A NOVEL MONITORING TECHNIQUE TO DEFINE CP CRITERIA FOR BURIED PIPELINES UNDER AC CORROSION CONDITION

The risks of alternating current (AC) corrosion and overprotection increasingly demand new criteria for cathodically protected pipelines. To assess the risk of AC corrosion, new cathodic protection (CP) criteria have been proposed based on DC/AC current densities measurements using coupons. The monitoring system designed for this project was based on the instant-off method, with steel coupons simulating coating defects on a buried pipeline. The problems associated with the instantaneous off-potential measurements have been attributed to a non-sufficient time resolution. In present study, it has been possible to determine the de-polarisation of steel coupon within a few milliseconds after disconnecting the coupon from the DC/AC power source, by increasing data acquisition rate. For this, a monitoring system was developed in order to measure the IR-free potential together with the DC/AC current densities. The monitoring system was utilized for both laboratory experiments and site survey to study the mechanism and the condition of AC corrosion, its mitigation and more importantly to define new CP criteria.