Understanding the Effects of Hydrogen, Hydrostatic testing and Mill- scale on SCC of Pipelines in Near-Neutral pH Environment

  • Author / Creator
    Shirband, Zeynab
  • Near-neutral pH stress corrosion cracks (NNpH SCC) are developed under coating disbondments on the external surface of pipelines where the surface of pipeline is exposed to the soil solution. NNpH SCC initiation and early stage of growth is mainly governed by the dissolution process of steel. However, laboratory experiments on polished specimens rarely reproduced cracks in near-neutral pH environment whose growth rates were consistent with what has been observed on pipelines in the field. So, there have to be other factors involved in the crack initiation process which have been ignored in most of the crack initiation studies. The presence of mill-scale and application of occasional over-load cycles during hydrotesting on pipelines in the field can explain this discrepancy. Furthermore, hydrogen involvement in this type of cracking, during both crack initiation and growth, has been confirmed in several studies. However, the presence of disbonded coatings on the pipeline, which can cause a CO2 concentration gradient along the disbondment and may further change the environment and the amount of hydrogen ingress into the steel, has been ignored in all previous experiments. Therefore, these tests may not represent the true role of hydrogen in near-neutral pH SCC without considering disbonded coatings. This study aimed at understanding the contribution of these important factors to NNpH SCC. In this study, the role of coating disbondment on hydrogen ingress into pipeline steels and the effects of mill-scale and hydrotesting on NNpH SCC crack initiation are investigated. To study hydrogen ingress, a specially-designed Devanathan-Stachurski double cell was used for hydrogen permeation measurements. This double cell simulated coating disbondment on one side of the specimen and at the same time measured hydrogen permeation on the other side. Furthermore, a comparative study was developed and crack initiation in mill-scaled and polished specimens and also in samples that were undergone hydrotesting and those, on which no hydrotesting was applied, was compared. It was found that a simulated coating disbondment acts to change hydrogen ingress along the disbondment. The results showed that although corrosion rate was lower under disbonded coating at narrower gap sizes (2 and 5 mm) between pipeline steel and the coating, the highest level of hydrogen occurred at intermediate gap sizes (5 mm). Also, it was shown that contrary to general belief, increasing CO2 concentration in solution did not increase hydrogen ingress into pipelines. It was also found that the frequency of crack initiation occurrence from pits was increased by the presence of mill-scale and by the application of multiple hydrostatic testing stress cycles on pipelines. The results of this study showed that crack initiation experiments on polished specimens and without considering real pressure condition, to which pipelines are exposed in the field, underestimated short crack growth rates in the near-neutral pH environment.

  • Subjects / Keywords
  • Graduation date
    Fall 2016
  • 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.