General corrosion of alloy 22: Experimental determination of model parameters from electrochemical impedance spectroscopy data

Models used to predict general corrosion damage of Alloy 22 high level nuclear waste (HLNW) containers must be deterministic, relying upon the time-invariant natural laws, because of long time scales involved in the predictions compared with the time over which the corrosion of this alloy has been studied. The point defect model (PDM) is one such model and requires high-accuracy experimental data to determine model parameters that will accurately predict corrosion over the long times required for HLNW disposal. Electrochemical impedance spectroscopy (EIS) and steady-state polarization data were collected in deaerated 4 M NaCl, pH 6 solutions at 30 degrees C and 60 degrees C. This work, which was carried out under a quality assurance program, emphasized data reproducibility and adherence to the steady-state condition in order to ensure confluence between theory and experiment. Thus, fresh electrolyte was continuously flowed through test cells allowing test specimens to remain undisturbed for up to 6 weeks during data collection. Parameters for the PDM have been estimated by optimizing the PDM on the impedance data, and the ability of the parameter values to account for the steady-state passive current density and barrier oxide layer thickness has been evaluated using phase space analysis. The calculated impedance gives generally good agreement with experimental data showing that the PDM is a valid approach.