Airframe Technology
Avoid it by effective detection and treatment practices
By Eric M. Smyth
The aircraft that we maintain on a daily basis are not getting any younger. In fact, many airframes are now older than the technicians who are maintaining them. Due to the advanced age of the aircraft fleet, corrosion control is no longer limited to the airlines or military aircraft that operate in merciless environments. It involves every aircraft type and location. As the aircraft metal tries to return to its original state, (powder in the ground), the structure of the airframe is compromised and structural failure could be imminent. So as maintenance technicians it is our job to find the corrosion, treat it, and return the airframe to its original integrity. This article will discuss some of the ways to do that.
Simple corrosion cell showing conditions which must exist for electrochemical corrosion.
What causes corrosion
Corrosion can be broadly classified into two types, electrochemical, and chemical. Electrochemical corrosion needs four items to propagate, an anode, cathode, electrolyte, and a current path. When dissimilar metals come into contact with each other, such as a stainless-steel fastener in an aluminum structural member, the aluminum will act as the anode and the stainless fastener as the cathode. When an electrolyte is introduced a current path has been provided between the two materials. Because the aluminum has a higher electrode potential than the stainless steel its corrosion rate will be accelerated. The greater the difference of the electrode potential, the higher the corrosion rate.
Types of corrosion
In the example stated earlier of the stainless fastener bolted into the aluminum, the corrosion will form around the fastener head and cause pitting and general decay of the surrounding material. This type of corrosion is called galvanic corrosion. It is an electrochemical attack and probably the most common found in airframes. If caught early, the structure can usually be repaired.
This chart indicates the relative corrosion resistance of a group of commonly used metals and alloys. When two metals are in contact, the metal that is higher on the chart acts as a sacrificial anode to the metal below it. Metals that are far apart on the chart will corrode more severely when they are in contact, particularly in a wet environment. For example, an unprotected magnesium part in close proximity to a stainless-steel part would deteriorate severely in the presence of moisture.
Intergranular corrosion can develop in metals that have been alloyed. High tensile aluminum alloys such as 7075 and 2014 that have been improperly heat treated are susceptible to this type of corrosion. The internal grain boundaries when exposed to an electrolyte will begin to act like an anode and a cathode and the corrosion process will begin. This type of corrosion is very dangerous, because the material breakdown cannot be visually confirmed and airframe failure can be the result. If intergranular corrosion is left unchecked, exfoliation of the part will proceed. The part will actually start to peel like a shale rock until all the material’s integrity is gone. The only repair for a part that has intergranular corrosion, or exfoliation, is replacement of the part.
Surface pitting can be caused by a direct chemical attack or galvanic action. If the surface of the metal has a dull appearance this is generally a direct chemical attack. Powder forming near a fastener or other areas of the aircraft is most likely galvanic action. If either of these is not corrected, surface pitting will result. If the pitting is left untreated the thickness of the metal can be severely diminished over time. Immediate cleaning and treatment are necessary.
Corrosion detection
When trying to identify the material for repair, each will have its own signature. Magnesium will have a white powder, aluminum can be recognized by its grayish white powder, and ferrous metals will have a brownish powder. Stainless steel is different in its mak eup, so is its corrosion signature. It will tend to corrode on the surface and appear to be dull.
Surface protection
Manufacturers and technicians can stop the corrosion process by breaking the chain of events that allows the decay to start. All of these processes do not allow the electrolyte to the surface of the metal, so the chain is broken.
Ninety nine percent pure aluminum has natural corrosion control properties. When exposed to the atmosphere it begins to form an oxide film. This film becomes so dense that air is not allowed to attack the underlying surface. While this is excellent for corrosion control, pure aluminum is too soft to use in aircraft structures. Manufacturers alloy aluminum with other materials to increase its strength, but this allows the material to be more susceptible to intergranular corrosion. When aluminum sheet is manufactured, a process of cladding is used to protect the aluminum alloy underneath. This cladding is a pure aluminum overlay of approximately 5 to 10 percent of the alloy’s thickness. This allows both strength and corrosion resistance in one sheet.
Another process that forms an oxide film is anodizing. This is an electrochemical process that artificially forms a hard oxide film on aluminum parts of a complex shape. To form this artificial surface, the parts are dipped into a charged bath of chromic acid and water, this process will form a hard oxide film on the component, and the part is now protected.
Since anodizing is not practical in field work, a pure chemical form of corrosion control is needed that can be used at the aircraft or bench. Alodizing is a chemical process that forms a hard oxide film on aluminum surfaces to protect the underlying material. To properly prepare the part for alodizing the piece must be cleansed with an acid etch. After the part has been fully rinsed with water, the part is ready to be treated. Alodine can be applied by brushing or spraying on the surface and letting it stand for the manufacturer’s recommended amount of time. The aluminum should have a yellowish to golden brown appearance when correctly applied.
For centuries paint and primers have been used for their aesthetic appeal but also for their ability to prevent corrosion. The paint and primer seal the metal beneath and will not allow the electrolyte to contact the metal, so no corrosion will occur. Paint must be inspected, as cracks in its surface will allow contaminants to seep to the metallic surface, allowing the corrosion cycle to begin.
Plating is another method to effectively control corrosion. For example, cadmium is electroplated to almost all of the steel hardware used in aviation today. If the plating is scratched and the steel is exposed, the cadmium will form an oxide that is very much like that of pure aluminum, so dense that no more corrosion is allowed. Steel firewalls are passed through a vat of molten zinc and rolled to form galvanizing. If the steel firewall is scratched in any way the reaction is very similar to the cadmium plating, it will form a dense oxide and will not allow damage to the firewall surface.
Corrosion repair
Once the corrosion has been identified, it needs to be repaired. This entails removal of the corroded surface with an abrasive of some sort. Aluminum and steel may be sanded with fine grit paper. Do not use steel wool on aluminum as it will embed steel particles and the corrosion will start again. Retreat the aluminum surface with Alodine and prime and paint as necessary. Steel may be primed and painted as well.
Since magnesium is the most anodic metal used in aircraft, you should only use nonmetallic materials for cleaning the surface such as stiff bristle brushes. The use of any type metallic devices will lodge small particles into the magnesium and will destroy any cleanup by allowing corrosion to start again. Once the corrosion damage is removed apply a chromic acid solution and rinse the part. After the part is dry recoat the parts as necessary.
Above: Corrosion in honeycomb structure.
Below: Corrosion in fuel cell.
When sanding the areas of corrosion, care should be taken to only remove the damaged area, so as not to weaken the material by decreasing its overall thickness, blending will help smooth the surface and allow for better paint adhesion.
Corrosion prone areas
Many areas of the aircraft structure are more vulnerable to corrosive effects than others. While performing inspections, areas that are normally open to the environment should receive extra attention. Landing gear and wheel wells are especially prone to the environment. Debris lobbed by the tires can chip the paint exposing the surface to the potential of corrosion. Also due to the mechanical complexity of the landing gear and wheel well areas, there are many areas that are hidden and can trap chemicals, dirt, and debris. Without close inspection of these areas, corrosion can go undetected.
Lap joints are numerous in aircraft construction. These allow moisture traps if the paint is cracked or the metal is unprotected. Inspect lap joints for paint bubbling or surface corrosion in order to catch this early and keep repairs to a minimum.
In summary
Because of the severe damage that can be caused by corrosion and the potential consequences of corrosion propagating to extreme levels, corrosion control needs to be at the forefront of aircraft maintenance. Many new products in the market are available to help deter corrosion and its effects. But only training in corrosion detection, identification, and repair will keep us ahead of this aircraft disease and prevent our aircraft from succumbing to death by corrosion.
This by far is not all the information on corrosion control. Many resources are available to increase your knowledge base on the subject; the FAA has AC 43.4A and AC43.13-1B which go in depth on the cause and repair of aircraft affected by this malady.
About the author: Eric M. Smyth is employed with the Department of Defense as a mechanic on Chinook helicopters. He is an A&P/IA and an advanced ground instructor with more than 20 years in aviation.
