BobsV35B(at)aol.com
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 Posted: Mon Apr 01, 2013 11:28 am Post subject: Corrosion prevention was Blind riveting ground to ai... |
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Good Afternoon Ed,
Thanks for the message!
Obviously, I was thinking Alodine and wrote Anodize!
Alumaprep and Alodine is just what I use. And that is only on parts BEFORE they are assembled.
I also have the high pressure, fine mist, fogger recommended by Corrosion X.
Gotta keep those molecules from heading back to dust!
Happy Skies,
Old Bob
In a message dated 4/1/2013 12:36:20 P.M. Central Daylight Time, bicyclop(at)pacbell.net writes:
| Quote: | Howdy Bob,
I think you're referring to alodine which is a conversion coating. The proper (by the book) way to prep aluminum for paint is to wet scrub it with alumaprep and scotchbrite, rinse thoroughly, and soak it in alodine until it starts to get that golden color and rinse again. Then it should be primed as soon as it is completely dry. The alumaprep cleans the surface. The alodine corrodes the surface layer (a few atoms deep) so that a barrier to further corrosion is created and provides a bit of "tooth" for the primer to follow. A problem with this process on completed structures is the difficulty of getting the phosphoric acid based alumaprep and alodine rinsed out from between riveted structures and every little nook and cranny. It especially tends to rear its ugly head on repaints and will later show up as filoform corrosion under the paint around rivet heads and lap joints.
I did it this way for much of my early build and used the solvent base Akzo epoxy primer that you can't get anymore. It would be an interesting experiment to compare the areas that I did that way to the stuff that I've just wiped with acetone and sprayed with self etching primer in twenty or thirty years.
Alodined aluminum without primer will probably perform about like alclad since both processes work by creating a very thin layer of aluminum oxide on the surface as a barrier to more damaging corrosive processes. As a prep process for paint, it used to be the only real way to get primers to grab onto aluminum. This was before epoxy primers. The instructions for these are now mostly about cleaning and sanding the surface before application. Completely cured epoxy primer is extremely tough and very difficult to remove which makes it a good long term undercoating for paint. By the way, the wash primer that Van's sprays quickbuild kits with is a form of self etching primer and is a lousy moisture barrier. It is intended to be painted over within hours, not left to stand on it's own.
Corrosion only occurs in the presence of an electrolyte, usually moisture contaminated with dirt, dust, and whatever airborne acids are around. The whole purpose of any of the available treatments to prevent or stop the spread of corrosion is to exclude air and the moisture it contains from the vulnerable metal. The barrier can be a few molecules thick or a nice coat of paint over well adhered primer. The latter is obviously more robust and less susceptible to tiny breaches. Another approach is to spray the interior with CorrosionX or equivalent every few years (after painting the aircraft please - it weeps from between joints for a long time).
Between joints is the hardest area to protect and that's why I recommend that if you're going to prime any aluminum, faying surfaces and extrusions are the best places to spend your energy. All steel, except for stainless, must be primed and painted in any case. Powder coating works well on steel, too.
Pax,
Ed Holyoke
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