ANOVELMONITORINGTECHNIQUE TO DEFINE CP CRITERIA FOR BURIED PIPELINES UNDER AC CORROSION CONDITION

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