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At normal atmospheric temperatures, metals do not corrode appreciably without moisture, but the moisture in the air is usually enough to start corrosive action.
The initial growth rate of corrosion is usually much greater than the propagation rate after a short period of time. This slowing down occurs because of the oxide film that forms on the metal surface. This film tends to protect the metal underneath.
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When components and systems constructed of many different types of metals must perform under various climatic conditions, corrosion becomes a complex problem. The presence of salts on metal surfaces (from sea coast operation) greatly increases the electrical conductivity of any moisture present and accelerates corrosion.
Other environmental conditions which contribute to corrosion are :
— moisture collecting on dirt particles,
— moisture collecting in crevices between lap joints, around rivets, bolts and screws. 3. CORROSION TYPES (Figures 1, 2 and 3)
The common types of corrosion that are encountered in aircraft maintenance are described below. In actual practice, it may be difficult to determine the exact type. The reason for this is that more than one type may be occurring in the same area, at the same time.
However, even though you may not be able to identify the exact type or types, you shall be able to determine that there is some kind of corrosion taking place. If impractical to replace assembly or component, contact authorized repair station.
A. Uniform corrosion
The surface effect produced by most direct chemical attacks (as by acid) is uniform etching of the metal. On a polished surface, this type of corrosion is first seen as a general dulling of the surface. If such corrosion is allowed to continue unremedied, the surface becomes rough and possibly frosted in appearance.
B. Pitting corrosion (Figure 1)
The most common effect of corrosion on aluminum and magnesium alloy parts is called pitting. It is first noticeable as a white or gray powdery deposit, similar to dust, which blotches the surface.
When the deposit is cleaned away, tiny pits can be seen in the surface. Pitting may also occur in other types of metal alloys.
C. Intergranular corrosion (Figure 2)
This type of corrosion attacks the grain boundaries of metals. A highly magnified cross—section of any alloy shows the granular structure of the metal.
This structure consists of quantities of individual grains, and each of these tiny grains has a clearly defined boundary and differs chemically from the metal grain next to it. The adjacent grains of different elements can react with each other as anode and cathode when in contact with an electrolyte.This conductive arrangement causes rapid selective corrosion at the grain boundary, thus destroying the solidity of the metal.
D. Exfoliation corrosion
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TB20 MAINTENANCE MANUAL 20 STANDARD PRACTICES AIRFRAME TB20飞机维护手册 20 标准施工机体(28)
