W. Richard Bowen and Nidal Hilal 4

4.8 CHARACTERIsATION OF METAL sURFACEs 133
to salt spray from roads during winter. Sanchez and colleagues [19] examined
the early stages of corrosion of high-strength steel in dilute (50 mM)
solutions of NaCl. The steels examined were of the type used for the reinforcing
of concrete structures, which are prone to corrosion when exposed
to salt water due to the porous nature of the concrete and consisting of
ferrite and cementite. The authors scanned the same area of the steel surface
for over 2 h and observed changes in the surface morphology due to
interaction with the corrosive media. Snapshots of the sample at different
times are shown in Figure 4.28.
Initially (t � 0) the polishing marks and scratches are still visible, but
disappear after 50 minutes, when the surface was observed to be increasingly
rough as the growth of oxides on the surface occurred. After approximately
50 min, and later, a series of ridges were observed to appear. The
authors concluded that this was the lamellar structure of pearlite (the
mixture of ferrite and cementite phases) appearing as the ferrite matrix
was selectively attacked by the salt solution. The change in the surface
roughness was monitored as a function of time, allowing the quantitative
assessment of corrosion of the surface, Figure 4.29.
Z: 24.1nm
X: 5.0µm
A B
Z: 40.1nm
X: 5.0µm
C D
Z: 21.6nm
Z: 151.8nm
X: 5.0µm
X: 5.0µm
FIgURE 4.28 Steel surface after varying lengths of exposure to 50 mM NaCl solution.
(a) 0 min; (b) 30 min; (c) 50 min; (d) 2 h 15 min. Reproduced from [19].