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...


Maintenance training
Maintenance training is a factor that can have direct impacts on corrosion damage. Trained, motivated mechanics are a crucial key in being able to find corrosion and treat it. Maintenance personnel must:
• Recognize corrosion inducing conditions.
• Be knowledgeable in corrosion identification techniques.
• Be knowledgeable in detection, cleaning, and treating corrosion.
• Know proper lubrication and preservation techniques for the aircraft structure and components.
Experience goes a long way in this area. Mechanics with type-specific knowledge and experience know the areas that are prone to corrosion development.
On type-specific experience, Tom Burt of Lincoln, Nebraska-based Duncan Aviation shares, "What we tell people is that as a major service center, one of the advantages we have is that we see a lot more of these problems with more frequency than a typical operator sees. For instance, if you’re talking about a big structural check on an airplane that might be done every six years, we might see 20 of those in a year, whereas an operator out there sees only one every six years. So, our inspectors get used to looking for this corrosion, and they get a little more skilled at knowing what to look for. What we would suggest to the operators is to get with some of the major service providers. Get access to their technical people and get some suggestions for what to look for. Quiz them on the corrosion hot spots, because every airplane has them."

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Corrosion inhibiting compounds

Corrosion inhibiting compounds are useful tools in corrosion control. Some products available on the market today, such as ACF-50 being applied in the photo on the left, are able to provide an enhanced level of protection against corrosion. Applied in a mist form to entire areas of the airframe, they fight corrosion in two ways. First of all, they displace any electrolyte (moisture) present thereby halting the corrosion process if it has already started. Second, they form a protective barrier preventing future moisture intrusion.

Photo courtesy of Lear Chemical Research Corp., 2001

Corrosion inspection
The primary method of corrosion detection is inspections performed on a regularly scheduled basis. Early detection and treatment of corrosion reduces repair costs, out of service time, and the possibility of flight related incidents.
All corrosion inspections should start by thoroughly cleaning the area to be inspected. This serves a dual purpose of removing any corrosive residues from the surface, as well as providing a clean slate for inspection.
Areas should be cleaned according to manufacturer’s recommendations. Technicians need to be cautious when working with steam cleaners. Damage caused by them can outweigh the advantages of cleaning dirt and grease away with ease.
"Steam cleaning may get the areas nice and clean, but the high-pressure forces moisture into areas where it can sit and over time cause problems," Burt explains. "We caution people that while cleanliness is important, be sure to clean in the right ways."

Visual inspection
By far, the most widely used method to inspect for corrosion is a visual inspection. It provides an effective way to detect and evaluate corrosion.
During a visual inspection, the mechanic looks and feels for the telltale signs of corrosion, whether it is evident in signs like corrosion by-products or paint defects, or other classic signs like bulging skin — indicating possible corrosion underneath the surface.
A good flashlight, mirror, and magnifying glass are helpful tools for corrosion inspection. Other useful tools may include borescopes, optical micrometers, and depth gauges.
During inspection, it is helpful to shine the light across the surface at a low angle of incidence to more easily detect corrosion. Shadows caused by bulges in the skin or corrosion around fasteners are highlighted when the light source is used this way.
Visual indications of corrosion will vary depending on the type of metal and the length of time it has had to develop. Direct by-products of corrosion, like white powdery residue on aluminum or magnesium structure or the familiar rust color of corroded ferrous material, are generally easy to identify. Other indications of possible corrosion would include dished and popped rivets, skin bulges, or lifted surfaces.
In addition to visual inspection, other non-destructive inspection (NDI) methods are useful in detecting corrosion. The different methods do have limitations and they should be performed only by qualified and certified personnel.

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Light shaping diffusers

With visual inspection being the most relied upon method of corrosion detection, a good light source is essential. Unfortunately, some flashlights have dark spots in them due to the shape of the filament and the way it reflects off the lens. A light with dark and light spots will make inspection difficult.
Light shaping diffusers like this one manufactured by Torrance, California-based Physical Optics Corporation, eliminate the problem of irregular lighting. When attached to a flashlight lens, they diffuse the light providing an even light pattern. This is particularly useful when using a low angle of incidence to inspect for corrosion.

Photo courtesy of POC, 2001

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