Pipline Cathodic Protection 2

[Metalmeister]

I inadvertently posted this to another question on the forum; so here it is again.

I'm looking to protect a coated carbon steel pipe that carries seawater. In addition to the inside coating, the design calls for sacrificial anodes placed strategically on the inside, and an impressed current cathodic protection system on the outside for the buried segment of the pipeline.

Is there any potential that the outside impressed current system will impeded or affect the inside galvanic protection system?

 

[brimmer]

The inside will not effect the outside and viceversa, remember each system would have its own seperate electrolyte which it is operating in.
As far as protection levels outside, there are guidelines for criteria (-850 mV CSE IR free the most common). As far as a spec requiring anodes in the pipe I won't comment...

 

[Metalmeister]

Thank you, Brimmer. I'm looking for a Star to give you. I was getting bogus info from an EE who had concerns about crossing potentials.

So now I'm really interested in your "no comment" on the sac anodes on the inside. The pipe is 12 ft. diameter, essentially a covered ditch with coal tar/epoxy coating on the inside. I told them to install enough manways to maintain/replace the anodes, but I thought the ID anodes were an unusual option.

 

[SJones]

The 'EE' is partially correct in terms of what happens at insulating joints/flanges with a conductive medium and EXTERNAL CP. External current WILL use the internal medium for conduction when conductivities and insulation arrangements are conducive (no pun!) to it. Regarding the question: applying internal CP will bring internal and external potentials (again no pun!) closer together reducing any interaction that may occur.

Steve Jones
Materials & Corrosion Engineer

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

 

[strider6]

just my opinion.. but i've never seen a CP system on the inside of a pipeline.. are you sure that it's necessary and that it will works?

S

Corrosion Prevention & Corrosion Control

 

[SJones]

Pressure vessels use anodes with varying degrees of success. The stated term 'ditch' would imply little or no flow in the 'pipeline'.

Steve Jones
Materials & Corrosion Engineer

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

 

[Metalmeister]

Thank you all for your responses; they have been helpful. the pipe is a welded construction, 12 ft diam discharge line back to the ocean. So the "ditch" comparison is more like no pressure, but plenty of flow.