Surface Rust on 316L SS Catwalk

Most likely the rust is due to the steel being a poor grade of 316SS.

There is quite a lot of this happening throughout the world market with the poorest quality control concentrated to a few specific countries.

BUYER BEWARE!

multiple posts

Surface rust on 316L SS

I think discussion was inadvertently shut down on this one. The duplicate thread was deleted. Just keep it civil -- as usual. Thanks.

That's asking a lot from a bunch of opinionated old engineers.

Civility is not a particularly prevalent characteristic among some of us.

Contradictory decisions were made. This copy of the thread was closed for a while, but still left showing to direct people to the second copy, which got quite a few replies. But then they deleted the informative thread and apparently reopened this copy.

Stainless steel break's your heart over and over. It's not truly "stainless" but relies on a trick of atomic physics to stay shiny. The stainless part is a layer a few atoms thick of chromium atoms that migrate to the surface when it has been polished. The surface is said to be "passivated". The chromium oxidizes (stable oxide) in contact with the air after some time which is which is why it makes a lousy mirror. 316L is a low carbon version that makes it weld more easily, and has more chromium than 304L. They both will rust if you let them. You are right about contact with regular steel, during manufacture, or in handling. Just scrape it on a steel table, truck floor, or anything and the scrape will rust. Also tightening down on a fastener, even if it is also stainless, will scrape through the chromium and rust. Welds must be polished and care not to over heat the base metal when doing larger welds. The layer can be enhanced by treating it in a kind of reverse plating process whereby more of the iron atoms are removed on the surface and the result is a thicker chromium layer. The same places also do "electro polish" which is a close cousin. Also if regular steel is processed near by, and the grinding dust settles on the stainless steel it will start the rusting process. The bad news is that those rusty spots will become sacrificial anodes and corrode at an accelerated rate into the metal. The only way to stop it is to grind it out and repolish. It doesn't need to be mirror, just a #4 sanitary is good. If the rust is flash, and still on the surface, Bar keepers, or some other acid wash might treat it. Some of the cleaners are citric acid, so safe to use. If you don't neutralize it it will come back to haunt you in a few months. There are portable "passivators" also that some folks use to treat welds. I recently restored some large scale outdoor stainless steel sculptures (Conservator/Sculptor) that had been hit by lawn mowers and had severe rusting on the scrapes. I used a Walter blendex wheel and a cotton buff with polishing compound. It worked great. I'm trying to convince the grounds keepers to put HDPE bumpers on the mowers to prevent steel to stainless contact...

Having worked in the food and dairy industry passivation is usually done in the food contact areas. Never heard about doing it on a platform or walkway. But with the walkway being 316L, which tells me that the walkway is in a corrosive environment.

An interesting tidbit here from my experience. and pushing off-topic has to do with electro-polishing.

Talking to USDA-3A new equipment inspector. Where some equipment is to be electro-polished (customer request but not needed), I asked the inspector about the surface finish, and he was aware that electro-polish actually does create a surface area that due to the process removes the ferrous material and leave the nickel and chromium that give its shine or luster. Under a microscope the surface area is actually rougher. And that's what I was told in my Physical Metallurgy class in college.

Since then I have done research understand that there is also a anodic leveling effect that reduces the highs points in the surface area. that give it a more sanitary finish.

Its quite a science, and working in the field, is something that one should be aware of.