Iron migration from the anode surface in alumina electrolysis

Applied Surface Science 265 (2013) 790–795

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Iron migration from the anode surface in alumina electrolysis Elena N. Zhuravleva a , Tatiana N. Drozdova a , Svetlana V. Ponomareva a , Sergei D. Kirik a,b,∗ a b

Siberian Federal University, Krasnoyarsk, 660041, Russia Institute of Chemistry and Chemical Technology SB RAS, Krasnoyarsk, 660036, Russia

a r t i c l e

i n f o

Article history: Received 24 August 2012 Received in revised form 11 November 2012 Accepted 24 November 2012 Available online 30 November 2012 Keywords: Corrosion mechanism Alumina electrolysis Metallic inert anodes in aluminum production

a b s t r a c t Corrosion destruction of two-component iron-based alloys used as an anode in high-temperature alumina electrolysis in the melt of NaF/KF/AlF3 electrolyte has been considered. Ni, Si, Cu, Cr, Mn, Al, Ti in the amount of up to 10% have been tested as the dopants to an anode alloys. The composition of the corrosion products has been studied using X-ray diffraction, scanning electron microscopy and electron microprobe analysis. It has been established that the anode corrosion is induced by a surface electrochemical polarization and iron atom oxidation. Iron ions come into an exchange interaction with the fluoride components of the melted electrolyte, producing FeF2 . The last interacts with oxyfluoride species transforming into the oxide forms: FeAl2 O4 , Fe3 O4 , Fe2 O3 . Due to the low solubility, the iron oxides are accumulated in the near-electrode sheath. The only small part of iron from anode migrates to cathode that makes an production of high purity aluminum of a real task. The alloy dopants are also subjected to corrosion in accordance with electromotive series resulting corrosion tunnels on the anode surface. The oxides are final compounds which collect in the same area. The corrosion products form an anode shell which is electronic conductor at electrolysis temperature. The electrolysis of alumina occurs beyond the corrosion shell. The rate limiting step in the corrosion is the electrolyte penetration through corrosion shell to the anode surface. The participation of the released oxygen in the corrosion has not been observed. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The investigations on inert anodes are connected with one of the main directions of the aluminum industry modernization [1]. Their use offers the prospects of the revolutionary change of environmental consequences and decrease the expenses of the aluminum production. However, according to literature data an optimal material to be introduced in the production has not been found yet [1–5]. In large-scale investigations a great number of ceramic, metal-ceramic and metallic materials are considered [1,6]. The most perspective from the economic and technological point of view are metallic alloys. They have high mechanical strength and low electrical resistance. Their main drawback is a corrosion in the cryolite-alumina melt under the conditions of anode polarization. The mechanism of anode corrosion was widely studied in literature [1,4,7–9]. Xiao [8] noted that at the normal electrolysis regime with metal-ceramic anode the corrosion was due to the chemical dissolving of the anode and reduction of the corrosion products on the cathode. The authors in [9] came to a conclusion that the destruction of composite materials followed a mechanism of intergranular migration of the cryolite deep into the anode surface. The

∗ Corresponding author at: Siberian Federal University, Krasnoyarsk, 660041, Russia. E-mail address: [email protected] (S.D. Kirik). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.113

interphase boundary was characterized by high current density, resulting in their fast and deep degradation. According to Samoilov et al. [7] the corrosion of metallic single-phase anode alloys consisted of Ni,Fe and Al follows two mechanisms. In the first mechanism the oxidation of the surface layer occurs giving NiFe2 O4 ; NiO; Fe2 O3 . The second mechanism includes the penetration of the cryolite melt along the grain boundaries deep into the anode. The mechanism of channel-diffusion degradation was described in [5]. It includes the saturation of the anode surface with oxygen and formation of oxides with the dopants of the anode alloy. A similar mechanism of the metallic anode destruction was described in [2] as a selective dissolution of alloy components. This results in the formation of a porous layer and saturation of the nearelectrode sheath with the components of one of the metallic phase without destroying the second one. Iron-based alloys are the most perspective from the point of view of the technology. To improve the corrosion and heat resistance some metals such as Ni, Cu, Cr, Al are introduced into anode composition. The data available in literature shows that the corrosion rate can be decreased down to 2 cm/year, which is considered to be a perspective result [4]. However, many important details remain unclear, for example, the mechanism of electrolysis, the role of the released oxygen in the anode corrosion, formation of spinel phases in the near-electrode sheath, the migration of iron ions from the anode surface and some others [10–12]. Further improvement

E.N. Zhuravleva et al. / Applied Surface Science 265 (2013) 790–795 Table 1 Alloy compositions used for anodes.

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detector. The areas of the electron microprobe analysis are pointed out in the images.

Sample

Doped elements

Concentration (wt.%)

Fe100 Ni10 Cu10 Cr10 Мn10 Al10 Ti10

– Ni Cu Cr Mn Al Ti

– 10 10 10 10 10 10

of the anode material requires of the specification of the mentioned details, and as a whole, the improvement of understanding the mechanism of corrosion. In the present paper the mechanism of iron migration from the anode surface into the melt is specified at the example of twocomponent iron alloys with Ni, Cu, Cr, Mn, Al, Ti. Using X-ray diffraction, scanning electron microscopy and electron microprobe analysis the role of electrolytic polarization of the anode surface as the initial corrosion process has been revealed. The scheme of the processes occurring on the anode including the corrosion mechanism have been suggested on the basis of the obtained results. 1.1. Experimental and research methods The electrolysis was carried out in the laboratory electrochemical cell with the vertical electrodes. The cylindrical rods with the diameter of 8–10 mm and length of 80–150 mm, were used as the anodes. The alloy chemical compositions are given in Table 1. The process was carried out under the galvanostatic conditions at the anode current density of 0.5 A/cm2 . The experiment duration was 10 h. Potassium-containing cryolite melt at a temperature 900 ◦ C was used as electrolyte. After the electrolysis the anodes changed their volume and shape considerably. Fragile layers formed at the surface. To avoid the losses of the outer layers the samples were first covered with plastic and after the plastic polymerization, were cut in the cross-section. The microsections were made using the automated polisher/grinder Saphir 520 (Germany) and consumable materials fabricated by Lam Plan (France) according to the techniques supplied by «Latemi» Co Ltd. The microstructural analysis was made using the microscope Axio Obserber.A1 m, Carl Zeiss. To determine the phase composition the X-ray diffraction patterns were scanned using the X-ray diffractometer X’Pert Pro (PANalytical) equipped with PIXEL detector, graphite monochromator in diffracted beam and fine long focus tube with copper anode, The electron microprobe analysis was made in the selected points using the scanning electron microscope (SEM) EVO 50 HVP (Carl Zeiss) with an energy-dispersive analyzer INCA Energy 350 (Oxford Instruments). The current was 200–230 рA and acceleration voltage - 20 kV. The images of the alloy microstructure were obtained on microsections using the back-scattered electrons (BSE)

2. Results and discussion The investigation of the anode corrosion was carried out on the sample group of the electrodes made of the alloys based on iron with the addition of 10% wt. dopants. Due to iron being the basis of the alloys under study (90 wt. % and more) special attention was paid to the iron behavior during the electrolysis. The anode made of the pure iron is taken for comparison. The external view of the anodes taken out of the electrochemical cell after ten hours of electrolysis are presented in Fig. 1a and b. When an anode was taken out of the melt there remains a shapeless near-electrode layer resulting from the undissolved particles and the electrolytic melt associated with this zone. The distributions of the iron contained oxides and fluorides for each alloy were obtained on the basis of semi-quantitative X-ray diffraction phase analysis (XRD) (Fig. 2). The XRD has revealed that the near-electrode layer consisted mainly of crystallized electrolyte components independently of the anode composition. Iron difluoride FeF2 was the main «non-electrolyte» fluoride which amount varies from 6% to 10%. No fluoride phases of the dopants, except aluminum, were observed. Iron oxides (II,III) were present in all the samples analyzed. Hematite (Fe2 O3 )–was the final form of the oxidation. Its content varied in the wide range from 3% for Al10 to 44% for Fe100. Iron could an partly be present in the phases with the spinel structure: FeAl2 O4 and Fe3 O4 , Me(Fe,Al)2 O4 . The spinels locate in the anode sheath. There was the correlation in the content of the Fe2 O3 and Fe3 O4 phases. As the former decreased, the latter increased. Since the final oxidation form was hematite Fe2 O3 , this correlation was due to the transition of Fe(II) into Fe(III), indicating that initially iron was present in the near-electrode sheath in the form of Fe(II). In the near-anode sheath other phases were also observed including the doping components: ␣-Al2 O3 (7–12%), FeCr2 O4 (6%), NiO (0.5%), Cu (0.5%), Fe2,50 Ti0.5 O4 (10%), MnO2 (1%). The anode samples after the electrolysis were subjected to the electron microprobe analysis to study the layer composition in more detail. Fig. 3 shows the images of the cross-section of the sample Fe100, Cu10, Al10. The central metallic part is surrounded by a rather thick circle of the corrosion and electrolysis products forming the near-electrode layer. A certain asymmetry of the nearelectrode layer is consistent with the orientation of the anode relative to the cathode. At the cathode side the layer is thicker and more porous. The anode corrosion in this area is more significant, resulting in asymmetry form. It is obvious that the observed asymmetry is caused by more intensive electrochemical processes. Besides the total shift of the «section center», there was the corrosion irregularity observed across the anode. The irregularity of the given type is likely to be caused by the local inhomogeneities in the

Fig. 1. The anode layout after the high-temperature electrolysis.

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Fig. 2. The histogram of the phase distribution for the products of anode degradation in the near-electrode layer for the anodes under study.

Fig. 3. The microsection patterns of the anodes Fe100 (а), Cu10 (b), Al10 (c), observed under the optical microscope.

anode alloy. The electron-microscopy images (Fig. 4) confirm the existence of such inhomogeneities. Fig. 4а presents the SEM image of the near-electrode layer structure of the iron anode (sample Fe100). The irregular boundary of the metal, as well as the presence of some metallic areas in the near-surface layer (spectra 1,2) illustrate non-uniform corrosion. Directly near the metallic surface iron difluoride–FeF2 (spectra 9, 11) as well as the areas filled with the electrolyte without any oxygen content species (spectrum 10) can be observed. It should be noteed a considerable number of cations Na+ , K+ , Ca2+ penetrating onto the anode surface. It is obvious that the cation penetration onto the surface of the metallic anode is only possible within large anion aggregates. It indicates electrolyte penetration onto the metallic surface. The iron difluoride layer propagates further from the metallic surface (spectra 6, 7, 8). The composition in points 4, 5 correspond to aluminum fluoride (AlF3 ) with the small content of

electrolyte and iron difluoride. In the peripheral area (spectrum 3) oxides are observed, which are most likely to be in the form of spinel FeAl2 O4 . In general, the near-electrode layer does not have integrity and is intercepted with some segments which have different composition including metallic anode fragments. Certain cylindrical zones are clearly visible in the structure of the near-electrode sheath for Cu10 sample (Fig. 3b). At higher magnification at least, four zones can be distinguished (Fig. 4b). The outer boundary of the near-electrode sheath consists of a homogeneous oxide layer (Fig. 4b, spectra 1–3), which is likely to be a combination of the spinel phases and hematite. Copper was not found both at the outer boundary and in the inner zones. The inner porous degradation zone adjacent to the metal is a result of the surface etching of the anode. This zone is a fine «net-sponge» of the metallic phase consisting of copper and copper oxide, whose cells contain the etching products: iron difluoride, aluminum trifluoride

Fig. 4. The SEM images for the samples Fe100 (а, the metal is below), Cu10 (b, the metal is below), Al10 (c, the metal is above). The points and rectangles indicate the areas of the electron microprobe analysis.

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Table 2 The results of the electron microprobe analysis. Spectrum

O

F

Na

Al

K

Sample Fe100 (the areas of imaging are given in Fig. 4а) Spectrum 1 Spectrum 2 62.23 25.20 Spectrum 3 77.99 0.73 13.20 3.99 Spectrum 4 75.10 6.17 9.52 1.68 Spectrum 5 78.07 0.45 Spectrum 6 79.98 0.36 Spectrum 7 Spectrum 8 75.64 1.41 Spectrum 9 75.30 0.58 Spectrum 10 72.21 10.99 10.37 0.58 77.56 Spectrum 11

Ca

Fe

0.25 0.65

2.42

100.00 100.00 12.56 3.85 6.89 21.48 19.66 22.95 24.11 3.43 22.44

Sample Cu10 (the areas of imaging are given in Fig. 4b) Spectrum

O

Spectrum 1 Spectrum 2 Spectrum 3 Spectrum 4 Spectrum 5 Spectrum 6 Spectrum 7 Spectrum 8 Spectrum 9 Spectrum 10 Spectrum 11

49.18 49.43 50.21 33.42

8.77 8.46

F

27.97 76.11 75.70 76.15 74.45 71.80 59.99 56.67

Na

1.01 0.79

Al 8.85 7.06 7.80 17.59 13.22 13.27 0.92 1.02 8.02 4.52

K

Ca

4.94 5.07 0.55

41.98 43.51 41.99 21.02 4.71 5.17 0.26 22.10 25.55 27.18 11.52 17.11

1.23

Fe

Cu

10.46 13.23

Sample Al10 (the areas of imaging are given in Fig. 4c) Spectrum Spectrum 1 Spectrum 2 Spectrum 3 Spectrum 4 Spectrum 5 Spectrum 6 Spectrum 7 Spectrum 8 Spectrum 9 Spectrum 10 Spectrum 11 Spectrum 12 Spectrum13

O

F

Na

75.12 79.28 79.38 79.17 79.57 80.27

Al

K

Ca

24.88 8.06 19.99

12.67 0.63 20.83 20.43 19.73

76.06 26.98 72.99 74.14 67.78

Fe

12.07

8.09 9.25

0.32

0.24

2.45

17.05

6.14

0.35

100.00 100.00 23.94 64.93 5.13 25.86 6.23

and a little amount of the electrolyte (Fig. 4b, spectra 9, 10, 11). The metallic fragments of the «sponge» interchange with continuous homogeneous layers. In whole, electron microprobe analysis data of the sample Cu10 (Table 2b) indicate that the anode destruction has occured within the metal regions enriched with iron giving corrosion tunnels filled with iron difluoride. In this form iron was carried to the periphery. The continuous region adjacent to the porous layer contains a salt mixture of fluorides: aluminum trifluoride, iron difluoride and the electrolyte fragments (Fig. 4b, spectra 5, 6). Further, depletion of aluminum fluoride occurs and the next layer mainly consists of iron difluoride (Fig. 4b, spectra 7, 8). The sample Al10 was subjected to corrosion to the highest extent. The metal boundary in the alloy (Figs. 3c and 4c) was considerably destroyed and loosed its cylindrical shape. This is likely to be connected with the fact that the doping component in the alloy was dissolved faster than the main one due to its position in electromotive series. This is evidenced by metallic iron inclusions in the near-electrode sheath (Fig. 4c, spectra 7,8). The electrolyte containing sodium, potassium and calcium cations is observed nearby the metal surface (Fig. 4c, spectra 11, 13). The corrosion products did not form the layered structure. In general, the near-electrode volume is filled by iron difluoride (Fig. 4c, spectra 1, 3, 9, 13) and aluminum trifluoride (Fig. 4c, spectra 4, 5, 6), which are not separated by a sharp boundary. More distantly from the metal surface

Fig. 5. Schematic presentation of anode processes, phase location and the forms of iron migration. gr5.

there are irregular inhomogeneous areas of the metal and oxides (Fig. 4c, spectrum 10). Similar results are observed in all the samples under study. The SEM data confirm that the anode destruction is caused by the penetration of the electrolyte into the electrode metallic boundary and the interaction of the electrolyte with the anode surface, forming iron difluoride and aluminum fluoride. The electrolyte etching of the surface occurs non-uniformly with the penetration into the inter-crystalline areas, causing roughness, porosity and tunnels. As the iron difluoride is carried to the peripheral area, its interaction with the oxyfluoride species of the melt occurs, forming the spinel phases and, at the final stage, hematite which is collected in the outer near-electrode sheath. Schematically the iron migration from anode surface is reflected in the fig. 5. The dopants introduced into the iron-based alloys in the amount of up to 10 wt.%, exhibit different activity during the anode corrosion. The introducing more electronegative elements into the alloy e.g. aluminum, titanium and chromium in accordance with the electromotive series leads to the active dissolving of the metallic base of the alloy with the formation of a number of oxides and spinels on the basis of these elements. Besides spinels aluminum leads to the formation of aluminum fluorides. Manganese has passivating action, forming an insoluble oxide shell on the anode. Nickel can be transformed into the oxide and fluoride phases at lower rates than iron forming a low-conductive shell. Copper does not interact with the cryolite-alumina melt components and does not form oxides and fluorides. 3. Mechanism The electrolytic production of aluminum is possible due to the dissolving of alumina in the cryolite melt [11]. The metal corrosion in the medium of fluorine salts in the presence of ionizing factors was earlier investigated in relation to the nuclear energy production [13]. Under the alumina electrolysis conditions the corrosion process proceeds much more intensively. The mechanism and kinetic of the multi-component alloy corrosion are defined by a number of factors. The composition and microstructure of the anode alloy, electrolyte composition, electrolysis temperature,

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voltage on the anode and some other are among them. The most considerable factor is the high-temperature alumina electrolysis. The detailed mechanism of alumina dissolving is still unknown [14,15]. It is possible to assume that during the dissolving process complex fluoride-oxygen aluminum anions are formed. [1]. The fact that NaF, giving free fluorine ions to the melt, does not dissolve alumina, indicates the significance of hexafluoroaluminate ions [AlF6 ]3− in the dissolving process. The application of square brackets here and below emphasizes the importance of octahedron coordination of aluminum atom. The recent review devoted to inorganic fluorides fixes that the octahedron fluorine coordination is the most stable and usual for aluminum [16]. The maximum solubility of Al2 O3 in the cryolite melt Na3 AlF6 , amounts to about 15% (wt.), which is close to the molar ratio Na3 AlF6 :½Al2 O3 . This allows one to nominally describe the dissolving process by the following reaction: Al2 O3(s) + 2[AlF6 ](melt.) 3− → [AlO][AlF6 ]2− (melt.) + [AlO2 ][AlF6 ](melt.) 4−

(1)

The oxygen-containing complex anions are carried to the anode under the action of the electric field. In the melt for the charge compensation highly charged anions are accompanied by cations. On the anode the anion «discharge» occurs in the process of the oxygen «oxidation»: [AlO][AlF6 ](melt.) 2− → [Al][AlF6 ]0 + 1/2O2 + 2e−

(2)

[AlO2 ][AlF6 ](melt.) 4− → [Al][AlF6 ]0 + O2 + 4e−

(3)

The hypothetical polarized particles [Al][AlF6 ]0 , which reflect association of multiply charged ions in the melt, interact with the cations Na+ or К+ , resulting in complex cations which, according to the charge migrate to the cathode: [Al][AlF6 ]0 + Na+ → [Al][AlF6 ][Na]+

(4)

On the cathode they are reduced with the deposition of metallic aluminum: [Al][AlF6 ][Na]+ +3e− → Al0 + [AlF6 ]3− + Na+

(5)

The released hexafluoroaluminate ions repeatedly participate in the process of alumina dissolving. As a result, the dissolved alumina, in a complex way, migrate first to the anode, and, then to the cathode, accompanied by hexaflouroaluminate ions and cations «compensating» the charge. The diffusion rate of oxyfluoride ions in the near-anode sheath is less than that of hexafluoro-ions due to the difference in their weight. Besides, the loss of their electro-diffusion mobility is due to the decrease of their charge. The corrosion on the anode acts as an accompanying process. It can be caused by three factors: electrochemical oxidation of the anode metal due to the applied voltage, chemical oxidation by the oxygen released in the main process, oxidation by fluoride due to the overpotential voltage on the electrode in the absence of the oxygen phases. The measurements given here show the absence of oxygen directly on the metal surface. The electrolysis is carried out with the excess of alumina under the conditions excluding fluorine release. One should note that in some cases the presence of oxygen on the boundary with the metal is recorded. The electrochemical process of anode etching at positive polarization implies the appearance of iron cations on the surface or other alloy species with lower ionization potential: Fe0 → Fe2+ + 2e−

(6)

As a result of the electro-diffusion migration the hexafluoroaluminate anions accompanied by sodium and potassium cations

for compensating their charges, reach the anode surface and interact with the charged particles located on it, in particular, with iron cation for the formation of iron difluoride according to the exchange reaction of the following type: Fe2+ + Na2 [AlF6 ]− → FeF2 + NaAlF4 + Na+

(7)

Here, the anions Na2 [AlF6 ]− , neutral molecules NaAlF4 [17,18] and some other ones can be considered as fluoroaluminate species, which bring fluorine to anode. It is worth to note that the atomic ratio F:Al does not decrease below 3. Actually electrolyte fluorine is «consumed» for the iron difluoride formation and sodium cation receives uncompensated positive charge. In its turn, a sodium cation, being the most mobile particle, diffuses from the near-anode sheath into the electrolyte and carries the positive charge. The compound NaAlF4 known as highly fugitive [19] can escape the anode via gaseous phase. The iron difluoride forms crystalline particles which are detected by the XRD and electron microprobe analysis. In the frame of discussed scheme the alkaline cations are the indicators of electrolyte penetration into metallic surface and corrosion process. Oxygen-containing particles penetrate to the anode surface to a smaller extent. However, they also can interact with iron ions or other alloy components, yielding oxides according to the approximate reactions: Fe2+ + Na2 [AlO][AlF6 ] → FeO + [Al][AlF6 ] + 2Na+ 3+

2Fe

+ 3Na2 [AlO][AlF6 ] → Fe2 O3 + 3[Al][AlF6 ] + 6Na

(8) +

Cu2+ + Na2 [AlO][AlF6 ] → CuO + [Al][AlF6 ] + 2Na+

(9) (10)

It is important to note that the hypothetical species [Al][AlF6 ] can transform to crystalline phase AlF3 , which is really observed in sample Cu10. Similar reactions describe the interaction of iron difluoride with oxyfluoride ions of aluminum with the formation of the observed spinel phases more distantly from the anode surface: FeF2 + [AlO][AlF6 ]2− → FeO + 2[AlF4 ]− FeO + 3[AlO][AlF6 ]

2−

→ FeAl2 O4 + [Al][AlF6 ] + 2[AlF6 ]

(11) 3+

(12)

There are experimental evidences that fluorides transform into oxides. Small oxide nucleation centers, which can be observed under additional magnification on boundary of fluorides and oxides, develop up to form oxides crust of anode area. Appeared oxides are dissolved poorly in electrolyte and are not subjected to electrolysis. Further oxidation of Fe2+ into Fe3+ observed on the anode can occur during the direct oxidation by the released oxygen or according to the electrochemical reaction: Fe2+ → Fe3+ + e−

(13)

And, further with the formation of magnetite and hematite: 2FeO+ + Na2 [AlO][AlF6 ] → Fe2 O3 + [Al][AlF6 ] + 2Na+

(14)

The experimental observation of hematite location on the external layer of near-electrode sheath prompts that the oxygen discharge locates in the same area distanced from metallic surface. The hematite, magnetite, iron-aluminum spinel phases migrate gradually to the periphery of the near-anode sheath due to corrosion process development and their poor solubility in the electrolyte. They form a crystallite suspension around the anode which is observed when taking the anode out of the melt. These oxides are not subjected to electrolysis because of solid state. The most probable location of the alumina discharging seems to be the boundary between of external oxide layer of near-anode sheath and electrolyte.

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4. Conclusion Thus, based on the XRD, SEM and electron microprobe analysis of the near-electrode sheath it is possible to reproduce the main stages of the corrosion of the iron-containing anodes at alumina electrolysis. The scheme is presented in fig. 5. Due to the electrochemical polarization of metallic surface the oxidation of the metallic iron into Fe2+ occurs on the anode surface. The formed ions interact with the electrolyte penetrating to the anode giving iron difluoride. Electrolyte diffusion to metallic surface seems to be rate limiting stage of the anode corrosion and the presence of sodium and potassium cations nearby the metallic surface is the indicator of corrosion. Iron difluoride is displaced by electrolyte from the metallic surface to more distance area where iron difluoride interacts with oxyfluoride polynuclear aluminum complexes, producing iron-aluminum spinels. Upon further oxidation magnetite and hematite are formed. The oxide phases migrate to the periphery of the anode area, forming a dense peripheral layer on the electrode. In whole it is become clear that kinetic phenomena more than thermodynamic manage the corrosion process. The presented results urge to the conclusion that the key direction in the “inert anode” problem is the inhibiting electrolyte diffusion to anode surface. Acknowledgements The authors grateful to prof. P.V.Polykov for fruitful discussion of the paper results and “RUSAL ETC” (the project “Electrolysis with inert anode”) for financial support and the permission to publish. References [1] I. Galasiu, R. Galasiu, J. Thonstad, Inert Anodes for Aluminium Electrolysis, Aluminium-Verlag, Dusseldorf, 2007, p. 207. [2] E.N. Lyndina, T.N. Drozdova, S.V. Ponomareva, V.S. Biront, A.O. Gusev, D.A. Simakov, Simulation and experimental study of the mechanism of degradation channel-diffusion of due to high temperature corrosion of anode metal alloy, Journal of Siberian Federal University. Engineering & Technologies 2 (2010) 139. [3] V.A. Kovrov, N.I. Shurov, A.P. Khramova, Y.P. Zaikov, Character and causes of corrosion destruction of inert anodes in the electrolysis of cryolite-alumina melts, Non-Ferrous Metallurgy (Rus) 5 (2009) 46.

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