A laboratory investigation of application limits of 13Cr steel was carried out for sweet downhole environments in the CO2 partial pressure regime up to about 10 bar (1 MPa). Two grades of 13Cr steel were studied, one complying with the API 5CT specification and the other a higher carbon version. Polarisation curves, taken at various times after immersion of the specimens in simulated brine, revealed an increasing pitting tendency for the higher chloride levels and indicated better performance for the API 5CT grade. This was in agreement with evidence for chromium depleted zones in the higher carbon grade, obtained from polarisation curves in sulphuric acid.

Weight loss exposure tests, in which the corrosion rate was also monitored by polarisation resistance in all cases, is temperature range. A mathematical formula was derived to describe the corrosion rate as a function of temperature and chloride content. For instance, if 0.1 mm/y is taken as an acceptable corrosion rate, 13Cr steel could be applied up to 125 °C in the presence of 150 g/L Cl-. In the exposure tests, the API 5CT 13Cr steel did not show pitting at the 0.1 mm/y boundary but the higher carbon grade did suffer from this form of attack.

Subject
Electrochemical impedance spectroscopy, Materials, Corrosion rate, Chromium, Partial pressures, Weight loss, Steel, Brines, Pitting, Carbon, Corrosion resistance, Chlorides, Corrosion potential
Keywords:
martensitic steel, 13Cr steel, Uns S42000, API 5CT, corrosion, CO2, brine, oil production, gas production, application limits
© 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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