Background: Pulsed amperometric detection is a relatively new method for detection of ions and especially non-electrolytes such as carbohydrates in aqueous solutions. Pulsed amperometric detection relies on a redox reaction while electrochemical impedance simply measures the real and capacitive resistant of the solution. There is a correlation between the real impedance of a solution and the ionic strength of the solution. Method: This work explores measuring real impedance of pure water as a function of temperature from 25.0 to 60.0 °C to determine the relationship between impedance and temperature. Maintaining temperature at 25.0 °C, solutions of sodium chloride, potassium carbonate, sodium sulfate acetate and bicarbonate have been measured using impedance spectroscopy. Results: Regression analysis shows that measuring anions using impedance spectroscopy and simple stainless steel cylinders that detection limits at the parts per trillion (ppt) level are possible. There was no statistical difference when comparing impedance values of the same concentration of acetate and chloride in solution, showing real impedance is not dependent on ion size. However, ions with higher charge do result in lower impedance measurements. Conclusions: This work establishes the use of simple, small, robust stainless steel cylinders and impedance measurements following separation for the identification and quantification of ions in solution.

A dissertation submitted in fulfilment of the requirement for the degree of Master of Science in Engineering, to the Faculty of Engineering and the Built Environment, School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, 2018 / The two hydrofluoric acid (HF) storage tanks used for holding 70% technical grade HF product at the HF plant at Necsa started leaking in March 2012. An evaluation of the failure was conducted in the form of a corrosion failure analysis. It was confirmed that a higher than usual nitric acid (HNO3) content in the technical grade HF stream changed the corrosion mechanisms typically experienced within the HF storage vessels, which then caused the tanks to fail. Immersion type corrosion experiments were done to safely simulate the corrosive environment experienced by the mild steel, stainless steel and nickel alloys used on site, and to predict the change in corrosion rates and characteristics associated with the HNO3 contamination in the HF production plant circuit. Since the corrosion resistance of mild steel in HF is heavily dependent on the thickness of the protective scale on the steel, a series of planned interval corrosion tests (PICTs) was done to reproduce and then examine the oxidefluoride barrier on mild steel coupons in pure 70% HF prior to corrosion tests. These shorter PICTs were also done on the stainless steel and nickel alloys and showed that the prepassivation step had a surface cleaning effect when exposed for only 24 h. Eleven day corrosion tests were conducted to establish the effect of HNO3 concentration and temperature on mild steel corrosion in 70% HF, and to determine the change in corrosion rates and mechanisms associated with HNO3 contamination (0.1-1% HNO3) of the downstream HF products. The corrosion was characterized by analysing the corroded coupons for mass loss, apparent corrosion rates, acid consumptions, visual observations of scale formation and pits, as well as depth profiles from scanning electron microscopy and energy dispersive spectroscopy analyses. Linear relationships were frequently observed when analysing mass losses for the coupons over time, making it possible to define corrosion rates in terms of first order reaction kinetics. The harshest corrosive condition for mild steel in HF was determined to be 1% HNO3 in 70% HF at a constant temperature of 25ºC. The corrosion characteristics of alloys used in the HF plant, as affected by HNO3 impurities (in the range 50–10000 ppm) in the final HF acid product (70% Technical grade) were successfully established. Normalized SA516 Grade 70 mild steel and Monel 400 were found not adequate for use as construction materials in a plant where HNO3 contamination was >100 ppm. However, the corrosion resistance of SS 904 L was suitable under these conditions and was recommended for applications in HF solutions where the presence of an oxygen-containing acid (e.g. HNO3) is consistent. It was recommended that Alloy 31, Alloy 33 or Nirosta 4565S, with higher chromium content (>20 wt% Cr), should be considered for construction material of the HF plant when HNO3 contamination becomes unavoidable. However, if the continued use of mild steel at the plant cannot be avoided, other inhibition strategies tailored to the selective consumption of HNO3 in the HF product stream need to be investigated. / XL2019

In this study, the effects of stress and strain rate on cyclic deformation, secondary hardening, martensitic phase transformation, crack initiation, and fatigue behavior of type 304L stainless steel are examined. A series of load and strain controlled uniaxial zero and non-zero mean strain fatigue tests were conducted with varying frequencies in order to investigate the effect of loading rate on fatigue behavior. The volume fraction of martensite was quantified for several tests using x-ray diffraction and electron backscatter diffraction. The loading rates were found to have a direct effect on the microstructure and fatigue behavior of the alloy investigated. Adiabatic heating from an increased rate of loading was found to effect martensite formation which is a major contributor to the secondary hardening phenomena associated with many austenitic stainless steels under cyclic loading. Also affected by the microstructural changes were cyclic deformation, crack initiation, microstructurally small crack growth, and fatigue behavior.

No description available.

Over time science has provided us with new materials and fabrication techniques making it possible to design and create more complex engineering components for service. If we are to include these materials in damage tolerant design efforts, engineers need to understand when/where degradation will occur in the engineering component. To do so it is imperative that micromechanical studies be conducted to understand the material behavior of the microstructural features including phases, build pattern features, and microstructural imperfections including cracks of new materials to validate any future modeling efforts. This dissertation will discuss the experimental and computational micromechanics of extruded and cast aluminum cerium alloys and selective laser melted 316L stainless steel. In Chapters 2 and 3, micromechanical experiments and computational efforts are carried out on extruded 52:1 Al-8Ce-10Mg alloy. Using in-situ scanning electron microscopy tensile testing microcracking is observed in Al11Ce3 intermetallic after yield in the bulk alloy. In-situ digital image correlation tests observe the load sharing characteristics between the Al(Mg) matrix and the Al11Ce3 intermetallic before and after microcracking. Finally, that failure process is determined to be coalesce of microvoids leading to ductile damage failure. These results are used to create an experimental-computational framework to develop a crystal plasticity finite element model for extruded Al-8Ce-10Mg alloys. The calibrated model is used to perform multiple simulations evaluate the possible effect changes intermetallic content and grain orientation texture have on the mechanical strength of the alloy. The experimental and computational framework are expandable to other material systems not just Al-Ce alloys. In Chapter 4, in-situ scanning electron microscopy tensile testing is used to investigate how the matrix and intermetallic phases contribute to the failure behavior alloy of cast Al-11Ce- 0.4Mg alloy. The in-situ tests shows that after multiple points of crack nucleation, crack coalescence causes the subsequent failure to occur in the Al(Mg) matrix phase of the alloy, as seen by tortuous behavior. The cause of this crack behavior is determined to be due to the high strength match between the matrix and intermetallic phase, strong metallurgical bond between the two phases, and the size effect created by large eutectic colonies created during casting. The results of the experimental work are used to propose a 3D multiscale computational model of cast Al-Ce alloys. In Chapter 5, micromechanical experiments are carried out on SLM 316L Stainless Steel with four different sets of varied processing parameters. Discontinuous yielding is observed in the lowest energy density sample caused by the strong [110] texture, optimal for dislocation slip, in the loading direction. The in-situ loading experiments are also able to capture the melt pool track deformation and crack formation that leads to the failure of these samples. This highlights the importance of micromechanical experiments for additive manufactured materials. / Doctor of Philosophy / As time has progressed new materials have been discovered that make it possible to design more complex parts for engineering design. To ensure the safety and reliability of these materials, engineers need to understand when/where damage will occur in a design. Micromechanical studies conducted at magnifications higher than human visible range allow engineers to explore where damage in materials initiates which would otherwise not be detected until after failure. The results of these studies can be used to build and test models of these materials. This dissertation will discuss the micromechanical studies of extruded and cast aluminum cerium alloys and selective laser melted 316L stainless steel. In Chapters 2 and 3, micromechanical experiments and computational techniques are performed on extruded Al-Ce alloys. In Chapter 4, the failure behavior of cast Al-Ce alloys is examined in active tension using scanning electron microscopy. Finally, in Chapter 5, selective laser melted 316L stainless steel is studied and the results highlight the importance of micromechanical experiments for the new age of metal 3D printing.

The performance of a coupled viscoplastic model of FSW has been evaluated over a variety of tool RPMs and feed rates. Initial results suggested that further optimization of the material parameters and an additional ability to model the thermal recovery of the material would improve the overall performance of the model. Therefore, an experimental/numeric approach was taken to improve and quantitatively compare the performance of the model based upon the thermal profile of the workpiece. First, an experimental method for obtaining real-time temperature measurements during Friction Stir Processing (FSP) of 304L Stainless Steel was developed. The focus of the method was to ensure that the obtained temperatures were both accurate and repeatable. The method was then used to obtain thermal cycle data from nine welds, each at different operating conditions ranging in tool rotational speed from 300 to 500 RPMs and in feed rate from 0.85 to 2.54 mm/s (2 - 6 in/min). Then a family of nine numerical models was created, each model corresponding to one welding condition. The performance due to improved convergence stability and the added thermal recovery term are also discussed. A gradient following technique was used to optimization and iteratively adjust nine material parameters to minimize the difference between the numerical and experimental temperature for the whole family of models. The optimization decreased the squared error between the numerical and measured temperatures by 76%. Recommendations are also made that may allow the optimization method to return greater dividends.

Background: the process of water quality monitoring is a critical quality assurance process. However, conventional methods include monitoring by a team of divers going to different depths to collect water samples. This method is expensive, time-consuming, and can lead to adverse consequences like a fatality.  Objectives: This study aimed to develop a robust, autonomous prototype for water quality monitoring. The idea was to measure water quality at a depth of 20 meters by designing an unmanned underwater drone with sensors for measurement and tracking.  Methods: a detailed literature review was conducted and trend watching, tech watching, and benchmarking were some strategies used to extract design ideas. After creating a good scope, various design ideas were iterated to solve the problem. Two concept designs of the underwater drones were selected and evaluated by a discussion matrix. The design idea that best fit the project requirements was selected and produced. Stress analysis was performed on the final design to check design performance at a depth of 20 meters.  Results: Design Fish 2 was chosen as the final design of the prototype. The selected design is manufactured from S316 stainless steel which is the material used for the shell while the rest is PLA.  Conclusion: The prototype has a hollow, cylindrical body that provides low hydrodynamic resistance, and the vertical design provides better stability when moving underwater. / Bakgrund: processen för övervakning av vattenkvaliteten är en kritisk kvalitetssäkrings process. Men konventionella metoder inkluderar övervakning av ett team av dykare som går till olika djup för att samla in vattenprover. Denna metod är dyr, tidskrävande och kan leda till negativa konsekvenser som dödsfall.Mål: denna studie syftade till att utveckla en robust, autonom prototyp för övervakning av vattenkvalitet. Tanken var att mäta vattenkvaliteten på 20 meters djup genom att designa en obemannad undervattensdrönare med sensorer för mätning och spårning.  Metoder: en detaljerad litteraturgenomgång genomfördes och trendwatching, techwatching och benchmarking var några strategier som användes för att extrahera designidéer. Efter att ha skapat en bra omfattning, upprepades olika designidéer för att lösa problemet. Två konceptdesigner av undervattensdrönarna valdes ut och utvärderades av en diskussionsmatris. Den designidé som bäst passade projektkraven valdes ut och togs fram. Spänningsanalys utfördes på den slutliga designen för att kontrollera designprestanda på ett djup av 20 meter.Resultat: Design Fish 2 valdes som den slutliga designen av prototypen. Den valda designen är tillverkad av S316 rostfritt stål som är materialet som används för skalet medan resten är PLA.Slutsats: Prototypen har en ihålig, cylindrisk kropp som ger lågt hydrodynamiskt motstånd, och den vertikala designen ger bättre stabilitet vid förflyttning under vatten.

A new kind of wave power generator is being developed at KTH Royal Institute of Technology which potentially can reach an efficiency of 98 %. However, this generator’s small air gap sets strict requirements on the stiffness of the structure to withstand the large magnetic forces. The structure, therefore, need to be both stiff and non-magnetic. To tackle that problem austenitic stainless steel will be used. Then again, austenitic stainless steel tends to become slightly magnetic because of impurities and mechanical stress. The purpose of this report is to study the magnetic properties of the austenitic stainless steel and observe how mechanical stress can change their properties. Moreover, economic and environmental aspects considering the use and production of the steel are studied. Two experiments were applied to measure the magnetic properties, using an LCR-meter and an electrical circuit with a current amplifier. Both methods showed that mechanical stress will result in changing the magnetic property of austenitic stainless steel. Some steel types were less affected by the mechanical stress applied leading to the conclusion that they are more effective when placed near the generator’s air gap. Regarding sustainable development, it is uncertain to determine the impact the generator has on the environment, mainly because of the steel types manufacturing process is unknown. On the contrary, the maintenance costs of the generator are predicted to be low and if the prototype fulfills the efficiency expectations it will have a huge impact on the future of wave power technology. / En ny typ av vågkraftsgenerator utvecklas på KTH som potentiellt kan uppnå en verkningsgrad på 98%. Denna generators lilla luftgap ställer dock strikta krav på strukturens styvhet för att stå emot de stora magnetiska krafterna. Strukturen måste därför vara både styv och icke-magnetisk. För att ta itu med det problemet kommer austenitiskt rostfritt stål att användas. Sedan tenderar austenitiskt rostfritt stål att bli något magnetiskt på grund av föroreningar och mekanisk stress. Syftet med denna rapport är att studera austenitiskt rostfritt ståls magnetiska egenskaper och observera hur mekanisk stress kan förändra deras egenskaper. Dessutom studeras ekonomiska och miljömässiga aspekter som beaktar stålets användning och produktion. Två experiment utfördes för att mäta de magnetiska egenskaperna, med användning av en LCR-mätare och en elektrisk krets med en strömförstärkare. Båda metoderna visade att mekanisk stress kommer att leda till förändring av den magnetiska egenskapen hos austenitiskt rostfritt stål. Vissa ståltyper påverkades mindre av den mekaniska påfrestningen som ledde till slutsatsen att de är mer effektiva när de placeras nära generatorns luftgap. När det gäller hållbar utveckling är det osäkert att bestämma vilken påverkan generatorn har på miljön, främst på grund av att detrostfria stålets tillverkningsprocess är okänd. Tvärtom förmodas att underhållskostnaderna för generatorn komme vara låga och om prototypen uppfyller effektivitetsförväntningarna kommer det att ha en stor inverkan på framtiden för vågkrafttekniken.

Abbott Vascular encountered strength and variability issues when attempting to resistively weld 304 Stainless Steel to equiatomic Nitinol. Initial observations suggested that passivation layer (Cr2O3, TiO2) formation affected the weld interface. One hundred 304 Stainless Steel/Nitinol pairs were allowed to oxidize in air at room temperature for allowed periods of time (.1, 1, 3, 5, 7, 12, 16, 24, 168, and 336 hours). Each pair was welded resistively with constant current. A Miyachi/Unitek Advanced Data Analysis Monitor (ADAM) recorded the peak resistance at the instance the weld was made. Resistances were compared to Instron 5900 tensile maximum break load (KgF). Use of optical microscopy and Scanning Electron Microscopy (SEM) revealed microstructural reduction of void size at the sample fracture surface (1-.5 µm). Literature suggested the existence of metastable precipitate forms at near equiatomic compositions within the theoretical temperature range (261.9-1425.2 0C). The Instron 5900 mechanically validated presence of precipitates, while Electron Dispersive X-Ray Spectroscopy (EDS) confirmed the existence compositionally. Literature confirms B19’ precipitates size increases with temperature. This suggests higher resistance samples will promote growth of precipitates due to increased heat input. Increased average particle size was observed with increased resistance (0-.3 µm). Crystal lattice inconsistencies between Nitinol parent phase (B2) and B19’ promote premature fracture due to increased misfit dislocation density. Therefore increased weld resistance promotes the growth of incoherent Ti3Ni4 precipitates which inhibit load bearing capabilities, causing premature failure.

This thesis evaluates the ability of laser surface melting to mitigate chloride stress corrosion cracking (See) of type 304 stainless steel. The effects of laser surface melting on microstructure, mechanical state, and corrosion behavior were examined. The major effect of laser surface melting of 304 stainless steel was found to be the introduction of tensile residual stresses on the order of the yield strength in the surface of the laser-melted regions. Exposure of laser-melted coupons to boiling magnesium chloride at 154°C revealed that the residual stresses were sufficient to cause failure by see processes in the absence of an external load. It was concluded that unless measures could be found to eliminate or reverse the residual stresses introduced by the laser melting process, the technique is not viable for mitigating chloride see in these alloys. / Master of Science