Corrosion 101: What Causes It?

Are service documents mandatory? No. Are they a good idea to implement? It depends, but for the most part, yes, and you’ll soon discover why.

Corrosion comes in all shapes and sizes. It is a natural process in which metals deteriorate due to chemical reactions with environmental elements, such as oxygen, moisture, and pollutants. In aviation, corrosion can manifest in various forms, including surface, pitting, intergranular, and stress corrosion cracking.

Each type poses significant risks, potentially weakening the structural integrity of the aircraft and leading to safety hazards. We’ve recently seen how corrosion affects wing spars like with the Piper PA-28 and Cessna 177 and 210, for example. These started as Service Bulletins (SBs) to inspect for corrosion and escalated to mandatory compliance in the form of Airworthiness Directives (ADs).

The implications are severe. Corrosion can lead to structural failures, increased maintenance costs, and, in worst-case scenarios, catastrophic accidents. Once you have the basics and understand the theory, you are ready to progress to the next level.

That’s right, it’s aircraft maintenance time, and here’s one example.

The Learjet is legendary among corporate aircraft—produced from 1964 to its unfortunate scuttling in 2021. Learjet became synonymous with business aircraft in the early days of private business travel.

“Learjet models are known for their exceptional performance, speed, and range,” said Sky Aircraft Maintenance. “Airframe issues can be a common maintenance concern for Learjet aircraft. Due to the high speeds and stresses placed on the airframe during flight, wear and tear can occur over time, leading to a need for structural repairs. This can include corrosion.”

On July 13, 2007, the Australian Civil Aviation Authority released AWB 57-004 Lear Jet Industries 45 Wing Corrosion. The agency said this correspondence was needed because “recent reports have been submitted indicating that corrosion has been found on the lower skin of both wings fitted to the Lear 45 aircraft. This corrosion resulted in the replacement of the entire lower-wing skins.”

Years later, in February 2019, Learjet, now owned by Bombardier, released a series of Service Bulletins addressing “wing spar inspection.” The reason? Corrosion had been observed on the lower-wing splice plates, requiring a more frequent interval to detect and correct protective coatings.

Remember our chat earlier about adhering to the manufacturer’s recommendations? 

Fast-forward a bit and the series of Learjet bulletins are now the FAA’s AD 2021-23-08.

What prompted this escalation? According to the AD, exfoliating corrosion was found on a particular Lear 45 upper surface of the lower center-wing, midspar splice plate during unrelated maintenance. The corrosion appeared to extend halfway through the thickness of the splice plate. Since the initial report, the FAA has received 23 additional accounts of corrosion from Learjet.

Jerel Bristol, owner of SEAL Aviation in Hollywood, Florida, was not surprised when the call came in. Bristol is aware of the trouble that Learjets have with wing-spar corrosion and knows the AD well. His team deploys to aircraft or ​AOG situations for mobile fuel leak repair, nondestructive testing, and structural repairs anywhere in the world.

During the center-wing inspection, a SEAL technician identified corrosion on the center-wing splice plate. I spoke with Bristol, and he said that it is a common area to find corrosion past repairable limits, which requires the replacement of the forward and aft splice plates.

After pulling the affected parts off the airframe, the SEAL team quickly repaired the area and replaced the damaged parts. The pictures reveal a sea of cleco fasteners. 

The guys buttoned up the Lear, and the owner was wheels-up again. 

The big question remains: What causes corrosion?

One follower commented on a SEAL Aviation webpage post about the Lear 45 repair, stating that brine used for deicing could have contributed to the corrosion. He is not far off. Environmental elements can contribute to aircraft corrosion.

These factors include:

• Humidity and moisture, particularly in coastal regions.

• Temperature fluctuations which can cause condensation.

• Exposure to deicing fluids and other chemicals.

• Poor maintenance practices and infrequent inspections.

The environment is not the only player in the corrosion game. According to Aviation Devices and Electronic Components (Av-DEC) in its article “Causes of Corrosion,” industrial pollutants are equally harmful and can be difficult to protect against.

These include several contaminants such as:

• Ozone (exposure from high altitude, motors, and welding)

• Carbon compounds (exposure from combustion engine exhaust)

• Sulfur dioxide (exposure from engine exhaust, smokestacks, and acid rain)

Operators and GA aircraft owners alike are well advised to take heed when an SB shows up in the mailbox. A recommended inspection, especially when it can be coordinated with other scheduled or unscheduled maintenance, may help ultimately reduce the total cost of ownership and down time if/when an issue becomes an AD.

Perhaps the best reason to take a closer look is personal safety and peace of mind through identifying a problem before it manifests in something tragic.

This column first appeared in the September Issue 950 of the FLYING print edition.