The importance of dissolution of MnS inclusions in environmentally assisted cracking (EAC) of pressure vessel steels in high temperature water is well recognized. However, no direct measurement of the crack tip chemistry that develops during EAC has ever been performed, and only estimates exist for the dissolution rate of MnS and the resulting sulfur levels in the crack. In this manuscript, microsampling of the crack tip solution in A533B low alloy steel (0.013% S) exposed to constant and cyclic loading in 288°C water under various test conditions has been used to obtain direct measurements of the crack tip sulfur concentration, while simultaneously monitoring crack length.

A reversing direct current potential drop method was used to continuously monitor crack length. Ion chromatography (IC) and inductively coupled plasma (ICP) were used to measure dissolved sulfur species in the microsampled solutions. Most experiments involved varying the corrosion potential (from = 0.12 to ≈ −0.5 Vshe) by changing the dissolved oxygen concentration between 10 ppm and 0 ppm (nitrogen deaerated) in high purity water. At 10 ppm oxygen, high crack growth rates were observed and the microsampled solutions contained between 1 and 2 ppm sulfur, about 10 times higher than at 0 ppm oxygen, where crack growth rates were low. Measurements of the room temperature solutions showed that most sulfur was present as sulfate, although in the high temperature deaerated water in the crack, MnS undoubtedly dissolves to form HS− and H2S. Decreasing the loading frequency from 10−4 to 10−5 Hz also lowered both the crack growth rate and the crack tip sulfur concentration. Very high microsampling rates effectively flushed the crack tip chemistry, decreasing the crack tip sulfur content and crack growth rate. This research provides a technique for directly determining the effects of MnS inclusion dissolution, corrosion potential, (bulk) solution impurities, solution flow rate, and loading conditions on crack tip chemistry and, in turn, on crack growth rate and thus can contribute significantly to the fundamental understanding of EAC.

Subject
Environmental cracking, Sulfur, Crack growth, Water, Corrosion rate, Sulfates, Corrosion cracking, Crack chemistry, Fatigue crack growth rate, Stress cracking, Cracks, Corrosion potential, Sampling
Keywords:
stress corrosion cracking, corrosion fatigue, crack chemistry, low alloy steel, MnS inclusions, water chemistry, high temperature water, micro-sampling, crack growth rate
© 1995 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).
1995
Association for Materials Protection and Performance (AMPP)
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