Corrosion Control

Corrosion Control Controlling corrosion damage through effective inspection and treatment By Joe Escobar September 2001 The average age of the aircraft in use today is getting older. Many mechanics are maintaining aged aircraft and...


Corrosion Control

Controlling corrosion damage through effective inspection and treatment

By Joe Escobar

September 2001

Corrosion ControlThe average age of the aircraft in use today is getting older. Many mechanics are maintaining aged aircraft and fleets that have special inspection and maintenance needs. Although there are many issues to consider when working with older aircraft, the most insidious of these is corrosion. It can slowly progress and become well-established before being detected. Just as cancer can be devastating to the human body, corrosion can spread to adjacent structure at a rapid pace once it takes hold. It can lead to fatigue, weakened structure, and have a catastrophic impact on public safety. Early detection and treatment are essential to controlling corrosion.

Basic corrosion cellWhat is corrosion?
ASA’s Dictionary of Aeronautical Terms defines corrosion as "an electrolytic action which takes place inside a metal or on its surface. The metal reacts with an electrolyte, and part of the metal is changed into a salt. The salt, which is the corrosion, is usually dry and powdery, and it has no strength."
Four factors are needed for corrosion to exist:
1. Presence of a metal that will corrode (anode)
2. Presence of a dissimilar conductive material (cathode) which has less tendency to corrode
3. Presence of a conductive liquid (electrolyte)
4. Electrical contact between the anode and cathode (usually metal-to-metal contact, or a fastener)

Elimination of any one of these factors will stop corrosion. The following are the common types of corrosion found on aircraft.

Uniform etch corrosion
This is the result of a direct chemical attack on a metal surface and involves only the metal surface. On a polished surface, this type of corrosion is first seen as a general dulling of the surface, and if the attack is allowed to continue, the surface becomes rough and possibly frosted in appearance.

Pitting corrosion
This is the most common effect of corrosion on aluminum and magnesium alloys. 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 or holes can be seen in the surface. Pitting corrosion may also occur in other types of metal alloys. The combination of small active anodes to large passive cathodes causes severe pitting. The principle also applies to metals that have been passivated by chemical treatments, as well as for metals that develop passivation due to environmental condition. ImageFuel cell areas are particularly prone to corrosion ImageExfoliation corrosion in a fuel cell dry bay Corrosion in honeycomb structure

Galvanic corrosion

Galvanic corrosion occurs when two dissimilar metals make electrical contact in the presence of an electrolyte. The rate at which corrosion occurs depends on the difference in the activities. The greater the difference in activity, the faster corrosion occurs. The rate of galvanic corrosion also depends on the size of the parts in contact. If the surface area of the corroding metal (the anode) is smaller than the surface area of the less active metal (the cathode), then corrosion will be rapid and severe. When the corroding metal is larger than the less active metal, corrosion will be slow and superficial.

Concentration cell corrosion
Concentration cell corrosion is corrosion of metals in a metal-to-metal joint, corrosion at the edge of a joint even though joined metals are identical, or corrosion of a spot on the metal surface covered by a foreign material. Another term for this type of corrosion is crevice corrosion. Metal ion concentration cells, oxygen concentration cells, and active-passive cells are the three general types of concentration cell corrosion.

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