rusting stainless
 

3 months ago i had a solarpanel mount made out of 316L in Guatemala.
After a few weeks i noticed the first traces, and now 3 months later, the whole thing is covered in rust. Not a thick layer, just a tiny layer that is easilly removed by hand with some 360 grain sandpaper.

Strange thing is, it is also happening to the stainless bolts and other mounting materials that i bought in different shops in guatemala so its not just the welder that screwed me with some lower grade stainless.

Is this normal for 316L? Is it guatemalan quality ? Or can it not be 316 at all ?

If this is normal, what are my lifeline stanches made of then ? They are ancient and have only a few spots on them. Other parts like winches, cleats and hatch hinches are similarly ancient and totally spotless.

Finally, what is the best i can do about it ? I am currently sanding it with 360grain, then 500 and finally i hand polish it with turtle wax. I have no acces to electric tools where i am right now and just materials from the mexican shopping centres.

Re: rusting stainless
 

It could be 316L active... But most likely it is some cheap 300 series like 304, or 300. Either way, NEVER SAND STAINLESS. whatever you had, you not have active stainless because you have sanded away the passivated (if there was one) layer. Giver everything a good clean, get the wax off of it, then dip it in a vinegar bath.

This will re-passivated it. It may still rust, but it should be much slower than before.

Re: rusting stainless
 

If it was oxidizing after 3 months, even in Central America, it was not passivated. It sounds like Chinese S/S. I wouldn't use it for anything critical or highly stressed.

Re: rusting stainless
 

Sometimes you get rust because the surface is contaminated with small ferrous debris. But that's usually a cosmetic problem not the type of rust you're describing I think

Re: rusting stainless
 

Bummer. You kow the answer to your question. Sorry to hear it. You can try to pacify, but its probably futile.

Re: rusting stainless
 

I think SJB and Minne have answered your questions. 316 will never rust, even if you did sand it (never do that, use a polish like Flitz). Have you tried the magnet test? Bummer, hope you didn't pay much for it.

Re: rusting stainless
 

Your stainless was probably supplied with a common 'mill finish' - the surface is rough, has many mill rolling marks, not polished.
Also, possibly the material was processed with plain steel tools, jigs and fixtures with the plain carbon steel contaminating the surface of the stainless steel.

For rust prevention and corrosion retardation, 300 series stainless has to be supplied in at least a 300 to 180 grit finish to remove processing scars and surface defects of manufacture by the mill followed by or purchased from the mill already 'sanded'/ground; then surface then can be 'passivated' to remove the remaining surface IRON. Simple passivation should only be done with a surface ground to at least that mill supplied 180 grit finish. For best corrosion resistance (rusting, etc.) is to grind all welds flat and planar to the surfaces followed by grinding, then finely sand (down to 600 grit) followed by power-buffing to a mechanical mirror bright surface to remove all scratches and mechanical blemishes, followed by electropolishing.

If you take 'any' 300 series stainless steel that is not mill or shop finished to the above, it will surface rust, especially on boats when in contact with sea water. Thats why 99.9% of the stainless steel you see on 'boats' is 'very shiny' - its highly polished to prevent rust and corrosion.

Re: rusting stainless
 

just googled about passivating. It seems like it is rather critical what exact acid solution is used and how long and what temperature etc. It is a 2 meter high and 2 meter wide construction that is impossible to immerge in a bath really unless i'd fill a swimming pool with acid... :(

Now that it is sanded anyway, i guess ill have to just keep doing this every 3 months or so. It should take many years before the amount of metal removed like this is getting any significant i figure. I could make a test case of the first coming months though, partly just seeing if it helps when i put some vigager onto it without being able to immerge it, partly leaving it turtle waxed and part of it completely untreated. Any other suggestions that i could try ?

Re: rusting stainless
 

when the pipes were new, they were actually shiny. Definately not sanded with 180. I see electro polishing unfortunately also includes emerging in a special bath. :(

Re: rusting stainless
 

continual waxing will help.

Or just decide to ultimately sand and polish it to a mirror bright finish - a quite laborious process but if taken in small steps one small area at a time with quality sand paper a power high speed wheel buffer and the proper 'white metal' ('tripoli', etc.) buffing compound you can 'eventually' make this mirror shiny and 'rust-free'.