PASSIVE BEHAVIOR OF TANTALUM

The passive behavior of tantalum has been studied at electrode potentials from 0 to 4 V sce in aqueous solutions of pH 0-13 at 25 degree C by quasi-steady (20 h) and transient polarization measurements, dc capacitance determinations and coulometric measurements. Tantalum exhibits a quasi-steady passive current varying only a little with the electrode potential, the pH and the presence of HF or F** minus . The transients accord with the Cabrera-Mott equation and fit reasonably with lambda equals 1. 5A and z equals 5 for the tantalum ion transfer at the metal-oxide interface. Capacitance data largely obey expectations from an 'insulator' model, revealing the main charge distribution at the passive tantalum electrode.