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Aluminum is an attractive metal for use as an anode in the cathodic protection of steels in seawater due to its low cost and high current capacity. Zinc is often used for its ability to readily corrode, but it has a low current capacity and it operates at very negative voltages, leading to hydrogen generation at the steel cathode, which may cause hydrogen embrittlement. Aluminum can operate at less-negative voltages, therefore reducing hydrogen generation, but it forms a passive oxide film, preventing the anode from corroding. Ga is added to aluminum in small amounts (0.1 wt%) to destabilize this oxide film and allow for active corrosion. The mechanism of how Ga activates Al is still not well-known, though there are prevailing proposals. A previous study noted a difference in behavior between Al-Ga master heats and the alloys that were later produced by re-melting them. This study is focused on characterizing the corrosion behavior of Al-0.1 wt% Ga in synthetic seawater, with samples from a master heat and two subsequent remelts. Galvanostatic, potentiostatic, and open-circuit tests were run, as well as galvanic coupling with 1123 steel. It was found that the remelted anodes behaved more consistently and maintained stable corrosion behavior for longer times than the master heat. X-ray Photoelectron Spectroscopy analysis showed elevated concentrations of Ga inside the oxide layer. The findings support the mechanism in the literature of discrete particles of Ga forming under the oxide film but do not support the mechanism of an amalgam layer formation. This project was funded by NACE International, Virginia Tech project number 457789. Master of Science

The ultimate goal of this research was to gain a more fundamental understanding of the effects of “active"? alloying elements on the performance of low voltage, aluminum, sacrificial anodes. We have developed an overview of elemental trends and a comparison with literature, in support of a larger program on predicting anode behavior. The broader impact of this work was to enhance the ability to control corrosion and reduce the likelihood of hydrogen embrittlement induced by cathodic protection on ships and marine structures. As compared to high voltage anodes, low voltage, aluminum, sacrificial anodes reduce the likelihood of causing hydrogen embrittlement or stress corrosion cracking when used to protect high strength steels. In this study, open circuit potential, potentiostatic, galvanostatic and Tafel tests were performed on eleven high-purity aluminum-based binary and ternary alloys containing Bi, Ga, In and Zn in an effort to understand the individual effects of each element and the interactions between these elements. The microstructures of the as-cast alloys and the corrosion surfaces after testing were characterized using electron microscopy. Current capacities and current capacity efficiencies were calculated from potentiostatic and galvanostatic data. Galvanic coupling data from Druschitz et al was plotted with average values from potentiostatic and galvanostatic tests on Tafel curves for all alloys. [1] Variance of weight loss, average galvanostatic current, and average potentiostatic potential of the Al-0.57 wt% Zn-0.55 wt% Bi alloy was evaluated. Indium and gallium had the most significant effect on corrosion behavior (per wt% added), followed by zinc and bismuth. Bismuth had only a small effect on the weight loss, galvanostatic current and potentiostatic potential. However during potentiostatic testing Al-Bi alloys showed a steady increase in current with time, indicating that larger effects may be seen at longer periods of time. In Al-Zn alloys preferential dissolution of the zinc-rich interdendritic regions was observed. The Al-5.3 wt% Zn alloy showed high current values, but also exhibited high weight loss and more adherent corrosion products. Interdendritic corrosion also occurred with the Al-5.3 Zn-0.011 In alloy. Also, non-uniform dissolution of the remaining primary aluminum dendrites by the formation of small holes was observed, possibly due to indium precipitates. Grain boundary attack and severe intra-granular pitting was observed in Al-In alloys. Small holes were also evident on the surface of pits, similar to those seen on dendrites with the Al-5.3 Zn-0.011 In alloy. The addition of Indium greatly shifted voltages to more negative values (-0.802 to -0.858 VSCE at 9 A/m²) and significantly increased the observed currents (42-83 A/m² at -0.730 VSCE). High potentiostatic current capacities were exhibited by Al-In alloys, Al-0.1 wt% Ga, Al-5.3 wt% Zn-0.011 wt% In, and Al-0.57 wt% Zn-0.55 wt% Bi. However some calculated current capacity values were actually above the theoretical values, possibly due to corrosion products affecting the weight loss measurements. Master of Science

We have created a three-dimensional, implicit finite difference model that can accurately calculate temperatures within the bulk of a sample during a friction stir fabrication process. The model was written in Wolfram Mathematica® 7 for Students, and allows for time-efficient calculation of thermal profiles. The non-dimensionality of the model allows for accurate refinement of the temporospatial mesh, and provides portability across material types. The model provides insight as to the mechanism of heat generation by qualifying the fraction of mechanical energy converted to thermal energy for different material types and sample geometries. Finally, our model gives an understanding of the effects of the heat transfer at the boundaries of the workpiece and suggests a backside heat loss localized at the center of the tool due to a decrease in thermal contact resistance. We have explored the effects of processing parameters on the performance of the friction stir fabrication process. The process has four stages; tool insertion, warm-up, bead formation, and steady-state translation. The tool insertion phase is characterized by a rapid increase in system horsepower requirements. During the warm-up phase, the mechanical energy of the rotating tip is converted to thermal energy. Once enough thermal energy has been transferred to the workpiece, the volume between the tip and the workpiece is filled by feedstock material. Finally, the tool is translated under relatively steady-state conditions. The success or failure of the process is dependent on adequate material delivery to the system. The horsepower requirements of the process depend on the material type and the rate of material delivery. We have explored the effect of processing parameters on the microstructure of the processed samples. Optical microscopy shows that the stratification of layers within the weld and the depth of the weld are both dependent on the processing parameters. EBSD analysis coupled with Vicker's microhardness measurements of the processed pieces show that the grain size within the weld nugget is constant over the range of processing parameters available to the system. Data also show that pressure and heat inherent in friction stir processing of strain-hardened Al5083 counteract strengthening of the temper of the alloy. Ph. D.

Nanometer thick layers of clay and polymer were formed on mica, silicon, and aluminum 2024-T3 alloy using alternating solutions of positively and negatively charged polymer and clay, respectively. Atomic force microscopy was used to observe the morphology of the composite films on mica and silicon. It was found that solution concentrations of clay above 0.02 weight percent lead to the uncontrolled deposition of clay platelets on the substrate's surface. By using solution concentrations of clay above 0.02 weight percent and ultrasonic agitation together it is possible to deposit a uniform monolayer of clay platelets on a mica substrate in £ 20 seconds. Ultrasonic agitation also produced crude patterns of montmorillonite platelets. Thin films of poly(diallydimethylammonium chloride) (PDDA) were made using concentrations ³ 2 weight percent of PDDA. It was found that the PDDA formed several unusual morphologies. Spherulites of PDDA were observed with AFM and the glass transition temperature of high molecular weight PDDA was measured using differential scanning calorimetry (DSC). Circular regions of positive charge were discovered on silicon wafers provided by three different sources. These areas of charge have never been reported in literature, but can easily be detected by placing wafers into solutions containing negatively or positively charged solutions of clay or polymer, respectively. The exact nature of these charged regions is unknown, but it is hypothesized that impurities on silicon wafers create the circular regions of positive charge. ISAM films made of a polyamide salt and a synthetic clay, Laponite RD®, demonstrated significant corrosion resistance on 2024-T3 Al alloys after 168 hours of salt spray testing. The ISAM films offered corrosion protection only if there was a significant layer of underlying surface oxide present, however. It was found that ISAM deposited films of polyarylic acid (PAA) and polyallylamine hydrochloride (PAH) may offer some corrosion resistance on 2024-T3 Al alloys, but these films' corrosion resistance is severely hampered by the presence of Cl- in the PAH solution. Funding from this project was gratefully received from the Materials Science and Engineering Department at Virginia Tech; Luna Innovations Inc; the American Chemical Society / Petroleum Research Fund #34412-G5 and the Environmental Protection Agency Contract #68-D-00-244. Master of Science