Aqueous corrosion of metals and alloys

To the average person, corrosion has always seemed a mysterious evil, associating a medium, or one of its components, a metal, with various forms of damage. Hence the abundant and ancient traditional nomenclature, based on one or other of these three aspects: corrosion by, corrosion of, and corrosion how:

• corrosion by air, seawater or oxygen, by the atmosphere or soils, by acids, bases or salts, by bacteria or molds, or by all the good and bad environments ;

• corrosion of iron, steels or cast irons, copper, bronzes or brasses, etc. ;

• uniform corrosion, pitting, crevice corrosion, etc.

Such a nomenclature has the advantage of perfectly expressing the observations, impressions or priorities of the materials user. On the other hand, it has the major disadvantage of sometimes being unmanageable as it stands. In fact, it results in the definition of several thousand combinations with no logical link between them. In both diagnostics and corrosion prevention, the mere consideration of these raw factual aspects would then result in thousands of unexplained and inexplicable recipes. The result is a truly hopeless situation.

In fact, only a scientific understanding of corrosion mechanisms can provide a simple approach to these complex problems, and above all, reliable solutions. As we shall see in paragraph 1, aqueous corrosion of metals is essentially electrochemical in nature. This explains why only an electrochemical framework can identify simple laws of behavior, and therefore also simple rules of prevention. We shall also see that such an electrochemical understanding of corrosion in no way requires an electrochemist by trade, or even by training. In fact, it requires only a few rudimentary concepts, and the aim of paragraph 1 is precisely to make it both natural and very simple to acquire. Similarly, the lack of links between the corrosion and electrochemistry professions means that international conventions in electrochemistry have no reason to be applied to corrosion. After a period of largely fruitless cohabitation, the international conventions in use in corrosion tend to revert to the original historical conventions, which are the opposite of the current official electrochemical conventions.

Generally speaking, understanding corrosion does not require any particular specialization in any one technique, but rather a multidisciplinary generalist culture. Whenever necessary, we'll remind you of a few essential elements, and refer you to the relevant articles in Techniques de l'Ingénieur.

Finally, corrosion is a discipline that has been and remains highly evolutionary. Until 1930, it had remained...