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Corrosion Mechanism

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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 environment (air, soil, water, seawater, ...) 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 corrosion can have specific vulnerabilities that need to be understood. 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.

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.

Similar electrical potentials may also be developed between two areas of a component made of a single metal as a result of small differences in composition or structure or of differences in the conditions to which the metal surface is exposed. That part of a metal component which becomes the corroding area is called the "anode"; that which acts as the other plate of the battery is called the "cathode" and does not corrode, but is an essential part of the system. In the corrosion systems commonly involved in buildings there may often be only a single metal involved, with water containing some salts in solution as the electrolyte. Corrosion may even take place with pure water, provided that oxygen is present. In such cases oxygen combines with the hydrogen generated at the cathode, removing it and permitting the reaction to go on.

Other agents, notably certain bacteria in the soil which remove hydrogen, can also act as depolarizing agents and thus promote the corrosion reaction. Steel, because of its low cost together with its many desirable properties, is the most common metal used in buildings. It can often be protected adequately by the application of suitable coatings. For certain purposes other metals more resistant to corrosion may be a better choice, depending on initial cost and expected service life. Metal components used in buildings can be grouped for purposes of discussing corrosion into four general categories: 1) those used on the exterior as cladding, roofing and flashings, 2) those incorporated in the construction as structural and reinforcing steel, masonry ties and damp courses, 3) those used in the services to a building as piping, storage tanks for hot water, drains and heating ducts, 4) those buried in the ground.

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.

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.