5.4 Properties of Electrodeposited CMAs
165
For the advantage of the higher corrosion resistance of the more noble layer, it
has to exhibit a threshold minimum thickness even if the layer thickness undulates
to a certain extent (which always happens for electrodeposited layers). Therefore,
the optimum layer thickness is typically above 15 nm (but multilayer illustration
images are usually given for much larger layer thicknesses only). In the literature
of corrosion research, the habit is to give the layer number of the CMA multilayer
within a fixed coating thickness; therefore, the layer thicknesses are often hidden
among the primary experimental data. The corrosion rate of multilayer coatings can
be 1–2 orders of magnitude smaller than the corresponding single-layer coating with
the same mean composition.
The multilayer approach is often applied for Zn-containing coatings on mild steel
by alloying Zn with an element that makes it a more efficient protecting layer at a
limited cost. These alloying elements used in multilayered deposit with layer thickness below 100 nm are Co [120, 121], Fe [122] and Ni [123–125]. All these nanolayered deposits were obtained with the single-bath method. There are no sufficient
structural data available to establish whether the composition modulation leads to a
structural change which may have a contribution to the enhanced corrosion resistance
of the coatings. For the corrosion protection efficiency, it is not necessary to produce
a layered system with sharp interfaces. Therefore, beside the conventional two-pulse
plating method, saw-tooth [121] and triangular [124] current–time waveforms also
proved to be suitable to deposit layered coatings with high corrosion resistance.
The surface roughness was found to decrease as the period of the multilayers was
diminished [123, 125], which is coupled with a larger lateral homogeneity. These
factors are likely to contribute to the good corrosion protection performance of the
multilayered Zn-based coatings.
The deposition of multilayered Cr/Ni coating as a protecting layer was carried
out from a Cr
3+ /Ni
2+ solution with a combination of current and solution agitation
modulation [126] where the solution agitation was applied during the plating of the
more noble metal (here, Ni). Even though Cr deposition is usually difficult from trivalent chromium baths, the deposits were smooth and crack-free, showing optimized
corrosion protection ability at the Cr(20 nm)/Ni(50 nm) multilayer structure. The
role of Ni is inverted for Ni x W 1−x /Ni y W 1−y multilayers where the layer with higher
W content has a higher corrosion resistance. Nevertheless, the enhanced protection
ability with nanoscale layer thicknesses was evidenced for this layer structure, too
[119].
5.4.5 Magnetic and Magnetoresistance Properties
of Electrodeposited Multilayers
Electrodeposited multilayers exhibiting ferromagnetism can be divided into two
major groups. In the first group, both layer types in the multilayer stack are ferromagnetic. In ferromagnetic/ferromagnetic (FM/FM) multilayers, one goal can be
165
For the advantage of the higher corrosion resistance of the more noble layer, it
has to exhibit a threshold minimum thickness even if the layer thickness undulates
to a certain extent (which always happens for electrodeposited layers). Therefore,
the optimum layer thickness is typically above 15 nm (but multilayer illustration
images are usually given for much larger layer thicknesses only). In the literature
of corrosion research, the habit is to give the layer number of the CMA multilayer
within a fixed coating thickness; therefore, the layer thicknesses are often hidden
among the primary experimental data. The corrosion rate of multilayer coatings can
be 1–2 orders of magnitude smaller than the corresponding single-layer coating with
the same mean composition.
The multilayer approach is often applied for Zn-containing coatings on mild steel
by alloying Zn with an element that makes it a more efficient protecting layer at a
limited cost. These alloying elements used in multilayered deposit with layer thickness below 100 nm are Co [120, 121], Fe [122] and Ni [123–125]. All these nanolayered deposits were obtained with the single-bath method. There are no sufficient
structural data available to establish whether the composition modulation leads to a
structural change which may have a contribution to the enhanced corrosion resistance
of the coatings. For the corrosion protection efficiency, it is not necessary to produce
a layered system with sharp interfaces. Therefore, beside the conventional two-pulse
plating method, saw-tooth [121] and triangular [124] current–time waveforms also
proved to be suitable to deposit layered coatings with high corrosion resistance.
The surface roughness was found to decrease as the period of the multilayers was
diminished [123, 125], which is coupled with a larger lateral homogeneity. These
factors are likely to contribute to the good corrosion protection performance of the
multilayered Zn-based coatings.
The deposition of multilayered Cr/Ni coating as a protecting layer was carried
out from a Cr
3+ /Ni
2+ solution with a combination of current and solution agitation
modulation [126] where the solution agitation was applied during the plating of the
more noble metal (here, Ni). Even though Cr deposition is usually difficult from trivalent chromium baths, the deposits were smooth and crack-free, showing optimized
corrosion protection ability at the Cr(20 nm)/Ni(50 nm) multilayer structure. The
role of Ni is inverted for Ni x W 1−x /Ni y W 1−y multilayers where the layer with higher
W content has a higher corrosion resistance. Nevertheless, the enhanced protection
ability with nanoscale layer thicknesses was evidenced for this layer structure, too
[119].
5.4.5 Magnetic and Magnetoresistance Properties
of Electrodeposited Multilayers
Electrodeposited multilayers exhibiting ferromagnetism can be divided into two
major groups. In the first group, both layer types in the multilayer stack are ferromagnetic. In ferromagnetic/ferromagnetic (FM/FM) multilayers, one goal can be