Large water wall panels and convection modules were aluminized by commercial vendors, using the pack cementation process. Variations in thickness and aluminum content were determined by optical microscopy, electron microprobe and NDE. Considerable variations were found in both. It was found that aluminized coatings containing more than 18% Al were resistant to sulfidation up to 500°C and also nonmagnetic. Thus, magnetic stand-off gauges can be used to monitor coating quality. Welding of aluminized low-alloy steels caused considerable cracking in the coating around the weld. Internal sulfidation along cracks was found to occur, especially when the pressure and the H2S content of the syngas were high. Aluminized coatings were also found to be susceptible to pitting corrosion by acidic condensates formed during downtime. It is concluded that the use of aluminized coatings in coal gas environments is probably limited to applications where the temperature and the H2S content of the syngas are low and the formation of acid condensates during downtime can be prevented.

Subject
Coatings, Materials, Substrates, Diffusion, Aluminum, Corrosion cracking, Gasification, Coating thickness, Syngas, Downtime, Sulfidation, Coal, Corrosion resistance
© 1986 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).
1986
Association for Materials Protection and Performance (AMPP)
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