Fundamentals

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some iron and that present near the top surface is predominantly in the reduced ferrous state. This is generally not a problem; however, those glasses containing NiO can exhibit small metallic droplets on the top surface that are cause for rejection. Based upon Fig. 2.14, this should not occur if the pO2 is maintained greater than 10-9 atm, assuming a maximum temperature no greater than 1100°C. Johnston and Chelko [2.103] proposed the mechanism of reduction of ions in glass by hydrogen diffusion through the glass to the reducible ions that act as immobile traps reacting with the hydrogen and stopping further diffusion.

2.4 CORROSION BY SOLID

Many applications of materials involve two dissimilar solid materials in contact. Corrosion can occur if these materials react with one another. Common types of reactions involve the formation of a third phase at the boundary, which can be a solid, a liquid, or a gas. In some cases, the boundary phase may be a solid solution of the original two phases. Again, phase diagrams will give an indication of the type of reaction and the temperature where it occurs.

When the reaction that takes place is one of diffusion as a movement of atoms within a chemically uniform material, it is called self-diffusion. When a permanent displacement of chemical species occurs, causing local composition change, it is called interdiffusion or chemical diffusion. The driving force for chemical diffusion is a chemical potential gradient (i.e., concentration gradient). When two dissimilar materials are in contact, chemical diffusion of the two materials in opposite directions forms an interface reaction layer. Once this layer has been formed, additional reaction can take place only by the diffusion of chemical species through this layer.

Solid-solid reactions are predominantly reactions involving diffusion. Diffusion reactions are really a special case of the general theory of kinetics (discussed in Sec. 2.8) since the diffusion coefficient, D, is a measure of the diffusion reaction

Copyright © 2004 by Marcel Dekker, Inc.

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Chapter 2

rate. Thus diffusion can be represented by an equation of the Arrhenius form:

(2.40)

where:

D = diffusion coefficient

Do = constant

Q= activation energy

R= gas constant

T = absolute temperature

The larger the value of Q, the activation energy, the more strongly the diffusion coefficient depends upon temperature.

The diffusion in polycrystalline materials can be divided into bulk diffusion, grain boundary diffusion, and surface diffusion. Diffusion along grain boundaries is greater than bulk diffusion because of the greater degree of disorder along grain boundaries. Similarly, surface diffusion is greater than bulk diffusion. When grain boundary diffusion predominates, the log concentration decreases linearly with the distance from the surface. When bulk diffusion predominates, however, the log concentration of the diffusion species decreases with the square of the distance from the surface. Thus by determining the concentration gradient from the surface (at constant surface concentration), one can determine which type of diffusion predominates.

Since grain boundary diffusion is greater than bulk diffusion, it would be expected that the activation energy for boundary diffusion would be lower than that for bulk diffusion. The boundary diffusion is more important at lower temperatures, and bulk diffusion is more important at high temperatures.

Chemical reactions wholly within the solid state are less abundant than those which involve a gas or liquid, owing predominately to the limitation of reaction rates imposed by slower material transport. The solid-solid contact of two different bulk materials also imposes a limitation on the

Copyright © 2004 by Marcel Dekker, Inc.