from EM to ED. As the power potential sweeps periodically from
the value found in the active region to that in the passive
region, the ion distribution near the surface of the electrode
changes with imposed potential and the polarization curve is
repeated. The two factors, local concentration gradient and the
induced opposite electric field, are therefore coupled together
in a complementary fashion. Thus, if the local concentration
near the electrode is not high enough as in the case of the active
region from EM to EP in Figure 1A, then the induced electric
field is also not important.
In addition, the static oxide film theory does not account for
experimental data obtained from other metals. Anodic
passivation is quite commonly observed for metals, although
the anodic curves2 are not as typical as observed for iron shown
in Figure 1. It is not surprising that different metals give
different passivation curves as they have different mobility. For
example, the mobilities of Na+ and Fe2+ are different. The fact
that anodic passivation is commonly observed for metals
emphasizes that it is more likely to be caused by the reversible
transport nature of these ions than by the formation of
irreversible passive oxide films. It is also worth noting that the
typical passive curve shown in Figure 1A for iron only appears
at electrolyte concentration ranges where the concentration is
dilute but still capable of conducting electricity, showing the
importance of transportation conditions (Figure 3 in the
Supporting Information). Indeed each scan is different (Figure
1B) because pH (Figures 1 and 2 in the Supporting
Information) and the concentration (Figure 3 in the
Supporting Information) change. When ion concentration is
high (Figure 3 in the Supporting Information) the passive effect
might be suppressed and there are enough ions to respond to
the driving potential to generate a current. The experiments
were conducted without solution stirring, which enhances the
concentration difference between different scans. Any supporting
electrolyte that is present changes the transportation
conditions and has a significant effect on current conduction.
Thus, at higher electrolyte concentrations with strong
conductivity, the passive curve is not so well characterized as
that in Figure 1A, consistent with the fact that transportation
effects are important for passivation. As the concentration
increases, the passive region will finally disappear at the end of
many of the subsequent scans even though the tarnished film
still existed, presumably because the ion concentration became
higher and more ions are available to respond to the outside
voltage and thus improve conductivity. Thus, the obstacles for
ion migration are then overcome, but not purely because of the
dissolution of the static passive oxide film. It is reasonable to
propose that transportation limitations result in an effective
“dynamic passive film” of an ion layer but not a dynamic
formation and disappearance of a real “static passive film”.
Although the oxide film has been characterized by some
techniques,2 it was also noted9,10 that the hydrated iron oxides
formed in the experiment are soft and do not prevent further
oxidation. So, the static film is not so effective that it can
prevent the generation of current. It is no surprise that the
oxide film could be characterized by sophisticated techniques as
the tarnished film on the scanned iron anode and on the iron
treated with concentrated sulfuric acid can be seen even by the
naked eye. Whether the active region from EM to EP or the
passive region from EP to ED in Figure 1A is involved, the
tarnished oxide is still there, showing little evidence for the
effect of the formation and disappearance of the static film but
rather showing the likelihood of the presence of a reversible ion
layer. The tarnished oxide could not fully prevent the
appearance of the active region in the anodic polarization
curve, which is experimentally proved by the reappearance of
the same curve pattern when repeating the experiment with the
same iron anode.
Our new theory concerning anodic curves can be applied to
more complex phenomena and this is discussed in detail in the
Supporting Information.