Making metal HNO3 resistant?

[Sonny Jim]

Had an idea regarding HNO3 production today, along the lines of the distilling XNO3 + H2SO4 method. I've been tearing my hair out about ways to obtain and shape suitable glass with no success, for an improvised distillation rig, but then this occurred to me. Could one not coat say, a regular copper or steel pipe with an acid resistant material? I first thought about dipping such a pipe in molten glass in the hope of a glass layer forming on the pipe's inside, rendering it acid proof. Then I thought maybe the pipe could be electroplated with something that would resist HNO3.

The idea sounds good to me, cause anyone can get metal pipes. They're also damn easy to put together in a gas proof way, and can be shaped etc etc much easier than glass can. I dont know whether this would work too well so I'll put it in the cooler. Can't do any harm that way.

[Trinitrotoluene]

One idea I had been thinking is maybe coat the inner metal with molten teflon, or even molten aluminium which is pure, melting point of that is around 1200*F.

But there is problems with coating with glass, one such issue is, glass have a fairly high melting point, so you would need sufficient heat to melt it. Another problem is when glass cools it may crack, so you may need to find a way to slowly cool the glass, so it woulden't crack, or fracture.

[Tuatara]

I recall seeing somewhere that both aluminium and stainless steel are suitable for making containers for red fuming nitric acid, though I imagine one would have to be careful which alloys were used.

I'd be hesitant to rely on coating something - one small pinhole you wont know about until its too late ...

[mongo blongo]

Wouldn't coat it with glass. Think that in operation there is going to be temperature changes which means the two materials are going to expand and contract differently. With something as brittle as glass you can guess what is going to happen. Electroplating sounds good but why not just use an aluminium pipe?:rolleyes: :D

[0EZ0]

Long time no see, mongo blongo:D.

The main problem I see with using commercially available piping is that it is not of high purity. Many types of Al pipe are alloys or have a substantial oxide content. If you could find a supplier that sells very high purity Al piping and accessories, then yes you would be set.

Coating pipe with glass would also be quite difficult and impractical when you can use normal lab glassware. Maybe you could use a light frame or similiar substrate to coat with glass. It would negate the process of forming and shaping the glass in the usual manner. Instead just dip and gradually cool.

You may even be able to form a clay rod and dip in molten glass. It would help to dissapate the heat evenly to prevent cracking. Once cooled you could scrape out the clay the leave the glass shell. A similiar technique is used when a glass bottle is filled with sand or powdered clay and cut using a heating method. The filling is used to help stop cracking due to uneven temperatures of the glass.

I think your best bet after finding a commercially available source of the right shaped glass is trying to form your own from glass sheet. It is a tedious process that would require some skill, but would be well worth the effort.

There is a pdf on the ftp somewhere with all the techniques of cutting and shaping glass. It is called Glassware.pdf and you could probably find it in the recent or general chem folder.

[TRUTHSEEKER]

I used to know a guy that was a glass-blower. He had a nice business making custom glassware for several labs. Last I heard he had retired from this. Anyway, learn to make your own glassware or find somone who can - I don't think it will be easy either way. He claimed it was pretty easy to do once you learned how and had the equipment.

[bubbling_beaker]

Hi,
Ive heard of coating metal with a black layer of something by dipping it in a mix of NaOH and NH4NO3 i think its the NH4OH that does it,
iirc its called nitriding or something, perhaps that will be acid proof .:confused:

[koolkitty89]

On wikipedia I've read that you can "denature" nitric acid with a small addition of hydrofluoric acid. This causes a film of corrosion resistant metal fluoride to form on the surface of metalic containers.

[tomu]

Using Teflon Coating on the steel pipe might be a way to go. See for specifics about Teflon Coatings: http://www2.dupont.com/Teflon_Industrial/en_US/products/selection_guides/coatings.html

A good description about using Teflon Coatings on steel can be found in the book 'Vestbusters' by Uncle Fester.

[Skean Dhu]

I recall trying to use a glass soda bottle, PTFE tape and aluminum tubing from the hardware a few summers back. It wasn't all that productive, however the setup I was using was extremely primitive and rather rushed. I did get a few< 5mL of what I assume was nitric acid. Most of the fumes didn't condense or were converted inside the tube, which after the synthesis was deamed a failure had considerable 'gunk' deposits inside it.

This could have been due to oils left on the tube during manufacture, due to impure alloy usage,or perhaps me not building up a sufficient Aluminum oxide layer before beginning the distillation attempt.

The undistilled liquid did however turn crystal clear and there were no suspended solids at all. Upon opening the seal the beads of KNO3+H2SO4 solution started fuming.
It might be worth revisiting if one were to anodize the aluminum tubing before hand.

Oezo: About commercial Al tubing containing a high oxide content. Isn't that what you want? I mean the entire reason Aluminum is used is because it forms surface oxides that protect the rest of the sample from corrosion; so in this case the inferior quality would be beneficial would it not?

Sonny jim: Pardon my asking but, What's so prohibitive about purchasing glassware from an online source? Theres no reason to go the whole nine yards and get every condensor under the sun if you're only going to be distilling acids. Heck you don't really need a traditional distillation setup, you could buy a simple retort. They are considerably less expensive, somewhere in the order of 40-50USD for 250, and 500mL respectivley(on eBay). Unless your entire goal is to use as few commercial parts as possible.

-edited for space between paragraphs

[Jacks Complete]

I think I'm going to move this to a better place.

Also, the purity of most aluminium alloys will be fine, as long as the oxide coating is Al2O3 which is pretty inert. Certainly the cheap aluminium tubing you see about the place will be, since it is mostly aluminium. Anodizing the completed tubes would seem to be a good way to ensure safety. Also, ensure joints are tight and won't leak or move, since scraping will expose fresh, highly reactive AL to the HNO3, which will do you a mischief. Don't solder the joints either, as the solder will react.

Glass blowing is easy, once you have the equipment (which isn't exactly common!). The hard part is actually getting enough skill so you can make something worthwhile (unless you want simple glass baubles!), and then the knowledge of how to prevent it shattering as it cools.

[meselfs]

On wikipedia I've read that you can "denature" nitric acid with a small addition of hydrofluoric acid. This causes a film of corrosion resistant metal fluoride to form on the surface of metalic containers.

Indeed, it also says that in Comprehensive Inorganic Chemistry, though for nitrogen dioxide.

Stainless steel of high quality will work. However: the "high quality" bit is important; I've read in engineering magazines (more then once) about events where there was too much carbon in the steel or it had a bad microstructure, and it leaked. So make sure it's high quality. Can't go wrong with inconel, or something like that.




Glass blowing is overkill here. Glass working, however, is the way to go. You can, with a little work, turn a straight piece of glass tube into a coil to fit in a cold water bath, and attach that to other glass pieces with PTFE. Currently I use plaster wrapped densly with PTFE tape for stoppers as they're easy to make and completely acid proof; however I'm thiking of repacing that with just solid PTFE since the tape doesn't stay in it's place all to well.

Also, plumber's PTFE tape may or may not be pure, my acid comes out green and I'm blaming the PTFE tape. Anyone else had this experience?

[Meawoppl]

I know that there is a way to apply a glassy layer with some clever sol-gel chemistry. We had to do it once to make some metal plates corrosion resistant. Looking at my notes I have the following recipe:

1.) Combine in a flask: 25.5g methylthyltriethoxysilane (to avoid expansion/contraction/cracking while drying), 8.1g tetraethyl orthosilicate (to bind the colloidal silica together?), and 14.3g colloidal silica.

2.) Stir for 5 min.

3.)While stirring add dropwise 0.3g HCl in 35% solution.

4.)Stir for 20 min.

5.) Add 123.5g isopropyl alcohol

That recipie makes a rather large batch of the hard coat, but is completly scalable to whatever you need.

We then dipped various metal plates in the solution and dried them and then fired at 200 C for about 40 min.

My notes tell me that the films were pretty thin. They caused those cool color gradients you sometimes see with thin oxide layers on the order of the visable wavelength of light (about 400-700 nm I think). The plates were fairly chemically resistant, but we never did anything with fuming nitric acid.

[lucas]

Also, plumber's PTFE tape may or may not be pure, my acid comes out green and I'm blaming the PTFE tape. Anyone else had this experience?

It may be your tape anyway but I have seen green nitric before, when water was added to red nitric.
Blue nitrous acid is a common impurity when adding nitric acid rich in NO2 into water. Of course the NO2 is yellow to red depending on concentration. When both are present I believe that is the cause of green nitric.