|Content Type||Conference Paper|
|Title||Cathodic Protection - It's All About Current|
|Authors||Dennis Janda,Baker Hughes Incorporated|
|Source||CORROSION 2010, March 14 - 18, 2010 , San Antonio, TX|
|Copyright||2010. NACE International|
|Keywords||cathodic protection, current measurement, in-line inspection|
The external pipeline corrosion control field has seen great change over the years from a regulatory standpoint, but very few advances in new technology to assist in monitoring the effectiveness of mitigation efforts. Recently a new tool for the corrosion professional's toolbox was introduced - an in-line inspection (ILI) tool that measures cathodic protection (CP) current flowing on a pipeline.
A.W. Peabody stated in his classic book Control of Pipeline Corrosion “When the amount of current flowing (to the pipe) is adjusted properly, it will overpower corrosion current discharging from all anodic areas on the pipeline and there will be a net current flow onto the pipe surface at these points. The entire surface will then be cathodic and the protection complete.” 1 The cathodic protection current measurement tool is an inline inspection tool that measures the voltage drop in the pipe wall that is caused by the accumulation and flow of cathodic protection (CP) current. This voltage is converted to current using Ohm's law and the data is graphically represented. This paper will explain the benefits of this new technology and present case histories of recent pipeline inspections.
Corrosion professionals charged with the task of protecting pipeline assets from corrosion have always understood that direct current plays an important role in cathodic protection. Until recently, these professional have not had the tools to accurate measure this current at all points on a protected pipeline. New technology is now available that accurately measures the accumulation and flow of cathodic protection current at all points on a pipeline. This paper discusses the role of direct current in cathodic protection and techniques to measure this current.
THE ROLE OF DIRECT CURRENT IN CP
The role that direct current plays in the corrosion of steel has been clearly understood for many years. It is widely accepted that corrosion takes place at the anode where current is discharging from the steel. Likewise, the role of direct current in cathodic protection has been clearly defined since 1823 when Sir Humphrey Davy installed the first cathodic protection systems on copper clad vessels for the British Admiralty. As stated earlier, Peabody theorized that making the pipeline a receiver of current in its entirety would effectively stop corrosion.
Corrosion professionals have a great deal of experience in applying this protective current to pipelines. Galvanic anodes and impressed current rectifiers have been employed to perform this task for many years. The challenge has been how to prove that the entire pipeline is in fact receiving current. Many techniques to measure this protective current have been employed. These techniques fall into two categories - measuring CP current flowing down the pipeline, and measuring the affect of CP current flowing in the soil to the pipe.
CP LINE CURRENT
Some of the methods used to measure CP current flowing on the pipeline include:
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