Role of structural factors in improving the corrosion resistance of refractories

Abstract

During service most refractories undergo chemical reaction with corrosive agents (usually with a liquid phase). The kinetics of this process, for certain chemical compositions of the refractory and liquid phase, and for a given working temperature, are determined by the structure of the material. The influence of the refractory's structure follows directly from the fact that its solubility obeys Fick's law, in which the magnitude of the surface of the reaction is directly connected with the structure of the refractory, and is understood as the total surface of the refractory reacting with the melt. For each individual case, it should depend on the volume and size of the pores in the refractory and on the capacity, of these pores to absorb the melt.The resistance of the refectories in contact with liquid phase depends also on the rate of suction (capillary impregnation) of the melt and the solution of refractory in it. From the theory of capillary suction in a porous body for laminar flow of a liquid in a viscous schedule, it is known that the depth of movement of the melt is described by the expression (ignoring the force of gravity) [1]:where I is the depth of penetration, K is a constant of the rate of penetration, and ~" is time.Furthermore, the mass of dissolved refractory, due to the reaction with the melt, depends on the process occurring with minimum rate. In fact, if the solution rate is high and the penetration rate is low, then the wear rate in the refractory is determined by the latter process. Conversely, if the rate of penetration is high and the solution rate low, then the decisive rate of the process is the rate of solution. Since in practice, in most cases the worn refractory has a working zone impregnated to a certain depth, the wear should be determined by the solution rate.The total volume of melt Q penetrating the refractory, according to Zagar [2], is calculated from the equation:where A is the area of the section of the specimen in the direction perpendicular to the impregnation front; K is a coefficient; Fief f is the effective porosity; reff is the effective pore radius; 3, is the surface tensionofthemelt; ~ is the viscosity of the melt; 0 is the wetting angle; and ~-is time. Equation (1) is suitable only for the qualitative evaluation of the impregnation process, since refractories have complex structures and pore-size distributions. However, in practice it is simpler to regulate the penetration through the structure factor (IIeff2reff) '/2 than by the others. With a reduction in the open porosity and pore size, there is a fall not only in the magnitude of the structure factor but also in the numerical value of the remaining factors, since the behavior of the melt in capillaries of large and small diameter is not appropriate, and according to Strelov [1] metallurgical slag does not enter into capillaries measuring less than 5 txm.In practice the open porosity of the refractory is more often reduced, and the pore sizes more rarely reduced. The latter react with the grain composition of the...

Characterization of alumina-magnesia-carbon refractory bricks containing aluminium and silicon

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“…8, showing the two typical populations present in this type of material: pores larger than 100 µm which correspond to packing defects, and pores < 100 µm located between fine particles in the matrix and inside particles. Table 3 shows the contribution of different pore sizes (D) in the range of small pores (< 10 µm), where the ranges of pores playing an important role in the resistance to fluid penetration (< 1 µm [13] and < 5 µm [17]) are included.…”

The effects of laser-induced modification of surface roughness of Al2O3-based ceramics on fluid contact angle

“…Thus, a reduction in wetting generally leads to increased corrosion resistance when the surface is in contact with liquid corrosive species (e.g. molten slags in incinerators), since penetration through access sites, and chemical degradation, are limited due to a decrease in the effective contact area between the corrosive species and the ceramic surface; for example, Kashcheev and Semyannikov [11] have derived an expression that relates the volume of molten slag penetrating a refractory ceramic surface to the contact angle between the surface and the slag. Therefore, the wetting properties of the surface can become critical in determining the corrosion of refractory ceramics in their application environments, influencing their service lifetimes.…”

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