The influence of a mixed radiation environment on the properties of the passive film on tungsten

The properties of the passive film formed on the tungsten (W) in H₂SO₄ (pH 1.6) during proton irradiation (spallation) were characterized with electrochemical impedance spectroscopy (EIS), surface enhanced Raman spectroscopy (SERS), and the Mott-Schottky (MS) method. EIS data were associated with an adsorption pseudocapacitance. Observed changes in the adsorption pseudocapacitance were consistent with thinning of the outer layer of the passive film. SERS experiments found that the spallation environment had no affect on the molecular state of the oxide. Results from MS experiments found that the oxygen vacancy concentration in films formed during proton irradiation were lower than those films formed in the absence of irradiation. In accordance with the point defect model (PDM) for oxide films, a decrease in the oxygen vacancy concentration indicates proton irradiation alters the passive film by either decreasing the oxygen vacancy flux or increasing oxygen vacancy diffusion. Calculations using the LAHET and MCNP transport codes have demonstrated that spallation reactions are capable of generating numerous energetic particles such as, electrons, neutrons, protons, and photons. Each of these species is capable of generating a wide variety of defects in the oxide film altering its electronic and transport properties and, thus, explaining the observed electrochemical phenomena.