The aggressive behavior of carbonic acid solutions upon various steels has been widely investigated, but is yet not fully understood. In order to achieve more knowledge of the mechanisms of CO2 corrosion, simultaneous diffusion and homogenous chemical reactions have been simulated close to a corroding iron electrode, using kinetic data for the reactions involving CO2-related species and water.

Many CO2 corrosion models predict mass transfer across the stagnant diffusion layer without considering contributions from homogenous chemical reactions.1-3  However, such reactions play a significant role when the equilibria of the CO2/H2O system are disturbed by concentration changes caused by diffusion. Attempts have been made to treat this phenomenon, using equilibrium constants in the routines involving chemical terms.4  Still, this thermodynamic approach fails to determine the concentration changes caused by chemical reactions, as equilibria will not always be obtained.

The recent model simulates the conditions in the stagnant layer close to a dissolving iron surface, on which heterogeneous reduction of H+-ions and H2CO3 occurs. One-dimensional diffusion is calculated using a finite element method. Homogenous chemical reaction rates are determined from rate constants and differential or integrated rate expressions. The diffusion/reactions computation procedure is iterated until convergence is reached. The results are compared to published corrosion rates.

Subject
Nernst layer, Water, Diffusion layers, Hydrogen concentration, Corrosion rate, Metal surfaces, Diffusion, Walls, Bulk solution, Constants, Ion concentrations, Corrosion modeling, Chemical reactions
Keywords:
CO2 corrosion, mass transport, reaction kinetics, modeling
© 1997 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).
1997
Association for Materials Protection and Performance (AMPP)
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