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Electropolish Current Density Issue

May 1, 2011

First, Thanks in advance for those that can assist in the solution.

I have setup a small electro polishing process to polish and brighten 316L Stainless Steel parts. Setup as follows:

50% sulfuric - 50% Phosphoric Acid bath (2 gallons total)
Power Supply - Sorensen 40-50 (0 to 40 Volts - 0 to 50 Amps adjustable)
Cathode - 304 SS two plates 8 X 8 inches - 128 inches total
Anode - 316L Parts range from between 1.4 Sq In to 38 Sq In

Situation:
I'm able to get an excellent finish on the smaller parts but per the Electroplating Engineering Handbook and other references, I'm using at or above the highest documented Amps. Good finish on my 1.4 Sq in (total surface area including top, bottom and edges) using 12 to 15 amps and 4.5 volts. Below this Amperage and I start to get pitting instead of polishing. (per the EEH 50 A to 500 A per Sq Ft is the recommended range).

Problem:
I assume that if I increase the voltage that I should be able to reduce my Amps and stay in the polishing range and out of the etching range (up to 12 v or so). While my power supply is adjustable on both V (0 to 40) and A (0 to 50) the Volts will not go over 5 V max. I can adjust the amps up to the maximum of 50 A but once the Volts hit 5 it stays there and only Amps increase. My power supply is fine as I can adjust volts up to the maximum while disconnected from the bath.

By the book my 50 A power supply should be able to polish my 38 SQ inch parts I'm unable to get out of the etching range with only 5 volts. At 45 amps the largest part I'm currently able to polish is about 23 Sq In. and that appears to be just barely out of the etching range.

Ideas:
Would changing my cathode to Copper allow more Voltage?
Other Ideas?

Thanks in advance for your assistance.
Jeff

Jeff Christman
Hobbyist - Seminole, Florida, USA

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May 5, 2011

One amp/sq in should be more than adequate. I think you have a chemistry problem.

50/50 is not the best for electropolishing stainless. You will find that 25% sulfuric, 75% phosphoric works much better.

Pitting can be caused by low specific gravity (too much water). The electropolish solution can absorb water from air. A reasonable estimate of water in the bath can be made by measuring specific gravity. A new 25/75 bath should be 1.7 or higher, and a used bath which has accumulated some dissolved metals should be near 1.8

Excessively high temp can cause pitting. I'd operate @ 90-120° F.

Keep it covered when not in use. Keep it warm ( about 90° F) when in use.

If your specific gravity is low, you can electrolyze for some time using scrap stainless to decompose some of the water.

Jeffrey Holmes, CEF
Spartanburg, South Carolina