What kind of corrosion would you call this? 1

[StoneCold]

I have an older reactor that is made of 316Lss. It has been exposed to acid numerous times. It is developing these black areas with pits. I wonder what you call them?
Obviously the tank needs replaced. I get that, but from a pursuit of knowledge aspect. What is happening there? Why is it turning black? Is some element being leached out?
I put a bunch of pictures in a pdf, hopefully you can download it.
Please tell me what you think.

Regards
StoneCold

 

[EdStainless]

You aren't going back with 316 are you?

Because of its association with welds I would suspect that it is either sensitization (local Cr carbides in the grain boundaries). What is the C content of the alloy?
Or a result of the heat tint from the welds.
The heat tint is very damaging as the oxide takes Cr out of the alloy in that location. If you try to remove it mechanically all that you do is hid it. It can only be removed by pickling or electropolishing.

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Plymouth Tube

 

[StoneCold]

EdStainless
Thanks for Replying.
No we are not going back in with 316. We are either going to do 2205 or if we can stomach the cost Hastelloy C2000. Though I have thought about cutting off the bottom head and re-using the rest of the vessel.
I think we could cut some coupons out of the side wall and do some testing on them to see if the rest of the vessel was affected. Even if we saved it, it would go in another less harsh service.

We could pickle the reactor but I am conserned that the pinholes are so deep that it is ruined. I think the shell is only 0.25" thick.

What would you do with it?

Regards
StoneCold

 

[EdStainless]

At this point scrap is the right thing to do.
If it were earlier in life you could grind the local areas and locally pickle with paste to clean the spots.
Trying to grind off heat tint is a fatal mistake. You do remove most of the oxide and some of the Cr depleted layer beneath it, but you just re-imbed this material into the surface.
I am guessing that your C level is low enough to avoid sensitization, unless the weld heat input was way too high.
It would be great to see some micros to go along with this.

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Plymouth Tube

 

[metengr]

Heat tint.

 

[TomEun]

Please do not make haste about the repair plan until you find the root cause. The black spots including pits show on both of the base metal and weld joint. It means that the damage mechanism may be come from the service condition. The black spots look a symptom of the initiation of pitting in the acidic condition.
I believe you may have to evaluate the acidic environment, such as what type, concentration, pH, duration time, frequency, chemical composition at normal and upset, temperature, etc.
Once the acidic service is proved as a primary corrosion mechanism, at least the in-kind replacement and repair welding should be avoided. New appropriate material selection which is based on the normal and upset conditions will be the best solution.

Thomas Eun

 

[StoneCold]

Metengr.
Are you saying that the original heat tint spots were chrome depleated and that is contributing to the holes and this black zone formation now? They are more than heat tint now for sure.
We often stir magnesium in these reactors so it wears off the original heat tint spots in a hurry. It would be super awesome to be able to have some time lapse pictures of the spots, but I don't have that. Pictures and explosion proof areas are hard to mesh.
Do you think that if we would have pickled the reactor after fabrication to remove the heat tint that it would have lasted longer?
I have another reactor about five years newer that I should probably investigate.

TomEun
We are definately looking at another alloy for the reactor. The sad thing is that it is not the main product that is the problem. It is when the tank is empty and they decide to use it for waste neutralization, or "mother liquor" storage. These odd unscheduled things that eat up the reactor.

Regards
StoneCold

 

[metengr]

Are you saying that the original heat tint spots were chrome depleated and that is contributing to the holes and this black zone formation now?

Yes.

 

[EdStainless]

Not only Cr depleted but the heat tint is a scale that will promote under deposit corrosion.
The abrasive nature of your process may polish the spots so that they don't look different, but the Cr depleted layer is still there.

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Plymouth Tube

 

[SJones]

http://www.outokumpu.com/applications/upload/pubs_113142858.pdf

http://www.nickelinstitute.org/TechnicalLiterature/Technical

Series/SpecifyingStainlessSteelSurfaceTreatments_10068_.aspx

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[vanci]

Next time around, use lower heat input (kJ/in) and backing gas (assuming it was GTA welded) regardless of what base material you decide to use.

 

[StoneCold]

It has a dimple jacket on it so the dimples are mig welded. It was fabricated by a company we no longer use. It would appear the the heat input was excessive, that and abuse has led us to this spot.

Regards
Brad Stone

 

[vanci]

Like EdStainless indicated, your current one is scrap. Going forward use a backing gas regardless of welding process. If applied corretly, should eliminate any evidence of heat tint. Lower heat input will reduce susceptibility of sensitization.

 

[racookpe1978]

What's the expense to the paper process of a separate sacrificial "dump tank" for those irregular times that the wrong liquor needs to be stored?

That is, buy an expensive stainless tank (or Hast-X tank) for expected service and expected liquors, then a cheaper tank for the short dump times of the waste fluid that screws up the good tank?

 

[StoneCold]

That would work, or a large stick to educate the offenders.

 

[dik]

vanci... 'backing gas'... is this an inert gas source to act as a coolant? I've not heard the term...

Dik

 

[EdStainless]

Backing gas or purge gas, The idea is to use Ar to prevent the oxidation of the metal on the back side of the part being welded.
On tanks I have seen it done with tape, Saran wrap (make sure to use real saran), and a tygon tube from the Ar source. The covers a few places near each other with one 'tent'.

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Plymouth Tube

 

[dik]

thanks...

Dik

 

[mrfailure]

I would like to throw a different idea out there for original failure mechanism to see if it makes sense in this system: MIC.

MIC attack is preferential to weld regions but will occur regionally, consistent with the observed damage. Acid attack and cleaning would remove the surrounding slime in this environment. If MIC occurred, there would have been stagnant water present at some point to start anaerobic bacterial growth, which subsequently would have attacked under the surface.

Would the reactor have seen sufficiently high temperatures during service to kill the bacteria (thus elimination MIC as a possibility)?

Regarding sensitization: The weld HAZ would have been affected but it would not account for the base metal attack observed.


Aaron Tanzer

http://www.lehightesting.com

 

[StoneCold]

mrfailure
Intersting idea. The reactor sees solvents regularly, one of them being tetrahydrofuran. So I think it would kill anything. The max temperature reached is 150C. The reactor sees water but it is city water mixed with acids or bases. I think that MIC is a very remote possibility. Now on the inside of the jacket that could be happening, but in the reactor I don't think so.
I welcome the discussion though.

regards
Brad Stone