Super Duplex Stainless Steels: Influence of Copper & Tungsten on the Passivity and Corrosion Resistance

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 rate

Effect of electropolymerisation conditions on the permeability of polyphenol films deposited on a vitreous carbon electrode

Polymeric films were prepared from alkaline (NaOH) phenol aqueous solutions on a vitreous carbon (VC) electrode by potentiostatic or galvanostatic electro-oxidation. Permeation through such films was studied by rotating-disk electrode using the ferricyanide redox couple, and by cyclic voltammetry using phenate ions. The influence of the electropolymerisation controllable parameters such as NaOH and phenol concentrations, potential or current applied, electrosynthesis time, temperature and hydrodynamic conditions (electrode rotation + solution magnetic stirring) on the permeability of these polymeric films was examined. Conditions for the removal of phenol by electropolymerisation are discussed on the basis of the permeability of polyphenol films obtained by electrosynthesis. Permeable films were formed for a concentration of free hydroxyl anion larger of 0.1 M. An increase of the temperature to 85°C favours the formation of highly permeable films, thus avoiding electrode fouling

Compositional Algorithms for Succinct Safety Games

We study the synthesis of circuits for succinct safety specifications given in the AIG format. We show how AIG safety specifications can be decomposed automatically into sub specifications. Then we propose symbolic compositional algorithms to solve the synthesis problem compositionally starting for the sub-specifications. We have evaluated the compositional algorithms on a set of benchmarks including those proposed for the first synthesis competition organised in 2014 by the Synthesis Workshop affiliated to the CAV conference. We show that a large number of benchmarks can be decomposed automatically and solved more efficiently with the compositional algorithms that we propose in this paper.Comment: In Proceedings SYNT 2015, arXiv:1602.0078

Coreference-Based Summarization and Question Answering: a Case for High Precision Anaphor Resolution

Approaches to Text Summarization and Question Answering are known to benefit from the availability of coreference information. Based on an analysis of its contributions, a more detailed look at coreference processing for these applications will be proposed: it should be considered as a task of anaphor resolution rather than coreference resolution. It will be further argued that high precision approaches to anaphor resolution optimally match the specific requirements. Three such approaches will be described and empirically evaluated, and the implications for Text Summarization and Question Answering will be discussed

Error Control of Iterative Linear Solvers for Integrated Groundwater Models

An open problem that arises when using modern iterative linear solvers, such as the preconditioned conjugate gradient (PCG) method or Generalized Minimum RESidual method (GMRES) is how to choose the residual tolerance in the linear solver to be consistent with the tolerance on the solution error. This problem is especially acute for integrated groundwater models which are implicitly coupled to another model, such as surface water models, and resolve both multiple scales of flow and temporal interaction terms, giving rise to linear systems with variable scaling. This article uses the theory of 'forward error bound estimation' to show how rescaling the linear system affects the correspondence between the residual error in the preconditioned linear system and the solution error. Using examples of linear systems from models developed using the USGS GSFLOW package and the California State Department of Water Resources' Integrated Water Flow Model (IWFM), we observe that this error bound guides the choice of a practical measure for controlling the error in rescaled linear systems. It is found that forward error can be controlled in preconditioned GMRES by rescaling the linear system and normalizing the stopping tolerance. We implemented a preconditioned GMRES algorithm and benchmarked it against the Successive-Over-Relaxation (SOR) method. Improved error control reduces redundant iterations in the GMRES algorithm and results in overall simulation speedups as large as 7.7x. This research is expected to broadly impact groundwater modelers through the demonstration of a practical approach for setting the residual tolerance in line with the solution error tolerance.Comment: 13 pages and 1 figur