Potential for Corrosion: Dissimilar Metals for RFWN Flgd connections

[steakman911]

Good day all.

I am working as a CSA level II (CWI) Welding inspector on a SAGD site in N. Alberta.
I have noticed in the various areas of our plant a number of instances of: eg 1-2" 150# A105N RFWN Flg coupled to a similar sized flange of A182 Gr 316/316L RFWN flg. The gasket used is a fairly common and quite typical (on this site), type: 316L SPWD inner/outer ring. The bolting material is usually either B7 / B7M type with corresponding 2H / 2HM nuts and at other times there is SS steel bolting...none of the bolting is isolated from its dissimilar mate so to speak.

With what little knowledge I have on galvanic corrosion, IMO these connections present a possilblity of corrosion when there is an elctrolyte present.

I would be curious to know what limitations ASME may have (if any), on this type of connection as well as the potential for corrosion. There are various fluids being transported through these connections: Glycol, Brine, Bitumen among others.

To me and using common sense, there must be some sort of isolation present. As an analogy, it's no different if one has an RV and never uses or replaces the Anode...say goodbye to your hot water tank.!

But is there somewhere where this is stated categorically.?

(a Nace course or two is on my to do list within the next 12 mo.)

Thank you,

TxG

 

[SJones]

I don't think you will find any pertinent requirements in the national design and fabrication codes employed as they generally expect such specific materials selection consideration to be made by the owner.

http://resource.npl.co.uk/docs/scie...ine_guides/pdf/bimetalic_corrosion_basics.pdf

http://resource.npl.co.uk/docs/scie...publications/online_guides/pdf/bimetallic.pdf

http://www.energypublishing.org/pub...book-upstream-oil-and-gas-production-plant-a5

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.

 

[moltenmetal]

Where carbon steel is already a marginal selection from a corrosion resistance perspective, a stainless/carbon galvanic couple of the sort you're describing is going to make that situation worse. Where carbon steel is adequately resistant, the galvanic couple may reduce the service life a bit, but probably not much. And in the services you describe, attempts to electrically isolate one pipe from the other will be futile. The pipe will be grounded/bonded in numerous locations which will make electrical isolation of the gasket and fasteners basically a waste of time.