CORROSION MECHANISM OF SnO2-BASED INERT ANODES

According to the extent of their damage in cryolite melts,the corrosion of SnO_(2)-based inert anodes can be divided into three types,slight corrosion,m&rked corrosion and catastrophic corrosion.The conditions under which these three types take place and their mechanisms are different.Our work shows that depletion of oxygen containing ions in melts,high density,electrolyte penetration into the electrode and intergranular corrosion contribute greatly to marked corrosion and catastrophic corrosion and that the reduction reaction by aluminium only attacks the electrode slightly.

MODEL OF CONDUCTIVITY IN DOPED SnO_(2)-BASE INERT ANODES FOR Al-ELECTROWINNING

The variations of chemical bonding characteristics in doped SnO,-base inert anodes with various dopants are studied using CNDO/2 quantum chemical calculations.A new model of conductivity is derived.which is checked with literature data and our experimental results.Proper dopants can be selected with the model to reduce the resistivity of said electrode effectively.

Preparation and Electrochemical Properties of Pb-0.3wt%Ag/Pb-WC Composite Inert Anodes

We prepared Pb-0.3wt%Ag/Pb-WC（WC stands for tungsten carbide,the same below） composite inert anodes by double-pulse electrodeposition on the surface of Pb-0.3wt%Ag substrates,and investigated the electrochemical properties of the composite inert anodes,which were obtained under different forward pulse average current densities from 2 A/dm2 to 5 A/dm2 and WC concentrations from 0 g/L to 40 g/L in bath.The kinetic parameters of oxygen evolution,corrosion potential and corrosion current of the composite inert anodes were obtained in a synthetic zinc electrowinning electrolyte of 50 g/L Zn2＋ and 150 g/L H2SO4 at 35 ℃,by measuring the anodic polarization curves,Tafel polarization curves and cyclic voltammetry curves.The results show that Pb-0.3wt% Ag/Pb-WC composite inert anodes obtained under forward pulse average current density of 3 A/dm2 and WC concentration of 30 g/L in an original acid plating bath,possess higher electrocatalytic activity of oxygen evolution,lower overpotential of oxygen evolution,better reversibility of electrode reaction and corrosion resistance in [ZnSO4＋H2SO4] solution.The overpotential of oxygen evolution of the composite inert anode is 0.926 V under 500 A/m2 in [ZnSO4＋H2SO4] solution,and 245 mV lower than that of Pb-1% Ag alloy;the corrosion current of the composite inert anode is 0.95×10-4A which is distinctly lower than that of Pb-1%Ag alloy,showing the excellent corrosion resistance.

EFFECT OF Cu-Ni CONTENT ON THE CORROSION RESISTANCE OF(Cu-Ni)/(10NiO-90NiFe204)CERMET INERT ANODE FOR ALUMINUM ELECTROLYSIS

（Cu-Ni）/（10NiO-90NiFe204） cermet inert anodes containing metal Cu-Ni0, 5, 10, 15 and 20 wt pct were prepared and their corrosion resistance to Na3AlF6-Al2O3 melts was investigated. The results indicate that the content of metal Cu-Ni has little effect on the steady-state concentration of Ni in the electrolyte and the values could not be used to effectively differentiate their corrosion resistance. The steady-state concentration of Fe decreases from 304×10^-6 to 168×10^-6 and that of Cu increases from 21×10^-6 to 71×10^-6 with the content of metal Cu-Ni increasing from 0 to 20 wt pct. Post-examination shows that metallic phase Cu-Ni is corroded preferentially during electrolysis and many pores are left at the anode surface. Considering the corrosion resistance and electrical conductivity, the cermet containing metal Cu-Ni 5 wt pct should be selected and studied further.

THERMAL STRESSES RELAXATION DESIGN OF Ni/NiFe_(2)O_(4) SYSTEM FUNCTIONALLY GRADED CERMET INERT ANODE

The thermal stresses relaxation of Ni/NiFe2O4 system functionally graded cermet inert anode for aluminum electrolysis was optimally designed. The transient thermal stresses of the inert anode under complex boundary condition during high-temp （955℃） electrolysis were calculated using the finite-element software ANSYS, the influence of different parameters on the distribution of the thermal stresses were analyzed. The results showed that, during the process of thermal shock, the thermal hoop tensile stress on the surface of the anode is very large, which is possibly the major cause of anode crack; when the radius of the anode is between 0.05-0.15m, a range that can be realized by recent manufacturing technology, the optimum composition distribution exponent p is 0.25; The hoop tensile stresses reduce with the decrease of anode scale and also decrease with the decrease of the convection coefficient between the electrolyte and the anode.

EFFECT OF STRUCTURAL PARAMETERS ON THE THERMAL STRESS OF A NiFe_(2)O_(4)-BASED CERMET INERT ANODE IN ALUMINUM ELECTROLYSIS

Inert anode has been a hot issue in the aluminum industry for many decades. With the help of FEA （finite element analysis） software ANSYS, a model was developed to simulate the thermal stress distribution working condition of an inert anode. To reduce its thermal stress, the effect of some parameters on the thermal stress distribution was investigated, including the anode height, the anode radius, the hole depth, the hole radius, and the radius of inner chamfer and outer chamfer. The results showed that in the actual working condition of an inert anode, there existed a large axial tensile stress near the tangent interface between the anode and bath, which was the major cause of anode breaking. Increasing the anode height and reducing the hole depth properly seemed to be beneficial for the stress distribution. With the increase of anode radius, the stress distribution became better first and then deteriorated, the reasonable value was between 0.045 to 0.06m. The hole radius had a significant effect on the stress and a smaller radius would reduce the thermal stress. The effect of the radius of the inner chamfer and the outer chamfer was less than other parameters.

Electrical Conductivity and Corrosion Resistanceof ZnFe _(2)O _(4) Based Materials Used as Inert Anodefor Aluminum Electrolysis

ZnFe 2O 4 and ZnFe 2O 4 based materials were tested to obtain the electrical conductivity and corrosion resistance in melting bath for aluminum electrolysis. The results proved that adequate additives, such as Ni 2O 3 CuO, Cu, ZnO and CeO 2 would increase the electrical conductivity, and the ZnFe 2O 4 based anodes with these additives were of good corrosion resistance. The current density on anode, the mole ratio of NaF/AlF 3 (MR) and the content of alumina in the bath effect the anode corrosion rate in different way.

EFFECT OF MnO_(2) ADDITIVE ON PERFORMANCES OF NiFe_(2)O_(4) SPINEL BASED INERT ANODE

The NiFe2O4 inert anode is synthesized by high-temperature solid-state reaction method using NiO and Fe2O3 as main raw materials and adding MnO2 powder as additive. Archimedes method using water immersion technique is used to measure the sintering performances of sampies. The static thermal corrosion rates of samples are measured by weight loss. SEM is employed for the observation of material microstructure, and phase structure of the sample surface after corrosion is determined by XRD. The experimental results indicate that a suitable MnO2 additive content is 2%, while the sintering performance is the best, and the static thermal corrosion rate is the lowest. Because of MnO2 dopant enriching at crystal boundary, the corrosion reaction of molten salt to crystal grain creates Mn2AlO4 phase, which is denser than NiFe2O4 phase, and prevents the cryolite molten salt to penetrate into the inert anode, thus reducing the corrosion.

SYNTHESIS OF NANO-ZnO PARTICLES FOR ALUMINUM METALLURGY AS INERT ANODE MATERIAL

Nano-ZnO particle was produced by evaporating zinc powders in air at air flow-rate from 0.2 to 0.6m3/h. Nano-ZnO particles was formed by the oxidation of the evaporated zinc vapor. X-ray diffraction shows the powders to be ZnO with lattice parameters of a=0.3249nm and c=0.5205nm. The particle size is dependent upon the transit time from the source to the collection area. The size of particles was ranged between 81 to 103nm. The average density resulted was 4.865g/cm3.Normal ZnO and nano-ZnO were investigated to use them in aluminum metallurgy as an inert anode material. A certain amount of both oxides were molded subsequently inserted to the molten cryolite-aluminum oxide to investigate the corrosive behavior of both oxides. When the sintering temperature increased up to 1300℃, the weight loss ratio rose to 5.01%-7.33% and up to 7.67%-10.18% for nano-ZnO and normal ZnO, respectively. However, when the samples in the molten cryolite aluminum oxide were put for long time, the corrosive rate was found to be higher. It was found that the corrosive loss weight ratio of nano-ZnO anode was much lower than the normal one made from ordinary-ZnO providing that the nano-ZnO is more possible to be use inert anode material.

Electrochemical preparation of the Fe-Ni36 Invar alloy from a mixed oxides precursor in molten carbonates

The Fe-Ni36 alloy was prepared via the one-step electrolysis of a mixed oxides precursor in a molten Na2CO3-K2CO3 eutectic melt at 750℃,where porous Fe_(2)O_(3)-NiO pellets served as the cathode and the Ni10 Cu11 Fe alloy was an inert anode.During the electrolysis,Ni O was preferentially electro-reduced to Ni,then Fe_(2)O_(3)was reduced and simultaneously alloyed with nickel to form the Fe-Ni36 alloy.Different cell voltages were applied to optimize the electrolytic conditions,and a relatively low energy consumption of 2.48 k W·h·kg^(-1) for production of Fe Ni36 alloy was achieved under 1.9 V with a high current efficiency of 94.6%.The particle size of the alloy was found to be much smaller than that of the individual metal.This process provides a low-carbon technology for preparing the Fe-Ni36 alloy via molten carbonates electrolysis.

Influence of Adding Y_2O_3 on the Structure of the Cu-Containing Cermet

The enriching of metals to the center of sample was found and investigated during the sinter process of the cermet.The enrichment is caused by addition with Y_2O_3.The enriching of copper is greatly in- fluenced by sinter temperature and time.The enriching of metal can significantly enhance the electrical con- ductivity of the cermet.

Evaluation of Ni-Fe base alloys as inert anode for low-temperature aluminium electrolysis

Isothermal oxidation behaviors of Ni–Fe(wt.%)and of the same alloy with additions of 10 and 15%Cr alloys in the air at 800℃and 900℃and their anodic behaviors in aluminum electrolysis system at 800℃were evaluated.The composition morphologies of oxide scales were characterized by XRD,SEM,and EDS.Results show that the scales formed on Ni–Fe alloy at both temperatures consisted of an inner(Ni,Fe)Olayer and an outer FeOlayer.For Ni–Fe–10Cr alloy,an external(Ni,Fe)O/FeOlayers and an internal oxidation zone were formed at 800°C,while a continuous CrOlayer forms at the internal oxidation zone/substrate interface at 900°C.A multilayer structure oxide of CrO/(Ni,Fe,Cr)O/(Ni,Fe)O/FeOwas formed on Ni–Fe–15Cr alloy at 800°C,while at 900℃the FeObecomes discontinuous disperses in the(Ni,Fe)Olayer close to the surface.Increases in oxidation temperature or Cr content for Ni–Fe–Cr alloys promote the growth of the inner CrOlayer and simultaneously reduce FeOcontent.After 4 h of electrolysis at an anode current density of 0.25 A cm,the oxidation resistance of Ni–Fe–15Cr anode is superior to the Ni–Fe anode.

Oxygen-evolving SnO2-based Ceramic Anodes in Aluminium Electrolysis

This study deals with SnO2-based ceramic anodes doped with Sb203 and CuO, aiming at contributing new data regarding their electrochemical behavior in cryolite melts. The performances of the anodes were evaluated by anodic polarization, cyclic voltammetry, and current efficiency and corrosion measurements. The investigation proves that the anodic process of SnO2-based inert anodes occurs at a low overvoltage and the oxygen discharge takes place in one step with an exchange of two electrons. The current efficiency and corrosion were proved to be dependent on the electrolysis parameters and composition of electrolysis bath. For a long term electrolysis, the dissolution of the anode in the cryolite-alumina melt produced small aluminium contamination（ca. 0.2%, mass fraction）.

Enhancing oxide scale growth and adhesion via electrochemically regulating ion diffusion

Pre-oxidation is a common surface treatment approach to enhance the corrosion resistance of met-als,which suffers from slow ion diffusion kinetics,bad oxide scale adhesion,and weak compactness.Herein,we propose an electrochemical oxidation approach in molten carbonate to improve the oxide scale growth and adhesion via electrochemically regulating ion diffusion in the formed oxide scale.An anodic polarization applied at the metal substrate enlarges the chemical potential gradient of oxygen and offers a favorable electric field across the metal/oxide interface to the oxide scale/molten salt interface,thereby speeding up the oxide scale growth rate in terms of increasing the diffusion rate of metal cations and oxygen ions through the oxide scale.At the same time,the oxide scale adhesion is remarkedly im-proved because the metal vacancies at the oxide/alloy interface are filled by the newly formed oxide.After being treated by electrochemical oxidation,the corrosion rate of the Ni-16Fe-4Mo alloys in molten Li_(2) CO_(3)-Na_(2) CO_(3)-K_(2) CO_(3) decreased ten-fold.Overall,electrochemical oxidation enables the rapid formation of an adherent and corrosion-resistant oxide scale on the metal substrate,offering a general and effective way to fabricate corrosion-resistant films to survive in various aggressively corrosive media.