W. Richard Bowen and Nidal Hilal 4

At pH 5.5, dissolution began to occur, but only at the step edges, not in the
centre of the crystal planes. At pH values below this, as the pH decreased
the dissolution rate increased, with a significant change in behaviour
occurring below pH 4.0. Below this value, dissolution also began to occur
at the flat crystal surfaces, although it was still most pronounced at the
edges. It was suggested that this behaviour was due to the presence of
oxygen in the step edges, which would be much more prone to attack by
solution protons than the flat surfaces, which are positively charged at the
higher pH values.
A number of other studies have used AFM alongside other techniques
to characterise a number of different coatings to protect metal surfaces
from corrosion and wear, including TiN and ZrN thin films [24]; Sn–Ni
and Sn–Cu alloy coatings [25, 26]; CrN films for enhanced wear resistance
[27] and zirconia-based primers as coatings on aluminium [28].
4.8.3 biofilms at Metal Surfaces
4.9 CONCLUsIONs 135
Yu and colleagues [29] examined the difference between nano- and
microcrystallization of stainless steel surfaces and the effect on the adhesion
of biofilms. Using an AFM tip coated in a synthetic peptide to simulate
a biofilm, adhesion measurements were made against nanocrystalline
and microcrystalline surfaces. It was found that the measured adhesion
in air was significantly reduced for the nanocrystalline surface compared
with the microscrystalline surface. These observations were matched by
similar results from the observations of attachment of whole bacterial
cells to the same surfaces in bulk solution.
4.9 COnCLUSIOnS
The many benefits of AFM in investigations of membranes and membrane
processes may be summarised as:
1. Atomic force microscopy can determine the key properties of synthetic
membranes: pore size distribution, surface morphology and
surface roughness, surface electrical properties and surface adhesion.
2. Correspondence between surface pore dimensions from AFM and
MWCO is good. In addition, AFM gives surface pore size distribution.
3. Operations in liquid and colloid probe techniques are particular
advantages of AFM.
4. AFM can establish the effects of changes in interactions over the surface
of membranes, e.g. due to local morphology.
5. AFM allows the visualisation of solute/membrane interactions.
6. AFM is a very useful asset in assessing the properties of membranes
during their development.