Onshore gas transmission lines are conjointly protected against external corrosion by cathodic protection (CP) and organic coatings. A coating may present defects such as holidays, rock indentation and/or disbondment. Those defects can generate a significant corrosion risk of the lines.

The experiments hereby presented have concentrated on the evaluation of the coating defects permeation to cathodic protection electrical currents and corrosive species (water and O2). Two electrochemical cells were designed to simulate coating defects. The first cells allowed the simulation of holidays and rock indentation; the second one, the simulation of large disbonded area with the possibility of electrolyte circulation in the gap. The cell also allows the control of the medium in an external compartment (simulating soil conditions) and monitor the medium evolution in the gap, containing a metallic coupon (simulating a coating disbonding). The metal potential, the oxygen concentration, and the pH of the medium under the coating disbondment were monitored using micro electrodes. In some instances weight loss or electrochemical measurement were carried out to assess the corrosion rate.

The results are discussed with respect to the effect of the type of defect on the corrosion risk and in the case of the second cell, the effect of electrolyte circulation in the gap. The comparison between the different defect types shows significant differences which must be taken into account for a proper corrosion prevention of aging lines by cathodic protection. Therefore, this paper points out which types of defects are the most dangerous and must be detected to prevent corrosion risks.

Subject
Holidays, Water, Coatings, Cathodic protection, Corrosion rate, Rocks, Cathodic current, Pipelines, Metals, Oxygen, Dissolved oxygen, Coating defects, Electrode potential
Keywords:
coating, corrosion, disbonding, holiday, pipeline, underground, cathodic protection
© 2004 Association for Materials Protection and Performance (AMPP). All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of AMPP. Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
2004
Association for Materials Protection and Performance (AMPP)
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