(Keynote) Optical in-Situ Detection of Passive Oxides on Metals
Authors: Ohtsuka Toshiaki 
Published: February 2020

The passivation and passive oxide films on metals and their in-situ detection were reviewed. Various optical techniques combined with electrochemistry have been used for the in-situ detection of the oxide film.
(1) Ellipsometry and refrectometry are available for the thickness measurement.
(2) Raman spectroscopy for the composition identification,
(3) AC impedance and AC potential modulation reflectance for discussion of semiconducting and dielectric properties.
For the investigation of the passive oxide film on metal electrodes, we must first consider whether the passive film-covered electrode system is placed under a stationary state or non-stationary state. Under the stationary state, the current on the passivated metal electrode is determined by a cationic transfer rate (dissolution rate) at the oxide/ solution interface. Because the ionic transfer rates are a function of the potential difference at the oxide/ solution interface and the interfacial potential difference is a function of solution pH, the dissolution rate can be described as a function of solution pH. Under the stationary state, the thickness of oxide film is kept constant and the ionic transfer rates at metal/ oxide and oxide/ solution interfaces and the ionic migration rate in the oxide film are equal to each other. When the stationary CD is the order of 0.01 μA cm–2 or less, time period to reaching the stationary state may be necessary for 1 d or longer and thus the stationary state cannot be easily reached in the experimental time. Usually we discuss the passive oxide films under the pseudo-stationary state condition. When one uses the results during the potentiodynamic condition, one should recognize the results under a non-stationary condition and the results are much different from those under the stationary state and much depend on the starting condition.
For iron, aluminum, titanium etc. on which n-type and insulating oxide films are formed, the surface passive oxide linearly grows in thickness with increase of anodic potentials. The ionic migration rate is determined by the electric field in the oxide film according to the high-field ionic migration model after Cabrera and Mott 1-5)

i = i0 exp [B(dE/dd)]
i0 is a function of densities of migrating species in oxide film and (dE/dd) is a electric field in the oxide film and approximately as described by ΔE/d where ΔE and d is a potential drop in the oxide film and thickness of the oxide film, respectrivily. For example, under constant current oxidation, ΔE/d should be constant and thus thickness (d) is linearly increased with potential increase 6, 7). Under constant potential oxidation, log(i) linearly decreased with (1/d) 8).
The difference between the semiconducting and insulating oxide films is seen in the higher potentials than an equilibrium potential of O2/ H2O redox. When the bang-gap energy of the semiconducting oxide is more or less 2.5 eV, the electron transfer between the inner conduction band to the redox species in the solution can occur via the tunneling process over the barrier of the space charge layer. The current corresponding to the oxygen evolution reaction emerges in the passive state and exponentially increases with increase of potential. When the band-gap energy is much larger, the tunneling process is inhibited and no current of the oxygen evolution reaction is observed. The oxide film of the insulating property such as the oxides on aluminum, tantalum, and silica can grow to several 100 nm thickness without evolution of oxygen gas.
For the passive oxide film on stainless steel, simple relation between CD and thickness is not establish, because the composition of oxide film changes with time, Cr component enriching with increase of time 9). For the passive film consisting of p-type oxide such as Ni passive oxide, the thickness is not described as a function of potential by a simple relationship 10).
References
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