Principles of Corrosion

CORROSION of metal is a chemical or electrochemical process in which surface atoms of a solid metal react with a substance in contact with the exposed surface. Usually the corroding medium is a liquid substance, but gases and even solids can also act as corroding media. In some instances, the corrodent is a bulk fluid in others, it is a film, droplets, or a substance adsorbed on or absorbed in another substance. 

All structural metals corrode to some extent in natural environments (e.g., the atmosphere, soil, or waters). Bronze, brass, most stainless steels, zinc, and pure aluminum corrode so slowly in service conditions that long service life is expected without protective coatings. Corrosion of structural grades of cast iron and steel, the 400 series stainless steels, and some aluminum alloys, however, proceeds rapidly unless the metal is protected against corrosion. As described in Chapter 1, corrosion of metals is of particular concern because annual losses in the United States attributed to corrosion amount to hundreds of billions of dollars. 

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In Section 1.4 it was pointed out that a fundamental principle of corrosion is that the sum of the rates of the cathodic reactions must equal the sum of the rates of the anodic reactions, irrespective of whether the attack is uniform or localised ... 

The principle of corrosion in a concrete sewer pipe is shown in Figure 6.5. 

For our purposes, the classification system is not as important as (a) recognizing that many different forms of corrosion exist and (b) understanding the fundamental kinetic processes behind these different types. To this end, we will first look at the important principles of corrosion and then see how they can be applied to some important types of corrosion. 

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