Understanding Corrosion in Water Pipelines: A Guide for Pipeline Designers


Instant-Off Potential

Last updated: December 7, 2015

What Does Instant-Off Potential Mean?

Instant-off potential refers to a standard cathodic protection (CP) measurement method. It is the polarized half-cell potential of an electrode taken immediately after stopping the cathodic protection current. This potential closely approximates the potential without an IR drop (i.e. the polarized potential) when the current was on. Instant-off potential represents an effective on-potential (with IR-drop compensation).

This potential can be used for the measurement of cathodic protection of a buried pipeline in the oil and gas industry.


Corrosionpedia Explains Instant-Off Potential

Instant-off potential is the measurement of the structure-to-soil potential made immediately following the interruption of the cathodic protection rectifier. In this measurement, the effect of voltage drops on a structure-to-electrolyte is determined by interrupting all significant current sources that may influence the half-cell potential. The measurement is done immediately after the switch-off of CP current to avoid the loss of polarization. This type of potential measurement provides greater accuracy than readings recorded with the current on. It is also commonly used to evaluate the level of cathodic protection achieved.

For example, in an instant-off potential measurement in a pipe-to-soil, the pipeline potential shows more negative than its true potential due to electrolyte IR-drop errors caused by cathodic protection. The instant-off potential measurement corrects these errors. The CP current is momentarily stopped to produce a “true” pipe-to-soil potential, which is free from undesirable IR drop effects and taken before any appreciable depolarization has occurred. When the CP current is turned off, the current lowers to zero (I=0 amps) and causes the IR drop. This measurement produces a correct measure of the level of protection afforded to the pipeline. When it is not possible to momentarily stop the CP current, an alternative method such as a corrosion coupon is employed.


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