Effects Of Alloying Elements And Media On The Hydrogen Evolution Behavior Of Magnesium Alloys

Magnesium（Mg）alloys have low density,high specific strength,and excellent processability,so they have been widely applied in engineering fields as lightweight materials.Because of high electrochemical activity,they have also received widespread attention in the fields of sacrificial anodes and magnesium seawater batteries.However,the low corrosion resistance and the existence of Negative Difference Effect（NDE）in electrolyte solutions greatly limit the practical application of Mg alloys.Because these features of Mg alloys are closely related to the hydrogen evolution behavior on their surface,it is of great theoretical and practical significance to study the key factors affecting the hydrogen evolution behavior and NDE of magnesium alloys.In this thesis,Cu and Sn elements,which are considered to have different influence on the hydrogen evolution process,are selected as alloying elements to prepare Mg-Cu and Mg-Sn alloys.The effects of Cu and Sn alloying elements and different aggressive ions in media on the electrochemical and hydrogen evolution behavior and the NDE of Mg alloys have been studied by the means of electrochemical tests,hydrogen evolution tests,weight loss,and microscopic analysis methods,etc..The main research results are as follows:（1）Mg-Cu（0.1,0.5 wt.%）,Mg-Sn（0.5,2 wt.%）,and heat-treated Mg-Sn T4（0.5,2 wt.%）alloys were prepared.The microscopic analysis results indicate that the Cu element mainly exists in the grain boundary of Mg-Cu alloy as Mg₂Cu second phase.With increasing the Cu content,the number and size of the Mg₂Cu second phase increases and the grain boundaries become more continuous.Sn element can refine the grains of the Mg alloy.After the solution treatment,the grain size of Mg-Sn T4 alloy increases and the Sn element is mainly dissolved in the Mg matrix nearly without any Mg₂Sn second phase.（2）In 0.6 M Na Cl,the cathodic process and corrosion rate of the Mg-Cu alloys are promoted apparently with an increase in Cu content.Under anodic polarization,the hydrogen evolution current density(i _H2)of the Mg-Cu alloys is significantly accelerated than that of Mg,showing the NDE phenomenon.The higher the Cu content,the stronger the NDE of the Mg-Cu alloys.The i _H2 of the Mg-Cu alloys increases with an increase in the anodic polarization current density（i）,but their NDE gradually weakens and approximates to that of pure Mg.In 0.6 M Na Cl,the Sn element can significantly inhibit the cathodic process and the corrosion rate of the Mg-Sn T4 alloys.Under anodic polarization,the i _H2 of the Mg-Sn T4alloys is much smaller than that of Mg,also showing the NDE phenomenon.The i _H2 of the Mg-Sn T4 alloys increases with increasing i,but their NDE is less affected by the degree of anodic polarization.The change rules of i _H2 and NDE coefficient（D_i）obtained by the potentiostatic polarization method are nearly the same as those obtained by the galvanostatic polarization method,but the later facilitates the use of D_i.Different alloying elements change the microstructure of Mg alloys and their electrochemical activities,and then,change their i _H2 and NDE.But the influence is weakened at large i.（3）With an increase in Cl^-concentration（[Cl^-]=0.01-0.6 M）of Na Cl solutions,the cathodic and anodic processes of Mg,Mg-0.5Cu and Mg-0.5Sn T4 alloys are all promoted,and their corrosion rate is significantly increased.When i<1 m A/cm²,their i _H2 increases with an increase in[Cl^-];while when i>1 m A/cm²,their i _H2 is less affected by[Cl^-].The NDE of the Mg and Mg-0.5Cu alloys is stronger in low[Cl^-]solutions,while that of the Mg-0.5Sn T4alloys has the opposite effect.In solutions containing PO₄^3-ions,the corrosion rate and i _H2 of Mg are inhibited to some extent,while those in 0.6 M Na₂SO₄ solution are very weak and its NDE is also very slight and less affected by i.Aggressive Cl^-ions have an important effect on the NDE of Mg.（4）Based on the reported theories and our experimental data,a qualitative model is set up to explain the effects of various factors on the hydrogen evolution behavior and NDE of the Mg alloys in this study.