Refractories and Industrial Ceramics

Vol. 53, No. 4, November, 2012

ASPECTS OF THE CORROSION OF REFRACTORIES IN STRUCTURED AGGRESSIVE MEDIA S. V. Mulevanov,1 V. M. Nartsev,1 V. A. Doroganov,1 E. A. Doroganov,1 and S. V. Zaitsev1 Translated from Novye Ogneupory, No. 7, pp. 42 – 44, July, 2012.

Original article submitted June 26, 2012. This article examines aspects of the testing of refractories for corrosion resistance by different methods. Studies were made of the corrosion resistance of refractories under dynamic conditions by using a special unit based on rotation of the test specimen in a melt of the corrodent (boron-bearing ultrahard flint, 65% PbO, 35% B2O3). It was determined that corrosion rate increases in proportion to the depth of immersion of the specimen in the melt. A new method is proposed for determining corrosion rate, the method making it possible to significantly shorten corrosion tests and improve the accuracy of the results. Keywords: corrosion, refractory materials, aggressive media, methods of evaluating the corrosion resistance of refractories.

The different types of aggressive media in which refractories are used can be conditionally divided into unstructured media (melts of metals and most salts), weakly structured media (melts of salt-bearing glasses and metallurgical slags with a low content of glass-forming components), and structured media (glasses having a high content of glass-forming components and containing additions, such as P2O5, B2O3, and ZrO2, that increase high-temperature viscosity). The corrosion of a refractory in an aggressive melt is a complex process that includes chemical reaction of the components of the melt with the refractory material to yield products that were absent previously, physical dissolution of the components of the refractory with a change in their phase state (including polymorphic transformation and the formation of solid solutions), and physico-mechanical wear (erosion); the latter is more pronounced for structured media. Here, high-temperature viscosity can serve as an integral index of the degree of structuralization of the refractory. High-temperature viscosity characterizes the internal friction between the layers of the melt during deformation in shear [1]. The chemical reaction of the refractory with the melt is a typical heterogeneous reaction which includes three main stages: delivery of the reacting components to the reaction surface by molecular diffusion or convection; direct reaction at the surface; the movement of the reaction products back 1

into the melt [2]. The rate of the reaction phase of the process depends on the composition of the refractory and the acidbase properties of the corrosive medium. Acid refractories are more stable in non-alkaline and low-alkali melts. In addition to accounting for the composition of the refractory and the acid-base properties of the aggressive medium, any evaluation of corrosion activity must also consider the activity of the oxygen anions. As was noted in [3], ions of free oxygen O2– are the most active anions. The ion fraction of free oxygen NO2– for glass and slag can be evaluated by means of the Temkin formula N O2– = NFeO + NMgO + NCaO – 2N SiO – 2 – 3NP O – 3NAl 2 5

2O3

.

The initial stage of corrosion of the refractory is directly dependent on the concentration of free oxygen in the melt. Within the contact layer, ions of free oxygen can be transferred to the solid phase of the refractory and form a low-viscosity melt, which accelerates the refractory’s destruction. As the thickness of the reaction layer subsequently increases, the corrosion rate comes to depend mainly on diffusion. The dependence of corrosion rate K on temperature T is exponential, i.e. it obeys the Arrhenius law [4]: K = K 0e

Belgorod State Technological University.

-

E RT

,

226 1083-4877/12/05304-0226 © 2012 Springer Science+Business Media New York

Aspects of the Corrosion of Refractories in Structured Aggressive Media where K0 is a pre-exponential multiplier; E is activation energy (a quantity that is determined experimentally); R is the universal gas constant. Thus, the higher the temperature and the lower the activation energy of the diffusion process, the more actively the corrosion process will take place. The components that are the most aggressive in relation to a zirconium-bearing refractory are B2O3 and F–. The compound PbO is especially aggressive, with corrosion rate increasing many-fold when it is present in high concentrations (such as in glass solders and optical glass STF-11, which contain up to 65% PbO). High aggressiveness is also manifest by the salt fluxes Na2CO3 and Na2SO4, which form sulfate lye during glassmaking. Counter-diffusion of the components of the refractory and melt takes place during the reaction. The alkali cations flow most rapidly from the glass-forming melt to the refractory, while the oxides of Mg, Ca, and Si exhibit less mobility. It has been established that in terms of mobility the components can be ranked as follows (the parentheses indicate the components that are inert in the intermediate layer) [4]: Na2O, MgO, CaO, SiO2 (Al2O3, ZrO2, Cr2O3). The most important indices that limit the process of physical dissolution are the content of the amorphous glassy phase, the composition and morphology of the refractory phases, the dimensions of the crystallites, the formation of concretions, and the presence of pores and voids (shrinkage cavities) that facilitate penetration of the melt into the deep layers of the refractory product. In accordance with the diffusional character of the process, the corrosion of refractories usually increases with a decrease in the viscosity of the melt. In this case, the extent to which the melt penetrates the pores and cavities is determined by the wettability of the material and the rate of the capillary phenomena which take place. The capillary effects in turn depend mainly on the surface tension at the phase boundaries [6]. The rate of mechanical erosion depends on the strength characteristics of the refractory product, its structural-textural features, and the viscosity characteristics of the corrosive medium [3]. There are several different methods of evaluating the stability of the refractory materials that are used in aggressive media (melts of metals, salts, glasses, and slags). To a significant extent, the accuracy of the results obtained by these methods determines the progress that is made in improving the chemical and phase compositions of refractories and their structural and technical characteristics. All of the methods presently used to evaluate the corrosion resistance of refractories can be classified [7] as static or dynamic. Static methods are those in which the corrosive medium does not move, while dynamic methods are methods in which the corrosive liquid moves relative to the refractory. One of the static tests that is used is the sessile-drop test. Here, a specimen of glass or slag is placed on a substrate made of the refractory being tested, heated to a stable temperature, and kept at that temperature for a prescribed length of time. This results in wetting of the refractory by the slag

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and their chemical reaction with one another. The contact angle is used to determine the wettability of the refractory by the corrodent and a prediction is made on the refractory’s durability. The impregnation method entails keeping the corrodent at a prescribed temperature inside a crucible made of the test refractory for an extended period of time and determining the depth of penetration of the crucible by the medium. In evaluating corrosion resistance by the immersion method, a refractory specimen of square or circular cross section is held for a long period of time in a melt having a prescribed temperature; the specimen is weighed before the test. The specimen is extracted from the slag after a certain length of time. The slag that adheres to the specimen must be melted or mechanically removed. The rate n, g/(cm2·sec), at which the refractory dissolves in the slag during the time of the test is determined from the formula n = m/St, where m is the amount of refractory that is dissolved, g; S is the area of the surface of contact between the refractory and the slag, cm2; t is the duration of the test, sec. All of the static methods have shortcomings: low sensitivity, lengthy test durations, and the complexity of accounting for concentration gradients of the components in the melt. There may not be a clear reaction boundary between the melt and the refractory, which would also significantly decrease the sensitivity and accuracy of the method. Dynamic methods are largely free of these problems. The most widely used dynamic method is the rotation method. Here, a 20 ´ 20 ´ 120 mm specimen of the test refractory with a square cross section is immersed in a melt of the corrodent at a prescribed temperature and rotated at a certain velocity for several days. At the end of the test, measurements are made to determine the diameter of the neck on the section of the specimen that underwent the most corrosion [8]. In accordance with the standard OST 3-4230–78, the linear rate of corrosion of the refractory Ky, mm/day, is calculated by means of the following formula in this method Kó =

( d av - d¢av ) ´ 24 2t

=

12Dd , t

where dav and d¢av are the average thicknesses of the specimen at the level of the upper boundary of the melt before and after the test, mm; t is the length of time the specimen is in the melt, days; Dd is the difference between the thicknesses of the specimen at the level of the melt before and after the test, mm. The problems with this method are the duration of the test, the sizable error in the determination of the difference in specimen thicknesses at the level of the melt, and the effect of possible fluctuations in that level. We have proposed an accurate express method of determining the rate of corrosion of refractories under dynamic

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Fig. 1. Diagram of unit for dynamic testing of the durability of refractories in structured corrosive media: 1) knob for controlling the position of the specimen; 2) corundum crucible; 3) melt of corrosive medium; 4) column of hoist; 5) refractory support; 6) base of column; 7) silicon-carbide heaters; 8) electric furnace; 9) refractory roof of furnace with opening; 10) specimen of refractory being tested; 11) clamp for specimen; 12) bracket of rotary drive; 13) electric drive to rotate specimen.

conditions [9]. The corrosion test is speeded up by conducting it in the most aggressive corrodent — boron-bearing ultrahard flint (65% PbO, 35% B2O3). Dynamic tests were performed on the unit shown in Fig. 1. A 0.5-liter corundum crucible containing cullet was placed in an electric furnace with silicon carbide heaters. A dye (0.2 wt. % CoO) was added to the glass in order to obtain a distinct corrosion boundary. The furnace was heated to 1400°C at a rate of 0.5 – 0.8 K/sec. The square test specimen, with dimensions of 20 ´ 20 ´ 250 mm, was secured in a special holder on the rotary drive. Then the drive was turned on and rotated at a speed of 8.5 rpm. The specimen was immersed in the molten glass in such a way that the bottom part of the specimen was submerged 30 – 35 mm deep into the melt. The tests were conducted over a period of 12 h. At the end of the test, the specimen was extracted and cooled to room temperature and cut lengthwise with a diamond disc. It is being proposed that the rate of destruction of the refractory be measured based on the reduction in the cross-sectional area of the part of the specimen located in the melt. The cross-sectional area of the specimen after testing is determined by computer analysis of a scanogram of the lengthwise kerf cut. In this case, without allowance for round-off errors the linear rate of corrosion is calculated from the formula Kó =

Dx S 0 - S 1 = t 2ht

where Dx is the linear corrosion, mm; t is the duration of the test, days; S0 and S1 are the cross-sectional areas of the specimen before and after the test, mm2; h is the depth of immersion of the specimen in the melt, mm. It follows from the above formula that the accuracy of the measurement of corrosion rate increases in proportion to the depth of immersion of the specimen in the melt. The method being proposed here for determining corrosion rate makes it possible to significantly (by several-fold) shorten the tests and appreciably improve their accuracy, which is especially important when studying the effect of modification of the surface of refractories through the application of coatings by magnetron sputtering or plasma deposition. The studies discussed here were conducted within the framework of the Federal Incentive Program “Scientific and Scientific-Pedagogical Cadre for an Innovative Russia” for the period 2009 – 20013. The overriding objective of the studies was “The Development and Processing of Composite Ceramic Materials,” GK No. 14.740.11.1076. REFERENCES 1. W. E. Lee and S. Zhang, “Direct and indirect slag corrosion of oxide and oxide-c refractories,” in: VII International Conf. on Molten Slags, Fluxes, and Salts (2004), pp. 309 – 319. 2. O. N. Popov, “Kinetics of the interaction of fused-cast refractories with melts of commercial glasses,” in: Research on Refractories for Glassmaking Furnaces: Symposium. GIS, Moscow (1984), pp. 8 – 17. 3. K. K. Strelov and P. S. Mamykin, Refractories Engineering: Text [in Russian], Metallurgiya, Moscow (1978). 4. E. N. Gramenitskii and A. M. Batanova, “Principles of the interaction of refractories with glass-forming melts in light of the theory of zonal diffusion,” Steklo i Keramika, No. 2, 9 – 13 (1996). 5. O. N. Popov and N. M. Galdina, “Glass resistance of electrofused baddeleyite-corundum refractories,” Ogneupory, No. 10, 54 – 59 (1972). 6. M. Velez, J. Smith, and R. E. Moore, “Refractory degradation in glass tank melters. A survey of testing methods,” Ceramica, No. 43, 283 – 284 (1997). 7. A. N. Smirnov and M. V. Epishev, “Methods of studying the processes involved in the interaction of slag melts with a refractory,” Naukovi Pratsi Donets’kogo Natsional’nogo Tekhnichnogo Univesitetu. Seriya Metalurgiya, No. 12 (177) 35 – 39 (2009). 8. I. D. Kashcheev, Testing and Inspection of Refractories: Text [in Russian], Intermet Inzhiniring, Moscow (2003). 9. S. V. Mulevanov, V. M. Nartsev, V. A. Doroganov, and S. V. Zaitsev, “Improving the methods used to determine the stability of refractories in aggressive media,” Novye Ogneupory, No. 3, 70 (2012).