192
6 Nanocrystalline Deposits
In contrast to the saturation magnetization, coercivity, H C , is rather sensitive to
the grain size. For microcrystalline materials, coercivity is inversely proportional to
the grain size (H C ~ d
−1 ). However, in nanocrystalline materials, the grain size is
often smaller that the exchange correlation length. This is the limit where the local
magnetocrystalline anisotropy of the grains with statistical orientation distribution
becomes negligible. In this case, the local magnetic anisotropy is averaged out and
the remagnetization process is determined by an effective anisotropy constant that
is proportional to d
6 . This means that nanocrystalline materials often show ultrasmall coercivity as compared to their microcrystalline counterparts. Even though
the transition from the H C ~ d
−1 regime to the H C ~ d
6 regime is well accepted
as nanocrystallinity occurs [27], it is scarce that the entire crystallite size range can
be mapped for one single material. For electrodeposited nanocrystalline materials,
the impurities accumulated at the grain boundaries may serve as pinning centres
that block the domain wall motion and may distort the behaviour as compared to
theoretical predictions.
Corrosion rate of nanocrystalline metals. As it is agreed in various reviews of the
corrosion of nanocrystalline metals [28–31], the crystallite size has a great impact on
the corrosion rate. As it was claimed above in connection with the thermal stability,
the nanocrystalline state is thermodynamically unstable, which may suggest that such
materials can dissolve in an aggressive medium at a higher rate than a similar material
with larger grain size. Although this is true, the higher inclination for dissolution does
not necessarily leads to an increase in corrosion rate. Due to the increased driving
force for dissolution, the key parameter is the fast formation of the passive layer as
compared to microcrystalline materials. Hence, the corrosion rate can be significantly
reduced for nanocrystalline metals as compared to their microcrystalline counterparts
due to the rapid passivation and the uniformity of the passive layer. In contrast, when
no passivity can take place or the corrosion rate estimation is performed with a highcurrent method, the dissolution rate of a nanocrystalline material is usually larger
than that of the corresponding coarse-grained metal.
In either of the above cases, nanocrystalline materials exhibit an advantage that
is related to the uniformity of corrosion. Due to the nanocrystalline structure, the
possibility to local corrosion attack that often leads to a fatal corrosion-related material failure is very limited, and the corrosion of a nanocrystalline metal is rather of a
laterally even rate without a development of deep cracks. This feature lends a high
potential to nanocrystalline coatings in corrosion protection.
6.2 Nanocrystalline Deposits of Metallic Elements
6.2.1 Noble Metals: Au, Ag and Pd
The electrolyte solutions used for electrodeposition of nanocrystalline gold include a
commercial sulphite-type solution with Au(III) [32] and an Au(I) solution prepared
6 Nanocrystalline Deposits
In contrast to the saturation magnetization, coercivity, H C , is rather sensitive to
the grain size. For microcrystalline materials, coercivity is inversely proportional to
the grain size (H C ~ d
−1 ). However, in nanocrystalline materials, the grain size is
often smaller that the exchange correlation length. This is the limit where the local
magnetocrystalline anisotropy of the grains with statistical orientation distribution
becomes negligible. In this case, the local magnetic anisotropy is averaged out and
the remagnetization process is determined by an effective anisotropy constant that
is proportional to d
6 . This means that nanocrystalline materials often show ultrasmall coercivity as compared to their microcrystalline counterparts. Even though
the transition from the H C ~ d
−1 regime to the H C ~ d
6 regime is well accepted
as nanocrystallinity occurs [27], it is scarce that the entire crystallite size range can
be mapped for one single material. For electrodeposited nanocrystalline materials,
the impurities accumulated at the grain boundaries may serve as pinning centres
that block the domain wall motion and may distort the behaviour as compared to
theoretical predictions.
Corrosion rate of nanocrystalline metals. As it is agreed in various reviews of the
corrosion of nanocrystalline metals [28–31], the crystallite size has a great impact on
the corrosion rate. As it was claimed above in connection with the thermal stability,
the nanocrystalline state is thermodynamically unstable, which may suggest that such
materials can dissolve in an aggressive medium at a higher rate than a similar material
with larger grain size. Although this is true, the higher inclination for dissolution does
not necessarily leads to an increase in corrosion rate. Due to the increased driving
force for dissolution, the key parameter is the fast formation of the passive layer as
compared to microcrystalline materials. Hence, the corrosion rate can be significantly
reduced for nanocrystalline metals as compared to their microcrystalline counterparts
due to the rapid passivation and the uniformity of the passive layer. In contrast, when
no passivity can take place or the corrosion rate estimation is performed with a highcurrent method, the dissolution rate of a nanocrystalline material is usually larger
than that of the corresponding coarse-grained metal.
In either of the above cases, nanocrystalline materials exhibit an advantage that
is related to the uniformity of corrosion. Due to the nanocrystalline structure, the
possibility to local corrosion attack that often leads to a fatal corrosion-related material failure is very limited, and the corrosion of a nanocrystalline metal is rather of a
laterally even rate without a development of deep cracks. This feature lends a high
potential to nanocrystalline coatings in corrosion protection.
6.2 Nanocrystalline Deposits of Metallic Elements
6.2.1 Noble Metals: Au, Ag and Pd
The electrolyte solutions used for electrodeposition of nanocrystalline gold include a
commercial sulphite-type solution with Au(III) [32] and an Au(I) solution prepared