Tham khảo tài liệu 'corrosion of ceramic and composite materials part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 106 Chapter 2 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. concluded that dynamic thermogravimetric studies provide insufficient data Copyright 2004 by Marcel Dekker Inc. Fundamentals 107 for calculation of reaction kinetics that the data are influenced by the .
