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.

 

[weldstan]

Is corrosion maximized in the F55 at the interface between it and the 625 shaft? If so I suspect a combination of galvanic mismatch and crevice corrosion. In the current environment I would suggest a seal material change much closer to that recommended by Ed. Although it is understood that the 625 seals on similar units are performing well.

 

[EdStainless]

Check pricing on an alloy "C"version (C22, 622, 686, 59), these are Ni-Mo-Cr alloys.
If you don't need strength at elevated temp then 625 offers less corrosion resistance and the pricing should be similar.

You said that you have general corrosion, I hope that this is actually widespread pitting.
If you actually have general corrosion you need to find out what is being used to clean or flush this system.
General corrosion is almost impossible to have in these alloys.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[SJones]

@nickypaliwal

If you want an opinion on 625 performance in seawater, it's in plentiful supply in the literature. Here are a couple of examples:

JS Lee, et al, UNS N06625: A Current Review Of The Literature, Materials Performance, September 2006, pp 36-40
F Martin, et al, Crevice Corrosion Of Alloy 625 In Natural Seawater, Corrosion, Vol 59, (6), 2003, pp 498-504
H Alves, et al, NACE Corrosion 2007, Paper 07215

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.

 

[Meshry]

Georgeverghese,
80C is the temperature of the pumped fluid. this will be the same as the seal chamber pressure at both sides (thanks to balance line).
I have seen the temperature trend for one month back. all the time this pump was running at 83C. I would say based in the other replies. super duplex is not the correct selection for this temperature??

 

[Meshry]

Weldstan,
You are right the corrosion is more at the interface wet parts, however as per your theory the 625 should be the first to go (higher potential) am I right??

 

[weldstan]

625 would not be the first to go, alloy 55 would be.

 

[georgeverghese]

If 83-85degC is the seal supply temp, that is unusually high. The external cooling on this seal system doesnt seem to be working..why ? What does your company rotating machinery engineer have to say about this ? Is the external cooling with a water cooled tube bank in the seal pot or with an external fin fan cooler?

Or perhaps the pumping ring in the pump isnt running as it should - is this a VFD operated pump, with operating speed too low to enable adequate recirculation of the seal fluid with the internal pumping ring ?

 

[Meshry]

Weldstan,
Why is that?? why would alloy 55 go first?

 

[Meshry]

Verghese,
83 is the pumped fluid temperature NOT the barrier fluid temperature.

 

[weldstan]

Because in your stated conditions, 625 is the more noble material.

 

[Meshry]

Weldstan, Thanks a lot

 

[EdStainless]

The problem with superduplex alloys is that if someone is slightly off in the heat treatment then the two phases will not have matching corrosion resistance. If the anneal temp is too high or low the elements will partition differently between the phases and either the austenite or the ferrite will end up with lower corrosion resistance.
Considering how often I have seen 625 suffer crevice corrosion in high chloride waters strongly suggest a higher performance alloy.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[nickypaliwal]

Thanks Edstainless.... I went through the documents highlighted above and many other articles/papers on internet.. The conclusion that I can make is that Alloy 625 is very resistant in seawater but can not be used for piping and equipment due to crevice corrosion.

 

[WKTaylor]

What alloy(s) does the USN use for shipboard piping systems?

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[EdStainless]

Ti and Monel 400 mostly, some 70-30 Cu:Ni also.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube