United States Marine Corps aircraft corrosion fundamentals state that “metal corrosion is the greatest threat to the soundness of metals and to the structural integrity of an aircraft.”
According to www.corrosion-doctors.com, the total annual direct cost of corrosion to the U.S. aircraft industry is estimated at $2.2 billion, which includes the cost of design and manufacturing, corrosion maintenance, and downtime.
Given that the aircraft industry is dealt such a blow by corrosion both financially and structurally on an annual basis, it is both worthwhile and imperative to take a closer look at this issue.
Defining this decay
Corrosion, by definition, is the electrochemical deterioration of metal as a result of its chemical interaction with the surrounding environment.
According to documents from the American Society for Non-Destructive Testing (ASNT), most forms of corrosion begin at the surface. The rate of corrosion depends on the nature of the metals involved.
Four conditions must be satisfied in order for corrosion to occur: the presence of a corrodible metal or alloy (anode), the presence of a dissimilar conductive material that has a lesser tendency to corrode (cathode), the presence of an electrolyte, and electrical contact between the anode and the cathode.
An electrolyte is any solution that conducts electrical current and contains both positive and negative ions. Fresh water, saltwater, acid, and alkaline solutions in any concentration will act as an electrolyte; acidic gas deposits, dirt, salt, and engine exhaust gases can dissolve on wet or damp surfaces, increasing the conductivity of the electrolytic solution and thereby increasing the corrosive reaction.
“Corrosive attack is often exacerbated by mechanical erosion of surface finishes caused by sand, rain, or mechanical wear,” state ASNT documents. “This can lead to stress corrosion cracking, corrosion fatigue, and fretting corrosion.”
Where to look
Some of the most troublesome areas where corrosion occurs are the battery compartment, engine exhaust streams, bilge areas, and landing gear and wheel wells. While drain holes are placed at low points to facilitate drainage of collected fluids and moisture, they can become clogged with debris or sealants — particularly if the aircraft is in an unleveled condition. “Another area of difficult access is behind all of the hydraulic lines and cable assemblies,” says Chuck Pottier, president of Zip-Chem.
Bruce McMordie, manager of the science and technology center for Sermatech International in Royerssord, PA, adds that “Hard-to-inspect areas are particularly in blind holes, ones with no drains in them. A cavity in which material can accumulate in time, even just condensation, is a high risk area for corrosion.”
Moisture and other corrosive agents can become trapped between layers of sheet metal in spotwelded skins and assemblies.
Zip-Chem formulating chemist Jason Smith says that corrosion is prevalent in overlapping surfaces. “Corrosion tends to occur between two pieces of metal. Steel and ferrous metal tend to have general rusting,” he says, “but few aircraft have mostly steel structures. Moisture will get in between two panels that are riveted together and it will start a corrosion cell because of electrolytes.”
Smith adds that corrosion does not occur with just ferrous metals. “Other substrates can experience corrosion, too,” he says.
Mark Pearson, general manager of Lear Chemical Research Corporation, says that intervals at which aircraft should be checked for corrosion are dictated by the type of aircraft being worked on, the maintenance manuals prescribed by the OEMS, FAA regulations, and the operational environment in which the aircraft is flown.
“Visual inspection has been relied on for quite a long time, but in the last 10 to 15 years there has been some noticeable improvements in corrosion detection equipment, processes, and procedures,” says Pearson. “From simple dye penetrant testing to eddy current testing, to thermal imaging or X-ray, there’s lots of choices that can be used in addition to good old-fashioned eyeballs.”
McMordie says that use of borescope inspection is an excellent way to detect corrosion in hard-to-reach areas. “Other methods are much more equipment-intensive, like X-rays and ultrasounds,” he says. “The problem with borescopes is that you have to rely on visual appearances, but looks can be deceiving. Typically you’re not working blind, though, because you can look to an engine guide and to field experience.”
Pearson says that thermal imaging can be done from the outside of an aircraft to avoid the need for disassembly. He recommends this method for detection of corrosion in between skin and lap joint areas and/or along joints where the skin meets the longeron.
“Getting into the wing section of a Cessna might be a little easier than getting past a lot of insulation in the wing section of a 737,” says Pearson. “It’s dependent on the aircraft and how the technicians are trained to perform the corrosion inspection process. Most engineers and maintenance personnel know the aircraft they’re working on fairly well and would know through familiarity where they should be looking.”
When beginning to treat corrosion, McMordie insists that the first place a mechanic should look is in the aircraft manual. “The manuals prescribe what is the base line of corrosion resistance that you need to make sure is intact on the part,” he says. “You may need to go above and beyond the requirements of a manual, so you look to the OEMs for tips on how to combat those. Beyond that, there are companies that offer customized solutions for unique applications.”
Smith also recommends looking to qualified products lists (QPLs) maintained by OEMs to determine what kind of product to use to treat corrosion. “You can’t throw one product at every problem. One product will not do everything or treat every kind of corrosion,” he says. “It’s one of the reasons why qualified products lists exist. If you used a corrosion inhibiting compound (CIC) for connectors on a boldly exposed inner panel, you would be disappointed in the results and wind up changing it out more often, because in addition to corrosion inhibiting characteristics, a lubricant is needed.
“Areas you can see easily, you can treat,” says Smith. “For those you can’t, you need to have some sort of compound to penetrate to fill in any voids or openings that could be a problem. The product’s ability to penetrate, wick, and get into nondirect line of sight areas is critical.”
Zip-Chem’s spray process for treatment of corrosion in hard-to-reach areas involves attaching a “formit” to a CIC can. The formit is a long, thin, straw-like attachment that allows the spray applicator to reach further into the aircraft. “You can drill a hole, take the formit into it, and fog the whole area,” says Pottier.
“The real problem with trying to counter corrosion is that you won’t be able to effectively counter it unless you can begin to understand why it’s occurring,” says McMordie. “Sometimes that’s not as easy to uncover as it might first appear, especially when you can have an overlay of many different effects. It’s a constant struggle, but continue to think wisely and carefully about what’s happening and that will lead you to find a way to stop it.”
The role of OEMs
“Many OEMS are looking to using CICs on their manufacturing floor because of the added value of having it on there already,” says Smith. “This extends the life of the aircraft, rather than telling customers to apply it while in service. They still have to do maintenance, but the aircraft can go much longer without serious corrosion problems.”
Pearson says that OEMs typically rely on engineering solutions to minimize corrosion through the airframe. “That can be in the form of placement of drain holes to drain off excess condensation that forms in the aircraft, the use of polymer sealants in between faying surfaces, and the use of paints and primers to protect the open surfaces of the metal,” he says.
“One need of corrosion protection compounds is for them to be transparent,” says Smith. “Primers and paints are opaque, so you can’t see corrosion until it presents itself.”
“If you’re going to reapply the recommended coatings that the manufacturer lays down in its maintenance manuals, then that’s a choice, unless the aircraft is under warranty,” says Pearson. “Many operators have seen that the replacement of the waxy-type films on aircraft that already have demonstrated or exhibited some sort of corrosion may end up with some reoccurring corrosion problems by reapplying those types of films onto surfaces, complex joints, and other structures where an electrolyte might be trapped inside.”
Smith points out that some major OEMs have discontinued applying waxy-type CICs. He expects that current waxy film technology will fade further as technology developed to neutralize trapped corrosion products to prevent ongoing corrosion becomes more prevalent.
Words of wisdom
“People need to think about corrosion prevention just like they do with changing their engine oil,” says Pearson. “That has to be on regular, prescribed intervals as the manufacturers have laid out, and there’s a reason for that. If some preventative maintenance is done in the form of corrosion protection, then the life and the structural integrity of the airframe is going to be extended.”
Smith contends that the regularly prescribed intervals spelled out by the OEMs are determined more by the performance of the CICs than any other factor. “As the CICs last longer, the interval between reapplication of the CICs increases,” he says. “Current CIC technology is intended to be much longer lasting and reduce the corrosion as well as the number of times that areas must be re-treated.”
Paying the price
According to www.corrosion-control.com, the cost of repairing corroded aircraft is 100 times the cost of preventive maintenance.
“It’s too inexpensive to have an aircraft treated,” says Pearson. “There’s not much you can do to the airframe that’s as inexpensive as a corrosion-prevention program that’s going to yield the amount of return in the form of peace of mind and longevity of their investment.
“Everybody hates working on corrosion,” says Pearson. “It is a metal cancer and it’s always sad to see an airframe deteriorate with that sort of disease. If we’re mechanics at heart, we want to see the equipment run right and work correctly and look good. Nobody likes to see chunks taken out here or there or gap patches put on. That’s an indication that the airframe wasn’t loved as much as it should have been.”