We also measured the depth of the corrosion pits using a 3D
measuring laser microscope. Fig. 10a shows the 3D image of a
small pit – which can be considered as the typical morphology of
a metastable pit or a pit in early stage – and a large pit – which
can be considered as the typical morphology of a fully developed
pit. The corresponding cross-sectional profiles of the small pit and
the large pit in acidic solutions with 0.05 mol/L NaCl are shown
in Fig. 10b. We can clearly see from Fig. 10a and b that the pit
depth remains almost unchanged with time. In acidic solutions
with 1 mol/L NaCl, the laser microscope image and the cross section
profile are shown in Fig. 11. The pit depth is typically smaller
than 1 m.