metalresearcher
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Registered: 7-9-2010
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Chlorate cell question : does S/S cathode oxidize ?
I made a chlorate cell with 400ml NaCl / KCl solution and powered it with 5-6 V and 8-10 amps .
I added a tiny amount (knifetip) of K2Cr2O7 to increase efficiency.
I use a stainless steel cathode and (temporarily) a graphite rod anode which I will replace my an MMO anode which I ordered via ebay.
But after running 1 hour a brown debris (first photo) appeared in the bath, presumably Fe2O3. I checked for sure that the polarity was correct but
indeed the (-) was connected to the SS spoon and the (+) to the carbon rod.
How is the Fe2O3 formed ? Attack of the SS by chlorine gas ? I have a sheet of Ti which I can also use as cathode which I plan to connect with a
(pure) silver strip (copper is attacked by Cl2) . Is this a good idea ?
[Edited on 2011-1-1 by metalresearcher]
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Xenoid
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Location: Springs Junction, New Zealand
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The brown colouration is caused by colloidal ferric hydroxide. This is one of the drawbacks to using stainless steel. Much of the corrosion seems to
occur in the headspace of the cell. This colloidal material is extremely difficult to get rid of, you can't filter it. You can ignore it or I have
caused it to flocculate using alum (swimming pool floc). When it is flocculated you can let it settle or filter it out.
I am using Ti or MMO cathodes with all Ti hardware in the headspace, the cells run clear except for a little white floc. I now totally avoid SS with
chlorate/perchlorate cells. Don't use SS saucepans, especially the cheap ones, to concentrate your liquors either. My wife threatened to divorce me
over that 
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garage chemist
chemical wizard
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Registered: 16-8-2004
Location: Germany
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Use a titanium cathode and have only titanium in the headspace. SS will corrode if it is not cathodically protected- this means that it will corrode
in the headspace. Silver would also corrode.
Just connect the Ti sheet where it's not reached by the electrolyte spray.
I solved all my filtering difficulties by acquiring some kieselgur.
A 3-5mm layer of this on a Por. 3 sintered glass funnel filters all kinds of flocculent or gelatinous precipitates.
I previously had much trouble filtering off the hydrated Cr(OH)3 that was produced when I neutralized my dichromate-containing NaClO4 cell liquor
after gassing with SO2 and boiling. It would instantly plug coffee filters and lab filter papers so that both gravity and suction filtration through
paper were impossible. Through a layer of kieselgur, the cell liquor comes out crystal clear and the filter layer can be washed very easily. Ferric
hydroxide is filtered equally well.
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MadHatter
International Hazard
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Registered: 9-7-2004
Location: Maine
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Carbon and/or graphite cathode
I always use carbon(gouging rod) cathode and never encountered erosion although
the same is not true at the anode.
From opening of NCIS New Orleans - It goes a BOOM ! BOOM ! BOOM ! MUHAHAHAHAHAHAHA !
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Arthur Dent
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When i was a movie theatre projectionist apprentice nearly 30 years ago, I remember briefly working with antique carbon arc projectors (some of the
very last because all had been converted to xenon lamp illumination) and I inherited a box-full of 30 cm long copper-plated solid carbon rods. When
breaking off the tip, I could peel off the copper sheath and I had a nearly foot-long, 3/8" rod.
I just realised these could make excellent cathodes! I hope I haven't thown them away, haven't seen those in a long time. I imagine that carbon rods
used for welding should be pretty good too?
Robert
--- Art is making something out of nothing and selling it. - Frank Zappa ---
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