Duplex S31254 with Inconel 625 in seawater application

[Mumawalde]

Hello,

I've been searching to fine the galvanic reaction between these two materials used in a flowing seawater application. The Duplex will have a very small exposed area and the Inconel will have a rather large exposed area, they are both in close contact with each other.

I have also been trying to place the Duplex on the galvanic series, any insight into that would be helpful as well. "Alloy 625" is clearly listed on most charts I have seen. For example:

http://www.rowantechnology.com/wp-content/uploads/2012/06/Galvanic_Series_Chart.jpg

Please shed some light on this for me.

Thank You!

 

[EdStainless]

Any high alloy stainless that is passive (not actively corroding) will have a very similar potential and therefore there will not be a risk of galvanic corrosion.
In a crevice corrosion situation the 625 may not be up to the task....

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Plymouth Tube

 

[SJones]

S31254 is not 'duplex;' it's a high alloy austenitic that generally goes under the generic title of 6Mo. Bimetallic corrosion guides, such as NPL Guide To Good Practice In Corrosion Control - Bimetallic Corrosion, place high austenitic alloys at more electropositive potentials than 625 rendering the 625 more likely to corrode in a 1:1 couple. This is supported by Kain & Vasanth, NACE Corrosion 2006, Paper 06282, who found that alloy 22 (not too dissimilar to 625) was corroded when coupled to 6Mo in seawater. This would suggest that the area ratio might be reasonably favourable in the case under discussion.

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.

 

[EdStainless]

I was trying to decide how to make that comment, Thanks, Steve
Of all of the various 6% Mo superaustenitic stainless grades S31254 is the least corrosion resistant.
N08367, N08925/26, and S32654 are all better alloys.

Steve, 22 has significantly better pitting resistance than 625. There have been many cases of 625 seals and flanges failing in seawater. I am not aware of any 22 (or 59 or 686) seawater failures.

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Plymouth Tube

 

[Mumawalde]

Thanks everyone! I totally misspoke, I realize that S31254 is not Duplex. We refer to it as 254 SMO. Sorry about that.

Please keep any comments coming.

 

[SJones]

@Ed - I was trying to point out that if the 22 was corroded by coupling to the 6Mo, then 625 is more likely to suffer the same fate for the same environment and area ratio.

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.

 

[Mumawalde]

We also have the ability to alter the area ratios. In the easiest case for us, there is nearly the same areas of the two materials, there would probably be more 6Mo than 625 in the local area. The 6Mo is the vessel material and the 625 is an installed sensor in the vessel nozzle.

Thanks again