ABSTRACT

Buried station piping at a pipeline pump facility with high soil resistivity experienced sub-criterion structure-to-electrolyte potentials post cathodic protection (CP) system commissioning. A prompt assessment of the external corrosion threat was required to determine if supplemental CP would be required, or if operating with sub-criterion structure-to-electrolyte potentials would be acceptable. One option to quantify the external corrosion threat on metallic structures is to utilize soil side electrical resistance (ER) corrosion probes. The corrosion growth rate obtained from soil side ER corrosion probes can be used to determine part of the external corrosion threat when CP system effectiveness is inadequate. However, soil side ER corrosion probes typically require a 12-month measurement time frame to establish representative corrosion growth rates. In this review, the project construction team needed to understand within a 3-month time frame if supplemental CP systems were required to alleviate sub-criterion structure-to-electrolyte potentials. A specialized soil side ER corrosion probe monitoring system was installed to obtain rapid corrosion growth rate data, which was used to determine the external corrosion threat being experienced on the buried station piping. This paper describes the quick response soil side ER corrosion probe system, data collection, and data analysis, and the decision-making process to not install supplemental CP systems. Additionally, the operating soil side ER corrosion probe data collected following the initial 3-month decision window is reported to validate the decision to operate with the sub-criterion structure-to-electrolyte potentials. However, at the end of the data collection period, anomalous results were found, which have been flagged for future investigation.

INTRODUCTION

Achieving structure-to-electrolyte potentials that meet criteria can be challenging in high resistivity soil environments. This is particularly so when measuring structure-to-electrolyte potentials for CP systems on buried station piping that is electrically continuous with a copper electrical grounding system. Determining if buried station piping meets the respective CP criteria can be supplemented through the use of CP corrosion coupons. However, when buried station piping is located in a high soil resistivity environment, the amount of CP current that can reach the structure-to-electrolyte boundary is insufficient, and results in low cathodic polarization on external pipe surface. Under these given circumstances alternative monitoring techniques were sought for quantification of the external corrosion threat when a particular CP system demonstrated limited effectiveness at providing sufficient cathodic polarization.

The use of soil side ER corrosion probes, which measure cumulative metal loss data, provide a measurement of the corrosion growth rate being exhibited upon a metallic structure, and in turn determine if sufficient corrosion prevention is being achieved on a metallic structure.1 It is important to note, that soil-side ER probes typically require a 12-month measurement time frame to obtain cumulative metal loss data that is representative of the corrosion growth rate being experienced on a structure.2,3 However, for this review it was necessary to know within a 3-month time frame, if supplemental CP systems would provide sufficient corrosion protection and alleviate concerns from sub-criterion structure-to-electrolyte potentials.

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