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Electropolishing Aluminium Current Density

Q. Hello,

I have tried electropolishing aluminum. For the experiment, I used a solution of sulfuric acid and phosphoric acid (20/80) and a standard laboratory power supply. The test pieces were 3 cm2, and the polishing voltage was 15V, with the current fluctuating around 4-5 amps (the power supply lacks current regulation). During testing, I let the pieces polish for 10 minutes. After polishing, I rinsed the piece with distilled water, then with an alkaline solution, and finally with distilled water again. The results were good, so I also tried polishing larger pieces, but the results were not as good. I suspect that the power supply does not provide enough current and tends to drop the voltage when a higher current is required.

What would be the correct current per cm2 for aluminum if I want to polish larger pieces? What would be the correct voltage? Are there any other good ideas or changes that could improve the results?

Thanks!

Erkki
- Estonia
July 24, 2024

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A. Hi Erkki,

If you recorded your variables correctly, you sort of answered your own question by successfully electropolishing at 4 to 5A / 3 cm2 = 1.33 to 1.67 A/cm2 [1235 to 1551 A/ft2]. However, I suspect that you measured something wrong because that is an awfully high current density. If you meant 3 dm2, then 1.33 to 1.55 A/dm2 sounds quite low but not quite unreasonable, whereas 133-155 A/dm2sounds quite high but not quite unreasonable, according to ASM's Surface Cleaning, Finishing, and Coating ⇨
and Wernick, Pinner, and Sheasby [on AbeBooks or Amazon or eBay affil links]

Voltage and amperage are not independent because V=IR; the voltage will be what it will be depending on anode-cathode ratio, spacing, solution conductivity, etc. 15V sounds high but possible. Luck & Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Ted is available for instant help
or longer-term assistance.

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