What kind of corrosion would you call this?
What kind of corrosion would you call this?
(OP)
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
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





RE: What kind of corrosion would you call this?
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
RE: What kind of corrosion would you call this?
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
RE: What kind of corrosion would you call this?
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
RE: What kind of corrosion would you call this?
RE: What kind of corrosion would you call this?
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
RE: What kind of corrosion would you call this?
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
RE: What kind of corrosion would you call this?
Yes.
RE: What kind of corrosion would you call this?
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
RE: What kind of corrosion would you call this?
http://www
Steve Jones
Materials & Corrosion Engineer
http://www.linkedin.com/pub/8/83b/b04
RE: What kind of corrosion would you call this?
RE: What kind of corrosion would you call this?
Regards
Brad Stone
RE: What kind of corrosion would you call this?
RE: What kind of corrosion would you call this?
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?
RE: What kind of corrosion would you call this?
RE: What kind of corrosion would you call this?
Dik
RE: What kind of corrosion would you call this?
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
RE: What kind of corrosion would you call this?
Dik
RE: What kind of corrosion would you call this?
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
www.lehightesting.com
RE: What kind of corrosion would you call this?
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
RE: What kind of corrosion would you call this?
Since the reactor is being scrapped, it would be a good idea to get a metallurgical analysis of the damage to identify failure mechanism. Tough to take corrective actions (including material and fabrication changes) without first knowing what happened.
Aaron Tanzer
www.lehightesting.com