Steady-state kinetics of formation of anodic oxide films on tantalum in sulphuric acid

On elementary theories, the ionic current i through the oxide is expected in the steady state to depend on the field strength E in the oxide (as measured by oxide overpotential/thickness of oxide) and the temperature T according to an equation of the form i = i0exp { – (W – qaE)/kT}, where i0, W and a are constants which have different significance according to the model, q is the charge on the ion, and k is Boltzmann’s constant. The relation has been investigated experimentally by a spectrophotometric method of determining the thickness of the oxide. It was found that the experimental results for dilute solutions could be represented within less than 1 % of E at 2490 Ả thickness by the above equation with qaE replaced by q(α + βE) E where α and β are constants with i0 = 108.24 A/cm2, W = 2.185 eV, α = 6.995Ả,β = – 3.35Ả/(107 V/cm) and q = 5e. The modifications that occur as the concentration of acid is increased have been investigated. In concentrated acid, although less charge is required to form a film of given optical thickness, the films have a lower a.c. capacity, and require a higher overpotential to produce a given ionic current density. The capacity of films formed to a given overpotential at a given current density is almost independent of the concentration of the acid. The significance of the results is discussed in connexion with how far the existence of a range of site parameters (in particular, activation energy and activation distance) in an amorphous oxide tends to explain them, or whether a more fundamental change in the equations used to describe ionic movement is required.