Question- Powder Coating of aircraft parts

[nomocom]

Powder coating has become a common finish for many non-aviation applications and a few aviation items. That probably isn't suprising, since many of us have likely seen powder coating peform well in challenging environments that would have removed other types of coatings quickly. The longevity, relatively low cost, and ease of application may be arguements for it's use, however there are a few concerns.

Once the part is put in place, say it is chipped at a later time. How is this part repaired? Is it removed, stripped, and recoated? Or is the affected area sanded and painted? My old Cessna repair manual says nothing about a powder coating as a refinishing method. Speaking generally, are repair methods limited to what the repair manual includes and what is described within AC 43.13?

Temperature may be a concern. Some aircraft parts are heat treated. It may be difficult to ascertain whether or not the common 400 degree powder coat bake will have any metalurgic effects, since we often don't have the engineering data for the part in question and most of us aren't metalurgists. How do we know for sure? For example, the Cessna Wittman type main landing gear. It would be fantastic to have the durability of a powder coat, but can we safely and legally apply powder to the MLG?

One other concern I've heard expressed is that powder coating may form such a tough, impervious and flexible membrane that the base material may be cracking or corroding, but problems are hidden by the powder coat. Is this a realistic concern?

First person experiences, peer reviewed studies, sharing of manufacturer guidance or FAA guidance would all be of interest.

[Hiperbiper]

There was an NTSB report last year concerning the crash of a Safari homebuilt helicopter. The cause was a powder-coated cyclic control stick fracturing.
On a personal note; I have seen several aluminum rims that were powder coated fracture at the dragstrip. Some people have good luck with the process and some don't. In any case it is not something I would like to have in the back of my mind while I'm flying my plane down the backside of a loop...

Chris

[nomocom]

There was an NTSB report last year concerning the crash of a Safari homebuilt helicopter. The cause was a powder-coated cyclic control stick fracturing. Chris

Chris
Do you have a link? I'm having no luck finding it. Closest I could find was a control rod failure, but wasn't powder coated......

[Jim Hann]

There was an NTSB report last year concerning the crash of a Safari homebuilt helicopter. The cause was a powder-coated cyclic control stick fracturing.
On a personal note; I have seen several aluminum rims that were powder coated fracture at the dragstrip. Some people have good luck with the process and some don't. In any case it is not something I would like to have in the back of my mind while I'm flying my plane down the backside of a loop...

Chris

Chris, is Gilbert Goodlett's accident the one you are thinking of? I attached it below. It was a non-fatal Safari accident in 2007, and it was apparently due to anodizing a control rod, not powdercoating.

Jim

Anodizing & Fatigue.pdf

[Hiperbiper]

Chris, is Gilbert Goodlett's accident the one you are thinking of? I attached it below. It was a non-fatal Safari accident in 2007, and it was apparently due to anodizing a control rod, not powdercoating.

Jim

Anodizing & Fatigue.pdf

Jim; you are correct; that is the crash I was (wrongly) refering to...

My apologies to all the powder-coaters I might have impuned.

Chris

[R. Novak]

This sounds like an interesting discussion -
Powder coating seems to be another topic where many people have a personal preference. Sort of like which finish paint to use, what type of engine oil, etc... So, I will add my two cents worth:
1) Inspections - Coming from the perspective of performing annual inspections and such, I would rather have a coat of paint instead of powder coat for the reasons listed previously by others.
2) Corrosion - the engineering department I work for recently completed extensive analysis (as in very expensive and time consuming) of powder coating with regard to corrosion, with the final result being that we will not powder coat over bare metal. Powder coating offers a very thick coating which provides good barrier protection, but there is more to an effective corrosion resistant coating than that. The good thing is that most aircraft, except for seaplanes in salt water, don't see the environment we were testing for. We consider powder coat as a top coat only (like a coat of enamel), to be used over a corrosion resistant primer, which is on top of the proper conversion coating for the particular base metal being used.
3) Fatigue - we are not aware of any hydrogen embrittlement issues, and would not think the baking temperatures would be an issue except for some heat treated parts.
4) Repairability - a coat of paint wins out for ease of repair.

Randy

[nomocom]

This sounds like an interesting discussion -
Powder coating seems to be another topic where many people have a personal preference. Sort of like which finish paint to use, what type of engine oil, etc... So, I will add my two cents worth:
1) Inspections - Coming from the perspective of performing annual inspections and such, I would rather have a coat of paint instead of powder coat for the reasons listed previously by others.
2) Corrosion - the engineering department I work for recently completed extensive analysis (as in very expensive and time consuming) of powder coating with regard to corrosion, with the final result being that we will not powder coat over bare metal. Powder coating offers a very thick coating which provides good barrier protection, but there is more to an effective corrosion resistant coating than that. The good thing is that most aircraft, except for seaplanes in salt water, don't see the environment we were testing for. We consider powder coat as a top coat only (like a coat of enamel), to be used over a corrosion resistant primer, which is on top of the proper conversion coating for the particular base metal being used.
3) Fatigue - we are not aware of any hydrogen embrittlement issues, and would not think the baking temperatures would be an issue except for some heat treated parts.
4) Repairability - a coat of paint wins out for ease of repair.

Randy

I agree whether to pow coat or not seems be driven by personal preference. Personal preference is great when it comes to colors of favorite foods, but when we are talking about a maintenance procedure, I'd like to think we can come up with more. I suppose that is why I started this thread. Along those lines, could you get some underlying data from the "extensive analysis". Without that, we are left trying to piece together the data and the logic train- an effort I'm lousy at and when you read the below, you can see I failed miserably, as I come to different conclusions than the engineering department.

Too Thick? It was suggested earlier in the thread that new polyurathanes can be laid down every bit as thick. I seemed to get verification of that yesterday. The local paint shop suggests 3 coats of polyurathane on top of 2 coats of epoxy primer. Now how thick that will be- not sure, but I'd think similar enough I have to toss the "thickness" argument. Should we only be painting thin, so we can see what is going on with the base metal?

The local powder shop does coatings for lots of different industries, one being whitewater equipment. These parts get banged against rocks, other equipment, tossed in the back of a trucks and bounced down backroads. He had some rafting frame tubes lying around. The superviser took a spare 1 inch tube that had been coated and as we toured the process, he wacked steel beams and other hard objects near our path. The result, a dented tube, but the coating stuck right to it, it didn't scrap off or peel. He explained not only does the powder cross link to itself, it also chemically bonds to the steel- which is exactly what you want if you are trying to prevent corrosion, no? What is a primer needed for when the powder has locked onto the steel surface?

3) Hydrongen embrittlement? Maybe a concern about paint removal on main landing gear? In relation to MLG, IMHO, there has been a generalized reaction that fails to consider stripper type. There are some newer acid strippers on the market that should NOT be used (H+ donors) but you can figure out which ones those are by reading the label or the MSDS. I stuck to the old style methylene chloride (basic pH) that has been used on airplanes for ages- aluminum and steel. Furthermoe, if a part is suspected of hydrogen embrittlement, one of the treatments is an oven to drive out the hydrogen, so at this point I see no connection between HE and powder coating. If there is one, please educate us.

4) Airframes based out of Alaska produces powder coated PMA'ed Piper MLG. I've chatted with one of their engineers. Continued airworthiness consist of inspection, then repair of any uncoated metal. Thing is, all that repair consists of is painting, rough it up, and throw some paint on. This touchup should be less frequent with powder coat, because it takes more abuse for it to come off. It would seem to follow that powder coating would win here, since it can have the same method of repair, but can be expected to need repaired less often.


Wes had a good point out earlier about the importance of maintaining the inside of the tube as well. Thanks for that. Do we agree outside versus inside condition and maintenance are independent of one another? Obviously both important! Turtle stated he had cut into tubing that looked good on the outside, but was rotten on the inside. Playing devil's advocate here, that is not a phenomenon unique to powder. Go on a used car lot and look for some nicely painted layers of rust. If you look long enough, you'll find some.

If we want to detect inside corrosion or cracks the earliest possible (without looking inside) should we not keep the outside bare? That would be silly. OK, maybe an eddy current inspection? That would be reasonable. What might shed some light on this concern is head to head testing between different coatings on steel that was forced to crack. Would the crack hide longer on a PC MLG or a polyurathane MLG? Need test results!

As the OP, I'd like to thank those that have contributed. Lacking any data otherwise, I'm heading in the direction of believing the only problem with powder coating MLG is that it is new. If I'm off track here, beat me up. I'll take it.

[Jim Hann]

Jim; you are correct; that is the crash I was (wrongly) refering to...

My apologies to all the powder-coaters I might have impuned.

Chris

No worries Chris, just want accurate info out there!

[Tom Downey]

The common routine for Cad plating on steel parts require baking after plating at 400 degrees, so I wouldn't worry about powder coating them, but heat treated aluminum, I wouldn't bake at any temps over 250 degrees. You would be required to be very well versed in the treatment process on the aluminum material, and stay well below the critical temps.
I know there are powders that will bake at 200 degrees.
As for the cracking problem, the new URA paint systems are as thick, and as pliable, so what's the difference?

[nomocom]

new URA paint systems are as thick, and as pliable, so what's the difference?

Tom,
I've googled URA paint systems and not getting much. Is it a propriatary product or a general term? I did find this http://www.hentzen.com/index.php/pro...rands/ura-zen/

I'm hearing 3 mils as a light coat and 5 mils as heavy from the local powder coating shops. I don't think there is much of an argument that powder is going to always be thicker than non-powder techniques.