What We Don’t Know (Can Hurt Us): Corrosion

"An airframe contains the elements necessary to turn it into a battery — all that’s lacking is an electrolyte" — Jim Van Gilder, founder, Corrosion Technologies

When two different metals are near each other and are bathed in an electrolyte, electrolysis occurs: that’s how a battery works. Electrons transfer from one metal to the other: that’s corrosion. Even "aluminum" (skin) and "aluminum" (rivets) are different alloys and unprotected metals at the places they meet, wetted by an electrolyte, will result in corrosion. Remove a skin from a 30-plus-year-old airframe and you will likely find a circle of oxidation around each rivet hole.

Calling airframe a "battery" is a stretch, but it’s true that all metal-to-metal points can act as tiny batteries, destroying an airframe ion by ion. Just expose them to moisture or other electrolytic elements, and the action begins.

What can we do? There are two basic defenses against corrosion; we need both. Manufacturers use primers and sealants (paint). Undamaged paint is the first line of defense. With use, however, paint breaks down. Exposure, vibration, turbulence and other normal activities attack it. Hairline cracks appear around stress points like rivets and fraying surfaces. Chips result from FOD, sand, stones and from removing inspection panels. Bug and bird strikes can also compromise the finish.

Owners and maintainers must systematically use a remedial product to displace moisture and leave a protective film that prevents attacks from other abusive elements found in the atmosphere.

Relief from the elements is a hangar’s job. When the aircraft is outside or in service, we protect the painted surfaces with frequent washing with non-corrosive cleaners and waxes. It’s a start, but it’s not sufficient. We need to keep electrolytes from reaching metal.

The most-common electrolyte is moisture, contaminated by airborne by-products of combustion produced from diesel/turbine exhausts. Other moisture comes from the lav, passengers’ drinks, and unseen condensation. Exhaust fumes from tugs, other airplanes, trucks and traffic introduce corrosive chemicals to common water, and what started life as dew or condensation becomes an aggressive electrolyte (and often a metal-eating acid in its own right). Know where to look and what to protect: nobody is surprised to drain water from fuel tanks; the same circumstances cause condensation to form inside the entire airframe.

There is no permanent cure for corrosion, and it can’t be prevented without constant attention. When it gets to the point of exfoliation, we must replace the structure; but corrosion can be interrupted indefinitely, even after it has a toe-hold, by the application of remedial and preventative chemical treatments.

One coating, used for years, is LPS-3, essentially a wax dissolved in solvent. When sprayed inside an airframe, the solvent allows it to spread; then it evaporates, leaving a barrier coat of wax on the surfaces. Its disadvantages include adding weight to the airframe and making inspections difficult, concealing flaws and collecting dirt. It is hard to remove, requiring a lot of solvent and a pressure sprayer (and atomized solvents in an enclosed structure can be dangerous). LPS-3 and other similar coatings have another serious disadvantage: they don’t stop the expansion of existing corrosion. In fact, they can lock the electrolytes inside of the coating, allowing it to continue.

Newer technology can interrupt corrosion’s progress for extended periods. There are two popular products dominating the market: ACF-50 from Lear Chemical in Canada; and CorrosionX Aviation, made by Corrosion Technologies, Garland, Texas. The technology is commonly referred to as FTFC, Fluid Thin Film Coating (or TFFC Thin Fluid Film Coating by trademark-holder Lear Chemical Research Corp., which makes ACF-50). These "oily" type treatments, properly applied at two-year intervals, can actually stop existing corrosion. Mark Pearson, managing director of Lear, explained that in 1985, when ACF-50 premiered the technology, it "was significantly different from the passive/barrier films because it actually penetrated corrosion to remove the electrolyte. With the process completed, ACF-50 acts like an ‘off switch’ for the corrosion process."

These fluids creep into minute, inaccessible areas like between lap joints and around rivet shanks, forming a dielectric barrier that stops the transfer of electrons, shutting down the "battery." (A couple of years ago, I noticed some dirty seams and rivet lines on an otherwise pristine pair of amphibious floats that I was featuring in an article. I asked the owner why the seams were dirty. "Dirt!" he said, plainly enough. "I sprayed the inside a couple months ago with CorrosionX and it’s still working its way out; but I’d rather have a little dirt than a little corrosion, any day." I noticed that it wiped off cleanly and I continued the photo shoot.)

CorrosionX is also available in a HD (heavy duty) form and is frequently used "off-label" to protect the exposed fittings on floatplanes, even though it does not officially carry the aviation ratings. (There is a mil spec, 81309E, that covers such products for aircraft use. Both ACF-50 and CorrosionX Aviation meet the mil spec.)

Other products, such as the Ardrox line (formerly known as Dinatrol), are used extensively in temporary and semi-permanent applications. These form moisture barriers that can withstand handling, de-icing and outdoor conditions for long periods of time. Pearson explained, "Passive coatings like LPS-3 and the Ardrox/Dinatrol products work fine on ‘new metal surfaces.’ That is why they are used by Boeing and Airbus at the factory level. But once the aircraft begins to fly, torsion and other factors such as expansion and contraction due to extreme temperature can cause these rigid to semi-rigid films to fail." These are specialized industrial products rather than broad-use consumer items, so consider them if you have need for this degree of specialization, and the capacity to use them properly.

Some of the things we do that we think are helping, really aren’t. Oil, for instance, is not a good moisture barrier. We all know that oil floats, yet we persist in thinking that it won’t float off our shiny surfaces when they’re exposed to water. It will.

WD-40 has its uses, particularly as a water displacer, adhesive remover, and as a cleaner for small parts, but is of little value as a preventative for corrosion. Its extremely high-solvent content (76%) also makes it a fire hazard in enclosed areas — but it can also come in handy as a starting fluid for tugs and other equipment! Review the label warnings.

When you consider a corrosion protection product, look for the one that will do what you most need it to do. Some are heavier; some contain more solids; some creep into smaller gaps; some last longer under certain conditions. (CorrosionX is an excellent light-duty lubricant, too, for cables, rod ends and suspension sliders.) Some are stickier; some "heal" better after being penetrated; some stand up to water better. The sales people for both ACF-50 and CorrosionX know their products and the competitions’ products.

One last consideration: packaging. Both CorrosionX and ACF-50 are available in aerosols or as bulk liquids for pump sprayers. With the pumps, you get more product (roughly 20 percent of the content in any aerosol is propellant), but you tend to waste more product, getting it to atomize and cover surfaces. The biggest disadvantage to aerosols is in shipping: airlines won’t carry them.

Choosing the best fluid thin film for a given application will take some time; but it’s better to apply either one right away (and then do the research) than to debate and corrode! Personally, I use both.

New Method for Inspecting Aging Aircraft Material fatigue, which can result in cracks, can compromise the structural integrity of an aircraft. At the University of Missouri-Columbia, a new method for identifying potentially dangerous cracks in aging aircraft has been developed. Using vibration technique named the Boundary Effect Evaluation Method (BEEM) and a scanning laser vibrometer, P. Frank Pai, professor of mechanical and aerospace engineering in the MU College of Engineering, can detect tiny cracks in various materials, including aluminum alloy and composite laminates. He said the inspection can be done with the aircraft fully intact and for less money than the traditional method. Peizoceramic patches are attached to the structure. Electrical voltage is applied and the patches produce small vibrations. The laser vibrometer scans the structure at uniformly distributed points measuring the vibration levels. The data is relayed to a computer and processed using a mathematical theory developed by Pai, which is the basis of BEEM. Once analyzed, the locations and sizes of any cracks are displayed on the computer monitor. This method allows for inspection of the entire aircraft, including fuel tank and wing and body junctions, while fully assembled. Other methods that are in development using high-power lasers produce high levels of heat, which Pai said can potentially damage the surface of the structure. Pai’s method uses a low-power laser, which causes no harm. It is portable, less expensive and quick, according to Pai.