Corrosion
All metals are affected to some extent by the atmosphere. This effect, which is called corrosion, is especially important in aircraft due to the loss of strength it causes. Corrosion reduces the strength and ductility of metals to an alarming extent if not restrained. In the relatively thin sections used in aircraft construction even a small amount of corrosion is unsafe. Metals have been developed that are corrosion resistant in themselves and they are very generally used when their other properties are suitable for the intended application. Such metals as Inconel, K Monel, Alclad and corrosion-resisting steels are in this category. In general these metals are given a protective coating of paint only when it is desired to carry out some particular color scheme. It has been generally established that corrosion is caused by the moisture in the air. A dry piece of metal in dry air will not corrode. In order the minimize the amount of condensation, it is necessary to adequately vent all the nookx and crannies, particularly inaccessible locations in seaplane hulls. Provision should be made for the inspection will detect corrosion in its initial stages, when it can be easily arrested before becoming dangerous. There are two distinct types of corrosion to which metals used in aircraft construction are subject. The first type is the eating away or pitting of the surface, as in the rusting of steel and iron. Practically all metals are subject to this type of corrosion when they oxidize in the presence of air. This type of corrosion is one that is not visible on the surface and is, therefore, very dangerous. It is called inter granular or inter-crystalline corrosion, because it eats its way internally through the metal around the grain or crystal boundaries. This type of corrosion is found in some aluminium alloys and some corrosion-resisting steels. Metal corrosion is the deterioration of the metal by chemical or electrochemical attack and can take place internally as well as on the surface. As in the rotting of wood, this deterioration may change the smooth surface, weaken the interior, or damage or loosen adjacent parts. Corrosion can cause eventual structural failure if left unchecked. The appearance of the corrosion varies with the metal. On aluminium alloys and magnesium it appears as surface pitting and etching, often combined with a grey or white powdery deposit. On copper and copper alloys the corrosion forms a greenish films; on steel a reddish rust. When the grey, white, green or reddish deposits are removed, each of the surfaces may appear etched and pitted, depending upon the length of exposure and severity of attack. If these surface pits are not too deep, they may not significantly alter the strength of the metal; however, the pits may become sites for crack development. Some types of corrosion can travel beneath surface coatings and can spread until the part fails.