Corrosion Protection

The generally accepted "best" approach (if there is such a thing) is acid etch and alodine, then epoxy primer. 

No need to apoligize for being a newbie.  You are way ahead of all those who have not yet made the attempt.  And, besides, we were all newbies once.

You first may want to ask yourself if you really need the added weight and effort of priming interior metal surfaces.  If you expect to hangar your plane in an area with a dry climate, it may not be worth the trouble.  On the other hand, if you expect to put your plane on floats, or even if you live in an area where rain and/or snow are common, priming would be a good idea. Of course, you will need to prime the exterior surfaces in any case, unless you are going to have polished skins (beautiful, but a lot of work).

Assuming the decision to prime has been made, the recommended prep is to etch and de-glaze the surface of your parts using something like Alumiprep, or a similar product, and a Scotchbrite pad.  Go over the entire surface with the Scotchbrite while immersed in the Alumiprep, then rinse with clear water.  Immediately immerse the rinsed part in Alodine as per the instructions on the bottle.  Aircraft Spruce has both of these products, as I am sure do Wicks and a number of other aviation suppliers. Hang the alodined parts out to dry and prime with a primer that is compatible with the finished coat that you expect to use.  Many high quality epoxy primers will meet this requirement. Be sure to read the Material Safety Data Sheets on the paints you use.  Much of that stuff is really bad for you and requires special protection when you use it.

I made large trays for dipping the parts by nailing a wooden frame made of 1x4s together and then lining it with 4 mil plastic sheeting. When you're done just throw the plastic away. Be sure to use gloves and goggles for safety, and old clothes, since you can't help but get the alodine on you.

For small parts fabricated at times when it was not convenient to go set up the Alumiprep-alodine process, I used a self-etching primer called Marhyde, which comes in rattle cans.

I would only disagree with the acid etch as stated in post 2.  Alodine (brush on or dip) and then prime with an epoxy based Zinc-Chromate primer.  However, if you want the best protection you'll need two coats of primer.  You should also install fasteners wet with this same primer and also install piece parts in the assembly with MIL-S-8802 based sealant between the two adjoining (faying) surfaces.  This can get cumbersome to do on a kit level production.  So two coats of primer should keep your aircraft like new for years.  The parts can and should be fully cured before assembly.  I would still install the fasteners wet. 

I am from Pittsburgh, Pa. I get little taste of everything: from fog to thunderstorms. I plan to use hanger to protect my plane.

 I don't plan to cover entire interior surface. I was look to cover area where two components come together, and  cockpit floor/ baggage area (just in case I spill coffee during turbulence )

 If the 2040 aluminum sheet is alcad, is it necessary to apply epoxy primer or zinc chormate to the areas that I just mentioned?

Yes.  Even Alclad needs primer.  And I would still paint the entire interior surface.  For these it can be brushed/rollered on. 

Ahhh, primer wars.  Here’s my .02...ALCLAD does not have to be primed as it offers good corrosion protection all by itself, as long as it doesn’t get scratched.  However, the ALCLAD oxide layer is thin and easily scratched, which leads many, including me, to go the primer route over everything.  Some only primer the faying surfaces which the surfaces that are in contact between two components (like between a rib flange and the skin).  Some don’t primer at all – Cessna didn’t until after 1994.  Like others have said, a lot depends on the plane’s environment.

Here’s what some consider being the ultimate method:

1.       Wash with dish soap and water – Why?  To remove grease and oils.

2.       Etch/Clean with alumiprep and a maroon scotchbrite pad—Why? Removes the ALCLAD layer which some primers have difficulty adhering to and removes any dirt/impurities that the soap didn’t get.  ALCAD is smooth and the problem is that most primers rely on mechanical bond to adhere to the substrate.  So that’s the purpose of etching and/or roughing with acid or sandpaper:  to give the aluminum more tooth for the primer to mechanically bond to.  However, this solution leads to another problem which is that doing this removes the ALCLAD corrosion protection, so this leads to step 3...

3.       Conversion coating (ie alodine-Note very hazardous, so something similar) Why? Re-establishes an  oxide layer  that won’t affect how the primers mechanical bond to the aluminum

4.       Prime with epoxy primer –number of coats depends on what the manufacturer says in the instructions, but regardless of how many coats, keep it thin. (Note: Epoxy has pretty much supplanted Chromates as the primer of choice)

This is not the only way to go, it’s just “a” way.  For example, some just shoot a rattle can of self-etching primer (like NAPA 7220) with little prep and seem to get good results.  For the best results, my advice is to follow whatever prep instructions the primer manufacturer recommends.  If properly maintained, your plane will most likely outlast you in the long run regardless of whether you prime or not.

YMMV.....

Todd

Building an RV-10 (N728TT reserved)

While slightly off topic, you should consider when you are discussing (faying) surfaces and sealing them with sealant.

Much discussion on the Yahoo Groups Thorplist WRT fraying surfaces from a strength perspective for joints assembled with PRC. Search the site using (faying surfaces) for wet wings. 

John Thorp T-18 designer spent a lot of time at the "wet-wing" engineering problem... and counseling Don Taylor (N455DT) around the world Thorp T-18 as he built the first set for N455DT. He was a highly reputable engineer and was
keenly aware of these issues... and knew how things would "likely" be built by an amateur. Due to the flexible, light-weight structure of the T-18, John Thorp INSISTED on added rivets between existing rivet locations within the outer wing [1/8" ONLY, skin-to-ribs] to improve sealing, account for fuel side-loads on ribs and account for joint load-reduction factors caused by added sealant.

Will Taylor (son) and engineer with Boeing wrote:

Folks...
As a Boeing engineer, I can definitely say that Boeing considers sealant [IE: BMS5-85] just that... the fact that is sticks and is strong enough to appear as good as structural adhesive is an illusion. Adhesives [epoxy] and fasteners [aluminum, titanium, steel] are extremely stiff compared to sealant [rubber]. Stiffness of a fastened joint strongly influences load
transfer: rubber between metal layers is degrading to the point that Boeing stress analysts have a fastener reduction factor when sealant is used. For this reason, and because of tight construction tolerances and the high cost of using sealant, Boeing's current policy is to assemble as many joints together "dry" as possible. ALL rivets in wings are precision BACR15x rivets installed in precision drilled/reamed holes, by machines... DRY... In every aircraft built today. Bolts and Lock-Bolts are mostly installed "dry"... but are put in "wet" with sealant only when essential for pressure tightness. Most major structural assemblies are then edge sealed, if necessary for pressure-tightness. Corrosion protection in dry joints is accomplished by multiple [primer/paint...or fuel-resistant primer, only] coatings on adjoining metal surfaces... and the extremely tight fit of all fasteners.

Just my 0.02 cents.

Well, sealing all the faying surfaces is an option.  In RV's the fuel tank faying surfaces all receive sealant (Proseal), but not for corrosion purposes.  I do know of one builder that used pPoseal over all of his faying surface intersections and took a huge hit in useful load.  Why?  Sealant, compared to primer, is orders of magnitude heavier, even with a light application.  As a result, I wouldn't recommend using sealant everywhere--just where the plans call for it.

Merry Christmas “Newbie” … Newbieism is relative ……Opinionsare worth every penny you pay for them …. although all the comments youreceived are definitely good and will protect your airplane from corrosion …. Iwould just like to add that I have inspected a lot of airplanes …. Some withnasty corrosion … most if even moderately taken care of do not suffer serious problems…. I really like a product called Corrosion X …. Not selling the stuff justhappy with the way it works …. Here is my opinion …. I would feel moreconfident with a totally treated airframe with Corrosion X  than with all the other mentioned cures …..The reason is the Corrosion X treats everything ….steel, alum, it wicks intolap joints, it is a lubricant, it doesn’t attract dirt, and it doesn’t smellbad etc.    All the others don’t ….. allyou need is a chip in steel and you have a spot for corrosion to start … I’veexperimented with bare steel and although it won’t repair the damage , it willstop corrosion dead in it’s tracks…. The stuff is great….. and I really don’twork for them, honest  ….. Happy NewYear  

Dwayne I could not agree more on your observation of the benifit of Corrosion - X or a similar product. Primer is NOT water proof. I have had some experience in SE Alaska in Float Planes operating in salt water. No amount of primer would keep these A/C from going to H--- FAST. They, the A/C, get treated at least semi-annually with a treatment of some sort of protectant to eliminate the water present between the mating surfaces. Well that's my .02 worth.

Chuck Garland

My son and I are currently building a Waiex conventional gear airplane and are using a Stewart Systems water-based primer on faying surfaces and especially in lower areas where water tends to collect (for example aft lower fuselage where skins mate with structural angles, and stub rudder near the tail wheel.) Exterior skin will be mostly polished aluminum, but we'll use the Stewart Systems etching, priming and probably painting products too on the few exterior areas to be painted, such as trim stripes, cowling, fiberglass tips and control surfaces. We are using ACF-50 on interior surfaces and are wet installing all the pulled rivets with ACF-50 too. ACF-50 is a non-toxic wipe on or spray on oily anti-corrosion liquid. We are planning to mist the ACF-50 into the various aircraft structural parts and surfaces every two years to maintain good anti-corrosion protection after we get the airplane built. Though we live in a relative dry region in Texas and plan to keep the airplane hangared, we don't want to take any chances with corrosion. We have some health issues that rule out using anything strongly toxic. After studying the Material Safety Sheets on all the products posted on this thread, plus a few others, we decided that ACF-50 and the Stewart Systems water-based products were the safest for us to use, while offering us an acceptable level of corrosion control.         

Hello everyone

I am a newbie too and don't claim to know everything about what i am about to post, so if someone knows something i don't, please help me out.  I am currently building a Waiex which is entirely 6061, so this is like compairing apples to oranges with your 2024 project, but i have decided not to do any corrosion protection on the inside surfaces for several reasons.  6061 is one of the most corrosion resistant aluminum alloys out there, my airplane will be hangered and since it is VFR exclusive it will never see the weather, none of the prototype Sonex had any corrosion protection applied, John Monnett recomeneds against it, i will save myself the money and extra time of doing it, and there will be no weight penatly.  I live in a beach town about 2 miles from the ocean, but we don't see any form of "salt fog" this far inland.  I have been asking everyone i know and posting to various forums about this for some time and have not been able to find one person who knew first hand of a 6061 airplane corroding away to an unflyable condition prematurely (under similar environmental conditions).  I understand that my plane would probably have a higher resale value some day (if i decided to sell it) if i did use the process, but i don't really care about that.  I think the only time i would adoline/prime the inside is this project was going to be put on floats.

Are any special measures taken for the pull rivets used by Sonex because of the disimilar metals (aluminum and steel)?

Thanks,

John

Taken directly from the Sonex Aircraft website:

With the stainless steel rivets against the aluminum skin, isn't there a galvanic corrosion problem?

No. We have been using this combination since 1972 and have not experienced galvanic corrosion.

I read one article somewhere that drew a correlation between the surface area of the rivets and the rate of corrosion.  The theory was the larger the surface area, the greater the potential difference that could be generated between the dissimilar metals.  It was also stated that is was theoretically possible that the size could be small enough to be below the energy level required for certain metal combinations to corrode.  I am not saying this is what is definitely occurring in the Sonex.. but i found that interesting.

I am also a newbie to the EAA's forum

this is one point where size does matter any 2 dislike materials,..the smaller the connection point the less electrolysis,..

some have applied different materials between larger dislike material contact points ,..adhesives ,..dielectric greases,..sheets of teflon ,..primers etc

now not to change the format of the discussion ,..but for anyone reading this  about anti corrosion and was wondering about 4130 CM Tubing,..one trick that I have done during the welding process,..I have drilled a 1/16th" hole at every intersection where 2 or more pieces are joined ,..,..I said tubing right? yes ,..ok the purpose of this "extra work" is once the fuselage is completely welded ,..all seams joined/sealed ,..I choose 2 locations for installation of a valve similar to a tire stem valve,one at each end of the fuselage,...after the fuselage is primed and had time to dry ,..I inject Argon in one of the valve stems while the core is out of the other ,..I do this for a 5 minute period to ensure I have voided all of the air/oxygen from the interior of the airframe,.. then close the opposite valve while still charging Argon into the other for additional 30 seconds,..I have done this on 4130 struts as well,..Argon being the inert gas that is it eliminated the ability of condensation/moisture to form in an atmosphere where there is no air/oxygen ,....so from that point on ..when this airframe goes from one temperature to another chances are it will not get any moisture/condensation with in its fuselage,..as for the exterior ,..stay on top of maintenance to include re priming any surfaces that get abused.

Chase

Except  that is contrary to ALL of the data and practices followed by ALL of the aircraft manufacturers like Boeing, Airbus, Cessna, SIkorsky....  All of these companies have found that steel rivets, bolts, and other fasteners in contact with ANY aluminum alloy will corrode either the aluminum or the steel.  As they corrode the clamp up deteriorates and then you get a fatigue problem.  Either way it is BAD NEWS.   The usual protection is to Cadmium plate the steel.  Even Stainless is affected.  An alternate to Cadmium is to coat the fasteners with vapor deposited aluminum.  Blind Rivets for aircraft with steel pull stems normaly have these stems coated with one of these methods. 

I recommend to all to take corrosion VERY seriously if you plan to own your aircraft for any length of time.

Chase:

Your method will work altough the Argon will likely dissipate out of your tubes over time.  If you used those holes for injecting a thinned primer and then tumbling the structure, you will get a primed surface on the interior.  I've also heard of good results with ordinary wood varnish (probably left over from wood wing construction).   Thinning is necessary to get good coverage and then good drainout.  of course you need to use an epoxy or other catalyzed primer for these methods.    

But you would still need exposure to an electrolyte.. right?  If it is a VFR exclusive airplane kept in a hanger is this still necessary?  The Monnett Moni hanging in the Smithsonian and EAA museums are still, to my knowledge, in great shape after 25-30 years

Nick, Yes you need some sort of electrolyte.  VFR does include rain and even ground mist can wick into these small gaps.  Seen it; seen the consequences.   Washing off the dust is another source.  Any source of water (and some vapors) can be all the electroyte needed.

J..