Superduplex erosion corrosion problem 1

[ErikHavinga]

Hello,

I am senior plant engineer in a chemical industry and like some help with following issue: A SAF2205 (duplex) FH-heatexchanger with SAF2507 (superduplex) tubes (76 pieces) and pipeplates (baffleplates, tierods etc are Duplex), already showes eroded/corroded pipes after 2 years in service. Shell side are light ends, T=170 C with acids (0<PH<1), Tubeside Brackish CW, 40 C. 5 pipes are already plugged We performed IRIS on the pipes and this showes a decrease up to 70% wallthickness as a combination of internal and external erosion. The internal corrosion was uniform through the whole lenghth of the tubes, regarding the external erosion the problem is most severe on the ends of the pipes where the hot vapour enters (impingementplates are applied). Also we face some erosion and pitting on the baffleplates. I read an article where was written that in acid environments the flowspeed must be as high as possible to avoid erosion/corrosion. Is that correct (we have low flowspeeds) and what is the explanation behind it. Furthermore is Incoloy 825 or Monel more suitable for this application?
Thanks for all reactions.

Erik

 

[EdStainless]

What was the previous material of construction and how long did it last? This actually sounds like good life for such severe service.

How much particulate is there in this system? Any precipitates forming in the fluids? Is there an impurity that is raising the conductivity(corrosivity) of you solution?
The velocity issue has mostly to do with the prevention of static areas. Uniformity is really the key to handling strong acids.

You will have great difficulty finding an alloy with higher erosion resistance. The only one that comes to mind is Ultimet, a Co based grade.

You must work to eliminate all corrosion. Is there a Ni based alloy that you know of that is immune to corrosion in your environment? 825 does not stand a chance. It is softer and does not have more Cr.

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Plymouth Tube

 

[MikeMet]

First, you need to provide much more information on the shell side acids to get a good answer. But I'll shoot my mouth off anyway:
Alloy 2507 has good corrosion resistance to some acids at 170C and very poor resistance to others. It appears you are condensing an acid that is corrosive to Alloy 2507, such as sulfuric acid at concentrations above about 20%. Add chlorides and sulfuric becomes more corrosive at lower concentrations. Alloy 2507 also has poor resistance to hydrochloric acids. I recommend you check the Sandvik website, they have very corrosion data on-line.
The temp of 170C is above the critical pitting temperature for Alloy 2507 in low pH, high chloride environments, so one might expect the pitting you see on the baffles.
The general ID corrosion down the length of the tube makes no sense to me if you are truly running brackish water. Alloy 2507 has excellent corrosion resistance to brackish water. Might make some sense if you have leaks that lowered the pH and you are getting under deposit corrosion. If the general corrosion is very uniform, I think you may have thinner tubes than you thought you should have.
Much of what you are calling erosion, may be velocity accelerated corrosion, especially on tubes under the inlet nozzle. One should always install impingement plates or dummy tubes under the inlet nozzle
Given all that, it sounds like you have a very aggressive tube side environment and Alloy 2507 does not have the necessary corrosion resistance. You absolutely have to know what acids are forming on the tube side before you can select a proper tube material of construction. I think it's doubtful that Incoloy 825 would work since it is generally inferior to Alloy 2507 in most acidic environments. Monel may work depending on acid forming.
You have a lot of home work to do. Good luck

 

[EdStainless]

another good source of data is the corrosion handbook here,

http://www.outokumpu.com/applications/documents/start.asp

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Plymouth Tube

 

[ErikHavinga]

Thanks for all responses. The Heat exchanger, built in 1993 has been constructed in Duplex with superduplex bundle. The former exchanger (in duty from 1963 – 1993) was made of SS316 and caused a lot of problems. At the end of its life-time, the bundle was made of SAF2207.
Something about the process. We are producing Versatic (Synthetic carboxylic) acids. The principal corrosive agent in this process is the catalyst, a mixture of Phosphoric acid (H3PO4), Boron Trifluoride (BF3) and water. Its corrosiveness is due not only to the H3PO4 but also to HF, which results from hydrolysis of the BF3. Furthermore in the pure catalyst service AISI 317 LMN (1.4439) = AISI 316 with extra Molybdenum an Nitrogen does an excellent job. It is the water which makes it very aggressive.
Is it possible that condensation on the pipes of sour water out of the Versatic can result in uniform corrosion?
Near the reaction-section we use Hastelloy C276 with good results. Only temperature is significant lower (110 degr. C). than in the Heat Exchanger. Also history (from labor-tests) shows that Inconel 686 is suitable.
Thanks for incoming reactions.

Regards
Erik

 

[strider6]

just some consideration:

1) if you have brackish cw @ 170°C then SAF 2507 is not suitable. It will be necessary to define what is the chloride conc but as someone else said the "safe" limit for SAF 2507 is much lower than 170°c unless you've a cw without any oxygen

2) if you have H3PO4, it will be necessary to define the conc but if the pH is below 1 i think is concentrated.., then SAF 2507 at 170°C will corrode as you can see form the table below:

http://www.outokumpu.com/applications/corrosion/view_details.asp?name=PH02

this document from NIDI deals with the material selection for H3PO4

http://64.224.111.143/technicallibrary/nidi/literature/pdf/415.pdf

hope this help u

S

Corrosion Prevention & Corrosion Control