Browsing by Subject "electrochemical"
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Item Development of Electrochemical Micro Machining(Texas A&M University, 2008-10-10) Srinivas Sundarram, SriharshaThe machining of materials on micrometer and sub-micrometer scale is considered the technology of the future. The current techniques for micro manufacturing mostly are silicon based. These manufacturing techniques are not suitable for use in demanding applications like aerospace and biomedical industries. Micro electrochemical machining (?ECM) removes material while holding micron tolerances and ?ECM can machine hard metals and alloys. This study aims at developing a novel ?ECM utilizing high frequency voltage pulses and closed loop control. Stainless steel SS-316L and copper alloy CA-173 were chosen as the workpiece materials. A model was developed for material removal rate. The research studied the effect of various parameters such as voltage, frequency, pulse ON/OFF time, and delay between pulses of the stepper motor on the machined profiles. Experimental data on small drilled holes agreed with theoretical models within 10%. Micro burrs can be effectively removed by optimal ?ECM. A sacrificial layer helped to improve the hole profile since it reduced 43% of corner rounding.Item Electrochemical characterization and time-variant structural reliability assessment of post-tensioned, segmental concrete bridges(2010-07-14) Pillai Gopalakrishnan, RadhakrisIn post-tensioned (PT) bridges, prestressing steel tendons are the major load carrying components. These tendons consist of strands, ducts, and cementitious grout that fill the interstitial space between the strands and ducts. However, inspections on PT bridges have reported the presence of voids, moisture, and chlorides inside grouted ducts as the major cause of accelerated corrosion of strands. Corrosion of the strands has resulted in PT bridge failures in Europe and tendon failures in the United States. As most of the PT bridges have high importance measures and the consequences of failure are significant, it is important to maintain high levels of safety and serviceability for these bridges. To meet this goal, bridge management authorities are in dire need of tools to quantify the long-term performance of these bridges. Time-variant structural reliability models can be useful tools to quantify the long-term performance of PT bridges. This doctoral dissertation presents the following results obtained from a comprehensive experimental and analytical program on the performance of PT bridges. 1) Electrochemical characteristics of PT systems 2) Probabilistic models for tension capacity of PT strands and wires exposed to various void and environmental conditions 3) Time-variant structural reliability models (based on bending moment and stress limit states) for PT bridges 4) Time-variant strength and service reliabilities of a typical PT bridge experiencing HS20 and HL93 loading conditions and different exposure conditions for a period of 75 years The experimental program included exposure of strand specimens to wet-dry and continuous-atmospheric conditions. These strand specimens were fabricated to mimic void and/or grout-air-strand (GAS) conditions inside the tendons. It was found that the GAS interface plays a major role in strand corrosion. The GAS interfaces that are typically located in the anchorage zones of harped PT girders or vertical PT columns can cause aggressive strand corrosion. At these locations, if voids are present and the environment is relatively dry, then limited corrosion of the strands occurs. However, if the presence of high relative humidity or uncontaminated and chloride-contaminated water exists at these interfaces, then corrosion activity can be high. The strands were exposed for a period of 12, 16, and 21 months, after which the remaining tension capacity was determined. The analytical program included the development of probabilistic strand capacity models (based on the experimental data) and the structural reliability models. The timevariant tension capacity predicted using the developed probabilistic models were reasonably consistent with the tendon failures observed in PT bridges in Florida and Virginia. The strength reliability model was developed based on the moment capacity and demand at midspan. Service reliability model was developed based on the allowable and applied stresses at midspan. Using these models, the time-variant strength and service reliabilities of a typical PT bridge were determined based on a set of pre-defined constant and random parameters representing void, material, exposure, prestress, structural loading, and other conditions. The strength and service reliabilities of PT bridges exposed to aggressive environmental conditions can drop below the recommended values at relatively young ages. In addition, under similar conditions the service reliability drops at a faster rate than the strength reliability.Item Reduction of Perchlorate and Nitrate by Aluminum Activated by pH Change and Electrochemically Induced Pitting Corrosion.(2011-08-08) Raut Desai, Aditya B.Highly oxidized species like perchlorate and nitrate that are released into the environment by anthropogenic activities are a source of concern as they have been known to contaminate groundwater. These species are extremely soluble in water and can migrate through aquifer systems, travelling substantial distances from the original site of contamination. Due to their high solubility, these oxy-anions cannot be treated using conventional treatment processes like filtration and sedimentation. Several treatment technologies are currently available to abate the human health risk due to exposure to perchlorate and nitrate. However, most of the existing treatment processes are expensive or have limitations, like generation of brines with high concentrations of perchlorate or nitrate. Aluminum can effectively reduce perchlorate and nitrate, if the protective oxide film that separates the thermodynamically reactive Al0 from most environments is removed. Aluminum was activated by pH change and electrochemically induced, pitting corrosion to remove the passivating oxide layer and expose the underlying, thermodynamically reactive, zero-valent aluminum. A partially oxidized species of aluminum, like monovalent aluminum, is believed to bring about the reduction of perchlorate and nitrate. This research studied the reduction of perchlorate and nitrate by aluminum that was activated by these two mechanisms. Results indicated that aluminum activated by pH change resulted in an instantaneous decrease in perchlorate concentration without any increase in chlorate or chloride concentrations, which suggests that the perchlorate might be adsorbed on the aluminum oxide surface. However, aluminum activated by electrochemically induced pitting corrosion can effectively reduce perchlorate to chlorate. Nitrate, on the other hand, was reduced completely to ammonia by both treatment mechanisms. The studies conducted in this dissertation suggest that aluminum can be effectively used as a reducing agent to develop a treatment process to reduce perchlorate and nitrate.