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Exfoliation is a form of intergranular corrosion. It shows itself by "lifting up" the surface grains of a metal by
the force of expanding corrosion. This occurs at the grain boundaries just below the surface of the metal.
Exfoliation gives the appearance of sheets of very thin metal separated by corrosion products. This type of corrosion is often seen on extruded sections. There the grain thicknesses are usually less than in rolled alloy form. Most exfoliation type corrosion is found on aluminum alloy ducting.
E. Galvanic corrosion (Figure 2)
Galvanic corrosion occurs when dissimilar metals are in contact. The contacting of these unlike metals provides an internal circuit. An external circuit is provided by the presence of a buildup of an electrolytic substance between these metals. An example is when aluminum components are attached with steel fasteners.
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F. Concentration cell corrosion (Figure 3)
Concentration cell corrosion occurs when two or more areas of the same metal surface are in contact with different concentrations of the same solution : moist air, water, chemicals, etc.
The general types of concentration cell corrosion are identified as : metal ion cells and oxygen concentration cells.
G. Filiform corrosion
Filiform corrosion is a "concentration cell" corrosion process. When a break in the protective coating over aluminum occurs, the oxygen concentration at the back or bottom of the defect is lower than that at its open surface. The oxygen concentration gradient thus established, causes an electric current flow and corrosion results. Filiform corrosion results when this happens along the interface between the metal and the protective coating and appears as small hairline tracks. Filiform corrosion generally starts around fasteners, holes and countersinks and at the edge of sheet metal on the outer surface of the aircraft. Filiform corrosion is more prevalent in areas with a warm and damp environment.
To help prevent filiform corrosion growth, it is recommended to :
- spray wash the aircraft at least every two or three weeks (especially in a warm and damp environment),
- wax the aircraft with a good grade of water repellent wax to help keep water from accumulating in the skin joints and around countersinks,
NOTE : Wax only clean surfaces. Wax applied over salt deposits will almost guarantee a trapped salt deposit which is capable of accumulating moisture and developing into corrosion.
- park the aircraft in hangar at relative humidity below 70 percent,
- fly the aircraft to promote aeration of the enclosed parts,
- ensure all vent holes are free of debris.
H. Stress corrosion cracking (Figure 1)
This corrosion is caused by the simultaneous effects of tensile stress and corrosion. The stress may be internal or external. Internal stresses are produced by nonuniform shaping during cold working of the metal. These stresses are also induced during pressing in bushings or shrinking a part for press fit.
　
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