Aluminum generally has excellent resistance to corrosion and gives years of maintenance-free service in natural atmospheres, fresh waters, seawater, many soils and chemicals, and most foods. This chapter explains why aluminum and aluminum alloys are naturally resistant to corrosion and describes the conditions and circumstances under which their natural defenses break down. It discusses the causes and forms of corrosion observed in aluminum alloys and the effect of composition, microstructure, processing history, and environmental variables such as impurities, fluid flow, surface area, pressure, and temperature.

This chapter is dedicated mostly to the metallurgical effects on the corrosion behavior of corrosion-resistant alloys. It begins with a section describing the importance of alloying elements on the corrosion behavior of nickel alloys. The chapter considers the metallurgical effects of alloy composition for heat-resistant alloys, nickel corrosion-resistant alloys, and nickel-base alloys. This chapter also discusses the corrosion implications of changing the alloy microstructure via solid-state transformation, second-phase precipitation, or cold work. It concludes with a comparison of corrosion behavior between cast and wrought product forms.

This chapter is dedicated mostly to the metallurgical effects on the corrosion behavior of corrosion-resistant alloys. It begins with a section describing the importance of alloying elements on the corrosion behavior of nickel alloys. The chapter considers the metallurgical effects of alloy composition for heat-resistant alloys, nickel corrosion-resistant alloys, and nickel-base alloys. This chapter also discusses the corrosion implications of changing the alloy microstructure via solid-state transformation, second-phase precipitation, or cold work. It concludes with a comparison of corrosion behavior between cast and wrought product forms.

Nickel-base alloys used for low-temperature aqueous corrosion are commonly referred to as corrosion-resistant alloys (CRAs), and nickel alloys used for high-temperature applications are known as heat-resistant alloys, high-temperature alloys, or superalloys. The emphasis in this chapter is on the CRAs and in particular nickel-chromium-molybdenum alloys. The chapter provides a basic understanding of general welding considerations and describes the welding metallurgy of molybdenum-containing CRAs and of nickel-copper, nickel-chromium, and nickel-chromium-iron CRAs. It discusses the corrosion behavior of nickel-molybdenum alloys and nickel-chromium-molybdenum alloys. Information on the phase stability and corrosion behavior of nickel-base alloys is also included.

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 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.

Amorphous alloys, because of their lack of crystallographic slip planes, are assumed to be insensitive to the selective corrosion attack that causes stress-corrosion cracking (SCC) in crystalline alloys. However, under certain conditions, melt-spun amorphous alloys have proven vulnerable to SCC due to hydrogen embrittlement. This chapter presents findings from several studies on this phenomenon, describing test conditions as well as cracking and fracture behaviors. It also discusses the effect of deformation on corrosion behavior, particularly for alloys without strongly passivating elements.

This chapter addresses the basic concepts important to understanding corrosion of metals. It begins with an overview of the three types of behaviors that a metal exhibits when immersed in an environment and of the four requirements of a corrosion cell. The chapter then covers the important characteristics of metals with respect to corrosion, namely the metallurgical characteristics, the inherent tendency to corrode, and the tendency to form insoluble corrosion products. The important characteristics of aqueous solutions with respect to corrosion are then addressed. The characteristics include: conductivity of the solution, acidity and alkalinity, oxidizing power, degree of ionization, and solubility in the solution. These characteristics, in combination with the characteristics of the metal, will determine the corrosion behavior of a metal/environment combination. The chapter concludes with a section on the determination of corrosion rates and corrosion rate allowances.

All materials are susceptible to corrosion or some form of environmental degradation. Although no single material is suitable for all applications, usually there are a variety of materials that will perform satisfactorily in a given environment. The intent of this chapter is to review the corrosion behavior of the major classes of metals and alloys as well as some nonmetallic materials, describe typical corrosion applications, and present some unique weaknesses of various types of materials. It also aims to point out some unique material characteristics that may be important in material selection, and discuss, where appropriate, the characteristic forms of corrosion that attack specific materials. The materials addressed in this chapter include carbon steels, weathering steels, and alloy steels; nickel, copper, aluminum, titanium, lead, magnesium, tin, zirconium, tantalum, niobium, and cobalt and their alloys; polymers; and other nonmetallic materials, including rubber, carbon and graphite, and woods.

The nonferrous alloys described in this chapter include aluminum and aluminum alloys, copper and copper alloys, titanium and titanium alloys, zirconium and zirconium alloys, and tantalum and tantalum alloys. Some of the factors that affect the corrosion performance of welded nonferrous assemblies include galvanic effects, crevices, assembly stresses in products susceptible to stress-corrosion cracking, and hydrogen pickup and subsequent cracking. The emphasis is placed on the compositions, general welding considerations, and corrosion behavior of these alloys.

Many factors must be considered when welding dissimilar metals, and adequate procedures for the various metals and sizes of interest for a specific application must be developed and qualified. Most combinations of dissimilar metals can be joined by solid-state welding (diffusion welding, explosion welding, friction welding, or ultrasonic welding), brazing, or soldering where alloying between the metals is normally insignificant. This chapter describes the factors influencing joint integrity and discusses the corrosion behavior of dissimilar metal weldments.

This chapter discusses the principles and procedures of electrochemical measurements used to investigate corrosion behaviors. It begins by presenting a diagram of a basic potentiostatic circuit, which consists of a working electrode and an auxiliary or counter electrode suspended in an electrolytic solution. It describes how corrosion potentials and current densities are measured and explains how to deal with various sources of error. It also explains how electrochemical impedance measurements are used and describes the underlying theory and procedures in some detail.

This chapter is a detailed study of the localized corrosion behavior of steel, copper, and aluminum alloys. It applies the basic principles of electrochemistry, as well as materials science and solid and fluid mechanics, to explain the causes and effects of pitting, crevice corrosion, stress corrosion cracking, and corrosion fatigue. It describes the underlying mechanisms associated with each process and how they relate to the microstructure of the metal or alloy, the physical condition of the surface, and other factors such as the coupling of the metal to a dissimilar metal or surface film.

This chapter discusses the erosion and erosion-corrosion behaviors of metals and alloys. It includes data primarily related to particle-laden gas streams impacting on the metal surface. It also covers properties and applications and provides guidelines for materials selection.

Containment materials used in power generating applications are subject to molten salt corrosion. This chapter reviews the data relevant to corrosion problems in molten salt environments. It describes the corrosion behavior of steel, aluminum, nickel, and titanium alloys in molten chlorides, molten nitrates, molten fluorides, molten carbonates, and molten sodium hydroxide.

Liquid metals are frequently used as a heat-transfer medium because of their high thermal conductivities and low vapor pressures. Containment materials used in such heat-transfer systems are subject to molten metal corrosion as well as other problems. This chapter reviews the corrosion behavior of alloys in molten aluminum, zinc, lead, lithium, sodium, magnesium, mercury, cadmium, tin, antimony, and bismuth. It also discusses the problem of liquid metal embrittlement, explaining how it is caused by low-melting-point metals during brazing, welding, and heat treating operations.

This chapter provides a brief account of galvanic corrosion, which occurs when a metal or alloy is electrically coupled to another metal or conducting nonmetal in the same electrolyte. It begins by describing the galvanic series of metals and alloys useful for predicting galvanic relationships, followed by a brief section on polarization of metals or alloys. The effects of area, distance, and geometric shapes on galvanic-corrosion behavior are then discussed. Various alloys susceptible to galvanic corrosion are briefly reviewed. The chapter also discusses various modes of attack that lead to galvanic corrosion, along with methods for predicting and controlling galvanic corrosion.

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 corrosion behavior of titanium, the types of corrosion that can occur, and the effect of alloying on corrosion resistance. It explains that, due to its tenacious oxide film, titanium has excellent corrosion resistance in oxidizing environments and that the resistance can be extended into the “reducing-acid” region by adding a small amount of palladium. It describes how different grades of titanium respond to different forms of attack, including uniform, crevice, and galvanic corrosion. It also identifies applications where corrosion is often a concern.

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.