Aircraft corrosion occurs as it does on any other metal surface that is exposed to the oxygen and humidity present in the environment, as well as being exposed to foreign materials like dust and different types of pollutants. Of course, metal corrosion is accelerated by certain conditions such as harsh environments. For example, dealing with corrosion is commonplace in oil rigs because of the corrosive environment in the open sea.
Given the nature of aviation operations, aircraft corrosion is also common. In the end, aircraft are also exposed to harsh environments, both when flying and on the ground. Fortunately, there are ways to control how corrosion affects aircraft, starting by understanding how it happens and the different types of aircraft corrosion that can be found.
If you are willing to learn everything you need to know about aircraft corrosion, we invite you to keep reading as we share all the details.
Generally speaking, aircraft corrosion is the rust that appears on the metal surfaces and components of an aircraft when the unprotected metal comes into contact with oxygen in the atmosphere.
However, aircraft surfaces and components are made of a variety of metals, and the rust may not appear in the typical reddish color as most people are used to identifying it. For example, aluminum is one metal that corrodes differently. As described by the Aircraft Owners and Pilots Association (AOPA), aluminum alloys “first shows as a whitish or gray “dulling” of the aluminum surface, then progresses to more and more severe pitting and eventual destruction of the metal.”
In general, corrosion is associated with any metal surface being exposed to the oxygen and humidity of the environment. Also, many people understand that the closer to the sea, the worse the corrosion will be because of the effect of the minerals present in the seawater. However, there are more causes for corrosion to appear on airframe structures and other aircraft components.
One common factor that causes corrosion in aircraft is foreign material like dust and grease, acid batteries, or cleaning solutions. All of these elements can cause corrosion in the airframe if they remain in place for prolonged periods of time.
The resulting corrosion damage causes crack propagation and stress accumulation and can lead to significantly decreased mechanical characteristics for an aircraft. As a result, the aircraft becomes unairworthy in a short period of time.
The good thing is that the effect of corrosion on aircraft happens progressively, so it can be treated to avoid major issues. However, sometimes corrosion can be hidden, and an untreated ongoing corrosion process can result in corrosive destruction, thus causing the aircraft system to fail.
In order to guarantee aircraft corrosion is treated in time, the types of aircraft corrosion must be known, so they can be identified during routine aircraft maintenance inspections.
The process of corrosion formation can differ from one metal to another. The process also depends on other factors such as the type of pollutant, the type of component, and whether there is a conductor involved or not.
In any case, here we are going to describe the five types of corrosion found in aviation. Let’s see them in detail.
This is one of the types of corrosion where conductor is involved as described above. Therefore, this type of electrochemical corrosion can cause extensive pitting damage. It is called dissimilar metal corrosion because it appears when different metal parts come into contact with each other in the presence of a conductor.
Dissimilar metal corrosion often takes place out of sight, making it particularly dangerous since it can go a very long time before it is identified and treated. So, it is recommended to disassemble parts and components whenever this type of corrosion is suspected to make sure it can be identified.
Also, a precaution that aircraft mechanics can take to reduce the chances of this type of corrosion involves not using steel wire brushes or steel wool when cleaning the surface of the plane, because this can result in tiny pieces coming off and getting deep into the surface of the aircraft, which always increases the odds that dissimilar metal corrosion will occur.
Stress corrosion occurs in parts that are exposed to high loads such as the engine crankshaft or the landing gear. It is usually the result of superficial corrosion or scratches that are undetectable because finding out about the corrosion is often tricky. Typical causes for stress corrosion on airplanes include tapered bolts, press-fit bushings, external loads applied on the components, cyclic loads, or even the manufacturing procedure itself.
During manufacturing processes, internal stress that may cause stress corrosion can be inadvertently induced. Many manufacturing companies carry out production using stress relief procedures. Nevertheless, there is still tension. Stress is externally introduced by riveting and welding on parts including bolts and rivets, clamping, or press fittings. In the case that a small error is made or a fastened part becomes over-torqued, internal strain can arise, thus inducing loads that could result in stress corrosion.
Intergranular corrosion is particularly tough because it affects the microstructure of an alloy, which is why it is often related to a lack of uniformity in the microstructure. Although intergranular corrosion often exists without surface evidence, it can lead to the lifting and flaking of surface metal over time. Extruded components may be particularly susceptible.
AOPA states that intergranular corrosion is “normally worst on 7000-series aluminum alloys (those with an appreciable amount of zinc, like wing spars, stringers, and other high-strength aircraft parts).” However, they also highlight that “this is not frequently found but is a particularly nasty type of corrosion. It can be difficult to detect, and once you see it, it’s too late: that piece of metal is toast.”
General surface corrosion or uniform attack corrosion are the more common forms of corrosion in aviation. Surface corrosion usually results from roughing, etching, or pitting a surface in metal often accompanied by a powdery deposit of corrosion product. Surface corrosion can occur either through direct chemical or electrical attacks. Sometimes corrosion spreads beneath surface coatings and is not detected by the rough surface or the powdered deposit.
Surface attacks (or corrosion) are usually caused by exposing parts of the plane to oxygen for longer periods. The most common causes include, but are not limited to:
Filiform corrosion is common in airplanes is because it mainly occurs on aluminum surfaces that have not been through the proper pre-painting process or covered with a corrosion-resistant alloy. In other words, the surface is not prepared for the polyurethane paint used on airplanes.
The moment it appears, it looks like “fine, worm-like lines of corrosion under the paint that will eventually lead to bubbling and flaking” AOPA describes.
Consequently, filiform corrosion is easy to recognize and it is actually easier to prevent than it is to remedy, something that is not common with other types of corrosion.
There are many ways corrosion can occur in aviation, but there are also good practices to avoid corrosion becoming a hazard.
For example, aluminum and magnesium alloys are frequently used in aviation, and they do not show the typical color seen in steel alloys when they corrode. Therefore, light surface corrosion can be disregarded, but the alloy structure may be affected and a bigger issue may appear in the longer term. So, thorough inspections are always recommended.
Also, dissimilar metal parts and two mating surfaces should not be cleaned with a steel wire brush since this may induce dissimilar metal corrosion.
As a general rule, preventing the aircraft surfaces and components from having contact with corrosion-producing agents present in the environment they operate is the best way to avoid corrosion formation. However, this is not always possible, especially in a corrosive environment like coastlines.
In any case, while we could expect manufacturers to use things like surface coating to avoid surface corrosion, and stress relief heat treatments to avoid stress corrosion cracking, it is always a good practice to carry out inspections to verify whether any corrosion inhibitors are needed and guarantee the airworthiness of the aircraft.
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