Anti-oxidation And Corrosion Resistance Of Fe-Ni-Al Alloys As Inert Anode In Aluminum Electrolysis

Instanding of evolving green house gas of CO2 from carbon anode,inert anode releaves O2 during the cource of aluminum electrolysis,which has signifigant advantages in energy saving and environment.Further more,if inert anode is put into commercial utilization,it can improve the automation of aluminum reduction cells operation and control,decrease the labour force and intensity,and save high-quality carbon.Thus the research and development of inert anode have be one of the focus and hotspot of primary aluminum industry.This dissertation mainly focuses on metallic inert anodes for aluminum electrolysis.Based on a fixed mass ratio of Fe to Ni(Fe/Ni=1.42),a series of Fe-Ni-Al alloy with different content of aluminum are made by intermediate frequency induction furnace.The oxidation and electrochemical corrosion behaviors of Fe-Ni-Al alloys at high temperature are studied,and their corrosion mechanisms are also discussed with the analysis results obtained by XRD,SEM and EDS.When the addition amount of aluminum is not more than 5.3%,the aluminum is evenly dissolved in the Fe-Ni alloy phase and the second phase does not appear.When the added amount of aluminum is large,52.9Fe-39.2Ni-7.9Al and 54.1Fe-37.3Ni-8.6Al,there exist the second phase,and solid solution of y_Fe,Ni-Al and y-AlNi3 presence in high aluminum content of Fe-Ni-Al alloys.The oxide film formed on the surface of the alloy was relatively unstable at a 750?and 900?,while the oxide film formed on the alloy surface was relatively stable at 800? And 850?.When the addition of Al is 5.3%in Fe-Ni-Al alloy,the stability of the oxide film is the best.Oxide film on the sample surface can ne divided into several layers,the outer oxide layer is mainly iron oxide,the component of inner oxide layer is related on the content of alumum.At lower aluminum content and temperature,the phase in the inner consists of A1 and Ni enrichment and Fe-Ni-O compound oxide which can change into NiFe2O4 at high temperature.At high aluminum content,the phase in the inner consists of NiAl2O4 and NiFe2O4 which stick to the outer layer better.The addition of Al has positive effect on improving the oxidation resistance of the alloy.When using Fe-Ni-Al alloy as anode for aluminum electrolysis,the initial cell voltage is relativly stable at 4.5 V to 5.5V.After 120 minutes later,the voltage increases,which indicates that the anode resistance increases.The purity of aluminum products obtained by this anodes is in an interval of 97%to 99%.By analysis the anode surface oxide film,there exists fluoride.The corrosion surface layer is multi-layer,the inner layer is Ni enrichment,while the outer layer is reched by Fe and O,which is due to the selected oxidation of Fe.After electrolysis,the thickness of the corrosion layer on the anode surface is relatively thick,nearly 2000?m.The oxide film contains Al2O3,NiO,NiFe2O4 and FeAl2O4.These protective oxide films make the corrosion resistance of the alloy better.Compared with the Fe-Ni alloy anode,the corrosion resistance of the anode was enhanced by the addition of Al,and the alloy anode with the addition of 5.3%Al was the best.By a linear sweep voltammetry test,it is observed that the linear voltammetric curves can be divided into three parts:the active solution region,the transition region and the passivation region,the passivation region is not obvious.The bimodal phenomenon in the transition region is due to the existence of two phases in the alloy.When the addition of Al is 1.4mass%,3.9mass%,5.3mass%and 7.9mass%,the Ep of the alloy is more negative,more easily into the passive state.The corrosion resistance of the alloy is relatively low and the corrosion rate is relatively slow.The Eb of 56.1Fe-39.5Ni-4.4Al and 54.1Fe-37.3Ni-8.6Al alloy is higher,and the passive film is less likely to be damaged.According to the Tafel curve,when the Al content is 1.4%,3.9%and 5.3%,the corrosion voltage of the alloy anode is higher and the corrosion resistance is better;When the addition of Al is 1.4%and 3.9%,the corrosion current density of the alloy is higher and the corrosion rate is faster.In conclusion,the electrochemical corrosion resistance of 56.8Fe-37.9Ni-5.3Al alloy is the best.