We characterize the dependency of hydrogen embrittlement crack initiation on potential of an ultra high strength steel. This is accomplished through (1) prediction of crack tip hydrogen concentration (CH,tip) as a function of material, geometry, and chemistry of the bulk environment, (2) definition of subsequent crack chemistry and potential, (3) development of a database of diffusible hydrogen concentration (CH,diff) as a function of local chemistry and potential, and (4) use of damage models that predict the threshold stress intensity for cracking (KTH) as a function of CH,tip. In this paper the primary material selected for study is UNS K92580 (AerMet 100), which is characterized by high purity (i.e., low sulfur and phosphorus content) and nano-scale strengthening solute zones and coherent precipitates. The experimental relationship between applied potential and the threshold stress intensity for cracking (KTH) has been established previously for UNS K92580 through slow strain rate testing in an aqueous environment over a range of cathodic and anodic applied potentials.1 The results reveal a window of applied potentials where there is reduced susceptibility to hydrogen environment embrittlement (HEE). Damage models for initiation and growth can justify this behavior based on the potential dependent value of CH,tip. However, the applied potential itself does not yield insight as to the exact conditions at the crack tip. Ohmic potential (IR) drop and electrochemical/chemical reactions in the crack can lead to a significantly different environment at the crack tip than on the bulk surface. Potential-pH conditions in the crack environment are defined as a function of material, geometry, and chemistry, and then connected to CH,tip, which can then serve as inputs to damage models for HEE initiation and growth.

Subject
Hydrogen induced cracking, Materials, Hydrogen concentration, Cathodic potential, Overpotential, Stress cracking, Acidity, Risk reduction, Electrochemical potential, Cracks, Electrode potential, Hydrogen, Hydrogen stress cracking
Keywords:
AerMet 100, hydrogen embrittlement, diffusible hydrogen, scaling law
© 2007 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).
2007
Association for Materials Protection and Performance (AMPP)
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