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Monel Corrosion

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TEV

TEV

Materials
Oct 31, 2002
100
What can cause grain boundary corrosion attack in Monel 400? I am aware that aerated hydrofluoric acid can cause stress corrosion cracking in Monel, but can it also attack the grain boundaries?

I am looking at some 316 stainless tubing that has been in service in HF gas. There is no evidence of deterioration of the ID surfaces, but preliminary metallographic observation of a longitudinal section shows areas that appear to have material removal by etching out the grains due to grain boundary attack, and there are also a few radial cracks that do not appear to follow the grain boundaries. I think the evidence points to a leak somewhere in the system, causing the HF to release into the (moist) atmosphere, thus the SCC, but does this explain the grain boundary corrosion, also?
 
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Could this be a form of de-alloying you are describing?

I have heard of de-alloying of MONEL in refinery service. (If I locate the source, I'll re-post the information.)

~NiM
 
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Intergranular Attack is common in some stainless steels, nickel and aluminum alloys. I believe the problem is segragation.

a)
Usually solution annealing is preformed to remove phases from the grain boundry regions that decrease corrosion resistance.

b)
Reduction of impurities & use of stabilizing agents.

I dont understand the next part of your question. It is worth mentioning that Monel does exihibit embrittlement in sour service.

Cheers

 
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TEV,

Monel resists reducing conditions pretty well, but not oxidizing. IGA (corrosion)frequently leads to IGSCC (corrosion+stress)in Inconel 600-don't know about Monel specifically.
 
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A point that I didn't mention in my original posting. The corrosion is on the external surfaces of the tubing, the surface that is not exposed to the HF contained gas.

Further metallography shows transgranular SCC cracking in some regions, and extensive grain boundary attack in adjacent areas. The grains are actually falling out of some of these latter areas.
 
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TVH,

What sort of environment is on the tube OD? Any sulfur/S compounds?
 
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The OD environment is "humid atmosphere". That is all that I can find out.

Does Sulfur segregate to the grain boundaries in Monel? Is that an indication of either or both of excessive Sulfur content or improper thermal processing?
 
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Special Metals claims that Monel alloy 400 offers exceptional resistance to HF in all concentrations up to the boiling point in the following bulletin:


ASM HANDBOOK Volume 13 Corrosion has a small section on Nickel alloys and HF:

alloy 400 showed transgranular SCC in the vapor phase of dilute HF solutions (up to 0.5% HF) at temperatures up to 95 C (Ref 38).

Reference 38 is the following:

S.W. Ciaraldi, M.R. Berry, and J.M. Johnson, Paper 98, presentation at Corrosion/82, Houston, TX, National Association of Corrosion Engineers, 1982.
 
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TEV (got it right this time!)

I had been thinking about how susceptible nickel is to S, since you seem to have a pretty severe OD environment there.

IIRC from the BWR Inconel 600 CERT (constant extension rate testing), the slowest testing gave IGSCC, followed by TGSCC, both in the same environment-O2 in 550 deg. F pure water. The SCC mode can be stress-dependent as well as chem. dependent.
 
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Problem solved! I received an old paper by Harry Copson and C.F. Cheng on observations of corrosion on Monel in HF environments. They showed that exposure of Monel to HF in oxidizing, moist conditions can generate both transgranular and intergranular stress corrosion cracking. So, what I had interpreted as intergranular corrosion is actually a form of intergranular SCC.

The reference is "Stress Corrosion Cracking of Monel in Hydrofluoric Acid" by Copson and Cheng, CORROSION, Vol 12, No. 12, 647-653 (1956) December.
 
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Additional Notes;

Monel 400 has excellent resistance to HF acid (whole concentration) under the 150 deg.C in the absence of oxygen but it is subject to IGSCC when exposed to wet vapor of HF in the presence of oxygen.

References

- Materials for Hydrofluoric Acid Service in the New Millennium, Herbert S. Jennings, Corrosion 2001, Paper No. 01345
- Materials for Receiving, Handling and Storing Hydrofluoric Acid, NACE Publication 5A171, 1983
- Safe Operation of Hydrofluoric Acid Alkylation Units, API RP 751, June 1992


Thomas
 
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