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Corrosion Control The act or process of dissolving or wearing metal away by a chemical action is corrosion. It can also be defined as the deterioration of a metal by reaction to its environment. Most metals are subject to such deterioration; however, the reaction can be minimized by using corrosion-resistant metals and finishes within the aircraft's design limits. In airframe structures this is done by using aluminum alloy sheets coated on both sides with pure aluminum (alclad). Internal structures are generally painted with an organic finish (carbon base). Cadmium or zinc plating, conversion coating, paint, or all three are used to protect steel (except most stainless steel), bronze, and brass. The paragraphs that follow discuss briefly the need for corrosion control and the kinds of corrosion. Need for Corrosion Control. Without protection from corrosion, an aircraft's capability and operational integrity are endangered. As a result, tactical and combat service support missions cannot be flown. Economy is another reason for corrosion control because in severe cases corrosion can weaken primary structures sufficiently to require their reinforcement or replacement. Either repair can be costly and time-consuming. Moreover, aircraft effectiveness is lost. Common Types of Corrosion. Many ways have been used for classifying corrosion. For simplicity, corrosion is discussed here under the titles most generally used. Uniform Etch Effect. Most direct chemical attacks, as by an acid, produce a uniform etch effect on the surface. This is first noticed on a polished surface as a general dulling or loss of polish. If the corrosion is not stopped, the surface becomes rough and possibly frosted in appearance. Pitting. On aluminum and magnesium alloys, the usual effect of corrosion is pitting. A powdery white or gray deposit is the first sign of pitting corrosion. When the deposit is removed, shallow pits or holes can be seen in the surface. Pitting corrosion can occur on other alloys as well. Intergranular. Intergranular corrosion occurs at the metal's grain boundaries. A magnified cross-sectional view of any alloy shows the metal's granular structure. Each grain has a clearly defined boundary differing chemically from the metal within the grain center. Adjacent grains of different elements reach an anode and cathode when in contact with a conductive medium such as moisture. Under this condition, rapid selective corrosion at the grain boundary takes place. Exfoliation. Exfoliation, a form of intergranular corrosion, shows itself by lifting a metal's surface grains. The lift is produced by the force of expanding corrosion products at the grain boundaries just below the surface. Exfoliation corrosion is generally seen on extruded sections where grain thickness is usually less than in rolled forms. Galvanic. Galvanic corrosion occurs when dissimilar metals are in contact and an external circuit is completed by moisture and contaminants. The result is a corrosion buildup at the bimetal juncture. Concentration Cell. Concentration cell corrosion takes place when two or more metal surface areas are in contact with different concentrations of the same solution. In this corrosion, three general types of concentrations are recognized. They are the metal-ion cells, oxygen-concentration cells, and active-passive cells. Stress. Stress corrosion cracking and fatigue corrosion are related in that the latter is a special case of the former. The simultaneous effects of tensile stress and corrosion cause stress corrosion cracking. Stresses can be internal or applied. Internal stresses are produced by nonuniform deformation during cold working, unequal cooling from high temperatures, and press and shrink fits, and application of rivets and bolts. The combined effects of cyclic stress and corrosion produce fatigue corrosion. No metal is immune to some reduction of its resistance to cyclic stressing if it is in a corrosive environment.
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