|
|
|
Book Review:
Corrosion of Aluminum and Aluminum Alloys
12-26-02
Publisher: ASM International
One of the reasons that aluminum is chosen for a number of applications is its corrosion resistance. But all aluminum alloys and products are not equally resistant to corrosion. Providing specific information for the practicing engineer on the forms of corrosion that can occur, test methods for determining alloy susceptibility, and measures for prevention is the focus of Corrosion of Aluminum and Aluminum Alloys.
Edited by J. R. Davis and published by ASM International in 1999, this 313 page book provides well-referenced but quite readable coverage of the key topics. Its emphasis is on the description of corrosion processes and their prevention from mostly a phenomenological perspective for the benefit of those dealing with day-to-day corrosion issues, rather than from the point of view of electrochemical theory. The provision of both a subject and alloy index assists the reader in using the book as a reference to find information on specific alloys.
The initial chapter provides an introduction to aluminum alloys as well as applications and aluminum industry background that is valuable to not only those unfamiliar with aluminum and the industry, but also as a summary for more experienced members of the community. Since Mr. Davis also was the editor of the ASM Specialty Handbook Aluminum and Aluminum Alloys, he has an excellent grasp on the most important general points to be made, and they are presented in a very concise and useful format. In particular, Figure 1, showing a comparison of aluminum alloy tensile strength properties with those of other metal systems, and Figure 3, a one-page flow chart illustrating aluminum primary and secondary production as well as end uses, are excellent. Three appendices, which provide information on the compositions of wrought and cast alloys, respectively, and also a description of the temper designation system supplement the information in this chapter.
The second chapter, entitled "Understanding the Corrosion Behavior of Aluminum", provides the scientific background for the corrosion characteristics of aluminum. It is noted that although aluminum is a fairly active metal, it generally has excellent corrosion resistance as a result of the aluminum oxide film formed on its surface. This film is stable over a range of pH from roughly 4 to 9, and thus most of the corrosion observed in aluminum is localized corrosion. More general, uniform corrosion can occur if the oxide film becomes unstable (under highly acidic or alkaline conditions) or is removed. Variables affecting the corrosion behavior of aluminum are classified into metallurgical and environmental, and a relative ranking of the general and stress corrosion cracking resistance of both wrought and cast aluminum alloys is given. An important point made in this chapter is "because many variables influence corrosion, the suitability of aluminum cannot be considered solely on the basis of a specific product or environment. A detailed knowledge of traces of impurities, conditions of operation, design of a piece of equipment, and alloy microstructure is essential."
Chapters 3-7 form the core of the book, and address the types of corrosion that can occur in aluminum and aluminum alloys. Each chapter provides a phenomenological description of the corrosion process plus information about the performance of various aluminum alloys and product forms followed by descriptions of testing procedures and specific prevention methods. In many cases, practical examples of the particular corrosion process are given that are useful illustrations. Specific topics in these chapters are:
- Pitting and crevice corrosion (Chapter 3)
Creation of a localized aggressive environment that breaks down the normally corrosion-resistant passivated surface of the metal. Pitting occurs at microscopic heterogeneities in the alloy, while crevice corrosion (and the special cases of water staining, poultice corrosion, and filiform corrosion) occurs at macroscopic heterogeneities. (Note that the topic of water staining is also addressed in the Aluminum Association publication TR-3-792601, "Guidelines for Minimizing Water Staining of Aluminum".)
- Intergranular and exfoliation corrosion (Chapter 4)
Corrosion in which an electrochemical difference between the grain boundaries and grain interiors can lead to accelerated attack.
- Galvanic, deposition, and stray-current corrosion (Chapter 5)
Accelerated corrosion because of electrical contact with a more noble metal (or due to an imposed electric current) in a conductive environment. It should be noted that galvanic corrosion accelerates but does not change the type of morphology of the inherent corrosion, and so might actually be considered an environmental influence instead of a type of corrosion.
- Erosion, cavitation, impingement, and fretting corrosion (Chapter 6)
Corrosion combined with mechanical processes, with attack resulting primarily from the removal of the protective oxide film.
- Environmentally assisted cracking (Chapter 7)
Cracking in which a combination of mechanical loading and environment result in the potential for premature failure below customary design stresses. Stress corrosion cracking occurs under sustained tensile loads, while corrosion fatigue occurs under cyclic loading. This chapter has a discussion of the proposed theories for environmentally assisted cracking as well as an extensive data summary for a number of aluminum alloys.
A broader discussion of corrosive environments is contained in the next chapter, addressing corrosion in the atmosphere and various types of water along with special environments such as microbiological, soils, building materials, food, and chemicals. (Note that the latter two topics are addressed more extensively in the Aluminum Association publication CFC-60-516105, "Guidelines for the Use of Aluminum with Food and Chemicals".)
Chapters 9 and 10 address the special topics of corrosion resulting from joining processes and corrosion of aluminum metal matrix composites, respectively. A comprehensive table providing relative corrosion ratings for various combinations of alloys joined with various fillers is a valuable resource contained in Chapter 9. The summary of corrosion performance of aluminum metal matrix composites contained in Chapter 10 is probably one of the most complete available in this type of general publication, considering that these materials are just beginning to see widespread application.
While the prevention of the various types of corrosion was covered in Chapters 3-7, a more general discussion is contained in Chapter 11. The role of design is highlighted. Also, a review of various types of corrosion protection methods, focusing on coatings, is given. A useful addition to this chapter is a listing of corrosion monitoring and inspection methods.
The closing chapter discusses corrosion testing methods. Again, there is overlap with earlier chapters, but having a listing of ASTM standards for corrosion testing in one place is a useful reference.
Corrosion of Aluminum and Aluminum Alloys is available from the Aluminum
Association Bookstore online at www.aluminum.org
as well as from ASM International at www.asminternational.org.
Article provided courtesy of The Aluminum Association - www.aluminum.org
|
|
|
