Aircraft Corrosion Protection, III
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.
If done properly the aircraft can last a lifetime, or more. And its resale value will not corrode either should you decide to properly have the materials protected against corrosion.
There are several methods to help prevent this process and each has their own typical application area, read more below to get a basic idea.
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.
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.
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 too. 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.
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 Scotch Brite 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 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.
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.