What is Corrosion?

We Don't Need No Steenking Corrosion!

CORROSION

Summary:

Corrosion, the deterioration caused by chemical reaction with the environment, affects materials as different as structural metals, ceramics, and wood, as well as works of art and artifacts from past civilizations. Corrosion technology is a field of study that focuses on the mechanisms of corrosion and on the design of protective schemes to prevent it or limit its extent.

ALLOYS

materials made by melting together two or more elements, at least one of which is a metal

ANODE

a site of chemical oxidation (loss of electrons)

CATHODE

a site of chemical reduction (gain of electrons)

CORROSION

a destructive chemical process; most often applied to the conversion of a metal to one of its compounds, for example, the corrosion of iron by oxygen and water to produce iron oxides (rust)

CRYSTAL

a type of solid in which the particles are arranged in a definite geometric lattice; the basic cell, reproduced in three-dimensional space, results in one of fourteen commonly found crystal types

ELECTROLYTE

a substance that exists in water as ions; the resulting solution can carry an electrical current

ELECTROMOTIVE SERIES

a tabulation of the metals in order of tendency to be oxidized (or reduced)

PATINA

a layer of corrosion products that forms on the surface of a corroding metal; the cover may or may not form a barrier to further attack

REDOX REACTION

a chemical reaction in which one reactant is oxidized (loses electrons) while another is reduced (gains electrons)

Overview

Corrosion is the deterioration of a material as a result of reaction with its environment, especially with oxygen. Although the term is usually applied to metals, all materials, including ceramics, plastics, rubber, and wood, deteriorate at the surface to some extent when they are exposed to certain combinations of liquids and/or gases. Common examples of metal corrosion are the rusting of iron, the tarnishing of silver, the dissolution of metals in acid solutions, and the growth of patina on copper. Most research into the causes and prevention of corrosion involves metals, since the corrosion of metals occurs much faster under atmospheric conditions than does the corrosion of nonmetals. The cost of replacing equipment destroyed by corrosion in the United States alone is in the billion-dollar range annually.

Corrosion is usually an electrochemical process in which the corroding metal behaves like a small electrochemical cell. Since the corrosion of iron by dissolved oxygen is, from an economic standpoint, the most important redox reaction occurring in the atmosphere, it will be used here to illustrate the electrochemical nature of the process. A sheet of iron exposed to a water solution containing dissolved oxygen is the site of oxidation and reduction half-reactions, which occur at different locations on the surface. At anodic areas, iron is oxidized according to the reaction

At the same time, oxygen molecules in the solution are reduced at the cathodic areas:

The two processes produce an insoluble iron hydroxide in the first step of the corrosion process:

Generally, this iron hydroxide is further oxidized in a second step to produce Fe(OH)3, the flaky, reddish-brown substance that is known as rust. Unfortunately, this new compound is permeable to oxygen and water, so it does not form a protective coating on the iron surface and the corrosion process continues.

All metals exhibit a tendency to be oxidized, some more easily than others. A tabulation of the relative strength of this tendency is called the electromotive series of metals. Knowledge of a metal's location in the series is an important piece of information to have in making decisions about its potential usefulness for structural and other applications.

The existence of anodic and cathodic sites on the surface of a piece of metal implies that differences in electrical potential are found on the surface. These potential differences have a number of causes. One important mechanism is oxygen concentration cell corrosion, in which the oxygen concentration in the electrolyte varies from place to place. An underground pipe that passes from clay to gravel will have a high oxygen concentration in the gravel region and almost no oxygen in the impermeable clay. The part of the pipe in contact with the clay becomes anodic and suffers damage. A similar situation is found where a pipe passes under a road. The section under the road (which is the more difficult to get at for repair) is oxygen deprived and will suffer the greatest damage. The cure for this is cathodic protection, which involves the use of a sacrificial anode such as zinc or aluminum. In this situation, the metal to be protected is connected electrically to a piece of scrap metal that will take its place as the anode. The anode is destroyed by the corrosion reaction, leaving the cathode intact. This technique is still used extensively to protect underground gas and water pipelines.

Concentration cells may also be formed where there are differences in metal ion concentration. A copper pipe in contact with copper ion solutions of different concentrations will corrode at the part in contact with the more dilute solution. This is an obvious problem when copper pipes are used to carry flowing water. Parts of the copper surface in contact with the more quickly moving fluid will be more negative and therefore anodic. This phenomenon plays an important part in the erosion corrosion of copper and its alloys.

Although most metals are crystalline in form, they generally are not continuous single crystals, but rather are collections of small grains or domains of localized order. Metal objects are formed from melts in which microcrystals form as the liquid cools and solidifies. In the final state, these microcrystals have different orientations with respect to one another. The edges of the domains form grain boundaries, which are an example of planar defects in metals. These defects are usually sites of chemical reactivity. The boundaries become anodic, while the grains themselves are the cathodes. The boundaries are also weaknesses, the places where stress cracking begins.

Perhaps the best known of all corrosion types is galvanic corrosion, which occurs at the contact point of two metals or alloys with different electrode potentials. An example of this might be brass detail in contact with copper hot-water pipes. The brass becomes anodic and suffers the loss of its zinc atoms. Brass in contact with galvanized steel is protected, while the zinc coating on the steel is first dissolved, leaving the steel open to attack for the same reason. An obvious area of concern is the use of one type of metal as bolts, screws, and welds to fuse together pieces of another metal. The combination to be desired is the large anode-small cathode combination. Bolts, screws, and so on should be made of the metal less likely to be oxidized so that the bolt or weld is cathodically protected.

Of great importance is the conductivity of the corroding solution. When large areas of the surface are in contact with a water solution of high conductivity, such as seawater, the attack on the anodic metal may spread far from its contact point with the cathodic metal. This is a less severe situation than that which occurs in soft water or under atmospheric conditions in which the attack is localized in the vicinity of the contact. In the absence of dissolved oxygen or hydrogen ions to maintain the cathode process, galvanic corrosion does not occur. It is possible to combine different metals such as copper and steel in closed hot-water systems with little corrosion.

The first step in preventing material corrosion is understanding its specific mechanism. The second and often more difficult step is designing a type of prevention. Some metals produce corrosion products that are insoluble, about the same size molecularly as the parent metal, and that crystallize in the same type of lattice structure. These are often able to become attached to the metal surface and form a protective coat against further corrosion. The patina that forms on copper is an example of this type of coating.

Other preventive measures involve the use of protective coatings and modification of the environment. Some trace impurities can significantly reduce the rate of corrosion and can be added in low concentration to the surrounding medium. Paint is the most common coating used to slow the rate of atmospheric corrosion. Many other materials, such as plastics, ceramics, rubbers, and even electroplated metals, can be used as protective coatings. The corrosion resistance of a metal can be greatly increased by the proper choice of alloys. For example, aluminum added to brass will increase its corrosion resistance.

Uses of the Technology

One of the large and economically important groups of industries that commit a considerable amount of time and money to corrosion prevention is the chemical process industry. Chemical process industries are found in such sectors as pulp and paper products, chemical and food processing, pharmaceuticals, metals, and semiconductor manufacture. Since these industries are characterized by widely varying service and environmental conditions, the selection of a corrosion-resistant process involves a complex analytical process.

Equipment that must be protected from atmospheric and service-related corrosion is coated in layers that vary from 0.5 to 200 mils of high-performance industrial paints, which may be organic or inorganic resin-based films. Although large numbers of these paint systems have been available for some time, changes in the late 1980's in federal and local regulations affecting the use of coatings have narrowed the field of available options. Industrial paints are composed of solid materials kept in solution, usually in volatile organic solvents. The paints are applied, and in a fairly short time, the solvent evaporates. Stricter environmental regulations have placed a limit on the percentages of volatile organic solvents permitted in a paint. As a result, new paints have been developed that have as high as 65 percent solids content as opposed to the 30 to 50 percent of the older paints.

Some of the generic classes of coatings used in recent years, such as pure vinyls or chlorinated rubbers, can no longer be formulated in compliance with regulations. Coal tar, which has long been used as an extending resin, is increasingly being regarded as carcinogenic and is less in demand. Similarly, products with large amounts of styrene are in decreasing usage because of reported toxicity. All of these facts indicate a strong future for the developers of new types of industrial paints.

The workhorse of the protective coatings industry is epoxies. These are resins that use a curative or hardening agent. Epoxies can be used to protect almost any substrate, including concrete and steel. Most tank linings are epoxies. Epoxies are relatively inflexible and are sensitive to ultraviolet radiation. Ultraviolet exposure degrades the outer surface layer to a chalky patina, which is an appearance effect only and does not change the corrosion-control capabilities.

When appearance is important, polyurethanes are used, typically as the top coat. These also offer high resistance to abrasion and have superior physical and mechanical properties. Because they are flexible, polyurethanes are more effective than materials that crack, such as cement. They are moisture sensitive and must be kept dry before curing.

Among the best coatings for steel are inorganic zinc silicates. These have a very high percentage of zinc dust in the silicate-binder matrix. They are known to prevent corrosion for five to twenty years, depending on the particular formulation and the exposure conditions. The zinc coating protects by a galvanic mechanism, in which the zinc corrodes preferentially to the steel. The zinc oxides that form provide a further barrier to corrosion.

Attracting attention in recent years because of their safety are water-based acrylics. These weather well and can be applied over concrete. They are useful and acceptable in environments that are not very aggressive.

Many aspects of the process chemical industries involve the handling of corrosive gases and liquids under varying conditions of temperature and pressure. Because of its low cost and strength, carbon steel is used in the construction of fluid-handling systems. The major problem is its poor corrosion resistance. The alternatives are stainless steels and other higher-alloy metals, which have costs that are proportional to the increase in their exotic properties.

Nonmetallic pipes made of plastic, glass, or Plexiglas are also available as alternatives to metal pipes. They are limited in use because of their lower physical strength and range of operating temperatures and pressures.

The best of both worlds is available in the form of plastic-lined pipes. These systems provide the corrosion-resistant plastic as the inner lining and the physically strong stainless steel as the exterior where the pipes are not in contact with the corrosive fluids. Their low cost (compared to high-alloy metals), ease of installation, and long service life make these pipes an important development in the field of corrosion technology.

A second interesting application of corrosion technology is the preservation of bronze artifacts from ancient civilizations. Articles made of copper and copper alloys such as brass and bronze have been in use since around the third millennium B.C.

The addition of tin to copper to produce bronze increased the strength of the cast object. Hammering the alloy produced better tools and weapons than those obtained from pure copper. Recipes for bronze composition have been found in writings of the fourth century B.C. Greeks as well as those of the early Chinese metallurgists.

The unique properties of bronze--good castability, corrosion resistance, and appearance--made it suitable for sculptures in Greek and Roman times. The Colossus of Rhodes, which was erected in 280 B.C., was thought to be of bronze. Roman coins were bronze, as was the dome of the Pantheon. Church bells were cast of bronze after about A.D. 600. Many examples of these ancient pieces are known to exist today as evidence of the civilizations that produced them. There is a need to devise protective measures against the more corrosive atmospheres of today's world so that the pieces may be passed on to future generations.

Copper is a relatively noble metal and has good resistance to corrosion. It will, however, react to form more stable compounds such as copper oxide. The rate of this process depends on the composition of the atmosphere and on the solubility of the corrosion products. Untreated copper will tarnish in a few weeks when exposed to air. This process occurs more rapidly in a polluted atmosphere.

The corrosion of bronze is similar to that of copper; however, the alloying elements, tin and lead, do have some effects, and bronze usually corrodes faster than copper. In atmospheres that contain chloride ion, the patina may contain copper chloride compounds, which react with water to form copper oxides and release the chloride to do further damage. This condition is known as bronze disease and is common in marine environments.

Benzotriazole has been well known as a corrosion inhibitor for copper and copper alloys since about 1950. It was first used in the treatment of artifacts of architectural and historical importance in 1967. It has become important in the storage of corroded articles and in the preservation of stabilized antiques.

There are a number of ways in which benzotriazole is used to inhibit corrosion. If a display case contains only metallic objects, corrosion may be inhibited by keeping the atmosphere very dry. Metallic objects in display cases that also contain fabric are protected by benzotriazole vapor in the case since some humidity is needed to preserve the fabric. Bronze objects may be stored in packaging material treated with benzotriazole, again to provide a vapor protection. Lacquers for temporarily coating newly excavated bronze artifacts also contain the preservative. The application of such varnishes has been successfully used in the National Archaeological Museum in Athens to protect statues. Benzotriazole may also form a protective compound with copper in the patina on a corroded surface. Future applications may include adding it to detergents used during preliminary cleaning of artifacts.

Context

Humans have most likely been trying to understand and control corrosion for as long as they have been using metal objects. The most important periods of prerecorded history are named for the metals that were used for tools and weapons (Iron Age, Bronze Age). Ancient writings contain instructions for the correct compositions to be used in alloying metals to obtain the most usable materials for needed objects.

Modern corrosion science has its roots in electrochemistry and metallurgy. Electrochemistry contributes an understanding of the mechanism that is basic to the corrosion of all metallic objects. Metallurgy provides a knowledge of the characteristics of metals and their alloys as well as the methods of combining the various metals and working them into the desired shapes.

The type of corrosion mechanism and its rate of attack depend on the exact nature of the atmosphere in which the corrosion takes place. In today's industrial setting, the waste products of various chemical and manufacturing processes find their ways into the air and waterways. Many of these substances, often present only in minute amounts, act as either catalysts or inhibitors of the corrosion process. The corrosion engineer then needs to be on the alert for the effects of these contaminants.

Many of the coatings used to prevent or slow metallic corrosion are organic resins. Thus, the outlook of a corrosion specialist must certainly be interdisciplinary, regardless of his or her preliminary education. It is this breadth that makes corrosion science a particularly interesting subject.

Although corrosion technology generally focuses attention on metals, most materials undergo corrosion to some degree. Much of this is accelerated as a result of the increases in pollutants in today's atmosphere. The effects of atmospheric pollutants on buildings and statues are well documented. Many of the remnants of the Greek and Roman civilizations, as well as statues crafted during the Renaissance, have suffered more damage in the twentieth century than in any previous century. The field of corrosion technology is expanding to include these types of material deterioration.

The corrosion problem is widespread, costly, and has no easy solution. The need for better protective coatings and linings will provide impetus to those industries that deal with materials development for years to come.

Atkinson, J. T. N., and H. VanDroffelaar. CORROSION AND ITS CONTROL. Houston, Tex.: National Association of Corrosion Engineers, 1982.
This small book is a general introduction to the field. It covers most of the relevant topics, such as testing, economics, safety factors, and control measures. Generally easy reading, but a background in general chemistry might be helpful for the introductory section.

McKay, Robert, and Robert Worthington. CORROSION RESISTANCE OF METALS AND ALLOYS. New York: Reinhold, 1936.
An old book, one that obviously does not cover the modern topics in corrosion technology. It is, however, very readable and gives a rather comprehensive treatment of the corrosion behavior of many metals and their alloys.

Mansfield, Florian, ed. CORROSION MECHANISMS. New York: Marcel Dekker, 1987.
A collection of papers on corrosion topics. The article on coatings by H. Leidheiser, Jr., is particularly interesting and readable.

Schweitzer, Philip A. WHAT EVERY ENGINEER SHOULD KNOW ABOUT CORROSION. New York: Marcel Dekker, 1987.
It is not necessary to be an engineer to appreciate this little book, which describes briefly most of the topics relevant to the field of corrosion.

Uhlig, Herbert H., and R. Winston Revie. CORROSION AND CORROSION CONTROL. New York: John Wiley & Sons, 1985.
A very readable book that has a good explanation of corrosion in the first few chapters. There are chapters on various types of corrosion protection as well as corrosion resistant alloys.

Walker, Robert. "Benzotriazole: A Corrosion Inhibitor for Antiques." JOURNAL OF CHEMICAL EDUCATION 57, no. 11 (1980): 789-791.

Walker, Robert. "Corrosion and Preservation of Bronze Artifacts." JOURNAL OF CHEMICAL EDUCATION 57, no. 4 (1980): 277-280.
The author describes the important part that copper alloys such as bronze and brass have played in the development of civilization and the efforts that have been made to protect them from further corrosion. Both articles can be read easily by persons with little scientific background.

Wranglen, Gosta. AN INTRODUCTION TO CORROSION AND PROTECTION OF METALS. New York: Chapman and Hall, 1985.
After a fairly technical introductory chapter, the author describes corrosion under various conditions, such as in soil and in dry gases. Also describes in generally nontechnical language the various schemes for corrosion protection. The chapters after the introduction should be understood by the general reader.