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Brown staining on exterior - after electropolishing the interior

I recently had a job which consisted of several large fabrications that required electropolishing internally. The items were made of 316 S/S and had been mechanically polished internally to 0.4 Ra before I received them.

We fitted the cathode and submerged them into the tank (sulfuric/phosphoric solution) and polished them. (There were no cathodes outside of the item.)The internal surfaces came out perfectly, but the external surface have a brown, streaky stain all over them.

The items came out of the bath like this - so it isn't a rinsing problem. It isn't possible to remove it by pickling nor by lightly scotching/finishing. I have come across this problem a couple of times in the past but has never been an issue - due to the external surfaces being mechanically polished, bead blast or lagged after we had finished.

This time, the customer wanted the external to look 'matte' as it did after the pickling process. I've spoken to other people about this phenomena and were told it was caused by 'stray currents' and that there was nothing you can do about it. has anybody else seen this - and do they know how to avoid it?

Neil Brook
surface finishers - Barnsley, South Yorkshire, UK
2003

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Mr. Brook,

Perhaps immersing the parts in a 30-40% by volume Nitric acid solution, room temperature,1-2 min., would remove the stain.
This is part of our process for electropolishing 316 stainless steel.

You may already be doing this as part of your process, but I thought I would suggest this.

Good Luck.

THOMAS KEMP
- Erie, Pennsylvania
2003