Chemistry Reference
In-Depth Information
concentration are shown in Fig. 5.7. All the currents increase with increasing fluoride
concentration but exhibit a maximum. The plateau currents, i.e., and which are
determined by the chemical dissolution rate of the oxide film, have been empirically
fitted to pH and fluoride concentration:
The general features of the
i-V
curves at potentials positive to those of in fluoride
solutions are relatively insensitive to the presence of anions such as or
939
The
effect of cations is, however, significant as shown in Fig. 5.8 for different types of
cations.
861,936
Thus, the dissolution rate of the oxide is different in solutions containing
different cations, which is attributed to the change of the aggressiveness of the elec-
trolyte since the property of the oxide is little affected by the electrolyte in which it is
formed. According to Hassan
et al
.,
861
the effect of cations on the dissolution kinetics
is via adsorption on the oxide surface resulting in an increased polarization of the Si-O
bonds. The largest increase in current is obtained with the cations that are most easily
adsorbed. The change in reaction rates in the presence of the cations can also be par-
tially attributed to the difference in the solubilities of the corresponding fluorides and
Thus, the large hysteresis on the
silicates.
936
i-V
curves of
and
may be due to
the formation of an insulating salt layer of and
may be related to their tendency
respectively, whereas
the decreased current on addition of
to form low-solubility fluoride compounds or complexes.
or
