Intergranular stress corrosion cracking (SCC) of alloy B-2 (UNS N10665) can occur in dilute acids at low temperatures if there is short range ordering and Ni4Mo in its metallurgical structure. Certain chemistries of alloy B-2 are particularly susceptible to the rapid kinetics of the Ni4Mo transformation, which can occur within minutes of exposure to temperatures of 650-750°C such as during welding. The severity of ordering is dependent on alloy chemistry and thermomechanical processing conditions, and can result in reduced ductility at 700°C, and subsequently to SCC.

SCC in the heat-affected zones of welds of fabricated equipment of alloy B-2 have been found to be associated with the presence of Ni4Mo and a short-range ordered structure. Laboratory tests confirmed that only a partially-ordered structure is needed for alloy B-2 to be highly susceptible to SCC in dilute sulfuric acid. A strong correlation was found between degree of ordering, and both susceptibility to SCC and reduced ductility at 700°C. Development of alloy B-3 (UNS N10675) overcomes this thermal instability of alloy B-2, and provides enhanced resistance to SCC in the as-welded condition. In addition to TEM evidence and ductility measurements at 700°C, a stress-corrosion cracking test in boiling 5 wt.% sulfuric acid has also been found useful in evaluating the susceptibility of Ni-Mo alloys to ordering, and to Ni4Mo formation.

Subject
Water, Intergranular stress corrosion cracking, Sulfuric acid, Reactors, Nickel based alloys, Molybdenum alloys, Structures, U bends, Heat, Caustic stress corrosion cracking, Alloys, Stress corrosion cracking, Ductility
Keywords:
Aging, ordering, short-range order, Ni4Mo, nickel-molydenum, stress corrosion cracking, ductility, thermomechanical cracking, annealing, alloy B-2
© 1996 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).
1996
Association for Materials Protection and Performance (AMPP)
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