Electropolishing Problem

Question:

Our company electropolishes 316 SS, primarily tubing, using a 65% phosphoric, 30% sulfuric acid solution at 120F. Our power setting is approximately 12 volts and 1,100 amps. A 10-min cycle time is used. Some of the parts come out with a “frosted” appearance. What causes this? J.B.

Answer:

Your e-mail does not give complete details about your electropolishing process, but there are a few things to consider.

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· Current distribution
· Cathode placement relative to the parts being electropolished
· Current density is not high enough
· Temperature stratification of the electro- polishing bath
· Composition stratification of the electro- polishing bath
· Stray currents in the tank

The problem you mention is more likely to occur if the ratio of SO4:PO4 is much greater than one and when the total acid is less than 80%. If the viscosity and the iron concentration of the electropolishing solution are too high, frosting may also be more of a problem.