CORROSION DETECTION LEARNING OBJECTIVE: Identify the types, forms, and characteristics of corrosion. Timely detection of corrosion is essential to any corrosion control program. Of course, corrosion can be detected after a part fails (if the equipment can be recovered). However, then it is too late to do anything about it other than to intensify inspections of other similar aircraft and SE. Inspection for corrosion should be a part of all routine inspections. On every aircraft and piece of SE, there are certain areas that are more corrosion prone than others. You should check these areas carefully. For the corrosion inspection to be thorough, you must know the types of corrosion likely to be found and the symptoms or appearance of each type of corrosion. Sometimes corrosion is hidden, and special detection methods are used to find it. Various aspects of corrosion detection are discussed in the following text. FORMS OF CORROSION Corrosion may occur in several forms, depending upon the specific function, size, shape and type of metal involved. Atmospheric conditions and the presence of corrosion-producing agents are also factors in the development of corrosion. The types of corrosion described in this section are the more common forms found on aircraft structures and SE. This text uses the most commonly accepted terms that describe the various types of corrosion. Uniform (Direct) Surface Attack The surface effect produced by the direct reaction of a metal surface with oxygen in the air is a uniform etching of the metal. The rusting of iron and steel, the tarnishing of silver, and the general dulling of aluminum surfaces are common examples of surface attack. On aluminum surfaces, if the surface attack is allowed to continue, the surface will become rough and eventually frosted in appearance. Figure 4-8 shows direct surface corrosion on an A-6 landing gear linkage system. Pitting Corrosion The most common effect of corrosion on aluminum and magnesium alloys is called “pitting.” The primary cause of pitting is the variation in structure or quality between areas on the metal surface in contact with a corrosive environment. Pitting corrosion is first noticeable as a white or gray powdery deposit, similar to dust, which blotches the surface. When the superficial deposit is cleaned away, tiny pits or holes can be seen in the surface. They may appear as shallow indentations or deep cavities of small diameter. Pitting may occur in any metal, but it is particularly characteristic of aluminum and magnesium. Figure 4-9 is an illustration of pitting corrosion. Crevice Attack or Concentration Cell Concentration cell corrosion is actually a form of pitting corrosion. Concentration cell corrosion is caused by the difference in concentration of the electrolyte or the active metal at the anode and cathode. When there are concentration differences at two different points in an entrapped pool of water or cleaning solution, anodic and cathodic areas may result. This results in the anodic area being attacked. Figure 4-10 shows the theory of concentration cell corrosion. Areas where there are crevices, scale, surface deposits, and stagnant water traps are prone to this type of attack. Concentration cell corrosion is controlled and prevented by avoiding the creation of crevices during repair work. It is also controlled with sealants and caulking compounds that eliminate voids that trap water. Intergranular Attack, Including Exfoliation All metals consist of many tiny building blocks called “crystals” (sometimes called grains). The boundaries between these crystals are commonly called “grain boundaries.” Intergranular corrosion is an attack on the grain boundaries of some alloys under specific conditions. During heat treatment, these alloys are heated to a temperature that dissolves the alloying elements. As the metal cools, these elements combine to form compounds. If the cooling rate is slow, they form at the grain boundaries. These compounds differ electrochemically from the material adjacent to the grain boundaries, and they can be either anodic or cathodic to the adjoining areas, depending upon their composition. The presence of an electrolyte results in attack of the anodic area. This attack can be rapid and exist without visible evidence. As the intergranular corrosion progresses to the more advanced stages, it lifts the surface grain of the 4-20