Growth behavior of surface cracks in pipeline steels exposed to near-neutral pH environments

  • Author / Creator
    Egbewande, Afolabi T.
  • This study is part of a larger study designed to help a consortium of pipeline operators replace current (very expensive) approaches to managing stress corrosion cracking (SCC) concerns in near-neutral pH (NNPH) environments with mathematical models that predict SCC growth rates. This could significantly reduce the cost of pipeline integrity management programs. The goal was to help find ways to improve the accuracy of existing models.
    NNPHSCC cracks are surface-type flaws. However, NNPHSCC was typically modelled as through-thickness cracks in previous laboratory studies. This was identified as a major reason why current NNPHSCC models are inaccurate. Therefore, this study was designed to model NNPHSCC cracks as surface-type flaws rather than through-thickness cracks.
    Results showed contrary to popular opinion that surface-type flaws propagated less rapidly than through-thickness cracks in NNPHSCC environments. Also, inherent variations in the local environment under a disbondment produce hydrogen concentration gradients that result in very high propagation rates at the open mouth of a disbondment. The propagation rate declines very sharply non-linearly distance away from the open mouth inside the disbondment. It was determined the environmental factor used to account for the contribution of the environment to crack propagation, could be up to ten times higher at the open mouth compared to other locations under the disbondment. Identifying these issues helps to guide NNPHSCC modellers in selecting more appropriate growth rates for SCC programs. A series of propagation rate ranges under various environmental and mechanical loading conditions were determined.
    Contrary to popular opinions, increased CO2 concentration in groundwater decreased crack propagation rates by intensifying (environmental) crack tip blunting. This delayed crack re-initiation from a dormant state. Under benign loading conditions, this helps to reduce/stop
    crack growth by driving towards dormancy. Mechanically blunting a crack tip was found to produce the same effect. Hydrogen enhanced localized plasticity (also called hydrogen enhanced low temperature creep) was found to be responsible for this blunting effect. Means of manipulating the mechanical loading factors to produce this effect were identified.

  • Subjects / Keywords
  • Graduation date
    Fall 2013
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.