Corrosion

Corrosion is the deterioration a material undergoes as a result of its interaction with its surroundings. Although this definition is applicable to any type of material, it is usually reserved for metallic alloys . Of the 105 known chemical elements, approximately eighty are metals , and about half of these can be alloyed with other metals, giving rise to more than 40,000 different alloys. Each of the alloys will have different physical, chemical, and mechanical properties, but all of them can corrode to some extent, and in different ways.

Corrosion is a natural phenomenon. When newly made steel is first exposed to the air, its originally shiny surface will be covered with rust in a few hours. The tendency of metals to corrode is related to the low stability of the metallic state. Metals occur either in the pure metallic state, the zero oxidation state, or in the form of compounds with other elements (they acquire positive states of oxidation). In the natural world, most metals are found as compounds with other elements, indicating the greater stability of their oxidized forms. For this reason, to obtain the pure metal from one of its compounds, it is necessary to put in energy. The reverse is true when a metal is exposed to its environment: it tends to release this stored energy through the processes of corrosion. This is rather analogous to what happens when an object is suspended at a point above the ground (equivalent to the metallic state). When allowed to fall or reach a stable state, it returns to a position of minimum energy on the ground (equivalent to the metal's oxidized state).

The chemical reactions that take place in corrosion processes are reduction-oxidation (redox) reactions. Such reactions require a species of material that is oxidized (the metal), and another that is reduced (the oxidizing agent). Thus the complete reaction can be divided into two partial reactions: one, oxidation; the other, reduction. In oxidation, the metal loses electrons. The zone in which this happens is known as the anode. In the reduction reaction, the oxidizing agent gains the electrons that have been shed by the metal, and the zone in which this happens is the cathode.

Corrosion processes not only influence the chemical properties of a metal but also generate changes in its physical properties and its mechanical behavior. This is why the effects of corrosion are manifested in a variety of forms. The most common form is uniform corrosion, whereby there is a generalized, overall "attack" of the entire exposed surface of the metal, leading to a more or less uniform reduction in the thickness of the affected metal. In contrast, there is the process of localized corrosion, in which an intense attack takes place only in and around particular zones of the metal, leaving the rest of the metal unaffected; an example is pitting corrosion. Some other forms of corrosion are stress corrosion cracking, galvanic corrosion, selective alloy breakdown, intergranular corrosion, fatigue, friction, erosion, cavitation, hydrogen enbrittlement, biocorrosion, and high temperature oxidation.

It is easy to find everyday objects that show signs of corrosion. A used automobile consists essentially of different metals, plastics, paints, metallic coatings, and so forth, which have been exposed to a variety of aggressive conditions. These materials are exposed to the action of atmospheric agents under conditions of high temperature, are incorporated in closed water circuits, or are subject to mechanical wear. As a result, we see many signs of corrosion in cars, chiefly on the bodywork and in the exhaust system, and especially after years of use.

Traffic signs, streetlamp standards, metal barriers and fences, and many other items seen along roads are usually manufactured in galvanized steel (normal mild steel with a zinc coating). On such items it is common to see "white" corrosion (zinc oxide), characteristic of the oxidation of a zinc coating, often with spots of red rust also appearing that indicate that the steel base is being corroded.

The sea and the salty atmosphere are saline media that are highly aggressive to metals. Marine structures such as ships, bridges, and drilling rigs and platforms usually show signs of severe corrosion unless they have been properly protected. Cars kept near oceans show signs of corrosion.

Alloys of copper used in the casting of sculptures, and used externally on some buildings, usually show a greenish coloring that corresponds to a layer of corrosion product known as copper patina that gives some protection against further corrosion.

The degradation of reinforced concrete in buildings is commonly caused by the corrosion of the steel reinforcing bars within the concrete, rather than by the loss of the mechanical properties of concrete.

Corrosion processes affect many areas of human activity in which metal products are used. In general, as levels of economic development increase, so do costs incurred as a result of corrosion. It is estimated that the costs attributable to the corrosion of metallic materials amount to 4 percent of the gross domestic product of the developed countries. And this cost, representing a loss of resources, would be even higher if methods of protection against corrosion were not so widely applied. It is estimated that because of this protection, populations are able to reduce these potential losses by a factor of about 30 percent.

Many methods for preventing or reducing corrosion exist, most of them orientated in one way or another toward slowing rates of corrosion. A series of methods that are based on depositing a layer of a second material on the surface of a metal structure to impede the structure's contact with an aggressive medium have been developed. The most prevalent of these is painting, and a wide range of protective paints is now available. As a general estimate, for every 100 m² of exposed metal surface, paint is applied to 90 m². Included among these surface covering methods are metallic surface treatments, such as chrome, nickel, and galvanized coatings, and inorganic treatments, such as chromates, anodizing coatings, and phosphate coatings.

Another method of protection uses inhibitors, which are substances added to the liquid medium, again to reduce rates of corrosion. Antifreeze liquids utilized in the cooling circuits of vehicle engines have inhibitor agents incorporated into their formulations in order to reduce corrosion problems.

Cathodic protection is an anticorrosion technique widely used in ships and in buried or submerged pipe work. This method seeks to reduce the rate of corrosion of the structure to be protected by joining it to "sacrificial" anodes. In other words, the structure is joined to another metal (an anode) that corrodes more readily, effectively diverting the tendency to corrode away from the structure.

As an alternative to using metals that must be protected by one or other of the methods described, engineers often have an option to use an alloy selected for having a greater resistance to corrosion caused by its surroundings. But better corrosion resistance usually comes at a higher materials and/or manufacturing cost. Engineers must take into account that the corrosion resistance of any alloy depends on both the medium and the working conditions. Hence, alloys with good resistance in one environment may have poor resistance in another, and their resistance is also likely to vary according to differences in exposure conditions, such as temperature or stress.

see also Oxygen.

F. Javier Botana

Bibliography

Bradford, Samuel A. (1993). Corrosion Control. New York: Van Nostrand Reinhold.

Fontana, Mars Guy (1986). Corrosion Engineering, 3rd edition. New York: McGraw-Hill.

Jones, Denny A. (1996). Principles and Prevention of Corrosion, 2nd edition. Upper Saddle River, NJ: Prentice Hall.

Roberge, Pierre R. (2000). Handbook of Corrosion Engineering. New York: McGraw-Hill.

Shreir, L. L.; Jarman, R. A.; and Burstein, G. T., eds. (1994). Corrosion. Boston: Butterworth-Heinemann.

Internet Resources

Information available from: <http://www.corrosion-doctors.org>.

Corrosion

Corrosion is the deterioration of metals and other materials by chemical reaction. Corrosion of metals is the most common type of corrosion and is a process involving an exchange of electrons between two substances, one of them being the metal . In this process, the metal usually loses electrons, becoming oxidized, while the other substance gains electrons, becoming reduced. For this reason, corrosion is classified as an oxidation-reduction or redox reaction.

While many redox reactions are extremely important and beneficial to society (for example, those that are used to make batteries), the redox reactions involved in corrosion are destructive. In fact, close to $200 billion dollars (or 4% of the gross domestic product) is spent in the United States each year to prevent or repair the damage done by corrosion. Economically, one of the most important metals to corrode is iron and one of its alloys, steel . Almost 20% of the iron and steel produced in the United States each year is used to replace objects that have corroded.

Most metals react with oxygen , but not all metals become subject to corrosion. Many metals react, but their rate of reaction is so slow that no significant corrosion appears visible. In other cases, the metal reacts with oxygen to form a metal oxide which then coats the metal and prevents further reaction with oxygen. A few metals, such as gold and platinum, are very unreactive and they are used in applications where their tendency not to corrode is important.

In general, both oxygen and water are necessary for corrosion to occur. For this reason, corrosion occurs much more slowly in desert climates that are drier. The process of corrosion is accelerated in the presence of salts or acids. In the northern United States and in Canada, winter is especially harsh on cars because there is plenty of water in the form of snow and roads are salted to prevent the formation of ice . Warm temperatures also speed the process of corrosion.

There are many ways in which we can minimize, and sometimes even prevent the occurrence of corrosion. One simple way is to paint the metal surface and prevent oxygen and water from having access to the metal surface. If the paint chips, however, the exposed surface can cause rust to spread beneath the surface of the metal. Another way to protect the metal surface is to coat it with a layer of zinc metal. Zinc is more likely to oxidize but it then forms a coating that adheres firmly to the zinc and prevents further reaction, protecting both the zinc and the metal beneath the zinc. Metals protected by zinc are said to be galvanized. Other metals that form protective coatings can also be used to protect metals that corrode. For example, in stainless steel, chromium or nickel is added to the iron. It is also possible to deposit a thin film of one metal on the surface of another metal. This process is called electroplating. Iron cans are electroplated with tin to form tin cans. The tin is less easily oxidized than the iron. If the shape of the metal object is such that it is difficult to galvanize or to electroplate, a piece of magnesium or zinc can be attached to the object. This piece of attached metal is called a sacrificial anode . It will preferentially corrode, keeping the metal (such as underground tanks or buried pipeline) intact and free from breakage. It becomes necessary to replace the sacrificial anode from time to time, but that is less expensive than replacing the whole pipeline.

Rashmi Venkateswaran

Corrosion

Corrosion is the deterioration of metals and other materials by chemical reaction. Corrosion of metals is the most common type of corrosion and is a process involving an exchange of electrons between two substances, one of them being the metal. In this process, the metal usually loses electrons, becoming oxidized, while the other substance gains electrons, becoming reduced. For this reason, corrosion is classified as an oxidation-reduction (redox) reaction.

While many redox reactions are extremely important and beneficial to society (for example, those that are used to make batteries), the redox reactions involved in corrosion are destructive. In fact, close to $200 billion dollars is spent in the United States each year to prevent or repair the damage done by corrosion to structures such as pipelines and bridges. Economically, one of the most important metals to corrode is iron and one of its alloys, steel. Almost 20% of the iron and steel produced in the United States each year is used to replace objects that have corroded.

Most metals react with oxygen, but not all metals become subject to corrosion. Many metals react, but their rate of reaction is so slow that no significant corrosion appears visible. In other cases, the metal reacts with oxygen to form a metal oxide, which then coats the metal and prevents further reaction with oxygen. A few metals, such as gold and platinum, are very unreactive and they are used in applications where their tendency not to corrode is important.

In general, both oxygen and water are necessary for corrosion to occur. For this reason, corrosion occurs much more slowly in desert climates that are drier. The process of corrosion is accelerated in the presence of salts or acids. In the northern United States and in Canada, winter is especially harsh on cars because there is plenty of water in the form of snow and roads are salted to prevent the formation of ice. Warm temperatures also speed the process of corrosion.

Corrosion can be minimized and even prevented. One simple way is to paint the metal surface and prevent oxygen and water from having access to the metal surface. If the paint chips, however, the exposed surface can cause rust to spread beneath the surface of the metal. Another way to protect the metal surface is to coat it with a layer of zinc metal. Zinc is more likely to oxidize but it then forms a coating that adheres firmly to the zinc and prevents further reaction, protecting both the zinc and the metal beneath the zinc. Metals protected by zinc are said to be galvanized. Other metals that form protective coatings can also be used to protect metals that corrode. For example, in stainless steel, chromium or nickel is added to the iron. It is also possible to deposit a thin film of one metal on the surface of another metal. This process is called electroplating. Iron cans are electroplated with tin to form tin cans. The tin is less easily oxidized than the iron. If the shape of the metal object is such that it is difficult to galvanize or to electroplate, a piece of magnesium or zinc can be attached to the object. This piece of attached metal is called a sacrificial anode. It will preferentially corrode, keeping the metal (such as underground tanks or buried pipeline) intact and free from breakage. It becomes necessary to replace the sacrificial anode from time to time, but that is less expensive than replacing the whole pipeline.

Rashmi Venkateswaran

Corrosion

Corrosion is the deterioration of a material, or of its properties, as a consequence of reaction with the environment.

In addition to corrosion of metals—the effects of soil , atmosphere, chemicals, and temperature serve as agents of corrosion for a number 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 and geochemistry .

Perhaps the earliest recognition of corrosion was the effect of seawater 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 English scientists 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 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.

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 (e.g., general corrosion, intergranular and pitting corrosion, stress corrosion cracking, corrosion fatigue, galvanic and cavitation corrosion, etc.)

Throughout the world, the direct and hidden costs of deterioration due to environmental corrosion amount to billions of dollars per year.

See also Atmospheric chemistry; Atmospheric pollution; Chemical bonds and physical properties; Chemical elements; Weathering and weathering series

cor·ro·sion / kəˈrōzhən/ • n. the process of corroding metal, stone, or other materials: each aircraft part is sprayed with oil to prevent corrosion. ∎  damage caused by such a process.