The Office of Pipeline Safety (OPS) of the US Department of Transportation requires all pipelines to be inspected regularly to ensure public safety. The reassessment interval currently in use was stipulated by OPS to be seven years, a single number regardless of differences in pipeline operational and environmental conditions. Since the reassessment interval can be determined by the size and the growth rate of a defect or a flaw in the pipeline and corrosion plays a key role, the goal of this work is to simulate mathematically the pipeline corrosion and compute the disbondment corrosion rate and potential. O2 is here assumed to be a dominant corrosive species. A simple algebraic equation and a corresponding simple measurement procedure have been developed. This procedure allows the pipeline operators to estimate conveniently the underneath corrosion potential and rate and can be considered for use in the external corrosion direct assessment procedure.

It has been found that aerated crevice corrosion rate or potential with or without CP can be reproduced by simple anodic or cathodic polarization in deaerated condition.

The steel potential (φ) at a distance (z) under disbondment from the holiday can be estimated by: φ−EOCPφ0−EOCP=e−C1za⁠. All variables in this equation can be measured or estimated. The crevice corrosion rate can be computed from the Tafel equation. “a” is the crevice gap and C1=12.303aρRp⁠. φ0 is the potential at the holiday; ρ is average resistivity of the soil solution in the disbondment. EOCP and Rp are respectively open circuit potential (OCP) and the linear polarization resistance of the substrate material in a simulated disbondment solution.

Subject
Holidays, Steel surfaces, Fluxes, Geometry, Corrosion currents, Density, Disbondment, Crevices, Corrosion modeling, Crevice corrosion, Steel, Electric potential, Anodic polarization
Keywords:
corrosion, steel, oxygen, crevice corrosion, pipeline, cathodic protection, anodic polarization
© 2006 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).
2006
Association for Materials Protection and Performance (AMPP)
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