Growth Mechanism of Thin Anodic Oxide Films on Tantalum

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Abstract
The growth of an anodic oxide film on tantalum metal immersed in diluted sulfuric acid has been investigated using potential and capacitance measurements. The tantalum metal was connected through an external resistance to a platinum electrode in the electrolyte. An analysis of voltage vs. time confirms the exponential field dependence of the ionic current for the later phases of oxide growth. Analysis of current vs. time measurements provides and where is the field coefficient, is the initial field in the oxide, and λ is the growth rate per unit current, provided that the emf of the reaction, , is known. can be determined by matching the ‐values derived from vs. to those derived from capacitance measurements. For sufficiently thin oxide films the growth rate exceeds the value expected from Faraday's law based on the external current which indicates the presence of internal electron currents for which the empirical expression with , was obtained. There are indications that either the inverse Tafel slope, , or the emf, , varies with film thickness in the range where the internal electron current is appreciable.

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