Controlling corrosion damage through effective inspection and treatment
By Joe Escobar
The average age of the aircraft in use today is getting older. Many mechanics are maintaining aged aircraft and fleets that have special inspection and maintenance needs. Although there are many issues to consider when working with older aircraft, the most insidious of these is corrosion. It can slowly progress and become well-established before being detected. Just as cancer can be devastating to the human body, corrosion can spread to adjacent structure at a rapid pace once it takes hold. It can lead to fatigue, weakened structure, and have a catastrophic impact on public safety. Early detection and treatment are essential to controlling corrosion.
What is corrosion?
ASA’s Dictionary of Aeronautical Terms defines corrosion as "an electrolytic action which takes place inside a metal or on its surface. The metal reacts with an electrolyte, and part of the metal is changed into a salt. The salt, which is the corrosion, is usually dry and powdery, and it has no strength."
Four factors are needed for corrosion to exist:
1. Presence of a metal that will corrode (anode)
2. Presence of a dissimilar conductive material (cathode) which has less tendency to corrode
3. Presence of a conductive liquid (electrolyte)
4. Electrical contact between the anode and cathode (usually metal-to-metal contact, or a fastener)
Elimination of any one of these factors will stop corrosion. The following are the common types of corrosion found on aircraft.
Uniform etch corrosion
This is the result of a direct chemical attack on a metal surface and involves only the metal surface. On a polished surface, this type of corrosion is first seen as a general dulling of the surface, and if the attack is allowed to continue, the surface becomes rough and possibly frosted in appearance.
This is the most common effect of corrosion on aluminum and magnesium alloys. It is first noticeable as a white or gray powdery deposit, similar to dust, which blotches the surface. When the deposit is cleaned away, tiny pits or holes can be seen in the surface. Pitting corrosion may also occur in other types of metal alloys. The combination of small active anodes to large passive cathodes causes severe pitting. The principle also applies to metals that have been passivated by chemical treatments, as well as for metals that develop passivation due to environmental condition. Fuel cell areas are particularly prone to corrosion Exfoliation corrosion in a fuel cell dry bay Corrosion in honeycomb structure
Galvanic corrosion occurs when two dissimilar metals make electrical contact in the presence of an electrolyte. The rate at which corrosion occurs depends on the difference in the activities. The greater the difference in activity, the faster corrosion occurs. The rate of galvanic corrosion also depends on the size of the parts in contact. If the surface area of the corroding metal (the anode) is smaller than the surface area of the less active metal (the cathode), then corrosion will be rapid and severe. When the corroding metal is larger than the less active metal, corrosion will be slow and superficial.
Concentration cell corrosion
Concentration cell corrosion is corrosion of metals in a metal-to-metal joint, corrosion at the edge of a joint even though joined metals are identical, or corrosion of a spot on the metal surface covered by a foreign material. Another term for this type of corrosion is crevice corrosion. Metal ion concentration cells, oxygen concentration cells, and active-passive cells are the three general types of concentration cell corrosion.
Intergranular corrosion is an attack along the grain boundaries of a material. Each grain has a clearly defined boundary which, from a chemical point of view, differs from the metal within the grain center. The grain boundary and grain center can react with each other as anode and cathode when in contact with an electrolyte. Rapid selective corrosion at the grain boundary can occur with subsequent delamination. High-strength aluminum alloys such as 2014 and 7075 are more susceptible to intergranular corrosion if they have been improperly heat-treated and are then exposed to a corrosive environment.
Exfoliation corrosion is an advanced form of intergranular corrosion where the surface grains of a metal are lifted up by the force of expanding corrosion products occurring at the train boundaries just below the surface. The lifting up or swelling is visible evidence of exfoliation corrosion. Exfoliation is most prone to occur in wrought products such as extrusions, thick sheet, thin plate and certain die-forged shapes which have a thin, highly elongated platelet type grain structure.
Filiform corrosion is a special form of oxygen concentration cell corrosion or crevice corrosion that occurs on metal surfaces having an organic coating system. It is recognized by its characteristic worm-like trace of corrosion products beneath the paint film. Filiform occurs when the relative humidity of the air is between 78 and 90 percent, and the surface is slightly acidic. Corrosion starts at breaks in the coating system and proceeds underneath the coating due to the diffusion of water vapor and oxygen from the air through the coating. Filiform corrosion can attack steel and aluminum surfaces The traces never cross on steel, but they will cross under one another on aluminum, which makes the damage deeper and more severe for aluminum. If filiform corrosion is not removed and the affected area treated and given a protective finish, the corrosion can lead to intergranular corrosion, especially around fasteners and at seams. Filiform corrosion can be prevented by storing aircraft in a relative humidity below 70 percent, using coating systems having a low rate of diffusion for oxygen and water vapors, and by washing aircraft to remove acidic contaminants from the surface, such as those created by pollutants in the air.
Factors affecting corrosion
Several factors affect the rate at which corrosion develops. These include design factors, operating environment, operational conditions, and maintenance training. Let’s take a brief look at each of these factors.
Some design factors lend themselves to the propagation of corrosion. Lap seams are vulnerable areas. They should be inspected closely at each maintenance task for the signs of corrosion. Spotting corrosion at the early stages in these areas can help avoid more costly repairs if the corrosion is allowed to develop further.
Poor drainage can lead to rapid corrosion development. Always ensure that all drainage holes are kept clear and promptly remove any standing water that may have been caused by blocked drains.
Dissimilar metal areas are always prone to corrosion. Again, the earlier corrosion can be detected and corrected, the less destructive the effect will be.
Flawed paint coating can lead to corrosion problems. This includes poorly applied paint as well as chipped or scratched paint. Areas susceptible to damage like landing gear and wheel wells should be inspected thoroughly for damaged paint.
Certain operational environments are more conducive to corrosion. Areas in proximity to the sea coast as well as high humidity areas are corrosion hotbeds. Industrial areas are also detrimental when it comes to fighting corrosion. FAA Advisory Circular 43-4A lists the global areas that are severe corrosion spots. Aircraft operating in these areas need to have a far more aggressive corrosion inspection and maintenance programs in place.
Certain areas of the aircraft are more susceptible to corrosion due to their operating conditions. Also included are those areas that are prone to spillage like the structure under lavatories and galleys. In addition, the cargo area may be more susceptible, especially if subjected to live animals or fresh or frozen seafood. Dissimilar metal areas like hinge points are susceptible to corrosion.
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."
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
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."
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.
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
Dye penetrant inspections aid in inspecting for large stress-corrosion or corrosion fatigue cracks. In this process a dye is applied to the clean surface to be inspected. The penetrant is absorbed into flaws by capillary action. Once it has been allowed to dwell the allotted time, the excess dye is removed and a developer is applied. The dye that was absorbed in the flaws is then drawn to the surface by the developer, giving a visual indication of the fault.
This type of NDI can be used for detecting cracks or flaws on or near the surface of ferromagnetic metals. A portion of the metal is magnetized and finely divided magnetic particles are applied to the object. Any surface faults create discontinuities in the magnetic field and cause the particles to accumulate on or above the imperfections.
Eddy current testing (primarily low frequency) is useful in detecting thinning of material due to corrosion and cracks in multi-layered structures. It can also be used to some degree for detecting corrosion on the hidden side of aircraft skins when used with a reference standard. High frequency eddy current testing is useful in detecting cracks that penetrate the surface of the structure.
X-ray inspection has limited uses for detecting corrosion due to the difficulty in obtaining the sensitivity required to detect minor or moderate corrosion. In an X-ray inspection, X-rays are passed through the material. A film placed on the opposite side of the material is exposed to these X-rays. The film is then developed and inspected. Areas of high density are indicated as underexposed areas, while areas of low density are indicated as overexposed areas. Trained personnel can interpret whether or not defects are present.
Ultrasonic inspection can detect corrosion damage on some surfaces. It is commonly used to detect exfoliation and stress-corrosion cracks.
Although corrosion will always be present on aircraft, especially on older ones, there are measures that can be taken to ensure that it is kept to a minimum.
An effective corrosion control program incorporates the following components:
• Inspection for corrosion on a scheduled basis.
• Thorough cleaning, inspection, lubrication, and preservation at prescribed intervals.
• Prompt corrosion treatment after detection.
• Accurate record keeping and reporting of material or design deficiencies to the manufacturer and the FAA.
• Use of appropriate materials, equipment, and technical publications.
• Maintenance of the basic finish systems.
• Keeping drain holes and passages open and functional.
• Replacing deteriorated or damaged gaskets and sealants to avoid water intrusion and entrapment, which leads to corrosion.
• Minimizing the exposure of aircraft to adverse environments, such as hangaring away from salt spray.
As mentioned earlier, in order for corrosion to occur, four conditions must exist: presence of an anode, presence of a cathode, presence of an electrolyte, and electrical contact between the anode and cathode.
Since it is not usually an option to remove either the anode or the cathode, the two common ways to prevent corrosion are to remove the electrolyte or to prevent physical contact between the anode and cathode.
Keeping the electrolyte from contacting the metal is one way to stop corrosion from forming. A good primer and/or paint coating is a good start. Not only is a good initial coating essential, but it should be inspected and maintained to minimize chips and scratches from allowing the corrosion process to begin.
Removing corrosive deposits like exhaust trail residue and salt spray are extremely helpful. Turbine and reciprocating engine exhaust are very corrosive, and regular cleaning aids in preventing corrosion from from setting up in these hot spots. These areas require more attention, especially at lap joints and access panels. Paint coatings should be inspected thoroughly, touching-up scratches and chips to prevent corrosion.
Preventing physical contact
Another way to prevent the onset of corrosion is to prevent physical contact between the anode and cathode. This is accomplished by applying a barrier between them. This can be in the form of primer, sealant, or other types of films.
Repair work, if not performed carefully, can set up corrosion areas. All metals should be treated as required. In addition, applying sealant between the metals and on the rivets during installation can be effective preventative measures.
This article has touched briefly on corrosion issues. Developing and implementing a corrosion control program is an essential part of any aircraft operation, helping to ensure that this structural cancer doesn’t progress into catastrophic results.
FAA Advisory Circular 43-4A — Corrosion Control for Aircraft
Lincoln, NE Facility
Battle Creek, MI Facility
Lear Chemical Research Corporation
P.O. Box 1040 Station B
Mississauga, Ontario L4Y 3W3
Physical Optics Corporation
20600 Gramercy Place, Bldg. 100
Torrance, CA 90501-1821