| Abstract: | Growth of thin transparent anode films on Sn in neutral media has been studied by measuring galvanostatic anodic charging curves in phosphate buffer, 0.1M KCl and 0.1M Na2SO4 solutions (pH 6.6–6.7) at low current densities. The experimental technique is essentially the same as that used in previous investigations on valve metals. Although the rise in the anode potential of Sn does not exceed 2.0 V, the shape of the anodic charging curve is identical to that observed on valve metals: being thus composed of a linear and a non-linear region. Application of the kinetics of galvanostatic anodization to the results on Sn show that: (i) the oxide formation rate is linearly related to the ionic current density i by a double logarithmic plot, (ii) the reciprocal capacity is linearly related to log i, and (iii) the Tafel behaviour is exhibited at constant charge. These relations indicate that the anode film growth occurs by an activation-controlled ion conduction under the influence of the electric field across the film phase according to an exponential law. Treatment of the results allows the estimation of some kinetic parameters of film growth, e.g.: (i) the constants a and b of the empirical relation between oxide formation rate and i, (ii) the constants A and B of the exponential law, (iii) the electric field which is of the order of 106 V/cm in phosphate, and 107 V/cm in both chloride and sulphate solutions, and (iv) the effective activation distance for the ionic jump over the energy barrier associated with cation transport within the film, whereupon relative (and not absolute) values can only be obtained. Comparison between the present results and previous ones (also on Sn) taken by potentiodynamic technique shows that while diffusion kinetics play an important role in the formation of thick anode films by the potentiodynamic technique, activation-controlled kinetics explain the present results on the galvanostatic formation of thin transparent anode films. |
