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Authors: J. Wang, A. Atrens

Division of Materials Engineering, The University of Queensland (UQ), UQ, St Lucia, 4072, Qld, Australia
Received 1 July 2003;accepted 12 August 2003

Abstract:

     Stress corrosion cracks (SCC) had been found in a natural gas transmission pipeline during a dig-up and inspection program. The question was raised as to whether the SCC was active or dormant. This paper describes the resultant investigation to determine if a particular service crack was actively growing. The strategy adopted was to assess the appearance of the fracture surface of the service crack and to compare with expectations from laboratory specimens with active SCC. The conclusions from this study are as follows. To judge whethera crack in the service pipe is active or dormant, it is reasonable to compare the very crack tip of the service crack and a fresh crack in a laboratory sample. If the crack tip of the active laboratory sample is similar to that of the service pipe, it means the crack in the service pipe is likely to be active. From the comparison of the crack tip between the service pipe and the laboratory samples, it appears likely that the cracks in the samples extracted from service were most likely to have been active intergranular stress corrosion cracks.

Keywords: Stress corrosion cracking;Pipeline failiure;Fractography;Simulation tests;Crack growth

Corresponding author. Tel.: +61-7-3365-3738;fax: +61-7-3365-3888
E-mail address: a.atrens@minmet.uq.edu.au (A. Atrens).

1. Introduction

     Stress corrosion cracks (SCC) had been found in a natural gas transmission pipeline during a dig-up and inspection program. The question was raised as to whether the SCC was active or dormant. This paper describes the resultant investigation to determine if a particular service crack was actively growing. The strategy adopted was to assess the appearance of the fracture surfaces of the service cracks and to compare with expectations from laboratory specimens with active SCC.
     This study was based on the following expectation regarding SCC. High pH SCC of pipeline steel is expected to be intergranular, by a mechanism of active path dissolution, with the crack aspect maintained by active crack-tip dissolution combined with passive crack sides. Immediate examination of a fresh SCC is expected to reveal a fracture surface at the crack tip, which has a very thin surface layer consisting of a passive film and other corrosion products. There would be a gradation of surface layer thickness, with increasing thickness with increasing distance away from the actively growing crack tip. The surface layer at the crack tip may be so thin, 2–100 nm, that it is not apparent during examination using optical micro- scopy and scanning electron microscopy so that the surface looks merely metallic in lustre.