Inconel 625 vs ASTM A182 F55 8

[Meshry]

Which alloy of these two has the higher corrosion resistance? Inconel 625 or ASTM A182 F55??
I understand, A182 is supper duplex alloy, meanwhile Inconel 625 is high alloy steel.

 

[EdStainless]

What environment? What temperature?
In almost every case the answer would be 625, but a C type alloy would even be better for most applications.

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P.E. Metallurgy, Plymouth Tube

 

[weldstan]

625 is not a high alloy steel. It is a Nickel-Chromium-Molybdenum alloy (nickel base alloy). Agree with Ed above as to corrosion resistance.

 

[pradipgoswami]

Hi Meshry,

The corrosion resistance is relative term, depending on media, operating conditions( pressure, temperature) and many more factors.
Attached are some information on both the alloys extracted from manufacturers data sheets& are worth looking at.
With respect to O&G(Production)environment by an large alloy 625 has better corrosion resistances,due to higher CPT, CCT, PREN values.
However there's is a huge difference in cost between F55 and 625 alloys.
F55 alloys on the other hand are quite versatile, with the reported applications in Offshore O&G, Downstream Applications, e.g. Refining& Petrochemical, chemical environments.
What is the intended service conditions for these alloys? Selection of the best alloy would influenced by many of the factors stated above.
Thanks.

Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
Ontario,Canada.
ca.linkedin.com/pub/pradip-goswami/5/985/299

 

[EdStainless]

Why the environment matters is that the super duplex is not usable above 600F, or below -40 (maybe even not below 0F depending on the application).
It has high strength, great resistance to aqueous corrosion, but can be difficult to weld (and the welds may have inferior properties).
625 has good strength up to 1400F and good toughness down to LN temps, and it welds well.

The cost between a superduplex, 625, and a C alloy (622, 686, 59) is not that great.
So pick carefully, there is no answer that is always correct

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P.E. Metallurgy, Plymouth Tube

 

[Meshry]

Gents,Thanks for your replies.
Actually my question is related to a mechanical seal installed on a multi stage water injection pump (Aquifer water).
it failed many times and the internal of the mechanical seal found heavily corroded. all the wet parts are ASTM A182F55.
the other pumps in the same facility/same application are equipped with Inconel 625 mechanical seal internals, this seal is working fine. so I am doubting the ASTM A182 F55 is not suitable for our application (corrosive environment).
the discharge pressure of this pump is 290 barg and seal pressure is 100 barg.
TSS is 20 mg/L.

 

[EdStainless]

How hot does the seal run?
You may be up against the critical crevice corrosion temp for the duplex.
We used to only use 622 for seals in high Cl waters (anything no fresh).

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P.E. Metallurgy, Plymouth Tube

 

[nickypaliwal]

What type of corrosion are you experiencing? Is it pitting? Do you have chances of oxygen ingress due to some reasons?

 

[Meshry]

Nickypaliwal,
I will try to get some photos for the corroded parts.
Yes Oxygen ingress is possible.

 

[Meshry]

EDstainless,
The working temperature is fluctuating between 72C to 85C.
Nickypaliwal,
could not arrange any photos due to legal issues, however the corrosion is almost a decay and metal loss. not only pitting.
Gents,
is it possible to have galvanic effect between super duplex and inconel625???? I have discovered that pump shaft is inconel625 meanwhile mechanical seal wet material is super duplex!!

Thanks

 

[EdStainless]

I don't see any risk for galvanic issues, the alloys are similar enough in corrosion resistance.
What pH range do you see?
I would move to a Ni alloy, and something better than 625.

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P.E. Metallurgy, Plymouth Tube

 

[nickypaliwal]

I agree that chances of galvanic corrosion is negligible. IN general Alloy 625 should be fine for water injection but if O2 ingress is expected more than 30-40ppb then shifting to a copper based alloy should be preferable.

Note that copper based alloys are not good in H2S service so should check before using it.

 

[georgeverghese]

Would say for seawater at 80degC, you'd look for a molybdenum content exceeding 6% - Mo content in 25Cr superduplex is only 4% while than in Inconel 625, Mo is 9% or so.

On the other hand, there may be some problem with the seal system heat removal operation, producing high temps on the contact surfaces in the seal.

 

[nickypaliwal]

@George : I believe that Alloy 625 should perform great in seawater due to high PREN but I am not sure as why the clients I am working have not recommended the same in any of their facility. I believe this may be due to crevice corrosion and MIC issues for Alloy 625 in sea water. 6Mo also has similar issues as highlighted in NORSOK M-001.

I am not very much aware of these materials in sea water service. Can you please advise and share your experience for use of alloy 625 in sea water ? Thanks in advance.

 

[Meshry]

Gents,
there is no sea water. the pumped fluid is aquifer water( with CL content). the seal barrier fluid is potable water.

 

[Meshry]

Can any one spot more light on crevice corrosion?

 

[EdStainless]

Crevice corrosion is the mother of pitting, it is much worse.
In general crevice corrosion will start at temperatures 20-40C lower than pitting.
All of these alloys rely on chrome to form the passive film that protects them (Mo and N make the film stronger and more self repairing).
The tighter the crevice the worse the conditions.
In the crevice you get oxygen depletion, so it is harder for the passive oxide film to be maintained.
You also get the concentration of corrosion products, one of which is hydrogen ions.
The result is dropping pH in the crevice.
In Fe bearing alloys (stainless and some Ni-Fe grades) we do corrosion testing in ferric chloride because it resembles what forms inside an active pit or crevice. The solution is high in Cl, and has a pH <1.
This is what happens in a crevice.

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P.E. Metallurgy, Plymouth Tube

 

[georgeverghese]

Am aware of OpCos using superduplex (with 3-5% Mo or so) for ambient temp seawater, but not at temps exceeding 80degC. Moreover, aquifer water may be higher in chloride content than seawater.

No info provided on seal system configuration. With what little I know, low chloride deoxygenated potable water should work okay for pressurised plan 53 with superduplex materials on the seal, since the aquifer water cannot migrate into the seal chamber.

Where is the 80degC experienced - seal supply or seal return fluid ?

 

[SJones]

Two of the four corrosion control options are: 1) change the material; 2) change the environment. Repeated seal failures suggest that one of these options needs to be deployed. The first thing to do would be to establish the specific cause of the seal failure. Unfortunately, this cannot be achieved on an internet forum; it will be achieved by a properly performed failure analysis exercise, probably involving an experienced laboratory facility.

Steve Jones
Corrosion Management Consultant

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

All answers are personal opinions only and are in no way connected with any employer.

 

[nickypaliwal]

Gents, Please provide your views on Alloy 625 use in sea water.