Corrosion Behavior Of Inert Anode Ni-Fe-Cr Alloys For Aluminum Electrolysis

Inert anode is little consumed in the alumnum electrolysis process.The total reaction of electrolysis is:Al2O3=2Al+3/2O2.There is not carbon consumed and oxygen is produced in the electrolysis process,while improving the working environment.Therefore,the inert anode has a series of advantages such as energy saving and environmental protection.The core of inert anode is the corrosion resistance of the material.The corrosion of an inert anode is not only related to its composition and structure but also its conditions of use.The metal alloy anode has the advantages of low cost,easy preparation and connecting.Therefore,this thesis focuses on the research of metal alloy anodes.This thesis mainly explores the corrosion behavior of Ni-Fe,Ni-Fe-l0Cr and NiFe-15Cr alloy anodes.The alloy anodes and pre-oxidised alloy anodes were electrolyzed at 960? and 800? respectively.The anodes were analyzed by X-ray diffraction(XRD),scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS)after pre-oxidation and electrolysis.The oxidation resistance and corrosion behavior of three alloy anodes were investigated.The research results are as follows:After oxidation at 960? for 10 h,the outer layer of oxide formed on Ni-Fe alloy is Fe2O3,followed by the inner layer of(Ni,Fe)3O4.The total oxide scale is the thickest.The oxide formed on Ni-Fe-Cr alloys includes the outer layer of Fe2O3,the middle layer of(Ni,Fe)3O4 and the inner layer of Cr2O3.The inner layer of Cr2O3 formed on Ni-Fe10Cr alloy is about 4?m.The inner layer of Cr2O3 formed on Ni-Fe-15Cr alloy is about 8 ?m,and the total oxide scale is the thinnest,indicating that Ni-Fe-15Cr alloy has the best oxidation resistance in air.It is found that the corrosion resistance of the alloy anode at 800? is better than that at 960?,indicating that low temperature electrolysis can reduce corrosion of alloys.At 960? electrolysis,the cell voltage of the pre-oxidized alloy anode is higher than bare alloy in electrolysis,because oxide scales of pre-oxidised the alloy anode is denser,so the resistance is greater and the cell voltage is higher.At 800? electrolysis,the cell voltage of the pre-oxidized alloy anode is lower in electrolysis.On the bare anodes,where no protective oxide scale is present,it is possible that these poorly conducting species as metal fluorides are freely generated during electrolysis,resulting in an increase in the cell voltage.When it is electrolyzed 960? and 800?,the difference between the cell voltage of the pre-oxidized alloy anode and the bare alloy does not exceed 0.7 V.It indicates that the pre-oxidized Ni-Fe-Cr alloy anode has good electrical conductivity.Through cross-section SEM and EDS,XRD analysis.After alloy anodes are electrolyzed at 960? and 800?,the oxide scales of pre-oxidised the alloy anode are more uniform and dense.The inner layer of the pre-oxidized alloy anode contains less F and O elements,indicating that it has better corrosion resistance.Therefore,it is necessary that alloy anodes were pre-oxidised treatment before electrolysis.The outer layer of oxide formed on Ni-Fe alloy is mainly(Ni,Fe)3O4,Fe2O3 and a few contents electrolyte such as Na3 AlF6,AlF3 etc.The difference is that the inner layer of oxide formed on Ni-Fe-Cr alloy is Cr2O3.Ni-Fe-Cr alloy anodes contain less O element after electrolysis,indicating that the Cr2O3 can improve the oxidation resistance.Therefore,the oxide scales formed on Ni-Fe-Cr alloy anodes exhibited a double-layer oxide structure primarily containing(Ni,Fe)3O4 spinel outer layer and Cr2O3 inner layer.The oxidation resistance of Ni-Fe-Cr alloy anodes was significantly improved in comparison with the Ni-Fe alloy anodes due to the formation of a continuous Cr2O3 inner layer.There are some gaps between the matrix and the oxide scales of Ni-Fe-15Cr alloy anodes,which is disadvantageous for its corrosion resistance and oxidation resistance.Matrix and oxide layer of Ni-Fe-10Cr alloy anode are better combined and the oxide scales is denser,So Ni-Fe-10Cr alloy anodes have better corrosion and oxidation resistance.In summary,the pre-oxidised Ni-Fe-10Cr alloy anode have better performance when electrolyzed at 800?.