We know that airplanes deteriorate in several ways: Paint fades, upholstery frays, plastic becomes brittle, and engines become weak. Similar deterioration takes place in the airframe, too, and the primary cause is corrosion. As our fleet ages, corrosion becomes ever more of a problem, a pervasive, insidious threat to the health and well-being of our airplanes. What's more, corrosion is showing up in places where it feared to tread before — a result of airframes lasting longer than the designers believed they would and, in some cases, lackadaisical maintenance.
How bad is it? According to several sources, where airplanes used to be junked due to crash damage or run-out powerplants, more and more are being sent to the boneyard with irreparable corrosion damage.
Chances are your airplane has at least some corrosion right now, perhaps a small portion well hidden, but the very way most aluminum airplanes are built invites corrosion of some sort. What we attempt to do is prevent or retard the corrosion to keep it from threatening the integrity of the airframe and its components.
Just what is corrosion? Simply stated, corrosion is an electrochemical (or mainly chemical) process that destroys metal. To occur, it requires three things: two types of dissimilar metals (which is exactly what an aluminum alloy is) and a conductor (or electrolyte) that makes electrons flow from one metal type to the other possible. Actually, corrosion can occur with only one type of metal present; if water is allowed to remain in contact with the alloy (such as when a leaky window allows rainwater to saturate insulating material) for extended periods, but corrosion involving two types of metals is more common in aircraft. You also see dissimilar metals subject to corrosion, like steel fasteners in an alloy panel; this is called galvanic corrosion. Unprotected, these two metal types will eat each other alive.
We can stop corrosion by keeping the electrolyte away from the alloys, or by preventing the dissimilar metals from coming in contact with each other. Several methods are used to prevent corrosion, including cladding the alloy surface with pure metals, plating steel fasteners, and painting the structure. The intent is twofold: to prevent direct, bare- metal-to-metal contact and to prevent the electrolytes from reaching the metal in the first place.
When corrosion begins to form, the by-products of the electrochemical breakdown (oxides, hydroxides, or sulfates, depending upon the original metal) take the place of the original material, which over time will dramatically reduce the strength of the part. If you remember the Aloha Airlines 737 that literally blew its top in 1988, you know the potential dangers of unchecked corrosion. (Inspection of that airplane showed that poor maintenance lead to substantial corrosion, which weakened the Boeing's fuselage to the point that repeated pressurization cycles literally tore it apart.)
Some forms of corrosion are widespread, where the effects of metal breakdown appear over a large area. Filiform corrosion, a light surface corrosion that looks a bit like cottage cheese under paint, is such a widespread type, often found in late-1970s Cessnas that were not properly prepared for the polyurethane paint applied to them. This type of paint is not as porous as others, and Cessna did not prepare the surfaces as called for by the paint manufacturer. As a result, any electrolyte left from the painting process, or introduced between the metal and the paint, would be held tight against the skin, promoting rapid corrosion. By now, though, many of those airplanes have been repainted, but any original-condition airplane from that era deserves close inspection.
Fumes from the airplane battery and exhaust gases — and their deposits — can rapidly corrode exposed aircraft skin. (Check out original Cessna 310s with their over-wing exhaust stacks, and you'll see what we mean; that area is a breeding ground for corrosion.) Other types of corrosion occur in isolation such as within the walls of pits in the paint or chips in chrome plating. This form of corrosion can spread rapidly under the paint. You might also see corrosion form in crevices, such as joints in wing skins, where water and airborne pollutants can be held against the metal.
Other types of corrosion include weld corrosion, which forms where the metal has been weakened by a weld (you see this often on engine mounts), and stress corrosion — constant working of a part through load cycles can cause existing corrosion to accelerate its spread. Similarly, fretting corrosion, caused when two surfaces are in light contact, is a type of corrosion whose proliferation is speeded by the mechanical working of the parts.
The usual progress of corrosion begins with the protective covering of the metal being removed, then the metal is exposed to moisture. Localized corrosion begins to eat away at the metal, a process known as intergranular corrosion. Finally, this corrosion reaches the point that the by-products of the corrosive action take up more space than the metal originally in place, a result called exfoliation. This can turn structural alloys into members about as strong as hot Velveeta.
Fortunately, inspection for corrosion is simple enough. (An FAA advisory circular, AC 43-4A, describes corrosion inspection and control for aircraft in great detail.) A close visual inspection will show the vast majority of corrosion. Look for grayish-white powder on aluminum and reddish deposits on ferrous metals. Bumps or blisters in paint signify corrosion occurring under the surface. Pay close attention to the trailing edges of control surfaces where the skins come together. Also, the insides of wheel wells on retractable models is a prime location for corrosion — not surprising given this area's exposure to acids, salts, gravel, and other mayhem-producing items.
Obviously, checking the outside of the airplane is easy, but it takes a bit more sleuthing to do a thorough job on the inside. Remove all the access panels and spend some time with mirrors and lights; again, you're looking for gray or white deposits on aluminum and trademark rust on steel.
Other areas on the airplane commonly subject to corrosion include the propeller, cylinder fins, areas around fuel tanks or bladders, piano-type control hinges, and the battery box. Propeller corrosion occurs in two basic areas: on the surface of the blade, which is constantly abraded and thereby exposed to the elements, and in the hub of constant-speed props. It's for the latter reason that recommended overhaul periods for constant-speed props ought to be heeded; undetected corrosion of the hub and blade-attach points could lead to catastrophic failure.
Removal of corrosion is the only sure fix once it's found. If, for example, you discover light surface corrosion, the remedy is simply to remove it with abrasive action (the specifics of which depend upon the metallurgy of the component), and then apply a protective coating to see that it doesn't recur. If the corrosion is severe enough to have removed a significant amount of metal, replacement or patching of the component would be called for. Most aircraft service manuals outline the proper guidelines for determining the extent of the necessary repair.
Some locations around the country are more conducive to corrosion than others. The military retires its airplanes to Davis-Monthan Air Force Base in Arizona for a very good reason. The arid atmosphere keeps the electrolytes at bay, stopping corrosion before it has a chance to begin.
Coastal locations (especially next to big cities, which tend to produce harmful airborne pollutants) are the worst, corrosion-wise. Salt-laden sea air makes a dandy electrolyte, needing only exposed alloys to begin the corrosion process with vigor. In these locales, a good paint job and frequent inspections are necessities. Airplanes left exposed along the coast, uninspected, and untreated, don't last long.
But corrosion can occur in any climate, so keeping up with the inspections is a must. Also, chemicals like ACF-50 (a commercially available anticorrosion treatment, usually applied during the annual inspection) can go a long way toward reducing corrosion. ACF-50 is unusual in that it not only will prevent future corrosion, but it will stop corrosion in progress. This chemical is a penetrating dielectric that soaks into the metals, neutralizing the electrolytes that are the catalysts for corrosion. It remains on the airframe as a coating, one that will dissipate over time. Treatment intervals depend upon where and how the airplane is stored — figure every one to two years.
Perhaps the best way to prevent corrosion is to protect the airplane from the elements, in a hangar, and treat it with ACF-50. If you don't have access to a hangar, make use of cabin covers and ensure that all the windows seal tightly to prevent moisture from attacking the fuselage from the inside out. Also, if you must park your airplane outside at a seaside airport, you should invest in frequent washing and waxing to keep the salty deposits from remaining in contact with the surface very long.
Because, after all, corrosion is a hungry beast, and it will feast at the slightest opportunity — a meal for which we as airplane owners get to pay.