Investigation of Surface Crack Growth Behaviour Under Variable Pressure Fluctuations in Near-Neutral pH Environments

  • Author / Creator
    Engel, Devin W.
  • The goal of this investigation was to study the corrosion fatigue behaviour and mechanisms of surface cracks, semi-elliptical in shape, when exposed to near-neutral pH environments conducive to the environments experienced in the field. To better simulate field conditions, while avoiding the financial and technical burdens of full-scale testing, a simple, yet effective test setup was implemented. A novel sample geometry possessing three surface cracks with their notches removed, dubbed the Surface Crack Tension (SCT) specimen, was used in combination with a simulated coating disbondment, with a gap size of 5 mm, inside of a sealed corrosion cell containing a ground water solution to study the effect of location inside of coating disbondments on crack growth behaviour. The corrosion cell used in the study was sealed from the atmospheric environment, and the bulk solution was purged with 5 % CO2 + N2 bal. away from the disbondment over the duration of the tests to maintain a pH of 6.29 and a deaerated environment. Underload-type variable amplitude loading schemes were designed and implemented to better simulate the pressure fluctuations that are experienced on a routine basis by pipelines in operation at locations where the majority of stress corrosion failures occur. It was found that crack growth rates (da/dN) were enhanced by up to several orders of magnitude when subjected to underload-type loading waveforms in comparison to constant amplitude loading for loading frequencies from 10 3 Hz to 10 1 Hz. Additionally, the crack growth rates for underload-type schemes comprising of either high R-ratio cycles (minor cycles) or static holds in between underload events were compared. The introduction of minor cycles into underload-type loading schemes demonstrated a greater enhancement in crack growth rates over constant amplitude loading, in comparison to the introduction of static holds between underload events. The enhancement in crack growth rates in the NNpH environments by segments of quasi-static or static loads between underloads were attributed to the time-dependent accumulation of hydrogen leading up to an underload. Minor cycles were determined to provide greater growth enhancements because of their cyclic nature. The crack growth rates for all of the waveforms tested, as well as the growth enhancement by variable amplitude loading, decreased as the distance into the coating disbondment increased. This was related to a CO2 gradient induced by the disbondment which resulted in a decreasing concentration of hydrogen towards the bottom of the coating disbondment. ICP analysis determined that there was a non-uniform Fe concentration distribution throughout the system. The non-uniform distribution of iron was determined to be a result of the transport, by gravity and diffusion, of Fe away from the Open Mouth of the disbondment. This lead to increased hydrolysis rates towards the bottom of the disbondment. In spite of the semi-elliptical geometry, with (a:c) aspect ratios of 0.17 – 0.21, it was determined that the depth growth rates were similar to the surface, with the surface growth rates exceeding the depth grow rate for the cracks located above the disbondment’s Open Mouth. This was rationalized to be an effect of crack closure effects and a steep hydrogen concentration gradient in the specimens’ thickness. A simulated hydrostatic test was also performed on the cracks to determine whether the test would retard or accelerate growth rates, with the results indicating that the plastic damage caused by hydrostatic testing could lead to either situation depending on the subsequent applied loads.

  • Subjects / Keywords
  • Graduation date
    Fall 2017
  • Type of Item
  • Degree
    Master of Science
  • 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.
  • Language
  • Institution
    University of Alberta
  • Degree level
  • Department
  • Specialization
    • Materials Engineering
  • Supervisor / co-supervisor and their department(s)
  • Examining committee members and their departments
    • Ivey, Douglas (Chemical and Materials Engineering)
    • Luo, Jingli (Chemical and Materials Engineering)
    • Chung, Hyun-Joong (Chemical and Materials Engineering)
    • Chen, Weixing (Chemical and Materials Engineering)