Supercritical water oxidation (SCWO) is a promising and very effective method for the oxidation of organic wastes. Its high solubility for organic compounds and its special physical properties (i.e. density, viscosity) result in a key advantage over standard processes like incineration. Due to the formation of HCl during oxidation of chlorine organics, a severe corrosion attack of most of the materials has been observed. This paper describes the results of investigations of nickel base alloy G-30 and Ducrolloy, a new PM Cr-Fe alloy, exposed at 400 bar and 420°C for up to 400 hours in a CH2Cl2/H2O2 mixture with about 2000 wppm of chloride.

The weight losses varied between 5 mg/cm2 for Ducrolloy and about 175 mg/cm2 for alloy G 30 after 400 hours of exposure. The kinetic analysis revealed that the weight loss of alloy G-30 followed a linear increase with time, whereas the Cr-base alloy Ducrolloy showed nearly no weight loss up to about 160 hours. After 400 hours in the CH2Cl2/H2O2-mixture, the oxide layers formed on both alloys were found to be about 20 μm on Ducrolloy and about 300 μm on alloy G-30. The oxide layer on Ducrolloy was homogeneous and therefore able to protect the alloy against strong SCWO corrosion, whereas a relatively brittle layer, with many cracks, was formed on alloy G-30.

Both alloys were preoxidized in air up to 200 hours prior to SCWO exposure. For alloy G-30 nearly no significant effect of the preoxidation on corrosion rate could be stated. On the other hand a reduction of weight loss by a factor of 3 was found for Ducrolloy for short time preoxidation, compared to the not treated material.

For both alloys a loss of alloying elements during exposure was obvious. Alloy G-30 lost nickel down to a depth of about 200 μm, whereas Ducrolloy exchanged its iron with the SCWO environment only within a 20 μm zone.

Subject
Materials, Corrosion rate, Reactors, Nickel based alloys, Oxide layers, Metal coupons, Iron, Mixtures, Supercritical water oxidation, Oxide scales, Weight loss, Nickel, Alloys
Keywords:
SCWO, supercritical water, corrosion, oxidation, chloride, high temperature, high pressure, nickel base alloys, chromium-iron alloys
© 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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