Preventing Corrosion

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

Ollie Krause

Well-Known Member
Joined
Feb 26, 2020
Messages
103
Hi Home Built Airplane Enthusiasts,

I'm having some trouble figuring out the best practices for corrosion prevention in ultralight aircraft. I cam across this HBA thread which was super helpful but it primarily focused on sealant and less about broad corrosion prevention methods. I forget which HBA thread I read it in but I've also heard that some people simply don't take any action to prevent corrosion which may occur over the long term with the mindset that they won't own the ultralight by then. I'm in the process of designing (and hopefully later constructing) an ultralight with 15 other high school students and we'd like to do everything we reasonably can to prevent corrosion on our airplane for the learning experience if nothing else. Our truss frame is going to be constructed of 6061 T6 but a lot of our U brackets and other small structural components will 304 stainless or 4130 chrome moly. I noticed this isn't an uncommon practice and galvanic corrosion shouldn't become an issue as long as the steel parts are used sparingly and are relatively small. We also live in the San Francisco Bay Area which has a relatively low humidity but we'd ideally like our ultralight to be able to survive in the more humid environments down south. For regular corrosion, I would assume the the 304 stainless and 6061 T6 aluminum shouldn't be at much risk but the 4130 might have some issues despite it's improved corrosion resistance due to the molybdenum. This leads to three main questions:

1) How should I prevent corrosion where steel (stainless or otherwise) interfaces with aluminum gussets? I've read that zinc chromate primer is common but what paints do people typically use? What if these are moving parts which may scrape off any primer and paint?

2) How should corrosion be prevented on 4130 parts and the spring steel rear suspension?

3) I've noticed a couple ultralights use stainless steel rivets into aluminum parts. This should be okay since stainless steel is more cathodic but I was wondering if this practice should be avoided when possible.

Thanks in advance for any help!
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
8,222
Location
Saline Michigan
Some folks get away with Alclad and no paint. Alclad is a thin layer of pure aluminum on the surface of your alloy.

Stainless is not. Under certain circumstances, some stainless steel will become brown, and 4130 WILL rust. Everything else is a good idea to paint with something if you do not live in a desert.

The usual approach in homebuilts is epoxy primer. The standby is solvent based from Stits, but there are waterborne from Stewart Systems and others. The solvent based stuff is great on steel - the waterborne stuff tends to flash rust the steel.

As for stainless in contact with aluminum, the contact point will galvanically corrode unless the stainless is passivated. Make sure your stainless rivets are passivated. Passivation is how some stainless components come, others you have to passivate yourself. Believe it or not, citric acid bath. Read about it online to get knowledge of how it works, recipes and soak times. Thoroughly degrease and dry before the citric acid bath.

High strength steel parts (springs qualify) should not see acids, clean with solvents then paint with epoxy primer.

Where parts contact and move, you will likely be lubricating them. Let the epoxy primer rub off and the lubricant protects the spot.

Billski
 
Last edited:

BBerson

Light Plane Philosopher
HBA Supporter
Joined
Dec 16, 2007
Messages
15,231
Location
Port Townsend WA
Ordinary oil based paint for metal like Rustoleum is fine for 4130. Lasts 30 years or more on outdoor tractors. Two or three coats.
Top coat with epoxy to dope proof the oil paint if fabric is to be glued to the steel tubes.
 

Ollie Krause

Well-Known Member
Joined
Feb 26, 2020
Messages
103
Some folks get away with Alclad and no paint. Alclad is a thin layer of pure aluminum on the surface of your alloy.

Stainless is not. Under certain circumstances, some stainless steel will become brown, and 4130 WILL rust. Everything else is a good idea to paint with something if you do not live in a desert.

The usual approach in homebuilts is epoxy primer. The standby is solvent based from Stits, but there are waterborne from Stewart Systems and others. The solvent based stuff is great on steel - the waterborne stuff tends to flash rust the steel.

As for stainless in contact with aluminum, the contact point will galvanically corrode unless the stainless is passivated. Make sure your stainless rivets are passivated. Passivation is how some stainless components come, others you have to passivate yourself. Believe it or not, vitamin C bath. Read about it online to get knowledge of how it works, recipes and soak times. Thoroughly degrease and dry before the vitamin C bath.

High strength steel parts (springs qualify) should not see acids, clean with solvents then paint with epoxy primer.

Where parts contact and move, you will likely be lubricating them. Let the epoxy primer rub off and the lubricant protects the spot.

Billski

Thanks for the great info! I did some reading and the vitamin C passivation seems relatively easy to do. Do you know if they sell pre-passivated rivets as I would imagine there would be a decent demand for them?
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
8,222
Location
Saline Michigan
Not my area, but I imagine if you can not get them already passivated, buying stainless with stainless mandrels can be home shop passivated. Make sure to neutralize and rinse thoroughly if you are doing this yourself.

Billski
 

wktaylor

Well-Known Member
Joined
Sep 5, 2003
Messages
350
Location
Midwest USA
4130 steel. Prepare steel for coating by carefully. blending-out sharp scratches/gouges, rounding-off corners and deburring/rounding-off all sharp edges. Abrasive-grit [fresh aluminum oxide] grit-blast-clean all surfaces/edges to a bright/uniform matte appearance. Wipe-off any dusty residue [clean/dry cloth] and immediately apply mist coat of zinc-rich primer. Build-up several mist-coat primer layers to attain ~0.001-to-0.002 dry film thickness... then apply a 'matching chemistry' topcoat for color [if desired].

BTW SStl passivation is done by acid. I prefer AMS2700 method 2 CITRIC ACID ['VITAMIN C' would be a slang-term for this method]... since it is 'friendly' [environment and users]. Immediately after water rinsing/drying apply a zinc-rich primer. CAUTION: traditional method 1 involves various forms of nitric acid... don't do-it.

Another method commonly used chemical is PPG-SEMCO Pasa-Jell 101 is used for cleaning-etching SStl to remove the existing passive film which interferes with paint adhesion. Brush-on with a chemical resistant brush; allow-to 'dwell' [time as required]; rinse-off with water . Immediately after water rinsing/drying apply a zinc-rich primer. Pasa-Jells [all 'kinds/purposes'] have to be handled carefully since they contain a strong mineral acids in a jell form [brush-on process is convenient] and can be dangerous if mishandled

NOTE.
Pasa-Jell 102 is for etch-cleaning aluminum in-prep for coatings.
Pasa-Jell-105 is for etch-cleaning aluminum in-prep for adhesive bonding.
Pasa-Jell 107 is for etch-cleaning Titanium in-prep for coatings and/or bonding.
 
Last edited:

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
8,222
Location
Saline Michigan
Thanks for catching that. Citric acid, not Vitamin C, and it is available in canning departments of grocery and hardware stores.
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
6,200
I wouldn't blast any aircraft part with sharp-cornered grit like aluminum oxide, silicon carbide or copper slag. Glass bead is spherical and even when broken it isn't hard enough to create the sharp-cornered pits that can lead to cracking.

Spark plug cleaner/testers use silicon carbide. I prefer fine glass bead. Not the coarser stuff. In either case, the plug electrode well and center electrode at the ceramic should be inspected with a light and magnifier to make sure none of that grit is stuck in there. It will fall out eventually, into the cylinder, and cause grievous scoring.

For steel parts it's hard to beat epoxy zinc chromate primer. Hard to find in some places that think everything, including coffee, causes cancer.

Paint strippers will cause hydrogen embrittlement in springs. They contain the acids that Billski refers to.
 

PagoBay

Well-Known Member
Joined
Nov 16, 2013
Messages
324
Location
US Territory of Guam
If suitable to your situation, consider Cortec or Corrosion-X or ACF-50. The latter two are equivalent products.

Zenith's Rudder Assemby Manual here: Jump to Page 15 for corrosion protection. Zenith factory supplies Cortec to its kit buyers.

I know Jon Croke of HomebuiltHELP videos likes ACF-50. See his video on corrosion in kit planes here:

Corrosion -X and ACF-50 are fogged in as a fine mist and proceed to creep into every space at a molecular level. Less weight than Cortec but has to be repeated. The largest flight school here uses ACF-50 to protect the interior spaces of the aircraft wings. Excess will drip out over time. Do not use these products before painting. I was traveling and had some surface corrosion on an unpainted aluminum wing. Came back four months later, the corrosion was completely arrested from day one. ACF-50 displaces salt water so great for anywhere near the ocean. Friend of mine uses Corrosion-X on his Aventura ultralight amphibian aircraft. After several years of salt water exposure, zero corrosion. But he is very meticulous in hosing down the airplane after each flight then using Corrosion-X. Prevention is the key.

Of course, the above won't address dis-similar metal issues specifically.

Matronics Forum has this post:
Be careful about using ACF-50 or Corrosion-X over Cortec, or else you might see your Cortec go bye-bye. Cortec needs to be topcoated to be
resistant to just about anything. What you might try doing it thinning some Cortec with alcohol to the consistency of 2% milk, and spraying it
liberally over everything inside. I have two lawn mowers that I leave out under some bushes as I have no garage space, and the old toro
has half the paint gone across it's deck (aluminum) and it was oxidizing like crazy, so I took a can of thinned cortec and slopped it all over it with
a paint brush a year ago and it seems to have stopped corroding. I also hit my daughters tricycle with it as it was rusting, and the rust
stopped as well.

Cortec is definitely not fuel resistant either, I dip tested some cured Cortec in 100LL and it dissolves it before too long as well.

I have a lot of cortec left over from building so I started using it on everything that is hinting at corroding around the House, and vehicles
and it works pretty good at just simply stopping corrosion even without any surface prep.

Another alternative which is better than ACF-50, and Corrosion X which are corrosion preventives meeting MIL-81309 (which are a light film
for avionics connectors, and light hardware), is to use one of the more durable products like CRC "Corrosion Shell". This is like a product we
used in the Navy on Grumman E-2C's for protecting exposed hardware we called it Grade 4, but I think it is the same thing. It dries to a brown
waxy film, much better protection than the lighter products, and doesn't run all over like ACF-50 and Corrosion X will.
 

Ollie Krause

Well-Known Member
Joined
Feb 26, 2020
Messages
103
If suitable to your situation, consider Cortec or Corrosion-X or ACF-50. The latter two are equivalent products.

Zenith's Rudder Assemby Manual here: Jump to Page 15 for corrosion protection. Zenith factory supplies Cortec to its kit buyers.

I know Jon Croke of HomebuiltHELP videos likes ACF-50. See his video on corrosion in kit planes here:

Corrosion -X and ACF-50 are fogged in as a fine mist and proceed to creep into every space at a molecular level. Less weight than Cortec but has to be repeated. The largest flight school here uses ACF-50 to protect the interior spaces of the aircraft wings. Excess will drip out over time. Do not use these products before painting. I was traveling and had some surface corrosion on an unpainted aluminum wing. Came back four months later, the corrosion was completely arrested from day one. ACF-50 displaces salt water so great for anywhere near the ocean. Friend of mine uses Corrosion-X on his Aventura ultralight amphibian aircraft. After several years of salt water exposure, zero corrosion. But he is very meticulous in hosing down the airplane after each flight then using Corrosion-X. Prevention is the key.

Of course, the above won't address dis-similar metal issues specifically.

Matronics Forum has this post:
Be careful about using ACF-50 or Corrosion-X over Cortec, or else you might see your Cortec go bye-bye. Cortec needs to be topcoated to be
resistant to just about anything. What you might try doing it thinning some Cortec with alcohol to the consistency of 2% milk, and spraying it
liberally over everything inside. I have two lawn mowers that I leave out under some bushes as I have no garage space, and the old toro
has half the paint gone across it's deck (aluminum) and it was oxidizing like crazy, so I took a can of thinned cortec and slopped it all over it with
a paint brush a year ago and it seems to have stopped corroding. I also hit my daughters tricycle with it as it was rusting, and the rust
stopped as well.

Cortec is definitely not fuel resistant either, I dip tested some cured Cortec in 100LL and it dissolves it before too long as well.

I have a lot of cortec left over from building so I started using it on everything that is hinting at corroding around the House, and vehicles
and it works pretty good at just simply stopping corrosion even without any surface prep.

Another alternative which is better than ACF-50, and Corrosion X which are corrosion preventives meeting MIL-81309 (which are a light film
for avionics connectors, and light hardware), is to use one of the more durable products like CRC "Corrosion Shell". This is like a product we
used in the Navy on Grumman E-2C's for protecting exposed hardware we called it Grade 4, but I think it is the same thing. It dries to a brown
waxy film, much better protection than the lighter products, and doesn't run all over like ACF-50 and Corrosion X will.
Wow thanks for all the great advice! We'll definitely get some Corrosion-X or ACF-50.
 

BJC

Well-Known Member
HBA Supporter
Joined
Oct 7, 2013
Messages
13,584
Location
97FL, Florida, USA
Ollie:

Just keep in mind that corrosion proofing via Corrosion-X adds weight and generally isn’t needed with 6061 alloy.


BJC
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
15,404
Location
Memphis, TN
ACF50/ Corrosion X Not great for glue and fabric. It’s all oil based. You are spraying oil with extra chemicals; make sure what touches is compatible. The question is how nice does it need to be? I don’t know now, but past Cessna did not primer internally their planes. Steel, paint is good. All sorts of levels. You could get fancy with powder coating. Paint the aluminum. Pour paint on the inside of tubes and slosh around or dunk in paint bath. Really unlimited options.
 

Daleandee

Well-Known Member
Joined
Sep 11, 2015
Messages
1,527
Location
SC
If suitable to your situation, consider Cortec or Corrosion-X or ACF-50.
Another vote for ACF-50 on aluminum structures. I use it in various places. It has many uses and comes in aerosol cans also. Great product!

1588718907595.png
I was warned that if I was going to paint my airplane I should do so before the ACF-50 was used as it gets into every crack and crevice and it will seep from every crack and crevice for sometime after application. It is also a great lubricant and is safe around electronics.
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
6,200
ACF50/ Corrosion X Not great for glue and fabric. It’s all oil based. You are spraying oil with extra chemicals; make sure what touches is compatible. The question is how nice does it need to be? I don’t know now, but past Cessna did not primer internally their planes. Steel, paint is good. All sorts of levels. You could get fancy with powder coating. Paint the aluminum. Pour paint on the inside of tubes and slosh around or dunk in paint bath. Really unlimited options.
Cessna started priming everything inside and out in '96. Before they stopped production in '86 they normally only primed the insides of floatplanes. A lot of 180s and 185s are primed internally.

Powder coating of steel would work if it's not a heat-treated part. It would ruin the temper of aluminum. 400+°F.
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
6,200
I was warned that if I was going to paint my airplane I should do so before the ACF-50 was used as it gets into every crack and crevice and it will seep from every crack and crevice for sometime after application. It is also a great lubricant and is safe around electronics.
Compromises everywhere in airplanes. Stop the corrosion at the expense of a lot of hassle at the next paint job.

One of the worst things to deal with when repainting is silicone. A lot of polishes have it, and the slightest trace of it makes the job a real pain. Fisheye everywhere. Even silicone sealant leaves a film that has to be removed with abrasives and solvents.

I once found ACF-50 on propeller bolt threads. The prop manufacturers specify the torque, and it's either for a dry thread or for a thread coated with a specified lubricant. ACF-50 on a dry-thread torque could easily lead to overstressing the bolt. With less thread friction the wrench will turn the bolt farther, putting more tension in the bolt. This is an example of using a great product in ways it was never intended for, and suffering unintended consequences.
 
Top