B7 bolt for SS flange 1

[Xeros]

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?

 

[strider6]

usually the bolt are hot dip galvanized for corrosion protection
in marine environments the cs bolt can suffer from galvanic corrosion.

S

Corrosion Prevention & Corrosion Control

 

[SJones]

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

 

[CoryPad]

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

 

[strider6]

this is a recomendation from BSSA

"Carbon steel bolts in stainless steel members should be avoided because the ratio of the area of the stainless steel to the carbon steel is large and the bolts will be subject to aggressive attack"

http://www.bssa.org.uk/topics.php?article=95

mil-std on galvanic corrosion

http://www.wbdg.org/ccb/FEDMIL/std889b.pdf

S



Corrosion Prevention & Corrosion Control

 

[ash9144]

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.