3.4.2 Anodic Polarization Curves
Figure 3.6 shows the measured pitting potentials versus solution layer thickness.
Over the thickness ranging from 40-280 µm, the pitting potential is independent of
solution layer thickness. The spread of breakdown potentials measured on the same
electrode configuration in a bulk solution of 1 M NaCl is also shown in Figure 3.6. There
is some indication that the thin film solution breakdown potentials are higher than those
in bulk solutions, but there is no clear trend.
In Figure 3.5 it can be seen that round and deep pits formed with an almost
hemispherical shape. In chapter 4 and in a publication by Tsutsumi et al. [20] pits grown
under thin layers of electrolyte were also found to be round but very shallow. However,
those pits were grown in concentrated electrolytes at OCP without potential control. The
pits in this study grew at a higher potential and much higher rate than the OCP pits. The
difference in pit morphology was likely related to the rate of pit growth.
Another factor that might have affected the pit morphology is the concentration of
the “bulk” environment. The experiments in this work were performed in chamber with
high humidity. The thin solution layer, which was initially in equilibrium with the humid
40
air environment (95% RH), became concentrated with ions as a result of the pitting
reaction. The highly concentrated solution layer was out of equilibrium with the humid
air and it rapidly absorbed water from the air, which was replenished by continual flow of
humid air through the chamber. In this fashion, the thin layer electrolyte essentially
became a thick layer and the concentration was diluted considerably.
Figure 3.8 shows images of a sample after anodic cyclic polarization immediately
after removal from the chamber and after rinsing and drying. The apparent crease was
associated with a thick precipitated corrosion product layer just beyond the edge of the
WE. The separation of anode and cathode by a very thin solution layer resulted in a steep
pH gradient and the formation of this precipitated product layer. The precipitated layer
prevented transport of metal cations to the CE, but did not inhibit current flow, which
continued during the experiment. The bubble evolution observed on the WE was
apparently hydrogen evolution, which was promoted by the need to maintain charge
neutrality in the thin solution layer.
The rapid absorption of water from the air into the thin solution layer did not
occur on naturally-formed pits in low humidity environment (34%) as shown in
Chapter 4. Therefore, it can be concluded that this phenomenon is associated with the
high relative humidity in the chamber. When the humidity was increased from 34 to over
90% in Chapter 4 the volume of the droplet also increased by water absorption which
diluted the highly concentrated droplet solution and the open circuit pit repassivated.
However, the solution volume increase was slower for OCP pits at high RH than for
potentiostatically controlled pits. Figure 3.4 shows that pits grew with a nominal current
41
density of 1.9 mA/cm2
which was the limit of the KPP. The actual current density was
much higher because the active pit area was a small fraction of the exposed area. The
OCP pits in chapter 4 exhibited an actual current density of 0.6 mA/cm2
. This difference
suggests that the higher pit growth rate under potentiostatic control in combination with
high RH leads to faster increase in ion concentration and therefore to faster water uptake