The authoritative public forum
  for Metal Finishing since 1989
  

  -----

320 grit-polished 316 stainless is rusting

2007

I at present have a problem with a finish on a stainless steel balustrade I have supplied,
The customer specified a 320 grit polish on a 316 stainless steel tube section; subsequently this section has shown signs of staining in a costal atmosphere, and tests have revealed that the finish has an Ra 1.5um. I am trying to find out what Ra value that 320 grit complies to so I can pursue the suppliers or the specifiers

Best Regards,

Chris Earp
Fabricator & Fitter - Crediton, Devon, England

Roughness in a coastal environment can be a factor because it can hold a variable amount of moisture that contains salt. That said, I will bet a cheap cup of coffee that the grit or the cabinet is slightly contaminated with another metal, probably iron which will impregnate the surface. Passivation will be a major help.

James Watts
- Navarre, Florida
2007

---

Coastal environments are very rough on stainless due to the chlorides in the air. Even well passivated 316 will start showing tea staining after a while, regular maintenance repassivation is often required. Also helpful is rinsing it off with fresh water regularly. And of course, a rougher surface will be more susceptible to corrosion than a smooth surface.

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois

2007

---

2007

I've been looking into stainless steel specs & finishes recently because we have got some staircases, walkways and balustrading to make, partly out of stainless steel, and want to avoid the experience of the original poster of this thread. A couple of things I found on the British Stainless Steel Associations web site (www.bssa.org.uk)- maintenance is required! they recommend cleaning every 6 - 12 months for 316 in a coastal environment.It is also recommended that surface finish should have a max. transverse Ra of less than 0.5 microns, equivalent to a no.5 or 2K finish.
So back to the original question - what surface finish would you expect from a 320grit belt grinding?
Re. passivation - is this actually done for general engineering & architectural purposes? Go to a stockist for some stainless tube & your choice is descaled, "dull polished" or "bright polished" and if they haven't got polished tube they'll take a descaled one and pass it through a belt grinding machine.I doubt they bother with acid dips. We use platers and anodisers but I'm not aware of any stainless steel passivators. Or am I just not looking hard enough?

Gary Middleton
- Ilford, Essex, United Kingdom

---

Gary,
Here in the states it's usually the plating job shops that will do passivation. Of course using a citric acid based product you may very well be able to just do it yourself. Let us know if we can help you.

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois

2007

Chris, after pickling and passivating you might consider electropolishing of the stainless steel balustrade. Grit 320 on 304 or even 316L will lead to staining in a coastal environment. Due to electropolishing you will achieve an easy to maintenance and corrosion resistance balustrade. If grit 320 followed by EP is to shiny you can consider ceramic blasting followed by EP.

Menno Kier Visser
- Heerenveen, Holland
2007