The corrosion and stress-corrosion cracking (SCC) of Alloy C-276 (UNS N10276) has been studied in high-temperature sour brines. The long-term effects of exposure to sour brines were evaluated using C-ring specimens tested at 400°F (204°C), 500°F (260°C), and 600°F (316°C) for up to 3 years. Microstructures of the C-276 samples were characterized before and after sour brine testing using TEM to evaluate the effects of initial microstructure and of temperature-related changes in microstructure on corrosion and cracking. The corrosion scales formed on exposed surfaces and within SCC cracks were characterized using electron microprobe, X-ray diffraction. Auger electron microscopy, and ESCA.

In the long term C-ring tests, Alloy C-276 was susceptible to SCC at temperatures above 500°F (260°C). The SCC crack mode was primarily intergranular at 500°F (260°C) and transgranular at 600°F (316°C). Microstructural analysis of samples tested for 9 months at 500°F (260°C) indicated that aging did not affect the degree of short range order or grain boundary precipitation. No significant effect of aging on dislocation substructure was observed. Indirect evidence for enhanced segregation of sulfur to grain boundaries was indicated in the STEM analysis. The constituents of the corrosion products varied with location inside and outside the SCC cracks. The corrosion scales were a complex mixture of oxides and sulfides consisting primarily of chromium and molybdenum oxides, and nickel sulfides with some molybdenum sulfide and iron sulfides.

A scenario for the onset of SCC in Alloy C-276 is presented based on the microstructural and corrosion product analyses.

Subject
Sulfur, Intergranular stress corrosion cracking, Metal surfaces, Nickel based alloys, Transgranular stress corrosion cracking, Sour brine, Corrosion scales, C ring stress corrosion testing, Scale formation, Grain boundary, Alloys, Stress corrosion cracking, Samples
Keywords:
stress-corrosion cracking, H2S, corrosion, brine, C-276
© 1992 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).
1992
Association for Materials Protection and Performance (AMPP)
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