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.”