Pipeline polarized potential attenuation is supposed to be IR-free pipe-to-soil potential distribution along a pipeline. Currently, most of the potential attenuation calculations in published documents or in CP training manuals are either a pure IR drop attenuation or attenuation that contains IR drop 1,2,3,4,6. Polarization and IR drop are caused by applying current to the pipe surfaces. When current density distribution along a pipeline is determined, the attenuation of cathodic polarization (IR free potential shift) can be established. Current density is restrained by both pipe to soil resistance and pipe polarization resistance. This article uses current density attenuation and polarization resistance to determine the polarization (IR-free potential shift) attenuation.
Shown on Figure 1 is a typical impressed current CP diagram. When the rectifier is first turned on, i.e. time t=0, there is no polarization yet. At that moment, the applied DC voltage is fully consumed by IR drops at anode (IRa0) and cathode (IRc0), plus original potential difference between anode and pipe (Eoca - Eocc). When t=0, the current is at the greatest value. Over time when polarization kicks in, due to adding polarization resistance, the current is gradually reduced. Figure 1 indicates that both IR drop and polarization resistance have significant impacts on the current (density), hence both IR drop and polarization resistance should be considered in current density attenuation calculations. This article provides a method to calculate the attenuation of ΔEp with the assistance of current density attenuation and polarization resistance. Currently, published documents only provide attenuation calculations for the moment of t=t0 which does not even include polarization or only provide the apparent potential shift (ΔEapp) attenuation which includes both IR drop and polarization.
Following are a brief review of some ISO 15589 as a example. The approaches from other published documents are similar.