During the entire anodizing treatment, i.e., from the onset
of the anodic oxide growth, a constant electrode temperature is
considered. This is ensured by the following experimental procedure,
which is systematically applied. Before introducing the
temperature controlling electrode holder into the electrolyte, the
temperature of the mounted electrode is set to the desired value.
Only after attaining a stable value of this electrode temperature
the holder is immersed into the solution. Due to considered temperature
differences between the electrode and the electrolyte
(electrolyte temperatures, different from the electrode temperature,
are evaluated) a minor deviation of the electrode temperature
from its intended value can occur. Therefore, the holder is given the
time to re-adjust the electrode temperature: from the moment of
immersion on, a fixed timespan of 90 s is waited before applying
the anodic current. Under all considered conditions this period is
sufficient to guarantee a constant electrode temperature from the
start of the experiments on.
Electrochemical impedance spectroscopy (EIS) is performed in
an electrochemical cell with a three-electrode setup including a
Pt grid counter electrode and a Ag/AgCl reference electrode. 0.5 l
of a 0.5 M Na2SO4 solution at room temperature is used as electrolyte.
An Autolab PGStat 30 potentiostat, in combination with
the Autolab Nova© software, is used. The following approach is
adopted for the EIS. Preliminary to the actual impedance analysis
the Open Circuit Potential (OCP) of the electrode is measured by
the potentiostat. During the subsequent impedance spectroscopy
10 mV peak-to-peak sinusoidal potential perturbations are applied
at the potential, corresponding to the determined value of the OCP.
Potential perturbations at 50 frequencies in the range from 0.01
up to 10 kHz are considered. Processing and fitting of the acquired
data is done using the ZView software of Scribner Associates,
Inc.
FE-SEM analyses are performed using a Jeol JSM-7000F FE-SEM.
Charging effects due to the non-conductive properties of the oxide
are avoided by covering the samples for FE-SEM analysis with a
1.5 nm Pt/Pd layer, applied by a Cressington 208 HR sputter coater
equipped with a Cressington MTM-20 thickness controller. On the
FE-SEM images the thickness of the observed cross-sectioned oxide
and barrier layers is measured using the Jeol Smile View software.
The values of these parameters are based on at least 55 measurements.