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Aircraft Corrosion
by Joseph E. Stump*
Ever since I heard Joseph Johns declare that "we are making biodegradable aircraft," I have been interested in corrosion of aircraft. This month's author has done an excellent job of presenting the basics of the corrosion processes in aircraft and he makes the subject interesting as well as useful.
Frank Iddings
Tutorial Projects Editor
INTRODUCTION
Aircraft corrosion has been a concern of aviation maintenance professionals since the Wright brothers first began putting wings on things. Unfortunately, the biggest problem facing human ingenuity and endeavor is humanity's own limitations. There is virtually nothing we fashion that does not wind up on the great cosmic scrap heap either eventually or immediately. This is no less true for today's advanced materials. When favorable conditions prevail, contemporary alloys begin to break down into metallic compounds such as oxides, hydroxides or sulfates. To put it another way, metals have a tendency to return to their natural states. Maintenance professionals like to call this condition corrosion; the customer likes to call this condition expensive; when left unchecked, the Federal Aviation Administration (FAA) likes to call this condition life threatening.
Corrosion is the electrochemical deterioration of metal as a result of its chemical interaction with the surrounding environment. While metal alloys continuously become more sophisticated and resistant to corrosion, they are also subjected to increasingly harsher operational environments and conditions. Jet engines are a good example.
Aircraft corrosion takes many forms and the ability of aircraft to resist corrosive attack can change dramatically with only minor environmental deviation.
This article will examine the corrosion process and the role of nondestructive testing (NDT) in detecting it.
Corrosion prevention is an ongoing task that is never completed.
THE FOUR ELEMENTS OF CORROSION
Before corrosion can take place, four conditions must be satisfied:
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 electrical contact between the anode and the cathode. The elimination of any of the above conditions will halt the corrosion process.
Electrolytes
An electrolyte is any solution that conducts electrical current and contains positive and negative ions. For example, fresh water, salt water, acid and alkaline solutions in any concentration will act as an electrolyte. Acidic gas deposits, dirt, salt and engine exhaust gasses can dissolve on wet or damp surfaces, increasing the conductivity of the electrolytic solution; this increases the corrosive reaction of the electrolyte.
How Corrosion Forms
Before corrosion can develop in an alloy, there must be a completed electrical circuit and flow of direct current. This sets up the initial galvanic action. An acting anode and cathode must also be present. This can be two dissimilar metals or two different areas of the same section of alloy.Corrosion always begins at the surface. For example, take the corrosion of iron. The iron atom gives up two electrons and becomes a ferrous ion with two positive charges. It goes into solution as a metallic ion via the electrolyte, which starts the corrosive reaction. Liberated electrons from the positive metallic ions flow to the cathode. Without this electron flow, no metal ions can detach from the anode. This establishes the electrical circuit of the corrosion process. The constant loss of positive metallic ions from the anode represents the eating away of the anodic material. The ongoing flow of electrons creates greater positive ionization and so the cycle continues.
The electrons reach the surface of the cathode material and neutralize positively charged hydrogen ions that become attached to the cathode. Some of these hydrogen ions become neutral atoms and will be released in the form of hydrogen gas. The release of positively charged hydrogen ions produces an accumulation of OH negative ions (an atom of hydrogen and an atom of oxygen with one extra electron). This process increases the alkalinity at the cathode and promotes the formation of tiny bubbles of hydrogen.
When cathodes and anodes are formed on a single piece of metal, the exact locations are determined by the imperfections in the material. For example, the lack of homogeneity in the metal, inclusions, internal stresses, surface imperfections, lapping of the material or any condition that can form a crevice, will set up a cathode/anode relationship. The only thing needed now is an unprotected surface and a suitable electrolyte.
TYPES OF CORROSION TO AIRCRAFT
Galvanic Corrosion
Galvanic corrosion occurs when two dissimilar metals make contact with one another in the presence of an electrolyte. The rate of corrosion depends on the nature of the metals involved. Some combinations corrode more rapidly than others. Metallurgists rely on a chart called the Galvanic Series to identify which metals tend to be anodic or cathodic in relation to one another and their level of activity.Surface area is critical as well. If the anode material is substantially smaller in area than the cathode, corrosion will be rapid. The reverse is also true Ñ a smaller cathode area will have a decreased corrosive effect.
Concentration Cell Corrosion
Concentration cell corrosion often focuses on metal to metal joints, even if the joined metals are identical alloys. Foreign material that masks metallic surfaces can often establish a corrosion cell condition, leading to corrosive attack. There are three general types of concentration cell corrosion: oxygen concentration cells, ion concentration cells and active/passive concentration cells.
Oxygen Concentration Cell Corrosion
This form of corrosion can be initiated anytime a deposit of sand, solution or other material produces localized low oxygen/high oxygen differentials across a metallic surface. Lapped metal on riveted or bolted joints is especially vulnerable to this form of attack.Oxygen cells can develop at any point where the oxygen in the air is not able to diffuse into the solution. Cells can also develop under gasket material, washers, wood, rubber and other materials that come in contact with metal.
Should corrosion resistant stainless steel become oxygen starved in any given area, the passivity of the steel can break down. The area of stainless steel that is freely exposed to dissolved oxygen becomes the cathode. Corrosion will now proceed in any adjacent areas where the oxygen source becomes depleted.
Cathodes are formed at areas of high oxygen concentration and anodes at areas of low concentrations. This can best be illustrated by observing a drop of saltwater placed on a polished steel surface. Within an hour or so, a ring of rust will form inside the drop (anode) while the outer edges (cathode) remain clear. The outer edges of the droplet absorb the highest concentrations of oxygen from atmospheric sources.
Filliform Corrosion
Filliform corrosion is a unique form of oxygen concentration cell corrosion. This form of attack occurs on metallic surfaces having an organic coating as its protective basis. It is characterized by a snakelike pattern of corrosive deterioration that forms beneath painted surfaces. Filliform attack tends to occur when relative humidity is high (78 to 90%) and surface conditions are slightly acidic. Figure 1 shows filliform corrosion on an exposed aileron fastener and one under paint.The corrosion finds its way to the metal through breaks in the outer surface of the coating or paint and works its way underneath. The corrosion continues to propagate due to the diffusion of water vapor and oxygen as air passes through painted surfaces that are in a compromised condition.
Filliform corrosion does a good job of attacking both steel and aluminum; however, its effect on the latter is more severe. The tracks left by the attack never cross one another on steel products, but they will intersect on aluminum, making the damage deeper and more insidious. Worse yet, if the condition is left untreated, or is improperly dealt with, it often develops into intergranular corrosion. On aircraft, the areas around fasteners and seams are the most vulnerable.
The standard methods of treating filliform corrosion involve the use of glass bead blast and mechanical buffing with abrasive materials. A coating system is then applied to unpainted surfaces to protect them from the diffusion of oxygen and water vapors.
Metal Ion Concentration Cells
Ion cells usually begin as a solution of water and ions of the parent metal with which the water is in contact. A high concentration of ions usually exists beneath faying surfaces where the solution is stagnant. Low metal ion levels are normally found adjacent to the crevice that is created by the faying surface. An electrical potential is established between the two points. The area with the lowest concentration of metal ions will become anodic and corrode. The area with the highest level of metallic ions will act in a cathodic manner.
Active/Passive Corrosion Cells
This type of corrosive attack affects metals that depend upon tightly bonded passive films, such as oxides, for corrosion resistance. Stainless steels, for example, are prone to attack by active/passive cells.The corrosion usually begins as an oxygen concentration cell. Salt water deposits on a metal surface in the presence of ample oxygen will form an oxygen concentration cell beneath particles of dirt, for example. The passive film becomes corrupted. Once the film is broken, the active metal beneath becomes exposed to corrosive attack. An electrical potential develops between the large area of passive film (cathode) and a small area of active metal (anode). A severe rapid pitting will be the result.
Intergranular Corrosion
This type of corrosion is most insidious and can represent a real hazard to aircraft aluminums. Intergranular attack originates along the grain boundaries of the material. This is chemically different from the metal within the grain center. Many alloying constituents migrate toward the grain boundaries during the metals solidification process. The grain boundary and grain center can react with one another as cathode and anode when in the presence of an electrolyte. As the grain boundaries break down, delamination and exfoliation can occur (Figure 2). Left unchecked, this could lead to catastrophic structural failure. High strength aluminum alloys, such as 2014 and 7075, have a higher susceptibility to this form of corrosion if improperly heat treated.Many stainless steels are prone to this kind of attack where chromium carbides precipitate to the grain boundaries. This lessens the chromium content adjacent to the grain boundaries, creating galvanic potential. Rapidly cooled austenitic stainless steels are especially prone to this kind of corrosion.
Exfoliation Corrosion
Exfoliation corrosion is an advanced stage of intergranular attack. The surface grains of the material are lifted up by the coercive force of expanding oxidation products at grain boundaries located just beneath the surface. This blistering effect is quite noticeable in aircraft aluminums and is most prevalent in wrought products such as plate, thick sheet and extrusions, where the grain structure of metal tends to be elongated (Figure 3).
Metallic Mercury
When metallic mercury comes in contact with aluminum alloys, rapid corrosion develops, causing severe pitting and intergranular attack that is exceedingly difficult to arrest. The aluminum becomes embrittled due to the formation of compounds that move rapidly across grain boundaries. If the aluminum is under load, the surface may exfoliate or split, creating a hazardous condition.X-ray testing is a good method of locating small trace amounts of spilled mercury. Being much denser than the surrounding aluminum, it is easily detected on radiographic film.
Corrosive Agents
The most prevalent corrosive agents on aircraft materials are acids, alkalis and salts. Water and the atmosphere act as the two most common media for these agents.By and large, moderately strong acids will severely corrode most of the alloys used in airframe structures. The most destructive of these acids are sulfuric, hydrochloric, hydrofluoric, hydrobromic and nitrous oxide compounds. Organic acids found in human and animal waste products are also detrimental.
Alkalis are not generally thought of as being as harmful as acids, but numerous magnesium and aluminum alloys are susceptible to corrosive attack by many alkali solutions. Washing sodas, potash and lime solutions can be highly detrimental to aluminum and magnesium alloys.
Salts are well known for their ability to promote corrosion on a wide variety of materials. The property that makes them so destructive is their ability to serve as an outstanding electrolyte. While some stainless steels may hold up in a salt environment, mild steels, aluminum and magnesium alloys corrode rapidly. A variety of other alloys are sorely affected by exposure to a salt environment as well.
The effects of the atmosphere on aircraft materials can be quite profound. The atmosphere contains ample supplies of oxygen and moisture, both of which are corrosive. Corrosion often results from the direct action of these two elements. Additional moisture alone, especially on ferrous alloys, can accelerate corrosive attack. Our atmosphere also contains a variety of other corrosive gases and contaminants that can hasten the development of corrosion products. Possibly the most common are oxidized sulfur compounds. When combined with moisture, they produce sulfur based acids that can induce severe chemical attack on a number of common aircraft alloys.
Marine atmospheres are highly injurious to most aviation related alloys. Marine air contains chlorides in the form of salt or droplets of salt saturated water. As a reminder, fresh water can be just as harmful as its marine counterpart. Fresh water often contains fluorides and chlorine, both of which promote corrosion. Dissolved minerals, gases and organic impurities determine the extent of its corrosive and electrolytic properties.
CORROSION AND THE ROLE OF MECHANICAL INFLUENCE
When a corrosive condition is aided by cyclic service loading, the corrosive attack becomes accelerated at a rate considerably beyond the normal progression of the corrosion itself. Environmental conditions, as well as alloy composition, greatly influence the corrosion's ability to react. Corrosive attack is often exacerbated by mechanical erosion of surface finishes caused by sand, rain or mechanical wear. This can lead to stress corrosion cracking, corrosion fatigue and fretting corrosion.
Stress corrosion cracking is a form of intergranular attack where localized stresses may be caused by internal or external loading. Internal stresses are usually the result of some manufacturing process or procedure that more often than not involves cold working of the material. Normally, stress levels in the material vary from zone to zone. In the areas where the stress level approaches the yield strength of the alloy, corrosion cracking is most likely to occur.
Interaction with compounds in the environment will induce stress corrosion cracking as well. Contact with sea water can also provoke stress corrosion fracture in high strength steels and heat treated aluminum alloys. Magnesium has a proven hypersensitivity to moisture and will stress corrode under high humidity conditions if not properly protected.
Corrosion fatigue failure is the result of cyclic loading combined with corrosive attack. It generally occurs in two distinct phases. Initially, the combination of corrosion and cyclic loading induces pitting in the material that ultimately leads to fracture. In the second step of the process, the material essentially becomes so fatigued that fracture propagation becomes a certainty.
Fretting is a condition that occurs when two surfaces under load that are not designed to come into contact with one another do so as a result of vibration or some other factor. When this occurs, damage to the protective film or finish on the metal's surface will result. The constant mechanical interaction leaves surfaces free from protection and open to the atmosphere or other corrosive influences. Deep corrosive pitting is likely to result if the condition is left unchecked.
AIRCRAFT TROUBLE ZONES
Aircraft can develop corrosion virtually anywhere, depending on its overall condition and geographic location. However, there are known trouble spots on any aircraft where corrosion has a much higher statistical probability of occurrence and routine testing and maintenance are a must.
Engine exhaust streams are a prime target for both jet and reciprocating engines. Exhaust gas residues are highly corrosive. Exhaust deposits can become trapped under seams, hinges and fairings where normal cleaning is ineffective. Mixed with rain, moisture or a high humidity atmosphere, exhaust residues become highly electrolytic, leading to conditions conducive to corrosion.
Without question, one of the best known trouble spots on any aircraft is the battery compartment. This is in spite of extensive venting, sealing and painting of the battery box area. Fumes that emanate from an overheated battery condition are extremely difficult to contain. Often, the fumes will disseminate to internal structures where unprotected surfaces become vulnerable to corrosive attack.
Lavatories and galleys present a problem as well. Behind lavatories, sinks and ranges, waste products, food and moisture tend to accumulate, causing corrosive conditions to prevail. Bilge areas under lavatories and galleys are particularly troublesome and regular maintenance in these areas is highly critical.
In fact, any aircraft bilge area is a trouble zone. A bilge area can be defined as a natural collection point for waste oils, hydraulic fluid, water, dirt or debris. Oil often hides water that has settled to the bottom of the bilge area, masking a potential corrosion cell.
Along with bilge areas, water entrapment or drain areas can be problematic. Drain holes are located at low points on the aircraft to facilitate drainage of collected fluids and moisture. They normally do not present a problem, except when they become clogged with debris or sealants or if the aircraft is in an unleveled condition.
Landing gear and wheel well areas take a real pounding. These areas of the aircraft are constantly exposed to mud, water, salts and flying debris from runways that inflict mechanical damage to protective coatings and surfaces. Areas of particular susceptibility are:
high strength steels the interiors of axles any exposed indicator switch or other electrical equipment crevices magnesium wheels, bolt heads, lugs and web areas exposed rigid tubing. Graphite composite materials can pose another set of corrosion complications when they come into contact with many of the alloys used in aircraft manufacturing. Graphite/epoxy materials make an excellent cathode, creating the potential for galvanic corrosion. When conditions are suitable, such as in a high humidity or saltwater environment, epoxy/graphite composites may become highly reactive. Sealant must be applied between the metal/composite interface to prevent moisture from initiating galvanic attack.
The frontal areas of aircraft engines often pose a corrosion problem as well. With the constant onslaught of abrasion caused by airborne dirt, flying debris, dust and gravel from runways, protective coatings and finishes take a real beating, exposing metal to the elements. Radiator cores and cooling fins on reciprocating engines are also vulnerable.
Spotwelded skins and assemblies are another area of high susceptibility. Moisture and other corrosive agents can become trapped between layers of sheet metal. This can occur at the time of manufacture, but that tends to be restricted to older aircraft. Corrosion eventually causes the skin to buckle or the spotweld to bulge outward, ultimately leading to fracture.
Rear pressure bulkheads are an area of real concern. The accumulation of fluids below the floor can result in severe corrosion damage. A good visual test may entail extensive disassembly of the aircraft fore and aft of the bulkhead area. Nondestructive testing methods such as ultrasonic, eddy current and radiographic testing are commonly used to detect corrosion. Severe corrosion conditions in the bulkhead periphery can lead to cabin pressure loss or worse.
THE ROLE OF NONDESTRUCTIVE TESTING
In addition to visual testing, other NDT methods play a major role in the detection and analysis of aircraft corrosion. Fluorescent penetrant, eddy current, ultrasonic, radiographic and magnetic particle testing have all been used in the detection of aircraft corrosion. As in other industries, the FAA mandates that only fully trained and qualified personnel perform these tests.
Fluorescent Penetrant Testing
Fluorescent penetrant testing is best suited for finding large stress corrosion or fatigue cracks open to the surface on nonporous metal alloys.
Magnetic Particle Testing
Magnetic particle testing is used for the detection of stress corrosion cracking on or near the surface of ferromagnetic alloys only.
Eddy Current Testing
Eddy current testing (low frequency application) is often used to detect material thinning due to corrosion, as well as cracking in multilayered airframe structures. Higher frequencies are used for the detection of cracks that can penetrate the surface of the airframe. High frequency techniques are also sometimes employed to detect the formation of corrosion that may lie beneath organic coatings.
Radiographic Testing
X-ray testing is another tool used for the detection of corrosion on aircraft structures, but its effectiveness can be rather marginal in detecting light corrosive conditions. This is largely due to the difficulty in obtaining the radiographic sensitivity necessary to detect corrosion in the early stages. The technical acumen of the test personnel involved becomes a consideration: the more experienced, the better. Moderate to severe corrosion conditions, as well as cracking, can be successfully detected using the radiographic method, provided geometric factors are not an overriding issue.
Ultrasonic Testing
Ultrasonic testing provides one of the most sensitive and accurate means of corrosion assessment available for a continuous thickness of material. Ultrasonic testing is commonly used to detect exfoliation, stress corrosion cracks and general thinning of material. For the most part, ultrasonic digital thickness meters are not considered reliable for the analysis of moderate to severe corrosion damage prior to removal of the corrosion products. There is little doubt that ultrasound is one of the most effective and commonly used methods to detect corrosion in the aviation industry. Its cost effectiveness, combined with versatility and portability, make it one of the most efficient tools in the NDT arsenal in the fight against corrosion.
CONCLUSION
Corrosion prevention is an ongoing task that is never completed. It is a constant sequence of cleaning, testing, preservation and lubrication. Corrosion must be detected and removed in the earliest possible stages to minimize damage to the aircraft and its component parts. Proper maintenance requires personnel who are professionally trained in the recognition of corrosion, its detection, identification and treatment.
* GE Inspection Services, 1211 Kona Drive, Rancho Domingquez, CA 90220; e-mail <joseph.stump@ps.ge.com>.
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