4.2. Difference between TAl, TH2SO4 and the actual anodizing
temperature
In this paper the influence of the temperature on the anodizing
process is considered by controlling and varying the electrode and
the electrolyte temperatures. However, the temperature, which is
referred to as actual anodizing temperature, is the temperature of
the barrier layer of the porous oxide. The considered electrode
and electrolyte temperature both affect the anodizing temperature,
though, due to experimental limitations the actual value of
the latter remains unknown. Despite the enhanced temperature
control by the considered electrode holder, the direct measurement
of this parameter is experimentally not possible; nor can
it directly be applied. Nonetheless, the electrode temperature is
believed to be a better approximation for the anodizing temperature
than the electrolyte temperature. The aluminium substrate
displays a very high thermal conductivity, whereas in addition
a strong adherence between the barrier oxide layer and the
underlying metal exists. The electrolyte is characterized by a significantly
smaller thermal conductivity, whereas inside the pores
heat transfer in the electrolyte also primarily occurs by thermal
conduction: due to the very small dimensions of the pores, the
electrolyte in the pores is assumed to be stagnant. Hence, the
influence of TH2SO4 on the temperature of the thin barrier oxide
layer is expected to be smaller than that of TAl. This hypothesis
is in line with the observed higher impact of the electrode
temperature than of the electrolyte temperature on the aspects
of the anodic oxide growth, related to the processes occurring in
the barrier oxide. Examples of the latter are the electrochemical
behaviour of the aluminium anode, the transport number of
the an- and cations, and the electrical field strength in the barrier
layer.