Fatigue of X65 steel in the sour corrosive environment—A novel experimentation and analysis method for predicting fatigue crack initiation life from corrosion pits

Farhad, Farnoosh, Smyth-Boyle, David and Zhang, Xiang (2021) Fatigue of X65 steel in the sour corrosive environment—A novel experimentation and analysis method for predicting fatigue crack initiation life from corrosion pits. Fatigue and Fracture of Engineering Materials and Structures, 44 (5). pp. 1195-1208. ISSN 8756-758X

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Abstract Oil and gas pipelines manufactured from API-5L Grade X65 steel are generally subjected to cyclic loading and their internal surfaces are frequently exposed to corrosive sour fluids. Exposure of pipelines to these environments often leads to localized corrosion (pitting) and decreased fatigue life. Corrosion pits are geometrical discontinuities that may promote fatigue cracking by acting as stress raisers. In order to optimize asset inspection and repair scheduling, it is important to understand the fatigue behavior of X65 steel and in particular, the ability to predict the crack initiation from corrosion pit. To achieve this level of understanding, conducting fatigue tests in an environmental condition replicating the field environment is important. This paper presents the test protocol and results of environmental fatigue testing using bespoke laboratory apparatus to undertake in situ corrosion fatigue tests in a sour corrosive environment under uniaxial loading. The environment selected represent processes that are likely to occur at internal surfaces of oil and gas pipelines exposed to production fluids. The tests were carried out on smooth samples to obtain S-N curve in this specific environment as well as on pre-pitted samples. An electrochemical method is used to create corrosion pits on the samples. Also, a model is proposed to predict the crack initiation life from corrosion pit, using a local stress-based technique, which has been validated by experimental test results. Post-test fractography was carried out by scanning electron microscopy (SEM). The performance of our approach is demonstrated. The innovation is anticipated to encourage other workers to employ similar small-scale tests requiring toxic and challenging test environments.

Item Type: Article
Additional Information: Funding information: The first author would like to thank BP, TWI Ltd., and Coventry University for the sponsorship of this project. The work was enabled through, and undertaken at, the National Structural Integrity Research Centre (NSIRC), a postgraduate engineering facility for industry‐led research into structural integrity established and managed by TWI Ltd. through a network of both national and international Universities. We also acknowledge Dr. Abdul Khadar Syed for his help with SEM images.
Uncontrolled Keywords: environmental fatigue testing, fatigue crack initiation, life prediction, oil and gas pipelines, pitting corrosion, X65 steel
Subjects: F200 Materials Science
H200 Civil Engineering
H300 Mechanical Engineering
Department: Faculties > Engineering and Environment > Mechanical and Construction Engineering
Depositing User: Elena Carlaw
Date Deposited: 04 Feb 2021 09:56
Last Modified: 31 Jul 2021 15:46
URI: http://nrl.northumbria.ac.uk/id/eprint/45366

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