As with everything related to building an airplane, there are many hotly debated topics… one of the hottest is what to do about corrosion protection.  Start a thread on any of the message boards regarding paint,  alodine, or whatever else you might come up with and you are guaranteed to have a topic that gets a life of its own.  Those of us building aluminum airplanes tend to discuss it frequently and there are usually a few different points of view on the topic:

1) It’s al-clad aluminum and therefore you probably don’t need corrosion protection (just slap it together and be done with it)

2) You need to clean, scrub, and alodine every part, then prime it

3) Use an etching primer

I think the first option is really only feasible if you live in Arizona or somewhere arid.  I live in the SF bay area, so I am near a large body of salt water and we have a lot of fog and rain.  As such, I don’t think I can skip the corrosion protection piece.  Also, I live in California, which means that etching primers, alodine (hexavalent chromium), and most automotive / aircraft paints are both difficult to acquire and impossible to dispose of here.  Which brings us to my criteria for corrosion protection:

1) Non-hazmat

2) Does not require excessive amount of time / energy to apply

3) Effective

4) Plays well with topcoats.

Impossible, right?  Well not quite.  There is a company out of Chicago called Sanchem that makes several non-toxic variations of a permanganate conversion coating for aluminum.  The first product is their 3400 line and it is a standard heated tank/dip type application similar to what one does with anodizing or alodining.  The second is their 6100 line (which is what I am using) and is cold applied.  The third is the 7000 line, which is the same product as the 6100, but in a weaker solution (I don’t know why they have this, but whatever.) Sanchem primarily sells to large corporate accounts, but they do sell their products in 1 gallon bottles so you don’t have to buy 55 gallon drums.  I have been using them since the start of my project and am very happy with the results so far.  Not only is the product non-toxic, but application, clean-up, and disposal could not be easier.  They also make several seals to go on top of the conversion coating for those areas that you do not plan to paint.

The 6100 product comes in three parts, 6100 A, 6100 B, and 6100 C.  To apply it you first rinse the aluminum with DI or RO water (distilled or reverse osmosis.  Some people have reported that tap water works, but water is cheap so I’ll stick with the known clean stuff), then spray or brush on the 6100C.  I use a household spray bottle that you can get on the plant aisle at any hardware store.  6100C is a mild acid and, using a scotchbrite pad, you scrub the aluminum to a break-free finish (well that is what it says in the instructions… I have no idea what “break-free” is supposed to mean in this context.)  I just scrub it until all of the pen marks, dirt, ink from the manufacturing process, and any other stuff comes off of it.  Usually it will have a brushed look from the scotchbrite scratching the aluminum.  The next step is to rinse it again with the DI water and then dry it.  I’ve found that if the piece is a little wet it doesn’t really affect the next step.  You also want to try to do the next step before the piece has a chance to build up an oxidation layer on it, although I’ve gone four  to five hours between the two steps and the next step still came out well.

The second step uses a 1:1 mixture of 6100A and 6100B.  6100A is a very dark purple color and 6100B is clear.  Once you mix the two you apply the liquid to the piece via whatever method you like.  I use foam brushes to apply it, you don’t need much and it just needs to go on evenly.  If the aluminum is cleaned properly you will see it start to change color within a minute or two.  It will turn a gold or light brown color and should not take more than 5 minutes to complete.  Incidentally, if you have some of the 6100 mixed up and it’s brown then throw it out… it’s gotten old.  One of the things I agonized over at first is that it does not change color evenly.  You will typically see some really dark spots and some lighter spots.  The important thing is just that it is no longer silver, but has a gold tint to it.  The conversion coating is only a couple of molecules thick, so as long as the color change is there you are good.  Sometimes there will be spots that you didn’t scrub well enough and there won’t be a color change.  That’s also ok, it just means more work.  For those spots, hit them again with the 6100C and scotchbrite, and then re-apply the A+B mixture until you get the color change.  Also, I’ve noticed that in my shop it is next to impossible to see the color change under the fluorescent lights that I have, so try to use some incandescent light or daylight.  Once you are happy with it, rinse it again and let it dry.  If you are planning to paint or prime this part, then you are done with it.  Sanchem says that you don’t have to paint immediately and that once the conversion layer is on you have excellent corrosion protection, so you really don’t have to paint at all.  To get the maximum protection, however, you will need to either prime or use Sanchem’s 3400 Seal #2.

The third step is to apply the 3400 Seal #2.  Again, I usually apply this with a foam brush.  It has ammonia in it, so it smells a bit like cat pee, but this is the only smelly bit in the whole process, and it’s not very strong.  You only need a very thin coat of this on the part, as long as it looks wet you are good.  Too much makes it a little bubbly and doesn’t give a nice smooth finish.  Once the thin coat is applied, you then MUST hit it with a heat gun.   It only needs about 200 degrees, so a hair dryer is sufficient if you don’t have a heat gun.   As you hit it with the heat gun you will see milky streaks and bubbles appear in it.  Keep the heat on it until it is all perfectly clear and let it sit for a few minutes.  The heat is what bonds the seal.  When you are done you will have a piece of aluminum with a gold tint that looks and feels like it is coated in plastic.

Finally, to dispose of the excess chemicals, you neutralize the A+B mixture with hydrogen peroxide (just takes the color out of it) and rinse it down the sink.  The 3400 seal #2 and 6100C can both be disposed of by just putting it down the drain as well.  These procedures are what Sanchem recommends.

I am using this process on all the internal parts of my RV-10.  I am about 2/3 of the way through the tail kit at this point and I have used approximately 1/2 gallon of each of the four parts.  Each part costs around $40/gallon and goes a long way.  Also, since it is a conversion coating and a very thin coat of the seal, it weighs quite a bit less than priming.  I’ve left the outer skins bare aluminum as I’ll let the paint shop deal with that part when I get to painting.