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
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,
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).
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
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
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.
I can tell you, the purple I've got (done at 15 volts) is really something
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
I'll check out the colour chemistry tonight.
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.
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:
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!
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!).
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.":
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...
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 :}
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