November 1, 1997
MARC E. COOK
Corrosion is an evil disease. It can strike aircraft young or old — although it definitely preys more widely on the elderly — and can leave its damage unseen. Left unchecked, corrosion can destroy an otherwise fine airplane and turn it into an unairworthy hulk. With the age of the general aviation fleet, the effects of this deterioration have become ever more acute. Few manufacturers building aircraft in the 1950s and 1960s could have predicted that their products would still be in use so close to the year 2000; indeed, few prepared the structures for such a long haul.
Ask around the maintenance circles and you'll hear that corrosion is a continuing threat to aircraft, even though the methods to stop its advance have been in place for a long time and the skills required to administer them are minimal. Still, aircraft are parted out — junked — every year because critical components have corroded almost to the point of failure. Often, the cost to replace the corrosion-induced damage exceeds the airplane's value, or the parts needed for repair are unavailable at any cost.
Corrosion is as natural as aging. It is, to put it simply, an electrochemical process that ablates metal, reducing its thickness and strength. Corrosion is also strongly linked to cracking; either condition may precipitate the other. When dissimilar metals are in contact with an electrolyte — usually water from rain or condensation, or from sources like airborne particulates, dirt, and oil — an electrochemical reaction takes place that permits electron flow from one metal to the other. Byproducts of corrosion — sulfates, oxides, or hydroxides — take the place of the original material, although these byproducts may be carried away by the electrolyte.
Dissimilar metals such as steel and aluminum are most prone to corrosion — remember, too, that the alloys used in aircraft skin and structure are in and of themselves dissimilar metals. Though dissimilar-metals corrosion is the most common, corrosion can also occur in the presence of only one type of metal. If the electrolyte remains in contact with the alloy of the fuselage skin, for example, corrosion can set in. This is why it's vitally important that window leaks are fixed immediately. Water trapped by insulation or portions of the interior can be held against the aluminum skin or steel structural members and create a corrosive environment. Fuselage drain holes should also be kept clear of debris.
Corrosion shows its face in several ways. Uniform corrosion is found as a general coating, and its spread is typically slow. Filiform corrosion is a subset of this type, displaying itself as a bubbly growth under the paint. For a period in the 1970s, Cessna improperly applied paint without adequate priming, so a lot of these airplanes had difficulties with filiform. Fortunately, time has probably caught up with these defective paint jobs; subsequent repaints, properly committed, will have fixed the trouble.
Most apparently clean airplanes suffer from concentrated corrosion. That is, patches of corrosion here and there, generally concealed under fairings, at skin lap joints, and in other places where normal discontinuities allow an electrolyte to enter and be exposed to the bare alloy. Pitting is one type of concentrated corrosion that usually starts as a small imperfection in the paint that eventually wears through the primer to the vulnerable alloy underneath. Water or other deposits held in the pit itself help to accelerate the corrosion. Pitting is also a form of intergranular corrosion, wherein the electrolyte promotes corrosion of the individual dissimilar grains in the alloy.
Exhaust stains, oil deposits, and battery fumes also help to promote corrosion. That's why it's always a good idea to keep the belly of the airplane clean; these deposits can either act as the electrolyte or hold another electrolyte in suspension against the metal, leading to corrosion. Make certain, also, that the battery box is properly sealed and that the area around the vent tube is adequately protected with paint or other coatings. Battery effluent is tremendously corrosive, so any evidence of leakage of the battery box or its associated plumbing should get your immediate attention.
Stress corrosion is something else to watch for. When the metal is subject to electrolyte infiltration as well as normal working stresses — every time metal bends, there are internal stresses — corrosion can develop rapidly. Moreover, two parts that are in light but constant contact will eventually wear through their protective coatings and will start to corrode as soon as any electrolyte is present; this is called fretting corrosion. Extreme corrosion, called exfoliation, is found when the byproducts of corrosion actually begin to take up more space than the sacrificial metal; the part distends dramatically.
Stopping corrosion is as simple as removing the electrolyte from the equation. Make certain that the two metals cannot pass electrons and corrosion cannot take place.
Tactics to keep corrosion at bay are mainly two: One, seal the surface so that the electrolyte may not enter the substructure; and, two, coat the adjoining metallic surfaces with a noncorroding material so that any electrolyte that may find its way in will be rendered ineffective. Cadmium plating of fasteners is an example of this tactic, as is the use of stainless-steel control cables. The strands of the cables are constantly rubbing against each other during normal use, so employing a corrosion-resistant material only makes good sense.
For most aircraft, the exterior paint and interior primer are the first guards against corrosion. By preventing the intrusion of the electrolyte, paints and primers never allow the corrosion to take place. Such surface finishes also prevent dissimilar metals from coming in direct contact with each other.
Cladding alloy panels with a thin layer of pure aluminum — called alclad — is used to prevent electrolytes from penetrating the surface and wrecking the metal underneath. Still, pure aluminum will suffer surface oxidation quickly in most environments, which is why polishing and protecting finishes are still necessary. And while it's unlovely, this surface oxidation actually makes up the aluminum's own protective layer. Once oxidation reaches a certain stage, it can progress no more; it's not pretty, but the alloy underneath will be safe.
Most of our airplanes are painted, however, so maintaining the paint film is of primary importance. Repair any paint chips as soon as you can. As one experienced mechanic told us, it's far cheaper to fix the paint now than to repair corrosion damage later.
It's worth noting that visual inspection is still the best way to determine whether your airplane has a corrosion problem. (Chances are very good that it has at least a little.) In your hunt for corrosion, look for grayish-white powder on bare aluminum and reddish deposits on ferrous metals. Bumps or blisters in paint signify corrosion occurring under the surface. Look carefully at fuselage and wing skins at the lap joints as well as at control surfaces, particularly the trailing edges. Watch for weeping rivets; once corrosion has set in, the ablated material will come out the top of the rivet and create a streak on the downwind surface.
Because the wheel wells of retractable airplanes are exposed to both atmospheric contaminants and exhaust gases from the engine, they are a prime hiding place for corrosion. Its spread is made worse by the fact that it's hard to keep a good anticorrosion coating on the exposed parts of the gear; frequent inspections are the only safe bet.
How often and how thoroughly you inspect your airplane depends a great deal on where you live, how often you fly, and the particular airplane. Some aircraft came factory coated on the inside surfaces with an anticorrosion primer; some did not. Some airplanes have better-quality paint jobs that reduce the possible areas of electrolyte penetration.
Large cities near the coast provide the most fertile breeding ground for corrosion. Sea air contains a tremendous amount of salt, which is an excellent electrolyte, and urban atmosphere is rife with airborne effluvia. A dirty location is also bad, because a constant coating of dirt and dust can hold electrolytes against the aircraft skin. Clearly, high and dry are the two best qualities of location. So-called desert airplanes really do tend to have less of a corrosion problem because the dry air is incapable of carrying the electrolytes in great quantity.
For airplanes based in coastal cities, a constant regime of inspections and preventative maintenance is in order. Hangaring helps tremendously, but is not a cure-all for corrosion. You must keep after any surface corrosion that you see by removing it quickly and repainting or recoating the adjoining surfaces to prevent the corrosion from recurring. Exactly how you carry this out depends on the metal in question, but the service manual (as well as the FAA's advisory circular on acceptable methods and practices) will help outline the various procedures.
Engine corrosion can wreak havoc, too. Typically, it's the ferrous parts inside the engine that suffer the most; Lycoming camshaft distress is directly related to corrosion. Frequent flying and ensuring that the oil temperature rises enough to "cook off" the condensation and acids that are a natural byproduct of combustion are essential. (An indicated oil temperature above 170 degrees Fahrenheit will do the job.) Extreme external corrosion can lead to cracked cylinder fins — involving cracks that can penetrate the part if left unchecked — as well as failures of brackets and clamps.
Propeller corrosion is also a serious problem. It can occur on the surface of the blade, as well as in the hub or at the mating surface of the prop and the crankshaft for fixed-pitch units. Constant-speed and fixed-pitch props have overhaul limits specified in both time in service and by the calendar. Most props get overhauled at the hour figure, but it may be the calendar limit that's most important. A frequently flown airplane will generally suffer less prop corrosion than one that sits most of its life. The moral here is to observe the calendar limits on props and have them overhauled accordingly. Take a trip to a local prop-overhaul shop to see what comes off supposedly airworthy airplanes if you need further inducement.
But what if you can't afford a hangar and/or a new paint job for your faithful airplane? Several successful products have proven to be quite effective in stopping existing corrosion and preventing future attacks. By brand, the two most popular are ACF-50 and Corrosion-X. Each of the companies making these products takes great pains to differentiate itself from the other. Nonetheless, the word from the field is that the two products are essentially interchangeable.
They work like this: As penetrating dielectrics, these treatments help to stop the growth of existing corrosion and, by remaining in contact with the metal, prevent other electrolytes from setting up shop. Understand that neither of these products will magically repair corroded metal, but if the spread of corrosion in your airplane is in the earliest stages, either of these products may well help prevent a future shock to the wallet. Another popular product is Boeshield, which is a waxier compound more likely to stay inside the airframe; it apparently does not work as well to displace water and seek out existing corrosion. It has the main benefit of not weeping out of the structure for a month or two — as do both ACF-50 and Corrosion-X. Most shops recommend that coast dwellers have the airplane treated every year or two.
These steps, in addition to keeping the airplane as clean as you can, will help push corrosion to arm's length in your maintenance battles. At least it'll be one less thing you as an airplane owner need to worry about.
E-mail the author at [email protected].
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