There are very few areas in which corrosion can have as catastrophic consequences as it can in aircraft production and maintenance. While a large number of aircraft accidents and incidents are due to human error, a significant number of them are caused by various structural defects, including the ones that have corrosion damage as one of the failure mechanisms.
Accidents caused by corrosion alone are very few in numbers, but corrosion can accelerate structural failure when combined with other damage mechanisms. (Some implications are presented in the article Effect of Corrosion on a Material's Tensile Strength and Ductility.) In particular, corrosion fatigue and stress corrosion cracking (SCC) have caused a number of aircraft failures over the years.
Furthermore, the changing economic landscape has put pressure on both military and civilian aircraft fleets, resulting in a far longer service life for all airplanes than ever before. The longer the aircraft is exposed to the elements and subjected to standard wear-and-tear, the more susceptible it becomes to corrosion and potential critical failure.
All of this places greater emphasis on proper construction, maintenance and corrosion prevention in aircraft.
Occurrences of Uniform Corrosion in Aircraft
Uniform corrosion is a type of electrochemical corrosion in which the attacked material corrodes more or less evenly over its entire face or surface. Even though it typically occurs when the exposed material is in contact with oxygen, it is not to be confused with oxidation, as it still requires the presence of an electrolyte to occur.
In order for electrochemical corrosion to occur there must be a difference in electric potential between two materials in the presence of an electrolyte. In the case of aircraft construction, this can occur when two dissimilar metals are joined, or where material is exposed to the environment and fluids. Where materials are joined by welding, the galvanic difference between different weld areas can cause more localized corrosion. (Be sure to read An Overview of Welded Joint Corrosion: Causes and Prevention Practices for more on this topic.)
Mechanisms That Cause Uniform Corrosion in Aircraft
Since uniform corrosion attack occurs over the entire surface, it can cause material to change color in the affected areas (in the case of iron and non-stainless steels, it creates noticeable red rust) and the surface can become rough and lose its metallic sheen.
By itself, uniform corrosion isn’t too dangerous. It is easy to spot, the deterioration rate is slow at the beginning, it is fairly easy to treat, and aerospace materials are not prone to it as a rule. However, if left untreated, it can lead to a significant loss of material mass and it can lead to other, more dangerous, types of corrosion.
In aircraft, uniform corrosion typically occurs in areas where a protective coating that was applied has been damaged or completely destroyed. While the first association would be the outside of the plane, this damage mechanism is actually often observed on cadmium plated steels found in combustion chambers where the anodic coating has completely worn down.
A fast flowing fluid can further augment the potential for damage by causing erosion. This can be a problem in hot, desert or tropical climates, where sand, salt and other particulate content in the air acts as an extremely abrasive medium. These particles, which the aircraft encounters at high speeds, can scrape the coating off exposed surface with relative ease.
Exposed outside surfaces on an aircraft are typically in contact with water moisture caused by condensation, which serves as an electrolyte. However, other fluids stored in the aircraft itself can also be a potential source of corrosion. The British European Airways Flight 706 crash that killed all 63 passengers and crew in 1971 was caused by corrosion of the lower part of the rear pressure bulkhead. It is believed that fluid contamination, possibly from a lavatory, was the cause.
Another possible corrosive agent is the different types of fire suppressants that are stored on board the aircraft. While this is extremely unlikely to occur, it has been shown that sodium bicarbonate, or any other suppressant that produces deposits of bicarbonate, carbonate, or sodium hydroxide can cause corrosion in aluminum alloys that are typically used in aircraft construction.
Luckily, these issues are unlikely to occur in modern planes due to updates in regulation, but it is still a good practice to bear this in mind when designing, servicing and inspecting aircraft.
Uniform corrosion can also appear in the electrical and avionics equipment of aging aircraft, as these parts are typically optimized more for performance than corrosion resistance.
Uniform corrosion is one of the less dangerous types that can occur in aircraft, but if left unattended or undetected it can severely reduce the cross-section of the corroded part and thus greatly impact structural integrity. Furthermore, if this type of corrosion occurs in the joint sections, rust particles can impede the movement of the joint in question, and cause bulging or separation of the joint - all of which can have a very negative effect on the aircraft performance and safety.
Maintenance and Prevention of Uniform Corrosion
In order for electrochemical corrosion to occur there must be a difference in electric potential: dissimilar materials that are in contact, areas of the same material that have slightly different composition, and different concentrations of an electrolyte. In all cases, one area acts as an anode and the other as a cathode, but the presence of an electrolyte is also necessary to cause corrosion.
In aircraft construction, eliminating and limiting these factors is in large part constrained by other prerequisites, both economic and performance-based. Since these issues can’t always be easily eliminated, the threat of corrosion can be greatly minimized by protecting the material's surface, vulnerable joints and connections.
Surface treatments, such as galvanizing, can be used, as well as coating with protective paint. If weight and situation allows (or requires) it, a protective plating can also be included. Since water and humidity are unavoidable, sealants, drain paths, drain holes and corrosion inhibiting paths must be created to mitigate the negative effects as much as possible.
Aluminum Alloys and Cladding
Due to their excellent strength-to-weight ratio, high-strength aluminum alloys are predominantly used to construct airframes, but a thin layer of pure aluminum is often applied as a coating to improve the anti-corrosive properties of these alloys.
For the same reason, clad aluminum is used for fuselage skin where functionality and weight allow for it. Even so, development of better alloys has led to the replacement of older materials that had poorer corrosion resistance. For this reason, 7150-T651 aluminum plates on upper wings have been replaced by 7055-T7751 plates in Boeing aircraft.
Other Corrosion Prevention Methods
Smaller aluminum components can also be anodized in order to increase the thickness of the protective aluminum-oxide film. When these options are unavailable, or the material doesn’t show good natural anti-corrosive properties, several other methods can be used:
- Chemical conversion coatings
Also known as chemical films and chromate conversion coatings, these compounds chemically react with metallic surfaces to create a corrosion resistant film. Different types are used for different metals, and these coatings also increase the adhesion of other paints and coatings that are applied afterwards.
The main purpose of sealants in aircraft is to isolate pressurized areas and prevent water, moisture, dust, salt and aircraft fluids from intruding into areas where they can cause corrosion. Sealants are applied around the windows, in fuel tanks, firewalls and joints that are exposed to the environment.
Two types of sealants are used in aircraft, poly-sealants, and silicone sealants. Polysulfide, polythioether, and polyurethane are all two-component compounds where mixing the base and a curing agent creates a rubbery solid. Silicone sealants have only one component, and they solidify when exposed to moisture in the air.
Aircraft paint is also a two-component coating, where both coats can have several layers applied. A primer coat serves to increase adhesion and inhibit corrosion, and a top coat increases the paint's durability, notably increasing weather and chemical resistance. (For more information about coatings, see Aviation Coatings for Corrosion Prevention.)
Corrosion is a significant problem when it comes to aircraft safety and functionality, but by itself uniform corrosion is not a huge threat. However, proper, timely, and professional inspection and maintenance is obligatory even for such an obvious and simple damage mechanism. In addition, regularly re-applying and controlling coatings and other anti-corrosive measures can greatly increase an aircraft's service lifespan.