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The EPA Lead and Copper Rule set Action Limits for lead and copper concentrations in potable water, but accelerated corrosion of lead in potable water systems due to a galvanic connection to copper remains a significant health risk to consumers. In addition to elevated lead release due to galvanic corrosion of lead-tin solder and leaded brass fixtures, partial lead service line replacements with copper pipe present long-term health concerns. Prior research has demonstrated that the effects of galvanic corrosion can be controlled by water chemistry, and the interplay between alkalinity, natural organic matter (NOM), and orthophosphate (added as corrosion inhibitor) may have a significant influence on corrosion of common lead plumbing materials. Results of bench-scale experiments demonstrate that in some waters galvanic corrosion can multiply lead release from lead pipe by up to 60 times, but other waters curtail the galvanic current and alleviate the effects of galvanic corrosion. Measurements of pH at the lead surface demonstrate that a corrosive micro-environment forms during stagnation in which the local pH drops to 3.0 or lower, demonstrating that the worst-case scenario for galvanic corrosion of lead occurs during long stagnation periods. In addition to water chemistry, flow pattern also has an impact on galvanic corrosion of lead. Conventional wisdom regarding lead release indicates that continuous flow results in the greatest mass of lead release, but reports of anomalously high lead concentrations after long periods of stagnation point to the contrary. In this experiment, continuous flow of chlorinated water through a Pb-Cu galvanic couple promoted Pb(IV) formation and resulted in potential reversal that caused lead pipe to become cathodic to copper and minimized lead release to water. In contrast, intermittent flow resulted in sustained galvanic attack, and a mass balance of Pb release indicated that a greater total mass of lead was released with intermittent flow. These results have important implications for assessing lead risk at the tap, especially considering long stagnation periods at facilities such as schools and increasing efforts for water conservation. Elevated copper release in potable water can cause aesthetic problems and mild health concerns and often occurs in new plumbing systems prior to the formation of a protective scale layer on the pipe surface. While solubility in new copper pipes tends to be controlled by an amorphous solid of high solubility, over time, the natural copper aging process results in the formation of a protective scale of much lower solubility, but the transition can be inhibited in waters with high levels of NOM. Experiments demonstrated that GAC treatment to remove NOM accelerates the aging process to a protective scale that provides a long-term reduction in copper release even after GAC treatment is terminated. Therefore, compared to pH adjustment and orthophosphate addition, which must be continued perpetually, GAC treatment may be a more holistically pleasing method of copper corrosion control. This approach could be useful in the commissioning of new buildings to facilitate rapid aging and avoid potential long-term copper corrosion problems. / Master of Science

There is emerging concern about the potential for elevated lead in water after water utilities conduct EPA mandated (or voluntary) partial replacements of existing lead service lines. Connections between dissimilar metals results in the accelerated corrosion of the less noble metal via galvanic attack, increasing metal concentrations in water and posing potential public health risks. Many practical problems associated with stopping galvanic attack between copper:galvanized iron and copper:lead via use of dielectrics have also been raised. Galvanic corrosion can be effectively stopped by isolating the dissimilar metals; however, completely eliminating electrical continuity may not always be practical or allowed by code. Instead, increasing separation distance between the two metals was hypothesized to considerably reduce galvanic corrosion. Galvanic corrosion and lead leaching were evaluated for lead:copper connections with varying separation distances while maintaining electrical continuity. Increased distance between lead and copper pipe dramatically reduced the galvanic current and the magnitude of lead release. Galvanized iron and copper connections were also investigated using various commercial fittings, and results verified that a controlling factor was separation distance between the two dissimilar metals. When considering the long-term behavior of partially replaced lead service lines, detrimental effects from galvanic corrosion worsened with time. Even when water was sampled consistently at moderate flow rate, the condition representing traditional partial service line replacement was 40% worse than a full lead service line. At elevated flowrates, lead concentrations and variability increased for partly replaced lead pipe versus full lead pipe due to reservoirs of lead rust formed via galvanic corrosion. At low flowrates, these negative impacts were not observed. Finally, crevices formed by the use of commercial couplings increased lead release. Overall, the results enhance practical understanding of galvanic corrosion impacts and use of dielectrics in water service lines and premise plumbing. / Master of Science

This study focused on the kinetic data measurements, such as diffusion coefficient D and exchange current density i_0 of the electrochemical reactions of corrosion products (Fe, Cr and Ni ions) and corrosive species (OH-), and corrosion studies of structural materials (SS 316H and Alloy 617), including static corrosion and galvanic corrosion, in molten MgCl2-NaCl-KCl and/or NaF-KF-UF4-UF3 salts in a temperature range of 600 to 800C. The study applied the semi-differential (SD) analysis method and innovative fitting method for the kinetic property data measurements in the multicomponent system of NaF-KF-UF4-UF3 salts. In molten MgCl2-NaCl-KCl salts, the measured D_(OH^- ) has the largest value followed by D_(〖Cr〗^(2+) ), D_(〖Fe〗^(2+) ), D_(〖Cr〗^(3+) ) and D_(〖Ni〗^(2+) ) at the studied temperatures, and none of the diffusion coefficients depends on the ion concentration in the studied concentration range and all of them followed the Arrhenius law. At the same temperature, the measured D_(Fe^(2+) ) and D_(〖Cr〗^(2+) ) values in molten NaF-KF-UF4-UF3 salts were slightly smaller than those obtained in molten MgCl2-NaCl-KCl salts. The non-linear curve fitting technique was applied to determine the exchange current density i_0, charge transfer coefficient α, limiting current density i_L and standard rate constant k^0 values. i_0 and k^0 followed the Arrhenius law. The obtained fundamental data can be applied to corrosion models which make the corrosion rate prediction possible in a static system from the experimental kinetic data. Corrosion studies of SS 316H and Alloy 617 in thermal purified molten NaF-KF-UF4-UF3 salts were performed for 120 hours. Based on the post-test analysis, the major metal species corrosion products were Cr, Fe and Mn in SS 316H tests, and Cr, Co, Ni in Alloy 617 tests. The measured UF4/UF3 ratio increased after corrosion tests because some of the U3+ was oxidized to U4+ by corrosive impurities and corrosion products during tests. Cr depletion and salt penetration were observed at grain boundaries (GBs) for both SS 316H and Alloy 617. For Alloy 617 specimens, the corroded area could be divided into two parts: the first part (near the surface) where Cr was completely depleted, and the second part (underneath the first part) where Cr was partially depleted. For SS 316H specimens, the average attack depth was larger than that of Alloy 617. Mo segregation was observed in the matrix of SS 316H specimens but was found to be enriched at GBs in the second part of Alloy 617 specimens. The corrosion study of Alloy 617 with time was also conducted for 72 hours and 32 hours, respectively. A thin layer composed of Fe, Co, Ni and Mo was found on the surface of the specimen, which was different from the previous 120-hour tests. In the salt, the concentration of Cr kept increasing with time, while for the other identified corroded elements, i.e., Fe, Co, Ni and Mo, their concentrations increased first, then decreased until becoming zero or stable. In the galvanic corrosion study of Alloy 617/graphite in molten NaF-KF-UF4-UF3 salts, the galvanic corrosion rate of Alloy 617 at 750C was about four times of that at 650C in the 2-hour tests, which indicated that temperature has a significant effect on the galvanic effect. In the 120-hour galvanic corrosion test, the galvanic corrosion rate became slightly larger with time in the studied system. Similar to the previous 120-hour Alloy 617 corrosion test, the corroded area of the post-test specimen was divided into two parts. The measured attack depth in both parts were much smaller compared with that in the 120-hour Alloy 617 test. This was because of the lower corrosive impurity concentrations in the salt used in the test. The salt in the galvanic corrosion test has been used in the previous corrosion test, during which the corrosive impurities were consumed, which made the salt less corrosive. Finally, it is necessary to point out that all the salts used in the present work were only thermally purified, which is effective in the removal of moisture but not in the removal of oxide impurities. Therefore, further studies are needed to understand the oxides' impacts on the corrosion behavior, especially on the salt penetration. / Doctor of Philosophy / Molten salt is a promising candidate that can be used as fuel and coolant in the molten salt reactors (MSRs). Besides, it can also be used as thermal energy storage, heat transfer fluid in the concentrated solar power plants, because it has high heat capacity, low vapor pressure, and high thermal conductivity. However, materials corrosion is a key concern of molten salt applications, and it is known that the corrosion by molten salts is mainly impurity driven. The impurities, such as moisture in the salts, can make the salt more oxidized, thus becoming more corrosive to corrode the structural materials. The present work focus on the kinetic property of metal specie corrosion products and non-metal impurity in the molten fluoride and chloride salts, which were directly related to the mass transfer and charge transfer process during the corrosion. Especially in the measurements of fluoride salts, innovative methods were applied which were confirmed to perform well in the multicomponent system (Fe and Cr ions coexisted). The static corrosion tests of SS 316H and Alloy 617 were conducted in molten fluoride salt at high temperatures. The main purpose was to study their corrosion behavior and understand the corrosion mechanisms. The corrosion rate of SS 316H was also estimated, which could be a crucial criterion in the material selection. In addition, the corrosion of Alloy 617 with time was also investigated. The metal specie corrosion product concentration change trends were obtained, and the corrosion behavior over the different corrosion stages was analyzed. Different corrosion phenomenon was observed in different corrosion test. Thus, they shed lights on the study of how the corrosion was developed during the corrosion process. Moreover, galvanic corrosion was another major corrosion type when two or more dissimilar materials were electrically contacted. The galvanic corrosion of Alloy 617/graphite was studied in the molten fluoride salts. The galvanic corrosion rate increased with the rise of temperature, which verified that temperature was a key factor that affected the galvanic corrosion. And the galvanic effect was also turned out to increase with time in the present study.

Chemical systems containing oxidants are widely used at various stages in semiconductor processing, particularly for wet cleaning and polishing applications. This dissertation presents a series of studies related to oxidative removal of materials in the Front-End-Of-Line (FEOL) and Chemical Mechanical Planarization (CMP) processes during IC fabrication. In the first part of this study, stripping of photoresists exposed to high dose of ions (1E16 As/cm²) was investigated in activated hydrogen peroxide systems. Stripping of photoresists (PR) exposed to high dose (>1E15/cm²) ion beams is one of the most challenging steps in FEOL processing. This is due to unreactive crust layer that forms on the resist surface during ion implantation. The use of hydrogen peroxide systems activated by metal ion or UV light, for disrupting crust formed on deep UV resist to enable complete removal of crust as well as underlying photoresist was investigated. A systematic evaluation of variables such as hydrogen peroxide and metal ion concentration, UV intensity, temperature and time was conducted and an optimal formulation capable of attacking the crust was developed. A two step process involving pretreatment with activated hydrogen peroxide solution, followed by treatment with sulfuric acid-hydrogen peroxide mixture (SPM) was developed for complete removal of crusted resist films. In the second part of this study, electrochemically enhanced abrasive removal of Ta/TaN films was investigated in solutions containing 2,5 dihydroxy benzene sulfonic acid (DBSA) and potassium iodate (KIO₃). This method known as Electrically-assisted Chemical Mechanical Planarization (ECMP) is generating a lot of interest in IC manufacturing. Ta/TaN films were abraded at low pressures (<0.5 psi) on a polyurethane pad under galvanostatic conditions. The effect of variables including pH, KIO3 concentration, and current density has been explored. In the optimized formulation, tantalum and tantalum nitride removal rates of ~170 A⁰/min and ~200 A⁰/min, respectively have been obtained at a current density of 1 mA/cm². The use of benzotriazole as a copper inhibitor was required to obtain Ta to Cu selectivity of 0.8:1. Additionally, the nature of the oxide film formed on tantalum during the electrochemical abrasion process was characterized.

Premise plumbing can contain copper and lead bearing fixtures, and although copper is considered primarily an aesthetic issue, the neurotoxic effects of lead present a significant public health concern. Utilities approach corrosion control in low alkalinity water by increasing the pH (>9) or adding a phosphate inhibitor at neutral pH. Phosphate inhibitors, pH and chlorine are known to affect lead and copper release through their direct action as corrosion inhibitors or oxidizing agents or through an indirect action via microbial growth. Biofilms are often an implied cause of premise plumbing corrosion, although little is known about their community structure or ability to store metals. The central hypothesis of this thesis is that biofilm contributes to lead and copper release in premise plumbing. This thesis addresses a unique gap in corrosion literature by integrating traditional corrosion chemistry methods with microbiological and molecular biology techniques. The experiments used three distinct approaches: (i). electrochemical cell experiments to determine the key factors in decreasing lead and copper corrosion in galvanically coupled systems while maintaining microbial control (ii). galvanic macrocells using premise plumbing components to examine the unintended consequences of adding a phosphate based corrosion inhibitor and (iii). an annular reactor study to examine the impacts of two commonly applied corrosion control strategies. In the electrochemical cell experiments, the pH 9.2 with zinc orthophosphate and chlorine treatment achieved both goals: decreased lead and copper release and limited microbial growth. In galvanic macrocells experiments with premise plumbing components, zinc orthophosphate addition was shown to be positively correlated with increased bulk water bacteria, biofilm growth and biofilm community structure as measured by DGGE. Biofilm was also observed to sorb 3-29% of lead and 3-16% of copper from the bulk water. The comparison of the two corrosion control strategies showed that the majority of lead released was in the particulate form, and the results further alluded to the potentially significant role lead particles play in biofilm formation.

Aluminized steel Type 2 (AST2), often used for culvert pipes, is subject to corrosion which is the most important durability limitation factor. It was desired to determine if the outer aluminized layer will retain passivity and if protective galvanic action will develop. Thus, corrosion of unblemished and blemished AST2 surfaces was investigated in simulated natural waters. Experiments with unblemished specimens showed passive corrosion rates (~0.06 µm/yr) in scale-forming, 0.01 M Cl- solutions but sustained corrosion in other less protective media (with rates 3~10 µm/yr). Corrosion was manifested macroscopically by discoloration and few macro pits, but it likely proceeded also microscopically at the Fe-rich inclusion space scale. For blemished specimens, the aluminized coating galvanically protected to some extent the steel in all solutions. However, in 0.01 M Cl- solutions, protection was delayed until after some steel corrosion had occurred. In some solutions, complete consumption of the outer aluminized coating around exposed steel was noted. Elsewhere, coating appearance was similar to that of the unblemished condition. Nominal durability projections made for 16-gage AST2 ranged from >100 yr for unblemished AST2 to ~10 yr for the blemished condition. The present findings were used as a first step in proposing refinements of presently used durability guidelines of AST2 culvert pipe. Cyclic cathodic polarization tests to examine O2 and H2 reduction at the Fe-rich inclusions showed significant hysteresis, more pronounced with decreasing scan rate. The effect was tentatively associated to the amount of Fe+2 being deposited during the downward scan, a hypothesis supported by results from a physical model. A static polarization model was formulated for the blemished configuration. Results matched experimental trends and permitted evaluating the effect of solution conductivity s beyond the experimental range. Exposed steel corrosion rates at the steel were increasingly large for decreasing s. For the lowest s, corrosion rates at the exposed steel center were distinctly larger than at the edge, consistent with experiments. An impedance behavior model was also formulated. Results showed frequency dependent current distribution and predicted relatively small artifacts that were and not evident experimentally, but should be considered when exploring other system conditions.

The objective of this study was to evaluate galvanic corrosion potential under various pH conditions, buffering capacities, and corrosion inhibitors including zinc orthophosphate (ZOP) and orthophosphate (OP). Bench-scale dump-and-fill experiments evaluated metals release from a lead and copper couple under stagnant conditions. Key findings from this study were that increasing DIC from 3 to 7 or 17 mg CaCO3/L significantly reduced lead release with or without corrosion inhibitor; however, the lowest lead concentrations were observed in water conditions with corrosion inhibitor addition. However, addition of 20 mg PO4/L as OP exacerbated lead release in some cases; though dissolved lead release was always below 28 µg/L, particulate lead was as much as 4 times greater compared to no corrosion inhibitor. Overall, this study demonstrated the potential of high dose ZOP and OP for lead corrosion control in drinking water, however, overdosing OP can lead to exacerbated particulate concentrations.