Chromate Free Priming

I am looking to be "overprotective" when it comes to preventing corrosion but I also don't want to get involved with high power hazmat products containing chromium. So the search begins...

After looking through the forum I ran across a previous post talking about DuPont's Corlar 13570S, a chromate free 2 part epoxy based primer. Seems to me a huge benefit, not only will it seal the interior from moisture, dirt, and other corrosion inducers, but it would do so without the carcinogen issue.

So if a chromate free primer is used, what about pretreatment. I also came across a 2003 study on non-chromate aluminum pretreatments.

http://www.jgpp.com/projects/pretreatments/documents/phase_i_.pdf

Based on this study, I was wondering if anyone has tried Alodine 5700 or a similar chromate free as a pretreatment before using an epoxy primer?

Again I am just in the initial planning sessions so I haven't even decided if I am going to go the epoxy primer route. That being said, please, let's not make this a debate on whether to prime or to use self-etching primers. Let's try to stay focused on chromate free pretreatments and primers, their use and any information you may have on corrosion protection.

There is no good alternative to chromate coating. Chromate will protect against all kind of corrosion. The best protection you will get by epoxy with chromate.

However, on the outside skins, the most common corrosion is filliform corrosion. A primer with zinc phosphate an oxide beneith an epoxy or polyurethane will give good protection against that.

SvingenB said:

There is no good alternative to chromate coating.

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I humbly disagree. Several builders (including myself) have used SanChem products and have had excellent results. Boeing performed tests with SanChem vs Hinckle (Alodine) and has some impressive results.

Here is my process. http://www.flightinnovations.com/emp.html

SanChem Products http://www.sanchem.com/index.html

PreKote is another alternative

PreKote is advertised as a chromate-free alternative to chromate conversion coatings. Boeing has approved this product for some applications. I was intrigued by the product and tried some on a replacement rudder fabricated for my -4. It is very easy to use, and appears to offer good adhesion. Of course, only time will tell for my application. Here's a link to the website for more info:

http://www.pantheonchemical.com/PreKote.htm

Dean Pichon
RV-4
Bolton, MA

smithhb said:

I humbly disagree. Several builders (including myself) have used SanChem products and have had excellent results. Boeing performed tests with SanChem vs Hinckle (Alodine) and has some impressive results.

Here is my process. http://www.flightinnovations.com/emp.html

SanChem Products http://www.sanchem.com/index.html

Click to expand...

Seems to me the sanchem needs a sealer because it is porous. This makes it similar to a typical etch primer. A goood non chromated etch primer contains phosphates and oxid and has a corrosion resistance approaching or better that of alodine. The etch primer is allso a conversion coating, but it is essential for proper protection that it is covered with an epoxy primer, preferably a chromated one due to the porous nature of the etch primer.

Another thing to consider is mechanical toughness. An epoxy priimer is exceptionally toough. When messing with bucking bars inside the skins, my experience (small but still) is that etch primer can get scratced off, but the epoxy primer holds it without even a mark.