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Penetration of Cathodic Protection into Pipeline Coating Disbondment

An experimental setup was designed to investigate the cathodic protection (CP) penetration into coating disbondment. Embedded detection probes underneath the disbondment enabled pH and steel potential measurements without disturbing the environment inside the disbondment.

Product Number: 51317--9146-SG
ISBN: 9146 2017 CP
Author: Li Yan
Publication Date: 2017
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The most effective method to mitigate corrosion on the external surface of a buried pipeline is to utilize a protective coating supplemented by cathodic protection (CP). Ideally these two systems work together so that if the coating disbonds or has defects allowing ground water solution to contact the pipe steel surface the CP system will continue to function to protect the pipeline. However the success of this approach greatly depends on the nature of coating failures. CP can only provide corrosion protection to the pipe with coating failures if sufficient CP current is able to reach the exposed pipe steel. For instance if CP current can penetrate into a coating disbondment then corrosion related damages may be prevented. Otherwise the exposed pipe steel could be susceptible to corrosion and environmentally assisted cracking. Due to this very reason disbondment of pipe coatings especially coatings with high impedances is considered to be the most troublesome coating failure mode. Therefore a comprehensive understanding of CP penetration into coating disbondment is essential to coating selection and management i.e. choosing the right coating for the right application and developing mitigation measures for coating failures.This study was intended to conduct a systematic investigation to examine the effect of coating property disbondment geometry and dimension and soil solution on CP penetration into coating disbondment. An innovative experimental setup was designed to enable in-situ monitoring of pH and potentials without disturbing the environment inside the coating disbondment. It was observed that all the three coatings studied namely fusion bonded epoxy (FBE) high performance powder (HPPC) and two-part epoxy showed shielding behaviour and the applied CP could not fully penetrate into the disbondment. The HPPC coating containing a FBE inner layer and a polyethylene outer layer exhibited the highest shielding behaviour. The variation of CP penetration within the entire disbonded area (6.5 cm in diameter) was trivial. The consumption of CP current at each given potential decreased with testing time. The extent of CP penetration was more dependent on the gap size of disbondment than other parameters.

Key words: CP penetration, coating disbondment, detection probe, migration of OH-, calcareous deposit

The most effective method to mitigate corrosion on the external surface of a buried pipeline is to utilize a protective coating supplemented by cathodic protection (CP). Ideally these two systems work together so that if the coating disbonds or has defects allowing ground water solution to contact the pipe steel surface the CP system will continue to function to protect the pipeline. However the success of this approach greatly depends on the nature of coating failures. CP can only provide corrosion protection to the pipe with coating failures if sufficient CP current is able to reach the exposed pipe steel. For instance if CP current can penetrate into a coating disbondment then corrosion related damages may be prevented. Otherwise the exposed pipe steel could be susceptible to corrosion and environmentally assisted cracking. Due to this very reason disbondment of pipe coatings especially coatings with high impedances is considered to be the most troublesome coating failure mode. Therefore a comprehensive understanding of CP penetration into coating disbondment is essential to coating selection and management i.e. choosing the right coating for the right application and developing mitigation measures for coating failures.This study was intended to conduct a systematic investigation to examine the effect of coating property disbondment geometry and dimension and soil solution on CP penetration into coating disbondment. An innovative experimental setup was designed to enable in-situ monitoring of pH and potentials without disturbing the environment inside the coating disbondment. It was observed that all the three coatings studied namely fusion bonded epoxy (FBE) high performance powder (HPPC) and two-part epoxy showed shielding behaviour and the applied CP could not fully penetrate into the disbondment. The HPPC coating containing a FBE inner layer and a polyethylene outer layer exhibited the highest shielding behaviour. The variation of CP penetration within the entire disbonded area (6.5 cm in diameter) was trivial. The consumption of CP current at each given potential decreased with testing time. The extent of CP penetration was more dependent on the gap size of disbondment than other parameters.

Key words: CP penetration, coating disbondment, detection probe, migration of OH-, calcareous deposit

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