Insulation gasket between dissimilar metals in instrument air service

[midris]

Hello,

Our Client has asked for an insulation gasket where we have dissimilar material for the piping flange connection. In our case the flange connection is between SS instrument flange and galvanized pipe, while as per our drawings we have a graphite gasket.

Is it required to provide insulation gasket in such a case when the service is instrument air (i.e. dry service)? My understanding is that in such a case, galvanic corrosion is not an issue in the absence of an electrolyte.

Thanks,
Maen

 

[TomDOT]

Is the client going to pay for it?

If so, just do it.

Perhaps they are concerned about potential upset conditions. Or external corrosion due to moisture intrusion. Whatever it is, if the insulating gasket is not detrimental and the client is paying for it - do what they want.

 

[midris]

Thanks Tom,

There is no clear requirement apparently and thus we arent payed for it (we would have done it if the client payed for it). This was listed as a punch point and as you are aware we try discourage the client from making unjustified demands here and there.

What do you mean by upset conditions? Is there a probable risk of onset of galvanic corrosion due to moisture intrusion?

 

[TomDOT]

"upset conditions" are when the process isn't within design parameters. Something went wrong, or you are in a cleaning cycle or whatever. I mostly see the term used for pipe and tank linings - maybe the tank is normally kept at ambient (20-40C) - but twice a year is steam cleaned at 100C.

In a personal example, our "house" compressed air is supposed to be clean and dry here. In the 8 years I've been here, we have had 2 occasions where we had gallons of rusty liquid water come through. This was later followed by enough powdered rust to clog our filter traps.

 

[midris]

I understand what upset conditions are, however, I wanted clarity on what upset conditions are applicable to instrument air service.

I asked earlier if there a probable risk of onset of galvanic corrosion due to moisture intrusion. Would corrosion be exacerbated by the attack of zinc from the galvanized flange on stainless steel as do zinc rich coatings?

Appreciate everyone's feedback.

Thanks,

 

[SJones]

It will be the zinc that is affected by connection to the stainless steel, as it will be in its connection to the carbon steel substrate. When the zinc has gone, the carbon steel will suffer. Insulation is one means of cutting down increased loss of zinc when the two materials are immersed in an electrolyte which could be external rainwater.



Steve Jones
Corrosion Management Consultant

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

All answers are personal opinions only and are in no way connected with any employer.

 

[midris]

Thank you Steve for the very informative response.

 

[Compositepro]

Graphite will cause galvanic corrosion of steel in the presence of an electrolyte.