13.6 Pursuits Related to the Extension of the Anodizing Conditions
493
13.6.2 Hard Anodization
Hard anodization was a well-established industrial process much before laboratoryscale mild anodization of regular PAA templates. However, it was notorious of irregular pore formation, and the process was hard to regulate. These problems have been
overcome in the last two decades, and hard anodization processes have been developed for some sulphuric acid [119–121] and oxalic acid [100, 122–124] baths. The
current density of hard anodization is about an order of magnitude larger than that
of mild anodization, which leads to a much faster PAA layer formation. Also, the
empirical porosity rule established for mild anodization is not valid for hard anodization; therefore, new pore size and porosity ranges became available. In order to avoid
the burning of the samples, the electrical control of the experiments is often different
from mild anodization. A gradually increasing voltage and the application of current
control instead of the voltage control are often mentioned as a tool to avoid cracking
of the samples and to increase the mechanical stability of the resulting PAA layer.
13.6.3 Disparity Between the First and Second Anodization
Steps
The ordinary thumb rule for PAA anodization was that the first (and later sacrificed)
anodized layer has to be prepared under identical conditions than that applied for the
second anodization in order to achieve the maximal regularity in the pore arrangement. Later, various strategies have been elaborated to destroy this rule. It is possible
to combine two mild anodization steps with different electrolyte solutions [125], but
the process may also involve hard anodization as the first step [126–128]. This leads
to a faster sample preparation than the pore ordering process during mild anodization.
The interpore distance and regularity of the pore arrangement is determined by the
first anodization, while the wall thickness depends on the second anodization step.
The natural interpore distance of the two steps must be close enough so that no pore
branching takes place in the second step. The advantage of the dissimilar conditions
in the first and second anodizations is that the tendency for good ordering in the hard
anodization (that is less controllable in the long run) can be coupled with the easily
performed mild anodization.
13.6.4 Increase in Anodization Temperature
Extensive efforts have been made in order to extend the anodization temperature
range to or above room temperature with retaining the regularity of the pore arrays,
especially for oxalic acid solutions [117, 129–137]. The motivation of this pursuit
partly originated from the reduction of cost (sparing the installation of a cooling
493
13.6.2 Hard Anodization
Hard anodization was a well-established industrial process much before laboratoryscale mild anodization of regular PAA templates. However, it was notorious of irregular pore formation, and the process was hard to regulate. These problems have been
overcome in the last two decades, and hard anodization processes have been developed for some sulphuric acid [119–121] and oxalic acid [100, 122–124] baths. The
current density of hard anodization is about an order of magnitude larger than that
of mild anodization, which leads to a much faster PAA layer formation. Also, the
empirical porosity rule established for mild anodization is not valid for hard anodization; therefore, new pore size and porosity ranges became available. In order to avoid
the burning of the samples, the electrical control of the experiments is often different
from mild anodization. A gradually increasing voltage and the application of current
control instead of the voltage control are often mentioned as a tool to avoid cracking
of the samples and to increase the mechanical stability of the resulting PAA layer.
13.6.3 Disparity Between the First and Second Anodization
Steps
The ordinary thumb rule for PAA anodization was that the first (and later sacrificed)
anodized layer has to be prepared under identical conditions than that applied for the
second anodization in order to achieve the maximal regularity in the pore arrangement. Later, various strategies have been elaborated to destroy this rule. It is possible
to combine two mild anodization steps with different electrolyte solutions [125], but
the process may also involve hard anodization as the first step [126–128]. This leads
to a faster sample preparation than the pore ordering process during mild anodization.
The interpore distance and regularity of the pore arrangement is determined by the
first anodization, while the wall thickness depends on the second anodization step.
The natural interpore distance of the two steps must be close enough so that no pore
branching takes place in the second step. The advantage of the dissimilar conditions
in the first and second anodizations is that the tendency for good ordering in the hard
anodization (that is less controllable in the long run) can be coupled with the easily
performed mild anodization.
13.6.4 Increase in Anodization Temperature
Extensive efforts have been made in order to extend the anodization temperature
range to or above room temperature with retaining the regularity of the pore arrays,
especially for oxalic acid solutions [117, 129–137]. The motivation of this pursuit
partly originated from the reduction of cost (sparing the installation of a cooling