First-principle Calculations And Experimental Study On Micro Galvanic Corrosion Behavior Of Precipitates In Mg Alloy Containing Rare Earth

Rare earth alloying is one of the effective ways to improve the mechanical properties and corrosion resistance of Mg alloys,but the second phase of rare earth usually forms micro couple with Mg matrix,which induces local corrosion of Mg alloys.In this paper,the experimental characterization and the first principles calculation are combined to study the mechanism of the influence of the light rare earth elements Ce and Nd and the heavy rare earth elements Gd and Y on the corrosion behavior of Mg alloys.The work function and surface energy change of the second phase and the base phase,the relationship between the electronic properties of the second phase and the matrix phase,the interface energy of the second phase and the matrix and the corrosion behavior of Mg alloys are investigated.Mg-0.3%Ce,Mg-0.3%Nd,Mg-3.5%Gd,Mg-3.5%Y binary Mg alloys containing rare earth were formed by adding 0.3wt.%Ce and 0.3wt.%Nd,3.5wt.%Gd and 3.5wt.%Y in pure Mg respectively.The morphology,composition and structure of precipitates in binary Mg alloys containing rare earth were studied by optical microscope(OM),scanning electron microscope(SEM),energy spectrometer(EDS)and transmission electron microscope(TEM).It is found that the grain size of Mg alloys is obviously refined after adding rare earth elements.The amount of the second phase in the alloys is small,and the corresponding rare earth phases are long and granular Mg₁₂Ce phase,irregular Mg₁₂Nd phase,elliptical Mg₃Gd phase and sporadic granular Mg₂₄Y₅ compound,respectively.Scanning electron microscope(SEM)and scanning electrochemical microscope(EC-AFM)were used to observe the corrosion of Mg alloys containing rare earth.It was found that the pitting corrosion was dominant in pure Mg,and the filamentous corrosion occurred in Mg-Ce and Mg-Nd alloys.The local corrosion diffused to the surrounding,leading to large area corrosion.The second phase Mg₁₂Ce and Mg₁₂Nd had no obvious micro galvanic effect in the corrosion process of Mg alloys.Combined with the electrochemical test results,it is found that the corrosion rate of Mg-Ce and Mg-Nd alloy is faster than that of pure Mg,and the corrosion rate of Mg-Ce alloy is faster than that of Mg-Nd alloy.The corrosion of Mg-Gd and Mg-Y alloys is characterized by pitting corrosion caused by micro galvanic corrosion.Mg₃Gd phase is used as cathode phase to promote the corrosion of surrounding matrix;Mg₂₄Y₅ dissolves as anode phase during corrosion.The corrosion resistance of Mg-Gd and Mg-Y alloy is better than that of pure Mg,and the corrosion resistance of Mg-Gd alloy is better than that of Mg-Y alloy.The first principles calculation results show that the addition of Ce and Nd reduces the binding energy.The density of states between the second phase and Mg matrix,indicating that the stability of the second phase is higher than that of the matrix,and the stability of pure Mg is the worst.The calculated results of electronic work function and surface energy of different crystal planes of the second phase and matrix phase show that the larger the work function is,the smaller the surface energy of corresponding crystal plane is.The electron work functions of Mg₁₂Ce and Mg₁₂Nd are close to those of the matrix phase,which is the main reason why their micro couple effect is not obvious.The test results of SKPFM are basically consistent with the variation of work function between the two phases.The addition of rare earth elements Ce and Nd reduces the electronic work function of Mg(0001)substrate.The electronic work function of Mg-Ce and Mg-Nd alloys decreases from 3.66e V to 3.53e V and 3.58e V respectively after adding Ce and Nd.The corrosion behavior of Mg-Ce and Mg-Nd is mainly affected by rare earth elements Ce and Nd,which represent the surface stability.The first principles calculation results show that the addition of heavy rare earth elements Gd and Y improves the stability of the second phase and matrix phase.By calculating the binding energy and electronic density of states of Mg₃Gd,Mg₉₅Gd,Mg₂₄Y₅,Mg₉₅Y,?-Mg,the order of stability between the second phase and the matrix phase is:Mg₃Gd>Mg₉₅Gd,Mg₂₄Y₅>Mg₉₅Y.The calculated results of different surface electron work functions of the second phase and the matrix phase show that the work function of Mg₃Gd is higher than that of the matrix phase,and it is not easy to lose electrons and act as the cathode phase in the corrosion process.The work function of Mg₂₄Y₅ is lower than that of the matrix phase,and it is easy to lose electrons and act as the anode phase,the results show that the interfacial energy between Mg₂₄Y₅ and matrix is higher than that of Mg₃Gd,and the micro galvanic corrosion is more intense.It can be seen that the micro galvanic corrosion behavior of the second phase and the matrix is mainly affected by the work function of the second phase and the interface energy of the two phases.