Finishing 316L stainless rails

I like the look and feel of mill-finished stainless tubing.
Is it really necessary to polish this for marine use?

 

Yes, polishing retards the effects of surface corrosion.

Polish the tube and all welds. No pinholes in those welds.

Polished and beadblasted stainles holds up ok.

If you must have the look investigate this

Aluminium is another choice. Ask your metal shop if they can work with it.

Anodized al is very durable

 

I would either have it electropolished (best) or passivated with citric acid.

 

As I understand it, the smoother stainless steel is, the less chance there is of crevice corrosion starting at the molecular level and moving it from "passive" state to "active" state.

And once it's "active," all bets are off.

 

If minor scratches and the need to revisit every few months is no problem, a mix of Ospho and any dish washing liquid with a scotch brite pad will do.

Scrub the mixture in and let it dry . The purpose of the Joy is simply to hold the Ospho in place an hour or so.

The Ospho gets the iron on the surface and prevents rusting for a good while.Wear gloves.

A sponge and Bartenders friend powder does almost as good and is a bit smoother

 

Some like it matte but others like it mirror like finish. To polish it, you need to sand it with very fine grit paper then buff it with a buffing cloth and that "green cake" you apply to the buffing wheel. I forgot the name of the chemical we use to remove the stain in welded joints but it is commercially available. 316 grade steel polishes well and hold it better than the 304.

 

To answer your question. Polishing it won't do much. 316 L is very good stainless if you like the brushed finish leave it that way

 

Thanks Alex. That's what I thought...

 

Wrong. Polishing stainless is required to minimize inter granular corrosion. Which is why stainless should be polished to the degree possible. With the best being electropolished.

 

Maybe in a nuclear power plant or food processing but a brushed 316 rail will look fine for the next 100 years if waxed a few times a year. The only thing that will make it look bad are mineral deposits it won't corrode or rust

 

Alex,

You are wrong. The rougher the texture of stainless the higher the likelihood of inter granular corrosion leading to pitting and crevice corrosion. While stainless is generally highly resistant to chloride corrosion, it is not resistant to agressive localized corrosion. The problem with leaving it rough is that while sometimes it may be fine, in others you could see failure in months, not years.

For a more scientific grounding see the following studies

1) Electropolishing of 316L stainless steel for anticorrosion passivation
2) http://www.sjsu.edu/faculty/selvaduray/page/recent/316LStainlessSteel.pdf
3) http://www.kepcoinc.com/downloads/articles/LC_electropolishing-stainless-steel-implants.pdf

 

Stumble, implants in a body are not quite the same application as railings in air.

Also intergranular corrosion firstly requires chromium carbide precipitation to occur. This should not be present in mill stock products but develops after hot work and slow cooling. 316L’s only real advantage over 316 is a reduction in chromium carbide precipitation after hot work through the lower carbon availability. But thin wall sections such as pipe and sheet even in 316 don’t tend to develop this anyway, they cool too quickly for it to form.

Not just grain boundary corrosion but any surface imperfection can give rise to crevice corrosion given the right circumstances, but you are not going to get crevice corrosion outside of immersed applications unless you retain or trap water somehow, which is not much of a concern with railings.



Sprit...

To stop any staining after welding, clean the weld zones with Nitric acid which removes most surface iron deposits formed during welding that would otherwise oxidize and stain.

I wouldn’t be concerned using matt SS tube for railings so long as it’s 316 or 316L or better. Even if it’s a bit rougher and you do find it stains a little just rub it down with fine sand paper and or polish it and you'll fix the problem immediately.

 

I'm just talking about the 316 as a boat railing. Can it work if not highly polished ? Of course it can. All of the technical metallurgy aside. I understand you would be correct in a much more serious application such as the human body , or an extreme pressure application but for a freaking rail on a boat 316 will be absolutely fine.

 

Sure it will work..and it wil corrode, tarnish. When you touch it with hand or clothing this stain will now be on you.

Hard to understand the choice of brushed ss for exterior marine use.

Grind and polish is the standard way to finish a weld. Unpolished welds will look agricultural after a few years.

 

Mike,

Just like you aren't going to get trapped salt water on clevis pins, shrouds, turnbuckles, chainplates, ect? An owner can do whatever he wants, but that is no excuse for giving bad information.

All the research and practical experience is clear... The more polished a stainless piece is the longer it will last in a salt water environment. Making the trade off between corrosion protection and visual effect is an exercise for the owner.