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Chapter 9
where and are standard electrode potentials of respective metals,
and are metal ions activities, and are cathodic overvoltages, and
are numbers of electrons, R is the gas constant, T is the absolute
temperature and F is the Faraday’s constant.
In the simple salt solutions, if the standard potentials of two metals are
close, and if overvoltages are negligible, changing the activities can bring the
discharge potential together. An example of this type is electrodeposition of
Sn-Pb alloys from fluoroborate solutions.
If the standard potentials are significantly different, changing activities of
metal ions cannot bring the discharge potential together. The most effective
way of bringing close together discharging potentials of two metals, which
are deposited simultaneously, is the formation of strong complexes with
metal ions. In this case, not only activities of metal ions, but also
mechanisms of deposition are changed. The complexing agents are chosen in
a way to reduce the activity of ions of more positive metal to a greater extent
then the activity of ions of less noble metal. It is important that the
overvoltage of more noble metal is higher than the overvoltage of less noble
metal. Among complexing agents, used in the electrodeposition of alloys,
cyanides, pyrophosphates, ammonia, fluorides, citrates, tartars etc. should be
mentioned. Sometimes, the addition of surface-active compounds to the
solution decreases the rate of reduction of more noble metals.
For the same composition, properties of electrodeposited alloys differ
from metallurgical (thermally) prepared alloys, which is a consequence of
differences in the crystallisation process. The electrodeposited alloys,
depending on the system, composition and electrolysis conditions may
represent true solid solutions, and as well they may contain different phases
consisting of various intermetallic compounds and of the mixture crystals of
pure components (eutectic-type of alloys).
The change in the phase structure for the alloys with the same com-
position is often observed with a change in the conditions of electro-
deposition. It is obvious, then, that the properties, including composition of
electrodeposited alloys are determined by the electrodeposition conditions.
The main conditions determining properties of electrodeposited alloys are
classified in the following groups:
Composition of the plating solution, which includes concentration of
metals being deposited, concentration of complexing agents,
concentration of conducting electrolyte, pH, concentration of additives
etc,
Operating conditions, which include current density, temperature and
bath agitation, type of current (i.e. constant, pulsating, reversing etc.),