This chapter develops a corrosion model that accounts for solution potentials and the effects of coupling between cathodic and anodic reactions. It begins by examining potential differences at various points (in the solution) along a path from the anode to the cathode area. It then presents a simple model of a galvanically coupled electrode, in which the metal is represented as an array of anode and cathode reaction surfaces. The chapter goes on to develop the related theory of mixed electrodes, showing how it can be used to predict corrosion rates based on measured potentials and current densities, polarization characteristics, and physical variables such as anode-to-cathode area ratios and fluid velocity. It also discusses the effect of corrosion inhibitors, galvanic coupling, and external currents, making extensive use of polarization curves.

This article describes in detail the process of corrosion control by cathodic and anodic protection. The discussion covers the basic concept of cathodic and anodic protection systems, their types and equipment used, and the advantages, limitations, and applications of these protection systems. The types of cathodic protection systems include sacrificial cathodic protection and impressed-current cathodic protection systems. Some of the technical problems associated with cathodic protection include the effects of stray currents on the corrosion of adjacent metal structures, the effects of the chemical reactions occurring at the surface of the protected structure, and the effects of cathodic protection on coatings.

Anodic protection is used on a smaller scale than other corrosion control techniques due to the fundamental electrochemistry involved. This chapter provides a brief history of the technique, discusses anodic protection use, and compares anodic and cathodic protection. The background and theory of anodic protection are summarized. In addition to briefly describing the various items used for each component of an anodic protection system, the chapter presents design concerns as well as applications of the system. Factors concerning the economic justification of anodic protection are also described.

This chapter lays the groundwork for understanding electrode kinetics associated with corrosion. It presents a simple but useful theory relating kinetics to the polarization behavior of half-cell reactions. The theory is based on the observation that electrode potentials vary as a function of current density or charge transfer in a given area. The chapter explains how to measure and plot electrode potentials and currents and how to interpret the resulting polarization curves. It also discusses the effects of concentration gradients, explaining how they cause diffusion and, in some cases, produce changes in electrode potential.

This chapter provides a thorough introduction to the electrochemical thermodynamics that govern electrode reactions associated with corrosion. It begins with a review of the thermodynamic criteria for the stability of chemical reactions based on Gibbs free energy and explains how energies of formation are determined using the oxidation of iron as an example. It then considers how iron reacts with hydrochloric acid, explaining how it can be expressed as two half reactions modeled as electrodes in an electrochemical cell. It goes on to describe the chemical reactions occurring at each electrode, accounting for different variables, mechanisms, and electrochemical effects. The chapter concludes with an in-depth review of Pourbaix diagrams, explaining what they reveal about the stability of metal-water systems and the formation of corrosion products.

This chapter discusses the principles of corrosion of metals in aqueous environments. The thermodynamics of aqueous corrosion is the subject of the first half of this chapter, which addresses concepts such as corrosion reactions and free-energy change, the relationship between free energy and electrochemical potential, the effect of ionic concentration on electrode potential, and the corrosion behavior of a metal based on its potential-pH diagram. The corrosion (potential-pH) behavior of iron, gold, copper, zinc, aluminum, and titanium are described. Understanding the kinetics of corrosion and the factors that control the rates of corrosion reactions requires examination of the concepts of polarization behavior and identification of the various forms of polarization in an electrochemical cell. These concepts, addressed in the remaining of this chapter, include anodic and cathodic reactions, the mixed-potential theory, and the exchange currents.

Corrosion involves chemical reactions in equilibrium that that are understood through principles of thermodynamics. In practice, the rate at which corrosion reactions occur is the most important consideration. This chapter deals with corrosion kinetics, which allows engineers to to understand rates of corrosion. The discussion covers two kinetic processes, namely electrode reactions at the cathode and anode and conduction of ions in the electrolyte. The chapter also provides information on passivation and its effect on polarization diagrams.

This chapter first covers some basic principles of electrochemical corrosion and then some of the various types of corrosion. Some of the more common types of corrosion discussed include uniform corrosion, galvanic corrosion, pitting, crevice corrosion, erosion-corrosion, cavitation, fretting corrosion, intergranular corrosion, exfoliation, dealloying corrosion, stress-corrosion cracking, and corrosion fatigue. The chapter discusses the processes involved in corrosion control by retarding either the anodic or cathodic reactions. The rate of corrosion is reduced by conditioning of the metal, by conditioning the environment, and by electrochemical control. Finally, the chapter deals with high-temperature oxidation that usually occurs in the absence of moisture.

Corrosion can be defined as a chemical or electrochemical reaction between a material and its environment that causes the material and its properties to degrade. In most cases, it refers to the electrochemical oxidation of metals accompanied by the production of oxides or salts of the base material. This chapter discusses the process of corrosion and how to prevent or mitigate its effects. It describes several forms of corrosion, including uniform, intergranular, pitting, crevice, and stray-current corrosion, and the effects of stress-corrosion cracking, corrosion fatigue, and selective leaching. It discusses the use of corrosion inhibitors, cathodic and anodic protection, pH control, and Pourbaix diagrams.

The basic concept for most methods of corrosion protection is to remove one or more of the electrochemical cell components so that the pure metal or metal alloy of interest will not corrode. Another widely used corrosion protection approach is to change the nature of the anode so that it becomes the cathode (cathodic protection). This chapter briefly reviews these methods of corrosion protection. The factors affecting corrosion behavior are covered. In addition, the chapter provides information on coatings and inhibitors, which are used in corrosion protection.

This chapter discusses the complex polarization characteristics of active-passive metals and addresses related problems in interpreting their corrosion behavior. It begins by presenting several experimentally derived polarization curves for iron, comparing and contrasting them with the iron-water Pourbaix diagram. It then explains how anodic polarization is extremely sensitive to the environment and, as a result, a reasonably complete curve for a given metal-environment system usually can only be inferred. It goes on to describe how such curves are constructed, demonstrating the procedures for a wide range of alloys and environments. The examples also show how factors such as alloy concentration, crystal lattice orientation, temperature, and dissolved oxygen affect corrosion behavior.

This chapter provides a detailed account of cathodic protection. It begins by discussing the fundamentals of cathodic protection followed by a description of the various types of cathodic protection. It then describes the origins, types, and alleged failures of cathodic protection criteria. This is followed by a section providing information on anode materials that are used for cathodic protection applications. General guidelines for designing the cathodic protection systems are also listed. Finally, the chapter presents various examples on cathodic protection of steel structures. The examples are selected to familiarize the design engineer with the steps to follow in selecting a specific corrosion-control method.

Organic coatings (paints and plastic or rubber linings), metallic coatings, and nonmetallic inorganic coatings (conversion coatings, cements, ceramics, and glasses) are used in applications requiring corrosion protection. These coatings and linings may protect substrates by three basic mechanisms: barrier protection, chemical inhibition, and galvanic (sacrificial) protection. This chapter begins with a section on organic coating and linings, providing a detailed account of the steps involved in the coating process, namely, design and selection, surface preparation, application, and inspection and quality assurance. The next section discusses the methods by which metals, and in some cases their alloys, can be applied to almost all other metals and alloys: electroplating, electroless plating, hot dipping, thermal spraying, cladding, pack cementation, vapor deposition, ion implantation, and laser processing. The last section focuses on nonmetallic inorganic coatings including ceramic coating materials, conversion coatings, and anodized coatings.

This chapter provides practical information on surface treatments that work by altering the surface chemistry of metals and alloys. It discusses the use of phosphate and chromate conversion coatings as well as anodizing, steam oxidation, diffusion coatings, and pack cementation. The chapter also covers ion implantation and laser alloying.

Magnesium occupies the highest anodic position on the galvanic series and can be subject to severe corrosion. The corrosion problem is due to the impurity elements iron, nickel, and copper. However, the use of higher-purity magnesium alloys has led to corrosion resistance approaching that of some of the competing aluminum casting alloys. This chapter begins with a general overview of magnesium metallurgy and alloy designations and moves on to discuss in detail the nominal compositions, mechanical properties, heat treatment, fabrication, and corrosion protection of magnesium casting alloys and wrought magnesium alloys. It also discusses the nominal compositions, properties, and applications of commercially pure zinc, zinc casting alloys, and wrought zinc alloys.

This chapter familiarizes readers with the basic concepts of corrosion, discussing chemical reactions, ion transfer mechanisms, electrochemical processes and variables, and the formation of solid corrosion products. It presents a simple but effective teaching tool, the elementary electrochemical corrosion circuit, using it to explain how electric potential differences drive the corrosion process and how corrosion rates vary in proportion to current density. The chapter concludes with a discussion on the importance of corrosion products, such as oxides and hydroxides, and how their formation can be a major factor in controlling corrosion.

This chapter discusses the conditions and sequence of events that lead to stress-corrosion cracking (SCC) and the mechanisms by which it progresses. It explains that the stresses involved in SCC are relatively small and, in most cases, work in combination with the development of a surface film. It describes bulk and surface reactions that contribute to SCC, including dissolution, mass transport, absorption, diffusion, and embrittlement, and their role in crack nucleation and growth. It also discusses crack tip chemistry, grain-boundary interactions, and the effect of stress-intensity on crack propagation rates, and describes several mechanical fracture models, including corrosion tunnel, film-induced cleavage, and tarnish rupture models.

This chapter discusses some important factors involved in the atmospheric corrosion of engineering materials. The discussion begins with a description of elements necessary for the operation of a galvanic corrosion cell and corrosion reactions, followed by the types of atmospheric corrosion attack. Some of the atmospheric parameters and their effects on the corrosion of several metals are then reviewed. The following sections provide information on air chemistry, principal pollutants inducing corrosion, thermodynamics as well as models for prediction of atmospheric corrosion, and use of Pourbaix diagrams. The phenomenon of precipitation runoff on the corroded metal surface is then discussed. The chapter also describes the role of microbes or bacteria in the corrosion of metals. It concludes by providing information on the trends in atmospheric corrosion research and methods.

Corrosion is a key subject for more or less all classes of alloys that fall within the broad definition of stainless steels because these alloys were developed with the intention of preventing corrosion. This chapter provides an introduction to the fundamentals of electrochemical theory as it pertains to corrosion resistance of stainless steels. The discussion provides an overview of electrochemical reactions, Faraday's law, the Nernst equation, galvanic versus electrochemical cells, corrosion tendency, and Pourbaix diagrams.

This chapter covers the corrosion behavior of beryllium in aqueous environments. It describes the chemical reactions that drive the corrosion process, the conditions required for equilibrium, and the factors that affect corrosion resistance. It discusses the stability of the native oxides that form on the surface of beryllium and their ability to withstand acids, bases, and corrosive agents found in rain and seawater. It explains how carbides, inclusions, ions, and impurities contribute to corrosion damage, particularly pitting, and how corrosion reduces the ductility and fracture strength of certain beryllium alloys.