Developments in High Temperature Corrosion and Protection of Materials

W Gao

• Publisher: Woodhead Publishing
• Year: 2008
• ISBN:
  9781845694258
• Topic: Technology & Engineering
• Subtopic: Mining Engineering
• Index: Technology & Engineering

High temperature corrosion is a phenomenon that occurs in components that operate at very high temperatures, such as gas turbines, jet engines and industrial plants. Engineers are constantly striving to understand and prevent this type of corrosion. This book examines the latest developments in the understanding of high temperature corrosion processes and protective oxide scales and coatings.Part one looks at high temperature corrosion. Chapters cover diffusion and solid state reactions, external and internal oxidation of alloys, metal dusting corrosion, tribological degradation, hot corrosion, and oxide scales on hot-rolled steel strips. Modern techniques for analysing high temperature oxidation and corrosion are also discussed. Part two discusses methods of protection using ceramics, composites, protective oxide scales and coatings. Chapters focus on layered ternary ceramics, alumina scales, Ti-Al intermetallic compounds, metal matrix composites, chemical vapour deposited silicon carbide, nanocrystalline coatings and thermal barrier coatings. Part three provides case studies illustrating some of the challenges of high temperature corrosion to industry and how they can be overcome. Case studies include the petrochemical industry, modern incinerators and oxidation processing of electronic materials.This book is a valuable reference tool for engineers who develop heat resistant materials, mechanical engineers who design and maintain high temperature equipment and plant, and research scientists and students who study high temperature corrosion and protection of materials.

• Describes the latest developments in understanding high temperature corrosion
• Presents the latest research by the leading innovators from around the globe
• Case studies are provided to illustrate key points

1

Introduction

W. Gao; Z. Li    The University of Auckland, New Zealand

High-temperature corrosion is one of the most important issues for materials selection, structure design and service life prediction of engineering parts which are exposed to high-temperature environments. The formation of corrosion products such as oxides (the most common form), carbides, nitrides, sulfides, or their mixtures, in general leads to the loss of load-bearing cross-section, decreases the reliability and stability, and finally shortens the service lifetime of the engineering components. Inward diffusion of oxygen, nitrogen, sulfur or carbon may also result in undesired precipitation of compounds which are normally brittle in nature. High-temperature corrosion therefore deteriorates materials and thus degrades the performance of engineering parts. Directly or indirectly, high-temperature corrosion processes also have an impact on environmental problems such as air pollution and global warming.

Studies of high-temperature corrosion of metals have a long history. The basis for high-temperature corrosion and oxidation was established in the 1920s and 1930s. The focus at that time was mainly on kinetics (Tammann), mechanical property of oxide scale (Pilling–Bedworth ratio), and stress measurement (Evans). Carl Wagner made significant contributions towards the defect structure of oxides and diffusion processes during oxide growth. His theoretical analyses using oxidation models of single-phase alloys, growth and development of stable oxide scales, and especially the modelling of the transition of internal and external oxidation, still influence and provide guidance to modern oxidation studies. From the 1950s to the 1970s an integrated, multidisciplinary approach was developed to study the oxidation of binary alloys and, more generally, commercial alloys. These research activities, supported by powerful modern analytical tools such as scanning electron microscopy, transmission electron microscopy, electron probe microanalysis, X-ray diffraction, X-ray photoelectron spectroscopy, Auger spectroscopy, secondary ion mass spectroscopy, etc., were focused on scaling kinetics, scale morphology, and microstructural and compositional characterizations. This phase of research lasted for a long period with a varying emphasis on defect structure, transportation mechanism, minor element additions and their effects on the growth and mechanical properties of oxide scales.

The last 20 years have seen a fast and steady increase of research into high-temperature corrosion. A quick survey with SCOPUS using the keywords ‘high temperature corrosion’ or ‘high temperature oxidation’ gave the following results: in 1985, 972 papers were published and 7756 patents applied for; in 1995, the figures increased to 1190 and 13,158, whilst in 2005 they increased to 3956 and 45,562 respectively. This is a direct result of the many investigations into high-performance structural materials for aerospace, energy and automobile applications, and also long-life corrosion-resistant bulk and coating materials for engineering components working in complex environments, such as oxidizing, sulfidizing, carburizing or chloridizing environments with or without additional mechanical action of solid particles on the material surface. Metals and alloys must be effectively protected against these corrosive environments through the formation of protective oxide layers with the characteristics of slow growth, high resistance to cracking and spallation, and strong re-healing ability. The prevention of high-temperature corrosive attacks on materials plays a critical role in aspects such as reliability, quality, safety and profitability of any industrial sector associated with high-temperature processes.

Two excellent books, High Temperature Corrosion by Professor P. Kofstad (1988), and Introduction to the High Temperature Oxidation of Metals by Professors N. Birks, G.H. Meier and F.S. Pettit (1983, 2006), described the high-temperature corrosion processes of metals and alloys at elevated temperatures, providing an expert treatment of the fundamental mechanisms involved in various high-temperature corrosion processes. To summarize the experimental developments achieved in this active area, many excellent review-type papers have also been published. They provide detailed information on defect structure and transport, selective oxidation, the reactive element effect, high-performance coatings, corrosion in molten salt, metal dusting, high-temperature erosion-corrosion, alloy and coating design strategy, and analytical technique.

The present book, Developments in High-temperature Corrosion and Protection of Materials, is intended to be a showcase for the current state of research activity in high-temperature corrosion of various materials. A number of leading researchers within universities, institutes and industries from Australia, Canada, China, France, India, Japan, New Zealand, Poland and the USA have contributed to this book, bringing together a wide range of studies within a single volume.

This book is divided into three parts: developments in high-temperature corrosion theories and processes, oxide scales and coatings, and practical case studies. In the second chapter, Professor Pieraggi describes the role of diffusion and mass transport processes in scale growth, and in particular, the potential influences of interfacial reactions and structures on the mechanical properties of oxide scales and oxidation behaviours of components. In Chapter 3, Professor He and his colleagues study the oxide nucleation and growth behaviour of typical single-phase binary alloys under low oxygen partial pressure, and propose a model describing the transition between the internal and external oxidation processes. The progress of high-temperature corrosion studies is heavily dependent on the characterization of microstructure, composition and phase structure of oxide scales and bulk materials. In Chapter 4, the applications of modern analytical techniques in oxidation and corrosion research are reviewed by Professor Graham with his extensive studies of metals, alloys and semiconductors.

High-temperature corrosion in atmospheres containing gases other than oxygen is reviewed in the following chapters. Professor Ramanarayanan and Dr Chun report on the metal dusting corrosion of Fe, Co and Ni and selected Fe-based, Fe-Ni-based and Ni-based alloys, and have formulated some general principles governing the metal dusting process. Engineering components used in the nuclear industry, power generation and the transport industry may be subjected to mechanical degradation due to wear. This situation becomes worse with a synergistic interaction of oxidation and wear at elevated temperatures. In Chapter 6, Dr Roy describes typical high-temperature tribological degradation processes including sliding wear, erosive wear and abrasive wear. Hot corrosion is a complex phenomenon leading to serious problems in engines burning fuels containing sulfur, potassium, sodium, vanadium, etc., or exposed to various salts. In Chapter 7, Professor Prakash reviews the chemistry of hot corrosion and summarizes the methods used to prevent this type of accelerated attack. Oxidation is also a problem in the steelmaking industry, since the formation of oxide scale on sheet or strip steels both leads to material loss and affects the rolling process. Drs Chen and Yuen from BlueScope Steel (Australia) summarize current understanding of the long- and short-term oxidation behaviour of steel in air and oxygen, and in reheat furnace atmospheres, and review recent studies of the mechanical properties of oxide scales and their deformation and fracture behaviour.

The high-temperature oxidation and hot corrosion mechanisms of layered ternary ceramics, such as Ti₃SiC₂, Ti₃AlC₂ and Ti₂AlC, are reported by Drs Lin, Li and Zhou in Chapter 9. Their unique crystal structure and microstructure have strong influences on the scaling behaviour of these materials, and make them promising candidates for high-temperature structural applications. The development and maintenance of a protective oxide scale are determined by many chemical and mechanical factors. Dr Chevalier discusses the formation and growth of protective alumina scales in relation to growth kinetics and mechanisms, phase transformations and reactive element effects in Chapter 10. Ti-Al based intermetallic compounds are receiving more and more attention recently due to their high mechanical strength at elevated temperatures and low density. However, their high-temperature oxidation resistance is inadequate, though they have a relatively high Al content in comparison with Ni-based superalloys. Why is the protective alumina scale hard to form on these materials? The reasons can be found in Professor Taniguchi’s Chapter 11 which reviews the oxidation behaviour of Ti-Al alloys. Methods commonly used to enhance the oxidation resistance of TiAl, such as microstructure control, composition optimization and surface treatment, are also presented.

Incorporation of particulate or fibre shaped reinforcements into metal matrices can significantly change their response to mechanical loads. The presence of reinforcing phases, however, may not always be advantageous to the oxidation resistance of composites, since the enhancement of high-temperature corrosion resistance is often not the main concern of composite design. This is particularly true for long fibre-reinforced composite materials without special treatments to the fibre-matrix interface. A comprehensive summary of the literature on the general oxidation properties of metal matrix composites is given in Chapter 12 by Drs Li and Gao. This chapter also discusses the development of Ti-based in-situ composites. It has been found that incorporation of alumina particles into the Ti-Al matrix through in-situ reactions has improved the mechanical stability of thermally grown oxide scales, thereby increasing oxidation resistance significantly. In Chapter 13, Dr Pint of Oak Ridge National Laboratory USA reports on the valuable experience gained on design strategies for oxidation-resistant high-temperature alloys. In particular, he discusses how to develop a protective alumina scale on NiCr and FeNiCr alloys without sacrificing their mechanical properties.

Coating is a common way to prevent high-temperature corrosive attacks while maintaining the mechanical properties of bulk materials for structural applications. In the next group of chapters Professor Goto reports that SiC coatings are finding practical applications for engineering parts working in harsh environments. While a better understanding of the intrinsic oxidation behaviour of SiC, i.e., passive oxidation, active oxidation and bubble formation, is still needed, he believes that high-purity SiC derived by chemical vapour deposition might be a suitable candidate. Surface micro-crystallization or nano-crystallization has also become a ‘hot’ topic since it has been found that reduction of grain size can promote the formation of protective oxide layers through enhanced grain boundary diffusion. In Chapter 15, Drs Peng and Wang review the oxidation of nano-crystalline coatings in relation to the formation, growth and mechanical response of oxide scales, while in the following chapter, as an important type of coating, Professors Xu, Guo and Gong summarize the developments in materials, process and failure of thermal barrier coatings for advanced gas-turbine engines with higher efficiency and improved durability.

High-temperature corrosion phenomena and their related failures are widely found in the chemical, electrical, energy and transportation industries. The corrosion processes in real situations may have a quite different character in comparison with laboratory experiments. Corrosion studies in service conditions are therefore necessary to define the nature of the attacks in order to design materials and structures with better performance. In Chapter 17, Professors He, Ning and Gao introduce the main corrosion problems commonly found in the petrochemical industry. The main effort in high-temperature oxidation research is to protect materials from attack. However, controllable growth of oxide may find important applications in areas such as the semiconductor industry. Thermal growth of native oxide, SiO₂, on Si through dry or wet oxidation has long been used to establish surface passivation layers and functional dielectrics as summarized in Chapter 18 by Drs Li and Gao. Syntheses of superconducting composites, oxide thin films such as ZnO and low-dimensional nanostructures are reported, together with some results showing the effects of the oxidation conditions on the photoluminescence property of thermally grown ZnO films. Dr Kawahara, in Chapter 19, reports on developments in materials and coatings used in waste-to-energy incineration plants and the testing and monitoring methods employed to acquire information for a greater understanding of corrosion mechanisms.

From the contents it can be seen that this book seeks to reflect current developments in high-temperature corrosion science, to chart progress in the design of new materials and new methods, and to achieve a balance of theory and practice. The contributions from industrial experts in this respect are very welcome. The Editors would like to thank all contributors for the energy and time spent in the preparation of chapters and their willingness to share their results and understanding. Finally, the Editors would also like to thank the Publications Coordinators at Woodhead Publishing, Ian Borthwick and Beatrice Bertram, the Project Editor Laura Pugh, the Commissioning Editor Rob Sitton, and the Editorial Director Francis Dodds, for their very effective work in developing the book and in organizing the delivery of each of its constituent chapters.

Part I

Developments in high temperature corrosion

2

Diffusion and solid state reactions

B. Pieraggi ENSIACET, France

2.1 Introduction

As schematized by Fig. 2.1, the growth of intermediate phases between two reacting phases is a more or less complex combination of interfacial reactions and diffusion processes. Figure 2.1 shows a reaction zone formed of two sublayers that can be of different microstructure and/or different composition. For both cases, the role of external and internal interfaces delimiting these two sublayers and separating them from the two reacting phases has to be considered, and the reactions occurring at these interfaces cannot be neglected in the analysis of reaction mechanisms.

2.1 Schematic representation of the growth of a reaction layer separating one reacting medium from a reacting solid and involving mass transport and interface reactions.

However, for a growing oxide layer, the main constraints on the reactions occurring at the external interface separating the oxide layer from the oxidant phase are linked to the fluxes of reactive species towards...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• Contributor contact details
• 1: Introduction
• Part I: Developments in high temperature corrosion
• Part II: Developments in protective oxide scales and coatings
• Part III: Case studies
• Index