46
2 Electrochemistry and Electrodeposition
the argument. The surface roughness is not a universal property of the surface since it
depends on which length scale it is measured. The surface roughness characterization
of a surface is based on a series of roughness measurements on a wide range of length
scales, and the growth process is described with a series of length scale-dependent
measurement for subsequent phases of the layer growth. A review of this field is
recommended for further reading on the topic [20].
2.12 Composition Aspects of Alloy Electrodeposition
2.12.1 Selection of the Variables
When polarography is discussed in an electroanalytical handbook, the reduction of
metal ions of various kind on mercury surface is often taken as a superposition of
parallel processes whose current densities are additive and which do not influence
each other. During the formation of an alloy, such thumb rules cannot be created.
In most cases, codepositing metals often impacts both the rate and the deposition
mechanism of each other. Due to the interaction in the solid phase, the Nernst equation
is not suitable to assess the potential range in which a particular metal can be deposited
as an alloy component. The thermodynamic relationship of the alloying free energy
on the deposition potentials can be found elsewhere [21]. Below, a phenomenological
picture of the codeposition process is offered that is hoped to give an efficient help
for the beginner electroplater.
First of all, it is essential to realize that the number of possible variables in metal
codeposition is quite large, and these variables are not independent. One can choose
either the concentration of the salt of a particular metal, the ratio of the concentration
of metal salts in the solution or the total metal ion concentration as a technical variable
while keeping any other concentration-related variable constant. Concerning the
electrical parameter of the deposition, either the electrode potential or the deposition
current density can be taken as the independent variable. Beside these variables,
other parameters such as the temperature and the hydrodynamic conditions have to
be constant. For the general discussion of the codeposition modes, the natural choice
is as follows. The total metal ion concentration in the solution is fixed and the metal
ion concentration ratio is treated as c 1 /(c 1 + c 2 ); i.e., with a similar expression as
the mole fraction in a binary solid. As an electrical parameter of the deposition, the
current density is chosen as a technical variable of the experiment. It is important that
the current density cannot be larger than the mass transport-limited deposition rate
of either of the metal ions if it is present in the solution alone. With this parameter
selection, both axes of the composition diagram ranges from 0 to 1, the abscissa
indicating the relative solution concentration of the ions of the less noble metal and
the ordinate referring to the mole fraction (y) of the same component in the deposit.
The line in a composition diagram where c 1 /c 2 = y 1 /y 2 is called the reference line.
2 Electrochemistry and Electrodeposition
the argument. The surface roughness is not a universal property of the surface since it
depends on which length scale it is measured. The surface roughness characterization
of a surface is based on a series of roughness measurements on a wide range of length
scales, and the growth process is described with a series of length scale-dependent
measurement for subsequent phases of the layer growth. A review of this field is
recommended for further reading on the topic [20].
2.12 Composition Aspects of Alloy Electrodeposition
2.12.1 Selection of the Variables
When polarography is discussed in an electroanalytical handbook, the reduction of
metal ions of various kind on mercury surface is often taken as a superposition of
parallel processes whose current densities are additive and which do not influence
each other. During the formation of an alloy, such thumb rules cannot be created.
In most cases, codepositing metals often impacts both the rate and the deposition
mechanism of each other. Due to the interaction in the solid phase, the Nernst equation
is not suitable to assess the potential range in which a particular metal can be deposited
as an alloy component. The thermodynamic relationship of the alloying free energy
on the deposition potentials can be found elsewhere [21]. Below, a phenomenological
picture of the codeposition process is offered that is hoped to give an efficient help
for the beginner electroplater.
First of all, it is essential to realize that the number of possible variables in metal
codeposition is quite large, and these variables are not independent. One can choose
either the concentration of the salt of a particular metal, the ratio of the concentration
of metal salts in the solution or the total metal ion concentration as a technical variable
while keeping any other concentration-related variable constant. Concerning the
electrical parameter of the deposition, either the electrode potential or the deposition
current density can be taken as the independent variable. Beside these variables,
other parameters such as the temperature and the hydrodynamic conditions have to
be constant. For the general discussion of the codeposition modes, the natural choice
is as follows. The total metal ion concentration in the solution is fixed and the metal
ion concentration ratio is treated as c 1 /(c 1 + c 2 ); i.e., with a similar expression as
the mole fraction in a binary solid. As an electrical parameter of the deposition, the
current density is chosen as a technical variable of the experiment. It is important that
the current density cannot be larger than the mass transport-limited deposition rate
of either of the metal ions if it is present in the solution alone. With this parameter
selection, both axes of the composition diagram ranges from 0 to 1, the abscissa
indicating the relative solution concentration of the ions of the less noble metal and
the ordinate referring to the mole fraction (y) of the same component in the deposit.
The line in a composition diagram where c 1 /c 2 = y 1 /y 2 is called the reference line.