Corrosion and Its Prevention

The corrosion of metals is one of the most significant problems faced by advanced industrial societies (Fig. 17.13). It has been estimated that in the United States alone, the annual cost of corrosion amonnts to tens of billions of dollars. Effects of corrosion are both visible (the formation of rnst on exposed iron snrfaces) and invisible (the cracking and resulting loss of strength of metal beneath the surface). The mechanism of corrosion must be understood before processes can be developed for its prevention. 

Although corrosion is a serious problem for many metals, we will focus on the spontaneous electrochemical reactions of iron. Corrosion can be pictured as a short-circuited galvanic cell, in which some regions of the metal surface act as cathodes and others as anodes, and the electric circuit is completed by electron flow through the iron itself. These electrochemical cells form in parts of the metal where there are impurities or in regions that are subject to stress. The anode reaction is 

Various cathode reactions are possible. In the absence of oxygen (for example, at the bottom of a lake), the corrosion reactions are 

These reactions are generally slow and do not cause serious amounts of corrosion. Far more extensive corrosion takes place when the iron is in contact with both oxygen and water. In this case the cathode reaction is 

The hydronium ions produced in this reaction allow the corrosion cycle to continue. 

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Butler, G. and H. C. K. Ison, Corrosion and its Prevention in Waters, Robert E. Krieger Publishing Company, Huntington, New York, 1978. 

To keep this book in some kind of balance, no further treatment of corrosion and its prevention - or of high-temperature dry corrosion - is feasible here, important though these themes are. 

Table 1.25 is reproduced from Corrosion and its Prevention at Bimetallic Contacts (H.M.S.O., London, 1958) by permission of the Director of Publications. The reader should note that it is recommended that the Table be used only in conjunction with the Introduction to the original publication. 

Corrosion and its Prevention at Bi-Metallic Contacts, Admiralty and Ministry of Supply Inter-Service Metallurgical Research Council, HMSO, London (1956)... 

Cavitation Corrosion and its Prevention in Diesel Engines, Symposium, lOth Nov. 1965, British Railways Board (1966)... 

Asanti, P., Corrosion and its Prevention in Motor Vehicles , Proc. Inst. Mech. Engns., 182, Part 3J, 73-79 (1967-68)... 

Fig. 21.1 Practical galvanic series of metals and alloys showing potentials on the hydrogen scale. (Note that the potentials shown are typical values that will vary according to the nature of the solution.) (after Butler, G. and Ison, H. C. K., Corrosion and its Prevention in Water, Leonard Hill, London (1966))... 

There are three areas of concern when corrosion and its prevention are considered. The three major factors are economics, safety and environmental damage. 

The cost to society of corrosion and its prevention is staggering it has been estimated to be about 200 billion per annum in the United States alone, corresponding to 800 per capita per year. Much of this amount could be saved by proper design and choice of materials, and by the use of existing prevention methods. The problem cannot be eliminated, however, since as pointed out above, corrosion represents the natural tendency of all systems toward a state of minimum free energy. 

Ordinary corrosion is the redox process by which metals are oxidized by oxygen, O2, in the presence of moisture. There are other kinds, but this is the most common. The problem of corrosion and its prevention are of both theoretical and practical interest. Corrosion is responsible for the loss of billions of dollars annually in metal products. The mechanism of corrosion has been studied extensively. It is now known that the oxidation of metals occurs most readily at points of strain (where the metals are most active ). Thus, a steel nail, which is mostly iron (Section 22-7), first corrodes at the tip and head (Figure 21-11). A bent nail corrodes most readily at the bend. 

K. S. Rajagopalan, Report on Metallic Corrosion and Its Prevention in India, CSIR. Summary published in the "The Hindu, English language newspaper (Madras), Nov. 12, 1973. 

Solid materials, in general, are more or less subject to corrosion in the environments where they stand, and materials corrosion is one of the most troublesome problems we have been frequently confronted with in the current industrialized world. In the past decades, corrosion science has steadily contributed to the understanding of materials corrosion and its prevention. Modem corrosion science of materials is rooted in the local cell model of metallic corrosion proposed by Evans [1] and in the mixed electrode potential concept of metallic corrosion proved by Wagner and Traud [2]. These two magnificent achievements have combined into what we call the electrochemical theory of metallic corrosion. It describes metallic corrosion as a coupled reaction of anodic metal dissolution and cathodic oxidant reduction. The electrochemical theory of corrosion can be applied not only to metals but also to other solid materials. 

The history and causes of corrosion in power-station boilers have been reviewed by Mann [11] case histories of corrosion and its prevention in industrial boilers have been presented by Frey [12]. 

We will discuss corrosion and its prevention in detail in Chapter 13. 

Schmitt, G., Biicken, W., and Fanebust, R., Modelling Micro-turbulences at Surface Imperfections as Related to Flow Induced Localized Corrosion and its Prevention, CORRO-SI0N/9I, NACE Annual Conference 1991, Paper No. 465. 

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