48
2 Electrochemistry and Electrodeposition
occur with the change in the concentration. For instance, the codeposition mode can
be regular as long as the more noble component is deposited alone, but once the
ion concentration ratio makes the codeposition possible, the mole fraction of the
less noble component in the deposit may increase as if the codeposition mode was
anomalous. Below, only the main codeposition mode will be dealt with shortly.
Equilibrium codeposition is characterized with that the incorporation ratio into
the solid is equal to the component ratio in the solution, and hence, the composition
line coincides with the reference line. The term also includes that the deposit is in
equilibrium with the solution, so there is no cementation process if the deposit is left in
contact with the solution that it was deposited from. Obviously, the standard potentials
of the constituent metal ion/metal systems must be nearly identical, otherwise the
independent equilibria could not be set at comparable metal ion concentrations. This
codeposition mode is very scarce. Brenner [1] brings only a few examples for this
deposition mode (Cu–Bi and Pb–Sn by using baths with appropriate anions), and it
practically never appears in the modern literature.
Regular codeposition takes place when the components practically do not influence the incorporation of each other into the resulting deposit. This deposition mode
looks like as if only the ion transport in the solution were responsible for the partial
current densities of the components. This may happen in two distinct cases: (i) The
deposit components are neither miscible nor they can form a metastable alloy but a
granular mixture is obtained with (nearly) clean phases of the codeposited metals.
Typical examples are, by listing always the more noble element first: Ag–(Fe,Co,Ni),
Pb–(Co,Ni), Au–(Fe,Co,Ni). (ii) The components are miscible and indeed form an
either thermodynamically truly stable or metastable alloy also during the codeposition process but the A–A, A–B and B–B atomic interactions are energetically so
close that it cannot overwrite the deposition preference originating from the list of
the standard potentials of the metal ion/metal redox systems. The most important
examples include the Cu–Ni and Ag–Cu systems.
For regular codeposition, if the solution is rich enough for the more noble component, it can be deposited as a pure metal. If the metal ion ratio in the solution changes
and the concentration of the more noble metal is too small to account for the current
density used for the deposition, the less noble metal starts codepositing. This is the
apparent starting point when the composition function leaves the abscissa of the
composition diagram. It is important to note that the regular one is the only means
of codeposition when a single component can be plated as a pure element, and this
is always the more noble one.
For both irregular codeposition and anomalous codeposition, the interaction
between the components is strong enough to achieve the deposition of the less noble
component also in the case when the transport of the more noble component would
be sufficient to provide a large enough flux so that it could be deposited alone in
the absence of the less noble component. The difference between the two modes,
concerning the composition, is that for anomalous codeposition the deposit is always
more rich with respect to the less noble component than the solution. Metal pairs
2 Electrochemistry and Electrodeposition
occur with the change in the concentration. For instance, the codeposition mode can
be regular as long as the more noble component is deposited alone, but once the
ion concentration ratio makes the codeposition possible, the mole fraction of the
less noble component in the deposit may increase as if the codeposition mode was
anomalous. Below, only the main codeposition mode will be dealt with shortly.
Equilibrium codeposition is characterized with that the incorporation ratio into
the solid is equal to the component ratio in the solution, and hence, the composition
line coincides with the reference line. The term also includes that the deposit is in
equilibrium with the solution, so there is no cementation process if the deposit is left in
contact with the solution that it was deposited from. Obviously, the standard potentials
of the constituent metal ion/metal systems must be nearly identical, otherwise the
independent equilibria could not be set at comparable metal ion concentrations. This
codeposition mode is very scarce. Brenner [1] brings only a few examples for this
deposition mode (Cu–Bi and Pb–Sn by using baths with appropriate anions), and it
practically never appears in the modern literature.
Regular codeposition takes place when the components practically do not influence the incorporation of each other into the resulting deposit. This deposition mode
looks like as if only the ion transport in the solution were responsible for the partial
current densities of the components. This may happen in two distinct cases: (i) The
deposit components are neither miscible nor they can form a metastable alloy but a
granular mixture is obtained with (nearly) clean phases of the codeposited metals.
Typical examples are, by listing always the more noble element first: Ag–(Fe,Co,Ni),
Pb–(Co,Ni), Au–(Fe,Co,Ni). (ii) The components are miscible and indeed form an
either thermodynamically truly stable or metastable alloy also during the codeposition process but the A–A, A–B and B–B atomic interactions are energetically so
close that it cannot overwrite the deposition preference originating from the list of
the standard potentials of the metal ion/metal redox systems. The most important
examples include the Cu–Ni and Ag–Cu systems.
For regular codeposition, if the solution is rich enough for the more noble component, it can be deposited as a pure metal. If the metal ion ratio in the solution changes
and the concentration of the more noble metal is too small to account for the current
density used for the deposition, the less noble metal starts codepositing. This is the
apparent starting point when the composition function leaves the abscissa of the
composition diagram. It is important to note that the regular one is the only means
of codeposition when a single component can be plated as a pure element, and this
is always the more noble one.
For both irregular codeposition and anomalous codeposition, the interaction
between the components is strong enough to achieve the deposition of the less noble
component also in the case when the transport of the more noble component would
be sufficient to provide a large enough flux so that it could be deposited alone in
the absence of the less noble component. The difference between the two modes,
concerning the composition, is that for anomalous codeposition the deposit is always
more rich with respect to the less noble component than the solution. Metal pairs