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http://hdl.handle.net/11375/27314
Title: | Influence of Fine-scale Niobium Carbonitride Precipitates on Hydrogen-Induced Cracking of X70 Pipeline Steel |
Authors: | Wojnas, Caroline Theresa |
Advisor: | Kish, Joey McDermid, Joseph |
Department: | Materials Science and Engineering |
Keywords: | Corrosion resistance;Hydrogen-induced cracking (HIC);High strength low alloy (HSLA);Nb content |
Publication Date: | 2021 |
Abstract: | The microstructure of steel is well known to affect hydrogen-induced cracking (HIC) susceptibility by having certain heterogeneities serving as effective hydrogen trap sites. A consensus on whether or not fine-scale niobium carbide (NbC), nitride (NbN) and carbonitride (Nb(C,N)) precipitates can behave as effective hydrogen traps has yet to be established. The H-trapping capacity of Nb precipitates in a Fe-C-Mn-Nb model steel was investigated with the goal of minimizing embrittlement effects and improving the design of X70 pipeline grade steel for sour service oil and gas applications. First, a heat treatment was applied to the model steel to change the Nb-based precipitate size distribution, which was subsequently characterized via transmission electron microscopy, electron energy loss spectroscopy, and atom probe tomography. The experimental heat treatment increased the number of fine-scale precipitates (<15 nm) that are ideal for APT characterization. NbN and NbC precipitates of various stoichiometries were confirmed within the steel. Further, a custom electrolytic H-charging device was designed, fabricated, and validated using thermal desorption spectroscopy. Additionally, the extent of galvanic corrosion between NbC and NbN and the steel matrix was determined using custom scaled-up particle matrix specimens. Potentiodynamic polarizations conducted using active and passivating electrolytes revealed the relative nobility of the materials. Both NbC and NbN particles were more noble than the steel matrix; thus, possessing driving force for galvanic corrosion, with the particles serving as cathodes. Future studies involving electrolytic charging of the steel in a D-based electrolyte coupled with atom probe tomography will facilitate the direct observation of H-trapping sites relative to various Nb-based precipitates and contribute to an improved understanding of the mechanisms governing HIC. |
URI: | http://hdl.handle.net/11375/27314 |
Appears in Collections: | Open Access Dissertations and Theses |
Files in This Item:
File | Description | Size | Format | |
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Wojnas_Caroline_T_2021 December_MASc in Materials Science and Engineering.pdf | 11.31 MB | Adobe PDF | View/Open |
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