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Stresses induced when pieces such as rivets and bolts are formed, are internal stress. These components can crack because of the internal stresses and this characteristic is aggravated by corrosion. This is why such cracking is called stress corrosion cracking.
I. Fatigue corrosion
Fatigue corrosion is a special case of stress corrosion caused by the combined effects of cyclic stress and corrosion. Repeated stresses create metal fatigue and crystallization cracks in which corrosion starts.
4. TYPICAL CORRODABLE AREAS
Aluminum appears high in the galvanic series of electromotive forces and its position indicates that it should corrode very easily. However, the formation of a tightly adhering oxide film offers increased resistance under mild corrosive conditions. Most metals in contact with aluminum form couples which undergo galvanic corrosion attack. Aluminum alloys are subject to pitting, intergranular corrosion and intergranular stress corrosion cracking.
 
A. Battery area
The battery, battery cover, battery box and adjacent areas (especially areas below the battery box where battery electrolyte may have seeped) are subject to the corrosive action.
If spilled battery electrolyte is neutralized and cleaned up at the same time of spillage, corrosion can be held to a minimum by using a weak boric acid solution to neutralize the potassium hydroxide (battery electrolyte) ; if boric acid is unavailable, clean the area with an abundance of cold water.
B. Flight control cables
Checking for corrosion on flight control cables is normally accomplished during maintenance practices. If corrosion is detected on the cable, it must be replaced.
C. Electrical installations
Corrosion in electrical systems and resultant failure can often be attributed to moisture and climatic condition.
Fungi destroy the resistance of electrical insulating surfaces. The corrosion of metal can be accelerated because of the moisture absorbed by fungi. The protection of electrical equipment against fungus can be ensured by moisture and fungus resistant varnish that complies with Specification MIL-V-173.
D. Piano type hinges
The construction of piano type hinges forms moisture traps as well as the dissimilar metal couple between the steel hinge pin and the aluminum hinge. Lubricants are often applied to reduce corrosion problems. Care and replacement of lubricants require special techniques specific to the particular lubricant being used - refer to 12-21-00.
CAUTION : SOLID FILM LUBRICANTS CONTAINING GRAPHITE, EITHER ALONE OR IN MIXTURE WITH ANY OTHER LUBRICANTS, SHALL NOT BE USED SINCE GRAPHITE IS CATHODIC TO MOST METALS AND WILL CAUSE GALVANIC CORROSION IN THE PRESENCE OF ELECTROLYTES.
E. Faying surfaces
The overlaps in fuselage (skins /frames, skins / spars, skins / skins) and the overlaps in wings (skins / ribs, skins / spars, skins / skins) are very sensitive areas to corrosion. Thorough inspect and protect these areas with particular attention to the wings in the area included between the main landing gears and the two wing splicings.
　
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