A C/D stage bleach washer made from 6 Mo austenitic alloy was exposed to high levels of active chlorine residuals of up to 400 ppm in the inlet pulp for several months in a row. Several stainless alloy coupons were immersed in the vat during that time. The results were compared to those obtained with identical coupons during the preceding low residuals operating regime. All the coupons were corroded in the artificial crevice sites to various degrees during the high residuals period. The attack on the washer was limited to pitting shower pipes, crevice corrosion of the vat sidewall above the pulp level, etching of some welds, and the corrosion of parts made from Type 316 stainless steel.

The results of electrochemical laboratory experiments using actual and synthetic pulp filtrate, indicated that under test conditions approximating bleach washer operation, crevice corrosion would start only when the corrosion potential Ecorr, produced by a surge in residuals, exceeds the value of protection potential Ep under given environmental conditions.

Subject
Cyclic polarization, Pulp, Molybdenum alloys, Metal coupons, Sodium chloride, Corrosion initiation, Acidity, Crevices, Crevice corrosion, Pitting, Chlorine, Protection potential, Corrosion potential
© 1993 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).
1993
Association for Materials Protection and Performance (AMPP)
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