248
7 Composites
words, we encounter a mixed electrode where the parts of the heterogeneous surface
mutually polarize each other. This is the case for the incorporation of various forms of
carbon nanotubes [74, 194]. Especially for Ni-based coatings, this polarization leads
to a better conservation of passivity. For Cr(C) coatings made with carbon nanotubes
[195], an improved passivity was observed due to the accelerated formation of the
protecting Cr 2 O 3 layer that adsorbed strongly on the nanotube-covered part of the
composite coating. Here, the modification of the mechanism of the passive layer
formation was a key factor of the decrease of the corrosion rate.
The incorporated non-conducting particles may also modify the oxidation
behaviour of the metal matrix. This effect was shown for Ni(SiC) coatings [196].
During the oxidation in air at 1000 °C, the formation of SiO 2 was a competing
process with the formation of the NiO scale, and the incorporated particles worked
as scavengers of the oxygen diffusing into the depth of the NiO surface layer. Hence,
the presence of the particles at the grain boundaries had a decelerating impact on the
oxidation of the metal matrix itself. This effect is clearly of chemical nature and may
work only for particles that themselves can be oxidized.
7.2.12 Metal–Metal Composites with Miscellaneous
Applications
Electrodeposited composites with dispersed metal particles deserve special attention
because the chemical nature of the particles plays a much more important role in
the occurrence of the resulting functionality than for any other type of dispersion
coatings. The metal–metal composites will be discussed below with attention to the
special functionality gained by the particle incorporation.
Various composites were synthesized with Ag particles to improve the antibacterial feature of the metal surface. The principle of the antibacterial effect is that
Ag
+ ions exert a strong bactericide effect. For achieving this effect, the coating must
corrode by emitting Ag
+ ions into the contacting media. In aerated conditions, the
corrosion takes place at the desired rate that is sufficient for impeding the bacteria
colonization. At the same time, the Ag
+ concentration far from the surface is usually
negligibly small so that Ag-containing bio-fouling surfaces are not considered to be
a major heavy metal source due to the small overall amount of Ag
+ released. The
concept of Ag particle codeposition is based on the retention of the protecting ability
of a conventional coating without modifying the chemical background of codeposition by doping with an antiseptic agent, also in the case where alloy codeposition
would not be possible due to the lack of an appropriate bath.
The coatings tested so far are all composed of metals used as typical coating
materials in various applications: Zn [197], Cu–Zn [94], Cr [198] and Ni [199].
The silver nanoparticle concentration in the bath was between 0.5 and 10 g/litre.
For achieving a bacteria growth inhibition efficiency of 90 to 100%, an Ag particle
loading of 1 wt.% was sufficient. Although a slight change in the coating structure
7 Composites
words, we encounter a mixed electrode where the parts of the heterogeneous surface
mutually polarize each other. This is the case for the incorporation of various forms of
carbon nanotubes [74, 194]. Especially for Ni-based coatings, this polarization leads
to a better conservation of passivity. For Cr(C) coatings made with carbon nanotubes
[195], an improved passivity was observed due to the accelerated formation of the
protecting Cr 2 O 3 layer that adsorbed strongly on the nanotube-covered part of the
composite coating. Here, the modification of the mechanism of the passive layer
formation was a key factor of the decrease of the corrosion rate.
The incorporated non-conducting particles may also modify the oxidation
behaviour of the metal matrix. This effect was shown for Ni(SiC) coatings [196].
During the oxidation in air at 1000 °C, the formation of SiO 2 was a competing
process with the formation of the NiO scale, and the incorporated particles worked
as scavengers of the oxygen diffusing into the depth of the NiO surface layer. Hence,
the presence of the particles at the grain boundaries had a decelerating impact on the
oxidation of the metal matrix itself. This effect is clearly of chemical nature and may
work only for particles that themselves can be oxidized.
7.2.12 Metal–Metal Composites with Miscellaneous
Applications
Electrodeposited composites with dispersed metal particles deserve special attention
because the chemical nature of the particles plays a much more important role in
the occurrence of the resulting functionality than for any other type of dispersion
coatings. The metal–metal composites will be discussed below with attention to the
special functionality gained by the particle incorporation.
Various composites were synthesized with Ag particles to improve the antibacterial feature of the metal surface. The principle of the antibacterial effect is that
Ag
+ ions exert a strong bactericide effect. For achieving this effect, the coating must
corrode by emitting Ag
+ ions into the contacting media. In aerated conditions, the
corrosion takes place at the desired rate that is sufficient for impeding the bacteria
colonization. At the same time, the Ag
+ concentration far from the surface is usually
negligibly small so that Ag-containing bio-fouling surfaces are not considered to be
a major heavy metal source due to the small overall amount of Ag
+ released. The
concept of Ag particle codeposition is based on the retention of the protecting ability
of a conventional coating without modifying the chemical background of codeposition by doping with an antiseptic agent, also in the case where alloy codeposition
would not be possible due to the lack of an appropriate bath.
The coatings tested so far are all composed of metals used as typical coating
materials in various applications: Zn [197], Cu–Zn [94], Cr [198] and Ni [199].
The silver nanoparticle concentration in the bath was between 0.5 and 10 g/litre.
For achieving a bacteria growth inhibition efficiency of 90 to 100%, an Ag particle
loading of 1 wt.% was sufficient. Although a slight change in the coating structure