Aircraft Corrosion Protection, II
Aluminum corrodes almost immediately forming an oxide layer that will protect it from further corrosion. For example: Place an aircraft in a salty environment near the ocean and the unprotected aluminum corrodes to a white powder if not handled properly.
There are a number of ways one can protect an aircraft against corrosion: painting, waxing, zinc-chromate priming, thin layer of pure aluminum (Alclad), anodizing and coating with a liquid protective solution.
Due to its properties, lightweight, strength and corrosion resistance aluminum is the metal of choice for aircraft. But it needs proper handling because even aluminum can not escape from corrosion. And will corrode very quickly after cleaning so any treatment must be swift to prevent corrosion becoming trapped under the base paint.
The excellent corrosion resistance properties of aluminum are due to the fact that it corrodes (reacts with oxygen molecules) very quickly. Aluminum forms a very thin film of aluminum oxide (Al2O3) which is about 2 nm (nanometer or 10-9) thick. And it has a very strong bond to its surface and, if damaged or scratched, reforms immediately in most environments.
This property is used when anodizing aluminum and this is very durable over the years, even near salty environments near oceans and such.
But this quick oxidation process is making it difficult to treat it properly before painting. After this thin oxide film has formed the corrosion process stops, also known as passivation.
Pure aluminum is mixed with small quantities of copper, zinc, manganese, silicon, or magnesium creating an alloy, see aluminum alloys for more info. Alcladding (which is a thin layer of pure aluminum on the alloy) is done as an anti corrosion measure on some of these alloys.
Sheets of 2024-T3 Alclad are a good example of this.
Types of corrosion
Corrosion is the disintegration of a certain material into its constituent atoms due to the effect of reactions with its surroundings by, among other things, air and moisture:
- Galvanic, where two dissimilar metals are in contact combined with an electrolyte
- Electrochemical, chemical reactions with air containing moisture and salt (sea, oceans) will contribute to corrosion
- Microbial, caused by micro organisms in the presence or lack of oxygen
- Temperature, a non galvanic form of corrosion under high temperatures combined with high corrosive byproducts of combustion (engines)
The above reactions will cause the aluminum to degrade in time if not protected properly.
Effects of corrosion
Corrosion of aluminum is usually localized: e.g. at the edge of a sheet or near rivets and can be seen by the formation of random pits and eventually a crack. If the sheet is painted you will see the layer of paint coming loose from the aluminum, it is easily peeled away with a nail.
When you see these indications its time to take serious action and stop the process.
The result is a loss of electrons from the metal and a weakening of the structure will take place. You will have to stop corrosion before it is too late and the aircraft becomes a safety factor and is considered a loss for the owner.
If this happens the paint must be removed form the complete surface possibly sandblasted and retreated before any paint can be applied again. Only then can you be sure that corrosion will not return any time soon.
Our next page discusses ways of anti corrosion treatment.