Growth Mechanism of Thin Anodic Oxide Films on Tantalum

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 false(Ifalse) vs. time false(tfalse) measurements provides BFo and λFo where B is the field coefficient, Fo is the initial field in the oxide, and λ is the growth rate per unit current, provided that the emf of the reaction, E , is known. E can be determined by matching the B ‐values derived from I vs. t 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 IE=IEo e−d/l with IEo∼3×10−4 normalamp/cm2 normaland 1∼3.0Aå , was obtained. There are indications that either the inverse Tafel slope, B , or the emf, E , varies with film thickness in the range where the internal electron current is appreciable.

Kinetics of Tantalum Corrosion in Aqueous Electrolytes, II

“…This suggests that B for these thin oxides depends on the ionic current density, I~, or on the voltage, V, and is not a constant as observed for thick anodic oxides where Eq. [18] and [14] appear to apply fairly well.…”

“…Thus we conclude that either the ionic current cannot depend on voltage and oxide thickness through the field F, or else the parameter E in Eq. [14] for F is not a constant (18), but changes from low values E _~ E1 to higher value E ---E2, as the oxide thickness increases and the ionic current decreases. The Mott-Cabrera theory ( 4) is based on an exponential dependence of the ionic current on the field (14).…”

“…Studies of anodic oxidation of tantalum and other valve metals lead to the relation (21) Ii = A exp BF [18] for oxide films which were generally much thicker than those encountered in spontaneous corrosion for oxide films. If this relation applies, our coefficient ~i is related to B by Eq.…”

Electrochemical behavior of tantalum in potassium hydroxide solutions

“…Ta2O5 thin films can be deposited by several methods, such as sputtering, anodic oxidation and chemical vapor deposition. In electrochemistry, the anodic oxidation of Ta and formation of Ta2O5 films was the subject of numerous investigations concerned with the growth of thin films in different acidic and neutral solutions [6][7][8][9]. So far, the passive and corrosion behavior of tantalum has been studied using several electrochemical techniques such as chronoamperometry [10], coulometry [11], linear polarization [12], cyclic voltammetry [13], galvanostatic polarization [14], electrochemical impedance spectroscopy (EIS), capacitance and photocurrent measurements [15][16][17].…”

Stability of anodic tantalum oxide films in NaOH solutions

“…Valve metals, in general, are known to form stable oxide films in aqueous media [2]. Tantalum is a typical valve metal and many investigations have been concerned with the growth of thin oxide films on its surface [3][4][5][6][7][8][9][10][11][12]. Other studies have been concerned with the effect of surface pretreatment on the thickness of the formed oxide [13,14].…”