E-Thesis 54 views 20 downloads
Super Duplex Stainless Steels: Influence of Copper & Tungsten on the Passivity and Corrosion Resistance / CHARLOTTE HOLLAMBY
Swansea University Author: CHARLOTTE HOLLAMBY
Abstract
This thesis has investigated the corrosion behaviour of three Super Duplex Stainless Steelgrades, FERR (UNS 32250), SAF (UNS S32750) and ZER (UNS S32760), with focus on theinfluence of copper (Cu) and tungsten (W) content on pitting and crevice corrosion. In addition,the influence of surface roughne...
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Swansea University, Wales, UK
2024
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| Institution: | Swansea University |
| Degree level: | Master of Research |
| Degree name: | MRes |
| Supervisor: | Sullivan, J. |
| URI: | https://cronfa.swan.ac.uk/Record/cronfa66845 |
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| Abstract: |
This thesis has investigated the corrosion behaviour of three Super Duplex Stainless Steelgrades, FERR (UNS 32250), SAF (UNS S32750) and ZER (UNS S32760), with focus on theinfluence of copper (Cu) and tungsten (W) content on pitting and crevice corrosion. In addition,the influence of surface roughness and deaeration of the environment was explored. Theexperiments involved potentiostatic polarisation techniques and microscopy to quantify thefrequency, area and depth of dissolution observed. Two mass loss models, Faraday’s Law & PitGeometry, were also implemented to further quantity the corrosion seen.The samples were exposed to a 3.5% w/v solution and heated from 65 °C to 85 °C at aramp rate of 40 °C/hour. The Critical Pitting Temperatures (CPT) showed a minimal 4 °Cdifference between the highest, of additional W content, ZER and the lowest, of low Cu content,SAF. FERR, which has increased Cu content, displayed substantial pit frequency, aligning withthe high recorded currents. The presence of metastable pitting was observed, contributing to pitrepassivation and lower pitting corrosion in SAF and ZER. Despite expectations, additional Win ZER did not produce significantly improved results. The mass loss calculations correlatedwell with potentiostatic results although several factors like lacy pits and pit shape variabilityimpacted accuracy.Samples were used from previous research that were ground to a smoother surface finish andexposed to a 3.5% w/v solution. A deaerated environment using a nitrogen inlet wascreated, and the samples heated from 65 °C to 85 °C at a ramp rate of 30 °C/hour. FERR andZER demonstrated improved performance in the deaerated conditions, attributed to smootherfinish reducing pit initiation sites, and decreased cathodic reactions in oxygen depletedenvironments. SAF exhibited contrasting results with increased dissolution observed.The samples were exposed to 3.5% w/v solution at temperatures of 85 °C, 75 °C and 65°C, utilising an o-ring to change the preferred method of corrosion from pitting to crevice. At 85°C, all grades exhibited substantial dissolution due to surpassing Critical Crevice Temperatures(CCT). FERR and ZER showed similar crevice depths, implying no significant impact of Cuor W additions. At 75 °C, a reduction in current and crevice depths were seen, with somesamples showing incomplete crevice formation around the circumference. At 65 °C, corrosionrates declined significantly, with some areas showing no crevice formation. FERR and ZERagain showed similar performance. SAF exhibited stable performance over the temperaturerange with potential for a broad range of temperature applications.Overall, this research highlighted insights into the complex relationship between composition,temperature and other factors of corrosion behaviour. While Cu additions did not show asignificant impact in aerated conditions, it showed much improvement in deaeratedenvironments. The addition of W had a more influential role in aerated conditions, althoughboth Cu and W additions performed similarly under the crevice corrosion mechanism. Thelower alloyed SAF seemed to have irregular behaviour across all experiments apart from crevicecorrosion. Additionally, a smoother surface finish and absence of oxygen became a crucialfactor for influencing corrosion rates |
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| Keywords: |
Materials, stainless steel, copper, tungsten, electrochemical |
| College: |
Faculty of Science and Engineering |
| Funders: |
Langley Alloys Ltd |