Study On Corrosion Mechanism Of Biological Magnesium Alloy Based On Electronic Work Function Theory

Magnesium alloy materials have been widely used in biomedicine because of their unique properties,such as light weight,small density,good process performance and biological performance.However,magnesium alloys are very easy to be corroded,which hinders its large-scale application.Corrosion problem is one of the many problems that people face in the production process of metal materials.The most typical type of corrosion is electrochemical corrosion.Through the research on the electrochemical corrosion of materials,the fundamental mechanism of corrosion is discussed,and the means that can really help materials enhance corrosion resistance are always the common pursuit and goal of scientific researchers in this direction.Electronic work function is a physical quantity closely related to gain and loss of electrons.By calculating the electronic work function of different terminal surfaces of various materials based on the first principle of density functional theory,the electrochemical properties of materials can be analyzed and explored,and the causes of corrosion phenomena can be further analyzed.On this basis,the causes and trends of electrochemical corrosion of materials can be revealed and judged.In this paper,the metal elements of life in the human body（R）,i.e.,Fe,Mn,Zn,Mo,W,Li,Sn,Zr,Sc,Y,Gd,Nd,Ca,Sr,and Ba,were chosed as the research objects,and the cell models of 15 Mg alloy solid solutions and 15 second phases were constructed respectively,then the stability,surface energy,electronic work function,and intrinsic potential difference of alloy were investigated.In addition,the effect of alloying element on the corrosion properties was analyzed.The main results obtains are as follows:（1）The cohesive energy and Formative energy of Mg based solid solution Mg₃₅R₁（R=Fe,Mn,Zn,Mo,W,Li,Sn,Zr,Sc,Y,Gd,Nd,Ca,Sr,Ba）was calculated.The results shown that Mg-Sn solid solution is easy to form,Mg-W and Mg-Zr alloys have a higher structural stability.It was found that a small amount of Mo,Mn and W can improve the electronic work function of alloy,while Ba,Sr and Ca can reduce the value of the electronic work function.The work functions of different models for Mg-Ca alloy were also calculated.It is found that the electronic work function of the alloy is generally determined by the first three layers of atoms on the surface,and the internal atoms have little influence on the electronic work function;the atomic concentration and surface orientation of the surface layer have an important influence on the change of the electronic work function of the alloy,while the specific distribution of the surface atoms has little influence on the electronic work function of the alloy.（2）In Mg alloys,some second phases are often introduced to improve the properties of Mg alloys.However,because of the potential difference between the second phase and the magnesium matrix,the corrosion of magnesium alloy will be seriously affected.So,the electronic work functions of the second phase of Mg₂Zn₁₁,Mg₂₄Y₅,Mg₂Ca,Mg₁₇Sr₂,Mg₂Gd,Mg Mn,Mg Zr₃,Mg₂Nd,Mg₂Sn,Mg₂Fe,Mg₁₇Ba₂,Mg Li,Mg Mo₃,Mg₃Sc,Mg W₃ in this paper were also calculated,the difficulty of electron escaping from various crystal surfaces is discussed and the intrinsic potential difference between the second phase and Mg matrix is calculated.The results show that different crystal surfaces have different effects on the electronic work function and the intrinsic potential difference,and even the same crystal surface will cause different corrosion due to the difference of the type and concentration of atoms.For example,the difference of electronic work function and intrinsic potential difference between the second phases of Mg Zn,Mg Mo and Mg W and the matrix is positive and large.Therefore,in the electrochemical environment,these surfaces are not easy to lose electrons,which can be used as cathodes,leading to the corrosion of Mg matrix.In addition,the accuracy of the simulation is verified by the experiment of Mg-30Ca alloy.It shown that the Mg₂Ca phase in Mg-30Ca alloy was corroded before the matrix phase in 0.9%Na Cl solution due to Mg₂Ca phase have a lower electronic work function than matrix.In this paper,the research on the relationship among the composition,atomic structure and electronic work function of Mg alloy by means of calculation and simulation has scientific guidance and reference significance for the design and development of Mg alloy composition.