Go Okamoto and Norio Sato
Three different methods, the galvanostatic, the potentiostatic and the rapid can be used to measure the rate of growth of the anodic oxide film on metals. By using these methods, the rate of growth of a passive oxide film on nickel was measured in ln H2SO4 at 40°C. The rate of growth i can be expressed in the form i=i0 exp (β' ε), where β'=zae α/kT and ε is the strength of the electric field which can be expressed as Δ E/d. The value of β'=40 × 10-3 coul./V·cm2 obtained by the galvanostatic method was considerably different from β'=416 × 10-3 coul./V·cm2 obtained by the rapid method. Under a certain potentiostatic condition, the value of β' was almost equal to that measured by the galvanostatic method except in the initial period immediately after the change of the potential when β' is larger than 250 × 10-3 coul/V·cm2 and rather close to the β' obtained by the rapid method. From these results, it may be presumed that the significance of the measured values are essentially different in the galvanostatic method and in the rapid method. From the β' of the galvanostatic method, the plausible value of a ≈ 2.3 Å,as the thickness of the energy-barrier in the rate-process, was obtained under the assumption of α=0.5.
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