RB “Doc” Hecker
EAA Technical Counselor #5453
One of the most important inspections we can do on our aircraft is to look for signs of metal corrosion. Metal corrosion contamination in our aircraft may take place internally or on surface structures. Corrosive deterioration of the original metal may change surface smoothness, weaken the underlying structure, or damage or loosen adjacent parts. The main source of corrosion in aircraft is related to water or salt containing water vapor in the atmosphere that combines with oxygen in an electrolytic process. Obviously, aircraft operated in or around fresh or salt water environments (e.g. seaplanes) are at high risk for corrosion. Not so well appreciated is that aircraft operated within proximity to a seacoast are also at risk for salt water vapor contamination. Chemical agents can also initiate corrosion by directly attacking the metal if improperly applied or removed, as in the case of oil, grease or exhaust residues, caustic chemical cleaners, battery acids, or left-over flux agents after structural welding or brazing. Frequent aircraft cleaning allows for removal of corrosion prone agents, and has the secondary benefit of allowing visual inspection of the metal structures to identify potential problems.
There are two general classifications of corrosion that include the specific forms that will be discussed here. These classifications are: Chemical attack and electrochemical attack. In all types of corrosion, the underlying metal is physically converted to a metallic compound such as an oxide, hydroxide or sulfate. Aluminum and magnesium alloys present with surface pitting and etching, usually with a grey or white powdery deposit. Copper and copper alloys leave a greenish or bluish deposit, while with steel or iron containing metals, reddish (ferrous oxide) or black (ferric oxide) deposits are noted. The process of corrosion always causes two simultaneous changes that are chemically known as oxidation-reduction reactions. Electrolytically, the oxidized attacked metal undergoes anodic change, and the corrosive agent is chemically reduced while undergoing cathodic change.
Some attempt is made by the manufacturer to provide some corrosion resistance to metal structures. These forms include cadmium plating of steel fasteners, where the cadmium coating is sacrificial, and pure aluminum cladding of aluminum alloy (Alclad) which is employed to minimize corrosion capability. Also, chemical alodining of aluminum alloys and chemical magnedining of magnesium alloys, along with treatment of steel components by etching and priming followed by quality paint or other protective coatings are typically used on primary and secondary structures to preserve and to protect metal surfaces.
Types of Corrosion
There are many forms of corrosion that are dependent on various factors such as the metal involved, the metal’s size and shape, its specific function, the atmospheric condition it resides in, and which corrosion-producing agents are present. Interestingly, thicker metal sections are more corrosion prone than thinner sections due to the change in physical characteristics after machining. The following are the more common forms of corrosion found in aircraft structures:
Surface corrosion typically appears as general roughening of the surface with pitting or etching, and may be accompanied by a powdery deposit of corrosion products. If the area of corrosion is beneath the surface coating, the first clue may be the lifting of surface plating or paint in small blisters (this is not paint failure!) due to the underlying pressure of the accumulating corrosion deposits. Due to the multi-linear, spider web type pattern of surface deformity, this is sometimes known as filiform corrosion. Magnesium and aluminum structures noted to have paint deformities should be immediately inspected for underlying corrosion.
Dissimilar Metal Corrosion;
In the presence of an electrolyte, dissimilar metals in contact with each other may initiate an electrochemical (galvanic) action that causes severe pitting and destruction. Typically this galvanic reaction is hidden from surface view and is found by disassembly and inspection. A dissimilar metal chart typically found in aircraft mechanic handbooks should be consulted to guide you regarding conflicting metal contact. Direct attachment of aluminum to steel surfaces will begin dissimilar metal corrosion unless protective measures are undertaken to adequately prepare the mating surfaces. These measures can include electroplating, metal spraying, chemical treatments, or special wrappings.
Intergranular corrosion is an insidious problem caused by an attack along metal grain boundaries, and is commonly a result of lack of uniformity of the metal grain in the alloy structure. Aluminum alloys and some stainless steels are prone to this form of corrosion. Very severe intergranular corrosion may cause the surface metal to exfoliate due to pressure of corrosion products within the grain boundaries that leads to delaminating of the surface metal or causing the metal to flake off.
Stress corrosion occurs due to the combined effects of sustained tensile stresses in a corrosive environment. Although stress cracking occurs in any metal system, it is especially found in aluminum, copper, stainless steels and high-strength (> 240,000 psi) alloy steels. This corrosion may be either transgranular or intergranular in nature, and usually follows cold working stress points. Areas of concern include aluminum alloy bell-cranks with pressed-in bushings, landing gear shock struts with coarse (pipe) thread grease fittings, shrink fittings and overstressed B-nut fittings. Inspection of the radius of bends in cold worked metals should be included in your corrosion check.
Fretting corrosion may be particularly damaging when two surfaces normally mated together begin to undergo motion relative to each other. The mated surfaces accumulate fine debris which causes further abrasion. The debris particles typically cannot escape the abrasive environment, and in the presence of water vapor, the destructive process is accelerated. Deep grooving resembling brinell marks can be identified, or pressure indentations may be noted. The so-called ”smoking rivet” indicates rivet loosening with the “smoke” consisting of a metal debris trail. This smoke trail signals inadequate metal-to-metal fixation with potential underlying corrosion that should be investigated within a short (25 hour or less service time) maintenance period. Smoking rivets should always be replaced.
Corrosion, no matter how slight, is physical damage to metal. As in other damage, corrosion damage is classified under four standard types: (1) Negligible damage, (2) damage that is repairable by patching, (3) damage repaired by insertion of new materials, and (4) damage that requires part replacement. The term “negligible” does not imply that no action is necessary – the corroded surface needs to be cleaned, treated and coated (e.g., painted) as appropriate. Negligible damage is defined as a change of a metal surface that is scarred, or has had the protective coating eaten away and the metal has noticeably begun to etch.
Cleaning an aircraft, and keeping it clean, is very important as the main way to detect evidence of metal corrosion is by doing a visual inspection. Any change in the usual color of a metal, or a change in a coating or paint finish, signals that a problem with metal corrosion may be occurring. Depending on the aircraft, there may be recurring problems noted with a particular make and model that lead you to do more frequent inspections. Examples of these types are sea planes, which by their nature are around moisture containing environments, conventional gear aircraft in which moisture collects in the tail section, and Cessna 200 series aircraft with foam core elevators and trim tabs. Hard to reach structures may require mirror inspection or the use of the newly available flexible fiber scopes. Mechanical methods such as a “coin tap” to detect a change in the “ringing” of the metal (dull report or thud), or the use of a sharp device (awl) can be helpful in detecting a change of integrity of a metal’s soundness. Non-destructive testing measures such as dye penetration methods can be employed if hidden corrosion or metal damage is suspected. These advanced inspection methods are best left to use under the supervision of a qualified airframe mechanic.
Corrosion removal is necessary to preserve the metal structure. The removal of corrosion requires that the surface covering over the area of corrosion must also be removed. Cleaning of the affected area to expose all of the area of suspected damage is necessary as extensive corrosion on any panel surface may necessitate treating the entire panel. The following five steps are essential during the removal process: (1) Cleaning and stripping of the corroded area, (2) removing as much of the corrosion products as practicable, (3) neutralizing any residual materials remaining in pits and crevices, (4) restoring protective surface films, and (5) applying the temporary or permanent coatings or paint finishes.
The treatment of corroded surfaces is based upon the type of metal that is being attacked. An aircraft mechanic’s manual should be consulted prior to treatment of ferrous metals, anodized surfaces, magnesium and titanium alloys, stressed steel components, and aircraft structures and surfaces with specialized coatings (e.g. Parco lubrizing). For the treatment of unpainted aluminum surfaces, a typical aluminum corrosion treatment sequence is as follows:
Remove oil and surface dirt with a mild cleaner using a stiff fiber brush prior to abrasive cleaning. Do not use steel or ferrous containing bristles when cleaning aluminum surfaces as these bristles will leave dissimilar metal residue on the cleaned aluminum surface.
Hand metal polish with a fine abrasive or quality metal polish. If a surface is particularly difficult to clean, a metal cleaner and brightening compound for aluminum can be used to accelerate the process in order to obtain a clean, bright finish.
Corrosion Inhibition Treatment:
Treat any area with surface corrosion with an inhibitive material such as alodine or one of the commercially available products. The treated area should be wiped down with a clean cloth.
Over coating / Waxing:
Treated unpainted aluminum areas should be finish protected with a coating of a quality water-proof wax.
Additional Notes for Aluminum Surfaces that will Ultimately be Painted:
Aluminum surfaces that are to be subsequently painted can be exposed to a more severe cleaning procedure that includes the application of a solutions of phosphoric acid (etching) and chromic acid (alodining) prior to the restoration of paint coatings.
Trademarked products such as LPS-3, ACF-50 and Corrosion-X are marketed as corrosion inhibitors for all refined metals. These products penetrate joints, rivets, seams, and hinges and chemically neutralize the corrosion prone environment by immediately removing moisture. All of these agents are touted to have the capability to remove saltwater, but they will not loosen any rivets or secured joints. These compounds are safe on metals, plastics, paints, and seals, and can be used to treat your metal surfaces in all types of environments. All of these products are clean and free of toxic and greasy residues. Additionally, only one treatment will neutralize (not remove) ongoing corrosion and continue to protecting your affected structures for up to two years.
Typical Examples of Aluminum Corrosion on a Control Surface Exposed to Salt Air
Figures Demonstrating Visible Corrosion
Figure 1 - Intergranular corrosion with failed rivet head beneath paper masking tape. This rivet head was easily removed with a finger nail!
Figure 2 - Surface corrosion with pitting.
Figure 3 - Surface corrosion with exfoliation. Figure 4 - Surface corrosion around pushrod
control cutout. Note deteriorated fiber lock
nut plates and improper drilling of rib flanges
for fabric support by pop-rivets instead of
RB “Doc” Hecker (EAA 789419) is a FAA Senior AME (20969) who retired from the US Army Medical Department in 1997 after 26 years of service. He holds a Private/Instrument certificate for ASEL and ASES. He has logged over 3,000 hours and prefers small, intimate airparks. He has restored a 1965 Cessna C210E (N4904U), a 1946 Taylorcraft BC12-D (NC43306), refurbished a 1947 Taylorcraft BC12-D (N43928), and is currently restoring a 1946 Aeronca 7AC (NC2241E). His other projects include building a RV-8 (N51TX) and preparing to help restore a Taylorcraft F-19 (N3556T). He has previously owned a Cessna C-172 (N61785), a Grumman AA-5B (N74447) and a Mooney M20C (N10AD). In his free time, Doc practices medicine in San Antonio, TX. He is a member of EAA Chapter 35 of San Antonio, TX, EAA Chapter 92 of Orange, CA, AOPA, and the Gulf Coast Wing of the Commemorative Air Force.
July 1, 2010