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 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.
Corrosion resistance
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 (about 2 nm of aluminum oxide, Al2O3) that has a very strong bond to its surface and, if damaged, re-forms immediately in most environments. 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 (thin layer of pure aluminum) 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:
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Effects of corrosion
Corrosion of aluminum is usually localized, at the edge of a sheet or near rivets and can be seen by the formation of random pits and eventually a crack. There will be a loss of electrons from the metal and a weakening of the structure will take place.
Corrosion Treatments
If left untouched, corrosion will attack the aluminum and a possible failure of the structure could result. Proper treatment will extend the life of the aircraft and keep its market value high, important when you plan to resell it.
Surface layers
The most common form of corrosion protection is applying a surface layer or coating, either in the form of paint or plating (think of chroming or alclad). This forms an barrier of corrosion resistant material which is easy to apply and cheaper to replace than the structure itself. During the build phase of an aircraft, zinc-chromate primer (the yellowish green stuff) is heavily used as protection and as a base for subsequent coats of paint.
Before any treatment with zinc-chromate or other primers the parts will need to be cleaned of ink, glue residue and or grease. Read text below on how to do that.
Electrolytic
Anodizing is an electrolytic passivation process (much like the normal oxidation of aluminum) and it thickens and strengthens the oxide layer and increases the wear resistance, it also provides a better surface for paint to adhere to. The oxide layer can be colored on request for cosmetic purposes and anodized parts are non conductive.
The process usually uses a direct current in an electrolytic solution so that a coating is build up with pores of 10 to 150 nm which then oxidize and can be colored with a dye. The final oxide layer thickness is around 0,5 um to 150 um and will need to be sealed.
Chemical
Alodine is a chemical treatment of aluminum to prevent corrosion, it is electrically conductive and can be used as a primer for layers of paint. It does not add weight or changes dimensions of the part that is being treated, where as paint does. It is easy to apply and needs no electricity (anodizing does) to treat the parts.
Parts may not be assembled (like with anodizing) and you will need a tank that can hold the biggest part in your kit to properly alodine. Keep temperatures above 21°C (70°F) for good results. Disposal of left over chemicals may be difficult.
Before alodining the parts will need to be cleaned with acetone to remove all ink and or glue or sticky stuff. Then use scotchbrite to remove the oxide layer and use a good dishwash detergent (dawn) to clean any remaining grease of the parts. If this has been done properly water will form an unbroken film, if it does not, it must be cleaned again to remove the grease. Then dry and immerse the parts in acid for three minutes, acid rinse and dip in the alodine tank for another three minutes. Then rinse and let the alodined parts settle for at least 24 hours for maximum strength.
Alumetron
Alumetron is a clear polymer coating that bonds to aluminum, its a water based chemical/mechanical process and the result is that treated parts are suitable for constant water immersion. Application is easy through either wipe with clean cloths or spray with a HVLP gun with fine tip. You will need to clean and degrease the parts before applying Alumetron.
Oil coatings
The inside of wings, tail sections or the fuselage can be treated by mineral or synthetic oil based solutions with a waxy compound, either dipped or sprayed. Capillary action makes sure that the solution reaches every corner and section. Will need to be reapplied every couple of years depending on the location of the aircraft. Read more about Boeings Boeshield T-9.
