Corrosion pitting in 316L

keezer · Aug 15, 2020

What you see below is a photo of a corrosion pit on the inside of a stainless steel tank made of 316L. Each crater is about the size of a tack weld.

I believe it is a result of a support brace welded there during the manufacturing process. There is a pattern of these things around the perimeter of the tank. It is likely that the manufacturer used "plain" carbon steel as a support brace, and just welded it to the stainless material :eek:

After removal of the support brace, the spots were ground off and remained hidden until exposed to a high salt product.

The question: how can this pitting be stopped? Would grinding out/drilling out the affected area and then backfilling with 316 rod do the trick?

Milland · Aug 15, 2020

You can look into spot-passivation too, to remove any free iron and carbon.

Local/Spot Passivation

Spot Passivation?

Do the same if you grind and weld - the important thing is to allow a "healthy" oxide layer to grow, iron if present inhibits this.

Phil in Montana · Aug 15, 2020

Is it worth fixing? It has the look that the corrosion will stop at this point...Phil

Milland · Aug 15, 2020

Phil in Montana said:
Is it worth fixing? It has the look that the corrosion will stop at this point...Phil

I dunno, but if I was delivering that to a customer or selling it on I'd sure want it to look better than that. I don't think it's stable, so corrosion could continue.

1953chevB · Aug 15, 2020

Corrosion Uggg

Once corrosion starts on/in stainless steel it penetrates into the parent material and it's contaminated, crazy poop starts and the end results with corrosion cracking , so those little spots have started to weaken the material
(stress load), a crack can propagate further and can in time can cause a failure. how long any ones guess.

In past live we would get stainless steel parts with corrosion and no matter what I tried could not remove it all
it would start corroding. (failed) ended up scraping it. mechanical clean, aluminum oxide blast, passivate, polish with red wheels. it all failed to remove all of it. is it possible yes some did pass. some didn't.

this is a tough call, I would save the tank by cutting the corrosion out completely and weld patch repairs but that's me.
make some plugs or patches to precisely fit in the removed damaged areas,
you know some of us have worked on old cars with corrosion (rust) on body panels, we cut out and replace it.
it's hard to stop rust damage once it starts, if you sand it or grind it weakens the sheet metal and looks like a POS, it just a PITA.

gbent · Aug 15, 2020

That red rust suggests to me the corrosion is still happening. What is the value of this tank? Is it a code vessel? I think my next call would be to the original manufacturer.

1953chevB · Aug 15, 2020

gbent said:
That red rust suggests to me the corrosion is still happening. What is the value of this tank? Is it a code vessel? I think my next call would be to the original manufacturer.

on second thought what is the application of this tank, this is very good suggestion.

keezer · Aug 16, 2020

Thank you all for your replies!

The tank is a blending tank at a food processing facility. It is only several years old, and is not a code vessel. The original manufacturer is in a far away place, and sadly has little care for the item after having been paid. Needless to say, this will be the last purchase from them. The corrosion is indeed still happening, and something must be done before it eats any more material away.

The tank wall is about 5mm (0.2"), the depth of the pit is about 1mm (0.04") or so. Perhaps rather than cutting a patch, the wall could be gouged, or ever drilled through and re-filled with weld?

1953chevB · Aug 16, 2020

Yes that would be a good option , be sure to remove all the corrosion.
Good luck

gbent · Aug 17, 2020

Find a local company that specializes in tank repair. Here, there is a local company certifies and repairs semi trailer tanks, and they do stationary tanks as well. They frequently repair corrosion pits in both code and non-code vessels to maintain minimum thickness. The company has a "R" stamp and the necessary inspection for qualified repairs.

BT Fabrication · Aug 17, 2020

Passivation cleaning with an acid will do the trick, you need to etch the carbon off it.

id carbide burr out a good amount of material around and beneth it. essentally gouge it out then fill with weld and blend. lots of work to fix it.

1953chevB · Aug 17, 2020

BT Fabrication said:
Passivation cleaning with an acid will do the trick, you need to etch the carbon off it.

id carbide burr out a good amount of material around and beneth it. essentally gouge it out then fill with weld and blend. lots of work to fix it.

BT
Passivation only cleans contamination on the surface, it will not remove corrosion in the parent material

keezer · Aug 19, 2020

Yup. Tried a passivation gel (nitric/acetic/hydrofluoric blend), no luck. Still bleeds red after exposure to humidity. Waiting on burrs to arrive.

Milland · Aug 19, 2020

keezer said:
Yup. Tried a passivation gel (nitric/acetic/hydrofluoric blend), no luck. Still bleeds red after exposure to humidity. Waiting on burrs to arrive.

Probably got a lot of the base steel mixed into the puddle when welding. May have to go a little deeper and wider than you'd want when clearing out the contaminated metal, but I'd hope less than a mm.

BT Fabrication · Aug 19, 2020

1953chevB said:
BT
Passivation only cleans contamination on the surface, it will not remove corrosion in the parent material

I might not have explained it well, carbide burr id say 1/8" deep, some passivation to remove any iron left after on the surface, re-weld with 316L rod then again clean and passivate the surface. and polish to get back into service. Not a simple or easy job as the base material is contaminated with low levels of iron that got mixed in when someone welded onto it. could have got it too hot also when trying to get a good tack without good enough shielding gas and burned out the chromium from the mix. Might have to weld the area and grind it out a couple times to dilute the contaminated section.

1953chevB · Aug 19, 2020

BT Fabrication said:
I might not have explained it well, carbide burr id say 1/8" deep, some passivation to remove any iron left after on the surface, re-weld with 316L rod then again clean and passivate the surface. and polish to get back into service. Not a simple or easy job as the base material is contaminated with low levels of iron that got mixed in when someone welded onto it. could have got it too hot also when trying to get a good tack without good enough shielding gas and burned out the chromium from the mix. Might have to weld the area and grind it out a couple times to dilute the contaminated section.

ahh OK
I normally cut out it completely, it gets in the grain structure, which in turns causes cracking.

BT Fabrication · Aug 20, 2020

YA the guy above said he didnt want to, I would also. it would be a better fix and faster.

keezer · Aug 21, 2020

If one was to cut the corroded spot out, how small could the cut area be? The affected areas are small - a couple of tacks side-by-side, and 6-fold symmetric about the circumference of the tank. What would be the smallest practical patch?

1953chevB · Aug 21, 2020

There is no limitation to size could be drill out if small, and plug weld.
It really depends on the affected area.

BT Fabrication · Aug 22, 2020

Reality, the smallest practical patch is the max size of the affected area. under 1/2" id just cut and weld filler back in. over that you can cut a patch to any size and blend back in much faster.

Corrosion pitting in 316L

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