Parabolic rate constant, corrosion

Chapter 10 deals with high temperature corrosion, in which the thermodynamics and kinetics of metal oxidation are included. The Pilling Bedworth Ratio and Wagner s parabolic rate constant theories are defined as related to formation of metal oxide scales, which are classified as protective or nonprotec-tive. 

This parabolic law, which indicates that diffusion is rate-limiting, is of overwhelming importance for scale formation. Wagner (1933) showed that the parabolic scale constant (and hence, rate of oxidation) can be calculated using the enthalpy of formation of the corrosion product, the electrical conductivity of the protective film and the transport number of the ions and electrons in the film. 

Let us now consider a maximum acceptable scale thickness of 1 pm after 10 kh operation. This thickness is expected to avoid spallation of the oxide scale and to prevent corrosion of the metaUic conduction paths between the grains. As shown in Fig. 5, the alloy support should have an oxidation rate constant lower than 10 g cm s . The same plot can also be used to conversely predict the expected lifetime of the support by knowing the parabohc rate constant of the alloy. One should notice that these calculations are based on an ideal parabolic oxidation behavior of the alloy. Some deviations may be expected due to Cr evaporation in air from the scale, which is enhanced in the presence of steam. This linear evaporation term is not introduced in Eq. (1). 

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