Corrosion in aviation

Definition: Corrosion is the deterioration of a metal as a result of chemical reactions between it and the surrounding environment.

Corrosion is a massive factor in the well-being of an aircraft. Even through thorough cleaning duties and repainting, corrosion will eventually appear. Corrosion occurs because metals tend to return to their natural state.

The age of an aircraft, type of environment, and hangar area, are all factors in corrosion developing. We'll take a look at what to look for and how to prevent typical corrosion.

Types of corrosion

The most common types of corrosion are easy enough to deal with - as long as you know about them.

Uniform corrosion

This is the most common form of corrosion. It is caused by exposing metal to oxygen. A metal surface is gradually exposed to oxygen as paint erodes during flight. In general, uniform corrosion is considered a very predictable issue. What we refer to as rust in day-to-day language is actually this type of corrosion.

Stress corrosion

Highly stressed parts with lots of movements or rotations such as crankshafts and landing gear may develop stress corrosion. In severe cases it can look like a tree branching inwards from an initial crack in a structure.

Deposit corrosion

Trapped moisture or other pollutants in small holes or crevices in a metal structure.

Intergranular corrosion

An uncommon type of corrosion which is considered extra hazardous. When you see it, it's too late to save the affected part.

Recognizing corrosion

Corrosion should be spotted during the pre-flight. Depending on the type of operation, the aircraft in question might be going through routinely corrosion inspections. When going through your pre-flight, use extra caution with the following areas:

• Water entrapment areas
• Battery compartments
• Frame welding points
• Wheel weels and landing gear
• Wing flaps
• Areas hit by exhaust
• Cooling air vents

Pay close attention to the edges of control surfaces where the skins come together. On a helicopter, the leading edge of the rotor blades as well as the tips are of extra danger, depending on the type. On an airplane and in general, the propeller, areas around the fuel tanks, and the battery box are especially conducive to corrosion.

Corrosion on aluminum will look like grey/white powder. On ferrous metals, corrosion will look like reddish deposits.

Blisters or bumps in paint are an indicator of corrosion under the surface (filiform). If you see chipped paint on an airframe, that's a good time to look into having that treated.

Advisory Circular 43-4B does an excellent job of describing corrosion inspections in detail.

Corrosion treatment

The most important thing about treating corrosion is to remember that preventing corrosion is easier than treating it. Although in the end, everything has to come back to nature eventually. Understand the environment the aircraft is operating in, as this is a big factor of how corrosion develops on it.

Store the aircraft in a hangar overnight if possible. If no hangar is available, use cabin covers and/or make sure all windows are tight and sealed to prevent moisture slowly attacking the aircraft form the inside out.

A great way of keeping an aircraft free of corrosion is to keep it away from moisture as much as possible (not including washes). Without moisture, a metal will last for a very long time. This is why man-made structures last for so long on the poles.

Use a rust inhibitor. ACF-50 is a thin film compound which penetrates the corrosion itself, lifting it off the surface of the metal, ultimately encapsulating it. The product can make its way through cracks, rivet heads, seams, and lap joints. This compound is normally applied during an annual inspection, and is sprayed under pressure.

Lastly, try to give the aircraft a wash after a day of flying, depending on the location of the operation. Flying over the sea or near shorelines exposes the aircraft to a lot of moisture and salt. Not a great combination!