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Corrosion Protection Options for Sheet Metal Construction

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helomech

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Hi folks:
I'd like to readdress Jake's original question, how to reduce the likelyhood of future corrosion problems during assembly of a new aircraft. The conditions under which he envisions the plane will operate makes it a matter of "how bad will the problem be?", as opposed to "will there be a problem?". The fact is, as a support vehicle for missionary work, this plane will most likely be rode hard and put away wet (literally), and maintained on a shoestring budget. Lots different than your "hunnert dollar hamburger" crate. I've often read posts in this forum advising beginners to carefully assess their requirements before settling on an aircraft type or build method, and I really believe this is a good case-in-point.

So I stick by my initial advice, exercise meticulous care during fabrication and assembly; avoid tool marks and other mechanical damage; and seal EVERY joint, fitting, faying surface, and fasteners that penetrate the exterior surface. Prime all other fasteners, including screws and bolts, that secure primary structure. Jake, you asked about post assembly treatments, I highly recommend the use of corrosion-x on the threads of fasteners securing inspection panels, fairings, cowlings, and accessory components. I wouldn't recommend it as a overall post assembly treatment, a better choice for treatment of interior metallic surfaces would be something conforming to MIL-C-85054, commonly known as AMLGARD. It goes on a bit sticky, but dries to a clear, durable finish, and is easily removed, if neccessary, with a little acetone without damaging the underlying surface coatings.

In reference to your concerns about Alodyne, it is indeed fairly nasty stuff, and shouldn't be fooled with by young children. On the other hand, it isn't nuclear waste either, and I really would recommend using it on any aluminum parts that you have to fabricate, or that didn't come pretreated from Zenith. Mitigating the dangers is a matter of a combination of common sense and technique, I'm sure that a lot of the folks on the forum can and will offer advice on how to use it safely. After alodyning, use whatever primer system best fits your budget, the primer's primary function at this point is to prevent mechanical damage to the oxide layer formed by the conversion treatment. By the way, in reference to an earlier post by Planecrazy, "roughing" and "etching" are not the same thing, etching is a chemical treatment used to remove microscopic particles of metal from the pores of the component, AFTER the working of the surface with abrasives, and before the application of a conversion coating and/or priming.

Is all this expensive? Maybe, I'd estimate the cash cost for the type of airplane you're building would probably add 350-500 bucks to the total, I'm guessing a small fraction of the total. Messy? Again maybe, but then aircraft maintenance (or manufacture) really isn't for the faint o'heart. And again, much of the mess can be mitigated by careful preparation, common sense, and proper technique.

All said and done, under the operational conditions you describe, I'd rather seal one up tight as described above, and then if I was low on money, I'd paint it with aerosol cans from Walmart. 'Cause I absolutely guarantee that my bill for a little surface corrosion clean-up would be orders of magnitude less than the one for fabrication and installation of a structural member that's been damaged by corrosion due to contaminant intrusion in a joint. Besides, as you get closer to completion, maybe we could hold a raffle to help pay for a spiffy paint job in the colors of the deserving charity that will become the owner.

In any event, I wish you the best of luck in this worthwhile endeavor. Happy riveting!

John
 

BBerson

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Alodine is not really that bad to get on your skin as far as I know. I have used hundreds of gallons a long time ago with no reaction. Best to wear gloves of course.

But disposing the waste is a concern today. What is done with the waste now?
BB
 

MalcolmW

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Hello, BB;

Alodine contains chromate ions (it's a chromate conversion solution, right?), and as such, can be disposed in a similar manner to chromic acid cleaning / conversion solution. This may be more than your need, but should do the trick:

Add a strong alkaline (baking soda, lye [sodium hydroxide], etc) to the dichromate /alodine solution until it becomes alkaline. Determine this by using litmus paper, which should be available from many sources, including Edmund Scientific, if you are in the US. This will cause the dichromate to convert to chromium hydroxide (very insoluble).

Either filter or let stand for a week to let the precipitate settle out. The supernatent liquid (the liquid above the sludge) can be washed down the drain with 50 times its volume of water. The precipitate (Chromium Hydroxide) can be washed with hot water to remove the sodium sulfate, dried, packaged and labeled, and then taken to a toxic waste disposal site or company.

If you choose to filter the solution to remove the sludge, a double layer of coffee filters should work, though lab grade filter paper would be best. (*Don't* use your coffee maker for this step!). At that point, you should be able to dispose of the liquid which should be chromium free and give the filtered sludge to a place that handles hazardous waste. The trivalent chromium is less toxic (according to my sources) but I still wouldn't just toss it down the drain.

For an alternative means of disposal of the chromium hydroxide sludge: Mix the wet sludge (precipitate) with dry concrete mix (right, the bagged stuff from Home Depot, Lowes, etc.) to form a stiff mix. Use for setting posts, making blocks, etc. This will tie up the chromium in a very insoluble form and harmless to the environment.

That's about it. I offer this in the spirit of information sharing and NOT as expert advice. If anyone can and wants to add to or correct any of this, I would welcome it.

Fly safe,
MalcolmW
 

BBerson

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Another idea for waste alodine liquid disposal would be evaporation. The big problem is containing the stuff to begin with. Perhaps a sheet of plastic laid on the floor could hold a few gallons of waste solution. It would evaporate in a few days or so depending on the humidity.
BB
 

stewart smith

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Hello,

There is a product that Boeing uses at the factory and we used it at the airline I worked for and it is excellent. It's name is COR BAN made ZIP chemicals corp in Calif.
It comes in a spray can or can be brush applied. It is almost clear in color with a slight red in it so that you can see it while spraying. Trust me, it works.

Smitty
 

proppastie

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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
Old thread, but did not answer my thoughts.....I was not planning to prime as weight and the adhesion of the fabric are issues to me. I have some 1201 Alodine and it is said it is good for corrosion protection. I do not want to epoxy prime. It seems a dip in a bath and water rinse should not be very hard. Any thoughts on Glass Bead blast at fabric glue interface of Alclad and no prime or Alodine only for corrosion protection?
 
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BBerson

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Wow, glass bead is too much work...
Alodine is to make primer stick better and needs be coated with primer, I think. Doesn't do much by itself. Etch and alodine are two separate steps.
First do the acid etch, then alodine. Scrub away fingerprint oils with the etch and scotchbrite till water lays flat and that gives visual proof of no oil on surface.
For more info on aluminum prep for structural bonding try reading the 3M data sheet for Scotchweld 2216 (online).
 

proppastie

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Wow, glass bead is too much work...
Alodine is to make primer stick better and needs be coated with primer, I think. Doesn't do much by itself. Etch and alodine are two separate steps.
First do the acid etch, then alodine. Scrub away fingerprint oils with the etch and scotchbrite till water lays flat and that gives visual proof of no oil on surface.
For more info on aluminum prep for structural bonding try reading the 3M data sheet for Scotchweld 2216 (online).
Glass Bead just the tops of the ribs that or Scotchbrite to give tooth to the Oratex glue...is my thought. I have the Henkel instructions, Alumiprep 33 and Alodine 1201..... it does say "substrate...." Some how I feel Glass Beading is easier than Scotchbrite. Somehow I feel if I Scotchbrite or Glass Bead I should then protect the metal as I will have scratched through the Alcad. Paint would be better but as you said the Cessna are still going with no paint or primer.
 

BBerson

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Glass Bead just the tops of the ribs that or Scotchbrite to give tooth to the Oratex glue...is my thought. I have the Henkel instructions, Alumiprep 33 and Alodine 1201..... it does say "substrate...." Some how I feel Glass Beading is easier than Scotchbrite. Somehow I feel if I Scotchbrite or Glass Bead I should then protect the metal as I will have scratched through the Alcad. Paint would be better but as you said the Cessna are still going with no paint or primer.
I thought you were talking about perimeter fabric bonding. I would not bond fabric to metal ribs. The fabric lift up puts the bond under peel loading. Even expensive Scotchweld for metal is only good for about 30 psi in peel load. Pop rivets are ugly, but they work.
 

proppastie

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I thought you were talking about perimeter fabric bonding. I would not bond fabric to metal ribs. The fabric lift up puts the bond under peel loading. Even expensive Scotchweld for metal is only good for about 30 psi in peel load. Pop rivets are ugly, but they work.
Yes perimeter fabric bonding,..The fabric has a glue to the structure, ribs and spars too, will still be rib-stitched. Just looking at the instructions....never done it. They want the aluminum roughed or good primer that does not lift, I do not trust glue or paint on aluminum but feel a glass beaded surface might be the best. At the airport today they said Alodine is a corrosion protection but paint on top is better.
 

mcrae0104

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Alodine is to make primer stick better and needs be coated with primer, I think. Doesn't do much by itself.
Alodine does improve adhesion but its other purpose is to inhibit corrosion by passivating the aluminum. More information can be found here under the conversion coatings section.
 

Matt G.

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Glass Bead just the tops of the ribs that or Scotchbrite to give tooth to the Oratex glue...is my thought. I have the Henkel instructions, Alumiprep 33 and Alodine 1201..... it does say "substrate...." Some how I feel Glass Beading is easier than Scotchbrite. Somehow I feel if I Scotchbrite or Glass Bead I should then protect the metal as I will have scratched through the Alcad. Paint would be better but as you said the Cessna are still going with no paint or primer.
Have you tried bead blasting thin aluminum sheet before? If you are not very careful you may warp the ribs.

A few years ago I added small wingtip wheels to my glider to make crosswind takeoffs a bit safer. I did a bunch of research trying to figure out how to prep the aluminum bracket to bond it to the wooden skid pad on the wingtip, and was told by a friend with more knowledge on the subject that all I needed to do was sand the aluminum clean with 220 grit sandpaper, wipe with an acetone-soaked white rag until the rag stays clean, and then IMMEDIATELY bond the bracket to the wingtip. I used West System epoxy. Awhile later, a friend whacked the tip wheel bracket pretty hard while helping me get the wing out of the trailer and broke it off. It tore out pieces of the wooden pad on the bottom of the wing; 100% of the aluminum side of the joint held. Your mileage may vary...
 

BBerson

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Alodine does improve adhesion but its other purpose is to inhibit corrosion by passivating the aluminum. More information can be found here under the conversion coatings section.
They call it a pretreatment conversion coating. If it is sold to be used without paint to protect it then that is new to me.

In any case, the primary corrosion protection is the alclad.
I stripped and repainted Cessnas from 50’s with no major corrosion under the paint. And they didn't alodine.
 

proppastie

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Have you tried bead blasting thin aluminum sheet before? If you are not very careful you may warp the ribs.
I needed to do was sand the aluminum clean with 220 grit sandpaper,
I do not know the grit size but the pressure is 90, no trouble with .025, the .016 is new so no minor corrosion. Probably have to use the sand paper or scotch-brite as the large LE etc do not fit in the cabinet . I am using some 40 year old .025 for splice plates etc. leftover from the MM1. There is minor corrosion on some of it and I wanted to stabilize it but not paint.
 

DangerZone

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I do not know the grit size but the pressure is 90, no trouble with .025, the .016 is new so no minor corrosion. Probably have to use the sand paper or scotch-brite as the large LE etc do not fit in the cabinet . I am using some 40 year old .025 for splice plates etc. leftover from the MM1. There is minor corrosion on some of it and I wanted to stabilize it but not paint.
Did you try to see if Cortec might have a product to suit your need..?

Cortec Corporation | Products

A click on 'Technology for Aircraft, Aviation and Aerospace' will download a pdf brochure with what they have.
 

Angusnofangus

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Etch, alodine, and prime is pretty much the aircraft industry standard. I think that Cessna's not priming was probably a money saving issue as they never thought their airplanes would last as long as they have. Therefore unprimed alclad was deemed good enough. It is good, but personally, I like etch, alodine, and prime.
 

proppastie

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Lars at Oratex wrote:

" In general The best is to Prime all Metal surfaces. Any BMS 10-11 material spec. type primer will do fine.

Those Self-etching, two-component primers are fine, they are by far the best way to prep an airframe.

Do NOT use any Zinc-Phosphate Rattlecan primers; And do Not use the Self-etching spraycan primers from Valspar either!


Epoxy paint is good and works fine for Oratex. Just scuff the surface with Scotchbrite and then wipe it clean with Alcohol before the glue is applied, let the Alcohol evaporate completely before the glue is applied. Do the same with powder-coating. Wipe in one direction only, then turn the cloth over etc. Do not wipe back and forth, that will just spread many contaminants evenly as they will stay behind when the alcohol evaporates...


In the last 6 month we have been testing another, very harmless primer that can be brushed on; So far so good...(not really long-term testing!)...
ABOUT THE 360 NT 100 GRAY PRIMER THE NEW PRIMER WE TESTED. THE STUFF STICKS TO IT LIKE HELL AND IS VERY HARMLESS TO USE IN A BUILDING ETC. WE BRUSHED IT ON. WHEN SCUFFED ORATEX-GLUE STICKS TO IT LIKE HELL.
IT SEEMS A GOOD ALTERNATIVE TO THE BMS 10-11 SPECIFICATION PRIMERS, AS THOSE ARE ALL toxic as hell.
Check the manufactures homepage, X-I-M Brand Page
if you choose this on aluminium, they say its good for metals: But the 360 NT they have is good for steel.
Keep in Mind there are two types!
360 NT 100 Gray Primer is a lower VOC alternative to the original 360 Primer. It is a phenolic modified alkyd primer, designed to inhibit rust and corrosion on iron, steel, and ferrous metals. Uses proprietary zinc compounds for excellent long term corrosion protection. Quick dry, Accepts alkyd or latex topcoats, Interior / Exterior
FOR BEST RESULTS: CAREFUL SURFACE PREPARATION AND CLEANING IS THE KEY TO A GOOD RESULT. BE SURE THE SURFACE IS THOROUGHLY CLEAN AND DRY, FREE FROM ALL GREASE, WAX, OIL, LOOSE PAINT AND DIRT OR LOOSE RUST."
 
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