The electrolyte is intensely stirred and does not stagnate near
the electrode surface. Nonetheless, this surface remains in contact
with the electrolyte and there is heat transfer from the electrode to
the electrolyte and vice versa. At the oxide–electrolyte interface a
thermal boundary layer develops, in which the temperature of the
electrolyte varies from the temperature at the surface of the oxide,
that is generally different from the substrate controlled temperature,
to the considered bulk electrolyte temperature. In this way
the oxide is partly shielded from the aggressivity of the electrolyte
when the electrode temperature is lower than the electrolyte at
elevated temperatures. This, for example, explains why in an electrolyte
at 65 ◦C a normal, consistent oxide layer is formed at an
electrode temperature of 5 ◦C—cf. Fig. 5(b).
On the other hand, the opposite occurs when considering high
applied electrode temperatures in an electrolyte at low temperature.
At an electrode temperature of 65 ◦C the oxide layer is
exposed to a locally warmer and thus more aggressive electrolyte.
The observed enhanced oxide dissolution and resulting collapse
of the porous structure at the outer surface in Fig. 4(d) on an electrode
anodized at an electrolyte temperature of 5 ◦C in combinationwith an applied electrode temperature of 65 ◦C are explained in
this way.
Despite the applied electrode temperatures the influence of
the electrolyte temperature on the anodic potential responses and
formed oxide layers, see respectively Figs. 3 and 6, persists. On
one hand this indicates that even by applying the electrode temperature
the influence of the electrolyte is not eliminated. On the
other hand, it is also an indication of the adequate convection in
the electrochemical cell. Due to a sufficient replenishment near the
electrode surface, the temperature of the electrolyte only differs
from its bulk value in a thin layer near the surface. If the forced
convection would be reduced during anodizing a larger boundary
region, with a local electrolyte temperature different than in the
bulk of the solution, might develop. In this case the influence of
the electrolyte temperature on the process is possibly also further
reduced.
The average oxide thicknesses (Fig. 6) display an increased
decline when temperatures of 45 ◦C, or higher, are considered. This
observation holds for the electrode temperature, as well as for the
electrolyte temperature, and becomes more pronounced at elevated
values for both temperatures. In general, a stronger decline
of the oxide thickness as a function of the electrode temperature is
observed, than as a function of the electrolyte temperature. For the
interpretation of this fact a distinction needs to be made between
two different situations.