ANION INCORPORATION AND ITS EFFECT ON THE DIELECTRIC-CONSTANT AND GROWTH-RATE OF ZIRCONIUM-OXIDES

Mechanically polished zirconium electrodes were potentiodynamically po larized in phosphate buffer solutions of various pH values and in 0.5 M NaOH. The results show that the shape of the I-E curves is independe nt of the solution pH. At relatively low scan rates, oxygen gas evolut ion was observed. The oxide film thickness was calculated from the val ues of the charge consumed in the anodic process assuming 100% current efficiency for oxide formation below oxygen evolution (lower values f or the current efficiency are assumed for potentials above oxygen evol ution). Capacitance measurements, together with the calculated oxide t hickness, were used to estimate values for the dielectric constant of the oxide. Two different values of the dielectric constant were obtain ed for the oxides formed in the range of potential below and above oxy gen evolution. Also, higher dielectric constant values were obtained w ith increasing solution pH. Anion incorporation was assumed to increas e the conductivity of the oxide films and, hence, decrease the dielect ric constant. A two-layer structure is proposed for the anodically for med oxide on zirconium in aqueous solutions; an anion-fi-ee layer near the metal and an outer layer containing the incorporated anions.