OXIDE-GROWTH AT PLATINUM ANODES WITH EMPHASIS ON THE PH-DEPENDENCE OF GROWTH

At platinum, analyses of the galvanostatic V - q transients (q = it) at various current densities and the dV/dq dependence on log i in H2SO4 solutions of different pH's have shown that, following the initial oxygen adsorption stage, the growth of the anodic oxide film in the potential region before significant oxygen evolution starts satisfies the Cabrera-Mott formalism of the high field assisted formation of ions and their migration through the oxide phase. For different current densities, the linear V - q lines intercept at a negative value of charge, q0. This charge is associated with oxygen adsorption at oxide-free surfaces before the onset of oxide film formation. Because the coverage with adsorbed oxygen at oxide surfaces is low, the charge for the initial adsorption at oxide-free surface has to be taken into account in considering the thickness of the oxide film and its growth. Analysis further shows that the exchange current density, io, and Tafel parameters, 1/α, in the rate equation for growth are not dependent on pH. The only parameter that depends on pH is the potential Vo at which the V - q traces at various constant current densities in a solution of the same pH intercept in extrapolation, i.e. V0 = V00 - (2.3RT/F)pH, where V00 is a constant independent of pH. V0 signifies the begining of oxide film growth. © 1979, The Electrochemical Society, Inc. All rights reserved.