Studies are made using two glass cells joined by an agar bridge to simulate conditions under and surrounding a tubercle on a metal surface in fresh water. By changing conditions in the "under tubercle" vessel while maintaining neutral, aerated water in the extra tubercle" side, it is possible to study galvanic effects between the two electrodes. Conditions simulated under the tubercle" include deaeration, addition of free sulfide, and addition of any of several different organic and inorganic acids. It is seen that no combination of these conditions produced galvanic currents sufficient to explain high corrosion rates on steel observed under tubercles in the field – except in the unusual case where the large, external electrode is fully passivated. In fact, lowering the pH on the "tubercle” side to 3 or below caused a reversal in galvanic current, whereby the small electrode in the low pH condition was actually undergoing cathodic reactions in support of anodic behavior of the larger, external electrode. In the case of 304 stainless steel electrodes, no conditions studied caused significant galvanic corrosion of the "tubercle" electrode except addition of large quantities of HC1.

Subject
Acids, Water, Metal surfaces, Acidity, Tubercles, Nitric acid, Oxygen, Sulfides, Stainless steel, Galvanic current, Chlorides, Electrodes, Electrode potential
Keywords:
tubercles, under-deposit corrosion, MIC, biocorrosion, bacteria, acid-producing bacteria, sulfide-producing bacteria, SRB, carbon steel, 304 stainless steel
© 1995 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).
1995
Association for Materials Protection and Performance (AMPP)
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