Galvanic Corrosion due to dissimilar Flange Joint in hydrocarbon service

[Sarahm554]

The flange connection between Duplex stainless steel and carbon steel is fitted in hydrocarbon service (well fluid) and flows from Duplex SS to carbon steel. What is the likelihood of corrosion while flowing from DSS to CS and vice versa? The operating pressure is approximately 20 barg, and the temperature ranges from 70 to 80°C.

 

[LittleInch]

What's the conducting medium between the two? Presumably the salty water in the well fluid?

Is the joint electrically insulated?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TugboatEng]

The flange faces themselves should not be affected by galvanic corrosion provided you have selected an appropriate gasket. The weld that attached the carbon steel pipe to the flange is going to be the region of most severe effect. The rate corrosion depends on the ratio of SS to CS. If you use a carbon steel flange to attach to a large SS tank you're going to have a much more severe effect than a small amount of SS in a large CS system. The direction of flow should not have effect on this. You may consider epoxy coating the flange face to just beyond the butt weld in the CS pipe to reduce the effect.

 

[SJones]

What internal corrosion control strategy is being deployed for the carbon steel? Externally, just paint both sides of the joint.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

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

 

[Sarahm554]

@LittleInch well fluid is mainly gas and contains 4% CO2, is regarded as saturated in both water vapors and hydrocarbon vapors. The joints are not electrically insulated and that is why I brought this for discussion.
@SJones There is corrosion inhibition on the upstream of dissimilar flange joint to protect the carbon steel pipework.

 

[LittleInch]

You've got more faith in corrosion inhibitor than I have. 4% CO2 and 80C. WOW.

I had 10% plus and 100C and we ran all the wellhead piping back to the GOSP in Duplex.

You need to have that injection 40-80D before the start of the C steel for the inhibitor to become effective. Do you have that?

But tug is right. Needs an internally coated spool piece and electrical isolation if you can get it.

what's the flange type and rating? I don't think I've even seen an isolating kit for API wellhead flange, but if any one knows Steve will.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SJones]

Tricky one this because it will hinge on two things:
1. Was the inhibitor qualified, selected, and dosed as a result of tests that also included a DSS/CS couple (also noting LI's above observation on inhibition development length)
2. How is oxygen being kept out of the system given the injection arrangement

For the insulation consideration: GPT industries offer good quality products

https://www.gptindustries.com/products/isolation-gaskets-kits/

, however, will they get shorted out through instrument and earthing connections anyway?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

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

 

[EdStainless]

I don't care how deep the well is, once the fluid hits the surface it will have oxygen in it.
The slightest little leak will allow the diffusion of oxygen into the system.
The other problem with inhibitor is the operator, are they willing to shut down if the injection system fails?
If not they could be exchanging years of life for days of service (I have seen worse also).

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Hajiwaji]

why is there DSS in the first place?

 

[TomEun]

In addition to the good answers above, you may also need two-step assessment below.

1. Does this circuit require galvanic corrosion prevention? (Need to check fluid conductivity and dissolved oxygen-present and future) Please use Galvanic Potential Table/Curve under similar service condition.
2. If required, what is the best prevention? (inhibitor, isolation kits, xylene coating, and/or others)