First-principles Simulation On The Polarization Patential/Current For Mg Alloy Corrosion

The rapid development of computational science provides help for material discovery.Magnesium alloy corrosion is a complex process.Traditional corrosion simulation mostly focuses on a single factor,which is difficult to make a quantitative prediction corresponding to the electrochemical polarization curves.Therefore,it is urgent to establish a comprehensive model.In this thesis,the first-principles simulation on the polarization potential/current for Mg alloy corrosion has been established by integrating factors,such as anodic dissolution reaction,cathodic hydrogen evolution reaction,medium adsorption,galvanic corrosion,oxide film,etc.And it could be applied for pure Mg,AZ,AZ-RE,and Mg-RE well.The alloying effects on magnesium alloy corrosion have been also discussed and clarified.The conclusions are as follows:(1)First-principles model on the polarization potential/current for Mg alloy corrosion has been established.The polarization potential should be:Ua(SHE)=[(ln10)RT(lg nFkT/h)+Gv+NA/N×S×Esurf]+[(10ln10)RT+0.65λ2nF]/-(λ1+λ2)nF.In which,[(ln10)RT(lg nFkT/h)+Gv+NA/N×S×Esurf],[(10ln10)RT+0.65λ2nF]are the anode and cathode contribution respectively;-(λ1+λ2)nF is a constant;n is charges transferred in the reaction;F,k,T,h,R are Faraday constant,Boltzmann constant,temperature,Planck constant and Gas constant respectively;NA,N,S,Esurf are expressed as Avogadro constant,surface atom number,surface area and surface energy.Normally,n could be 2,the anodic dissolution transfer coefficient λ1 should be 0.36,cathodic hydrogen evolution transfer coefficient λ2 should be 0.5,activation energy Gv could be 3.03eV.The quantization formula for Ua would be:Ua(SHE)=[(ln10)RT(lg 2FkT/h)+3.03+NA/N×S×Esurf]+[(10ln10)RT+0.65F]/-1.72F,which agrees with the corrosion potential of magnesium alloy in the polarization experiment.The polarization current would be:lgJtot=lgJa+ΔlgJ0+km ×∑(fm×|ΔEm|)+ki ×∑(fi×|ΔEi|)-γc×(W-ΔGs-ΔGF)/(ln10)RT.In which km ×∑(fm×|ΔEm|)is matrix intercrystalline planes galvanic corrosion contribution;ki×∑(fi×|ΔEi|)is second phase galvanic corrosion contribution,-γ ×(W-ΔGs-ΔGF)/(ln10)RT is protective film contribution.For common cases,km is 12.17,fm is 0.5;ki is 12.17,ΔEi is intergranular potential difference between the matrix and second phases,fi is 0.5(fi,AlRE+fi,MgRE is 0.5),and γ is 0(for Mg-RE alloy,γ is 0.01).The corrosion current quantitative formula would be:lgJtot=lgJa+ΔlgJ0+6.09|ΔEm|±6.09|ΔEi|,and for rare earth magnesium alloy should be:lgJtot,=lgJa+ΔlgJ0+6.09|ΔEm|±6.09|ΔEi|-0.01c×(W-ΔGs-ΔGF)/(ln10)RT.The prediction value is coherent to the corrosion current of magnesium alloy in the polarization experiment.(2)First-principles model on the polarization potential/current for Mg alloy corrosion is now applied for pure magnesium,AZ-RE magnesium alloys and rare earth magnesium alloys corrosion.The calculated value is consistent with the experimental value well.The alloying effects on magnesium alloy corrosion have been clarified as:the major alloy elements(such as Al and Zn)affect the corrosion potential,the matrix corrosion current between the crystal planes and galvanic corrosion due to the second phase.Furthermore,the major alloy elements as rare earth also decrease corrosion current because of the oxide film.Therefore,the major alloying element has a great influence on corrosion performance.Minor alloying element affects second phase galvanic corrosion current greatly if ΔEi of minor alloying element phase(such as ΔEi,Al4Sm,ΔEi,Al4Tb,ΔEi,MgRE)has a lower value than one of the major alloying element phase(such as ΔEi,Mg17Al12).