A semi-empirical crevice corrosion initiation model of the waste package outer barrier (WPOB) was developed using the long-term corrosion potential data for Alloy 22 in a wide range of test conditions, and the crevice repassivation potentials of creviced Alloy 22 samples obtained from the short-term CPP tests in a wide range of conditions. The crevice corrosion initiation model considers that crevice corrosion of the WPOB may occur when the steady-state corrosion potential (Ecorr) is equal to or greater than the crevice repassivation potential (Ercrev), that is, ΔE (= Ercrev - Ecorr) ≤ 0.

The model analyses have shown that the WPOB is not subject to crevice corrosion for conditions with neutral to alkaline pH solutions or with significant concentrations of inhibitive ions such as nitrate. The WPOB is potentially susceptible to the initiation of crevice corrosion if an acidic chloride-containing solution with relatively lower concentrations of inhibitive ions contacts the waste package while it is at elevated temperatures. The experimental data and model analyses that are available to date indicate that the lower bound of the nitrate to chloride concentration ratio for the immunity of Alloy 22 to localized corrosion initiation in chloride-containing brines is about 0.2. Because only nitrate ions are accounted for in the crevice corrosion initiation model for the inhibitive effect, the model results for solutions with significant amounts of other potentially inhibitive ions such as carbonate, phosphate and sulfate (in addition to nitrate ions) are conservative.

Subject
Water, Repassivation potentials, Acidity, Crevices, Ion concentrations, Nitrates, Corrosion modeling, Crevice corrosion, Localized corrosion, Alloys, Chlorides, Wastes, Corrosion potential
Keywords:
Alloy 22, waste package corrosion, nuclear waste disposal, corrosion potential, critical potential, crevice repassivation potential, localized corrosion susceptibility, inhibitive ion effect
© 2004 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).
2004
Association for Materials Protection and Performance (AMPP)
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