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Corrosion Protection Options for Sheet Metal Construction

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Jman

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Ok guys, I'm in need of some corrosion protection advice. I've read the recent thread Planecrazy J started concerning his Cessna 150 project and there is some good info in there. I'd like this thread to be about corrosion protection measures that should be used from the beginning of the build. Here is my situation.

I'm building the CH-801 which is built from primarily 6061 T6 aluminum sheet. It will be used by an aid / mission organization presumably in tough conditions. I don't know where it will be based yet but I'm going with the worse case scenario of a jungle environment within 5 miles of the ocean.

Here are some of the common methods I've seen being used by others:
1. Napa self etching rattle can primer over bare aluminum.
2. Alumiprep -> Alodine -> Zinc Chromate Primer
3. 2 part Epoxy Primer over bare aluminum.

I'm leaning toward number two but I'm open to suggestions. A one step, easy application, easy clean-up solution would really be nice, but not at the expense of the corrosion protection performance. The self etching primer is attractive but as Wally mentioned in the other thread, it needs to be top coated to truly provide protection.

Has anyone used or heard anything about the Stewart System Primer Sealer? It has the advantage of being a single component system but they claim as good protection as two part epoxy primers. Seems pretty expensive though.

So I guess what I'm asking for is informed advice about which method of corrosion protection is going to work in a salt environment yet be easy to apply. Being able to forgo the Acid Etch and alodine would be nice, but if they truly add benefit I'm more than willing to use them.

Thanks in advance.
 

bmcj

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Hi Jake,

Primer is called "primer" because it primes the surface for painting. By itself, it does provide some level of protection but it is pourous by nature and will still allow penetration by corrosive elementss over time. I would suggest careful preparation plus an epoxy topcot for a better seall. Pay particular attention to the joints as they are the areas that trap moisture and also the hardest areas to get paint into when sprayed. Just make sure you do not add so much paint that you impact your payload or W&B.

Bruce :)
 

PTAirco

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Even with epoxy primer, you cannot forgo the etching step. It will adhere just as badly as any primer without removing the oxide layer first. I agree it is a bit of a nuisance having to do this, but if you want to see just what a difference it makes, make a few test panels, using different processes. You will see how easily any paint will flake of un-etched aluminium.

With sheet metal airplanes it's usually a case of drilling on assembly, then taking everything apart, deburring, cleaing/etching, priming and then final assembly. It is very tempting to rush ahead and put the stuff together immediately - don't do it. I usually build up a good pile of parts, until it's worth getting the spray gun out, clean a big batch at a time and prime it. In the long run it adds only a littel time to the project.

With small steel parts, I have taken to dipping them in enamel, rather than spraying them. I have dozens of small hooks made from welding rod, with rows of nails in a shelf edge to suspend the parts. I wirebrush welded parts, dip them in metal prep for a while, rinse, dry with a heatgun and while they are still warm dip them into the enamel. Let the excess drip off and hang them up to dry. If you thin the enamel just a little bit, the excess runs off nicely and the finish looks almost powder coated. far quicker and more thorough than spraying them individually and the paint gets into every nook and cranny.
 

BBerson

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PT is is correct. The phosphoric acid scrub is a cheap easy way to remove mill oil and finger prints etc. and get a good bond no matter what kind of paint is used.

I owned an airplane paint shop. We always used etch to get the bare metal clean. The phosphoric acid etch works well to clean old paint also (for repainting).
BB
 

Jman

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Thanks for the comments guys, it's much appreciated. If you use a two part epoxy primer, is a top coat still recommended? I would think so. The Primer/Sealer from Stewart systems looks promising. It's a primer but seals like a top coat as well.

Which phosphoric acid do you recommend? Alumiprep seems to be easy to obtain. Is there a cheaper alternative? What do you use to wipe the etch onto the parts, a sponge or rag? Or do you spray it on with a squirt bottle? Or is it best to bath it in a plastic container?

Please keep the comments and experiences coming. Thanks!
 

BBerson

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We used the phosphoric acid etch from the local paint dealer and sprayed it on with a weed sprayer. But that was doing an entire airplane. A small panel can be done just dipping a scotchbrite pad in a bucket. The srubbing is key. Keep scrubbing until the water lays flat, this flat water indicates an oil free surface.
BB
 

PTAirco

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Enamel-dipped steel parts;

Back in the 30, when the British were very much into high strength steel structures, all their finished parts, spars and all, were put through an enamel bath and the finish baked on. You could have any colour as long as it was black.
 

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bmcj

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Enamel-dipped steel parts;

Back in the 30, when the British were very much into high strength steel structures, all their finished parts, spars and all, were put through an enamel bath and the finish baked on. You could have any colour as long as it was black.
Hey PT... better get your vision rechecked for color blindness. Those pieces don't look anywhere close to black. :gig:

Bruce :ban:
 

handprop

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Nice looking parts PT!

Are there ever concerns about thickness of the finish when riveting parts together that might cause structural issues? Just wondering, the wings for my project are going to be aluminum so iv'e been studying everything I can about proper methods but haven't come accross that yet.

Mike
 

Jman

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Well...I've been doing some thinking and looking over what you guys have written and think I'm going to give the following method a try:

Alumiprep (or equivalent from the paint store) -> Alodine -> Stewart Systems Primer / Sealer.

I'd be interested to hear any reasons for or against this combination as well as any alternatives. I really like how non-toxic the stuff is. I haven't asked the Stewart guys if they recommend alodine or not. I'll report back what they say. Thanks guys.

Jake
 

Jman

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I spoke with Dan out at Stewart Systems about their Primer / Sealer product. Really nice guy. He was out spraying an aircraft when I called.

My question to him was about prepping the material for the primer. He strongly recommends Etching then rinsing prior to applying the primer. I asked about alodine and he does not recommend it. He says it's a messy step that does not add any real benefit over what the primer / sealer will provide.

Dan also mentioned that they just finished a 2 hour DVD on using their products. You can get a free copy by going to their website and asking for a copy via email.

The non-toxic nature of the product is pretty important concidering my kids are going to be a big part of the project. I think I'll give this system a try.
 

planecrazy J

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Glad for your continuation of corrosion treatment, Jake. I need the indepth knowledge on this issue. Like I said aviation supplies are hard to aqquire here and pretty expensive so I,ve to settle for the affordable and easily accessible.BB, Wally and the other guys are allways ready to share their experienced and knowledge and I,m never out of questions.
After cleaning away oxidation with hardware store grade phosphoric acid the next step is alodyne which I haven,t been able to source.
1. Could the surface be roughened(etched) with a wire brush or sandpaper to prepare for primer or alodyne is must?
2 . After brushing away the paint and white powdery stuff under it what should I do with the riveted joints to ensure corrosion does not remain there?
3.What can be done to protect hard to reach areas such as the inside of elevators, tail, flaps etc from corrosion?
The surface of these parts were affected with peeling of paint and a the white powery layer.
4.Does this layer amounts to loss of substancial strength of the sheet and how much "thou" of an inch?
Glad for some answers fellas . bear with me BB and Wally.
 

helomech

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Hi Jake:
I'm new to posting on the forum, although I've been a reader for years. thought I'd throw in my 2 cents, as I do a good deal of corrosion control work on army aircraft, as well as the repairing of damage related to the effects of corrosion.
first, as both PTAirco and bmcj began to allude to in their previous postings, the truly corrosion resistant aircraft begins in its fabrication and assembly stage. Corrosion on wide areas of skin and accessible areas on structural members usually isn't an issue, and when it does occur can always be remedied relatively easily if detected during routine inspection. It's the corrosion that forms in entrapment areas and between faying surfaces, where corrosion effects are hidden, that's truly the enemy. Design (in this case, outside of your control), fabrication (partially within your control), and final fit and assembly (100% up to you), will have a far greater effect on the aircraft's corrosion resistance then all the primers and sealants that you care to dope on. MAKE SURE YOUR PARTS FIT!
As an inspector, I'm an absolute fanatic about riveting technique and rivet hole preparation. Improperly prepared holes lead to improperly driven rivets, which in turn allow for the possibility of contaminant intrusion, as well as corrosion and damage related to stress risers and fretting. ALL faying surfaces, unless it is necessary to provide an electrical ground, should be coated with a catalyst curing sealing compound prior to assembly, and all rivets driven through an exterior surface should also be coated prior to installation. I recommend the use of a sealant conforming to mil spec PRF-81733, with a consistency of "A". Rivets used on interior surfaces should be installed "wet" with primer.
I could go on and on, but don't want to get into a rant. But I can't emphasize enough that proper assembly will go a looong way toward corrosion proofing your airplane, regardless of what brand or type of surface coating you use. I do have to disagree with your primer dealer about the need to do a conversion on your bare metal prior to priming. Not only is it my experience that it enhances both the corrosion resistance and the ability of the surface to "hold" primers and paints, but it also an "acceptable practice" IAW the FAAs advisory circulars on aircraft inspection and repair. By the way, both AC 43.13-1B (aircraft inspection and repair) and AC 65-15A (A&P mechanics airframe handbook) are available free on-line from the FAA website, good general info for everyone. Chapter six of AC 43.13 is entirely devoted to corrosion control.
Jake, if I remember right you are an army pilot, so you also have a wealth of info available to you through your friendly QC section. Have the guys there hook you up with the tm 1-1500-344 series (aircraft cleaning and corrosion control) and the tm 1-1500-204 series (general aircraft maintenance). Also, a visit to your Hazmat person should allow you to familiarize yourself with the common materials used during repair of the aircraft assigned to your unit.
I wish you luck on your project, I'm hoping to get a chance to discuss a few particulars with you related to your specific project.

John
 

BBerson

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The information John provided is all good but keep in mind the military has a much higher standard and cost is not an issue. Most Cessnas have NO interior paint or faying surface protection. The seaplane option that includes faying surface paint adds much cost.

As far as how much corrosion is acceptable.... a difficult question... almost all Cessnas have surface corrosion. Try looking for the FAA advisory circular about corrosion.
BB
 

wally

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What heliomech said will for sure provide excellent corrosion protection.

The Gulfstream V airplanes I used to see being built have a multi-step finish on all structures concluding with a two part primer and on assembly all faying surfaces are coated with a Proseal sealant (put on with a roller and cures in 8 hours) using fasteners that are installed and driven wet with primer. This prevents moisture from getting between any pieces which can lead to corrosion. They do this so they can offer an airframe with I believe a lifetime corrosion warrantee.
Wally
 

BBerson

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The amount of protection needed depends on the alloy and environment. The 6061 alloy Jake is using probably doesn't need anything in a dry climate. I usually paint the 2024 and 7075 spar parts but not the skins.
Another option is the spraying of ACF-50 on an annual or 2 year schedule. This is common here in the coastal air.
That Proseal on the rivets is messy.
BB
 

Jman

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John, Wally, and BB,

Excellent comments, thank you for taking the time to share your experience. The admonition to stay in the books is right on, and I'm doing so as we speak. I did looked at the AC 43.13-1b prior to posting on the subject but got the impression it was geared more toward methods of identifying and repairing the corrosion, not for the initial application. I'll take a look at the other military pubs you referenced as well.

I took a look at the MSDS for alodine. Nasty stuff and, if I do end up using it, I won't let the little ones work around it.

Has anyone here used the "after the fact" corrosion protection that Wally mentioned? I was considering either ACF-50 or Corrosion-X on an annual basis as an insurance plan if the aircraft ends up based in a very tough environment. I have heard of folks using it alone with no other corrosion protection measures. I've not heard the long term effects however. Sure would make life easy though.
 

Jman

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2 . After brushing away the paint and white powdery stuff under it what should I do with the riveted joints to ensure corrosion does not remain there?
Here is what the AC 43.13-1B Chapter 6-135a says: "When corrosion is found around a fixed fastener head, the fastener must be removed to ensure corrosion removal. All corrosion must be removed to prevent further corrosion and loss of structural strength. To reduce the recurrence of corrosion, the panel should receive a chemical conversion coating, be primed, and have the fasteners installed wet with sealant."
 
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