The Passivity and Breakdown of Beryllium in Aqueous Solutions

The passivity of 98.0% pure beryllium has been studied in ambient temperature aqueous solutions as a function of pH. Below solution pH 2, potentiodynamic polarization curves exhibited active dissolution at all applied anodic potentials. In solution pH 2 to 12.5 it is shown that beryllium exhibits passive anodic behavior at potentials below 0.6 V vs. (saturated calomel electrode) SCE. Anodic polarization above 0.6 V SCE in the pH range of 2 to 12.5 led to a “stepped increase” in the passive current density followed by oxygen evolution. This stepped increase was associated with a change in the specific resistivity of the passive film above 0.6 V SCE as determined by electrochemical impedance spectroscopy (EIS). EIS results were analyzed in the context of an equivalent circuit model which incorporated two time constants, one associated with the oxide film and the other associated with the electrode/solution interface. From these EIS experiments, the oxide growth rate was determined to be 6.0 Å/V over the potential range of 0 to 4 V. At higher anodization potentials the growth rate appeared to be somewhat lower. Breakdown of passivity was studied in chloride and fluoride solutions. In chloride, the attack was found to be localized while in fluoride, the attack was localized only at concentrations below . Above , uniform attack was observed. In the pH range of 2 to 12.5, the pitting potential of beryllium in chloride environments was found to be independent of pH. In solutions containing both fluoride and chloride ions, the presence of the fluoride increased the passive current density of beryllium, but had no effect on the pitting potential