close interval survey 1

[corrosion10]

I just stated working in cathodic protection industry and I am confused about the mechanism of close interval survey. Would someone help me or point some reference for me? Really appreciate your inputs.

I understand that we try to measure the potential difference between the pipe and the soil. But I am confused how the potential difference could change along the pipe. For the pipe, the pipe section is electrically continuous which guarantee the same potential throughout the pipe. On the other end, a standard half cell with a standard potential is used. Why would the potential change on earth?

 

[SJones]

Because the potential measured is that of the steel surface in contact with the electrolyte (soil). The soil is not exactly the same at all points along the pipeline neither is the steel surface. It would appear a good time to purchase a copy of Peabody's Control Of Pipeline Corrosion or similar text.

Steve Jones
Materials & Corrosion Engineer

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

 

[corrosion10]

thank you very much, Steve.

But in order to do what you said we are doing, I would imaging we need to move the connection of the pipe. For example, initially both half-cell and pipe are connected at location A0. Both the half-cell and the pipe connection should move to location A1, A2, A3 and etc along the pipe. Right now, only the half-cell moves. So what we are really measuring is the soil potential at location A0, A1, A2, A3 against the pipe potential at location A0.

Also, I do not quite understand the notion of soil potential. I thought one side of the voltage meter is connected to the pipe and the other to the half-cell. Therefore, we are measuring the potential between the half-cell and the pipe. And the potential of the half-cell should stay constant no matter where you put it. Isn't that the purpose of a half-cell?

Do I make myself understood? I must have missed something critical in my thinking, but I can not figure out what.

 

[brimmer]

The readings change becasue they are affected by corrosion current flowing to or from the pipe. These small currents cause changes in voltage in the soil beside the pipe, which are detected.
You don't have to move the connection to the pipe because in a welded continuous same material pipeline there would be no significant voltage drop through the pipe between your test points.

 

[corrosion10]

Thank you guys for your replies.

How about discontinuous pipe then? Can CIS be performed if the pipe is not continuous? If not, why is that?