My 316 is rusting! what can I use to passivate?

A. The picture is not good enough to tell, but from the size ,shape and spacing of the spots, I will guess that someone used a wire brush on it. Another guess is that the brush was made out of 400 series SS, which will rust. Tiny tips of the 410 are stuck in the finish of the 316 and form a weak battery of corrosion.
A couple of firms that advertise at this site sell a citric acid based passivation that might work. An outside possibility is that naval jelly ⇦ this on eBay or Amazon [affil links] might work. A further guess is that you will have to clear coat the ss after you remove the indications of rust.
You may have to wet sand the entire rail with decreasing grit size paper.

A. Joe,

Your surprise about the rust-stains on 316 or 304 in outdoor-applications surprises me.
In my region (Belgium/Europe) practically all outdoor SS 316 or 304 is getting stained after some period, if it is not electropolished.

In my experience, electropolishing is the only finish that realises a rust-free surface for many years in outdoor applications in practically all circumstances.

Of course there are situations and applications where simply passivated SS will remain free of rust; regular cleaning with water and soft detergent is a good help to keep SS in good shape.

A. Mr. Colosi:

I agree with Mr. Watt's assessment of what could be causing the rust stains. Also, if any of the materials you used to finish or machine the 316 rails were previously used on carbon steel, carbon steel particles could have been transferred onto the rail surfaces, which would then rust. The treatments suggested by Mr. Watts would help in this case. Hope this helps.

Thomas Kemp

A. I feel for you.
The first thing to realize is that Stainless steel is not stain proof, only stain resistant. it stains less than regular steel.

While I agree with James, keep in mind that its not just tooling that can cause contamination, We do Stainless Steel railing every so often and while our primary focus may be Aluminum, we clean the shop particularly well prior to starting a Stainless Job (to get rid of any steel dust) and enforce a strict anti-contamination policy.

Grinders/ saws/ tables/etc. that have been used on other jobs must be blown clean prior to being used (not just changing the grind/sand disks) Another source of contamination is handling - Make sure that employees do not handle steel and then handle Stainless.. The oils from their hands mixed with steel dust create rusty fingerprints.

Clean up- after finishing wiping the job down with a clean (new)cotton cloth and a soap and water solution. let that dry and look for stains.. remove any staining and wipe down again with a acetone ⇦ this on eBay or Amazon [affil links] Warning! highly Flammable! or lacquer thinner ⇦ this on eBay or Amazon [affil links] Warning! highly flammable! cloth.check for stains again, the less you have to do the better off you are.

Starting by Eliminating potential sources of contamination, finish by ensuring no environmental contaminants. - a good clearcoat can help with that.

Passivation - We have a local company that specializes in that for the space industry. Fortunately we have had to use them.

Be aware that passivation and/or pickling can change the color/ finish of the job.

Good luck.

A. SS Discolouration,

Only way I know to determine exact mechanism is by X Ray Photoelectron Spectroscopy.

Good Luck from Ireland

A. First verify alloy type then verify if passive state.
Stainless steel testing kits are available for both.