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comes from an analysis of thermal diffusion is that a diffusion flux will set up a thermal gradient in an isothermal system.

When an elastic stress gradient is present along with a concentration gradient, a potential term must be included in the equation for total flux, just as was necessary for the thermal gradient. Thus the total flux of atoms in a particular direction is increased (or decreased) over that due only to concentration differences. This effect is called stress-assisted diffusion.

Diffusion is probably the most important rate-controlling step when one is evaluating the kinetics of a reaction by thermal analysis. Diffusion in the gas phase is about 104 times greater than that in the liquid phase. For a more complete description of diffusion, the reader is referred to any one of the texts on diffusion [2.154, 2.155].

2.10SUMMARY OF IMPORTANT CONCEPTS

1.It is the total surface area exposed to attack that is important.

2.Grain boundary diffusion is more important at low temperatures, and bulk diffusion is more important at high temperatures.

3.One need not have data on a specific material, chemically, but only on one of identical structure to estimate its dissolution characteristics.

4.One must remember that it is not the total porosity that is important, but the surface area of the total porosity.

5.If sufficient flow of a transpiring gas along a thermal gradient is present, dilution of the corrosive gas at the hot face may lower the corrosion rate to an acceptable level.

6.The characteristics of the corroding glass are more important than the solid parameters in corrosion.

7.Ceramics with an acid/base character similar to the liquid will tend to resist corrosion the best.

8.The importance of the zero point of charge (zpc) in corrosion is that it is the pH of maximum durability.

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9.The spontaneity of a reaction depends upon more than just the heat of reaction. To predict stability, one must consider also the entropy.

10.If the reaction is spontaneous, the change in free energy is negative, whereas if the reaction is in equilibrium, the free energy change is equal to zero.

11.The real problem with predicting whether a reaction may take place or not is in selecting the proper reaction to evaluate. Care must be taken not to overlook some possible reactions.

12.Since the corrosion of ceramics in service may never reach an equilibrium state, thermodynamic calculations cannot be strictly applied because these calculations are for systems in equilibrium. Many reactions, however, closely approach equilibrium, and thus the condition of equilibrium should be considered only as a limitation, not as a barrier to interpretation of the data.

13.There is a general tendency for oxides to be reduced at higher temperatures at constant oxygen partial pressures. One should be aware that any metal will reduce any oxide above it in the Ellingham diagram.

14.Unit activity should be applied only to species in the pure state.

15.The most important parameter of corrosion from the engineering viewpoint is the reaction rate.

16.Diffusion coefficients depend upon the composition and structure of the material through which diffusion occurs.

17.The rate of the reaction expressed as the rate of change of concentration, dc/dt, depends upon the concentration of the reactants.

18.The discrepancies between the experimental data and the theoretical models are often due to nonspherical particles, a range in sizes, poor contact between reactants, formation of multiple products, and the dependency of the diffusion coefficient upon composition.

19.Arnold et al. [2.141] concluded that dynamic thermogravimetric studies provide insufficient data

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for calculation of reaction kinetics, that the data are influenced by the experimental procedures, and that the results are uncertain.

20.The enthalpy of the reaction is often sufficient to raise or lower the sample temperature by as much as 1000°C.

21.The flow of material by diffusion is proportional to the concentration gradient and is directed from the region of high concentration to one of low concentration.

22.In isometric crystals, the diffusion coefficient is isotropic, as it is in polycrystalline materials as long as no preferred orientation exists.

23.In most real cases, the diffusion coefficient can vary with time, temperature, composition, or position along the sample, or any combination of these.

24.Silica-forming reactions are the most desirable for protection against oxygen diffusion.

25.A diffusion flux will set up a thermal gradient in an isothermal system.

2.11ADDITIONAL RELATED READING

Vetter, K.J. Electrochemical Kinetics; Academic Press, New York, 1967.

Bockris, O.M.; Reddy, A.K.N. Modern Electrochemistry; Plenum Press, New York, 1970; Vol. 2.

Shaw, D.J. Charged Interfaces. Introduction to Colloid and Surface Chemistry, 3rd Ed.; Butterworths, London, 1980; 148–182. Chp. 7.

Marshall, C.E. The Physical Chemistry and Mineralogy of Soils: Soils in Place; Wiley & Sons: New York, 1977; Vol. II.

Reviews in Mineralogy: Mineral-Water Interface Geochemistry;

Hochella, M.F. Jr., White A.F., Eds.; Mineral. Soc. Am., Washington, DC, 1990; Vol. 23.

Burns, R.G. Mineralogical Applications of Crystal Field Theory;

Cambridge University Press: Cambridge, 1970.

Shackelford J.F., Ed.; Bioceramics, Applications of Ceramic and Glass Materials in Medicine; Trans Tech Publications: Switzerland, 1999.

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Reviews in Mineralogy: Health Effects of Mineral Dusts. Guthrie, G.D. Jr.; Mossman B.T., Eds.; Mineral. Soc. Am., Washington, DC, 1993; Vol. 28.

P.G.Shewmon. Diffusion in Solids. J.Williams Book Co., Jenks, OK, 1983.

2.12EXERCISES, QUESTIONS, AND PROBLEMS

1.Discuss the reaction products that may form and how they may relate to any interfacial reaction layer formed.

2.If a “unified theory of corrosion of ceramics” were to be developed, what structural characteristic would be included and why?

3.Look up the vapor pressure of several materials to confirm the concept that covalent materials vaporize more quickly than ionic materials due to their higher vapor pressure.

4.Why does the corrosion rate decrease when a thermal gradient is present?

5.The Arrhenius equation has been used to represent the temperature dependence of corrosion. Discuss when this equation is most appropriate and why.

6.Discuss the difference between direct and indirect dissolution. What other terms are used to describe these types of dissolution?

7.What is the most predominant parameter in the equation for corrosion rate under free convection? Why is this parameter more predominant than the others?

8.Discuss the various problems relating to the experimental verification of the galvanic corrosion of ceramics.

9.Describe how the cross-linking of silica tetrahedra affect corrosion in silicates by aqueous solutions.

10.How does pH affect the corrosion of crystalline ceramics and how does this relate to isoelectric point (IEP)?

Copyright © 2004 by Marcel Dekker, Inc.