Metal finishing Q&As since 1989

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Stainless Steel: which is more important, surface finish or grade?

December 4, 2011

Hi,

What is more important, the surface finish of stainless steel or the grade of the stainless steel i.e., 304 or 316. We have to make some bicycle shelters for an inland town from stainless steel and they are too large to electropolish. Cost is a consideration so we would prefer to use 304. We are considering finishing the 304 stainless to .5 Ra. I have seen 316 corrode that had a poor surface finish and 304 remain bright that had a good surface finish.

Tom

Tom McCormack
- Ireland

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Inland location, either 304 or 316 should be just fine. The danger is corrosion which is initiated by surface contaminants, of which metal contamination, especially iron, are the most likely.

Surface finish from the mill is not a major factor.

If electropolishing is out of the question, then consider passivation, which will usually cost less. In fact there are companies, perhaps one near you, which can do passivation on-site.

If welded on-site, passivation of the welds is virtually mandatory. A web search for passivation paste will turn up DIY materials.

Jeffrey Holmes, CEF
Spartanburg, South Carolina
December 23, 2011

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Tom, having a very low surface roughness will initially give you a reflective surface, but it will tarnish and go dull with time. If at all possible, I would recommend using electropolishing and although your shelter may be too big to fit into an electropolishing tank, it is possible to use brush plating technology as an electropolishing process, so you may want to make enquiries on that route. At least electropolished stainless will remain bright and reflective much longer than mechanically polished surfaces. Regarding the grade of stainless, I would go along with either 304 or 316 - both are regular workhorses and shouldn't give any trouble

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
December 28, 2011