Preface

The science and engineering of gas-solid, liquid-solid, and solid-solid processes, which can involve high temperature reactions with oxygen, sulfur, nitrogen, carbon dioxide, water vapor, molten sulfates, chlorides, carbonates, vanadates, fluorides, etc., has evolved greatly over the past 50-60?years. Numerous symposia and colloquia have been held on the subject and have been widely attended by researchers from all over the world. One of the many factors enabling advancement of our understanding of the field is the creation and evolution of new, sophisticated instruments and techniques that allow a better analysis of scale compositions and structures.

The numerous details of high temperature corrosion () have led to thousands of publications over the years and to the writing of only relatively few books on the subject. Moreover, the early books suffered from lack of extensive data on various metals and alloys, the nonexistence of good experimental equipment, and an emphasis on materials that were important in the era in which the books were written. Many universities now have courses on oxidation and other forms of HTC, but the availability of texts continues to be limited. The following books have been very useful:

• N. Birks, G.H. Meier, and F.S. Pettit, Introduction to the High-Temperature Oxidation of Metals, Cambridge University Press, Cambridge, UK (2006).
• E. Fromm, Kinetics of Metal-Gas Interactions at Low Temperature - Hydriding, Oxidation, Poisoning, Springer-Verlag, Berlin, Germany (1998).
• A.S. Khanna, High Temperature Oxidation and Corrosion, ASM International, Materials Park, Ohio, USA (2002).
• P. Kofstad, High Temperature Oxidation of Metals, John Wiley, New York, USA (1966).
• P. Kofstad, High Temperature Corrosion, Elsevier, London, UK (1988).
• G.Y. Lai, High-Temperature Corrosion and Materials Applications, ASM International, Materials Park, Ohio, USA (2007).
• S. Mrowec, Defect and Diffusion in Solids, Elsevier Science Publications, London, UK (1980).
• M. Schütze, Protective Oxide Scales and Their Breakdown, The Institute of Corrosion, John Wiley, Chichester, UK (1997).
• M. Schütze and H.J. Grabke, eds., Metal Dusting, Carburization and Nitridation, EFC 41, Woodhead Publishing Ltd., Cambridge, UK (2006).

The main differences between the present book and the competitive books listed above are:

• None of them cover most of the HTC types (e.g. oxidation, sulfidation, nitridation, molten salts, fuel-ash corrosion, H2S/H2 corrosion, molten fluoride/HF corrosion, carburization).
• Many books on HTC concern fundamental studies of material-gas processes. In other words, aspects related to diffusion in HTC oxidation are well treated, but the books provide little engineering data to help engineers make informed material selection decisions.
• The present book covers fundamental science and engineering of HTC, in a balanced way, so that an academic researcher or PhD student or an engineer in industry will find it of interest.
• The present book includes up-to-date information accompanied by more than 1500 references, 80% of which cover the last 25?years.
• The present book includes details on systems of particular engineering importance at this time, namely, on the corrosion of high temperature fuel cells, and nuclear power plants.
• The present book includes an up-to-date summary of experimental methods, many of which have not been covered in earlier books.

The current book has 20 chapters, whose contents are briefly described hereafter.

The first chapter begins to examine historical aspects of the development of HTC. Then, the three key components of the corrosion phenomena frequently encountered in high temperature industrial processes are summarized. In the following sections, materials and their design for use at high temperatures, common and not so common hostile environments at high temperature, and the barriers (films, scales) that separate the material and environment and that, in many situations, are necessary to develop the desired corrosion resistance are presented. HTC occurs behind walls as high temperature processes are usually shielded from their environment. However, it can be lifetime and performance determining in a number of everyday examples, its minimization requiring a proper understanding of the underlying principles of corrosion, which involve plant engineering, metallurgy, chemistry, materials science, system design, or failure analysis. So, the academic and industrial impacts of HTC form the themes of the following sections. In this context, corrosion economics, safety, environment damage, and corrosion management are aspects of particular concern.

Basic high temperature phenomena that require attention are changes in dimensions, microstructures, mechanical properties, and other topics under the heading of physical metallurgy. It is no longer possible to neglect the metallurgical aspects of the corrosion problem. Imperfections in an essentially perfect structure, solidification, alloys, iron and steel, deformation and recrystallization, and fracture and fatigue are described in Chapter 2. It is expected to provide an introduction to physical metallurgy primarily intended for undergraduate students at universities and polytechnics, but, additionally, industrial technicians or engineers will be aided in identifying their in-plant corrosion problems. Seventy-three figures fully support the text, illustrating the fundamental points.

An important tool analysis of HTC problems is equilibrium thermochemistry, which, although not predictive of kinetics, allows to ascertain which reaction products are possible, whether or not significant evaporation or condensation of a given species is possible, the conditions under which a given reaction product can react with a condensed deposit, etc. This standard method of thermochemical analysis is commonly used in metallurgical equilibria involving reactions between a gas and a solid as a function of temperature, and the complexity of the ambivalent oxidation usually dictates that the thermochemical analysis be represented in graphical form. Chapter 3 deals with six types of thermochemical diagrams: Ellingham diagrams, integral free energy-concentration diagrams, vapor species diagrams, stability diagrams for one metal/two nonmetals, 2D stability diagrams for two metals/one nonmetal, and phase stability diagrams for two or more metals. The subject of thermodynamics (or phase equilibria) of metal-fused salt systems in high temperature corrosive environments has been treated in a manner exactly analogous to aqueous solutions by the formation of Pourbaix, i.e. plots of electrode potential (oxidizing potential) versus basicity, being treated in this chapter by constructing and properly detailing an oxidizing potential basicity diagram for iron in molten sodium sulfate.

The lattice of an actual crystal always contains imperfections or defects. Important properties of the crystal, such as diffusion of ions, result from the existence of defects, and, as a consequence, mass transport in the scale formed by oxidation can be explained on the basis of its defect chemistry and crystal structure. An understanding of reaction mechanisms in HTC requires a precise knowledge of defect structures in solids. The growth of a compact or a porous scale, as well as its structure, is largely determined by the presence of the lattice defects. This is analyzed in Chapter 4 on lattice defects in metal compounds. The considered topics include point defects and defect structures, defect reactions and relevant examples, defect equilibrium constants and their significance for defect reaction including electronic imperfections.

Besides its scientific interest, diffusion is of enormous practical relevance for industry and life, ranging from steelmaking, growth of oxide scales, sintering, and high temperature creep of metals to oxide/carbon dioxide exchange in the human lung. Chapter 5 is confined to diffusion in condensed matter, namely, in metals, binary alloys, and oxides. Emphasis is on very basic fundamental aspects, the contents being roughly characterized by the headings general theory of diffusion, diffusion coefficients, Matano-Boltzmann analysis, Kirkendall effect, Darken analysis, factors influencing diffusion, impurity diffusion in metals, grain boundary diffusion in metals, diffusion in solid oxides, morphology of reaction products, and measurement of diffusion parameters. The chapter includes many references for readers at the forefront of the subject; it is primarily intended for graduate students and corrosion scientists.

Many studies from both theoretical and experimental viewpoints and fundamental approaches made along the last 70?years suggested, with sufficient credibility, electrochemical mechanistic models for the observed corrosion at high temperature. Thus, the chemical reactions established at solid-solid, solid-liquid, and solid-gas interfaces during the growth of corrosion products can be visualized as oxidation-reduction electrode processes; in other words, it is acceptable that high temperature oxidation, sulfidation, halogenation, nitridation, carburization, and molten salt corrosion are processes of electrochemical nature. In Chapter 6, basic aspects of traditional electrochemistry, solid-state electrochemistry, and molten salt electrochemistry are described to show how HTC can be further understood and mitigated. Headings include electrochemical...