TECHNICAL PROPOSAL. Principal Investigator: R. G. Kelly Institution: University of Virginia

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1 TECHNICAL PROPOSAL Corrosion Co-op Thrust Area: Corrosion in Thin Layers of Moisture and Deposits Project Title: Evolution of Solution Layer Chemistry in the Presence of Dust Principal Investigator: R. G. Kelly Institution: University of Virginia Introduction Much of the expected corrosion that will occur in the repository will occur under atmospheric corrosion conditions, rather than full immersion. The waters that seep through the tuff will either drip or condense on the surface of the waste package with the conditions under which that occurs depending strongly on the final design and the environmental conditions assumed. The corrosion modes that represent the most threat to waste package integrity are those that are localized (pitting, crevice corrosion, stress-corrosion cracking). There has been limited fundamental study of localized corrosion in thin electrolytes. Most atmospheric corrosion research has focused on the more uniform corrosion exhibited by nickel and copper when exposed to atmospheres of differing relative humidity (RH), temperature, and pollutant gas type and concentration [1-5]. In order to validate models of WP performance, the plausible extremes of the WP environment and subsequent corrosion need to be established. To accomplish that requires a combination of experimental and computational work that focuses on the controlling variables and establishes a scientific basis for the prediction of the stabilization of localized attack on the WP. This stabilization involves the connection between a stable, robust cathodic area and the localized corrosion site. The robustness of the cathodic area depends in large part on the chemistry, electrochemistry, and transport properties in the thin electrolyte that forms, which will include dust on the WP surface. Understanding the atmospheric electrochemistry requires a determination of the extent to which cathodic and anodic sites under thin films and covered by deposits communicate with one another. The degree to which an anodic site (e.g., a pit) can interact with the surrounding cathodic sites will determine the maximum rate of localized corrosion, as rarely are the kinetics within the localized corrosion site the limiting factor, at least at early portions of a pitting cycle. There has been limited fundamental study of localized corrosion in thin electrolytes. The environmental conditions of atmospheric corrosion are similar to crevice corrosion in the sense that there are geometric restrictions to the mass transport along the surface as well as homogeneous chemical reactions taking place that can alter the electrochemical kinetics. The main differences are that there is the delivery of additional cathodic reactants (oxygen) occurs across the entire surface, and atmospheric systems are by their nature under open circuit conditions (i.e., the surface is not externally polarized). Proposed Work Objective: Establish a scientific basis for the determination of the plausible extremes of environments on WP in the presence of submonolayer amounts of dust, and the extent to which the rates of localized corrosion can be supported by cathodic reactions within the dust layer.

2 Task 1: Design and construction of experimental setups for (a) establishing dependence of thin solution chemistry on gaseous environment, (b) current and potential distributions. There is a strong need to validate models of the environment on the waste package (EOWP). Validation requires direct measurements of accessible parameters and their dependence on environmental variables. A multi-pronged analytical approach will be used that involves the quartz crystal microbalance, capillary electrophoresis, and microfabricated conductivity sensors. The quartz crystal microbalance (QCM) will be used in two main ways. It will be used to determine the equivalent water layer thickness on metallic surfaces exposed to elevated temperature ( C) gaseous environments containing water vapor (30 < RH < 95%) including the effects of the variables of interest on the amount of water on the surface. Of particular interest will be the effects of sub-monolayer amounts of Yucca Mountain dust on the water layer thickness and reversibility to decreased RH. If the QCM stability is unacceptable at high temperatures, a Cahn microbalance will be used. Our laboratory has a state-of-the-art capillary electrophoresis system that can determine the ionic composition of solutions of nl volumes. A recent project at Virginia resulted in the development of microfabricated solution conductivity sensors [6]. These will be incorporated into the exposure system to determine the conductivity of the electrolyte layers as a function of the experimental variables. Close cooperation with the task involving environments and current distribution in thicker deposits (Payer/Landau) will benefit both tasks through the sharing of the experimental methods. The task will result in an integrated experimental system for the study of surface solution layer composition and conductivity. Task 2: Measurement of steady state solution layer chemistry as a function of gas environment above surface, temperature, relative humidity, and dust composition. The composition of the electrolyte present at the surface controls the electrochemical kinetics for a given material. The most important aspect of the composition is the types and concentrations of ionic species. Understanding the dependence of this composition on the important variables will provide important boundary condition information for both measurements and modeling of corrosion processes. Capillary electrophoresis (CE) has been widely used for the analysis of ionic species, particularly in the area of biochemistry [7]. In the last decade, its use has been expanded to the analysis of solutions removed from highly occluded sites such as crevices [8,9], cracks [10], exfoliation sites, as well as from surfaces exposed to gaseous atmospheres [11]. In a study using a related method, Dante and Kelly [11] analyzed the effects of RH and substrate on the speciation of nitrogen compounds in the thin electrolyte layer. In the proposed work, we will generally follow the approach used by Dante [11] in which metallic surfaces (gold and copper)

3 were exposed for different times to a constant RH and pollutant gas concentration. The coupons were then removed, rinsed in a small volume of high-purity water and analyzed for ion content. By using a quartz crystal microbalance, we were able to determine the amount of water that had been present on the surfaces while in the chamber and combine that information with the CE analyses to establish the concentrations of the species detected. The effects of temperature on equilibria will be taken into account using thermodynamic analysis software from OLI Systems. [12] In the proposed work, we will expose coupons with differing amounts of Yucca Mt dust at different temperature/rh combinations ( C, <30 to 95% RH), as well as exposure to different gases (especially different concentrations of CO 2 ). A microbalance will be used to estimate the amount of absorbed water. Planar conductivity sensors fabricated using methods developed for microelectronics manufacture will be used to measure the solution layers formed in the presence and absence of dust. Steady state conditions will be emphasized. Close cooperation with the task involving environments and current distribution in thicker deposits (Payer/Landau) will benefit both tasks through the sharing of the data. The data on solution compositions will be fed to the localized corrosion and life prediction modeling tasks. This task will result in an experimental mapping of the dependence of solution layer composition and conductivity on gas environment above surface, temperature, relative humidity, and dust composition. These data will provide a scientific basis for establishing the plausible extremes of environment. In addition, the data will be used in the overarching life prediction task. Task 3: Modeling of steady state solution layer chemistry as a function of gas environment above surface, temperature, relative humidity, and dust composition. One of the primary challenges in life prediction under repository conditions is the need to develop models that link different processes. These processes occur over very different length and time scales, making computational linkages difficult. One linkage which impacts the corrosion of the WP is that between the in-drift chemistry and the environment of the waste package surface. The solution environment on the WP is determined by the interaction of the electrochemistry of the WP surface and the thin solution that forms, which may be affected by the presence of dust or deposits. There is a need to develop improved means of linking the indrift chemical environment to the conditions on the waste package surface as affected by the important variables. The work of Graedel [13-15] on developing a multi-regime model of atmospheric corrosion corrosion chemistry (GILDES) will serve as a framework for our modeling. The GILDES (Gas-Interface-Liquid-Deposition-Electrodic-Solid) model describes the six regions and their interactions with mathematical formulations based on a combination of fundamental principles and parameterization of data. As published, it provides a strong framework for the transport and reaction in the five of the six regimes (gas through deposition and solid). Given sufficient thermodynamic and kinetic data, these regimes can be handled. In the Electrodic

4 regime, the model used is overly simplistic and do not represent the behavior of real systems well. For example, Tafel behavior is assumed in the Electrodic regime. Even for carbon steel exposed to an aggressive atmosphere, this representation is not realistic. For the corrosionresistant alloys of interest, Tafel behavior is a particularly poor choice. We will adapt the GILDES framework to the relevant conditions at the repository, incorporating more realistic electrochemical kinetics. The in-drift chemical environment will be used as the inputs for the Gas regime. The indrift chemistry is determined by the amount of seepage water and its chemistry, which is controlled by its interactions with geology, and the thermohydraulic effects of the heat released from the WP. In addition, the extent of air exchange of the drift (e.g., due to forced ventilation) will affect the in-drift gaseous environment which is part of the in-drift environment. Our model will be exercised to determine the steady state solution layer chemistry as a function of gas environment above surface, temperature, relative humidity, and dust composition. The experimental data in Task 2 will be used to validate the model. Once validated, the model will be exercised to expand the domain of variable space over which plausible extremes of environment can be determined. Electrochemical kinetics will be input from the localized corrosion tasks, whereas information on reduction reactions and water chemistry will be accepted from other tasks within the thin film and deposit tasks. Interactions with the task studying thicker deposits (Payer/Landau) will ensure for internal consistency and compatibility in addition to leveraging the programming efforts through the sharing of computer code. This task will result in steady state solution layer chemistry as a function of gas environment above surface, temperature, relative humidity, and dust composition for comparison with the experimental results from Task 2. Task 4: Simulation of current and potential distributions in the presence of an active corrosion site (crevice, pit, stress-corrosion crack) as a function of thin layer thickness and current demand of active site. As discussed above, the mass transport conditions for atmospheric conditions are similar to those for crevice corrosion. Thus, the extension of models of crevice chemistry and electrochemistry represents an efficient approach to modeling of the interactions between the WP surface on which cathodic reactions dominate and the localized corrosion sites where anodic processes dominate. Unfortunately, most crevice corrosion models have several limitations with regards to the conditions of interest: (a) difficulty handling the non-linear electrochemical boundary conditions characteristic of actual corroding interfaces, (b) difficulties with inclusion of new physiochemical data without extension reprogramming, (c) inability to consider open circuit conditions (i.e., the mouth of the crevice must be assumed by most models to be held at a constant potential), (d) limited coupling to conditions outside the crevice, and none explicitly couple the crevice to a gaseous atmosphere.

5 We have recently adapted our crevice corrosion model, CREVICER [16-18], to atmospheric corrosion conditions, including the interactions between anodic sites and sacrificial metallic claddings [17] and to an organic coating that releases inhibitors at a rate dependent on ph [18]. The code can handle polarization curves that reflect the pitting-limited passivity exhibited by most corrosion-resistant alloys in bulk environments, as well local-chemistrydependent active behavior known to occur within localized corrosion sites. The model maps the spatiotemporal chemical and potential fields within a crevice in two dimensions, taking into account diffusion, migration, and generation of species, including those by homogenous chemical reactions. Its object-oriented design breaks a program into discrete, independent units called objects, which can be reprogrammed and debugged separately. It is through these objects that information is passed into and out of the solving portion of the code. This encapsulation hides the internal functionality of the object away from other parts of the code. The version of CREVICER adapted for atmospheric conditions [18] overcomes the limitations described above. We will further extend the code to consider the restricted mass transport conditions in a thin film or wet deposit that is coupled to an active corrosion site. It will also be linked to the corrosion chemistry model described in Task 3. This task will have substantial interactions with the task establishing the kinetics of localized corrosion sites (Scully) as well as the task considering similar issues, but in thicker dust/deposit layers (Payer, Landau) and the task considering reduction reactions (Payer, Gervasio). This modeling will provide a scientific basis for determination of both the maximum rate of localized corrosion and the conditions that must be maintained in order for propagation of the localized corrosion to become a structural integrity issue. References 1. M. Benarie, F. L. Lipfert, Atmos. Environ., 20, (1986). 2. T. E. Graedel, J. Electrochem. Soc., 136, 193C (1989). 3. J.-E. Svensson, L.-G. Johansson, Corros. Sci., 34, 721 (1993). 4. D. Persson, C. Leygraf, J. Electrochem. Soc., 142, 1459 (1995). 5. J.-E. Svensson, L.-G. Johansson, Corros. Sci., 38, 2225 (1996) 6. X. Wang, R. G. Kelly, J. S. Lee, M. L. Reed, MRS Symposium Vol 657, EE5.31, (Fall, 2000). 7. J. P. Landers, ed., Handbook of Capillary Electrophoresis, Boca Raton, CRC Press (1994). 8. C. S. Brossia, R. G. Kelly, Corrosion Science, v. 40, pp (1998). 9. C. S. Brossia, R. G. Kelly, Corrosion, v. 54, pp (1998). 10. K. R. Cooper, R. G. Kelly, J. Chromtag. A 850, (1999). 11. J. F. Dante, R. G. Kelly, J. Electrochemical Soc., 140, pp (1993). 12. J. J.. Kosinski, A. Anderko, Fluid Phase Equilibria (2001) T. E. Graedel, Corr. Sci., 38 (12), (1996). 14. T. E. Graedel, Corr. Sci., 38 (12), (1996). 15. J. Tidblad, T. E. Graedel, Corr. Sci., 38 (12), (1996). 16. K. C. Stewart, Ph.D. Dissertation, University of Virginia (1999). 17. F. J. Preusel, R. G. Kelly, Symposium Z: Mechanisms in Electrochemical Deposition and Corrosion, Editors: J.C. Barbour, R.M. Penner, P.C. Searson, MRS Proceedings Volume 781E, 9 pp (2003). 18. H. Wang, F. Preseul, R. G. Kelly, Electrochimica Acta, Vol 49/2, pp