CORROSION AND ITS CONTROL
* corrosion can be defined as the degradation of a material due to a reaction with its environment. * Degradation implies deterioration of physical properties of the material. This can be a weakening of the material due to a loss of cross-sectional area, it can be the shattering of a metal due to hydrogen embrittlement, or it can be the cracking of a polymer due to sunlight exposure. * Materials can be metals, polymers (plastics, rubbers, etc.), ceramics (concrete, brick, etc.) or composites-mechanical mixtures of two or more materials with different properties. Because metals are the most used type of structural materials most of this web site will be devoted to the corrosion of metals. * Most corrosion of metals is electrochemical in nature.
TYPES OF CORROSION
* UNIFORM CORROSION
* GALVANIC CORROSION
* CONCENTRARION CELL CORROSION
* PITTING CORROSION
* CREVICE CORROSION
* FILLI FORM CORROSION
* INTERGRANULAR CORROSION
* STRESS CORROSION CRACKIN
* CORROSION FATIGUE
* FRETTING CORROSION
* EROSSION CORROSION
* This is also called general corrosion. The surface effect produced by most direct chemical attacks (e.g., as by an acid) is a uniform etching of the metal. On a polished surface, this type of corrosion is first seen as a general dulling of the surface and, if allowed to continue, the surface becomes rough and possibly frosted in appearance. The discoloration or general dulling of metal created by its exposure to elevated temperatures is not to be considered as uniform etch corrosion. The use of chemical-resistant protective coatings or more resistant materials will control these problems. * While this is the most common form of corrosion, it is generally of little engineering significance, because structures will normally become unsightly and attract maintenance long before they become structurally affected. The facilities shown in the picture below show how this corrosion can progress if control measures are not taken. Galvanic Corrosion
* Galvanic corrosion is an electrochemical action of two dissimilar metals in the presence of an electrolyte and an electron conductive path. It occurs when dissimilar metals are in contact. * It is recognizable by the presence of a buildup of corrosion at the joint between the dissimilar metals. For example, when aluminum alloys or magnesium alloys are in contact with steel (carbon steel or stainless steel), galvanic corrosion can occur and accelerate the corrosion of the aluminum or magnesium. This can be seen on the photo above where the aluminum helicopter blade has corroded near where it was in contact with a steel counterbalance. Concentration Cell Corrosion
* Concentration cell corrosion occurs when two or more areas of a metal surface are in contact with different concentrations of the same solution. There are three general types of concentration cell corrosion: * metal ion concentration cells
* oxygen concentration cells, and
* active-passive cells.
* Metal Ion Concentration Cells
* In the presence of water, a high concentration of metal ions will exist under faying surfaces and a low concentration of metal ions will exist adjacent to the crevice created by the faying surfaces. An electrical potential will exist between the two points. The area of the metal in contact with the low concentration of metal ions will be cathodic and will be protected, and the area of metal in contact with the high metal ion concentration will be anodic and corroded. This condition can be eliminated by sealing the faying surfaces in a manner to exclude moisture. Proper protective coating application with inorganic zinc primers is also effective in reducing faying surface corrosion. * Oxygen Concentration Cells
* A water solution in contact with the metal surface will normally contain dissolved oxygen. An oxygen cell can develop at any...
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