ABSTRACT

Tests were performed to investigate the effect of soil and disbondment configuration on CP penetration into coating disbondment. An innovative experimental setup was employed to measure the pH and potential of pipeline steel under a disbonded coating without disturbing the environment inside the disbondment. It was revealed that CP penetration was dependent on CP level, environment inside the disbondment, and distance from the holiday. CP penetration increased with increasing CP level, and decreased with increasing pH and distance from the holiday. For a circular FBE coating disbondment, the existence of clay soil in the solution acted as a physical barrier for OH- generated inside the disbondment to diffuse out, and lead to a very alkaline environment around the holiday area, i.e., pH 14. The presence of soil would cause an inhibiting effect to CP penetration. Majority of CP current was consumed at the holiday and vicinity, and only a very small portion of CP current could reach beyond 10 mm into the FBE and HPPC coating disbondment even at CP level of -1126 mVSCE. CP current could reach deeper into a narrow tunnel-shaped disbondment than into a circular-shaped disbondment with similar conditions, i.e., similar sizes of holiday and disbondment gap. Tunnel-shaped coating disbondment was subject to hydrogen accumulation inside the disbondment. In particular, HPPC coating disbondment had a higher tendency to trap hydrogen gas than FBE coating disbondment. The trapped hydrogen gas could effectively block the CP current from penetrating into the disbondment. The critical parameters required for the occurrence of near neutral pH SCC, i.e., near neutral pH and potentials close to Ecorr, could be achieved inside the tunnel-shaped HPPC coating disbondment under elevated CP levels of -926 mV and -1126 mV due to the presence of trapped hydrogen gas.

INTRODUCTION

The most effective method to mitigate corrosion on the external surface of a buried pipeline was to utilize a protective coating supplemented by cathodic protection (CP). Ideally, these two systems should work together so that if the coating is disbonded or has defects allowing electrolyte to contact the pipe steel surface, the CP system would continue to function and provide corrosion protection to the pipeline. However, the success of this approach greatly depends on the nature of coating failures. Corrosion protection to the pipe with coating failures can only be achieved if sufficient CP current is able to reach the exposed pipe steel.1-5 For instance, if adequate CP current is able to penetrate into a coating disbondment, then corrosion related damages may be prevented. Otherwise, the exposed pipe steel can be susceptible to corrosion and environmentally assisted cracking.

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