|Written by Jim Cavanagh|
|Wednesday, 19 September 2012 12:42|
One word that an airplane owner hates to hear is “corrosion!” It immediately creates images of rust, flaky metal, and, well, dollar signs. And well it should. As bad knees and arthritis affect humans, corrosion is the debilitating scourge of aircraft. Once it is started, it can be difficult to stop, and many airplanes have been grounded permanently because of the wrong kind of corrosion on the wrong place.
There are many kinds of corrosion. Most people think of rust, but “rust” is only associated with the oxidation of the ferrous metals, such as iron, steel, cast iron, and even stainless steel. If there is iron in the alloy, there is a very good chance that it will rust in some circumstances, and a certainty in others.
With aircraft, there has been a number of airworthiness directives (AD) created because of corrosion. Some have been necessary; others, such as an AD on Piper Cherokee center section spars a few years ago, can be the kneejerk reaction of a zealous middle management FAA bureaucrat. Still, an AD is an AD, and while in place, it becomes a burden to mechanics and owners alike.
How the term “corrosion” is defined within a specific instance depends totally upon the inspecting mechanic. This is, in all reality, a subjective thing, and is based upon the individual’s interpretation of corrosion as outlined in any regulation. As with many other aspects of aircraft maintenance, this is an important consideration, as the person making the determination is the person with the ticket, ergo, the person with the power. When you deal with a mechanic on a regular basis, you don’t want to ruffle feathers, and, honestly, if you go to a hundred shops, you will probably get a hundred determinations. But you got to go with your mechanic for the future, if nothing else.
Kinds of Corrosion
Corrosion can take many forms in an airplane. You will mostly and nearly always find a combination of rust, acid corrosion, saltwater corrosion, and dissimilar metal (galvanic) corrosion. All of them are different, caused by different factors or conditions, but nearly all are destructive.
On an airplane, the most cursory inspection will find rust on bolt ends and heads, particularly around landing gear and areas of the plane that get wet. Iron alloys always rust first and very quickly. Rust is the reaction of iron and oxygen in the presence of water or air moisture, forming an iron oxide or an iron oxide-hydroxide. Iron oxide forms on the surface of the part, so what you actually see is the residue of the process that is occurring below this surface. Salt and water accelerate the rate of the corrosion through an electrochemical reaction, so high humidity and salt air can cause premature corrosion on ferrous parts.
A speck of rust here or a speck there won’t really hurt anything. Usually, the exposed parts are the corroded parts, and the “meat” of the fastener is usually capable of doing its job, at least for the time being. Washers and torque pressures will keep most moisture out of the structural areas. Unfortunately, the FAA doesn’t see it this way. Most ADs state that the presence of any corrosion whatsoever requires replacement of the part. If there is no specific definition, then Part 43-13 can be used to determine the acceptability of the extent of the corrosion. On steel parts, this is 10 percent of the thickness of the part.
Here is an interesting example: The shoulder bolts that fasten the wings of Grumman/Gulfstream/American General aircraft are covered by such an AD, and the bolts were $28 each 20 years ago. Some mechanics looked at these cadmium (cad) plated bolt heads and if they saw a touch of rust where the last wrench nicked a corner, they’d just let it go. The mechanics that followed the letter of the law grounded the airplane until new bolts were installed (A relatively simple and quick procedure, by the way.) Some owners pulled out the bolts and had them cleaned and replated, but this procedure is a real no-no in aviation, because there is no traceability of the part or the process to ensure it is compatible and meets specs. Any small garage plating company can plate about anything, but how do you know if their techniques are correct? Should a part become embrittled by the process (hydrogen embrittlement occurs when you plate parts with chrome or some sort of a zinc process, and the part won’t break, but, instead, will fracture because the molecular matrix loses elasticity and becomes brittle.), it could fracture, and you could lose a wing with no warning.
Most aviation hardware is cad plated. Cad plating is a process that does not affect the metal itself; cad plating is very hardy in a normal to semi-wet environment. Cad plating is under a lot of scrutiny these days because of the toxicity of the cadmium. However, in aviation, it is priceless because of its inherent qualities. It can be applied very thinly and is very durable. It is solderable, can be dyed different colors, has galvanic compatibility with aluminum, and can be reliably torqued when necessary. Its next best replacement is gold plating, which is expensive and is not paintable, as is cadmium.
Dissimilar metal corrosion is the next most common form of corrosion on our airplanes. Called galvanic corrosion, there are three parts necessary for this to occur. First, the parts have to be electrochemically reactive metals. Secondly, there must be an electronically conductive path. Finally, there must be a conductive path for the metal ions to move from the anodic to the cathodic metal. Anodic means a positive charge, and cathodic describes negatively charged metal ions.
Metals are grouped in a “galvanic series,” meaning their reactivity. The lesser reactive metals are called noble, and if you ever come across this term, it means staid or passive. There is also what is called the anodic index, which allows you to predict the compatibility of metals by grouping them according to their electrochemical reactivity.
The aluminum on our airplanes can be either bare or Alclad. Pure aluminum is resistant to corrosion in that the surface layer oxidizes (aluminum oxide) and forms a protective barrier, protecting the structurally strong underlying material. Like the patina you get on copper, it provides a barrier to moisture and chemicals and protects the underlying metal strata. The most common aluminum alloys we use in aircraft that are resistant to corrosion are 6061, 6063, and 7075.
In 1927, Alcoa developed Alclad, a heat-treated aluminum alloy containing aluminum, copper, manganese, and magnesium that is bonded to the structural aluminum. Alclad is highly resistant to corrosion but is heavier and usually used sparingly and only where needed.
For galvanic corrosion, water or even moisture can be the conductor.
Scientifically, when you immerse two galvanic metals in an electrolytic solution, you are building a battery. The solution creates half of an electrical loop, and when the metals are connected, the full loop is created, starting the reaction. Typically, aluminum is the anodic metal, sacrificing itself, while the cathode gains metal as a protective layer. We see this type of corrosion where a steel part is bolted to an aluminum part with no plating, paint, or nonmetallic washer or separation. The aluminum begins to flake and turn to powder. This isn’t that serious when you use a steel pop rivet with your cooling baffling, but it is when you bolt an aluminum spar to other structural parts with non-plated or unprotected steel fasteners.
Although we think of corrosion as something that occurs naturally because of the environment or the nature of metal, there is also corrosion that occurs chemically. This can be acidic, occurring around a battery where sulfuric acids or vapors condense on the metal and begin an etching process, or even where our good old red hydraulic fluid mixes with water and forms a corrosive mixture. A far more prevalent form of this is the spotting and flaking you see when saltwater corrosion attacks an airplane. Any airplane operating in salty or brackish water has to be very careful, as it is an insidious type of corrosion. It will attack the metal in places where it is impossible to be seen during a normal inspection.
Years ago, I saw a Grumman Traveler that had been in a saltwater environment with no surface protection on the insides of the skins. Literally millions of corrosion nubs and flaking in seams were pervasive throughout this airplane. The owner used a stainless steel wire brush in an electric drill and then vinegar to clean the corrosion where he could, and then treated with zinc chromate primer. He was careful to use a stainless-steel wire wheel to do this, because a regular wire brush would imbed iron particles in the softer aluminum and cause even more dissimilar metal nasties.
Another way to treat aluminum is to alodyne it. This is the gold coloring you see on many brackets and some baffling. It is an electrochemically bonded coating that is highly resistant to saltwater corrosion and oxidation. Small parts can be done in your basement, and it is fairly simple. DuPont is a supplier. Aluminum can be protected by paint or alodyne, or by a mechanical or chemical separation. Once any corrosion is started, it should be nipped in the bud, because it becomes aggressively pervasive throughout the structure. Zinc chromate has been the traditional aviation anti-corrosion paint, but some of the new etching primers (Variprime, for example) work well. Sherwin-Williams had a wash that was very lightweight, and you simply mopped it on, but I haven’t seen this for a while. Old-timers say to just wipe the aluminum with an oily rag every once in a while.
CorrosionX is an excellent corrosion preventative for our small aircraft. Available in bulk or in spray cans, it coats and sticks to the surface and prevents moisture contact. Although this causes some weeping, or what the company calls migrating, this very activity proves that the material is working its way into the hard-to-reach places, including under rivet heads, between ribs and skins, and around skin overlaps.
A newer product developed for the airlines and more commercial planes is called NavGuard. It is thicker and does not migrate. I have used CorrosionX with great satisfaction, but I haven’t used NavGuard, so I can’t tell you about its workability.
I have used another product, RejeX, to clean some surface corrosion off of aluminum. It worked well, and the metal is now protected.
Although this article is focused on airframe corrosion, there also is a definite concern about engine corrosion. Engine corrosion can be much more expensive in the long run, and, depending on how much the airplane flies, it can be 25 or more years before it is detected.
An engine is an air pump, for the most part, and it is mostly hollow; therefore, it’s full of air. It contains heated air and cold air and a list of chemicals that is nearly endless. The products of internal combustion wreak havoc with an engine, throwing incompletely burned hydrocarbons, gas, water, and myriad other chemicals into the crankcase. Thermal cycles, both within the engine and environmentally, can create conditions that cause rust to form on the ferrous parts of the engine, nominally the crankshaft, camshaft, and lifters.
The best way to avoid rust in an engine, according to Lycoming SBL180B, is to run the engine up to temperature at least every 30 days. However, this is old technology.
Ed Kollin, a chemist with Aircraft Specialties Lubricants, has devoted his professional life to developing fuels, oils, and additive packages that enhance performance and address items that plague our expensive engines. His newest product, CamGuard, has taken over aviation as the premier anti-rust oil additive package.
CamGuard addresses four problems that are shared by all internal combustion engines: Camshafts and lifters wear out prematurely due to wear and spalling; these same parts and the crankshaft suffer from corrosion; varnish turns to sludge and then, with heat, turns to carbon; and seals leak.
Furthermore, tests have shown that corrosion can start in an engine in as little as four hours after shutdown in the worst conditions. A consistent protective measure is needed throughout the engine and its oil change cycle to ensure protection from internal corrosion. Kollin developed CamGuard to deal with each of these four items, using very sophisticated chemistry and 11 components. This product, desiccant plugs, desiccant in the exhaust pipes, and taping up the intake and exhaust pipes will preserve an engine for the winter season. For day-to-day protection, regardless of how much the airplane sits, having this additive in the oil will provide peace of mind that is well worth the price of the product.
Don’t Freak Out
So when your mechanic tells you that you have a bit of corrosion, don’t let it freak you out completely. A little corrosion, particularly on steel, is easy to address and fairly normal. If you have any more than this, before you let your mechanic do anything, I advise that you contact the manufacturers of the products that are specifically made for the different kinds of corrosion found.