A field study was performed to assess the influence of local soil conditions (biological and chemical) on the ability to achieve "adequate" cathodic protection and reduce corrosion, including microbiologically influenced corrosion (MIC). The performance of four coatings was also assessed. Two sites were chosen based on inherent differences in soil corrosiveness. Site A was adjacent to a pipeline site at which MIC was implicated in the corrosion process leading to a failure. Site B was a site adjacent to the same pipeline (one-quarter mile from Site A) that had shown significantly less corrosion. The test system matrix consisted of four coating groups [fusion bonded epoxy (FBE), polyethylene-backed tape (PBT), coal tar enamel (CTE), and bare] and three levels of cathodic protection (native potential, -0.85 volts, and -1.2 volts vs. Cu/CuSO4) at Sites A and B. Intentional holidays were created in the coatings of the coupons which were placed at pipe depth and cathodically maintained for seven months. The results showed that levels of chemical species important in the MIC process were higher in corrosion products than in the nearby soil. Differences in pitting corrosion were shown to be due to an interaction between local soil conditions and cathodic protection level. A threefold increase in mean maximum pit depth was observed on native potential coupons at Site A as compared to Site B. Increased levels of cathodic protection reduced the mean maximum pit depth on coupons at both sites, however, those at Site A were significantly deeper that those at Site B for both -0.85 volts and -1.2 volts vs. Cu/CuSO4. With the exception of iron, no significant chemical differences were observed in soils from both sites. Levels of sulfate-reducing bacteria (SRB) were higher in soils from Site A than Site B. Levels of SRB were also higher in corrosion products from Site A coupons than Site B coupons. High levels of acid-producing bacteria (APB) and aerobes were found in soil and corrosion product samples from both sites. All coatings performed well, though some differences in current requirements were observed between coating groups.

Subject
Holidays, Pits, Electric current, CP coupons, Cathodic protection, Cathodic potential, Metal coupons, Corrosion products, Cathodic sites, Pitting, Corrosion coupons, Soil, Microbiologically influenced corrosion
Keywords:
Cathodic Protection, Coatings, Microbiologically Influenced Corrosion, Gas Pipelines, Biocorrosion, Biological Corrosion
© 1993 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).
1993
Association for Materials Protection and Performance (AMPP)
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