Powder Coating Al80 Tanks

[LanceRiley]

Hi,

My AL80s are due for hydro and I would like them to be powdercoated. what should we do first?

1. Hydro test, Powder coat , the o2 cleaned.?

this is what i was thinking of doing. i assume we don't want the insides of the tanks immersed?

or has anybody tried this?

 

[CuriousRambler]

Powder coat isn't an immersion process. You electrically ground the tank, and have a powder spray gun with a high voltage charged tip. The HV tip imparts a charge to the dry powder, which is then electrically attracted to the tank. After it's sprayed, you have to bake the whole thing around 400F.

As far as I know, tank manufacturers all prohibit exposing aluminum tanks to anything over a couple hundred degrees, and powder coat certainly exceeds that limit.

 

[Scuba Cobra]

Correct. The curing temperature is above that of the temperature that condoms tanks.

 

[michael-fisch]

T6 heat treated Alum (the aluminum that Luxfer uses) can be kept at 180°C for many hours without changing the characteristics of the alum alloy. At 200°C the characteristics have already changed after 15 minutes.
If your powdercoating operation can promise and prove that the baking process is kept at 180°C or less, you are good to go.
If the temp reaches 200°C, the aluminum is still worth a few dollars, but not as a pressure vessel.

Michael

 

[CuriousRambler]

T6 heat treated Alum (the aluminum that Luxfer uses) can be kept at 180°C for many hours without changing the characteristics of the alum alloy. At 200°C the characteristics have already changed after 15 minutes.
If your powdercoating operation can promise and prove that the baking process is kept at 180°C or less, you are good to go.
If the temp reaches 200°C, the aluminum is still worth a few dollars, but not as a pressure vessel.

Michael

Would you honestly trust a powder coat shop's word that they'll keep you cool enough? Not only that, but would you trust their oven's temperature controller is accurate and reliable enough to stand behind their promises? Will you bet your life that the guy who told you it'll work out passed the word along to the next shift who's actually doing the work? Powder coat is not a precision process. If their oven is stable +/- 10%, at 180C, you've already destroyed your tank. You have to remember, this is not a situation where 180C is acceptable but 200C isn't. Is 185C OK? What about 190? For how long? How stable is the oven? How precise is the specific batch of alloy your tank came out of?

This isn't a game of "it could be OK if..."

Powder coating tanks, without proper means to fully heat treat them after the fact, is unsafe. It's that simple.

I'm all for knowing the limits, and playing within the reality rather than the published "keep the least common denominator safe" policies. This isn't that situation. You can powder coat with a heat gun, or a propane torch, or a retrofitted electric kitchen range. There is little to no precision required for most powders (some are more sensitive than others, in terms of finish quality), so nobody is going to pay for equipment that delivers pointless expensive precision. Your life-sustaining pressure vessel you wear on your spine ABSOLUTELY requires precision.

You can paint your tanks. Somebody in another thread mentioned vinyl vehicle wraps for cheap. Options exist. Powder coat isn't one of them.

 

[michael-fisch]

No, I almost trusted my Powder coat shop's word as to the max temps they would be baking with. They do a couple of tons of aluminum every day. But I also put a dab of temp changing paint on the inside of each tank that changes color at 180°C. My 6 AL80s came through with the paint not having changed color.
T6 heat treating is exactly defined, if you do it any other way it isn't T6 aluminum.

Michael

 

[Superlyte27]

It's been about 20 years since I was a hydro guy, but if I recall correctly, a powder coated tank was immediately condemned. I'm sure someone here will know for sure and can correct me if I'm wrong.

 

[michael-fisch]

It's been about 20 years since I was a hydro guy, but if I recall correctly, a powder coated tank was immediately condemned. I'm sure someone here will know for sure and can correct me if I'm wrong.

I would hope so. The difference between doing it right and creating a bomb is very small. If your shop is not working to aerospace standards, you will not be happy .

Michael

 

[CuriousRambler]

Yes, T6 is specifically defined in how to meet that specification. What is does not do is specifically define how to ruin that treatment after the fact. Tank manufacturers publish numbers that are not to be exceeded. I don't have those numbers handy, but recall seeing someone link to them in a discussion regarding using a hair dryer to remove stickers, and it was something incredibly low like 165F.

I'm not saying heat treating safely is impossible. I am saying there's no shop on earth I'd trust to do my tanks, ever. And I regularly work with an extremely advanced coating shop. I think it's irresponsible to suggest to anyone that powder coating tanks IS acceptable.

 

[michael-fisch]

Yes, T6 is specifically defined in how to meet that specification. What is does not do is specifically define how to ruin that treatment after the fact. Tank manufacturers publish numbers that are not to be exceeded. I don't have those numbers handy, but recall seeing someone link to them in a discussion regarding using a hair dryer to remove stickers, and it was something incredibly low like 165F.

I'm not saying heat treating safely is impossible. I am saying there's no shop on earth I'd trust to do my tanks, ever. And I regularly work with an extremely advanced coating shop. I think it's irresponsible to suggest to anyone that powder coating tanks IS acceptable.

Unfortunatly I'm afraid tha the 165°F was written by somebody with a dive shop monkey's education. Unfortunatly it happens all the time, and most of the time we believe what they tell us in the gasstation or at the pump.
I had access to the raw data and results of testing done as part of a engineering groups doctoral thesis that was successfully defended.

Michael