Coating for Pressurized Liquid Oxygen
10-14-2009, 12:21 PM, (This post was last modified: 10-14-2009, 12:56 PM by jsheridan.)
#1
Coating for Pressurized Liquid Oxygen
I am new to the forum, but as an ex-pat Brit living abroad, it's nice to see a UK site as a significant hub for discussing this topic!

I've been involved in projects which have used HVOF before, but every previous time I was just observing. I think it should work here, but this particular project has me a little concerned.

For a variety of reasons which I won't bore you with, we are building a fairly large pressurized tank to contain liquid oxygen (@ -225 deg C) which needs to be made from a high-nickel content steel (~18% nickel in this case).

The tank itself will be routinely pressurized to over 500 psi.

And best of all, its going to be mounted on the deck of an ocean-going ship, exposed to all the elements which the Gods can throw at it -- hence one of the reasons for the nickel steel.

I'm looking for a high-endurance coating solution for the inside of this tank. I was thinking 300 Stainless Steel, possibly Plasma Flame Sprayed, perhaps 2-3mm thick to get really good coverage.


One of my key concerns is that I don't know what the application process might deposit within the stainless layer which might be reactive. It *absolutely must not* contain any volatile contaminants, because at that pressure the Oxygen (especially the gaseous Oxy bubble at the top of the tank) will be extremely reactive in the presence of even trace elements which can possibly burn. If there is any sort of 'fuel' for the Oxy to react with, a minor impact as an ignition source could possibly turn the whole thing into a "bad day" for anyone on-board the ship at the time.

So, we are already planning very tight control of the whole environment during manufacture of the tanking and during the coating application process too, so that (at least) isn't a big concern during the application process. My concern is that I want to ensure the application process itself isn't introducing any unexpected reactive elements to the final coating material, perhaps as a by-product of the applicator's functioning. I don't know how they actually work, so I just can't tell.


Another concern I have is that given the maritime conditions, I want to reduce the chance of anode/cathode corrosion around the piping connections too, so I'm looking to make everything out of the same materials -- and that means having a coating process which can be applied inside pipes (although, luckily in this case, the pipes will be fairly large-diameter ones).

Given the particularly difficult task & situation in which this needs to perform, I wanted to get some real expert advice from others before proceeding into the design phase. So I'm asking here for any assistance you guys might be able to offer.

I appreciate any help you can provide.

Jay.
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10-15-2009, 03:11 AM,
#2
RE: Coating for Pressurized Liquid Oxygen
Hi Jay

Sign0016 to the Surface Engineering Forum.

Are you sure you need a coating? I'm not aware of any thermal sprayed coatings used inside LOX tanks. Anti-corrosion coatings on exteriors yes. Thermal spray coating deposits should only contain essentially what was in the original spray powder/wire apart from some oxidation products. Coatings are very often are sealed with organic sealers afterwards, which in this case would not be advisable.
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10-19-2009, 10:01 AM, (This post was last modified: 10-19-2009, 10:06 AM by jsheridan.)
#3
RE: Coating for Pressurized Liquid Oxygen
Thanks for the welcome Gordon!

I consulted with an expert Mech Eng who specializes in handling of cryogenic LOX for over 30 years -- he's worked for NASA since the Apollo days. His advice was that we're talking about a container with well over 500 tons of highly pressurized pure oxygen (along with a hydrogen tank right next to it too) which would make for a very effective bomb if anything ever went wrong. Given how potentially dangerous it could be, he told us not to take any chances at all. He was actually the chap who suggested a stainless coating (or cladding) might be a good option, which is the reason why I'm here asking for more details about the application process -- we're still very much in the early 'research' phase of the project, so all ideas are still on the table.

You mention "oxidation products". Any chance you could expand on that a little for me?

Can I also ask for a bit more information regarding the 'organic sealers' too? What is their function? Are there alternatives? There is an industry practice within the space community that these sorts of tanks have to be "LOX Clean" prior to use. We're still investigating the specific requirements involved in that, but from what I've learned so far, most forms of even trace oils and sealants need to be completely removed from the tank walls prior to use -- I suspect this will probably be one such substance.

Thanks for your time and expertise on this, it is helping inform our decisions.

Jay.
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10-20-2009, 03:29 PM,
#4
RE: Coating for Pressurized Liquid Oxygen
Hi Jay

Quote:You mention "oxidation products". Any chance you could expand on that a little for me?

Lets say you spray a nickel chromium alloy powder in normal atmospheric conditions, the coating would be a nickel chromium alloy containing a certain amount of nickel and chromium oxides. Other contamination may be possibility if the spraying environment is not clean.

Quote:Can I also ask for a bit more information regarding the 'organic sealers' too? What is their function?

Most thermal spray coatings will have some degree of porosity. Sealing this porosity is sometimes necessary to stop the environment permeating through the coating and reacting with the substrate.
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11-17-2009, 09:10 PM, (This post was last modified: 11-17-2009, 09:11 PM by jsheridan.)
#5
RE: Coating for Pressurized Liquid Oxygen
Gordon,
I just wanted to follow-up this topic with you to let you know that we're going to go ahead with this process for our LOX tanks and I appreciate your help in answering my early questions.

Having selected this for the LOX system, we are now investigating whether we can use a similar process for our LH2 tanks as well.

I wanted to ask what you know about Hydrogen Embrittlement in specific context of using this coating process?

To avoid Hydrogen being embedded into the coating structure, we are considering using either an Aluminium or Zinc coating and a Nitrogen or Helium plasma process. We are also looking at doing the work (inside the tanks) using an automated system in a purged nitrogen environment to reduce the chance of getting any hydrogen into the mix.

Do you know if anyone has tried anything similar before? Any idea what success they have had?

Jay.
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11-19-2009, 03:50 PM, (This post was last modified: 11-19-2009, 03:59 PM by djewell.)
#6
RE: Coating for Pressurized Liquid Oxygen
One of the advantages of thermal spray is that it does not produce the hydrogen embrittlement of the substrate like electroplating does. The coating you described sounds unnecessarily expensive. If, as you fear, something were to strike the tank, a 2-3 mm thick coating will likely crack and delaminate. In fact, coatings that are that thick are difficult to apply with any bond strength because the internal stresses in the coating can often rip them apart. I would rather see thick walls on the tank to eliminate the possibility of coating failure. If you decide to proceed with the application, an HVAF/HVOF coating will be denser to protect the base metal from interaction with the LOX.
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11-21-2009, 06:35 PM,
#7
RE: Coating for Pressurized Liquid Oxygen
Hi Jay

Quote:I wanted to ask what you know about Hydrogen Embrittlement in specific context of using this coating process?

Only come across concerns regarding hydrogen embrittlement prone substrates for aerospace applications where plasma spray parameters are limited to argon or argon/helium only. I think this is a case of "belt and brazes" or "just in case" as I don't think there is any evidence of hydrogen absorption or hydrogen embrittlement of substrates from normal atmospheric thermal spray processes.

As for the coatings, absorption of gases (hydrogen, oxygen and nitrogen being the main ones of concern) is far more likely during spraying process than that of a cold substrate. The only real case off concern that I have come across is with spraying off titanium and its alloys. The presence of oxygen and or nitrogen during spraying certainly has a noticeable embrittling effect on the coating. I assume hydrogen has similar effects.

Quote:To avoid Hydrogen being embedded into the coating structure, we are considering using either an Aluminium or Zinc coating and a Nitrogen or Helium plasma process. We are also looking at doing the work (inside the tanks) using an automated system in a purged nitrogen environment to reduce the chance of getting any hydrogen into the mix.

Do you know if anyone has tried anything similar before? Any idea what success they have had?

Certainly possible, using the part itself to form inert spray chamber. Also consider arc spray with argon or nitrogen atomising gas instead of air.

I am still unsure as to the reasons for wanting to coat the inside or these tanks.
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