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jpsmith123
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TiH2 Substrates
Rosco, is it your idea that if the Ti substrate is hydrided, the cobalt oxide may be able to be electrodeposited without having the surface of the
substrate become passivated by an unwanted oxide layer?
When I look at patent #4153742 (attached), I get the idea that it might work.
(BTW I didn't have any luck with the platinum wire...the oxide wiped off rather easily).
Attachment: 4153742.pdf (365kB)
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Rosco Bodine
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For a pretreatment of a Ti substrate , the etching with oxalic acid followed by hydriding is probably unsurpassed
as surface preparation mechanically and chemically , regardless of whatever material is applied next .
The hydriding was first being considered as a *general* preplating treatment , a followup to etching which would hopefully provide a transient
immunity to oxidation for the Ti , so zero or a reduced growth of Ti oxide would occur before whatever coating went on next .
The technique is useful especially when a metal plating
is to be applied to titanium , and a metal to metal interface
can be achieved . This possibly can be done in the plating bath , by stepping the negative voltage alone , or changing the composition of the
electrolyte , or transferring the hydrided Ti quickly to the plating bath from
the etching and hydriding bath . The hydriding would probably also provide an ideal surface for electroless deposition of whatever metal might wish
to be plated onto the titanium , producing a metal to metal interface
simply by dipping the hydrided titanium into a solution of the salt .
With the case where a cobalt salt solution is used , cobalt metal would likely flash plate electrolessly onto the hydrided Ti upon immersion , and
later as you begin anodic electrodeposition the cobalt metal would be oxidized first , followed by additional oxide being electrodeposited .
What exactly would be the interface boundary composition
I'm not sure , perhaps a cobalt - titanium substituted spinel would form an interface and bonding layer , your guess is as good as mine .
[Edited on 28-11-2007 by Rosco Bodine]
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jpsmith123
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The examples discussed in patent# 4153742 are amazing. As per Table 1, just treating the Ti substrate cathodically at 0.05 mA/cm^2 for one hour
doubled the useful life...that's almost unbelievable.
I've had a piece of Ti wire running at 100 mA/cm^2 for about two hours now. Maybe in another hour or so I'll put in in the CoSO4 tank and see what
happens.
Edit:
It actually seems to be working! Apparently some cobalt got onto the surface before the surface passivated.
Edit:
It's still going strong! At a fixed anode voltage, the current is falling off only very slowly, as the oxide layer builds up; and it really is
building up.
For purposes of comparison, I did the exact same thing to another piece of Ti wire only without the hydriding step, and it passivated in seconds.
I'll let it go another hour or so and then test the deposit for adhesion.
Edit:
The coating is on there pretty good...far better than on the graphite or platinum wire. If I rub it very hard, a small fraction of the coating comes
off.
I will try the same process with another piece of wire tomorrow, which I'll first sand with 220 grit paper; hopefully the roughness will boost the
adhesion somewhat.
(BTW, in re-reading Beer's patents, he does give some examples of MMO coatings electrodeposited onto Ti substrates, but he uses AC rather than DC.
Maybe the AC is what keeps the surface from passivating).
[Edited on by jpsmith123]
[Edited on by jpsmith123]
[Edited on by jpsmith123]
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Rosco Bodine
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Hydriding Method for Polished Titanium
One of the patents I found is probably better for polished titanium surface preparation where a metal is intended to be plated , as it does not deeply
attack the titanium . It still provides some of the benefit of a hydriding treatment but does not agressively etch and pit and roughen the surface ,
so it is a gentler treatment as would be used for a mirror plating of silver , gold , or perhaps platinum .....and other metals such as copper ,
nickel .....or cobalt . This may not
be so useful for the electrode portion of the anode , but
could be useful for the electrical connection area at the
top of the anode , or for other special use . The same
activation dip in SnCl2 would likely be as effective for producing the nucleation sites for "silver mirroring" of
a connection area , on the Ti having been hydrided by
the more aggressive method as would be used on non-polished surfaces .
See US2801213 attached
Attachment: US2801213 PLATING_TITANIUM.pdf (79kB)
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jpsmith123
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Oh well, back to the drawing board.
Well that was short lived  . It looked great, and worked great, for a little while.
Then the coating started coming off.
I'm going to try again, this time I'm going to electrodeposit from an acetate solution and see what happens.
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Rosco Bodine
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You might try the application of a mixed nitrate and low temperature bake like the patent described . That low heating may be required to fully
develop the interface by decomposition of any residual hydride . Or if it is a spinel
that forms the interface layer , it might require that minimal sort of temperature range to form .
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jpsmith123
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What about electrophoretic deposition?
I'm increasingly wondering if electrophoretic deposition may be the answer? The advantages would be not having to deal with messy precursors and
eliminating the multiple drying and baking cycles (in Dann2's approach).
Many of the relevant electrode patents mention it as an acceptable means of coating the substrate.
Here's a paper that discusses the general process a little bit:
http://www.tms.org/pubs/journals/JOM/0001/Zhitomirsky/Zhitom...
[Edited on by jpsmith123]
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Rosco Bodine
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That looks like it wouldn't offer any advantage because of
the film cracking reported at a third of a micron thickness buildup that was reported for oxide films .
I am still betting that the hydrided titanium is receptive
to some sort of "conductive chromating" scheme where
the Ti itself can be chemically compounded at its surface to an electrically conductive surface layer . That could be exactly what is happening with
the use of the manganese nitrate - vanadium pentoxide mixture . The patent hasn't declared it in big bold letters that a conductive and chemically
resistant interface has been produced , which
is probably a molecular layer of a substituted spinel , or a solid solution of mixed oxides headed in that direction .
But something to that effect must have been achieved .
For this sort of "surface treatment" a temp of 150C is
actually pretty mild . From the test reports concerning
anodes produced by the method , it produces superior results compared with other schemes . It may be unecessary to go any further for production of a
perchlorate anode . However , if it is found necessary
to do more ......
It would be interesting to try one or two
coatings and low temperature 150C bakings with that nitrate mixture , followed by a multiple layer
coating with PVA thickened ammonium stannate - ammonium antimonate , or alternately the syrupy inorganic
polymer which is a long chain antimony substituted tin oxide-oxychloride sort of material . And perform the usual
higher temperature baking between coats for building a vitreous DTO coating . That scheme might produce a
nearly bulletproof DTO semiconductor layer encasing the
substrate . On top of that could be applied whatever
catalytic anode coatings are desired , by different methods . An oxidative soak deposition of cobalt oxide
could be applied , and then baked . Or an electrodeposited cobalt suboxide could be applied ,
or an electrodeposited PbO2 could be applied to complete the anode .
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Rosco Bodine
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Something Wonderful !
It appears that the TiH2 is itself the conductive boundary
layer . The conductivity of TiH2 is little different from the metallic Ti .
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi...
The TiH2 is not extremely reactive like some other metal hydrides . TiH2 is thermally stable till about 450 C . So a hydrided surface on Ti is a
perfect candidate for reaction with baked on coatings which could seal the surface . With something like the doped tin oxide vitreous coating , the
hydride would protect the titanium from oxidation into the temperature range where the DTO would effectively blanket and seal the substrate . There
would be no titanium oxide layer forming during baking . The TiH2 itself is the coating which blankets the titanium during the first baking . And
after that it is covered .
But it is also a perfect candidate for chemical reaction with
oxidizing coatings which could form spinels or dope the
minimal controlled thickness layer of Ti oxide as it forms .
[Edited on 30-11-2007 by Rosco Bodine]
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jpsmith123
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Another Hydride Related Patent
The attached patent describes a "Process For Electro-plating Nickel On Titanium".
In this case the inventor merely hydrides the Ti cathodically, and then immediately plates it in a standard nickel plating bath.
What's interesting to me is that he only hydrides the Ti for 10 minutes at 10 mA/cm^2.
Anyway it seems there is an added benefit to this, and that is the electrocleaning effect of the hydrogen bubbles. Notwithstanding the benefit of the
hydride per se, any subsequent coating process may benefit from this intense cleaning effect on the substrate.
BTW I am presently doing an experiment to see how well the titanium can be hydrided in distilled vinegar (I ran out of oxalic acid); so far it seems
to be working good except it takes a significantly higher voltage to drive it.
Attachment: 4127709.pdf (175kB)
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Rosco Bodine
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I am still trying to remember and find the old reference where I first saw this hydriding of titanium described .
IIRC it used HCl and heating and cathodizing for several hours . The etching that is the first step is the obstacle ,
as once that passivation layer is gone , then the Ti soaks up H2 like a sponge . It is so absorbent of hydrogen that over a long time the Ti will
severely embrittle into grains and crumble , simply falling apart as a structural disintegration.
[Edited on 30-11-2007 by Rosco Bodine]
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Xenoid
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| Quote: | Originally posted by jpsmith123
I've had a piece of Ti wire running at 100 mA/cm^2 for about two hours now. Maybe in another hour or so I'll put in in the CoSO4 tank and see what
happens.
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@ jpsmith123
Are you getting hydrogen evolved at this current density, Rosco's hydriding process implies no or very little hydrogen is evolved. If I used that
current density, my tank (jar) would be frothing over in bubbles.
@ Rosco
I have taken a Ti rod which was roughened and has been sitting in very dilute HCl for about 2 months and attempted this hydriding process! I'm using
about 5% H2SO4 and a graphite rod anode. I'm using a lab power supply (which unfortunately only resolves to the nearest 10mA) operating in constant
voltage mode.
I started at 0.0V / 0.0A and increased the voltage in 0.1 volt steps. Nothing was observed until I got to 1.6 volts, when a tiny stream of hydrogen
began to erupt from a single point on the rod, at this stage the current was still reading zero (ie less than 10mA). From this point on H2 evolution
increased as I ramped up the voltage, at 2.1 volts I finally got a reading for current of 10mA. I continued up to 2.7 volts and at this stage the
current was reading 230mA with copious H2 evolution.
I turned the voltage down to 1.4 volts (ie just below H2 evolution) and measured the current with a separate meter, it was only about 0.5 mA. I have
left it in this state for 3 hours now and nothing much seems to be happening!
Note: The 1.6 volts where H2 evolution commenced corresponds well with the bottom of the corrosion zone on the Pourbaix digram.
Regards, Xenoid
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Xenoid
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Pourbaix Diagrams
@ Rosco
In the "Electroplating Iron" thread you produced two Pourbaix Diagrams for Ti.
Why are they so different? In particular the area marked Ti++ (corrosion) is quite a bit smaller in the second diagram. I'm having trouble
interpreting the areas marked (Ti IMMUNE) and TiH2 in light of what has been said previously.
Also, you still haven't answered my question  (in another thread) about the
lines around the Ti++ area, marked 0, -2, -4, -6, what do they represent?
Regards, Xenoid
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Rosco Bodine
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Here's the link for the page where the diagrams were found . I am not exactly sure what those lines indicate
except possibly regions where some overlap occurs with
multiple oxidation state reactions are occurring simultaneously .
http://www.engr.sjsu.edu/WofMatE/projects/srproject/srproj5....
The lower diagram shows the effect of dissolved hydrogen , I am not sure if this means the hydrogen in the electrolyte or the adsorbed hydrogen in the
Ti .
Leaving a sample of Ti in dilute HCl probably has little effect .
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jpsmith123
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Yep I was getting quite a lot of bubbles.
BTW, earlier today I cathodized another piece of Ti wire (this time in distilled vinegar), and I noticed that the current was slowly increasing, such
that after about an hour it had gone up by about 10%. I'm wondering if the scrubbing action of the H2 bubbles might have dislodged some surface oxide,
thereby lowering the resistance a little bit?
Did you notice anything like this Xenoid?
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Xenoid
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@ jpsmith123
@ Rosco
Sorry! I seem to be getting confused here!
So, I do need to have copious H2 evolution for hydriding to occur! Holding the voltage at just below H2 evolution is just for extra surface
preparation. After doing this for a while I then turn up the current to say 20mA/cm^2 for a few hours, is this correct?
I have sanded and roughened another Ti rod and put it in conc. HCl (hardware store 290g/litre), it is happily slightly bubbling away and the solution
is turning mauve (TiCl3), can I safely assume this is etched? If this is so, why are people using exotic materials like oxalic acid and HF for etching
Ti?
Regards, Xenoid
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Rosco Bodine
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This is one of those scenarios where inadequately filtered rectified AC that would hammer the thing with some ripple
could actually work better than smooth DC .
On the hydrogen evolution I found a patent reference which I posted in the other thread .
I have a figure for hydride formation on Ti alloy 6Al 4V
at room temperature in 1N H2SO4 (~5% H2SO4) ,
current density
50 mA/cm2 100 uM depth at 12 hours , 150@16 , 230@24
no further depth development for longer times .
from US5178694
So yeah let it rip even at 50 mA per sq. cm.
I think here the color change by visual observation
is going to be the tell on what you have .
Personally , I think I'll try the oxalic for the etch ,
and then use the 5% H2SO4 at the 50 mA rate
because there's data to reference that is probably reliable
on the thickness of the layer developed .
I'd bet 5 to 15 minutes is all it's going to take
for cleanly etched Ti . But you could do some sample tests
and see what holds up best with whatever you are applying next .
[Edited on 30-11-2007 by Rosco Bodine]
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Xenoid
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| Quote: | Originally posted by jpsmith123
(BTW, in re-reading Beer's patents, he does give some examples of MMO coatings electrodeposited onto Ti substrates, but he uses AC rather than DC.
Maybe the AC is what keeps the surface from passivating).
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How can AC deposit anything? Surely the net effect is zero! Depositing on one cycle and removing on the next... ad infinitum! Can someone please
elaborate.
Regards, Xenoid
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jpsmith123
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Probably the metal is deposited on the negative half-cycle and oxidized on the positive half-cycle.
BTW I was interrupted today and couldn't do it, but tomorrow I plan on trying to plate cobalt on the TiH2 surface and then oxidize it in a subsequent
step.
[Edited on by jpsmith123]
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Rosco Bodine
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Not AC , I said *rectified* AC , pulsed DC , not fully filtered and regulated DC but noisy spiky DC . The impulses would hammer the surface pretty
good and might help with the hydriding . It would be like painting a wall with a paintball machinegun instead of a paint roller .
[Edited on 30-11-2007 by Rosco Bodine]
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Xenoid
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@ Rosco
The reference to AC was from an earlier post of jpsmith123, regarding Beer's patents for MMO coatings!
Regards, Xenoid
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Rosco Bodine
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Sorry for misunderstanding .
I have an HCl etching description for CP unalloyed Ti
from a patent US5435896 .
Using 18% HCl , etch for 1 hour at 95-100 C .
How well this etch may work for alloyed Ti is unknown .
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Rosco Bodine
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Here's some more information concerning hydriding .
Evidently electrolytic hydriding is greatly accellerated
in a hot electrolyte as compared with the room temperature rate . I had originally thought there was
a temperature of about 80 C needed to get hydriding
to proceed well and this source supports that figure .
So I would say , if hydriding isn't proceeding well at
room temperature , try a hot electrolyte and that should kickstart it .
http://www.azom.com/details.asp?ArticleID=1239
Excerpt from article:
When cathodically impressed or galvanically induced currents generate nascent hydrogen
directly on the surface of titanium , the presence of moisture does not inhibit hydrogen absorption of this type.
Laboratory experiments have shown that three conditions usually exist simultaneously for hydriding to occur:
1. The pH of the solution is less than 3 or greater than 12; the metal surface must be damaged by abrasion;
or impressed potentials are more negative than -0.70V.
2. The temperature is above 170°F (77°C) or only surface hydride films will form which, experience indicates,
do not seriously affect the properties of the metal. Failures due to hydriding are rarely encountered below this temperature.
(There is some evidence that severe tensile stresses may promote hydriding at low temperatures.)
3. There must be some mechanism for generating hydrogen. This may be a galvanic couple, cathodic protection
by impressed current, corrosion of titanium, or dynamic abrasion of the surface with sufficient intensity to depress the metal potential below that
required for spontaneous evolution of hydrogen.
[Edited on 2-12-2007 by Rosco Bodine]
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jpsmith123
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Well the hydriding in acetic acid didn't work too well. I couldn't deposit cobalt oxide over it.
Then just for the hell of it I cathodized a piece of Ti wire in the Co Acetate solution and deposited an adherent layer of cobalt metal.
Edit:
Ok I guess I'll put it back in the acetic acid and anodize it to see if the cobalt layer can be oxidized.
Edit:
It's back in the acetic acid being anodized at a current density of 5 mA/cm^2 and it's not passivating...I see no oxygen bubbles and it's turning a
very dark color.
Edit:
The layer of apparent oxide just rubbed right off...nfg at all.
It's amazing that the cobalt was stuck on there so well, but the oxide wouldn't stick to it.
Anyway, I just now took another piece of Ti which I sanded and washed two days ago (and then just left it laying around), and plopped it into in the
Co Acetate solution as anode, without any further treatment, and it is now building up a nice layer of cobalt oxide...not a hint of passivation.
[Edited on by jpsmith123]
[Edited on by jpsmith123]
[Edited on by jpsmith123]
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Rosco Bodine
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The hydride is not likely to form in acetic acid ,
maybe in sulphamic , more likely in oxalic .
But because these are not good electrolytes ......
it really probably is best to first etch with oxalic
or hot HCl , and then transfer with cathode connection
made into 5% H2SO4 . The H2SO4 allows use of a sheet lead anode which is also convenient .
From an electrical and chemical process standpoint ,
this is the most logical sequence for etching and hydriding .
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