electropolish procedure - lab scale

I found a method for small-scale electropolishing:
Variable DC supply, 0-24V to 1A
Solution of 60% phosphoric acid, 20% sulphuric acid, and 20% water as
electrolyte. Stainless rod to serve as cathode, part to be polished
attached to anode. Start with 100 mA for a few seconds; inspect part
with magnifier.
I was proposing to use a variac with a 120V to 24V transformer and a
simple bridge rectifier with appropriate capacitors... Set whatever
voltage gives me 100 mA using the variac. Does anyone have any
experience doing this on a small scale? Any electropoloishing tips
and tricks would be greatly appreciated. Thanks for your help!
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I use electropolishing as a surface preparation for laboratory X-ray measurement of residual stress. I've found plenty of suggestions and anecdotal reports of what to use in various situations, but I've not come across any decent theoretical models of the electropolishing process that would allow predictions about the optimum conditions. From my experience with ferritic steel, common acid electrolytes such as the sulphuric/phosphoric/water seem to work well for low carbon content steels, and give a rougher surface with higher carbon steels. I also found the electrolyte sometimes goes purple, and I don't know why. For rail steel (about 0.7wt% C), I've only managed proper surfaces with 8% perchloric acid in acetic acid. This is just speculation, but I think the reason is that when the Fe atoms are 'oxidised' during the electropolishing, this only means oxidation in the sense that they ionised and go into solution. C atoms, on the other hand, must literally react with oxygen and be burnt to C02 (or maybe H2CO3). So for higher carbon content steels, you need a chemical that liberates oxygen. If perchloric cannot be used, other possibilities might be chromium(VI) oxide, hydrogen peroxide or potassium permanganate electrolytes, although I've never heard of the latter being used in electropolishing.
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Joe Kelleher

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