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low.safety.standards
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[*] posted on 19-10-2018 at 05:42
HF + HNO3


Hello People,

I've got a bit of NaF which I need to turn into HF to use as an inhibitor for HNO3. I'm making HNO3 to use as an Oxidizer along with Turpentine/Aniline Fuel for an hypergolic rocket engine. HNO3 eats through most things including my tanks which are double walled stainless steel (thermos), I need to add HF to create a fluorate layer on the walls to prevent corrosion. Literature says 0.5% by weight so we're talking milligrams. How can I do this safely?

My thinking is to add NaF + H2SO4 to a copper tube, hammer shut one side and have the rest of the tube go through some cooling and end up in a Teflon tube that'll be submerged in HNO3, any better ideas? also thought of using glass and throwing it away later.

Would just adding the salt work? That would be MUCH better than making HF.

[Edited on 19-10-2018 by low.safety.standards]
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[*] posted on 19-10-2018 at 06:13


Theoretically, just adding the salt should work, but I doubt it is soluble in fuming nitric acid. The nitric acid would dissolve a bit of the metal, but the fluoride in solution should immediately precipitate the dissolved metal ions making a passivating coating of metal fluoride.

If your application can tolerate 5% water in your nitric acid, then you might also consider using commercial 10% HF as an inhibitor. Some italian stores sell it where I live as rust remover (it looks like this and a bottle costs 1 Euro https://goo.gl/images/Cj8fCL)

[Edited on 19-10-2018 by CobaltChloride]
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[*] posted on 19-10-2018 at 06:36


Quote: Originally posted by CobaltChloride  
Theoretically, just adding the salt should work, but I doubt it is soluble in fuming nitric acid. The nitric acid would dissolve a bit of the metal, but the fluoride in solution should immediately precipitate the dissolved metal ions making a passivating coating of metal fluoride.

If your application can tolerate 5% water in your nitric acid, then you might also consider using commercial 10% HF as an inhibitor. Some italian stores sell it where I live as rust remover (it looks like this and a bottle costs 1 Euro https://goo.gl/images/Cj8fCL)

[Edited on 19-10-2018 by CobaltChloride]


The more moisture the more corrosive the HNO3 gets to my stainless steel walls, I wanted to use the dry salt or HF gas. I really don't wanna throw red fuming nitric acid in NaF to see what happens, I need more info.
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[*] posted on 19-10-2018 at 06:53


Nothing bad would happen if you just combine the fluoride and nitric acid (as long as you add just 0.5% NaF), but in order for it to work, the nitric acid needs to be able to dissolve it. I doubt it can, but you should try it because the nitric acid could also react with the NaF producing HF and precipitating NaNO3. In this scenario the HF dissolves in the nitric acid.

https://chemiday.com/en/reaction/3-1-0-8404

[Edited on 19-10-2018 by CobaltChloride]
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[*] posted on 19-10-2018 at 07:14


Try dissolving NaCl in your nitric acid, see what happens, I guess this could predict what happens with NaF.
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[*] posted on 19-10-2018 at 07:29


Quote: Originally posted by CobaltChloride  
Nothing bad would happen if you just combine the fluoride and nitric acid (as long as you add just 0.5% NaF), but in order for it to work, the nitric acid needs to be able to dissolve it. I doubt it can, but you should try it because the nitric acid could also react with the NaF producing HF and precipitating NaNO3. In this scenario the HF dissolves in the nitric acid.

https://chemiday.com/en/reaction/3-1-0-8404

[Edited on 19-10-2018 by CobaltChloride]


Yeah I saw that too, but no other mention that it would work. A couple miligrams of NaNO3 wouldn't be a problem for me, I can peristaltic pump the IRFNA out of the reaction vessel with a Teflon hose and leave the solids on the bottom. I didn't want to make any tests before getting solid inputs because any steel/copper vessel that I have would corrode and leave impurities that would probably prevent me from seeing any NaNO3 or NaF leftovers. Now I realized I have a couple 2cm teflon sticks here, I'll drill into them to make a short thick walled test tubes. I can let it react for a few days and later test the precipitate solubility in alcohol (NaNO3 is soluble, NaF is not) or try to melt it (NaNO3 = 308º, NaF = 993º).

I'll try to setup this soon as possible. Any idea of the solubility of HF in HNO3 ?
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[*] posted on 19-10-2018 at 07:49


Inner dimensions are 50cm X 12mm

tef.jpg - 142kB
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[*] posted on 19-10-2018 at 10:53


@Tsjerk: That isn't really a good comparison. Red fuming nitric acid would very rapidly and vigorously oxidize the chloride to Cl2. On the other hand, fluoride should be totally unreactive towards HNO3.

@low.safety.standards: I'd suggest putting a bit of NaF and fuming nitric acid in your desired proportions in a small PP tube (urine sample tubes are great for this) and swirling the mixture for a couple of minutes. Afterward, decant the acid from the leftover powder and let the powder dry in air. After this is done, dissolve the powder in water and then mix the obtained solution with CaCl2 (road salt) solution. If there still is fluoride in there, you should get a white precipitate. If most of the fluoride has been transferred to the acid solution, you shouldn't get any precipitate.

I don't know the exact solubility of HF in HNO3, but it should be pretty high considering that HF is the very, very polar.


[Edited on 19-10-2018 by CobaltChloride]
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[*] posted on 21-10-2018 at 23:09


You can buy solid NH4HF2. Sometimes it is on eBay, but there are online sellers as well who sell this chemical. I expect this to be more soluble than NaF and adding a tiny pinch of this to the HNO3 may do the job. The salt itself is easy to handle. It is a dry solid, not particularly hygroscopic and it does not give off any fumes of HF below 100 C or so.



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[*] posted on 23-10-2018 at 09:44


Googling 'HNO3 + NaF Corrosion' I found several papers on corrosion-testing using NaF as fluoride ion source for HNO3. Acid concentrations varied and couldn't find solubility info. I'll try and find a small piece of austenitic stainless steel I can cut in 2 and test w/wo NaF for a few weeks.
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[*] posted on 24-10-2018 at 01:59


Can HF+HNO3+Au make fluorauric acid?



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