Corrosion is the deterioration of a metal as a result of chemical reactions between it and the surrounding environment. Both the type of metal and the environmental conditions, particularly gasses that are in contact with the metal, determine the form and rate of deterioration. All metals can corrode. Some, like pure iron, corrode quickly. Stainless steel, however, which combines iron and other alloys, is slower to corrode and is therefore more resistant to corrosion. But it in the end, everything corrodes.
Aluminium corrosion resistance is very good in untreated aluminium. Untreated aluminium has very good corrosion resistance in most environments. This is primarily because aluminium spontaneously forms a thin but effective oxide layer that prevents further oxidation.
It is the exposure of bare metal to oxygen and many other elements that cause corrosion so that means it happens all the time and everywhere. However, some conditions are more hostile then others on your plane metal parts. If you live in San Francisco where the moisture is high all the time and you live near the beach where salty conditions are high, it is more recommendable to prime then in other dry locations.
The aluminum skins used on RV aircraft are all 2024-T3 alloy. They are “alclad”, meaning that both sides of the sheet are coated at the mill with pure aluminum.
This forms aluminum oxide, a corrosion resistant material whichneed not be primed or painted for adequate service as an airframe material. However, if the airplane is to be kept in a salt- air environment, or if a greater margin of corrosion protection is desired, priming the entire inside of the airframe is a good idea.
Priming will add cost, weight and time to your project. If you decide to go with a polished alumnium outside, then priming is not possible off course but nothing prevents you from priming the inside.
The main bar aluminum used in the RV is 2024-T4 alloy, and is not corrosion treated.
Pre assembled spars are gold anodized and need no additional protection. Additional priming is not required for these gold looking parts.
The other bar, angle and tube used is 6061-T6 alloy and also is not corrosion treated.
All aluminum bar, angle, and tube used in the RVs, especially the main spar bars, must be primed or anodized to assure corrosion resistance.
I started my project with a zinc chromate primer from Spiess Hecker (Permafleet® 1:1 Korrosionsschutz Wash Primer 4130 based on polyvinylbutyral) which is an etching primer.
Applying the zinc chromate is done with a HVLP paint spray gun and is a very dangerous business.
Zinc chromate primer is using Chromate 6 as main component and this is one of the most serious cancer causing elements on the planet.
In dry form it's harmless, but if you spray paint it or scuff a surface and inhale the powder, you are playing with your life.
I used zinc chromate on the empennage kit and the skeleton of the wings.
In the end, it scared the living hell out of me and I decided to change system. The conditions in my place are not that hostile and I decided that the risk was just not worth it.
After a long time of looking and weighing of pro's and cons, I decided to go with a mechanical primer from Akzo Nobel. I heared that this is also the primer they use on the new airbusses so I guess it will also be good enough for my plane :)
The type is Epoxy primer CF 37112. Epoxy Primer CF 37112 is a 3-component chromate free epoxy primer and adheres to steel and aluminum.
The process of applying is very similar. You mix the exact quantities of paint , hardner and thinner to activate the product and apply thin layers of paint on the aluminum.
The advantages of the zinc chromate primer were that it actually etches itself in the material. Small scratches in the material will not cause corrosion as the protection is deep within.
The mechanical primer is as the word says a mechanical layer or a cover over the material. An scratch in the primer exposes the material again and will start corroding.
