CS to SS flange connection the problem is not solved yet

[msessi]

Dear All


My query is if I have above ground piping carbon steel connected to above ground stainless steel , the fluid is a produced water, shall I install a isolation kit at each flanges connection to avoid any galvanic corrosion ? note that the CS and SS lines can be connected somewhere by structure , earthing... I can not find this topic in Nace standards. The electrolyte can be the internal fluid for internal surface or rain water /snow for external surface?

I made many research in forums because no guidance in Nace stds and there are different opinions for the need or not of isolation kit.

Thank you for your help.

 

[sk10009]

You can’t avoid galvanic corrosion at carbon steel pipes connected SS pipes in the water service. Periodically, inspection should be needed to check thickness of carbon steel pipes nearby Stainless steel flange.
Others also might have corrosion over long time within a natural environment.

 

[moltenmetal]

If the produced water is corrosive enough to the carbon steel that the carbon steel itself is freely corroding at a marginally acceptable rate, a stainless to carbon steel transition may accelerate local corrosion of the carbon steel even further and the resulting corrosion rate may be unacceptable. But in that case, the problem isn't the galvanic joint really- it's the use of a marginally acceptable base material (the carbon steel) in a corroding service.

If the carbon steel is not freely corroding, it is unlikely that a galvanic joint in the run of a line (i.e. two pipes of equal sized, flanged to one another without isolation) is going to make it that much worse. Small carbon steel parts attached to large stainless steel parts immersed by a common, conductive eletrolyte, are most vulnerable.

Previous threads have posted many excellent references, particularly the EuroInox document and the National Physical Laboratory (UK) document.

Flange isolation kits do nothing if the pipe itself must be continuously grounded/bonded for fire prevention reasons.

 

[msessi]

Thank you for your prompt reply

the plant (water treatment) is containing produced water lines in carbon steel NACE 0.175 (sour) and the interface between those line and SS are generally at chemical injection or cleaning fluid... which are in SS, so the Spec break is at check valve or normally closed valves.

So we are all agree that if my lines are mounted in the same module , skid rack , flanges kit will be unnecessary. But today what is the risk of making such connections, if I inject some fluid which are in SS or a duplex into a carbon steel line where is my principal fluid? note that the size ratio vary from 1/10 to 1/2 (SS to CS pipe size) it depends if it chemical or citric acid ....

 

[moltenmetal]

An injection quill is a small cathode (stainless or duplex) inserted into a large anode (the pipe), which isn't a problem. If your pipe is not resistant to the partially diluted chemical, or to the heat of dissolution, it will be attacked- but the galvanic nature of the joint between the injection quill and the pipe isn't the cause for that problem. In that case you'd be better off with a stainless or non-metallic-lined section of pipe where the injection or mixing occurs.