Doctor of Philosophy
Shoesmith, David W.
Noël, James J.
Carbon steel is the material of choice for transmission pipelines for the oil and gas industry across the globe due to its mechanical properties, and low cost. Carbon steel is only resistffxidant, and the types of anions present in the exposure environment. Transmission pipelines are coated with non-conductive polymeric coatings and can be protected by cathodic protection in order to mitigate exterior corrosion. The primary goals of this thesis were to determine the role of anions, cathodic charging with hydrogen, and steel microstructure on different failure modes of carbon steel.
The corrosion mechanism of carbon steel in the presence of carbonate/bicarbonate and sulphide was investigated electrochemically and under corrosion conditions. The role of sulphide in chemically destroying protective iron oxides was investigated both electrochemically and under corrosion conditions. In near-neutral to slightly alkaline conditions, sulphide can chemically prevent the formation of iron oxides at [SH-] < 10-6 M [SH-] by reducing FeIII oxides to defective FeII species which subsequently dissolve. The ability of carbon steel to resist passive film breakdown was found to be primarily dependent on pH and [SH-].
The role of cathodic charging on accelerating anodic dissolution and interfering with passive film formation was investigated using hydrogen permeation and voltammetric experiments. A pearlitic and ferritic microstructure was found more susceptible to trapping than a dominantly ferritic microstructure containing bainite. As trapped hydrogen desorbs from the metallic lattice, atomic H is able to reduce FeIII to FeII and produce a highly defective surface oxide that led to accelerated anodic dissolution and a degraded passive film.
The role of microstructure on crack propagation was investigated by electron back scatter diffraction on micro compact tensile specimens. The material was determined to be compromised of primarily high energy high angle random grain boundaries which showed no preference for crack propagation. The crack propagated inter-granularly at grain boundaries between two ferrite grains and trans-granularly when encountering either pearlite in air and corrosion experiments and possibly when encountering pits during corrosion experiments.
Summary for Lay Audience
The direct cost of corrosion accounts for roughly 3% of the world’s gross domestic product. Corrosion within the oil and gas industrial sector poses added risks to the environment and to people. This sector currently utilizes multiple approaches to mitigate external corrosion, such as the application of protective coatings. However, it is possible for corrosion coatings to fail which can lead to a wide variety of failure mechanisms. Should the external corrosion protective layer fail, steel can produce a naturally occurring protective oxide layer. However, the exposure environment at the exposed steel surface will dictate the nature of the corrosion process and, if sufficiently aggressive, destroy any passive oxide. Two species that can be extremely detrimental to structural integrity are sulphides, produced by microbial corrosion, and atomic hydrogen, produced by the reduction of water. This thesis investigates the role of sulphide and atomic hydrogen in the degradation of surface oxides and the acceleration of steel corrosion.
Goldman, Maxwell, "Electrochemical Investigation into the External Failure Modes of Oil and Gas Pipelines" (2019). Electronic Thesis and Dissertation Repository. 6709.