E-Thesis 151 views 49 downloads
Influence of Copper (Cu) and Tungsten (W) additions on the passivity and breakdown of Super Duplex Stainless Steels in chloride-containing environments / JOSHUA AGBESEYI
Swansea University Author: JOSHUA AGBESEYI
Abstract
This thesis has been produced as a result of academic (Swansea University) and Industrial (Langley Alloys Ltd.) collaboration, facilitated through Swansea University’s M2A organisation under the Masters by Research (MSc) scheme of study. The scope of this investigation was set in the hopes of contri...
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Swansea, Wales, UK
2023
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| Institution: | Swansea University |
| Degree level: | Master of Research |
| Degree name: | MSc by Research |
| Supervisor: | Williams, Geraint. and Wint, Natalie. |
| URI: | https://cronfa.swan.ac.uk/Record/cronfa63723 |
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| Abstract: |
This thesis has been produced as a result of academic (Swansea University) and Industrial (Langley Alloys Ltd.) collaboration, facilitated through Swansea University’s M2A organisation under the Masters by Research (MSc) scheme of study. The scope of this investigation was set in the hopes of contributing to the current collective understanding and literature surrounding the localised electrochemical breakdown activity in Super Duplex Stainless Steels (SDSS). With specific consideration being given to highlighting the influence of Copper and Tungsten alloying additions on SDSS’s observed passivation and breakdown tendencies, when immersed and anodically polarised in a range of acidic & alkaline chloride media. This would, ideally, give an idea into how said additions impact and dictate the consequential states of nobility in the dual-phasic microstructure and bulk-alloy. In pursuit of these aims, it was the scope of this study to electrochemically investigate 3 different high Cr-content (25wt%) SDSS alloys of identical composition (excluding Cu- and W wt%) in various chloride-containing aqueous media. Consequently this would help ascertain their strengths/weaknesses in given conditions, with specific consideration to their addition-differences which would then be attributed to observed differences corrosion features with specific element additions. The alloys investigated in this study were UNS-S3550 (Ferralium® 255) UNS-S3750 (SAF 2507) and UNS-S3760 (Zeron®100). Under external anodic-polarisation these alloys were tested in various deoxygenated electrolyte solutions of 0.6M NaCl(aq) at a maintained ambient temperature of 25°C, in both alkaline and acidic conditions ranging from pH 3 to 11. Adjustment of pH was achieved via drop-wise addition of NaOH and HCl. PP and SVET analysis was employed to help quantify, map and explain the bulk & localised ionic-transport behaviour (respectively), both under open-circuit and non-polarised conditions. Findings from bulk anodic current characterisation using potentiodynamic polarisation were in good agreeance with observations made in a previous iteration of this study. That is to say it was found that in these conditions, anodic current increases typically associated with the phenomena of passive layer (PL) breakdown were mainly a result of transpassive dissolution kinetics, spotted with instances of localised pitting. Post-corrosion microscopy showed the anodic dissolution to unfold as a surface-wide, selective etching of constituent phases, typically attacking the less noble ferrite phase. This was later further supplemented with localised in-situ SVET scanning, showing the corrosive damage to evolve as an anodic “front”, steadily progressing across the active region of the corroding surface until stoppage of polarisation. Hysteresis loop data from potentiodynamic polarisation (PP) sweeps demonstrated a clear benefit of Cu-additions in mitigation of pitting damage in both acidic and alkaline chloride media. W-additions were also found to be beneficial for suppression of corrosion damage when compared to non-Cu/W containing grades (UNS-S3750). |
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| Keywords: |
Corrosion, Super Duplex Stainless Steel, Tungsten, Copper, Chloride, SVET, Potentiodynamic Polarisation |
| College: |
Faculty of Science and Engineering |