Identifying Titanium

I've got some titanium wire which might be 'commercially pure titanium' but could be 6:4 titanium (6% Aluminium, 4% vanadium). How can I confirm if it's
the pure type or not?
Thanks,
Scrim
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Tricky. A chemical analysis would easily show up the Al and the V, but would cost you. A photomicrograph would show the partial beta grain structure in the 6-4, compared with the entirely alpha structure in the CP Ti, but would require expert work to polish and etch the sample and interpret the results, and some fairly sophisticated microscope equipment.
For a quick and dirty test, physical properties would seem to be your best bet. However, be aware that "CP titanium" is actually quite a range of materials, with small variations in oxygen content giving a significant variation in physical properties. Also, 6-4 may vary a little from different manufacturers, and in different forms. However, FWIW:
Densities (CP 4.5, 6-4 4.42 g/cc) are too similar, as are melting points (both around 1670 deg C) and Young's modulus (116 GPa and 105-120 GPa respectively). However, UTS (220 MPa and 935-1035 MPa respectively), and electrical resistivity (5.54 x 10^-5 and 1.68 x 10^-4 ohm-cm respectively) are sufficiently different to allow a clear distinction even with fairly simple equipment.
For further research, suggest you try www.matweb.com (which is where I got the above data).
David
--
David Littlewood

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writes

Hi David, First, thanks for the help. Really I was just hoping there might be some well known simple way to tell them apart. I guess I'll have to work with the basic physical properties, but first, I now have a chemical clue. I've been experimenting with colourful titanium anodising with mixed results (just one very impressive success that I haven't been able to repeat so far. As part of the surface preparation I've been stripping the oxide film with hot concentrated sulphuric acid due to unavailability of Hydrofluoric acid. That acid now contains a pale blue precipitate. I think this is most likely caused by Titanium or Vanadium, but I haven't been able to get any typical colours for compounds of either metal so far. I report back if I have any success. Cheers, Alan
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Both Ti and V are capable of forming blue salts. V in particular can have valency 2,3,4 or 5 and produces a bewildering variety of green, blue, yellow, violet etc. colours. Without doing a lot of research, can't tell you what would be expected from H2SO4, but my money would be on it being vanadium.
You really don't want to mess with hydrofluoric acid unless you know exactly what you are doing, have all the right equipment, and somewhere safe to do it. It can cause horrific and painful bone damage by skin absorption.
Have fancied trying anodisation of Ti for ages, and even went to the trouble of acquiring a 150V power supply last year for that purpose.
David
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David Littlewood

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writes

I can tell you, the purple I've got (done at 15 volts) is really something to see! I know hydrofluoric's got it's own set of particularly nasty properties. Since I got the anodising to work well once without it hopefully at some stage I'll work out a reliable process without it. Do you happen to know by any chance if both pure titanium and 6:4 stuff anodise well. I'll check out the colour chemistry tonight. Scrim
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The book I have suggests a 10% solution of ammonium sulphate as electrolyte, "but other solutions can be substituted". I don't know any reason why the 6-4 alloy should not show colours on anodising in the same way as pure Ti, but I can't give you any authority on that.
You obviously know all about the technique and have seen the charts, but for the benefit of others reading this, the oxide film increases in thickness as voltage is increased, and different interference colours are produced. For Ti, the colours remain useful up to about 85 volts, and once done at a higher voltage are unaffected by lower ones, so by playing around with voltage and immersion depth you can get interesting gradations. Niobium and tantalum are also very effectively coloured using this technique, up to about 130v and 170v respectively.
David
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David Littlewood

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writes

Hi, Thanks for all the ideas. If I had some known samples of pure and 6:4 titanium to compare my bits to, just bending them would be good enough for me, or comparing the effects of acid and so on. Electrolysis as a cathode might do it too. Thanks again. For anyone interested here is a useful picture of titanium wires anodised at 1 volt steps up to 100V. Most places seem to suggest that the exact form of electrolyte used doesn't matter, but here 0.1% sulphuric acid is compared to Diet Cola and the sulphuric is obviously better: http://elm.bme.unc.edu/~bob/bob_titanium.html
I've dug out an old (1932) chemistry book and learnt a bit. It says titanium reacts with sulphuric acid to produce a violet solution. Well my light blue precipitate in concentrated sulphuric acid completely dissolves to a violet solution when diluted by adding to some water. Vanadium apparently only dissolves in hydrofluoric and oxidising acids (like nitric I assume), so the fact there is no precipitate in dilute sulphuric seems to suggest the metal is vanadium free but I wouldn't like to bet too much on it!
Scrim
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From the figures I saw, the Young's modulus of the two is similar, you would need to measure the UTS to get a big difference.

Nice stuff. One thing I fancied trying is to withdraw a specimen from the bath while gradually increasing the voltage, to get a smooth gradation of colour (bit like a birefringent quartz wedge under crossed polarisers). Do you get any problem with corrosion of your cathode? The book suggests a platinised titanium basket (yeah, like we all have one of those in the garage) but I guess the stainless steel will be OK if you use the right grade and don't leave it in the acid when not positive. Interesting to know, that was one thing putting me off. How big a piece did you use? did you have any measure of current density? If you do get any corrosion, or Fe+++ contamination of the bath, try using the 10% ammonium sulphate solution recommended in my book.

The only titanium I have available is 10-2-3, a high-strength beta alloy (10% V, 2% Fe, 3% Al), and I don't have the chemicals to test as you did. However, be aware that concentrated sulphuric acid is an oxidising acid (though not as powerful as nitric acid) and that vanadyl sulphate (VOSO4), a tetravalent vanadium compound, is blue. Don't know whether this helps or confuses :-)
The colours of some transition metal complexes can be strongly influenced by the exact conditions they find themselves in, so identifying things just by visual inspection can be misleading. The classic example IIRC is that some cobalt salts in aqueous solution change between pink and blue on warming (but I'm relying on a memory from 25+ years ago here, so don't quote me!).
Interesting stuff.
David
--
David Littlewood

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writes

Hi,
I recommend you start at maximum voltage and lower the rod and voltage together as when you do it the other way 'round some electrolysis definitely occurs on the wet metal above the general level of the liquid which messes things up. I had a go but with poor results - as I say I only got anodising to work properly once and couldn't reproduce the result, although I'm confident I just need to spend a few hours on it to sort that out. I had a graphite rod (eBay) for a cathode, so not corrosion problems. Unfortunately the current indicator on my power supply (2.2 amps max) needs fixing, but judging by the noise from the transformer inside I would say that there is an initial current surge as the oxide quickly grows, and then the current drops away. If you're worried about the surge damaging your power supply I suspect it will be fine to just connect a resistor in series with the circuit to limit the short circuit current to a safe value. One variable in forming these films is whether to ramp up to the maximum voltage during electrolysis or just use maximum straight away. One place I read said "ramp up over 10 minutes". A series resistor will have the effect of ramping up the electrolysis voltage as the current through the circuit falls but won't affect the maximum voltage significantly if the current falls to a sufficiently low level . One curious thing I noted during the hour or so I spent anodising was that sometimes the titanium at the anode generated gas and sometimes not. I had a feeling this was just a voltage dependant effect i.e. it bubbles at higher voltages, but something I saw has left me sceptical of that. I'm surprised you say Sulphuric is oxidising. I got the idea to use it from here where it says "Reducing acids such as HCl and H2SO4 can be used as well as oxalic acid to etch titanium. Usually, you'll need elevated temperatures say 80~100 degrees C and fairly strong acid concentrations.They have the advantage of being much faster than oxalic.": http://www.finishing.com/134/32.shtml What do you make of this?
I think in the end I'll work out/find some test for vanadium or aluminium and use that to test if my Titanium is pure or 6:4, but that's going to have to wait a few days...
Scrim
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At work we use Ti bolts inside the cathode of a Sulphuric based anodising system. We changed supplier and they sent the aircraft grade bolts instead of the pure Ti ones. The alloyed metal eroded in the solution after a few weeks (can't remember the exact duration), something we'd never seen with the pure Ti bolts.
So, perhaps put a bit on wire as a cathode in some sulph, turn on the power and leave it for ages, if it dissolves, it's not pure :}
Stu G
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Scrim wrote:

Blue - could be anything. But there is an easy test to distinguish 6-4 from CP titanium - bend a bit. CP is much softer.
--
Peter Fairbrother



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