A condition assessment was performed on the Calvert Cliffs Nuclear Power Plant Unit 1 Main Steam Condenser 6% molybdenum stainless steel tubing. This work consisted of eddy current (EC) examination, in-situ visual examination of tubes, and destructive analysis of pulled tubes. The assessment objectives were as follows: to determine the tubing degradation mechanism, to render the condenser leak-tight, to identify methods of improving condenser life management, and to develop a basis for projecting the remaining condenser life. Once the corrosion mechanism was identified, seawater corrosion tests were performed on UNS N08366 and N08367 tubing using vinyl sleeves, Type 316 stainless compression fittings, and epoxy paint as crevice formers.

Based on eddy current examination results, greater than 95% of the 49,500 tubes in the condenser had no detectable degradation. Over 1200 tubes were plugged with either eddy current indications measuring greater than 60% wall loss or with obstructions. One waterbox (11 A) contained 52% of all eddy current indications with over half of these in one section (B).

Destructive analysis of pulled tubes revealed crevice corrosion, a form of localized corrosion that initiated under deposits. This type of corrosion occurred most often and had the deepest penetration in the weld seam. This was attributed to molybdenum and chromium segregation in the as-welded tubes and to deleterious secondary phases formed during weld solidification.

Subject
Electric current, Weld attack, Materials, Vinyls, Tubes, Walls, Corrosion initiation, Seawater, Condensers, Crevices, Tubing, Corrosion losses, Crevice corrosion
Keywords:
UNS N08366, UNS N08367, tubing, crevice corrosion, seawater, seam-welded, eddy current, main steam condenser
© 1994 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).
1994
Association for Materials Protection and Performance (AMPP)
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