3.5 Scanning Probe Methods
73
The operation mode of an AFM apparatus depends on in which interaction regime
of the tip and the test specimen is utilized (see Fig. 3.7, right). In non-contact mode,
the interaction is attracting, and the tip oscillation is around the distance of maximum
attracting force. In the tapping mode, the tip touches the surface at the end of its
motion towards the surface, while in the contact mode, the tip contacts the specimen
surface throughout the entire test, and hence, interaction turns to be repulsive. The
danger of damaging the surface increases in the order of the list of operation modes
described above.
For the observation of surface topography, the measurement of force in the direction of the surface normal is of prime importance. Nevertheless, the measurement
of lateral forces is also possible, which allows one to test the adhesion strength of
namometric objects or even molecules to the surface. By using magnetic tips, we
arrive at magnetic force microscopy (MFM) that measures the magnetic interaction
between the tip and the surface and accounts for local field strength.
3.6 Corrosion Studies
The reason why corrosion test must be dealt with is that the original version of
several sorts of these tests used to be developed for materials much simpler than
the nanostructures synthesized nowadays. Therefore, the blind application of some
corrosion tests may produce misleading results.
Non-electrochemical corrosion tests involve the exposure of the specimens studied
to a certain media (salt solution, steam etc.) without either controlling or measuring
the electrode potential of the samples. The absolute rate of general corrosion can
be determined from the weight loss of the specimen. The corrosion performance,
however, is defined somewhat differently from the rate of general corrosion. For
instance, the time elapsed until the appearance of red rust is an important parameter
for rating of relative corrosion performance of various coatings on iron-containing
specimens. Other corrosion test methods are designed to assess the inclination of a
material to some specific mode of corrosion. One of these methods involves the reaction with FeCl 3 solution and the areal density of cavities formed bear information
on the inclination to pitting corrosion. Many non-electrochemical test methods are
standardized, and various fields of industry develop their on corrosion test protocols.
These tests are applicable for nanostructured materials in the case only when the
samples comply with the requirements of the standards (for instance, a nanocrystalline coatings of large surface area are suitable for such tests, while individual
nanoparticles are not).
Concerning electrochemical corrosion studies, some methods apply a large potential perturbation. The most common method is to record Tafel polarization data for
both the cathodic and anodic direction as referred to the corrosion potential. Although
the manner as corrosion test is displayed is quite analogous to that presented in
Fig. 2.5b for an equilibrium reaction, there are fundamental differences between the
two measurements. While in an equilibrium reaction, the cathodic and anodic currents
73
The operation mode of an AFM apparatus depends on in which interaction regime
of the tip and the test specimen is utilized (see Fig. 3.7, right). In non-contact mode,
the interaction is attracting, and the tip oscillation is around the distance of maximum
attracting force. In the tapping mode, the tip touches the surface at the end of its
motion towards the surface, while in the contact mode, the tip contacts the specimen
surface throughout the entire test, and hence, interaction turns to be repulsive. The
danger of damaging the surface increases in the order of the list of operation modes
described above.
For the observation of surface topography, the measurement of force in the direction of the surface normal is of prime importance. Nevertheless, the measurement
of lateral forces is also possible, which allows one to test the adhesion strength of
namometric objects or even molecules to the surface. By using magnetic tips, we
arrive at magnetic force microscopy (MFM) that measures the magnetic interaction
between the tip and the surface and accounts for local field strength.
3.6 Corrosion Studies
The reason why corrosion test must be dealt with is that the original version of
several sorts of these tests used to be developed for materials much simpler than
the nanostructures synthesized nowadays. Therefore, the blind application of some
corrosion tests may produce misleading results.
Non-electrochemical corrosion tests involve the exposure of the specimens studied
to a certain media (salt solution, steam etc.) without either controlling or measuring
the electrode potential of the samples. The absolute rate of general corrosion can
be determined from the weight loss of the specimen. The corrosion performance,
however, is defined somewhat differently from the rate of general corrosion. For
instance, the time elapsed until the appearance of red rust is an important parameter
for rating of relative corrosion performance of various coatings on iron-containing
specimens. Other corrosion test methods are designed to assess the inclination of a
material to some specific mode of corrosion. One of these methods involves the reaction with FeCl 3 solution and the areal density of cavities formed bear information
on the inclination to pitting corrosion. Many non-electrochemical test methods are
standardized, and various fields of industry develop their on corrosion test protocols.
These tests are applicable for nanostructured materials in the case only when the
samples comply with the requirements of the standards (for instance, a nanocrystalline coatings of large surface area are suitable for such tests, while individual
nanoparticles are not).
Concerning electrochemical corrosion studies, some methods apply a large potential perturbation. The most common method is to record Tafel polarization data for
both the cathodic and anodic direction as referred to the corrosion potential. Although
the manner as corrosion test is displayed is quite analogous to that presented in
Fig. 2.5b for an equilibrium reaction, there are fundamental differences between the
two measurements. While in an equilibrium reaction, the cathodic and anodic currents