Mechanisms of inclined stress corrosion cracks

Abstract

Inclined high pH stress corrosion cracking (SCC) is a type of intergranular environ- mental cracking in gas pipelines which di ers from typical SCC by propagating at an angle from axial-radial plane. Prior investigations of Australian and Canadian inclined SCC colonies have not provided a clear indicator of the mechanism be- hind the abnormal crack growth direction. This thesis addresses the issue of why SCC has inclined in the cases in Australia and Canada, as well as the implications of this inclination on industry management techniques. This research was also a project under the energy pipelines cooperative research centre as a collaboration between industry and university, with various publications and industry reports produced as a result of the work. The phenomenon of inclined SCC is investigated primarily through the use of simulations along with some supporting experiments. A survey of the literature is rst conducted to identify possible mechanisms that could cause inclined SCC. The simulations developed then have the aim of rst testing those mechanisms for feasibility with a developed crack path model validated with existing eld data, and then clarifying the e ect of that mechanism on SCC crack growth rates and interaction. Key results include that the likely mechanism governing SCC inclination is crack- tip strain enhanced electrochemistry, where the highly strained areas around the crack tip cause an increase in current density and hence dissolution rate. Another result is that growth rates of inclined SCC should always be slower than straight SCC to a depth of approximately 1mm for the surveyed cases, but that the growth rates could be higher after the rst 1 mm depth of growth depending on the degree to which current density is a ected by strain. Lastly, existing SCC interaction guidelines were shown to still be valid for inclined SCC in the conditions reported in prior studies on SCC colonies from Australia and Canada. The simulations on inclined SCC interaction were also extended to consider conditions outside of those reported in prior studies to demonstrate the regions where existing interaction guidelines would be at risk of being unconservative. The ndings of this thesis not only add to the literature of SCC research with various publications, but also could be made use of with the real world applications of pipeline SCC management and pipeline manufacture to control inclination, as has been documented in various reports to industry. Future work which could bene t the knowledge gained from this work is also suggested. In particular, some experiments are suggested which could provide further information as to the sensitivity of crack inclination to the material, electrochem- istry, and loading conditions.