The corrosion behavior of Re microelectrode in several aqueous solutions has been inferred from open circuit capacitance, C and potential, E measurements in the absence and presence of O2, at 25°C. Inspection of the steady-state potential, Est reveals that the metal surface is covered by an oxide film whose composition is likely to be ReO2. In all examined solutions except H3PO4, and CH3COOH, the pre-immersion surface oxide film dissolves, whereas in these latter two solutions it grows. The presence of the dissolved oxygen acts bi-functionally on the rhenium electrode. Its effect is more pronounced in-the slightly corrosive solutions such as Na2SO4. The rate of oxide film growth has been measured in acetic acid solutions and found to increase directly with the acid concentration. The relative thickness of the formed oxide film at the steady-state condition (after about 90 minutes of immersion), viz. C-1 at ac frequency, f = 20 k Hz, increases with c.
A comparison of the oxide formation rates coefficients derived from capacitance measurements with those derived from potential measurements reveals that the oxide structure change with the solution composition; probably the degree of hydration of the oxide decreases with an increase of acid concentration, c.
Complex plane analysis of the impedance gives the dielectric loss angle of the oxide. It was found to be in the range 40 - 58° for c in the range 1.0 - 50 mM.