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Corrosion Protection Test

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Jman

Site Developer
Joined
Oct 22, 2002
Messages
1,881
Location
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.

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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.

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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.........
 
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