Research On The Electrochemical Behavior And Discharge Performance Of AZ Magnesium Alloys In Magnesium-based Electrolytes

Magnesium/manganese dioxide?Mg/MnO₂?dry batteries possess several outstanding advantages,such as long storage life,good capacity retention,wide operating temperature and high thermal stability.The bottlenecks limiting the development and application of Mg/MnO₂ battery mainly come from two aspects.Firstly,the anodic efficiency is restricted by the relatively low corrosion resistance of magnesium toward various aqueous electrolytes.Besides,the delayed action effect is induced during discharge by passive film formation on the anode which hampers the transmission of ions and electrons.For this reason,magnesium-based electrolytes with additives were investigated to improve the discharge efficiency and the delayed action of the anode in this work.The localised corrosion behavior of magnesium alloy in different electrolytes was also monitored in-situ by the scanning vibrating electrode technique?SVET?,and the corrosion mechanism was preliminarily discussed.In addition,the theoretical expression of the delay-time curves was improved to render it more consistent with the experimental data.The main conclusions are as follows:?1?The addition of 0.04 mol·L^-1 NaHCO₃ into 2 mol·L^-1 Mg?ClO₄?₂ could effectively reduce the corrosion rate of AZ31B magnesium alloy and improve its electrochemical activity,which is a promising approach for reducing the delayed action of magnesium anode in Mg/MnO₂ primary battery.Even after 16 d of aging,the potential dip was only about 0.1 V,which is negligible.The addition of NaHCO₃enhanced the utilization efficiency of the anode by 10 percent during galvanostatic discharge.It can be inferred from the results of FT-IR and XPS analysis that the components of the corrosion products on the electrode immersed in Mg?ClO₄?₂ were a mixture of MgO,Mg?OH?₂,Mg?CO₃?₂ and Mg?ClO₄?₂.While with the addition of 0.04mol·L^-1 NaHCO₃,the component of Mg₅?CO₃?₄?OH?₂·4H₂O appeared.?2?AZ31B magnesium alloy was found to be the most promising anode material in2 mol·L^-1 Mg?NO₃?₂ electrolyte for Mg/MnO₂ primary battery.For the Mg?NO₃?₂-MgSO₄ composite electrolyte,MgSO₄ played a dominant role in the discharge process at small current density,while Mg?NO₃?₂ functioned as a leading role for the discharge characteristics at large current density.The normalized impedance diagrams for MgSO₄,Mg?NO₃?₂ and Mg?NO₃?₂-MgSO₄ indicated that the corrosion mechanism of the alloy in these three kinds of electrolytes was independent with immersion time.In addition,MgSO₄ played a leading role on the corrosion of magnesium in the composite electrolyte.?3?The electrolyte composed of 1.86 mol·L^-1 Mg?NO₃?₂+0.14 mol·L^-1 MgSO₄+0.08 mol·L^-1 Na₂WO₄ is one of the cost-effective method to solve the voltage delay in Mg/MnO₂ primary battery,which is expected to replace the traditional relatively expensive electrolyte of Mg?ClO₄?₂.The AZ31B magnesium alloy had higher corrosion resistance and electrochemical activity in the composite electrolyte with the addition0.08 mol·L^-1 Na₂WO₄.After immersion for more than 2 d,the delayed time was only0.3 s.The efficiency of the anode was up to 80%,with an increase of 10%compared with the blank composite electrolyte at a low current density of 2.5 mA·cm^-2.It can be inferred from the results of FT-IR and XPS analysis that the composition of the corrosion products on the AZ31B magnesium electrode in the blank composite electrolyte is MgO,Mg?OH?₂,MgCO₃,MgSO₄ and Mg?NO?₃.While for the electrolyte with addition of 0.08 mol·L^-1 Na₂WO₄,the corrosion products were mainly composed of MgO,Mg?OH?₂,MgCO₃ and MgWO₄.?4?The SVET experimental results indicated that the evolution of the localised corrosion behavior of AZ31B magnesium alloy in 2 mol·L^-1 Mg?ClO₄?₂ solution without and with 0.04 mol·L^-1 NaHCO₃ was under cathodic control.After 8 h of immersion,the localised corrosion rate was stable.The presence of HCO₃^-ions had no significant effect on the corrosion mechanism.The localised corrosion behaviors of AZ21 and AZ31B magnesium alloys in the composite electrolyte were mainly controlled by NO₃^-ions,and the variation of the corrosion rate was controlled by SO₄^2-,which was consistent with the results obtained by the electrochemical EIS method.SEVT line scans revealed that each active site with a strong anodic current density was surrounded by a strong micro cathode,the results were consistent with the cathodic hydrogen evolution model.?5?The majorization for the theoritical expression of the delay-time curves was realized through introducing the model that parameters of the specific resistances and film thickness vary with time during the discharge process.When the changes of the specific resistances and film thickness over time correspond to the Rayleigh distribution function and logarithmic function respectively,the simplified theoretical formula could be successfully modified.The fitting degree of the modified formula to the experimental data could reach to about R²=0.96,which had a good result.