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Corrosion Protection Test

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Jman

Site Developer
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Oct 22, 2002
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Pacific NW, USA!
I'm writing this thread to document an experiment I've decided to run on the durability and corrosion protection capability of several popular primers for homebuilt aircraft. I don't have the time or money to test all of the primers that folks are using on their projects, so I've narrowed it down to 5 that are either popular or seem to have promise for use in my project. Please keep in mind I am not a scientist nor am I attempting to definitively prove anything. Here is how I will test these products.

Each product has been applied to two separate test coupons. A test coupon is a 2x2 6061 T6 .032 square with a 20mm X 2” rectangle of the same material riveted to it using A4 Avex rivets provided by Zenith Aircraft Company. The product was applied to the coupons and allowed to dry for 4 weeks prior to riveting together. All edges were left rough and not deburred.

Scratch Test: In order to ensure a fair scratch test I used a simple method to repeat the same scratch with varying amounts of pressure. I drove a finish nail through the end of a 2x4 so that the tip protruded about 1/8th inch on the other side. I clamped each test piece to the bench and positioned a weight on the 2x4. Once in position I slowly pulled the 2x4 across the test coupon. I made three scratches for each type of primer. One using a 6.3lb weight on the 2x4 to scratch 1 coupon and a 1.3lb weight on the 2x4 to scratch the other coupon. As an after thought I decided to make one more scratch using only the wight of the 1lb 2x4. After making the “calibrated” scratches I went ahead and put a nice deep scratch by hand making an X across the test scratches. I wanted to make sure I got to bare aluminum for the corrosion test detailed below.







Bend Test: After the scratch test I decided to see how the coatings would hold up to bending. If I had a dimpling machine I might have dimpled the surface instead. I took each of the coupons and bent the top corner down 90 degrees by pressing them hard against my work bench. The radius looked to be about 3/16th of an inch. Not exactly scientific, but what the heck. All primers held up well and did not crack or flake.

Corrosion Test: Now for the main event. I created a low budget device designed to wet the coupons with ocean water for 1 hour 4 times a day. Salt fog testing is the Mil Spec standard for corrosion testing but I really didn't want to spend the money for a pump with enough pressure to create the misting action. Instead, the coupons are positioned in a container that is flooded with sea water for 1 hour. After an hour the sea water drains out and the coupons are allowed to drip dry. This is repeated every 6 hours.

After 2 weeks of testing I'll remove one of each of the coupons and drill out the rivets to observe the faying surface between the two pieces and the rivets themselves. I'll leave the other coupons in place for long term observation.






Here are the primers I've decided to test for durability and corrosion control properties:

PTI Rattle Can Zinc Chromate primer: Although toxic, Zinc Chromate primer seems to be the standard by which all other primers are judged. I'm not keen on using Zinc Chromate in my project but I wanted something to judge the other primers by. Depending on how the test turns out, I may change my mind.

Method of application: 1. Degrease. 2. Alodine 3. 1 mist coat then 2 full coats. 4. Dry 4 weeks before riveting.

Impressions prior to testing: This primer goes on pretty easy. I was impressed with how little spray it took to get full coverage. A couple of short sprays completely covered the small aluminum coupons. There was no tendency to sputter or clog like the Tempo brand Zinc Chromate I've tried in the past. That stuff was a nightmare. At nine dollars a can it was a little steep compared to the other two rattle can primers I'm testing though. The alodine I used to prep the coupons was very expensive at about 30 bucks a quart. It was difficult to get an even coat of alodine, even on the small parts. I can't imagine trying to do a spar.

2 part Epoxy (mfg unknown): The distributer I purchased my zenith tail kit from offered to prime the interior of my kit with 2 part epoxy. I took them up on the offer so most of my parts already sport this primer. I had to discard some parts so I had some pieces to cut up to add to the experiment. Many folks swear by epoxy primer for it's durability and it's ability to seal the surface for good corrosion protection. I'm not too keen on it because it requires mixing and, unlike the water based systems, the unused portion cannot be poured back into the container for later use. I'm adding it just to see how it does, however I doubt I will use it on the rest of the project.

Method of application: I'm not sure how it was applied because I was not there when they did it.

Impressions prior to testing: Don't really have any because I did not apply this primer. We'll see how it holds up in the testing.

Stewart Systems EkoPrime primer sealer: I was initially attracted to this primer because of it's low toxicity and the fact it's a water based primer. It's a bit of a pain to apply because it needs to be reduced with water and then sprayed with an HVLP sprayer. I've actually sprayed some parts with this primer and didn't really care for finish quality. I'm still tweaking my HVLP gun and thinning technique so the jury is still out.

Method of application: 1. Degrease. 2. Etch with Stewart Systems EkoEtch. 3. 1 mist coat and two full coats. 4. 4 weeks drying time before riveting.

Impressions prior to testing: I've had quite a few problems getting a good finish with this primer. No matter what settings I use I'm still getting a very bumpy finish. I'm chalking it up to my inexperience and I'm sure with some practice I'll get the hang of it. I don't really care for having to thin it and spray with an HVLP sprayer. The small parts have a tendency to get blown around by the forceful spray of the HVLP gun. All of this is technique of course. Clean up is easy but not nearly as easy as turning the rattle can over and spraying until it's clear.

NAPPA 7220 Self Etching Primer: This primer is VERY popular on the Vans forum. It's main advantage is its ease of application. It's a self etching primer so theoretically the metal doesn't require any special preparation prior to spraying other than a good degreasing. There are some that claim this primer provides zero corrosion control. It's very popular with the vans guys though so I'll give it a try.

Method of application: 1. Degrease 2. 1 mist coat then 2 full coats. 3. 4 weeks drying time before riveting.

Impressions prior to testing: This primer goes on very smooth with only a slight tendency to run if sprayed on too thick. Nothing to worry about. The finish is pretty smooth and it has a pleasing medium gray color. It was nice being able to skip the etching step. It dried to the touch within a few minutes, which was nice.

Krylon Tough Coat S00340 Industrial Primer: I've only seen this primer mentioned once on the Vans forum but it seems to have some promise to me. Krylon claims it meets the MIL TTP 1757A specification which includes such testing as water immersion, Hydrocarbon resistance, and 1000 hour salt fog testing. Sounds good to me. With a name like Tough Coat it's got to be tough too right? We'll see.

Method of application: 1. Degrease. 2. Etch with Stewart Systems EkoEtch. 3. 1 mist coat and two full coats. 4. 4 weeks drying time before riveting.

Impressions so far: I was amazed at how smooth this primer goes on. The finish is satin smooth and it's dry to the touch within a minute or two at 85 degrees temperature. There was no tendency to run at all. In fact, as soon as you spray it on it looks dry. If I had anything bad to say about the application it would be that it has a bit more over-spray then the Zinc Chromate and the NAPA rattle can primers. Cost was very reasonable as I paid only $5.50 for a single can with free shipping.

Bare aluminum control piece: A single coupon of bare .032 6061 T6 aluminum. We'll see what a pure Atlantic ocean water bath does to naked aluminum.


TEST RESULTS:

Coming Soon.........
 

jumpinjan

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Joined
Oct 3, 2004
Messages
313
Location
Dayton OH
.
NAPPA 7220 Self Etching Primer: This primer is VERY popular on the Vans forum. It's main advantage is its ease of application. It's a self etching primer so theoretically the metal doesn't require any special preparation prior to spraying other than a good degreasing. There are some that claim this primer provides zero corrosion control.
Jake, they are correct, a bare layer of etching wash primer has no corrosion protection. Only epoxy has corrosion protection.
Jan
 

142yx

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Joined
Dec 8, 2009
Messages
96
Location
Centennial, CO
Interesting, I am really curious as to what the results are going to be.

I think I am leaning heavily in the direction of not using any form of corrosion protection on the inside surfaces of my Waiex.. can't find any information on the internet that really tells me it will be worth all of the effort and the designer recommends against it.

If someone knows something I don't and thinks this will be a big mistake I am still all ears though.
 

paulgy80

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Joined
Mar 24, 2008
Messages
51
Location
Little Snoring, Norfolk
Love the testing program, can't wait to see the outcome. I have a piece of 6082T6 on test coated with Bulleye 123, water based primer. It has been outside now for over 1 year, and I live about 5 miles from the sea, whilst it is not a highly corrosive environment unprotected steel rust pretty quick.

So after 1 year in a North Norfolk climate the single brushed layer of 123 is undisturbed, no visable corrosion, and has taken a bend without cracking or peeling. Oh and preparation was just a wipe with panel wipe, no scuffing of the surface.

Paul
 

MalcolmW

Well-Known Member
Joined
Jan 21, 2007
Messages
118
Hello,

I'm not a coatings man, however, I have some experience with aluminum, adhesive bonding and environmental testing which involved surface preparation.

It would seem to me that the passivating process that comes from a chromate conversion on the surface of aluminum should offer a very stable substrate and barrier to further corrosion. I seem to remember that environmental exposure testing resulted in significant corrosion to the untreated aluminum, however, the aluminum with the chromate conversion surface held up well.

Consequently, if this holds up in your application, it would be the thinnest and lightest weight corrosion protection for your structure (within the aircraft's skin). In addition, a chromate conversion surface provides an excellent base for paint adhesion.

Details of chromate conversion have been discussed previously, however, if there are questions on the process, I'm sure I can dig a description out my files (wherever they are).

Always do some testing, and fly safe,

MalcolmW
 

MalcolmW

Well-Known Member
Joined
Jan 21, 2007
Messages
118
Jman Jake;

I am most impressed with your testing program. It replaces opinion with hard data, and I'm quite interested in hearing the results. As I mentioned above, I believe that chromate conversion offers corrosion protection, and a good substrate for paint adhesion.

All the best, and thank you for sharing your efforts with us.

MalcolmW
 

Tom Kay

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Joined
Aug 10, 2007
Messages
398
Location
Ottawa Canada
Jake;

As far as I'm concerned, this kind of thread is exactly why these forums exist. Well done, and I am really curious to see the results.

I have a couple thoughts, maybe you can reply. First, is it typical to wait 4 weeks after priming, to get to the riveting stage? If so, that kind of takes away from the "spontaneity" of building an aircraft (I'm kidding about that word). But it could really mess with one's schedule. Are there primer/corrosion systems that are usable more quickly, on the order of hours, or overnight cure?

Second, how do these primers protect against corrosion, with dissimilar metals, such as aluminum touching 4130? This is a question I've been asking in another thread, and I realize this may be outside the scope of your home lab project. But do you have any knowledge or experience with 2 different metals and protecting them?

Thanks again for posting this. Tom.
 

MalcolmW

Well-Known Member
Joined
Jan 21, 2007
Messages
118
Hello, Tom;

I can only speak to 'priming' using chromate conversion or its variations.

Normally, once aluminum has been treated with alodine (or chromic acid), it is rinsed with distilled, deionized or filtered rainwater and air dried. Once completely dry (allow twenty-four hours in air), either bond using a well-tested adhesive or use in chosen application.

As for placing aluminum against a dissimilar metal, the danger of galvanic corrosion is always present. If the environment is exposed to the weather, i.e., water can enter, then the two dissimilar metals must be isolated or separated by an 'insulator' or a non-conductive material. This can be an organic coating, a plastic shim, or an organic liquid (lubricating oil).

Note that the presence of electrolytes in the water accelerate the rate of galvanic corrosion. Pure water (rainwater) can cause galvanic corrosion in this set of circumstances, however, at a much reduced rate.

If the location is within an interior cavity with no chance of water entering, aluminum can be placed against a dissimilar metal.

Recognize that vibration always exists in operating aircraft, and should there be any degree of freedom of movement between aluminum and 4130 steel, the aluminum loses (wears more rapidly).

I hope this provides one of several answers that you may need. I cannot adequately address service / performance of other primers previously mentioned.

Fly safe,

MalcolmW
 

zk-jkw

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Joined
Apr 20, 2009
Messages
136
Location
NZ
Hey Jake, did you get any conclusive results from this test. Looks really useful. Thanks
 

Tom Kay

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Aug 10, 2007
Messages
398
Location
Ottawa Canada
Yeah Jake, I've been checking back into this thread for a while. Quite curious about the results.

Thanks, Tom.
 

Jman

Site Developer
Joined
Oct 22, 2002
Messages
1,881
Location
Pacific NW, USA!
Ha! Well, my contraption ran out of salt water when I was in Afghanistan. My plan for having it refilled while I was gone didn't work out. I'll see if I can post some picture of what happened up until it ran dry.
 

Tom Kay

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Joined
Aug 10, 2007
Messages
398
Location
Ottawa Canada
Jeez Jake;

If you lived by an ocean, you'd have that problem solved. Could you, for the sake of science, move house?

OK, I'm bad. But if you do get a chance to show us some results, even at the stage where your torture chamber ran dry, we might get some good general info out of it. We have salt fog chambers at work, and they really do destroy untreated aluminum quickly. If the chambers weren't so busy, I would do some tests myself, especially where aluminum meets 4130.

Thanks, and off to the salt water store with you!

Cheers, Tom.
 
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