Corrosion in oilfield produced waters is dependent upon the formation, or lack thereof, of corrosion product scales. This is especially true for environments that contain small amounts H2S, which form mackinawite (FeS). Iron sulfides have been described as both protective and aggressive in their effects on further corrosion. This study describes the thermodynamic calculations that can be used to predict when mackinawite will form and describes several "simplifying" assumptions that are commonly used that are not valid for this application. The separate effects of bulk solution pH, temperature, pressure, CO2 and HCO3− concentration, and the activity and fugacity coefficients upon the critical H2S level required for mackinawite formation are also presented. For pH values approaching neutral conditions, the critical H2S value can be extremely small (<10−9 mole fraction). Some reasons why mackinawite is not observed as a corrosion product under these conditions are discussed.

Subject
Mackinawite, Hydrogen concentration, Metal surfaces, Iron sulfides, Solubility, Corrosion reactions, Partial pressures, Acidity, Corrosion products, Constants, Fugacity, Oilfields, Corrosion mechanisms
Keywords:
Sour Corrosion, H2S Corrosion, Mackinawite, Slightly Sour Corrosion, Corrosion Mechanisms
© 1994 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).
1994
Association for Materials Protection and Performance (AMPP)
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