10 High-Temperature Electrochemical Synthesis of Nanostructured Coatings. . .
171
the theoretical value, which indicates that the charge-transfer step is reversible. The
latter is also confirmed by the following experimental facts: the deposition and halfwave potentials are independent of the polarization shift rate up to 0.2 V/s, and the
concentration dependence of the equilibrium potential of (1) is described well by
the Nernst equation.
The number of electrons participating in the electrode process was also determined from the difference in the peak and half-peak potentials of the stationary
I–V characteristics E p/2 – E p = 2.2RT/nF. For different oxide concentrations and
polarization rates from 0.05 to 0.2 V/s, we have n = 1.9–2.1 (for NiO) and 1.7–2.0
(for CoO).
A transition from a reversible to a quasireversible electrode process is observed
at a polarization rate higher than 0.5 V/s. This is confirmed by a deviation of the i p –
v 1/2 dependence from the directly proportional dependence and by the appearance
of the dependence of the peak and half-peak potentials on the polarization rate.
Thus, the analysis of the experimental data suggests that reversible equilibria and
processes involving nickel and cobalt(II) can occur in the sodium tungstate melt.
10.3.2 Combined Electroreduction of Nickel (Cobalt)
and the Oxide Forms of Molybdenum(VI)
(Tungsten(VI)) from a Tungstate Melt and the Synthesis
of Nickel (Cobalt)–Molybdate (Tungstate) Alloys
The addition of molybdenum(VI) oxide to the nickel-containing tungstate melt
induces the dimolybdate-ion reduction wave
Mo 2 O 7
2+
+ 6e → Mo + MoO 4
2–
+ 3O
2– ,
(10.2)
which was studied in detail in [6, 7]. The difference in the potentials of nickel and
molybdenum deposition is 0.09–0.115 V at 1173 K. Unlike halide and halide–oxide
melts, nickel exhibits a more precious behavior than molybdenum in an oxide melt.
A Na 2 WO 4 –2.5 mol % MoO 3 melt was used as a stock electrolyte to
deposit alloys of various compositions. After the electrolyte had been cleaned by
electrolysis, we electrodeposited molybdenum coatings using a molybdenum anode
in the temperature range 1123–1173 K at a cathodic current density ranging from
0.02 to 0.15 A/cm 2 to determine the structure of the molybdenum deposits. It was
found that, at the current density from 0.02 to 0.1 A/cm 2 , the molybdenum coatings
have a columnar structure, a thickness of 100–150 μm, and a microhardness of 1.8–
1.9 kN/mm 2 . Further electrolysis does not increase the coating thickness: dendrites
or isolated powdered deposits grow. The depth profiles of the molybdenum and
nickel concentrations in the coating and nickel substrate indicate interdiffusion
of the elements of the coating and substrate, which provides their good adhesion
(Fig. 10.6).
For the cathodic codeposition of metals to form an alloy, the molybdenum anode
was replaced by a more inert nickel electrode, and electrolysis was carried out at
171
the theoretical value, which indicates that the charge-transfer step is reversible. The
latter is also confirmed by the following experimental facts: the deposition and halfwave potentials are independent of the polarization shift rate up to 0.2 V/s, and the
concentration dependence of the equilibrium potential of (1) is described well by
the Nernst equation.
The number of electrons participating in the electrode process was also determined from the difference in the peak and half-peak potentials of the stationary
I–V characteristics E p/2 – E p = 2.2RT/nF. For different oxide concentrations and
polarization rates from 0.05 to 0.2 V/s, we have n = 1.9–2.1 (for NiO) and 1.7–2.0
(for CoO).
A transition from a reversible to a quasireversible electrode process is observed
at a polarization rate higher than 0.5 V/s. This is confirmed by a deviation of the i p –
v 1/2 dependence from the directly proportional dependence and by the appearance
of the dependence of the peak and half-peak potentials on the polarization rate.
Thus, the analysis of the experimental data suggests that reversible equilibria and
processes involving nickel and cobalt(II) can occur in the sodium tungstate melt.
10.3.2 Combined Electroreduction of Nickel (Cobalt)
and the Oxide Forms of Molybdenum(VI)
(Tungsten(VI)) from a Tungstate Melt and the Synthesis
of Nickel (Cobalt)–Molybdate (Tungstate) Alloys
The addition of molybdenum(VI) oxide to the nickel-containing tungstate melt
induces the dimolybdate-ion reduction wave
Mo 2 O 7
2+
+ 6e → Mo + MoO 4
2–
+ 3O
2– ,
(10.2)
which was studied in detail in [6, 7]. The difference in the potentials of nickel and
molybdenum deposition is 0.09–0.115 V at 1173 K. Unlike halide and halide–oxide
melts, nickel exhibits a more precious behavior than molybdenum in an oxide melt.
A Na 2 WO 4 –2.5 mol % MoO 3 melt was used as a stock electrolyte to
deposit alloys of various compositions. After the electrolyte had been cleaned by
electrolysis, we electrodeposited molybdenum coatings using a molybdenum anode
in the temperature range 1123–1173 K at a cathodic current density ranging from
0.02 to 0.15 A/cm 2 to determine the structure of the molybdenum deposits. It was
found that, at the current density from 0.02 to 0.1 A/cm 2 , the molybdenum coatings
have a columnar structure, a thickness of 100–150 μm, and a microhardness of 1.8–
1.9 kN/mm 2 . Further electrolysis does not increase the coating thickness: dendrites
or isolated powdered deposits grow. The depth profiles of the molybdenum and
nickel concentrations in the coating and nickel substrate indicate interdiffusion
of the elements of the coating and substrate, which provides their good adhesion
(Fig. 10.6).
For the cathodic codeposition of metals to form an alloy, the molybdenum anode
was replaced by a more inert nickel electrode, and electrolysis was carried out at