Stress corrosion cracking (SCC) susceptibility of austenitic Type AISI 304L, AISI 316L stainless steels and nitrogen alloyed Type AISI 301 steels in 50% CaCl2 solution at 373 K (100°C) under open-circuit potential and cathodic polarization conditions was studied using a constant load test method. The electrical properties of the surface films formed were monitored as a function of time and potential by means of the Contact Electric Resistance (CER) method. The steady state strain rate (ε˙ss) obtained from the corrosion elongation curve (elongation vs. time curve) showed a linear dependence on time to failure (tf). This means that ε˙ss can be applied as a parameter for prediction of tf. Based on creep measurements in a non-corrosive environment and studied solution, the influence of environment on ε˙ss showed more than fivefold increase in the creep rate, which is a clear indication of the role of creep in SCC mechanism. This environmentally enhanced creep was found in the presence of high resistance passive film on the surface. Nitrogen alloying was found to enhance clearly the passivation.

A new model is used to explain and describe the observed phenomena of SCC. The rate determining step in cracking is the generation of vacancies by selective dissolution. The effect of electrochemical polarization on creep is due to a change in the vacancy flux through the oxide film to the metal. Cathodic polarization resulted in decreasing strain rate by reducing the corrosion rate and suppressing the generation of vacancies, which made strain localization more difficult. Vacancies generated by corrosion are first consumed by dislocations which enhances creep, but later agglomeration of vacancies leads under applied stress finally to cleavage-like SCC.

Subject
Cathodic polarization, Strain rate, Corrosion rate, Stress cracking, Vacancies, Stress, Nitrogen, Elongation, Steel, Creep, Passivation, Stainless steel, Stress corrosion cracking
Keywords:
stress corrosion cracking, creep, life prediction, passivation, selective dissolution
© 1997 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).
1997
Association for Materials Protection and Performance (AMPP)
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