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A Study Of The Depolarization Phenomena In Soils And Remanence Of Corrosion Protection After CP Interruption

Picture for A Study Of The Depolarization Phenomena In Soils And Remanence Of Corrosion Protection After CP Interruption
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Oil and gas buried pipelines are protected against corrosion by both organic coatings, a passive protection system, and cathodic protection, an active protection system. When coating defects occur, CP controls the corrosion of the exposed steel surface. From an operating point of view, cathodic protection interruptions can occur on the network during interventions, consignments, or technical problems. Literature indicates that during CP interruption the corrosion rate of the metal remains lower than its free corrosion rate. Application of CP confers a remanence of protection to the metal. The objective of this study is to determine the safe duration for cathodic protection interruptions depending on environmental and cathodic protection conditions.

Product Number: 51322-17649-SG
Author: Clémence Rogier, David Sellam, Tiphaine Lutzler, Laurent Henry, Ahmed Fakhry, Yves Zannier, Elisabeth Fleury, François Castillon
Publication Date: 2022
Industries: Cathodic Protection , Corrosion Monitoring and Control , Coatings
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Cathodic protection (CP) conveys an active protection against corrosion to pipeline steel surface in case of coating defect. This work studies depolarization phenomena that occurs after CP interruption. The term “depolarization” refers to the pipes returning to its free potential value after cessation of CP. Here we present a laboratory study in soils, using metallic coupons to simulate pipeline coating defect behavior with metallic surface exposed to the electrolyte. After CP interruption the potential value of the steel coupon doesn’t return to initial value before CP (around -0.6 V/CSE) but remains higher (up to -0.2 V/CSE) for a long period of time (up to 11 days). This effect can be attributed to the formation of a passivation layer at the metallic surface due to pH increase during CP application. This passive layer confers a remanence to the protection against corrosion of the metallic surface after cessation of CP. This study focuses on understanding this depolarization behavior depending on various parameters: level of CP, time duration of uninterrupted CP, shape of the metallic coupon, composition of the soil media. The objective is to give insight to pipeline operators as to the safety of cathodic protection interruptions on the network depending on environmental conditions.

Cathodic protection (CP) conveys an active protection against corrosion to pipeline steel surface in case of coating defect. This work studies depolarization phenomena that occurs after CP interruption. The term “depolarization” refers to the pipes returning to its free potential value after cessation of CP. Here we present a laboratory study in soils, using metallic coupons to simulate pipeline coating defect behavior with metallic surface exposed to the electrolyte. After CP interruption the potential value of the steel coupon doesn’t return to initial value before CP (around -0.6 V/CSE) but remains higher (up to -0.2 V/CSE) for a long period of time (up to 11 days). This effect can be attributed to the formation of a passivation layer at the metallic surface due to pH increase during CP application. This passive layer confers a remanence to the protection against corrosion of the metallic surface after cessation of CP. This study focuses on understanding this depolarization behavior depending on various parameters: level of CP, time duration of uninterrupted CP, shape of the metallic coupon, composition of the soil media. The objective is to give insight to pipeline operators as to the safety of cathodic protection interruptions on the network depending on environmental conditions.

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Product Number: 51322-17635-SG
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There are several ways to validate the performance of a cathodic protection (CP) system for buried pipelines. Over the years, pipeline networks and their corrosion challenges have become increasingly complicated, not least due to the many sources of both AC and DC interference that affects CP operation. Also, the various measurement techniques that can be applied to test CP effectiveness has increased over the years. Finally, the sheer number of buried pipeline miles has been constantly increasing.
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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.
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The electrical conductivity of the electrolyte is one of the key parameters in the electromechanics of corrosion. Highly conductive electrolytes will permit more current and increase corrosion rates. Conversely, resistive electrolytes will enable less current to flow until the necessary conditions for corrosion are no longer satisfied or slowed.
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