zerodan
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Titanium cathode passivation after stopping the cell
I made a chlorate cell that I've ran twice so far and I've noticed that my cathode is developing a white layer of presumably TiO2 which is not
conductive when tested with a multimeter.
When pulled fresh out of the cell the cathode looked pristine, then I put it in my drawer for the night and next day it tarnished.
I never left it in the cell unconnected and after stopping the cell I remove the cathode then wash the brine off with tap water, then wash in
deionized water bath, then dry with paper towel, I thought that would be a sane procedure but It seems to have been futile.
Anyone experienced anything similar with their Ti cathode or know how to remove the TiO2 layer (I dont have HCl).
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mysteriusbhoice
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use citric acid and see if it dissolves and this sounds like your Ti is not grade 1 or 2 Ti but one of the other inferior grades of Ti metal which
explains the corrosion.
buy new cathodes and better ones or just dont use Ti cathodes and use stainless steel since your only making chlorates and anyway stainless steel will
improve your yields anyway due to chromium release especially with pH controlled passive or buffered for the amateur cells.
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zerodan
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I was working with the assumption that my cathode was pure Ti but after todays run I think this would explain may things.
I got weird white flakes/crystals at my cathode which I thought were KClO3/NaClO3 but they didn't behave that way at all. I scraped them off the
cathode onto a piece of paper and as a result got a dry white/gray powder. Checked it with wet PH paper, got very basic like 11+ and the paper got
bleached a little bit. Then mixed with charcoal but it wouldn't burn at all. Then tried small sample with H2SO4, no yellow color and no reaction at
all, it didn't even dissolve.
At first I thought it was TiO2 but the ph and bleaching was out of place.
The idea that the cathode was not pure titanium helps answer the why but I'm still curious as to what was that powder.
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mysteriusbhoice
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Probably whatever metal oxide like aluminum etc and maybe some TiO2.
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zerodan
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The thing is metal oxides don't have wild ph or bleach things
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macckone
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It could be titanium dioxide with tiny amounts of sodium hydroxide and hypochlorite.
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B(a)P
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Quote: Originally posted by mysteriusbhoice  | | anyway stainless steel will improve your yields anyway due to chromium release especially with pH controlled passive or buffered for the amateur
cells. |
Do you have a reference for this, or have you observed it yourself? I thought the chromium needed to be hexavalent? Or is hexavalent chromium
generated at the cathode of a chlorate cell?
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mysteriusbhoice
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| Quote: Originally posted by B(a)P | | Quote: Originally posted by mysteriusbhoice | | anyway stainless steel will improve your yields anyway due to chromium release especially with pH controlled passive or buffered for the amateur
cells. |
Do you have a reference for this, or have you observed it yourself? I thought the chromium needed to be hexavalent? Or is hexavalent chromium
generated at the cathode of a chlorate cell? |
When I ran with it the chromium seems to leech out due to some stainless being exposed to the gases and not in the solution as the solution slowly
turns yellow but more than just bleach also adding vitamin C makes it turn green so yea.
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B(a)P
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| Quote: Originally posted by mysteriusbhoice |
When I ran with it the chromium seems to leech out due to some stainless being exposed to the gases and not in the solution as the solution slowly
turns yellow but more than just bleach also adding vitamin C makes it turn green so yea. |
So the chromium is going into solution from the cathode, but by what means? It is then presumably converted to hexavalent chromium at the anode? I
would love to understand the mechanisms for this, could anyone point me in the direction of an appropriate reference that would cover it? What is the
reaction between ascorbic acid and hexavalent chromium that yields a green solution?
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mysteriusbhoice
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| Quote: Originally posted by B(a)P | | Quote: Originally posted by mysteriusbhoice |
When I ran with it the chromium seems to leech out due to some stainless being exposed to the gases and not in the solution as the solution slowly
turns yellow but more than just bleach also adding vitamin C makes it turn green so yea. |
So the chromium is going into solution from the cathode, but by what means? It is then presumably converted to hexavalent chromium at the anode? I
would love to understand the mechanisms for this, could anyone point me in the direction of an appropriate reference that would cover it? What is the
reaction between ascorbic acid and hexavalent chromium that yields a green solution? |
The cathodes made of stainless steel tend to corrode wherever it does not touch the solution especially if your cell is closed lid. and vitamin C is
known to reduce chromium 6 into chromium 3 and you only need a little to show it.
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B(a)P
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I would still like to understand the chemistry in more detail, if anyone could assist I would be most grateful.
So the chromate/dichromate forms a thin protective layer on the titanium cathode of a chlorate cell to prevent the conversion of hypochlorite back to
chloride. Does this still hold true at the stainless steel cathode? There are many forms of stainless steel, does this process hold true for them all?
Edit: fixed typo
[Edited on 28-11-2021 by B(a)P]
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macckone
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Stainless steel by definition contains a minimum of 10% chromium.
So it should hold true. It does not take much chromium.
With platinum electrodes the chromium in solution increases current efficiency.
According to literature chromium in solution decreases current efficiency with a lead-dioxide on titanium anode.
But stainless steel keeps the solution chromium low enough that it works properly and increases current efficiency.
Therefore allowing the stainless to sit in solution will contaminate the solution with too much chromium, but it will plate out at the cathode
eventually.
Adding vitamin C will accelerate plating.
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Hexabromobenzene
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F*ck. Chromates are carcinogenic( lungs cancer). HDPE container very difficult to make sealed. Some aerosols leaked in your room.
But good news whan concentration chromates on solution with stainless cathode is low(not more 100 mg liter) and it gives low concentration chromates
in air(PEL 6+ Chromium 5 microgram per cubic meter) and exposition at weeks or months not enough to increase cancer risk
I think chromate concentration maximum at the beginning electrolysis due to high hypochlorite concentration
I have a question. At what concentration, the chromates begin to paint the solution?
[Edited on 1-12-2021 by Hexabromobenzene]
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yobbo II
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The Chromates thing again....
IMO they are not worth the bother. Your milage may vary.
Use small cathodes.
Down load this file and read the section on cathodes.
http://pyrobin.com/files/ChloratesAndPerchlorates.zip
Yob
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mysteriusbhoice
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yea IMO chromates are pretty inconsistent as sometimes they appear to increase yield and others not rly but maybe other factors come into play. I
think CaCl2 is superior as it does the job of coating the cathode and HCl can be added safely without any offgassing of chlorine due to some Ca(OH)2
in solution and on the cathodes which react with it fast. I also dont want them because when you run a perchlorate cell and the liquor is yellow then
the run time is gonna be shit.
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