Typical Mg Alloys Of EW75 And ZK60

As the lightest structure material of metal,Mg and Mg alloys are popular because of their many advantages,such as low density and high specific strength and stiffness.However,the electrode potential of Mg is too low to keep Mg and Mg alloys from corroding,and the oxide film on the surface is not compact to protect Mg and Mg alloys,which limits the application of Mg and Mg alloys.Presently,Mg-RE alloys have become increasingly attractive.Mg-RE alloys exhibit high strength and good creep resistance,especially at elevated temperatures.Meanwhile,REs change the microstructure and improve the corrosion resistance of Mg alloys.Therefore,Mg-7Gd-5Y-1Nd-0.5Zr(EW75)and ZK60 have been chosen as the materials to study the influence of second phase,twins and texture on the corrosion behaviour.The micro-galvanic corrosion between matrix and second phases of cast EW75 has been studied.It is found that the second phases are more active than matrix in cast EW75.In 3.5 wt.%NaCl solution,the second phases act as anodes and dissolve preferentially.Initially,the second phases dissolve deeper with increasing immersion time.Finally,the micro-galvanic corrosion between the second phases and matrix becomes weak due to the coverage of corrosion products on the alloy's surface.The size,distribution,and volume fraction of second phases are adjusted using different heat treatment processes.It is found that the corrosion resistance of EW75 alloy first becomes better with increasing the volume fraction of second phases.However,when the second phases along the grain boundaries are dissolved into matrix entirely,the corrosion resistance of the alloy slightly declines.Overaging EW75 alloy exhibits the best corrosion resistance owing to the improved surface film by the substantial and homogeneous second phases.In the case of ZK60,many rod-shaped second phases appearing as the eutectic phases along the grain boundaries are dissolved during the homogenizing treatment process.The quantity of second phases increases with increasing heat treatment temperature and retaining time.Consequently,the corrosion resistance of ZK60 alloy decreases with increasing quantity of second phases.The influences of twins on the corrosion resistance of EW75 and ZK60 alloys have been studied.It is found that the micro-galvanic corrosion between twins and matrix occurs in both alloys.In EW75,the micro-galvanic corrosion improves the property of the surface film and increases the corrosion resistance of alloy.Differently,in ZK60,the micro-galvanic corrosion deteriorates the corrosion resistance of alloy due to the poor property of surface film.Mg-RE alloys have no strong texture.Thus,only the influence of texture on corrosion resistance of extruded ZK60 has been studied.It has been found that the ED plane is {1010}texture,TD plane is {0002} texture and there is no strong texture in ND plane.Hydrogen evolution and weight loss measurement results exhibit the best corrosion resistance of TD plane and the worst corrosion resistance of ND plane.The reasons for the corrosion resistances of three planes are that:1)orientations of ND plane are non-unified and that of TD and ED planes are unified;2)the Zr-enriched areas in ND plane are centralized and that in TD and ED planes are dispersive,the Zr-enriched areas in TD plane are most continuous.The surface films naturally formed in air and corrosive medium on the surface of EW75 have been studied.It is found that both surface films formed in air and corrosive medium contain RE elements.The surface film formed in corrosive medium exhibits special colors,namely,golden on the cast and homogenizing treatment EW75,and black on the overaging EW75.The surface and cross-sectional morphologies of the surface film on the overaging EW75 show that the film keeps intact and protective in a long term of immersion.Meanwhile,self-healing ability of the surface film has been found during immersion test.