Stainless Steel Ball Valve with Galvanised Bolts 1

[posture]

Hi

I have a client who is insisting that we use A320 L7 bolts to secure 316 stainless ball valves. Bolts will be screwed , through a stainless steel 316 flange and into threaded blind holes on the valves bodies so I can't use insulation on the full bolt lenght.

I know stainless is more nobel then the galv steel, so does this mean that the bolts will get "eaten" pretty quickly?

Enviroment is offshore but valves will not be immersed.

Thanks

Trouble

 

[posture]

Sorry I should have added, that the client has specifed that the A320 L7 bolts be galvanised and centrifuged

 

[cloa]

You just need to not have actual fluid contact both the bolt and valves for there to be even a possible issue.

 

[rmw]

I think your client is depending on the integrity of the galvanized coating to protect the bolts and that the zinc present in the coating will "enhance" their nobility. This would work if one never had to put a wrench on the bolts. Once the galvanized coating is breached due to a variety of possible reasons, the exposed CS is then a very small surface area of sacrifical metal in a sea (no pun intended) of more noble metal surrounding it.

If the environment is offshore, then it is a Marine environment and there will be salt in the air. So unless this is a desert marine environment where it never rains and the metal never will go below atmospheric dew point, expect the possibility of the presence of an electrolyte for this galvanic couple and design accordingly.

rmw

 

[SJones]

According to BP in 1999, as reported by DNV in their 2002 report, Bolting & Fastener Application In The Offshore & Process Industry - State Of The Art, 2002-3301, where they have used galvanised bolts in CRA piping systems, there has been no accelerated loss of zinc at mating faces. It's still not a long term (10+ years) solution though.

Steve Jones
Materials & Corrosion Engineer

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

 

[posture]

All

Thanks for the help!

Trouble