Sciencemadness Discussion Board

Titanium oxide strip

Swede - 11-10-2012 at 05:51

Titanium metal forms a fierce oxide layer that must be removed prior to any attempt at plating or coating. Most of the methods I've read about use HF. Usually, the acid itself. sometimnes, the fluoride ion in the form of ammonium bifluoride.

The only fluoride I have is sodium. Is there anything to prevent its use when combined with a mineral acid like HCl or HNO3? Or is a conversion to the ammonium salt possible?

I am aware of the toxicity of fluorides. Any suggestions on the mechanical and chemical stripping of Ti oxides from the metal is appreciated. I am going to attempt differing formulations of MMO, which is a "paint & bake" process rather than an electroplating, but the cleanliness of the Ti is still critical.

I have abrasive blasters (sand & glass bead), all the mineral acids except HF, NaF, and a variety of typical lab chems. Thanks!

kristofvagyok - 11-10-2012 at 11:16

HF is a nasty chemical... I would suggest to work in a polymer bottle with max. 40% acids.

To convert it to ammonium. fluoride is easy, just add some ammonia to the HF (use excess NaF to avoid the exess HNO3). Only catalytic amount of NH4F if needed, so a little ammonia is enough.

As I remember Titanium is a bit soluble in cc HCl (with a little H2O2?, I don't remember exactly), if the goal is to remove the oxide layer isn't that easier? Just boul it in a solution like that for a few mins, much less toxic than HF.

P.S.: wear gloves and a face shield if you work with cc HF, also a fume hood is a good point. And always have some Ca(OH)2 solution nearby if anything happens...

UnintentionalChaos - 11-10-2012 at 11:44

One can purchase ~1M HF in many walmarts as Whink iron stain remover in the brown bottle (intended for clothing, IIRC). I would describe it's reaction with titanium metal as vigorous and immediate. I see no reason why acidified fluoride salt wouldn't work just as well and it should be very dilute if you just want to strip the oxide coat.

1M isn't the sort of thing you want to spill on yourself, but it certainly wont put you in the hospital unless you do something -really- stupid. :cool:

There is a product designed for metalworkers who desire to anodize their titanium or niobium. I believe it's a mix of fluoride salt and a peroxysulfate (unsure if it's peroxymonosulfate or peroxydisulfate). I'll check back when I can dig up the name of the stuff and an MSDS.

Don't forget to degrease the metal with acetone, hexanes, or whatever first.

EDIT: found it. The product is called Multi-Etch and is a combination of sodium fluoride and ammonium persulfate. A hot solution of this apparently removes the oxide layer from valve metals quite easily and is roughly pH neutral.

[Edited on 10-11-12 by UnintentionalChaos]

Bot0nist - 11-10-2012 at 14:42

I am pretty sure I saw a few videos on making various anodes in which a good boil in concentrated HCl(aq) did the trick.

DDTea - 11-10-2012 at 19:48

In an old gig, I would titrate TiO2 pigment samples to determine their purity. To get the material into solution, we fused the pigment in a sodium peroxide flux and then dissolved it in an HCl/H2SO4 mix. This was usually enough. Sometimes, we cheated: if some TiO2 was unfused, we'd add a few drops of conc. HF. Then it would jump into solution.

[Edited on 10-12-12 by DDTea]

watson.fawkes - 11-10-2012 at 19:56

Quote: Originally posted by Swede  
Titanium metal forms a fierce oxide layer that must be removed prior to any attempt at plating or coating
The problem I see with eliminating the passivation layer is that it will immediately reform upon oxygen exposure. Mechanical pre-cleaning is probably as good as any method for getting rid of the bulk of it, but unless you're working in vacuum, you'll still get that layer.

Since you're planning on using a paint and bake process, consider painting on as the first layer a reactive species that becomes active only at high heat. Then you've stripped the oxide layer while it's underneath the coating layers, protecting it from atmosphere. I doubt there's need to carry away the reaction products if they can be simply incorporated into the interfacial region without harm. This is desirable, if possible, in order to avoid bubbling and disruption of the coating. Another benefit is that the same reducer will reduce some of the first MMO layer. The good result would be an intermetallic layer with a density gradient from the substrate Ti to metals in the MMO, overlapping with an oxide gradient. It should lead to a strong bond if you can figure out the chemistry.

That said, I don't have any particular idea of the chemistry needed to get that result. This first layer wouldn't need to be very deep at all, though, since it only has to react away a quite thin oxide film.

hyfalcon - 12-10-2012 at 02:15

I think I've read a patent on here about using oxalic acid to prep titanium before coating. Let me dig and see If I can find it.


Here we go.

Attachment: US3650861 Etching_TITANIUM_by_Oxalic_Acid (1).pdf (134kB)
This file has been downloaded 624 times

Swede - 13-10-2012 at 07:42

Hey thanks everybody. WF - much of the paintable MMO solutions are HCl based to begin with, so I'm thinking an HCl strip (concentrated, hot) perhaps with some fluorine ions in the mix, followed IMMEDIATELY by the first coating, would do the job.

There is a slight possibility of setting up a glove box with argon or nitrogen, but that may be overkill.

MMO is produced by a series of paint & bakes, usually from 5 to 15, with 300 C bakes between coats, and a final bake upwards of 500 C. As soon as coating one is in place, further oxides should be inhibited.

Hyfalcon, excellent patent, many thanks.

multi etch

holms1975 - 21-12-2012 at 05:25

Quote: Originally posted by UnintentionalChaos  
One can purchase ~1M HF in many walmarts as Whink iron stain remover in the brown bottle (intended for clothing, IIRC). I would describe it's reaction with titanium metal as vigorous and immediate. I see no reason why acidified fluoride salt wouldn't work just as well and it should be very dilute if you just want to strip the oxide coat.

1M isn't the sort of thing you want to spill on yourself, but it certainly wont put you in the hospital unless you do something -really- stupid. :cool:

There is a product designed for metalworkers who desire to anodize their titanium or niobium. I believe it's a mix of fluoride salt and a peroxysulfate (unsure if it's peroxymonosulfate or peroxydisulfate). I'll check back when I can dig up the name of the stuff and an MSDS.

Don't forget to degrease the metal with acetone, hexanes, or whatever first.

EDIT: found it. The product is called Multi-Etch and is a combination of sodium fluoride and ammonium persulfate. A hot solution of this apparently removes the oxide layer from valve metals quite easily and is roughly pH neutral.

[Edited on 10-11-12 by UnintentionalChaos]


Hello
I live outside US and it is impossible to buy Multi etch
But i can buy sodium fluoride and ammonium per sulfate for etching titanium
Can anyone tell how to mix this two chemicals???
Thanks

holms1975 - 21-12-2012 at 05:35

Hello
I live outside US and it is impossible to buy Multi etch
But i can buy sodium fluoride and ammonium per sulfate for etching titanium
Can anyone tell how to mix this two chemicals???
Thanks

gerarddekker - 16-3-2016 at 11:33

Quote: Originally posted by holms1975  
Hello
I live outside US and it is impossible to buy Multi etch
But i can buy sodium fluoride and ammonium per sulfate for etching titanium
Can anyone tell how to mix this two chemicals???
Thanks


If you live in Europe, you can get a multi-etch clone here : http://www.biketweaks.com/titanium-etching-multi-etch-europe