The criteria and methods given by AMPP/NACE SP0169 and TM0497 rely heavily on the use of Instant-Off potential measurements to assess cathodic protection levels on pipelines. Despite wide acceptance of these standards, there is little or no data to demonstrate that an Instant-Off potential accurately represents the polarized potential that exists across the metal/electrolyte interface of a pipeline under normal operating conditions. This paper summarizes laboratory testing that was conducted to demonstrate the accuracy of Instant-Off potentials. Test results show that Instant-Off potentials significantly underestimate pipeline polarized potentials. Examples are given where the reliance on Instant-Off potentials can lead to pipeline damage.
Cathodic protection (CP) is used to prevent external corrosion on underground pipelines. The effectiveness of CP is commonly evaluated by measuring the DC potential (voltage) of the pipeline with respect to a reference electrode located on the surface of the earth above the pipeline. Criteria used to interpret pipeline potential measurements are given by Section 6 of NACE SP-0169-2013 "Control of External Corrosion on Underground or Submerged Metallic Piping Systems"1.
It is recognized that an error exists in the pipeline potential measurement, since current from the CP system causes a potential gradient to exist in the earth in the vicinity of the pipeline. The measured potential can be expressed as:
(Equation)
Where:
Vx = Measured pipeline potential at distance x from the pipeline
V0 = True polarized pipeline potential (i.e., the potential across the pipeline/electrolyte interface)
I = Current flow in the earth between the pipeline surface and the location of the reference electrode
R = Resistance of earth between the pipeline surface and the location of the reference electrode
Equation 1 shows that the error between the measured potential (Vx) and the true potential (V0) is equal to I·R. This error is known as ‘IR Drop in the electrolyte’ or simply IR drop. Consideration of IR drop is required for valid interpretation of pipeline potential measurements, since this error tends to overstate the level of CP that exists on a pipelinei. All revisions to RP-0169 made after 1986 require that the IR drop be considered in potential measurements.