Digesters used in Kraft pulping were originally believed to be free of corrosion problems. However, approximately 20 years ago a catastrophic accident gave a signal that corrosion was indeed a problem and more attention must be paid to the inspection and protection of digesters. The major concern at that time was caustic stress corrosion cracking (SCC) of carbon steel, which was frequently found in the impregnation zones.

Pulping techniques are evolving strongly due to increasing demands for efficiency. Common for almost all methods is that higher temperatures, increased alkali levels and lower lignin contents are used, particularly in the lower part of the digester. At the same time, mill closure and recycling are increasing chemical load and, consequently, corrosion of carbon steel, in the form of rapid thinning, is nowadays taking place in extraction and wash zones, i.e. in the zones that previously were relatively free of corrosion.

The behavior of carbon steel in various cooking environments has been studied by using an on-line Contact Electric Resistance (CER) unit to determine the passivity of the carbon steel of digesters and impregnation vessels. The results from the CER unit will reveal the corrosivity of the cooking liquor.

This paper will present in situ CER results of various wood species in different cooking environments through out the pulping world. The main purpose of this research is to determine whether steels are able to passivate under chemical and thermal distraction in different zones of pulping process. Also the best potential ranges of passivity are defined.

Subject
Digesters, Anodic protection, Cooking, Metal surfaces, Liquor, Free corrosion potential, Wood, Metals, Steel, Passivation, Carbon steel, Polarization, Oxidation
Keywords:
anodic protection, carbon steel, passivation, digester corrosion, on-line corrosion determination, continuous kraft pulping
© 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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