Corrosion Science and Technology

David E.J. Talbot, James D.R. Talbot

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351259903
• Edition: 3
• Topic: Technology & Engineering
• Subtopic: Mechanical Engineering
• Index: Technology & Engineering

Twenty years after its first publication, Corrosion Science and Technology continues to be a relevant practical guide for students and professionals interested in material science. This Third Edition thoroughly covers the basic principles of corrosion science in the same reader-friendly manner that made the previous editioninvaluable, and enlarges the scope of the content with expanded chapters on processes for various metals and new technologies for limiting costs and metal degradation in a variety of commercial enterprises not explored in previous editions. This book also presents expertly developed methods of corrosion testing and prediction.

1

Overview of Corrosion and Protection Strategies

Metals in service often give a superficial impression of permanence, but all except gold are chemically unstable in air and air-saturated water at ambient temperatures, and most are also unstable in air-free water. Hence, almost all of the environments in which metals serve are potentially hostile and their successful use in engineering and commercial applications depends on protective mechanisms. In some metal/environment systems, the metal is protected by passivity, a naturally formed surface condition inhibiting reaction. In other systems, the metal surface remains active and some form of protection must be provided by design. This applies particularly to plain carbon and low alloy irons and steels, which are the most prolific, least expensive and most versatile metallic materials. Corrosion occurs when protective mechanisms have been overlooked, break down or are exhausted, leaving the metal vulnerable to attack.

Practical corrosion-related problems are often discovered in the context of engineering and allied disciplines, where the approach may be hindered by unfamiliarity with the particular blend of electrochemistry, metallurgy and physics which must be brought to bear if satisfactory solutions are to be found. This overview indicates the relevance of these various disciplines and some relationships between them. They are described in detail in subsequent chapters.

1.1Corrosion in Aqueous Media

1.1.1Corrosion as a System Characteristic

Some features of the performance expected from metals and metal artefacts in service can be predicted from their intrinsic characteristics assessed from their compositions, structures as viewed in the microscope and past history of thermal and mechanical treatments they may have received. These characteristics control density, thermal and electrical conductivity, ductility, strength under static loads in benign environments and other physical and mechanical properties. These aspects of serviceability are reasonably straightforward and controllable, but there are other aspects of performance which are less obvious and more difficult to control because they depend not only on the intrinsic characteristics of the metals, but also on the particular conditions in which they serve. They embrace susceptibility to corrosion, metal fatigue and wear, which can be responsible for complete premature failure with costly and sometimes dangerous consequences.

Degradation by corrosion, fatigue and wear can be approached only by considering a metal not in isolation but within a wider system with the components, metal, chemical environment, stress and time. Thus a metal selected to serve well in one chemical environment or stress system may be inadequate for another. Corrosion, fatigue and wear can interact synergistically, as illustrated in Chapter 5, but for the most part, it is usually sufficient to consider corrosion processes as a chemical system comprising the metal itself and its environment.

1.1.2The Electrochemical Origin of Corrosion

From initial encounters with the effects of corrosion processes, it may seem difficult to accept that they can be explained on a rational basis. One example, among many, concerns the role of dissolved oxygen in corrosion. It is well known that unprotected iron rusts in pure neutral waters, but only if it contains dissolved oxygen. Based on this observation, standard methods of controlling corrosion of steel in steam raising boilers include the removal of dissolved oxygen from the water. This appears to be inconsistent with observations that pure copper has good resistance to neutral water whether it contains oxygen or not. Moreover copper can dissolve in acids containing dissolved oxygen but is virtually unattacked if the oxygen is removed, whereas the complete reverse is true for stainless steels. These and many other apparently conflicting observations can be reconciled on the basis of the electrochemical origin of the principles underlying corrosion processes and protection strategies. The concepts are not difficult to follow and it is often the unfamiliar notation and conventions in which the ideas are expressed which deter engineers.

At its simplest, a corroding system is driven by two spontaneous coupled reactions, which take place at the interface between the metal and an aqueous environment. One is a reaction in which chemical species from the aqueous environment remove electrons from the metal; the other is a react...