What causes corrosion on Iron and Steel?

Principles of Corrosion.
Corrosion may take place by direct chemical attack or by electrochemical (galvanic) attack; the latter is by far the most common mechanism. When two dissimilar metals that are in electrical contact are connected by an electrolyte, an electromotive potential is developed, and a current flows.

The magnitude of the current depends on the conductivity of the electrolyte, the presence of high resistance “passivating” films on the electrode surfaces, the relative areas of electrodes, and the strength of the potential difference. The metal that serves as the anode undergoes oxidation and goes into solution (corrodes).

When different metals are ranked according to their tendency to go into solution, the galvanic series, or electromotive series, is obtained. Metals at the bottom will corrode when in contact with those at the top; the greater the separation, the greater the attack is likely to be.

Table 4-14 is such a ranking, based on tests by the International Nickel Company, in which the electrolyte was seawater.

The nature of the electrolyte may affect the order to some extent. It also should be recognized that very subtle differences in the nature of the metal may result in the formation of anode-cathode galvanic cells: slight differences in composition of the electrolyte at different locations on the metal surface, minor segregation of impurities in the metal, variations in the degree of cold deformation undergone by the metal, etc.

It is possible for anode-cathode couples to exist very close to each other on a metal surface. The electrolyte is a solution of ions; a film of condensed moisture will serve.

Corrosion Prevention.
An understanding of the mechanism of corrosion suggests possible ways of minimizing corrosion effects. Some of these include:
(1) avoidance of metal combinations that are not compatible,

(2) electrical insulation between dissimilar metals that have to be used together,

(3) use of a sacrificial anode placed in contact with a structure to be protected (this is an expensive technique but can be justified in order to protect such structures as buried pipelines and ship hulls),

(4) use of an impressed emf from an external power source to buck out the corrosion current (called cathodic protection),

(5) avoiding the presence of an electrolyte—especially those with high conductivities, and

(6) application of a protective coating to either the anode or the cathode.  

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