10 High-Temperature Electrochemical Synthesis of Nanostructured Coatings. . .
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the electrosynthesis of powders of molybdenum and tungsten carbides and to the
considerable enhancement in the plasticity of molybdenum and tungsten having
nickel and cobalt additions.
Therefore, it is important to study the electroreduction of nickel and cobalt
against the background of tungstate–molybdate melts in both the theoretical and
practical aspects, the more so as, in oxide melts (unlike halide–oxide melts), these
metals are more precious (inert) than molybdenum (tungsten), and, hence, the range
of substrates for deposition becomes larger and includes different steels. Moreover,
these melts need no protective medium over a bath.
10.2 Materials and Methods
The main procedure of the study was chronovoltammetry with potential sweep rates
ranging from 5 × 10 −3 to 5.0 V/s. The experiments were carried out in air in a
reactor of quartz or a heat- and corrosion-resistant steel. A platinum crucible served
as an anode and a container for a melt in chronovoltammetric measurements. An
alundum crucible was used in the experiments on alloy deposition. Platinum, nickel,
and cobalt electrodes served as indicator electrodes. A platinum/oxygen electrode
0.8Na 2 WO 4 –0.2WO 3 |Pt, air, was a reference electrode. A thin-walled alundum tube
[8] was used as a membrane separating the melt of the reference electrode from
the melt under study. Sodium tungstate was of special purity grade. Molybdenum,
nickel, and cobalt oxides were of reagent grade. Reagents were dried before use for
10–12 h in vacuo at 200–250 ◦ C and then calcined at 400–450 ◦ C.
The kinetic parameters of the electrode process were diagnosed and estimated
using the theory of stationary and nonstationary electrode processes [8].
Plates of MPG-7 graphite, nickel, copper, St. 3 steel, and grade 45 steel with
a surface area of 2.0–3.5 cm 2 served as substrates for the electrodeposition of
metals and alloys. The deposits prepared were studied by X-ray diffraction, electron
microprobe analysis, and metallography using DRON-4.0, Cameca, and Neophot21 instruments, respectively. The microhardness was measured on a PMT-3 device.
The initial electrolyte was purified via electrolysis at 850–900 ◦ C using an anode
made of a metal that is like to ions in the melt.
10.3 Results and Discussion
10.3.1 Electrochemical Behavior of Nickel(II) and Cobalt(II)
Against the Background of a Tungstate Melt
The I–V characteristics in the tungstate melt containing nickel(II) or cobalt(II)
oxide exhibit a reduction wave at the potentials ranging from −0.7 to −0.8 V
and from −0.8 to −0.9 V, respectively (Fig. 10.1). An increase in the oxide
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the electrosynthesis of powders of molybdenum and tungsten carbides and to the
considerable enhancement in the plasticity of molybdenum and tungsten having
nickel and cobalt additions.
Therefore, it is important to study the electroreduction of nickel and cobalt
against the background of tungstate–molybdate melts in both the theoretical and
practical aspects, the more so as, in oxide melts (unlike halide–oxide melts), these
metals are more precious (inert) than molybdenum (tungsten), and, hence, the range
of substrates for deposition becomes larger and includes different steels. Moreover,
these melts need no protective medium over a bath.
10.2 Materials and Methods
The main procedure of the study was chronovoltammetry with potential sweep rates
ranging from 5 × 10 −3 to 5.0 V/s. The experiments were carried out in air in a
reactor of quartz or a heat- and corrosion-resistant steel. A platinum crucible served
as an anode and a container for a melt in chronovoltammetric measurements. An
alundum crucible was used in the experiments on alloy deposition. Platinum, nickel,
and cobalt electrodes served as indicator electrodes. A platinum/oxygen electrode
0.8Na 2 WO 4 –0.2WO 3 |Pt, air, was a reference electrode. A thin-walled alundum tube
[8] was used as a membrane separating the melt of the reference electrode from
the melt under study. Sodium tungstate was of special purity grade. Molybdenum,
nickel, and cobalt oxides were of reagent grade. Reagents were dried before use for
10–12 h in vacuo at 200–250 ◦ C and then calcined at 400–450 ◦ C.
The kinetic parameters of the electrode process were diagnosed and estimated
using the theory of stationary and nonstationary electrode processes [8].
Plates of MPG-7 graphite, nickel, copper, St. 3 steel, and grade 45 steel with
a surface area of 2.0–3.5 cm 2 served as substrates for the electrodeposition of
metals and alloys. The deposits prepared were studied by X-ray diffraction, electron
microprobe analysis, and metallography using DRON-4.0, Cameca, and Neophot21 instruments, respectively. The microhardness was measured on a PMT-3 device.
The initial electrolyte was purified via electrolysis at 850–900 ◦ C using an anode
made of a metal that is like to ions in the melt.
10.3 Results and Discussion
10.3.1 Electrochemical Behavior of Nickel(II) and Cobalt(II)
Against the Background of a Tungstate Melt
The I–V characteristics in the tungstate melt containing nickel(II) or cobalt(II)
oxide exhibit a reduction wave at the potentials ranging from −0.7 to −0.8 V
and from −0.8 to −0.9 V, respectively (Fig. 10.1). An increase in the oxide