The role of acetate (Ac−) ion on the rate of CO2 corrosion has been investigated. The presence of Ac− in oilfield brines can significantly increase the rate of corrosion of carbon steel. In the presence of Ac−, the corrosion rate can increase even if the pH increases. Many corrosion prediction models cannot accommodate this observation. On a ppm basis, Ac− increases the corrosion rate more than the HCO3− decreases it. The presence of Ac− only affects the corrosion rate and not the corrosion mechanism. The mechanism by which HAc increases the corrosion rate may be explained by its ability to both decrease the pH and solubilise Fe2+, thus reducing the iron carbonate film thickness. Ac− has been shown to attack existing iron carbonate films and make them thinner. If local film thinning were to occur this would result in localised corrosion. Nitrate, another small anion, did not increase the corrosion rate. Brines containing Ac− can be inhibited using conventional oilfield inhibitors. The presence of Ac− can result in positive errors in the determination ofHCO3−⁠. The routine analysis of HCO3− and Ac− need to be undertaken carefully otherwise significant errors can result. Some commercial laboratories have been shown to be incapable of these ‘routine’ procedures. All of the above could result in the corrosion rate being significantly underestimated and consequently the specification of too low a corrosion allowance and/or the implementation of an insufficient inspection and corrosion control strategy.

This is the acetate double whammy:

  1. Acetate may lead to an overestimation of the HCO3− and thus an underestimation of the corrosion rate for CO2 only.

  2. Acetate will cause the corrosion rate to be higher than for CO2 only.

Subject
Corrosion rate, Predicted corrosion rate, Titration, Iron carbonate films, Sodium chloride, Bicarbonates, Acetates, Acidity, Corrosion modeling, Salt concentration, Corrosion inhibitors, Brines, Electrodes
Keywords:
Carbon dioxide, acetic acid, acetate, bicarbonate, corrosion prediction, inhibition, mechanism
© 1999 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).
1999
Association for Materials Protection and Performance (AMPP)
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