B7 bolt for SS flange 1

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

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:




Steve Jones
Materials & Corrosion Engineer

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

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"

mil-std on galvanic corrosion

S



Corrosion Prevention & Corrosion Control

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.