316, Electropolish or hand polish for corrosion in Sea

[Capntom]

I recently purchased some items used on my boat. They are welded 316 SS tubing that has been polished with some kind of rubbing compound. I was told that this is better than Electropolishing. I think this salesman is mis-informed. I was educated that Electropolishing is the best way to finish 316 SS for corrosion protection because it dissolves the free iron on the surface leaving chrome to form chromium oxide and prevent corrosion. This is especially true when welding has been performed. Can anybody confirm???

 

[MikeHalloran]

The nitric acid passivation that is supposed to precede mechanical polishing selectively removes iron so that the polished finish will not turn red.

Electropolishing uses an electroplating bath, with the current reversed. I'm getting the impression that it removes iron, or something, far beyond the surface. I'm not real impressed with its corrosion performance, especially near welds, especially on parts that don't drain or rinse well.




Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[EdStainless]

EP is much better than mech pol in terms of corrosion resistance. Mech pol results in smearing of the surface and the formation of lots of laps and crevices.
The big deal with EP isn't the iron removal, that is a side benifit. The real issue is the cleaning and building of a very stable passive film.
EP on a mill finished part will perform better than a mech pol even when it is very rough.
The other issue that you have though is the use of 316 in seawater. This may be common pratice, but there is no reason to expect 316 to stand up to any seawater exposure. It doesn't have enough pitting resistance.
Make sure that water cannot be trapped anywhere and keep everything clean.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

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