The interactions between engineering materials and their environment which give rise to stress
corrosion cracking are reviewed and industrial examples from the petrochemical industry are
described. In one of the examples, cracking took place in carbon steel exposed to pressurised
gas containing carbon monoxide, carbon dioxide and water. The crack morphology in this
system was studied by metallography of samples from industrial gas processing plants and the
crack growth rates were determined using precracked specimens. Constant extension rate
tests, U-bend specimens and potentiodynamic studies were used to evaluate alternative
materials and inhibitor additions in CO-C02-H20 environments. Electrochemical noise was
accessed as a technique to monitor sec on line.
It was found that the CO-C02-H20 system was characterised by a time dependent adsorption
of carbon monoxide at anodic and cathodic sites. The adsorption produced a critical balance
between crack tip corrosion rate and the repassivation process comparable to the behaviour at
the active-passive transition zone in more conventional systems. The anodic passivation
exhibited a breakdown potential near to -400 mV (Ag/ AgCI) that defined the zone of sec
susceptibility. Inhibition by CO and hence sec was virtually independent of CO partial pressure
provided there was a sufficient reservoir of CO. The addition of commercial film forming
inhibitors did not greatly influence the system and sec was still observed in CO-C02-H20
environments to which inhibitors had been added. Steels containing alloy additions of more
than 9 % chromium were found to be resistant to sec but austenitic-ferritic weld joints cracked.
A low alloy 3% nickel steel performed well in the constant extension rate tests but was not
wholly resistant to sec.
The morphology of stress corrosion cracks in CO-C02-H20 mixtures was influenced by carbon
monoxide partial pressure. More corrosion was observed on the crack walls at low carbon
monoxide partial pressure and widened cracks resembling •mesa• corrosion were common. This
increased corrosion was probably due to difficulty in maintaining passivity in the crevice formed
by the growing crack. Similarly, crevicing in precracked specimens appeared to inhibit sec and
no crack extension was observed. Electrochemical noise proved to be a useful tool for
monitoring. / Thesis (PhD)--University of Pretoria, 1994. / gm2013 / Materials Science and Metallurgical Engineering / PhD
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:up/oai:repository.up.ac.za:2263/32324 |
| Date | January 1994 |
| Creators | Heaver, Edward Ernest |
| Contributors | Sandenbergh, R.F. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 198 pages, PDF |
| Rights | © 1994 University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria |
| Relation | D13/9/994/gm |
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