W. Richard Bowen and Nidal Hilal 4

132 4. INvEsTIgATINg MEMbRANEs ANd MEMbRANE PROCEssEs
Vignal et al. [18] also studied the electropolishing of steel surfaces
using the AFM. They observed that under a specific set of conditions
(in perchloric acid and monobutyl ether), a very regular network of hexagonal
cells of approximately 100 nm periodicity and 10 nm depth was
observed, Figure 4.27.
This hexagonal pattern was described as being extremely sensitive
to the applied voltage and was explained as being ‘prints of convective
cells…localised in a resistive sub-layer of 100 nm thickness in the viscous
shell’ [18].
4.8.2 Corrosion of Metal Surfaces
Almost all metal surfaces at some point in their lifetime either routinely
or sporadically come into contact with corrosive media, whether
it be aqueous solutions of high ionic strength or solutions of low pH. As
a result, their behaviour when coming into contact with such chemicals
and their ability to withstand corrosion play a significant part in their
lifetime, durability, efficiency and appearance.
One of the most common corrosive liquids present in the environment
are aqueous NaCl solutions. Equipment used in the desalination industry,
food processing and any equipment or structures in the maritime or nearmaritime
environment will routinely come into contact with salt water,
not to mention the corrosive effect on cars and roadside installations due
500 nm
FIgURE 4.27 Image obtained by AFM of regular hexagonal cells on steel surface
formed after electropolishing, observed by Vignal et al. [18].