High-temperature corrosion thermodynamics

The basic types of thermodynamic diagrams useful in interpreting the results of high-temperature corrosion experiments have been discussed. While no attempt at a complete discussion of this subject has been made, it is hoped this chapter will facilitate a clearer understanding of the high-temperature corrosion phenomena to be described in the remainder of this book. 

G. H. Meier, N. Birks, F. S. Pettit, and C. S. Giggins, Thermodynamic analyses of high temperature corrosion of alloys in gases containing more than one reactant. In High Temperature Corrosion, ed. R. A. Rapp, Houston, TX, NACE, 1983, p. 327. 

High temperature corrosion and oxidation will be covered in a separate chapter. However, some basics of the thermodynamics will be addressed here. Consider a metal M reacting at high temperature with O2 ... 

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. 

The high-temperature corrosion of metals and alloys in sulfidizing-oxidizing environments has been reviewed by Stroosnij-der and Quadakkers (1986), Gesmundo et al. (1989), and Grabke et al. (1989). On the basis of equilibrium thermodynamics, the transition from oxidation to sulfidation of alloy constituent B should occur when ... 

The overall simulation of high-temperature corrosion processes under near-service conditions requires both a thermodynamic model to predict phase stabilities for given conditions and a mathematical description of the process kinetics, i.e. solid state diffusion. Such a simulation has been developed by integrating the thermodynamic program library, ChemApp, into a numerical finite-difference diffusion calculation, InCorr, to treat internal oxidation and nitridation of Ni-base alloys [10]. This simulation was intended to serve as a basis for an advanced computer model for internal oxidation and sulfidation of low-alloy boiler steels. 

Generally, the generic driving force of high temperature corrosion processes can be separated into (i) transport mechanisms, i.e., solid-state diffusion in most cases, and (ii) thermodynamics of chemical reactions. The commonly used, phenomenological way to treat diffusion processes is the application of a second-order partial differential equation (Pick s second law) formulating a relationship between the derivative of the concentration of a species c after... 

A further serious limitation is that diagrams evaluated from thermodynamic data at 25°C have little relevance in high-temperature aqueous corrosion, but it is now possible to construct diagrams that are applicable at elevated temperatures from data obtained at 25° C see Section 2.1). 

Lefrancois, P. A. and Hoyt, W. B., Chemical Thermodynamics of High Temperature Reactions in Metal Testing Corrosion , Corrosion, 19, 360t (1963)... 

The outstanding characteristics of the noble metals are their exceptional resistance to corrosive attack by a wide range of liquid and gaseous substances, and their stability at high temperatures under conditions where base metals would be rapidly oxidised. This resistance to chemical and oxidative attack arises principally from the Inherently high thermodynamic stability of the noble metals, but in aqueous media under oxidising or anodic conditions a very thin film of adsorbed oxygen or oxide may be formed which can contribute to their corrosion resistance. An exception to this rule, however, is the passivation of silver and silver alloys in hydrochloric or hydrobromic acids by the formation of relatively thick halide films. 

These sort of problems make it difficult to obtain reliable high temperature data on the aqueous chemistry of transition metal ions. Unfortunately the necessary timescales for even the simpler experimental studies are frequently too long for a Ph.D. student to make reasonable progress in 3 years from scratch or for industrial researchers to make much reportable progress before the patience of those supporting the work is exhausted. Results can be reported far more rapidly from, for example, corrosion experiments and since corrosion theories are in general of so little predictive value, each relevant alloy/electrolyte combination needs its own study. In such circumstances it is hardly surprising that thermodynamic studies have been (with a few notable exceptions) relatively poorly supported, while corrosion data continue to be amassed without any reliable thermodynamic framework within which to understand them. 

A feature of corrosion studies which has been stressed recently (2) is the complete failure of laboratory tests on their own to predict how reliable operation of some nuclear steam generators can be maintained. At least a part of this problem is likely to arise from different redox and/or pH conditions imposed by the solution in autoclave tests and in plant conditions and many low level contaminants could be involved. In view of what has been said earlier concerning the role of Mo(VI) in stagnant water it is clear that some data, at least on the thermodynamics of aqueous Mo species, should be sought at high temperatures. 

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