An Introduction To Corrosion

Suppose that you lose your pen. As a result you cannot write because you do not have a writing instrument that functions. That simple device, your pen, has a miraculous effect on your ability to perform. It is in this connection that we want to discuss the science and technology of corrosion. We live in a highly industrialized country. We all use parts which are extensions of us, and those parts periodically wear out, break and malfunction. Much of that is due to corrosion.

Each year, a tremendous amount of money is lost to corrosion in the United States. Figures change each year, but a reasonable estimate for the year 2006 is 400 billion (that is "billion with a B") dollars. This is of the order of 2-4 % of the Gross National Product. This amount is more than the cost of all of the natural catastrophes that take place in the United States each year. It is more that all of the hurricanes, tornadoes, volcanoes, floods, even including the infamous Katrina of New Orleans.

A good definition of corrosion is: A destructive attack (Chemical Breakdown) on a material by its environment. This definition includes the partial or complete wearing away, dissolving, or softening of any substance by chemical or electrochemical reaction with its environment. We normally think of metals when we think of corrosion, but a good definition is one which deals more broadly with materials in general to include any material such as metal, wood, plastics, etc. Admittedly, the most familiar example of corrosion occurs with rusting of iron. It is a complex chemical reaction in which the iron combines with both oxygen and water to form an oxide. As a matter of practical concern, the corrosion of metals is more problematic than that of other materials.

The general types of Corrosion include uniform, electrochemical, galvanic, concentration cell, erosion, embrittlement, stress corrosion, filaform, corrosion fatigue, intergranular, fretting, impingement, dezincification, and chemical reaction. For convenience we will categorize the various kinds of corrosions into eight different forms. In fact, one could have twenty or fifty-two or whatever. In any event we could have some subcategories if needed. Those eight forms are (1) Galvanic, (2) Crevice, (3) Pitting, (4) Intergranular, (5) Leaching, (6) Erosion- Corrosion, (7) Stress Corrosion Cracking, and (8) Hydrogen Attack (Embrittlement).

GALVANIC: Galvanic corrosion can result when a metal is in contact with another dissimilar metal. In order for galvanic attack to take place, there must be four things present. First, there must be an anode. This is the material which corrodes (e.g., the formation of rust takes place if the metal is iron). Second, there also must be another electrode called the cathode. Third, an electrical connection must exist between the anode and cathode through which electrons can flow. Finally there must be an electrolyte through which chemical ions can flow. This is generally an aqueous (water) solution yet even damp soil can make an excellent electrical conductor. The arrangement of these four specific components is always necessary for an electrochemical chemical cell to function. However in the case of galvanic corrosion, the anode and cathode are clearly dissimilar metals, copper and zinc, iron and brass, or mild steel and cast iron, for examples.

CREVICE: Consider a sheet of stainless steel that has been immersed in the ocean for some time, years perhaps. It has had a bolt with a washer on it to hold it in place. We notice that corrosion has taken place underneath the washer. The reason for the corrosion is that a crevice had been created under the washer. Stagnate water, an electrolyte, accumulated in the crevice. An electrochemical, corrosion cell had been produced and corrosion resulted. This cell is called a differential aeration, or oxygen concentration cell. It results because there is a difference in the composition of the electrolyte under the washer and that outside of the washer. There is a higher oxygen concentration in the solution that surrounds than washer that there is under the washer. It may surprise you to know that corrosion takes place at the lower oxygen concentration whenever we have an oxygen concentration cell. You may have associated corrosion with a higher oxygen concentration which is a true general statement. But, remember, in the case of a crevice, or a pit, there are two oxygen concentrations in the electrochemical cell that has is such that corrosion takes place at the lower oxygen concentration..

PITTING: Some materials are more subject to pitting than others. Imagine that we find a stainless steel spoon lying on the beach near the ocean where it has been for several days. It has corroded. Little pits have formed. Pitting corrosion is a form of corrosion that most of us see on almost a daily basis. This corrosion cell, i.e., the electrochemical mechanism of the corrosion, is very similar to that of crevice corrosion. There is a stagnant solution at the bottom of the pit, i.e., a lower oxygen concentration is outside of the pit than inside, resulting in another oxygen concentration cell. Corrosion takes place at the bottom of the pit. Do you recall why? Generally, as the pit gets deeper, the solution at the bottom of the pit becomes more stagnant creating more driving force to promote corrosion. A pit is said to be self-catalyzing.

INTERGRANULAR: Consider a stainless sheet that has been welded to another. Along each side of the weld you see a corrosion attack called weld decay. This attack takes place by intergranular corrosion. That is, the attack is into the boundaries of the metallic grains that make up the metal. As the metal is heated during the weld, chromium is precipitated out of the heated grains and deposits in the grain boundary, an area that separates the grains and is burdened with impurities. Again, the components of a chemical corrosion cell are the result.

LEACHING: Selective leaching corrosion is corrosion accelerated by the selective leaching of an alloying element out of the alloy matrix. The most common form of this type of corrosion is Dezincification, the selective leaching of zinc out of the brass matrix. Brass is made of zinc and copper. Zinc is considerable more corrosive than is copper. In certain cases, e.g., when brass is exposed to an aggressive environment, the zinc will corrode preferentially and leaching zinc from the brass alloy leaving behind a weak network of copper. It may look strong but is has been severely weakened.

HYDROGEN EMBRITTLEMENT: In general, �embrittlement corrosion� is corrosion that causes a ductile material to fail without localized yielding or shearing. More specifically, hydrogen embrittlement assumes several different forms with a general similarity. This damage takes place at the cathode, an area that we normally think is safe from corrosion � remember the saying that �corrosion takes place at the anode�, but it does not in this case. Hydrogen ions are reduced to hydrogen molecules at the cathode. Those atoms usually pair-up to become hydrogen molecules. These molecules harmlessly bubble off as hydrogen gas. However, some metals are very susceptible to letting hydrogen atoms permeate into the grains. This is done while the hydrogen exists as a atom, before it becomes a molecule. Once inside the metal the hydrogen atoms can do all sorts of mischief that results in hydrogen damage.

There are five methods of corrosion prevention or control that are generally used. They are: (1) Material Selection, (2) Environmental Change, (3) Cathodic and Anodic Protection, (4) Coatings, and (5) Design.

Material Selection: Material selection consists of obtaining a material which will do the job that we want done. One might think that the job that we want done is obvious, that it is to stop corrosion. But corrosion control is almost always an economic situation. Assume that you have some steel that is corroding. Perhaps you could stop the corrosion if only you were to use some noble metal such as platinum. �But platinum is too expensive,� you say. It is this type of economic consideration that we have to take into account. We select the material which is best for the job. This includes cost and strength as well as corrosion resistance considerations.

Environmental Change: Sometimes we can change the environment in which the corroding material is enclosed. When we think of environment, we might think of temperature and the possibility that we can control it. We might consider the concentration of solutions and the ions in the solution. Or we might think of our ability to add inhibitors to provide some control over the corrosion. An everyday example of a corrosion inhibitor of which almost everyone is aware is the inhibitor put into automobile-engine cooling radiator.

Cathodic and Anodic Control: Wouldn�t it be nice if all we had to do was to turn a switch and find that we had stopped corrosion? We have the capacity to do just that with cathodic and anodic protection methods. Cathodic protection is used extensively worldwide as a corrosion control method. Anodic protection is used to a lesser extent, but nevertheless has some very interesting possibilities with respect providing corrosion protection. Cathodic protection results when we impress a negative potential onto the material we want to protect. Anodic protection is possible in very specific cases when we impress a positive potential. However this must be done with quite stringent controls.

Coatings: When we think of coatings, many of us think of paints. But in addition to paints there are other types of coating such as metallic coatings, electro-plating, galvanizing, etc. Coatings are frequently use for cosmetic purposes. Look around you. Do you see anything this is not coated? You see coatings almost everywhere. They generally have two purposes. One is cosmetic; the other purpose is corrosion protection. The latter purpose is of particular interest to engineers. The corrosion engineer has a wide range of coatings from which to select.

Design: Perhaps you think of control by design as changing the design in some extremely clever way. But one might simply design a material to be bigger and thicker and simply let it corrode away. Normally design is simply a common sense approach such: how to drain tanks, where to locate a plant, and how to install various parts of a plant, etc.

President and Chief Technical Officer: Earl L. Pye, BS, MS, Ph.D., PE --- Earl is a world renowned consultant in the areas of coatings and corrosion control. He has built a reputation and a business based on his customer-centric approach to consulting.

See his Profile on Experts.com.

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