Kinetics high-temperature corrosion

This is a general equation that should be used in the development of high-temperature electrochemical kinetics and corrosion measurements if the processes of mass transport are already taken into account. 

The purpose of this review paper is to survey the principles of high temperature oxidation or high temperature corrosion. A typical situation is that of a metal exposed to a hot gas which can act as an oxidant. In many cases the oxidation product forms a layer which separates the reactants, the metal and the gas atmosphere. Under special conditions, the kinetics are diffusion controlled, i. e,, the rate of the reaction (the rate of oxide thickness growth) depends on the diffusion of species, ions and electrons, through the layer (sometimes called a tarnish layer). Actually when a metal or alloy is exposed to a corrosive gas, the reaction kinetics may be controlled by one or more of the following steps ... 

The high-temperature corrosion behavior of these engineering alloys by aU possible kinds of attack has been described in a book by Lai (1), which gives extensive information on the mechanisms and kinetics of corrosion and the necessary background for the material s selection in the industry. 

The kinetics of high-temperature corrosion differs from those of corrosion at ambient temperature in three respects ... 

Parabolic Reaction Kinetics Parabolic reaction kinetics describe high-temperature corrosion where the rate determining step is diffusion. This is often associated with protective scales and with internal penetration by such corrosive species as oxygen, carbon, or sulfur in certain materials. 

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. 

If an alloy is to have acceptable resistance against high-temperature corrosion, it must react with the environment to form a continuous and adherent slow-growing scale which has sufficient mechanical properties to withstand the effects of both growth and thermal stresses. As discussed in Chapter 3 of this volume, ideal protective scale growth obeys diffusion-controlled, parabolic kinetics, i.e. ... 

Vapor species that form in any given high-temperature corrosion situation often have a strong influence on the rate of attack, the rate generally being accelerated when volatile corrosion products form. Gulbransen and Jansson have shown that metal and volatile oxide species are important in the kinetics of high-temperature oxidation of carbon, silicon, molybdenum, and chromium. Six types of oxidation phenomena were identified ... 

In the present study the characterization of the effect of specimen geometry and the effect of exposure under thermal cycling conditions on the oxide scale failure mechanisms were the main objectives. Kinetics of corrosion product formation as well as changes in the chemical composition of both spalled oxide scale segments and internal corrosion products were determined to evaluate the material s resistance to high-temperature corrosion attack. 

The first part of the modelling consists of models for the salt-induced high-temperature corrosion system. Observations confirm the model initially described by Spiegel [8, 9] (Fig. 29.5). Therefore, the corrosion sequence can be subdivided into four different parts that correspond to the kinetic steps observed in the experiment ... 

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