3.3. Influence of temperature on the formation ratio
The influences of the electrode and the electrolyte temperatures
on the formation ratio of the barrier layer thickness is evaluated on
electrodes anodized under three different combinations of electrode
and electrolyte temperatures:TAl = TH2SO4 = 5 ◦C(referred
to as ‘TAl05TH05’), TAl = 5 ◦C at TH2SO4 = 65 ◦C (‘TAl05TH65’),
and TAl = 65 ◦C at TH2SO4 = 5 ◦C (‘TAl65TH05’). For each of these
electrodes the thickness of the barrier layer is determined
by both electrochemical impedance spectroscopy and FE-SEM
analyses.
The Bode plot, containing characteristic experimental spectra
recorded on the three types of electrodes by EIS, is presented in
Fig. 7. These spectra reveal the presence of two time constants.
The time constant in the low frequency regime is associated with
the capacitive behaviour of the barrier layer, whereas that in the
high frequency range accounts for the capacitive behaviour of the
porous oxide structure [27–30]. The equivalent electronic circuit,
3962 T. Aerts et al.
Fig. 7. Experimental Bode plot of the characteristic impedances of the three different
considered types of electrodes.
considered for the interpretation of the experimental data, is based
on the circuit proposed by Hoar and Wood [28] and is presented
by the model on the left in Fig. 8 (the solid lines). The two constant
phase elements (CPEs) CPEwall and CPEbarr respectively represent
the capacitive behaviour of the porous oxide structure, and of the
barrier layer. For both features a CPE is considered instead of a ‘pure’
capacitance to be able to account for their possible non-ideal capacitive
behaviour [31,32]. The remaining components Rsol, Rpore and
Rbar respectively represent the resistive behaviour of the solution,
of the solution in the pores, and of the barrier layer. Due to the
difference in their frequency dependency it is possible to separate
the contributions of the barrier layer and of the porous structure to
the overall frequency response of the oxide layer. For example, the
spectra of Fig. 7mainly contain information on the impedance of the
barrier layer; to be able to fully observe the impedance response
of the porous structure higher frequencies should be considered.
Therefore, the general equivalent circuit at the left in Fig. 8 can be
simplified to the equivalent model depicted at the right-hand side
in this figure. Based on the latter, the capacitive behaviour of the
barrier layer is determined.