W. Richard Bowen and Nidal Hilal 4

Dfractional
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
4.8 CHARACTERIsATION OF METAL sURFACEs 127
CA202, ES404
S5-20
T5-20
0.00
0 5 10 15
JHA / k
20 25 30
FIgURE 4.22 Fractional deposition of humic acids at different operating fluxes. The
intercepts on the x-axis indicate the critical fluxes.
applications in (bio) process industries. Many environments to which
they may become exposed are potentially corrosive, and thus more
corrosion-resistant surfaces may have greater operational lifetimes. In
addition, surfaces may become fouled, leading to a reduction in operating
efficiency, especially for surfaces involved in the desalination and
water treatment industries. This section will concentrate on the use of
AFM to characterise the surface features and morphology of metal surfaces
of interest in process engineering applications.
One of the most commonly used materials in process equipment is
stainless steel in various forms. In Figure 4.23, by way of example, are
shown three 3-dimensional 50 � 50 �m scans of different steel surfaces of
difference roughness characteristics, imaged in air [12].
These are: (a) a ‘highly polished’ stainless steel surface with an optically
fine finish, (b) the surface of a stainless steel sample disk, such as
that commonly used for affixing AFM samples and (c) a sample stub
similar to that in (b), which has undergone significant pitting owing to
corrosion. Surface (b) contains ridges caused by the mechanical polishing
of the surface, which are not visible in (a). For the corroded sample,
there is clearly a much greater variation in the height of surface
features and a greater number of asperities. This can be seen from the
roughness parameters obtained from the height data contained in these