Explanations for the Passive Region
The solution is neutral whether there is current or not.
However, this does not mean that there is no concentration
gradient near the double layer especially when a potential is
applied. To understand why there is no current in the passive
region between EP and ED (Figure 1A), two factors are
important, namely, the concentration gradient between the
electrode and bulk solution and the electric field between
electrodes, both of which provide a driving force for ion
transportation. A higher concentration gradient increases the
ion diffusion rate and produces a higher electric current.
Likewise, as another independent effect, a higher electric field
increases the ion migration rate and also produces a higher
electric current. It seems contradictory that the current is low at
the passive region where the concentration gradient and the
electric field are high. Here we propose an explanation by
considering the local concentration gradient and an induced
opposite electric field.
First, from the perspective of the local concentration
gradient, it can be considered that the concentration gradient
between the bulk solution and the electrodes is high in the
passive region. Even though the concentration gradient is high
at the diffusion layer in the higher-potential region, the local
concentration gradient near the electrode in the electrical
double-layer is low because a large number of ions are
concentrated within this region, and the ions cannot diffuse out
because they accumulate very fast and are held in position by
the higher voltage, though they will be released on reducing the
voltage.
An important feature of a series circuit is that the current is
the same everywhere. So, the low local concentration gradient
near the surface of electrodes is also responsible for the low
current in the passive region. Diffusion limited by high voltage
as described above should be the basis for the success of any
kinetic explanation.
Second, from the perspective of the electric field, it can be
assumed that the electric current is high when the electric field
is high as long as the insulating oxide cannot be formed quickly
on the anode, which is the case in the passive region. However,
at high anodic potential, the concentration of iron ions is high
near the anode in the double-layer as shown in Figure 2, and
the iron ions are congested near the anode being unable to
release their charges (or electrons for iron metal) due to their
close proximity to each other. Usually, high potential
differences lead to rapid electrochemical reaction. Here,
however, the dynamics caused by the congested ions hinders
the electrochemical process. This also results in a low local
concentration gradient because the ions cannot diffuse out
efficiently; that is, the chemical reaction at the electrodes is slow
and the diffusion of the ions is also slow because of the outside
potential, thus the driving voltage VA′D′ provided in Figure 2
maintains different concentrations of anions and cations at
cathode and anode, respectively, and cannot release the charges
of the ions. In other words, the Fe2+ ions are trapped on the
iron surface because of the driving voltage VA′D′. Thus, when
there is no current as in the passive region for a typical case,
Fe2+ is retained near the anode as shown in Figure 2.
The charge on the electrode should migrate to the electrode
surface. The outside voltage, EA′D′, provided by the outside
circuit keeps the ions on the surface of the electrode and they