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B7 bolt for SS flange

B7 bolt for SS flange

B7 bolt for SS flange

(OP)
Somebody said that using B7 (without protection) bolt for external SS flange (no thermal insulation) shall be inhibited because this is a dissimmilar metal contact and galvanic corrosion could occur (because rain water could be electrolyte).
Therefore isolation kit or coating shall be applied for preventing galvanic corrosion.

However, B7 bolting has been used for SS Pressure Vessel and there is no problem until now.

How about member's opinion?
In addtion, what type of coating should be used, if I applied coating for bolting?

RE: B7 bolt for SS flange

That somebody could be right, particularly if it is at an offshore location.  It's a perennial question.  Shell still appear OK with having B7 fasteners in a stainless steel piping system.  Perhaps they believe that a good dose of thread lubricant will see off any bimetallic corrosion issues.  BP seem to be happy with the performance of galvanised fasteners in stainless steel systems according to DNV Report 2002-3301.  NACE Publication 02107 will give you a review of coatings for threaded fasteners. The UK Ministry Of Defence are keen on paints when there is a bimetallic couple:

http://www.dstan.mod.uk/data/02/862/01000200.pdf

 

Steve Jones
Materials & Corrosion Engineer
http://www.linkedin.com/pub/8/83b/b04
 

RE: B7 bolt for SS flange

PTFE coatings are used for offshore applications, and would be good for reducing galvanic corrosion issues.  One producer is Whitford, one product is Xylan:

http://www.whitfordww.com

RE: B7 bolt for SS flange

Coatings on bolts sounds like a good idea and is done with the hope of saving a few pennies.  The coating lasts right up to the point you start putting the nut on the thread and then it is damaged and your protection is gone.

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