Role of Alloying Elements in the Electrochemical Corrosion Behavior of Multiphase Alloys

An electrochemical testing protocol is being developed to measure the corrosion behavior of waste forms made by alloying metallic fuel waste that remains after electrochemical processing of spent nuclear fuel with cladding hulls, contaminated hardware, and added trim metals. The release of radionuclides from alloyed waste forms must be predicted over the service life of disposal systems to ensure regulatory limits are met. Understanding the corrosion mechanisms of host phases containing the radionuclides and a scientific basis for experimentally measured durabilities will provide confidence in long-term performance predictions. Developmental studies conducted during in the past have led to a mechanistically-based model for predicting long-term corrosion kinetics for iron-based alloys and the laboratory testing protocol to parameterize that model. Electrochemical tests were conducted on alloy/oxide composite and metal waste forms in various electrolyte solutions to impose a range of chemical effects (primarily pH and Cl-) and a potentiostat was used to impose a surface potential representing the solution Eh. Potentiodynamic (PD) scans and Potentiostatic (PS) tests were performed to characterize the corrosion behavior. The corrosion currents were measured at various imposed potentials, with electrochemical impedance spectroscopy (EIS) performed daily to measure the electrical properties of the corroding surface. Surfaces of the electrodes were characterized by SEM/EDS before and after the electrochemical tests to compare and identify the active-passive phases. Solutions collected during and at the end of the PS tests were analyzed using inductively-coupled plasma mass spectrometry (ICP-MS) to correlate mass releases with the corrosion currents. Results from electrochemical tests demonstrate the use of these results in performance modeling. Analytical functions were derived for modeling the Eh and pH dependence in the degradation model. Surface stabilization corresponding to the Eh-pH stability regions of passivating oxides decreases corrosion rates by ~100X and is attributed to the role of alloying elements Cr, Ni, and Mo. Solution analyses used to relate electrochemical corrosion currents to mass release rates are correlate with surface characterizations that identify the active phase responsible for the release. Finally, Equivalent circuit models of EIS responses can provide confidence in using measured electrochemical kinetics to model waste form performance.