Sciencemadness Discussion Board

Cause of carbon anode erosion in electrolytic cell

RawWork - 9-4-2018 at 13:51

Has anybody ever discovered what is the cause of carbon anode erosion? Why does it becomes powder? Is it chemical or physical cause?

I found out that carbon cathode does not erode. Does this mean something? I think it could mean that erosion is not caused by any gas or any nascent gas evolving from electrode, but only Oxygen and Chlorine maybe.

I watched on How it's made or somewhere similar that carbon electrode is not pure carbon but some binder is added like clay or CaSO4. Does this mean that if it was pure carbon made by melting or crystalization, it would be resistant? Still it's miracle why cathode doesn't erode. Maybe there are more than one causes.

I would rather investigate this so i may be able to use something cheap and nontoxic and common like C, instead of Pb, Mn, Pt, MMO...

What about glassy carbon, there are little information about it, especially it's resistance to erosion as anode?

walruslover69 - 9-4-2018 at 14:34

The erosion is due to exfoliation on the carbon surface. This is a mostly physical phenomenon that is catalyzed by a chemical one. At the anode the carbon surface is often oxidized to some degree. this can disrupt what would normally be perfect layers of graphite. The evolving gas from electrolysis peels away or exfoliates these graphite sheets. This is actually one way that graphene quantum dots can be made. After putting current through some types of solutions with carbon electrodes the solution will have reasonable amounts of these graphene quantum dots that will fluoresce a nice blue color.
Carbon doesn't melt, it sublimes. If kept under pressure it will melt but only at ~3800K so it can't easly be cast. Glassy carbon is much more resistant to erosion because it structure doesn't consist of those graphite sheets that easily exfoliate.

yobbo II - 17-4-2018 at 14:12

The effect o f pH on graphite wear in a chlorate cell process attached.

[Edited on 17-4-2018 by yobbo II]

Attachment: graphite.pdf (575kB)
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PyroPlatinum - 22-1-2019 at 18:46

I would like to ask a question regarding this subject.
Would a piece of cloth or fine sieve around the graphite anode remedy a bit of this problem? Or would it interfere in the process of chlorate production at a point of making it impossible?
And the chlorate would precipitate outside this "layer of protection"?

walruslover69 - 23-1-2019 at 05:31

I would be skeptical that it would help, the best thing you can do to reduce the problem is lower the amperage or increase the surface area of carbon, its all about the current density. I don't think it would interfere to the point of making it impossible, it would probably reduce your efficiency by reducing diffusion. Therefore you would have a high concentration around your electrodes that could then be reduced/oxidized to other side products.

hissingnoise - 23-1-2019 at 05:54

Impregnating your anode with linseed oil greatly slows down its rate of exfoliation... more on that here ...

markx - 28-1-2019 at 06:34

Impregnation with binding agents and oils can offer some reduction in anode erosion rate, but lowering of cell temperature, pH control and proper current density shall aid even more.
Graphite shall erode inevitably, but it should be seen as a minor sideeffect given its availability and rather low cost. A bulky graphite anode under more or less optimal operating conditions can serve for a considerable number of synthesis runs without any really major losses in mass. It's rather the cleaning up of the resultant black cell liquor that poses an inconvenience. The carbon dust can sometimes be very persistant about passing filtering media.
Of course there are different graphite grades available that can display vastly differing erosion rates under cell conditions. A random pick from these might not hit the best candidate.