51313-02088-A Real-Time AC/DC Measurement Technique for Investigating AC Corrosion of Pipes and Its Effect on CP Effectiveness

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Product Number: 51313-02088-SG

ISBN: 02088 2013 CP

Author: Frank Cheng

Publication Date: 2013

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With the increasing development of buried pipelines and the high-voltage power transmission as well as the rail transit systems pipelines encounter frequent interference by alternating current (AC). It is acknowledged that the corrosion rate of most metallic structures can be accelerated in the presence of AC interference. Particularly there have been increasing reports of AC-induced corrosion and failures of oil/gas pipelines in recent years.

One of the primary technical challenges in AC corrosion measurements is that the recorded voltage signals contain both AC and DC components. It would be impossible to derive the intrinsic information of AC corrosion by analysis of these mixed signals. Moreover although AC corrosion rate of pipelines is directly related to AC current density rather than AC voltage the reliable online measurement technique for AC current density has been still lacking.

In this work a real-time signal data acquisition (DAQ) technique was developed to separate the DC and AC components from the recorded signals. The mechanism of AC corrosion was analyzed based on analyses of the effects of AC current density on DC potential shift most positive AC voltage and most negative AC voltage as well as the corrosion rate of the steel. A model was developed to illustrate mechanistically the AC corrosion process and the parametric effects. Moreover an on-site technique for measurement of AC current density was proposed. Furthermore the effects of AC current density on cathodic protection (CP) were evaluated and the new CP design was recommended for the testing system.