Chemistry Reference
In-Depth Information
Because the hydrogen termination process takes a considerable amount of time,
it is an important element contributing to the oscillation process.
The rate of hydrogen termination varies with crystal orientation, is fastest on
the (111) surface, and thus determines the probability of a current burst on sur-
faces of different orientations.
According to this model, as illustrated in Fig. 8.63 cycle of the processes occur involv-
ing active dissolution, oxide formation and dissolution, and hydrogen termination for
a reaction unit on a growing pore bottom.
1087
The current burst model is potentially powerful in providing explanations for
many mechanistic and morphological aspects involved in the formation of PS.
However, as recognized by Foll
et al
themselves, it would be extremely difficult for
such a unified model to be expressed in mathematical form because it has to include
all of the conditional parameters and account for all of the observed phenomena. Fun-
damentally, all electrochemical behavior is in nature the statistical averages of the
numerous stochastic events at a microscopic scale and could in theory be described by
the oscillation of the reactions on some microscopic reaction units which are tempo-
rally and spatially distributed. Ideally, a single surface atom would be the smallest
dimension of such a unit and the integration of the contribution of all of the atoms in
time and space would then determine a specific phenomenon. In reality, it is not pos-
sible because one does not know with any certainty the reactivity functions of each
individual atoms. The difficulty for the current burst model would be the establishment
of the reactivity functions of the individual reaction units. Also, some of the assump-
tions used in this model are questionable. For example, there is no physical and chem-
ical foundation for the assumption that the oxide covering the reaction unit is
.
