Study On The Behavior And Mechanism Of Mn Inhibiting The Corrosion Effect Of Impurity Fe In Mg Alloys

Magnesium alloys have broad prospects in automobile manufacturing and other fields because of their high specific strength,and they have been widely used in bioabsorbable implant materials due to their excellent biocompatibility.However,the inevitable impurity element Fe in Mg alloys will form electric couple corrosion with Mg matrix in water or humid environment,resulting in the poor corrosion resistance of Mg alloys,and further limiting their application in industrial fields.However,the electric couple corrosion resistance of Fe in Mg alloys can be improved by adding Mn.Therefore,it is virtally necessary to explore the behavior and mechanism of Mn inhibiting the corrosion effect of impurity Fe in Mg alloys,to provide additional guidelibes for the applications of Mg alloys in industrial fields.In this work,the corrosion behavior of industrial Pure Mg and Mg-Mn alloys in 0.6 mol/L NaCl solution was investigated by hydrogen evolution,electrochemistry tests,optical imaging in situ and scaning electron microscopy in situ.It is expected to reveal the mechanism of Mn inhibiting the corrosion effect of impurity Fe in Mg alloys.The experimental results can be concluded as follows.Based on the pandat phase diagram calculation software,it can be calculated that the alloy elements that could effectively inhibit Fe are Mn,Si and Zr,respectively.According to reports in related literatures,Si is harmful to the corrosion performance of Mg alloys,and Zr will precipitate to the bottom of the crucible since it can form metal compounds with Fe during smelting.When Mn is added in Mg alloys,the impurity Fe can be dissolved into Mn to form Mn(Fe)solid solution,thus improving corrosion resistance of Mg alloys.However,it is still limited studied how Mn improve the corrosion resistance of Mg alloys,and what is the influence of Mn/Fe ratio on corrosion performance of Mg alloys.Therefore,in this work,Mn is selected as the alloy addition element to explore the mechanism of Mn inhibiting the corrosion effect of impurity Fe in Mg alloys.The results of corrosion performance tests show that the corrosion rates of hydrogen evolution of both Mg-0.5Mn and Mg-6Mn alloys are lower than that of Pure Mg.Furthermore the corrosion rate of Mg-6Mn is lower than that of Mg-0.5Mn.Obviously,it indicates that the addition of Mn can effectively improve the corrosion resistance of Mg alloys,and the inhibition effect of diverse Mn/Fe ratio in Mg alloys is clearly different.The results of electrochemical tests and optical images exhibit that the surface film on Pure Mg and Mg-0.5Mn samples rupture,while that on Mg-6Mn samples is perfectly preserved during immersion in the 0.6 mol/L NaCl(48 h).Based on tafel measurement,it is found that the cathode hydrogen evolution rate of Pure Mg samples increases with the increase of soaking time,and that of Mg-0.5Mn firstly remains unchanged and then increases during soaking in 0.6 mol/L NaCl solution for 20 h.However,the cathode hydrogen evolution rate of Mg-6Mn samples obviously decreased.Clearly,it demonstrates that the addition of Mn can inhibit the hydrogen evolution ability of the impurity Fe in Mg alloys,and the inhibition effect is diverse in Mg alloys with different addition of Mn content.The results of SEM in-situ show that the impurity element Fe can be dissolved into Mn and form Mn(Fe)phase in Mg-6Mn alloy.Although Mn(Fe)phase still has a strong ability of cathode hydrogen evolution at the initial stage of corrosion,it will oxidize and produce insoluble and dense Mn3O4 corrosion product film as of corrosion time increased,thus inhibiting the hydrogen reduction reaction of Mn(Fe)phase.Therefore,the addition of Mn can effectively weakens the electric couple corrosion caused by Fe,thus significantly improving the corrosion resistance of Mg alloys.