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AC 43.13-1B CHG 1 9/27/01 6-14. CONCENTRATION CELL COR- ROSION. Concentration cell corrosion, (also known as Crevice Corrosion) is corrosion of metals in a metal-to-metal joint, corrosion at the edge of a joint even though the joined met als are identical, or corrosion of a spot on the metal surface covered by a foreign material. Metal ion concentration cells and oxygen con centration cells are the two general types of concentration cell corrosion. (See figure 6-6.) a. Metal Ion Concentration Cells. The solution may consist of water and ions of the metal which is in contact with water. A high concentration of the metal ions will normally exist under faying surfaces where the solution is stagnant, and a low concentration of metal ions will exist adjacent to the crevice which is created by the faying surface. An electrical potential will exist between the two points; the area of the metal in contact with the low con centration of metal ions will be anodic and corrode, and the area in contact with the high metal ion concentration will be cathodic and not show signs of corrosion. b. Oxygen Concentration Cells. The solution in contact with the metal surface will normally contain dissolved oxygen. An oxy gen cell can develop at any point where the oxygen in the air is not allowed to diffuse into the solution, thereby creating a difference in oxygen concentration between two points. Typical locations of oxygen concentration cells are under gaskets, wood, rubber, and other materials in contact with the metal sur face. Corrosion will occur at the area of low oxygen concentration (anode). Alloys such as stainless steel are particularly susceptible to this type of crevice corrosion. 6-15. ACTIVE-PASSIVE CELLS. Metals which depend on a tightly adhering passive film, usually an oxide, for corrosion protection are prone to rapid corrosive attack by active- passive cells. Active-passive cells are often re ferred to as a type of concentration cell corro sion. However, the active-passive cell is actu ally two forms of corrosion working in con junction. The corrosive action usually starts as an oxygen concentration cell. As an example, salt deposits on the metal surface in the pres ence of water containing oxygen can create the oxygen cell. The passive film will be broken beneath the salt crystals. Once the passive film is broken, the active metal beneath the film will be exposed to corrosive attack. (See figure 6-7.) Rapid pitting of the active metal will result. This reaction can become locally intense due to several factors. First the reac tion is augmented by the affected area, since the proportion of the exposed base metal is small compared to the surrounding non reactive metal. This effectively concentrates the focal point of the reaction, often resulting in deep pits in a short time and a greater rate of corrosion. FIGURE 6-6. Concentration cell corrosion. Page 6-6 Par 6-14PDF Image | AFS-640
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