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http://hdl.handle.net/11375/29040
Title: | Corrosion and its Control of Stainless Steel in Hot Pressurized (Subcritical) Alkaline Water |
Authors: | Asare, Elliott |
Advisor: | Kish, Joey Zeng, Yimin |
Department: | Materials Science and Engineering |
Publication Date: | 2023 |
Abstract: | The use of hydrothermal liquefaction (HTL) to convert wet biomass and organic biowaste into marketable biofuel has gained significant traction in recent years. HTL is operated in harsh environments due to the presence of hot pressurized water medium and catalysts as well as inorganic and organic corrodents released during the conversion. Alloys considered for HTL core component construction require suitable corrosion and SCC resistance to withstand the HTL process conditions (250-374°C and 4-22 MPa). Austenitic stainless steels typically exhibit suitable corrosion resistance in hot pressurized (sub-critical) water but are costly and susceptible to SCC under certain conditions. Ferritic-martensitic steels, contrarily, are less costly alternatives with lower SCC susceptibility but are more prone to corrosion. Given the unpredictable nature of SCC, the hypotheses underpinning this research is that it may be more practical to lower the corrosion susceptibility of ferritic (Fe-Cr) alloys than to lower the SCC susceptibility of austenitic Fe-Cr-Ni alloys to support materials selection and design for HTL core component construction. Specific research objectives devised to address this hypothesis include: i) determine how effective conventional surface treatments (mechanical grinding, sandblasting, shot peening, and chemical pickling) applied to P91 ferritic-martensitic steel (Fe-9Cr-1Mo) and austenitic Type 304 stainless steel are in reducing corrosion in simulated HTL alkaline water, ii) determine how effective increasing the surface Cr content, either by increasing bulk Cr content of ferritic Fe-Cr alloys or applying a chromizing surface treatment, is in reducing corrosion in simulated HTL alkaline water, and iii) determine the relative SCC susceptibility of ferritic (Fe-Cr), with and without a chromized surface coating, and austenitic (Fe-Cr-Ni) stainless steels in simulated HTL alkaline water. Bulk immersion testing coupled with post-exposure surface analyses by electron microscopy revealed that corrosion proceeded with the formation of a protective double-layer oxide film, the degree of protection depended on the structure and composition of the inner layer. A barrier layer comprised of Cr2O3 was the most protective, followed by a Cr-rich (Fe,Cr)3O4 spinel, and then a Cr-lean (Fe,Cr)3O4 spinel was deemed the least protective. None of the surface treatments applied to either P91 ferritic-martensitic steel or Type 304 stainless steel reduced corrosion relative to the mechanical abrasion baseline. In contrast, increasing the Cr content (from 9 wt.% to 21 wt.%), albeit in Fe-Cr alloys, was particularly effective in reducing corrosion by sufficiently increasing the Cr content of the inner (Fe,Cr)3O4 barrier layer to an extent required for improved protection. However, the lowest corrosion coincided with the formation of a Cr2O3 barrier, as exhibited by Alloy 33 (Fe-33Cr-32Ni) and chromized Type 409 stainless steel. The point defect model (and closely associated mixed conduction model) for oxide growth was used to account for the differences in corrosion observed. A similar approach applied to U-bend SCC samples revealed a susceptibility but only for austenitic stainless steel under certain conditions: i) significant cold work and associated deformation band formation, ii) Ni enrichment at the oxide/metal interface, and iii) S and Cl incorporation into the inner barrier layer. A slip dissolution-type model was used to account for the differences in SCC susceptibility observed. |
URI: | http://hdl.handle.net/11375/29040 |
Appears in Collections: | Open Access Dissertations and Theses |
Files in This Item:
File | Description | Size | Format | |
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Asare_Elliott_K_finalsubmissionSep2023_PhD.pdf | 12.19 MB | Adobe PDF | View/Open |
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