Aircraft are built from a variety of materials. There is one group of materials out of these, metal that is subject to deterioration due to corrosion. To a certain extent plastics may also deteriorate but are not subject to corrosion like metals. While wood and fabric suffer from the effects of aging and deterioration, fewer of these types of materials are used in modern aircraft.
Corrosion in metal is basically caused by the addition of water between two different metals; this is also a problem with alloys (which is basically several metals combined) if water is present. Corrosion is often difficult to detect but once the process begins the deterioration and degradation of the metallic materials can be rapid. An example of this is a Piper single engine aircraft that was inspected specifically for corrosion.
Twelve months later, at the next periodic inspection, severe corrosion was indicated. The difference between the two occasions was that prior to the corrosion inspection the aircraft was parked in a hangar and washed occasionally; after the inspection the aircraft was parked out in the weather and was washed constantly.
Metal once it is corroded is no longer the same alloy or compound it was made to be. It has little strength, becomes brittle and in some cases is soluble in water. Corrosion is therefore a major problem that must be removed and controlled, if possible. Corrosion needs to be detected and treated early, otherwise the only cure will be replacement of the affected part with new material. Like the old saying, as far as corrosion is concerned, “prevention is better than cure”.
The Corrosion Process
Most modern aircraft are manufactured from aluminum alloys. While this material is light and strong, it does have the drawback of being susceptible to fatigue and corrosion, particularly in a salt-water environment. Other alloys such as steel and magnesium are often used in the manufacture of aircraft components. These are also readily corroded in the presence of water. Water on these occasions may be as little as airborne particles on a humid day or as much as total immersion or a soaking by rain. Rain is not pure water but is in fact a solution of acidic compounds such as carbonic acid. In some areas of the world where pollution has become a problem rain can cover your aircraft with highly acidic solutions, all without you realizing it.
Oxidization
All metals in the presence of oxygen will react in some way; in some cases this reaction is quite violent. In the case of aluminum, the surface of the metal reacts with the oxygen in the air and forms a thin layer of aluminum oxide; this in fact prevents further contact between the metal and the oxygen in the air. Once this layer, which is soluble, is broken, more metal is oxidized. If the oxide layer is constantly broken or removed, then the oxidization process continues until the majority of metal is oxidized and no longer has any strength. Most metals after processing begin to corrode or oxidize in some way. They are inclined to turn back into the raw product they came from; i.e, iron turns into iron oxide, which is basically what iron ore consists of. Oxidization is a natural process but it can be stopped or slowed.
Galvanic Reactions
Whenever two dissimilar metals are near each other in the presence of an acidic liquid a reaction takes place. The reaction causes one of the metals to slowly disintegrate, while the other has its characteristics changed, in most cases What this means in an aircraft structure is that when two metals are separated by a liquid, such as water, there will be a reaction between the two, causing corrosion. For example, when a steel undercarriage component is bolted to the aluminum structure of an aircraft, there will be a corrosive chemical reaction. This is very prevalent where the engine mounts contact the firewall of an aircraft. Compounding the problem is the fact that most aircraft metals are alloys. Alloys will always have two dissimilar metals present. If through wear and tear or damage (even minor) an acidic substance is placed between these metals, a galvanic reaction will take place. This means that the internal structure of the aircraft metal itself will begin to corrode “from the inside out”. In most cases this internal reaction will be unobservable until it is too late. The only alternative then is to replace the component, which is a very expensive option. Any time corrosion is treated considerable time and expense will be incurred, so the best way to deal with it is to prevent it. If this is difficult, then early detection and treatment is necessary. This is a simple explanation of what corrosion is and what causes it.
Metal Coating Coating processes such as galvanizing, cadmium plating, chroming and anodizing are widely used through aircraft. Possibly the most common is aluminum cladding; processed metal such as ALCLAD is used throughout aircraft manufacture and repair. The process involves coating sheets of aluminum alloy with a thin layer of pure aluminum; this layer reacts with oxygen very quickly forming aluminum oxide, which is corrosion resistant. The major problems with all of these processes are that these are very expensive and once the coating is worn the core metal is subject to corrosion. The coatings are very difficult to repair and often the component requires complete replacement once the coating is worn off. One further problem with these processes is that during manufacture water can be trapped between the coating and the core metal, giving rise to corrosion right from the start
object correctly could cause major problems in the future. Remember paint is not applied to an aircraft just to look good but to protect the metallic components from corrosion. This means that the quality of the preparation, the undercoating and the type of painting process used are of vital importance. 
Preparation Before painting commences, the area to be protected must be prepared. This is the most important part of the painting process and must never be left out or condensed, Do not cut corners. Cutting corners may create future expensive problems. Preparation requires that the area be ‘stripped’ back to ‘bare’ metal. This area and the surrounding area should be inspected for corrosion, damage and cleanliness. All corrosion must be removed prior to painting; this may require extensive rework of the area. The area should then be treated with the appropriate preparation compound. Some metals require that all oxidization be removed while other metals require that oxidization be promoted. You should address the approved data to determine which is required. Once the area is prepared a primer should then be used to ‘seal’ the area. Some primers and undercoats will actually absorb and retain water; the data will give you the information you need. Always use the primer and undercoat, which is compatible with the metal surface and the paint you will be using. In cases where paint will not be applied special waterproof primer is used. This is usually olive green in color. When two surfaces are to be in contact with each other it is important to prepare and protect each surface prior to assembly. In some cases the atmosphere in the building must be controlled for the process to be effective. Painting and preparation must never be performed outside. To be effective, paint must be even over the entire area and must seal the area.
Other Coatings Application of grease, oil, wax, polish or any other temporary protection compound can be easy to apply and is relatively cheap. These compounds are all temporary and must be replaced at regular intervals or whenever the underlying metal is revealed
Some Simple Rules for Corrosion Prevention
- Regularly inspect the aircraft for corrosion.
- Remove and treat all corrosion no matter how small.
- Maintain the paint and other coatings in good condition.
- Keep the aircraft indoors, wherever possible.
- Repair paint other protective coatings as soon as possible.
- Use the correct washing compounds.
- Use the correct polish.
- Remove all water from areas where it may collect inside and outside of the aircraft.
- Prepare all areas in accordance with the approved data prior to application of permanent corrosion preventative coatings.
- If an aircraft is to be stored, corrosion inhibiting compounds should be applied and reapplied at regular intervals, these intervals are generally shorter in areas where there may be salt or pollution in the air.
- Stored aircraft should be inspected regularly for corrosion and then treated immediately.
