Corrosion of refractories

Thermal stability and corrosion resistance are greater in refractories than in other ceramics. This is because refractories operate at higher temperatures. Corrosion, being a chemical reaction, get enhanced by higher temperatures. Corrosion most often determines refractory life, and often enough sets the temperature limit on the contained process. 

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Corrosion of refractories and ceramics involves consideration of (i) acid-base effects involved in the corrosion of refractory (ii) verification of the thermal stability of each constituent (iii) calculation of the free enthalpy of all possible reactions that might occur in the corrosion of the refractory or ceramic compound. Kinetic data are also useful in understanding the corrosion and selection of the refractory compound for a specific application. 

All these processes which are based on mass transfer, are essentially affected by heat transfer and melt convection. The undesirable inherent processes involve corrosion of refractories and vaporization of the volatile glass components. 

The glass melting process always involves a fairly intensive melt flow it must be taken into account when melting in pots or tanks, but is of special significance in the case of continuous tank furnaces where it is a prerequisite of correct furnace function. Melt flow has the positive effect of accelerating the mass and heat transfer. Increased corrosion of refractories and possible carry-over of unmelted batch into the refining and working zone are its undesirable consequences. 

The main factors taking part in the corrosion of refractories by melts can be summarized as follows ... 

Although many different types of investigations in the area of alkali corrosion of refractories have been performed, there still appears to be doubt regarding the best method for testing. In addition, there are few comparative studies in the literature showing the differences in alkali corrosion of the different alumino-silica refractories. Studies concerning silicon carbide bricks exposed to alkalies are practically nonexistent. 

Decomposition of batch materials can produce extremely large quantities of gases such as CO2, SO3, NOx, H2O, etc. Reactions with metals in contact with the melt can generate oxygen, carbon dioxide, or hydrogen by electrolytic reactions. Corrosion of refractories can open previously closed pores to the melt, releasing the gas contained in those pores into the melt. Residual carbon in refractories, or carbide refractories such as SiC, can react with oxide melts to form CO2 or CO. The products of all of these reactions can agglomerate to form bubbles. 

Reduction of temperatures of glass melting and pouring on the account of boron additives specially included into an initial solution. This will allow a decrease in the melting temperature up to 850°C and thereby lessen corrosion of refractory, electrodes, water-cooled elements, and sensors of control and measuring instruments. 

DiStefano, J. R., Corrosion of Refractory Mettds by Lithium," Oak Ridge National Laboratory Report, ORNL-3551, March 1964. 

It is well known that corrosion of refractories used in the superstructure of glass melting furnaces can occur due to reaction with components of raw batch (also known as batch carryover) such as silica sand and soda ash and also from vapor phase species, such as NaOH. While there is a standard test available from ASTM for corrosion of refiectories from vapors (C987), there is no standard test available for testing corrosion from batch carryover. The ASTM standard, C987, requires the use of either alumina or platinum crucible for melting batch components that produce vapors, such as sodium carbonate. For the purpose of this study, the authors chose to prepare crucibles directly from the fusion-cast AZS and vibro-cast AZS products. 

Fig. 1.25 General view of chemical corrosion of refractories by liquid phase...

Volume effects of the reactions should also be taken into accotmt (not only in gas corrosion of refractories). The reactions of oxidation of nitride-bonded silicon carbide side lining are positive. The positive volume effect of the reaction may play a positive role, diminishing the open porosity (Table 1.11). However, on the other hand, it may cause tensile strains, which may result in either spalling or cracking of the refractory (Fig. 1.26). 

Corrosion of refractories by melts takes place in reduction pots and the cast house. The melt of liquid metal or the bath contacts the surface of the refractory wall, made from bricks or from castables. The chemical interaction between the constituents of the melt and the constituents of the refractory takes place, and all chemical principles of interaction between liquid and sohd reactants should be taken into account. The chemical nature of reactants (acid-base) may also be a factor. 

Brosnan D. Corrosion of refractories. In Charles S, editor. Refractories handbook. New York CRC Press 2004. p. 39-77. 

Rigaud M. Corrosion of refractories and ceramics. In Winson R, editor. Uhlig s corrosion handbook. New York Wiley 2011. 

Sodium on the surface of molten aluminium may also add its input in the corrosion of refractories. Reactions (3.13) and (3.14) proceed with a negative volume effect, which also doesn t promote the formation of continuous alumina film ... 

In industry, there is a belief [2, 3] that the corrosion of refractories by molten Al is inversely proportional to the alumina content in the refractories. This point of view is not quite correct, because alumina can react with magnesium, giving alumina magnesia spinel. The reaction (Table 3.4) proceeds with a positive volume effect, which may cause spalling and cracking. 

DEM CENATS 15418 2006-09. Methods of test for dense refractory products - Guidelines for testing the corrosion of refractories caused by liquids German version CEN/TS 15418 2006. 

H. J. Oel Max Planck Institute) It has generally been observed that water vapor enhances reactions in which Si02 is present in one form or another and in which other oxides are involved. I am especially thinking of the corrosion of refractory materials by molten glass and slags. It seems there exists some interaction between Si02 and H2O to cause this. 

Redox reactions are more common in the corrosion of refractories— for example, there are redox agents in combustion atmospheres. 

Corrosion of refractories can be defined for the purposes of this discussion as follows Corrosion of refractories is refractory wear by loss of thickness and mass from the exposed face of the refractory as a consequence of chemical attack by a corroding fluid in a process in which the refractory and the corroding fluid react, approaching chemical equilibrium in the zone of contact between the refractory and the fluid. 

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