mysteriusbhoice
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Hydride Embrittlement of Titanium Cathode.
So my old titanium cathode used in my NaClO3 cell has been in use for about several Thousand hours seems to be crumbling at the edges where bits
started to fall off in my latest runs.
The surface is no longer shiny but is dull gray and crusty.
I would like to know how long these things usually last because they seem to die way sooner than the anodes which still look brand new and have zero
losses of conductivity despite running about as long.
I plan to replace the mesh electrode with a plate one whilst keeping the mesh anodes since those have better current distribution.
Titanium mesh cathode tho is HORRIBLE!! it seems since it also exposes more surface area to whatever effect is causing it to crumble.
edit:
I would really like to know what causes embrittlement to occur and how to mitigate it.
I run no chromates on my cell which could be the reason why ?? but my cathode is titanium and not steel.
I did test the titanium and it seems to not give that many sparks though they are white when hit by a rotary tool so I guess it could just be a bad
cathode made of poor quality titanium.
The buss bars on the anodes tho gave white sparks when sanding em down so that could be it but any other reasons and conditions that can cause hydride
embrittlement can help me figure out someway to mitigate it whilst my replacement arrives.
[Edited on 6-8-2020 by mysteriusbhoice]
[Edited on 7-8-2020 by mysteriusbhoice]
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symboom
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Well I know titanium absorbs hydrogen to form titanium hydride I think sodium dichromate is usually added to prevent this process along with pH,
temperature and voltage used.
I run no chromates on my cell which could be the reason why ?? but my cathode is titanium and not steel. I think steel also suffers from Hydrogen
embrittlement.
https://patents.google.com/patent/US2861015A/en
[Edited on 7-8-2020 by symboom]
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mysteriusbhoice
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pH is controlled however my method for pH control deposits Ca(OH)2 on cathode which may accelerate this process.
I really dont want to add chromates to this since I will be boiling this stuff near other stuff where I dont want chromates to be in and freezing it
aswell in the freezer.
I did read on something called vacuum annealing or heating in inert atmosphere but it sounds like more trouble than its worth.
I wonder if baking the electrode at low temp 190-200 Celsius will be fine.
[Edited on 7-8-2020 by mysteriusbhoice]
[Edited on 7-8-2020 by mysteriusbhoice]
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markx
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Ti is quite suceptible to hydrogen uptake and embrittlement. Electrochemical H2 production creates a tremendous surface potential for the formed
hydrogen to penetrate into the cathode material. This potential is comparable to thousands of bar statical pressure to achieve the same outcome.
Cathodic hydrogen generation is used to saturate certain metals with the gas to levels which are unobtainable by other methods. So it is quite
predictable that the cathodes exhibit severe H2 related effects in a chloralkali cell. In fact one can often visually observe Ti plate cathodes (even
beefy ones with a thickness of several mm) bend and curve outwards from the anode side as this direction receives the most current density and hence
also the most hydrogen pressure to saturate the metal. Cristal lattice shall expand as a result and force the previously straight plate into an
impressive arch. The process is mostly reversible as hydrogen shall escape the metal quite quickly and heating mildly shall make the process rather
instantaneous. Just leaving the curved plates to rest in ambient air for a few days shall often rectify them into the initial configuration.
Given enough cycles of expansion the defects of the cristal lattice shal manifest themself as permanent damage to the metal and create centers for
corrosion....thus the plate shall begin to erode and cruble in the end. There is not really much one can do to prevent this from happening, but to be
honest the process is quite slow and a thin plate of CP Ti is not expensive enough to try and save it at all cost. It should just be exchanged, once
it is evident that irreversible damage has occurred.
Exact science is a figment of imagination.......
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mysteriusbhoice
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Quote: Originally posted by markx  | Ti is quite suceptible to hydrogen uptake and embrittlement. Electrochemical H2 production creates a tremendous surface potential for the formed
hydrogen to penetrate into the cathode material. This potential is comparable to thousands of bar statical pressure to achieve the same outcome.
Cathodic hydrogen generation is used to saturate certain metals with the gas to levels which are unobtainable by other methods. So it is quite
predictable that the cathodes exhibit severe H2 related effects in a chloralkali cell. In fact one can often visually observe Ti plate cathodes (even
beefy ones with a thickness of several mm) bend and curve outwards from the anode side as this direction receives the most current density and hence
also the most hydrogen pressure to saturate the metal. Cristal lattice shall expand as a result and force the previously straight plate into an
impressive arch. The process is mostly reversible as hydrogen shall escape the metal quite quickly and heating mildly shall make the process rather
instantaneous. Just leaving the curved plates to rest in ambient air for a few days shall often rectify them into the initial configuration.
Given enough cycles of expansion the defects of the cristal lattice shal manifest themself as permanent damage to the metal and create centers for
corrosion....thus the plate shall begin to erode and cruble in the end. There is not really much one can do to prevent this from happening, but to be
honest the process is quite slow and a thin plate of CP Ti is not expensive enough to try and save it at all cost. It should just be exchanged, once
it is evident that irreversible damage has occurred. |
Yea im getting a replacement anyway since the Ti mesh has started to fracture down the middle due to heavy use in dozens of runs of the cell.
Its been about more than 20 runs and yea thanks for this tip about how to reverse the hydride formation which will help the new replacement last a bit
longer than this one.
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