The investigation of passivating oxide layers on zirconium metal using small single shat anodic current pulses

Passivated metals such as Al, Nb, Ta and particularly Zr were anodically polarized in aqueous sulphate solution by single shot intensiostatic charge pulses, and the overtensions recorded on CRO photograms. The initial slope of the overtension/time curves is given by the capacitance of the passivating oxide film. The slope decreases when, with rising electrical field strength across the oxide film, the ionic current flow increases. In agreement with the known theories (Wagner; Mott; Verwey et al.) a straight line results, from plots of the overtension measured at charge densities of 10^-4C/cm² vs. the logarithm of the current density. From the slope of this straight line the thickness of the passivating layer can be obtained without interference of the surfaces roughness. For the measurement of the thickness of the passivating oxide film of an individual specimen surface, two successive pulses were applied, the first of 3-3 x 10^-5 A/cm² and the second of 3-3 x 10^-3 A/cm² current density; the pulse duration was so chosen that each pulse represented the same charge
density, 10^-4 C/cm². Thus the modifications of the original surface state of the electrode metal caused by each pulse were negligible. From the difference of the maximum overtensions and the difference of the logarithms of the current densities of the two pulses the thickness of the passivating oxide film of the investigated sample can be calculated. The method was checked measuring zirconium samples after gravimetrically controlled oxidation. The results obtained applying the method to differently prepared metal surfaces are discussed.