All metal is prone to corrosion over time. The rusting process can be delayed, but it is inevitable. Corrosion affects everything from pipes and panels to cars and planes. Special paint is usually used to fend off rust in airplanes for as long as possible, but the rate of corrosion depends on the age of the plane, the environment it's based in, whether or not it's kept in a hangar, and how often it's cleaned. Corrosion in airplanes looks different than the red-orange rust which typically comes to mind. When aluminum breaks down, corrosion appears a dull white or gray at first and progresses to increasingly severe pitting before eventually eating its way through the metal, according to the Aircraft Owners and Pilots Association. If ignored, corrosion can make a plane unfit to fly in just a few years.
Types of corrosion
Uniform surface attack
The most common form of corrosion in airplanes, uniform surface attacks are caused by exposing the metal to oxygen. This can happen when paint wears off of the wing skin or the fuselage. Corrosion can be accelerated by poor pre-paint preparation at the factory, fumes, acid, pollutants or high humidity.
This type of corrosion tends to be the worst on 7000-series alloys with lots of zinc, which is usually only used in high-strength parts like the wing spars and stringers, according to the Aircraft Owners and Pilots Association. Because it doesn't occur very frequently, it can be difficult to detect. Unfortunately, once the corrosion is visible, it's already too late, and the metal will need to be replaced.
Stress corrosion occurs in parts like landing gear or engine crankshafts which are under immense stress. It can develop from a scratch in paint or from existing surface corrosion. Undetected stress corrosion is often the cause of crankshaft failures. Stress corrosion cracking can occur as a result of improper materials selection, inadequate maintenance, thermal transient conditions or lack of consideration of residual stresses, according to O'Donnell Consulting.
Crevice corrosion or deposit corrosion
This type of corrosion occurs in areas where moisture and other pollutants get trapped, like lapped skin joints or rivets on an oil-stained belly. FeO says the best way to prevent this is to fully drain and dry any anything exposed to liquids and to avoid creating stagnant conditions. You can also simply seal existing crevices in overlapping joints using continuous welding.
Filiform corrosion tends to show up as fine, worm-like lines of corrosion under the paint which eventually lead to bubbling and flaking. It's usually found on aluminum surfaces poorly prepared for polyurethane paints, according to the Aircraft Owners and Pilots Association. A particular batch of single-engine Cessna's built from 1977 through 1982 are especially susceptible to filiform corrosion due to a problem with the paint, so a special program to help owners called SP79-3S was set up by the company to help.
Geographic locations prone to corrosion
Since moisture causes most of the common types of corrosion, airplanes based in coastal regions are at the highest risk. Hangaring a plane is a good idea in areas like the Pacific Northwest and the Gulf Coast. Frequent washing and regular treatment with rust inhibitors such as ACF-50 can help to slow the corrosion process as well, says the Aircraft Owners and Pilots Association.
Thoroughly inspect your plane for corrosion. Using a magnifying glass can help ensure you don't miss anything. Look for white or gray powder on aluminum and red or orange deposits on steel and iron. Bubbling or bumpy paint is an indicator for subsurface corrosion like filiform; the Aircraft Owners and Pilots Association says filiform will look like cottage cheese under paint. Focus on trailing edges of control surfaces where the skins come together and the inside of wheel wells on retractable models. When checking for corrosion on the interior of your plane, you'll need to remove all inspection plates and look around with a flashlight and mirror. High-risk parts include the propeller, cylinder fins, areas around fuel tanks or bladders, piano-type control hinges and the battery box.
Treatment for corrosion
If you weren't able to prevent corrosion, there are ways to treat it. If the corrosion is minor and on the surface of your plane, you can remove it with abrasion using sandpaper and an after-treatment of corrosion inhibitor, primer and paint. Sandpaper is the best option, though a bit more laborious, because steel brushes can ruin paint jobs. If the corrosion has advanced past a certain point, you may simply opt to replace the part entirely.
Spotting the signs of corrosion early on can save you time, effort and money down the line.