Contact corrosion flange - bolting

[Spero]

I've seen several piping-spec's where galvanised stud-bolts are used in combination with stainless steel flanges.
Therefore I would like to pose a question about contact corrosion between the galvanised bolts-nuts-washers and the stainless steel flanges. Is it not neccesary to worry about this, or are measurements needed (for instance: stainless steel stud-bolts & nuts, plastic washers, protection against electrolite (rainwater))

I also notice that some spec's describe the use of washers, and others don't. What could be an explanation and what is recommended?

Thanks,

 

[DSB123]

Spero,
Galvanised studbolts used with SS flanges not a good combination. Zinc will attack SS especially if within a fire and compromise the structural integrity of the flanges.

 

[moltenmetal]

From a pure galvanic corrosion perspective, the stainless steel flange is more noble than the zinc plating, so it's the zinc that's vulnerable. But the zinc is there to protect the underlying stud/nut material which is ALSO less noble than the stainless steel and hence susceptible to the same galvanic corrosion attack.

The liquid metal embrittlement concern noted above is a little overblown in my opinion. The same spec which calls out the galvanized hardware for the flanges probably requires/permits galvanized pipe shoes etc., and for moderate temperature SS lines, personally I don't see the problem with that.

If you don't agree with that, there's always B8 stainless or fluoropolymer coated hardware as a zinc-free alternative.

 

[unclesyd]

As I've stated many times our plant has about 90% process piping that is Austenitic SS, 304L, 316, where absolutely forbid the use of galvanized fasteners, HDG, Mechanical, and Electrolytic. This is not say it hasn't been tried on projects where we have been forced by higher ups to use them and where we did it cost many times the initial savings to replace them. Aside from the problems posted above we have we have gotten some very poor quality coatings on the fasteners, some that broke on the initial tightening, H2, and some over sized that would tighten to the required torque with out stripping or binding.
Our case many be unique as we required the highest possible utility possible since taking a process system down coast many thousands of dollars due to lost production which can't be recovered as the plant is at 100% capacity.
Adding to the galvanized fasteners we have been forced to use ductile iron backup flanges, another bad decision for our process system. Again many thousands of dollars to replace them.
I would go with B* Class 2 under 1 1/8" and B* above. We do use some Green and Blue Teflon coated B7 on SS non-process lines like pure water, process air, etc. We loose a few SS fasteners to galling, but the use of thread lubricants mitigates the problem somewhat.

 

[moltenmetal]

unclesyd: we too avoid the use of galvanized flange studs/nuts (for some of the same reasons you've stated). We do not avoid galvanized out of fears of LME or corrosion damage to the SS flanges, the latter of which was the OP's original concern with the practice. The spec the OP has in hand calls out galvanized components as required, so the owner whose spec this is either doesn't know of or doesn't share our concern.

We're perfectly comfortable with galvanized pipe shoes and other galvanized support components in low to moderate-temperature services.

 

[Spero]

Thanks for all of the reactions.
The Teflon coated bolting we've used before, but they are quite expensive. (like Stainless steel) SS bolting always needs adequate greasing because of the fast "gripping". But I also think it is wise to use SS bolting on SS flanges.

In certain cases we also use galvanized pipe supports on SS pipes, but we wrap the piping to avoid contact corrosion.