Corrosion
Corrosion is the deterioration of a material, or of its properties, as a consequence of reaction with its environment. The direct and hidden costs of corrosion amount to billions of dollars per year throughout the world.
There are several schools of thought regarding the inclusion of the term "corrosion." Some would limit the term to the corrosion of metals, while others include the effects of soils, atmospheres, chemicals and temperature upon all types of materials. The need to understand and control corrosion has given rise to the new sciences of corrosion technology and corrosion control, both of which are solidly based upon chemistry.
The corrosion of metals is caused by the electrochemical transfer of electrons from one substance (oxygen for example) to another. This may occur from the surfaces of metals in contact, or between a metal and another substance when a moist conductor or electrolyte is present. Depending upon the conditions, various types of corrosion may occur. These include, general, intergranular and pitting corrosion, stress corrosion cracking, corrosion fatigue, galvanic and cavitation corrosion, impingement attack, fretting corrosion, hydrogen embrittlement, graphic corrosion, dezincification and parting, and biological corrosion.
The mechanism of corrosion in plastic materials is different than in metallic materials. Corrosion of plastic materials involves modifications to the molecular chains that form the material. Corrosion in plastic materials involves both physical ageing and chemical degradation. The primary causes of corrosion through physical ageing involve the action of solvents, loss of material, or cracking due to stress. The main causes of chemical degradation are oxidation, photodegradation (the effect of ultraviolet rays on the material), and heat. The chemical attack of plastic materials is known as solvation. In this case chemical agents penetrate the plastic, causing swelling, softening, charring, crazing, delamination, blistering, embrittlement, discoloration, dissolving, and ultimate failure.
Perhaps the earliest recognition of corrosion was the effect of sea water and sea atmospheres on ships.Salt water, continual dampness, and the growth of marine life such as marine borers, led to the decay of wooden hulls. Because of its toxicity, copper cladding of the hulls was widely used to discourage marine growth. In 1824, to protect the copper from deterioration, the team of Sir Humphrey Davy (1778-1829) and Michael Faraday (1791- 1867) applied zinc protector plates to the copper sheathing. This was the first successful application of cathodic protection, in which a more readily oxidized metal is attached to the metal to be protected. This procedure was widely used until after World War II when most hulls were replaced by steel or newer materials.
With the development of the industrial age, and the increased use of iron, the oxidation of iron, or rust, forced the development of steels and the search for new metals and metal coatings to protect surfaces. This gave birth to the science of corrosion control that involves measures of material selection, inhibition, painting, and novel design. In turn, the growth of chemical industries also led to attempts to solve the problem of corrosive effects encountered in the manufacture of hazardous inorganic and organic compounds.
The advent of the nuclear and space age has brought on new environmental corrosion challenges. Both new and old materials can now be stressed, in a matter of minutes, due to high launch temperatures or the super cold of outer space. Materials may also suffer long-term exposure to various types of radiation or ionization. Material failure under these conditions can have disastrous results.
Not all research on corrosion, however, is oriented toward its prevention. In today's modern world, we are overrun with plastic waste and refuse and the high cost of its disposal. By applying knowledge gained on the processes of corrosion, engineers can now plan intentional degradation in advance. The most common and welcome example of this is the development of "biodegradable" plastics, a product that will grow in importance in the future.
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