202
6 Nanocrystalline Deposits
media [113] but no beneficial effect of nanocrystallinity could be identified for nonpassivating media [115].
Electrodeposition of nanocrystalline zinc was aimed at mainly because of the hope
for improving the corrosion behaviour of Zn coatings. The exchange current density
of zinc is higher than that of the iron group metals, and hence, the application of
additives was expected to have relatively little impact on the grain size of Zn. Nevertheless, the application of a number of additives proved to be effective in reducing the
grain size of electrodeposited zinc also under d.c.plating conditions. With d.c.plating,
nanocrystallinity of Zn could be achieved with the application of either a suitable
additive or a combination of several additives in either acidic [116] or alkaline [117]
solution. The grain size obtained was around 45 and 22 nm, respectively. If pulse
plating was applied, the application of additive was still indispensable for achieving
a grain refinement [118–122], but the grain size achieved was not always smaller
than that achieved with d.c. plating (20–60 nm). Nanocrystallinity was found to lead
reduced surface roughness and improved corrosion resistance of Zn deposits.
6.3 Electrodeposited Nanocrystalline Alloys
Formation of essentially any electrodeposited alloy in nanocrystalline form is a relatively easy process. The works dealing with nanocrystalline alloys are so numerous
that only a few representative examples can be shortly discussed here with a limited
set of representative examples, in particular, studies in which nanocrystallinity is of
high importance. For other examples, the reader is advised to make a literature search
for the specific system.
6.3.1 Ni–Cu Alloys
Although Ni and Cu are both of fcc phase with ~2% lattice mismatch and they exhibit
a wide range of miscibility, their codeposition with a single phase is hindered and
often leads to two-phase deposits when a non-complexed bath is used [123]. Hence,
the application of a complexing agent is essential. Since the reversible potentials of
the Cu
2+ /Cu and Ni
2+ /Ni systems are quite much apart, the appropriate complexing
agent makes a stronger complex with Cu
2+ than with Ni
2+ , hence promoting their
intermixing in the solid phase. Traditionally, this complexing agent is citric acid
[124–133], but a few other carboxylic acids have also been tested [134]. The baths
used for Cu–Ni codeposition always contain Ni
2+ in a much larger concentration
than Cu
2+ , corresponding to the normal codeposition character of the system. The
metal ion ratio ranged from 2 to 50. The complexing agent is applied in at least an
equimolar concentration with the copper salt (for citric acid, the concentration was at
least 0.2 mol dm
−3 ). The pH of the bath was in two distinct ranges, 4–5 for acidic and
6 Nanocrystalline Deposits
media [113] but no beneficial effect of nanocrystallinity could be identified for nonpassivating media [115].
Electrodeposition of nanocrystalline zinc was aimed at mainly because of the hope
for improving the corrosion behaviour of Zn coatings. The exchange current density
of zinc is higher than that of the iron group metals, and hence, the application of
additives was expected to have relatively little impact on the grain size of Zn. Nevertheless, the application of a number of additives proved to be effective in reducing the
grain size of electrodeposited zinc also under d.c.plating conditions. With d.c.plating,
nanocrystallinity of Zn could be achieved with the application of either a suitable
additive or a combination of several additives in either acidic [116] or alkaline [117]
solution. The grain size obtained was around 45 and 22 nm, respectively. If pulse
plating was applied, the application of additive was still indispensable for achieving
a grain refinement [118–122], but the grain size achieved was not always smaller
than that achieved with d.c. plating (20–60 nm). Nanocrystallinity was found to lead
reduced surface roughness and improved corrosion resistance of Zn deposits.
6.3 Electrodeposited Nanocrystalline Alloys
Formation of essentially any electrodeposited alloy in nanocrystalline form is a relatively easy process. The works dealing with nanocrystalline alloys are so numerous
that only a few representative examples can be shortly discussed here with a limited
set of representative examples, in particular, studies in which nanocrystallinity is of
high importance. For other examples, the reader is advised to make a literature search
for the specific system.
6.3.1 Ni–Cu Alloys
Although Ni and Cu are both of fcc phase with ~2% lattice mismatch and they exhibit
a wide range of miscibility, their codeposition with a single phase is hindered and
often leads to two-phase deposits when a non-complexed bath is used [123]. Hence,
the application of a complexing agent is essential. Since the reversible potentials of
the Cu
2+ /Cu and Ni
2+ /Ni systems are quite much apart, the appropriate complexing
agent makes a stronger complex with Cu
2+ than with Ni
2+ , hence promoting their
intermixing in the solid phase. Traditionally, this complexing agent is citric acid
[124–133], but a few other carboxylic acids have also been tested [134]. The baths
used for Cu–Ni codeposition always contain Ni
2+ in a much larger concentration
than Cu
2+ , corresponding to the normal codeposition character of the system. The
metal ion ratio ranged from 2 to 50. The complexing agent is applied in at least an
equimolar concentration with the copper salt (for citric acid, the concentration was at
least 0.2 mol dm
−3 ). The pH of the bath was in two distinct ranges, 4–5 for acidic and