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9/27/01 AC 43.13-1B CHG 1 c. Corrosion is quickened by high- temperature environments that accelerate chemical reactions and increase the concentra tion of water vapor in the air. d. Electrolytes (electrically-conducting solutions) form on surfaces when condensa tion, salt spray, rain, or rinse water accumulate. Dirt, salt, acidic gases, and engine exhaust gases can dissolve on wet surfaces, increasing the electrical conductivity of the electrolyte, thereby increasing the rate of corrosion. e. When some of the electrolyte on a metal surface is partially confined, (such as between faying surfaces or in a deep crevice) the metal around this area corrodes more rap idly. This type of corrosion is called an oxygen concentration cell. Corrosion occurs more rapidly because the reduced oxygen content of the confined electrolyte causes the adjacent metal to become anodic to other metal surfaces on the same part that are immersed in electro lyte or exposed to air. f. Slime, molds, fungi, and other living organisms (some microscopic) can grow on damp surfaces. Once they are established, the area usually remains damp, increasing the pos sibility of corrosion. g. Manufacturing processes such as ma chining, forming, welding, or heat treatment can leave residual stress in aircraft parts and can cause cracking in a corrosive environment. 6-3. COMMON CORROSIVE AGENTS. Substances that cause corrosion are called cor rosive agents. The most common corrosive agents are acids, alkalies, and salts. The at mosphere and water, the two most common media for these agents, may also act as corro sive agents. a. Any acid will severely corrode most of the alloys used in airframes. The most de- structive are sulfuric acid (battery acid), halo gen acids (hydrochloric, hydrofluoric, and hy drobromic), nitrous oxide compounds, and or ganic acids found in the wastes of humans and animals. b. Alkalies, as a group, are not as corro sive as acids. Aluminum and magnesium al loys are exceedingly prone to corrosive attack by many alkaline solutions unless the solutions contain a corrosion inhibitor. Substances par ticularly corrosive to aluminum are washing soda, potash (wood ashes), and lime (cement dust). c. The major atmospheric corrosive agents are oxygen and airborne moisture. Corrosion often results from the direct action of atmospheric oxygen and moisture on metal nd the presence of additional moisture often accelerates corrosive attack, particularly on ferrous alloys. The atmosphere may also con tain other corrosive gases and contaminants, particularly industrial and marine salt spray. d. The corrosiveness of water depends on the type and quantity of dissolved mineral and organic impurities and dissolved gasses (particularly oxygen) in the water. One char acteristic of water that makes it corrosive is its conductivity. Physical factors, such as water temperature and velocity also have a direct bearing on its corrosiveness. 6-4. MICRO-ORGANISMS. a. Bacteria may be either aerobic or an aerobic. Aerobic bacteria require oxygen to live. They accelerate corrosion by oxidizing sulfur to produce sulfuric acid. Bacteria living adjacent to metals may promote corrosion by depleting the oxygen supply or by releasing metabolic products. Anaerobic bacteria, on the other hand, can survive only when free oxygen Par 6-2 Page 6-3PDF Image | AFS-640
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