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Researchers seek to predict chemical reactions over thousands of years. When ordinary steel gets wet, the alloy composed primarily of iron rusts. Less widely understood is what happens at a molecular scale. Scientists have found that oxidation occurs at the interface between water and steel to form oxides and hydroxides making up the rust, often accompanied by loss of solid material into solution, or dissolution.
The processes that contribute to the phenomenon of corrosion involve the transfer of ions and electrons across this interface. In corrosion, charge transfer leads to degradation of the solid. Charge transfer enables the storage of chemical energy in batteries, which are designed to be used as a source of electrical energy when needed. Indeed, how ions and electrons travel across the interface is of interest to electrochemists working on improving the lifetime and reliability of batteries, especially the lithium ion battery, which will be a critical element of hybrid and plug-in electric vehicles. An electric double layer resides at the solid-water interface, so insights into the boundary's nature might help researchers increase battery capacity and lifetime. In a battery the charges must be transferred past the double layer, requiring an understanding of how the interface enables the battery to do work or store energy. Dave Wesolowski, an ORNL geochemist, is leading a multidisciplinary research team trying to understand the structure and dynamics—the relative positions and motions of ions and electrons—of the electric double layer. To obtain this information, the researchers are using both ORNL's supercomputer and Spallation Neutron Source. Wesolowski explains the electric double layer this way: "When a metal or oxide is exposed to water, the electrical charge distribution in the water phase is not uniform, inducing electrons and atoms to redistribute at the surface. As the surface becomes charged, water dipoles, which have separated positive and negative poles, and metal ions in the solution are attracted to the surface, forming an electric double layer. Away from that interface, the solid and liquid phases are electrically neutral without a net charge." "Chemical reactions occur easily in bulk water, but at the interface water molecules are rigidly attracted to the surface, resulting in a barrier to reactions there," Wesolowski says. "All reactions that occur between a solid phase and liquid phase must occur in the interfacial region. In the region we call the interface, the structure and dynamics are not like the structure and dynamics of the bulk solid phase or the bulk liquid phase." In recent years Wesolowski and other scientists have had the opportunity to use both experimental and computational tools to probe the interfacial region. Wesolowski's team has used the powerful X-rays of the Advanced Photon Source at Argonne National Laboratory to conduct surface spectroscopy and scattering. They also probed interfacial dynamics using backscattering neutron spectrometers at the SNS and at the National Institute of Standards and Technology. They coupled these probes with the computational capabilities of the Cray XT3 supercomputer at ORNL. "By merging experimental and computational capabilities, my collaborators and I better understand the structure and dynamics of mineral-water interfaces," Wesolowski says. "This is an exciting frontier." ORNL and Penn State University researchers have been using the Cray XT3 supercomputer at the Laboratory to model 48 water molecules on 48 titanium dioxide surface units. "We have used a million dollars worth of computing time to get one trillionth of a second of interfacial dynamics," he notes. "We can perform classical simulations over much longer time scales with much less computer time. By using high-level calculations and X-ray probes of surface structure to calibrate classical simulations, we can produce dynamics from the calculations." The plan is to link the dynamics results of the calculations with the results of experimental probes of surface dynamics using incoherent neutron scattering at SNS. Neutrons interacting with the hydrogen atoms in the water molecules will give a significant signal that should be valuable to theorists. According to ORNL corrosion expert Pete Tortorelli, the science being studied with respect to the electric double layer is also at the core of what controls dissolution of solids in water (one form of corrosion) or when protective (passive) surface films form. Corrosion-resistant metals such as stainless steels rely on the formation and maintenance of protective oxide layers to fend off extremes of chemical and electrochemical reactivity. For example, a protective oxide film in an aggressive water environment must withstand a potential of ~107 volts per meter without breaking down.
In a pressurized-water reactor the fuel cladding consists of zirconium alloy, which reacts with water to form zirconium oxide as a passivating layer. If the cooling water temperature rises to 340°C and approaches 374°C, the critical temperature of water, researchers find they no longer can model the reactor's neutron flux. The reason: the dissolved lithium borate introduced into the primary coolant as a thermal neutron moderator adsorbs onto the zirconium oxide on the fuel cladding surface. The high concentration of boron at the fuel surface muddles the prediction of fuel burnup rates—measures of the number of nuclear fission events that have taken place in the fuel. Wesolowski, along with retired ORNL chemist Don Palmer, received funding from the Electric Power Research Institute to study the high-temperature ion absorption characteristics of zirconium oxide. He has also been supported by the Department of Energy's Office of Civilian Radioactive Waste Management to investigate ion adsorption on spent nuclear fuel surfaces for applications in nuclear waste disposal. The projects are examples of macroscopic phenomena that scientists would like to understand at the molecular level. Meanwhile, Tortelli studies high-temperature corrosion. "For our studies of metals, 1000°C and above is extreme," he says. He characterizes work with water environments, 300°C and above as extreme, with pressure as high as 85 atmospheres. One of the important challenges for researchers is predicting with accuracy the changes likely to occur when the environment of radioactive materials stored at a proposed U.S. nuclear waste repository is subjected to anticipated temperatures, pressures and reactivity over an extreme time—a million years. The repository is located at Nevada's Yucca Mountain, which is made of "tuff"—rock consisting of somewhat stratified volcanic ash. The plan for the Yucca Mountain repository is to accept spent nuclear fuel from the nation's commercial nuclear reactors and to encapsulate high-level defense nuclear waste in glass logs. The materials would be placed inside stainless steel canisters, then enclosed in an outer canister of a highly-corrosion-resistant nickel alloy. A titanium metal "drip" shield would be put in place to keep water or falling rock from damaging the canister surfaces. "What defines the environment of the Yucca Mountain repository as 'extreme' is not only temperature and reactivity but also time," says Wesolowski. "When scientists consider changes that could occur in glass logs and on spent fuel surfaces for times beyond recorded history, they cannot model every possible reaction with certainty. "Metals in oxidizing environments and glass containers are, at least in theory, not stable materials. While the metals may survive intact for exceedingly long times, they nevertheless might eventually corrode." The stakes involved, and the need to understand such extreme possibilities with greater certainty, are ample motivation for researchers.—Carolyn Krause
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