Corrosion and material degradation

Corrosion or degradation involves deterioration of material when exposed to an environment resulting in the loss of that material, the most common case being the corrosion of metals and steel by water. The changes brought about by corrosion include weight loss or gain, material loss, or changes in physical and mechanical properties.

Metal corrosion involves oxidation-reduction reactions in which the metal is lost by dissolution at the anode (oxidation). The electrons travel to the cathode where the reduction takes place, while s move through a conducting solution or electrolyte. A positive and a negative pole, called the cathode and the anode respectively, are thereby created with a current flow between them. Thus the process of corrosion is basically electrochemical.

For corrosion to occur, certain conditions must be present. These are: (1) a potential difference between the cathode and the anode to drive the reaction; (2) an anodic reaction; (3) an equivalent cathodic reaction; (4) an electrolyte for the internal circuit; (5) an external circuit where electrons can travel. Sometimes, polarization of the anodic and the cathodic reactions must be taken into consideration. Polarization is a change in equilibrium electromagnetic field of a cell due to current flow. It has been reported that polarization may retard corrosion, as in the accumulation of unreacted hydrogen on the cathode.

In the corrosion of iron in water, the reactions differ according to whether or not oxygen is present. The common reactions that take place in a deaerated medium are essentially an oxidation reaction releasing ferrous ion into solution at the anode and a reduction reaction releasing hydrogen gas at the cathode. In the presence of oxygen, a complementary cathode reaction involves oxygen being reduced to water.

Degradation of concrete, on the other hand, depends on the composition of cement and the aggressive action of the water in contact with it. Some forms of corrosion may be visibly apparent, but some are not. Surface corrosion, corrosion at discrete areas, and anodic attack in a two-metal corrosion may be readily observed. A less identifiable form, erosion-corrosion, is caused by flow patterns that cause abrasion and wear or sweep away protective films and accelerate corrosion. Another form of corrosion which involves the selective removal of an alloy constituent requires another means of examination. Cracking, a form of corrosion which is caused by the simultaneous effects of tensile stress and a specific corrosive medium, could be verified by microscopy.

Some measures adopted to prevent corrosion in metals are cathodic protection, use of inhibitors, coating, and the formation of a passivating film. Protection of concrete, on the other hand, can be achieved by coating, avoiding corrosive pH of the water with which the concrete is in contact, avoiding excessive concentrations of ammonia, and avoiding deaeration in pipes.

See also Hazardous waste sites; Seabed disposal; Waste management

[James W. Patterson ]

RESOURCES

BOOKS

Dillon, C. P. Corrosion Control in the Chemical Process Industries. New York: McGraw-Hill, 1986.

Fontana, M. G., and N. D. Greene. Corrosion Engineering. New York: McGraw-Hill, 1967.

Weber, W., Jr. Physicochemical Processes for Water Quality Control. New York: Wiley-Interscience, 1972.