Corrosion of Cast Iron Sections of Gas-Collecting Bells of Ecosoderberg Electrolyser

The article presents research results focused on high-temperature gas corrosion of sections of EcoSoderberg electrolyzers’ gas-collecting bells (GCB) made of high-strength VCh50 cast iron with spherical graphite. The gravimetric method was used to study the specific mass losses of the sections due to corrosion. The microstructure of cast iron, structure, chemical and phase composition of corrosion products were studied using optical, electron microscopy and electron microprobe analysis. It was established that the specific weight loss of the sections during operation reaches 0.36–0.46 g/(cm2 month). Corrosion of cast iron sections of EcoSoderberg electrolyzers’ GCB is characterized by high unevenness by area. There are cases of decommissioning sections due to local through “burnouts” with a weight loss of 19–24 kg. With relatively uniform corrosion, the maximum allowable weight loss of the sections is 25–30 kg. To make predictive estimates based on experimental data, dependence of the sections’ mass loss on the operating time was obtained. It was found that the corrosion products of the sections consist of iron oxides and alloying elements of cast iron. Most samples are characterized by increased content of C, S, F, K, Al, and Na. Corrosion products have a pronounced layered structure and contain a large number of defects in the form of pores and cracks. The layers differ in chemical, phase composition, and macrostructure. All the studied samples are characterized by cyclic alternation of relatively dense layers of iron oxides Fe2O3 and Fe3O4 and more porous layers between them. The layers are characterized by increased content of C and F. Sulfur is evenly distributed over the thickness of corrosion products. The studied samples of corrosion products have high defectiveness, friability, large number of pores, cracks, discontinuities, and low adhesion to the surface of cast iron. This is due to the presence of phases and compounds with different coefficients of thermal expansion. The mechanism of corrosion product layer formation was established and scientifically proved.


INTRODUCTION
Presently, Russian manufacturers producing primary aluminum actively adopt EcoSoderberg technology [1]. Its obvious advantages are decreasing of the level of hazardous effluents of fluorides, dust, sulfur dioxide and tarry material, increasing of daily average efficiency of covering of electrolyzers [2]. Realization of EcoSoderberg technology required introduction of changes into the construction of electrolyzers, particularly of gas-collecting bell (GCB) and significantly modified its operation conditions [3].
Analysis of operation experience of sections of GCB of EcoSoderberg electrolyzers by JCS "RUSAL Krasnoyarsk", made of high-strength cast-iron VCh50 with spherical graphite using cast technologies, showed that temperature of gases in central cupola of gas removal system reaches 800°C, and maximal temperatures of cast iron sections constitute 600-620°C, minimal values are at level of 220-230°C.
Growth of service temperatures in combination with the aggressive impact of gas medium led to intensification of the corrosion processes of cast iron sections of GCB of EcoSoderberg electrolyzers. This caused reduction of their operational lifetime due to local burnouts. Most importantly, in the view of providing of quality of primary aluminum, is contamination of its melt with admixtures of iron [5], which are supplied with products of corrosion of cast iron section of GCB. According to data of work [6], about 20% of all the iron supplied to melt of primary aluminum from non-resource sources is introduced exactly by corrosion products of cast iron sections of GCB Thus complex researches of the processes of hightemperature of gas corrosion of sections of GCB of EcoSoderberg electrolyzers made of high-strength cast irons VCh50 with spherical graphite were carried out.

METHODOLOGY
The methodology included estimation using gravimetric method of specific mass losses of parabolic sections S-8BM (E) during operation from a unit area A (g/cm 2 ) and from a unit time δm (g/(cm 2 month)). Mass of new sections before operation and after operation (at the same time the inner surface was cleaned from remainders of electrolytes and products of corrosion) was determined using scales. The obtained difference of masses was used in computation of A and δm. Chemical composition of cast iron and products of corrosion were determined using X-ray fluorescence spectral analyzer. The microstructure of cast irons and products of their corrosion were studied using methods of optical (microscopes OLIMPUS-GX 51 F and Laboet-I1), electron microscopy (scanning electron microscope (SEM, TESCAN VEGA 3)) and electron microprobe analysis (EMPA, OXFORD AZtec). Microsections were made of samples, cut from sections, using sandpapers of different grain size and polishing on cloth using chromium oxide. Four percent alcohol solution of nitric acid was used to reveal the microstructure of cast irons.

RESULTS AND DISCUSSION
Estimation of specific mass losses of sections during operation established that they reach 0.36-0.46 g/(cm 2 month). Figure 1 presents dependence of mass loss on operation time.
Increasing of operation time ( Fig. 1) leads to decreasing of intensity of corrosive processes. This phenomenon is connected with the layer formation of corrosion products on the surface of cast iron, which decelerates diffusive processes [7].
The process of high-temperature gas corrosion of cast iron sections of GCB of EcoSoderberg electrolyzers is characterized by high irregularity by the area on the inner surface (Figs. 2a, 2b). Cases of withdrawal of sections from operation due to local through "burnouts" (Figs. 2a, 2c) at mass loss of only 19-24 kg are observed. At relatively regular corrosion (Fig. 2d), maximal acceptable mass loss constitutes 25-30 kg.
Graphite in the new section of GCB is represented in the form of spheres (Figs. 3a, 3b), prevailing average size by the whole cross-section of samples constitutes 46-60 μm. The microstructure-ferrite + perlite + graphite (high-strength cast iron on ferrite-perlite base).  Shape and view of graphite particles of cast iron samples differ before and after operation. Prevailing average size of graphite of cast iron samples after operation from the side of operating edge is 67-103 μm (Fig. 3c), the particles themselves have a shape of irregular sphere. Graphite particles from the opposite side of the sample is of more regular size, mainly 54-74 μm (Fig. 3d).
Microstructure photographs of surface layers of cast iron sections contacting with the electrolyzer atmosphere distinctly show regions of oxidation of metallic mass along graphite additions, which is in good agreement with the research results [8].
The chemical and phase composition study of corrosion products of cast iron sections showed that they mainly consist of iron oxides and alloying elements of cast iron. Most samples are characterized by the presence of increase content of С, S, F, K, Al, Na. X-ray phase analysis confirmed the presence of oxides of iron-magnetite (Fe 3 O 4 ) and hematite (Fe 2 O 3 ) as well as elpasolite (potassium, sodium, aluminum, fluoride-containing compounds Ka 2 NaAlF 6 ) and weberite (Na 2 MgAlF 7 ).
Hematite is formed at the expense of oxidation of magnetite. At the same time, hematite and magnetite form a lattice structure of intergrowth, which generates intracrystalline stress leading to formation of microcracks in crystals, filled with aluminofluorides and glass. Corrosion products may contain carbon in the form of small graphite plates.
Investigation of the microstructure and chemical composition of corrosion products using scanning electron microscopy and electron microprobe analysis showed that they possess clear layered structure and contain numerous defects in the form of pores and cracks. Layers differ by chemical, phase composition and macrostructure. Cyclic alternation of relatively dense layers of iron oxides Fe 2 O 3 and Fe 3 O 4 and more porous embedments between them is typical for all the studied samples. The embedments are characterized by increased content of carbon and fluoride. Sulfur is distributed quite regularly by the thickness of corrosion products.
The distinguishing features of the researched samples of corrosion products are high defectiveness, looseness, the presence of large amount of pores, cracks, continuity violations (Fig. 4), low adhesiveness to the surface of cast iron. This is the result of the presence of phases and compounds having different coefficients of temperature expansion.
Cyclic changes of the section temperature during operation lead to appearance of structural and phase stresses in corrosion products, which promote its loosening.
Importantly, when temperature of section changes from 220 to 620°C, a layer of wustite is formed on the surface of cast iron in corrosion products at temperatures over 560-570°C, which is decomposed into iron and magnetite at lower temperatures. Cyclicity of this process negatively impacts continuity and protective properties of the layer of corrosion products.
Carbon in embedments of corrosion products is also supplied by gas atmosphere, which results from the interaction of melt aluminum with carbon oxide 4Al + 6CO = 2Al 2 O 3 + 6C; 2CO = CO 2 + C contains carbon in the form of graphite or carbon black forming coal froth [9,10] as one of the products.
The presence of carbon in embedments is also connected with parallel processes of decarbonization of cast iron in gas medium containing oxidizing and deoxidizing components.
According to work data [8,11], as a result of decarbonization, oxide film may be partially deoxidized, become loose and thin, may puff up and get covered by accretions filled with soot carbon. Figure 5 presents the scheme of dynamics and the mechanism of high-temperature gas corrosion of cast iron sections of gas-collecting bell off EcoSoderberg electrolyzers.
The initial state (Fig. 5a) corresponds to microvolume of the new section, whose surface is not covered by corrosion products. Operation under the influence of temperature lower than 570°C and oxidizing medium leads to the layer formation of corrosion products from hematite and magnetite (Fig. 5b) on the surface of cast iron. Further due to structural, phase and thermal stresses cracks, chips and other continuity violations appear in it (Fig. 5c). Their formation is intensified by growth of cast iron, cyclic changes of section temperature, increased thickness of the layer of corrosion products.
Continuity violations open the contact of the cast iron surface with atmosphere.
Atmosphere also including solid particles (such as carbon, for example in the form of carbon black or graphite), penetrates the cavity, where these particles are deposited on the walls, and oxidizing aggressive gases form a new layer of hematite and magnetite (Fig. 5d) on the surface of cast iron and oxidize magnetite surrounding the cavity to hematite. At the same time, walls of the cavity, containing compounds with various chemical and phase compositions, begin acting as a strong stress concentrator with changing of section temperature and cracks forming in them serve as the next channels for penetration of oxidizing atmosphere (Figs. 5e, 5f).
In this way, multiple formation of such cavities and continuity violations takes place in macrovolume, which leads to loose layer formation of corrosion products on the section surface. Gravity forces and gas fluxes partially destroy corrosion products, which additionally intensifies corrosive processes and contaminates the malt of primary aluminum with admixtures of iron. Transition to temperature domain over 570°C and further cooling are important. In such conditions, wustite, decomposing at cooling, is formed on the surface of cast iron. This process causes additional stresses, which promote the violation of protective properties of the corrosion product layer. The presence of carbon in cavities under specific conditions promotes deoxidizing processes, for example, deoxidation of hematite to magnetite. Fluoride-containing and sulfur-containing components of the atmosphere are intensifiers of high-temperature gas corrosion and increasing of their content in gases contacting with cast iron sections of gas-collecting bell leads to their faster destruction.
Researches carried out in this work complement and in many respects confirm the mechanisms and peculiarities of corrosion of cast iron sections of GCB presented in works [5,12,13]. Corrosion process intensification of cast iron sections of GCB of EcoSoderberg electrolyzers is primarily caused by increasing of their service temperature and transition to application in anode mass of coxes with higher content of sulfur and vanadium [5,[14][15][16][17][18][19][20].

CONCLUSIONS
The processes of high-temperature gas corrosion of GCB sections of EcoSoderberg electrolyzers made of high-strength cast iron with spherical graphite VCh50 were researched. It was established that specific mass losses of sections during operation reach 0.36-0.46 g/(cm 2 month), and corrosion of cast iron sections of GCB of EcoSoderberg electrolyzers is characterized by high irregularity by area. Withdrawal cases of sections from service in virtue of local through "burnouts" at mass loss of 19-24 kg are observed. At relatively regular corrosion, maximal acceptable mass loss of sections constitutes 25-30 kg.
Mass loss dependence of sections on time was obtained to realize predictive estimations based on experimental data. It was established that corrosion products of sections consist of oxides of iron and alloying elements of cast iron. Most samples are characterized by the presence of increased content of С, S, F, K, Al, Na. Corrosion products have a distinct layered structure, as well as contain large number of defects in the form of pores and cracks. Layers differ by chemical and phase composition as well as macrostructure.
All the studied samples are characterized by cyclic alternation of relatively dense layers of iron oxides Fe 2 O 3 and Fe 3 O 4 , as well as more porous embedments between them. The embedments are characterized by increased content of carbon and fluoride. Sulfur is distributed uniformly by the thickness of corrosion products. The peculiarities of the researched samples of corrosion products are high defectiveness, looseness, the presence of large number of pores, cracks, continuity violations, and low adhesion to the surface of cast iron. Thus, this is caused by the presence of phases and compounds having different coefficients of temperature expansion.
The formation mechanism of layers of corrosion products was established and scientifically substantiated.