After a metastable pit is formed, it will either repassivate
or propagate. In the initial stage of propagation, walls of the
metastable pit are anodic sites at which metal dissolution takes
place. To keep electric neutrality, chloride ions must migrate from
the outer bulk solution into the pit. Therefore, the chloride ions
will gradually accumulate around the anodic sites, leading to high chloride concentration at the pit mouth. Part of the chloride ions
may form salt films with ferrous ions, while the rest diffuse away.
The chloride ions move in the horizontal direction under the effect
of concentration differences, and the sites where chloride ions
reach will immediately become an anodic site due to the low pH
in the solution. The diffusion of chloride ion towards all directions
from the pit mouth provides a good explanation for the extremely
regular shape of the large pit. The proposed pitting mechanism is
illustrated in Fig. 12. When the Cl− concentration is very high, however,
pitting corrosion is more intense on the steel surface whereas
the developing pit will be covered by a lot of corrosion products.
Thus the pit depth is extremely small and micro-crevices can form
between the corrosion products and the steel surface. This microcrevice
canforma local chemistry gradient andaccelerate the active
dissolution of steel in the crevice [61], which is confirmed by the
SEM images in Fig. 7f and the unsmooth cross-sectional profile
of pits in Fig. 11b.When the pH is very high (pH > 12), the chloride
ions may also diffuse from the pit mouth towards a horizontal
orientation, but the site where a chloride ion reaches will remain
passivated due to the lack of critical local chemistry thatis essential
for metal dissolution, as observed in high pH in bulk solutions. This
will explain the small pit sizes observed in alkaline solutions.