Low hydrogen overvoltage cathode and process for the production thereof

A cathode of sufficiently low hydrogen overvoltage is provided which is useful in electrolysis of water or of an aqueous alkali metal chloride solution such as a sodium chloride solution. A process for producing the cathode is also provided. The low hydrogen overvoltage cathode has an electroconductive base material coated with an alloy layer containing nickel and molybdenum, the alloy layer containing the nickel at a content ranging from 35 to 90% by weight and the molybdenum at a content ranging from 10 to 65% by weight. The alloy laser has an X-ray diffraction (CuK.alpha. line) pattern with a main peak at an angle ranging from 42 to 45.degree. with a peak half width ranging from 0.4 to 7.degree.. One process for producing the low hydrogen overvoltage cathode of the present invention involves plating an electroconductive base material by an arc discharge type ion plating method. Another process for producing the low hydrogen overvoltage cathode of the present invention involves co-electrodepositing nickel and molybdenum onto an electroconductive base material in a plating bath.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a low hydrogen overvoltage cathode for the 
electrolysis of water or an aqueous alkali metal chloride such as aqueous 
sodium chloride, and also to a process for producing the low hydrogen 
overvoltage cathode. 
2. Description of the Related Art 
Industrial electrolysis of water or an aqueous alkali metal chloride 
consumes a large amount of electric power, so that various energy saving 
techniques are being developed for industrial electrolysis procedures. 
"Energy saving techniques" means techniques which result in a substantial 
decrease of the electrolysis voltage which techniques can include 
decreasing the theoretical electrolysis voltage, solution resistance, 
diaphragm resistance, cathode overvoltage and anode overvoltage. In 
particular, the mentioned overvoltages, which largely depend on the 
electrode material and the electrode surface state, have attracted the 
attention of many research scientists, and many developments have been 
made in this area. 
For instance, in the ion exchange process for sodium chloride electrolysis, 
a decrease of the anode overvoltage has been actively studied. 
Consequently, anodes have been developed which do not involve problems 
regarding anode overvoltage; such anodes are in wide use industrially. 
Many proposals have also been made regarding low hydrogen overvoltage 
cathodes, namely active cathodes which can have their hydrogen overvoltage 
lowered by 200-250 mV in comparison with a conventional iron cathode 
exhibiting a hydrogen overvoltage of 400 mV. For example, a hydrogen 
absorbing alloy or a platinum group metal oxide has been deposited on an 
electrode base material surface (Japanese Patent Laid-Open Publications 
59-25940 and 6-146046). Further, a coating layer of an alloy of a 
transition metal such as iron, cobalt and nickel, tungsten or molybdenum 
has been formed by plating the same on an electrode base material surface 
(Japanese Patent Publication 40-9130). However, the electrodes having a 
hydrogen absorbing alloy or a platinum group metal oxide deposited thereon 
use an expensive material, which results in high cost, whereas while the 
latter electrodes covered with an alloy of a transition metal, etc., can 
be produced at low cost, they are not sufficient in reducing the hydrogen 
overvoltage. Thus, both types of electrodes still involve problems. 
To improve electrodes plated with an alloy of iron, cobalt, nickel or 
molybdenum, a water-soluble polyamine has been added to the alloy plating 
bath (Japanese Patent Laid-Open Publication 55-65376). However, this 
involves disadvantages in that the polyamine is soluble only over a narrow 
pH range which makes control of the plating bath difficult on an 
industrial scale. Further, the decrease of the hydrogen overvoltage is 
still insufficient. 
Most of the active cathodes to date comprise an electrode base material and 
a catalyst layer of a specific composition formed thereon to decrease the 
hydrogen overvoltage. The coating layer is formed in various ways. For 
example, a catalytic substance can be electrically deposited by wet 
plating from a bath containing a dispersed active substance or containing 
a dissolved metal salt as disclosed in the aforementioned patents; a 
catalytic metal substance in a molten state can be directly sprayed onto a 
base material (Japanese Patent Laid-Open Publication 61-41786); a metal 
salt solution can be applied onto a base material, dried, and subjected to 
reduction or other treatment to form a catalytic substance layer (Japanese 
Patent Laid-Open Publication 61-295386); etc. However, in the wet plating 
method the alloy composition for coating is limited due to differences in 
electrodeposition potentials which is a disadvantage. Further, the 
composition of the active substances or the metal components in the 
plating bath tend to change over the time of plating, requiring strict 
control of the bath to obtain a homogeneous alloy layer in a stable 
manner. On the other hand, in the last two methods, alloy formation is 
difficult with elements having a large difference in vapor pressure 
because of the high temperature treatment required for coating, and an 
amorphous or fine crystalline structure of high performance cannot readily 
be obtained because of enhanced crystallization in the high temperature 
treatment, which is disadvantageous. To avoid crystallization, a 
sputtering method has been proposed (Japanese Patent Laid-Open Publication 
7-268676). However, the sputtering method still has the problem that the 
film formation rate is low. 
SUMMARY OF THE INVENTION 
The inventors herein made comprehensive studies to solve the above problems 
involved in low hydrogen overvoltage cathodes. Consequently, it was found 
that a low hydrogen overvoltage can be attained using a cathode produced 
by an arc discharge type ion plating technique in which target atoms are 
vaporized and ionized, and the resultant catalytic substance is deposited 
to coat a base material. 
It has also been found that a cathode covered with a composition and 
structure having a low hydrogen overvoltage performance can be produced by 
a wet plating technique by controlling the composition and the pH of the 
plating bath without complicating a conventional plating system by bath 
additives. 
An object of the present invention is to provide a low hydrogen overvoltage 
cathode for electrolysis of water or an alkali metal chloride such as 
sodium chloride. 
Another object of the present invention is to provide a process for 
producing the above cathode. 
The low hydrogen overvoltage cathode of the present invention comprises an 
electroconductive base material coated with an alloy layer containing 
nickel and molybdenum, the alloy layer containing nickel at a content 
ranging from 35 to 90% by weight and molybdenum at a content ranging from 
10 to 65% by weight, and showing, upon X-ray diffraction with a CuK.alpha. 
line, a main peak at an angle ranging from 42 to 45.degree. with a peak 
half width ranging from 0.4 to 7.degree.. 
One process for producing the low hydrogen overvoltage cathode of the 
present invention comprises plating an electroconductive base material by 
an arc discharge type ion plating method with a target containing nickel 
at a content ranging from 35 to 90% by weight and molybdenum at a content 
ranging from 10 to 65% by weight at a potential on the electroconductive 
base material ranging from -100 to 50 V with the introduction of a gas 
containing at least one of hydrogen, carbon, nitrogen and oxygen as a 
reaction gas. 
Another process for producing the low hydrogen overvoltage cathode of the 
present invention comprises co-electrodepositing at least nickel and 
molybdenum onto an electroconductive base material in a plating bath, the 
plating bath containing nickel ions, molybdate ions, and a complexing 
agent at an Mo/(Ni+Mo) ratio ranging from 5 to 20 mol % at a total 
concentration of nickel ions and molybdate ions ranging from 0.1 to 0.5 
mol/l in the plating bath kept at a pH ranging from 7 to 9. 
The alloy layer preferably contains at least one of the 4d transition 
metals, noble metals, and lanthanide elements in an amount of from 0.1 to 
10% by weight in addition to nickel and molybdenum.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The electroconductive base material to be coated with the alloy layer in 
the present invention includes nickel, iron, copper, titanium, stainless 
steel and other metals which are resistant to caustic alkali. The shape of 
the electroconductive base material is not limited, and it may be in a 
shape suitable for the cathode of an electrolytic cell, for example, in a 
shape of a flat plate, a curved plate, an expandable metal, a punched 
metal, a net and a perforated panel. 
The electroconductive base material is preferably subjected to a 
conventional pretreatment such as degreasing, vacuum heating and ion 
bombardment. For strengthening the adhesion between the base material and 
the alloy layer, is plating of the base material with a suitable nickel 
alloy or deposition of electroconductive fine particles of carbon, a 
platinum group metal or the like, onto the base material is effective to 
roughen the surface. 
The alloy layer preferably has a thickness in the range of from 5 to 500 
.mu.m, since a thinner alloy layer is not effective enough for reducing 
the hydrogen overvoltage and a thicker alloy layer is liable to come off. 
The processes for forming the alloy layer of the present invention are now 
explained specifically. 
One process is arc discharge type ion plating (AIP) and another process is 
wet plating. 
The AIP technique is first described. The target used for the AIP is 
prepared in the same manner as those in usual ion plating. The target 
elements are physically mixed by means of a ball mill or the like, and 
shaped by press molding by CIP (cold isostatic pressing), HIP (hot 
isostatic pressing) or a like method. The method for preparation of the 
target is not limited, provided that the target elements are mixed 
uniformly and finely. The elements are not necessarily required to be 
alloyed in the prepared target. 
In the AIP technique, the composition of the coating alloy is nearly the 
same as the composition of the target, so that the coating composition can 
be controlled as desired by controlling the composition of the target. 
Nickel and molybdenum, which have vapor pressures which greatly differ, 
cannot readily be formed into a coating alloy layer by thermal spraying 
conducted at a high temperature. However, such elements which differ 
greatly in vapor pressure and which are not suitable for thermal spraying 
can readily be alloyed according to the process of the present invention 
by vaporizing the target atoms at a relatively low temperature by arc 
discharge. 
The alloy layer thickness can be controlled readily by the time of layer 
formation. The nickel-molybdenum alloy layer is formed at a rate of 
several microns for 10 minutes. This rate of alloy layer formation can be 
raised by simultaneously using plural targets. Thus, a thick alloy layer 
can readily be formed in comparison with other ion plating techniques or 
sputtering techniques. 
Using the AIP technique, the alloy layer having the composition of the 
present invention is obtained by controlling the target composition and 
the layer forming conditions. Specifically, a target is employed which 
contains nickel at a content of from 35 to 90% by weight and molybdenum at 
a content of from 10 to 65% by weight, and the layer formation is 
conducted by applying a potential of from -100 to 50 V to a base material. 
In the case where at least one of the 4d transition metals, noble metals, 
and lanthanide elements is to be incorporated into the alloy layer, a 
target is preferably used which contains the intended element other than 
nickel and molybdenum in an amount of from 0.1 to 10% by weight, in 
addition to nickel and molybdenum. 
The layer formation is conducted with the introduction of a reaction gas 
containing at least one of hydrogen, carbon, nitrogen and oxygen. The 
hydrogen-containing gas is a gas containing hydrogen atoms as a gas 
component, including H.sub.2 and H.sub.2 O. The carbon-containing gas 
includes CH.sub.4 and C.sub.2 H.sub.8. The nitrogen-containing gas 
includes N.sub.2 and NH.sub.3. The oxygen-containing gas includes O.sub.2 
and CO. The reaction gas is not limited to those mentioned. By arc 
discharge type ion plating under the aforementioned conditions, a low 
hydrogen overvoltage cathode can be produced which comprises an 
electroconductive base material coated with an alloy layer containing 
nickel and molybdenum at a nickel content of from 35 to 90% by weight and 
at a molybdenum content of from 10 to 65% by weight, and showing, upon 
X-ray diffraction with a CuK.alpha. line, a main peak at an angle ranging 
from 42 to 45.degree. with a half width ranging from 0.4 to 7.degree.. 
The potential applied to the base material is more preferably in the range 
of from -60 to 30 V. 
In the ion plating, the target atoms are ionized and deposited onto the 
base material to cover it. At a potential of the base material outside the 
potential range of the present invention, the kinetic energy of the 
coating ions is excessively large which causes a significant temperature 
rise of the base material by collision of the ions against the base 
material, making impracticable the formation of a coating layer of the 
crystal structure set forth in the claims. Further, at a larger absolute 
value of the potential of the base material, the layer composition 
deviates greatly from the target composition to make the formation of the 
intended composition of the alloy layer impractical. 
The wet plating technique will now be explained. In the wet plating 
technique, the counter electrode for the plating is not especially 
limited, and soluble electrodes such as a nickel plated electrode and 
insoluble electrodes such as a platinum plated electrode and a titanium 
plate plated with platinum may be used as the counter electrode. 
For producing the alloy layer of the composition and structure of the 
present invention, the plating bath composition for the wet plating is 
controlled to be within a specified concentration range. Specifically, the 
plating bath is controlled so as to contain nickel ions, molybdate ions, 
and a complexing agent at an Mo/(Ni+Mo) ratio ranging from 5 to 20 mol % 
at a total concentration of nickel ions and molybdate ions ranging from 
0.1 to 0.5 mol/l. The sources of nickel and molybdenum are not especially 
limited. The nickel sources include nickel salts such as nickel sulfate, 
nickel chloride, and mixtures thereof. The molybdenum sources include 
sodium molybdate, potassium molybdate and ammonium molybdate. The 
complexing agent is not especially limited, and may be any complexing 
agent which can readily form a complex with nickel ions. The complexing 
agents include citric acid, tartaric acid and pyrophosphoric acid. The 
amount of the complexing agent is not especially limited, but the amount 
of the complexing agent is usually an amount of from 0.1 to 2 moles per 
mole of the total of the nickel ions and the molybdate ions in the plating 
bath. 
The pH of the plating bath should be controlled to be within a specified 
range in order to produce the alloy layer of the composition and structure 
of the present invention. Specifically, the pH is controlled to be in the 
range of from 7 to 9. The chemicals for adjusting the pH are not limited, 
and include inorganic acids such as sulfuric acid and hydrochloric acid, 
and inorganic bases such as sodium hydroxide and aqueous ammonia. 
The composition and structure of the alloy layer of the present invention 
also depend on the plating bath temperature and the plating current 
density. These are controlled by selecting conventional conditions as 
shown in the Examples in Japanese Patent Publication 40-9130, Japanese 
Patent Laid-Open Publication 55-65376, etc. The plating bath temperature 
is selected to be in the range of from 20 to 70.degree. C. At a lower 
temperature the plating efficiency will be lower, and the process is 
uneconomical whereas at a higher temperature the resulting alloy coating 
layer becomes disadvantageously brittle. The plating current density is 
preferably in the range of from 2 to 20 A/dm.sup.2. At a lower plating 
current density the molybdenum content of the alloy layer will be lower 
than the specified range of the present invention, which causes a high 
cathode overvoltage, whereas at a higher current density the plating 
efficiency is lower, and the process is uneconomical. 
In wet plating, the intended performance of the alloy layer can be obtained 
by observing the above conditions, independently of using a third 
component which has been added to increase the surface layer present in 
the plating bath which is incorporated into the alloy layer. 
The alloy layer coating the surface of the electroconductive base material 
in the present invention should comprise at least nickel and molybdenum 
and show a peak in its X-ray diffraction pattern with a half width ranging 
from 0.4 to 7.degree.. To achieve such a half width, the temperature 
during and after the formation of the alloy layer is very important. If 
the alloy layer is treated at a temperature above 150.degree. C., the 
crystallinity of the alloy becomes higher and the half width deviates from 
the above specified values. For example, a nickel-molybdenum cathode, 
which is produced by flame spraying, as described in Japanese Patent 
Laid-Open Publication 55-100988, is always treated at a high temperature, 
producing an alloy layer having a diffraction peak half width outside the 
specified value range of the present invention. Thus, heat treatment at a 
temperature higher than 150.degree. C. during or after the alloy layer 
production prevents formation of a crystal structure having a peak of the 
specified half width of the present invention or destroys the crystal 
structure thereof, which results in an electrode giving a significantly 
high cathode overvoltage. Therefore, heat treatment after plating is 
undesirable. In particular, a heat treatment at 150.degree. C. or a higher 
temperature sharpens the X-ray diffraction peak, and causes the formation 
of molybdenum single crystals or intermetallic compound crystals of nickel 
and molybdenum to change the crystal structure, leading to a remarkably 
high cathode overvoltage. 
The composition of the alloy coating layer is preferably in the range of a 
nickel content of from 40 to 85% by weight and a molybdenum content of 
from 15 to 60% by weight, more preferably a nickel content of from 45 to 
80% by weight and a molybdenum content of from 20 to 55% by weight, in 
accordance with the present invention. At a nickel content or molybdenum 
content outside the claimed range, the region of simple nickel or simple 
molybdenum becomes larger to prevent nickel-molybdenum alloy formation, 
resulting in a remarkable increase of the overvoltage. Even at a nickel 
and molybdenum content within the claimed range, an alloy having an X-ray 
diffraction peak outside the claimed peak position range or the claimed 
half width range is different in crystal structure from an alloy showing 
the desired low hydrogen overvoltage, and results in a high overvoltage. 
The hydrogen overvoltage is further advantageously lowered by incorporating 
at least one of the 4d transition metals, noble metals and lanthanide 
elements in an amount of from 0.1 to 10% by weight into the 
nickel-molybdenum coating layer. 
The present invention is described more specifically by reference to the 
following Examples without limiting the invention in any way. 
EXAMPLES 1-7 
The samples of Examples 1-7 were prepared by arc discharge type ion plating 
using a target composed of 60% by weight nickel and 40% by weight 
molybdenum (50 atom% Ni and 50 atom% Mo) and plating onto a nickel plate 
as a base material (40.times.50 mm.sup.2) whose surface had been degreased 
and cleaned. The arc type ion plating was conducted using the ion plating 
apparatus SIA-400T (manufactured by Show Shinku K.K.) at a vacuum of 
1.times.10.sup.-3 Torr at an arc current of 100 A for 50 minutes to form a 
coating layer. An electrode was thus prepared which had an Ni-Mo alloy 
coating layer about 20-30 .mu.m thick on the base material. The layer 
formation conditions are given in Table 1, and the properties of the 
coating layers are given in Table 2. 
The alloy composition of the coating layer was determined using an X-ray 
microanalyzer, and is given by calculation on the basis of the Ni 
concentration+Mo concentration=100. The position of the main peak and the 
half width were derived from the CuK.alpha. X-ray diffraction pattern. The 
hydrogen overvoltage was measured by the current interrupter method at 
90.degree. C. in a 32.5% sodium hydroxide solution at a current density of 
40 A/dm.sup.2. FIG. 1 and FIG. 2 show, respectively, the X-ray diffraction 
pattern of the coating layers obtained in Example 3 and Example 6. 
COMATIVE EXAMPLES 1-2 
Coating layers were formed in the same manner as in Example 1 except that 
the potential of the base material was set at -300 V. The layer formation 
conditions and the layer properties are given, respectively, in Table 1 
and Table 2. The resulting coating layers had a half width outside the 
claimed range, showing overvoltages of as high as about 280-320 mV. FIG. 3 
shows the X-ray diffraction pattern of the coating layer obtained in 
Comparative Example 2. 
EXAMPLES 8-10 
The samples of Examples 8-10 were prepared by arc discharge type ion 
plating using a target composed of 60% by weight of nickel and 40% by 
weight of molybdenum or a target further containing 5% by weight of silver 
or lanthanum in addition to nickel and molybdenum. The layer formation 
conditions are given in Table 3, and the properties of the resulting 
coating layers are given in Table 4. 
EXAMPLES 11-14 
Coating films were formed using four kinds of targets having compositions 
of 10-65% by weight molybdenum, balance nickel, under a vacuum of 
1.times.10.sup.-3 Torr at an arc current of 100 A for 50 minutes under the 
conditions given in Table 5. The properties of the formed coating layers 
are given in Table 6. 
COMATIVE EXAMPLES 3-4 
In Comparative Examples 3 and 4, the targets employed had a composition of 
95% by weight nickel and 5% by weight molybdenum or 25% by weight nickel 
and 75% by weight molybdenum. The coating layers were formed in the same 
manner as in Example 11. The layer formation conditions are given in Table 
5, and the properties of the coating layers are given in Table 6. In 
Comparative Example 3, the overvoltage was high since the contents of 
nickel and molybdenum were outside the claimed ranges. In Comparative 
Example 4, the overvoltage was high since the contents of nickel and 
molybdenum and the peak position were outside the claimed ranges. FIG. 4 
shows the X-ray diffraction pattern of the coating layer obtained in 
Comparative Example 4. 
EXAMPLE 15 
A plating bath was prepared which contained 0.228 mol/l of nickel sulfate 
(hexahydrate) 0.012 mol/l of sodium molybdate (dihydrate) and 0.344 mol/l 
of trisodium citrate (dihydrate). The pH of the bath was adjusted to 8.0 
by the addition of aqueous 28% ammonia. The electrode base material was a 
nickel disc plate (electrode area of 78.5 mm.sup.2) which had been 
degreased with alcohol and etched by nitric acid. The counter electrode 
was a nickel plate. 
The plating was conducted at a bath temperature controlled at 50.degree. C. 
at a current density of 5 A/dm.sup.2 for 24 minutes to prepare an 
electrode having a nickel-molybdenum alloy deposited on the electrode base 
material. As a result of measurement using an X-ray microanalyzer, the 
alloy layer was found to contain molybdenum at a concentration of 39.0% by 
weight. The main peak of the CuK.alpha. X-ray diffraction pattern of the 
alloy layer was at an angle of 43.7.degree., and the half width thereof 
was 5.3.degree.. 
The hydrogen overvoltage was measured with this electrode in a 32.5% sodium 
hydroxide solution at 90.degree. C., and was found to be 108 mV at a 
current density of 40 A/dm.sup.2. 
EXAMPLES 16-22 AND COMATIVE EXAMPLES 5-13 
These experiments were conducted in the same manner as Example 15 regarding 
the nickel source, the molybdenum source, the complexing agent, the 
electrode base material, the pretreatment of the electrode base material, 
the counter electrode, the measurement method of the molybdenum 
concentration in the alloy layer, the measurement method of the X-ray 
diffraction pattern and the hydrogen overvoltage measurement. 
In Examples 16-17 and Comparative Examples 5-6, the alloy layers were 
prepared by changing the molar ratio Mo/(Ni+Mo) in the plating bath. Table 
7 gives the molybdenum concentrations, the main peak positions and the 
peak half widths of the alloy layers obtained, and the hydrogen 
overvoltage of the resulting electrodes. In Table 7, the hydrogen 
overvoltage was higher in Comparative Examples 5 and 6 since the 
Mo/(Mo+Ni) molar ratio was outside the range of the present invention. 
Similarly, in Examples 18-19 and Comparative Examples 7-8, coating layers 
were formed on the electrode base material by changing the total 
concentration of nickel and molybdenum in the plating bath. Table 8 gives 
the molybdenum concentrations, the main peak positions, the peak half 
widths and the hydrogen overvoltages of the resulting alloy layers. 
In Examples 20-22 and Comparative Examples 9-10, coating layers were formed 
on the electrode base material by changing the pH of the plating bath. 
Table 9 gives the molybdenum concentrations, the main peak positions, the 
peak half widths and the hydrogen overvoltages of the resulting alloy 
layers. As shown in Table 8, the hydrogen overvoltage was higher in 
Comparative Examples 7 and 8 since the total concentrations of nickel and 
molybdenum were outside the range of the present invention and, as shown 
in Table 9, the hydrogen overvoltage was higher in Comparative Examples 
9-10 since the pH of the plating bath was outside the range of the present 
invention. 
Coating alloy layers were separately formed and heat treated in the air at 
150.degree. C. for one hour. Table 10 gives the positions and half widths 
of the main peaks and the crystal structures of the alloy layers 
identified by X-ray diffraction patterns the hydrogen overvoltages of the 
electrodes. Table 10 shows that the heat treatment at 150.degree. C. 
narrowed the peak half width and gave rise to a new diffraction peak of an 
intermetallic compound of Ni.sub.4 Mo and caused a rise in the 
overvoltage. 
FIGS. 5, 6, and 7 show, respectively, the X-ray diffraction patterns of the 
alloy layer of Example 16, Comparative Example 5 and Comparative Example 
11. 
It has been shown that the active cathode produced according to the present 
invention exhibits an overvoltage as low as 110-150 mV in electrolysis at 
90.degree. C. and a current density of 40 A/dm.sup.2 in a 32.5% sodium 
hydroxide solution, and has excellent cathode properties. Such cathode 
performance is achieved by an electrode comprising an electroconductive 
base material coated with an alloy layer containing at least nickel and 
molybdenum, the alloy layer being produced by controlling the production 
conditions so that the alloy layer contains molybdenum at a content 
ranging from 10 to 65% by weight, and shows only a peak in the X-ray 
diffraction pattern there of with a CuK.alpha. line at an angle ranging 
from 42 to 45.degree. with a peak half width ranging from 0.4 to 
7.degree.. 
The cathode of the present invention lowers electric power consumption in 
the electrolysis of an aqueous alkali metal chloride solution to 
contribute greatly to energy savings in the chlorine-alkali industries. 
TABLE 1 
______________________________________ 
Coating Layer Forming Conditions 
Target Base 
composition material Vacuum Arc 
(weight %) potential 
Reaction degree current 
Ni Mo (V) gas (Torr) (A) 
______________________________________ 
Example 
1 60 40 -40 Steam 1 .times. 10.sup.-3 
100 
2 60 40 -40 Nitrogen 
1 .times. 10.sup.-3 
100 
3 60 40 -40 Oxygen 1 .times. 10.sup.-3 
100 
4 60 40 -40 Oxygen 1 .times. 10.sup.-3 
100 
5 60 40 0 Oxygen 1 .times. 10.sup.-3 
100 
6 60 40 20 Oxygen 1 .times. 10.sup.-3 
100 
7 60 40 40 Oxygen 1 .times. 10.sup.-3 
100 
Comparative 
Example 
1 60 40 -300 Steam 1 .times. 10.sup.-3 
100 
2 60 40 -300 Oxygen 1 .times. 10.sup.-3 
100 
______________________________________ 
TABLE 2 
______________________________________ 
Coating Layer Forming Conditions 
Alloy 
composition Hydrogen 
(% by weight) Peak Peak overvoltage 
Ni Mo position half-width 
(mV) 
______________________________________ 
Example 
1 61.7 38.3 43.5.degree. 
1.0.degree. 
127 
2 59.5 40.5 43.6.degree. 
0.9.degree. 
128 
3 59.8 40.2 43.6.degree. 
0.6.degree. 
141 
4 62.8 37.2 43.6.degree. 
1.2.degree. 
125 
5 62.4 37.6 43.7.degree. 
1.8.degree. 
121 
6 63.2 36.8 43.6.degree. 
1.2.degree. 
123 
7 62.9 37.1 43.7.degree. 
0.8.degree. 
137 
Comparative 
Example 
1 51.4 48.6 43.4.degree. 
0.3.degree. 
319 
2 50.8 49.2 43.5.degree. 
0.3.degree. 
285 
______________________________________ 
TABLE 3 
______________________________________ 
Coating Layer Forming Conditions 
Target Base 
composition material Re- Vacuum Arc 
(weight %) potential 
action degree current 
Example 
Ni Mo Ag La (V) gas (Torr) (A) 
______________________________________ 
8 60 40 -- -- 0 Oxygen 
3 .times. 10.sup.-3 
100 
9 57 38 5 -- 0 Oxygen 
3 .times. 10.sup.-3 
100 
10 57 38 -- 5 0 Oxygen 
3 .times. 10.sup.-3 
100 
______________________________________ 
TABLE 4 
______________________________________ 
Properties of Coating Layer 
Alloy composition 
Peak Hydrogen 
(% by weight) position Reaction overvoltage 
Example 
Ni Mo Ag La potential 
half-width 
(mV) 
______________________________________ 
8 61.5 38.5 -- -- 43.8.degree. 
1.8.degree. 
103 
9 60.2 36.4 3.4 -- 43.7.degree. 
2.2.degree. 
82 
10 58.4 37.7 -- 3.9 43.8.degree. 
2.8.degree. 
88 
______________________________________ 
TABLE 5 
______________________________________ 
Coating Layer Forming Conditions 
Target Base 
composition material Vacuum Arc 
(weight %) potential 
Reaction degree current 
Ni Mo (V) gas (Torr) (A) 
______________________________________ 
Example 
11 87 13 -40 Oxygen 1 .times. 10.sup.-3 
100 
12 82 18 -40 Oxygen 1 .times. 10.sup.-3 
100 
13 43 57 -40 Oxygen 1 .times. 10.sup.-3 
100 
14 38 62 -40 Oxygen 1 .times. 10.sup.-3 
100 
Comparative 
Example 
3 95 5 -40 Oxygen 1 .times. 10.sup.-3 
100 
4 25 75 -40 Oxygen 1 .times. 10.sup.-3 
100 
______________________________________ 
TABLE 6 
______________________________________ 
Properties of Coating Layer 
Alloy 
composition Hydrogen 
(% by weight) Peak Peak overvoltage 
Ni Mo position half-width 
(mV) 
______________________________________ 
Example 
11 88.5 11.5 43.8.degree. 
0.5.degree. 
146 
12 83.2 16.8 43.6.degree. 
0.8.degree. 
135 
13 42.9 57.1 43.6.degree. 
0.6.degree. 
149 
14 37.7 62.3 43.7.degree. 
0.6.degree. 
149 
Comparative 
Example 
3 96.8 3.2 43.6.degree. 
0.5.degree. 
252 
4 22.4 77.6 40.7.degree. 
0.6.degree. 
273 
______________________________________ 
TABLE 7 
__________________________________________________________________________ 
Effects of Mo/(Ni + Mo) Ratio in Plating Bath 
Example Comparative Example 
15 16 17 5 6 
__________________________________________________________________________ 
Plating bath composition 
Ni ion (mol/l) 
0.028 
0.228 
0.228 
0.228 
0.228 
Mo ion (mol/l) 
0.012 
0.0268 
0.057 
0.0012 
0.114 
Citrate ion (mol/l) 
0.344 
0.344 
0.344 
0.344 
0.344 
Mo/(Ni + Mo) (mol %) 
5.0 10.5 
20.0 
0.5 33.3 
Ni + Mo concentration 
(mol/l) 
0.24 
0.26 
0.29 
0.23 0.34 
Plating conditions 
Current density 
(A/dm.sup.2) 
5 5 5 5 5 
Temperature (.degree.C.) 
50 50 50 50 50 
Plating time (min) 
24 24 24 24 24 
pH 8.0 8.0 8.0 8.0 8.0 
Mo concentration 
(wt %) 
39.0 
41.2 
45.0 
9.7 68.5 
in alloy layer 
X-ray diffraction main peak 
Position 43.7.degree. 
43.7.degree. 
43.7.degree. 
44.2.degree. 
43.7.degree. 
Half width 5.3.degree. 
6.0.degree. 
6.0.degree. 
1.0.degree. 
5.7.degree. 
Hydrogen overvoltage 
(mV) 
108 120 127 298 220 
__________________________________________________________________________ 
TABLE 8 
__________________________________________________________________________ 
Effects of (Ni + Mo) Total Concentration in Plating Bath 
Example Comparative Example 
18 19 7 8 
__________________________________________________________________________ 
Plating bath composition 
Ni ion (mol/l) 
0.456 
0.114 
0.912 
0.057 
Mo ion (mol/l) 
0.038 
0.0095 
0.076 
0.00475 
Citrate ion (mol/l) 
0.688 
0.172 
1.380 
0.138 
Mo/(Ni + Mo) (mol %) 
7.7 7.7 7.7 7.7 
Ni + Mo concentration 
(mol/l) 
0.50 0.12 0.99 0.06 
Plating conditions 
Current density 
(A/dm.sup.2) 
5 5 5 5 
Temperature (.degree.C.) 
50 50 50 50 
Plating time (min) 
24 24 24 24 
pH 7.8 7.7 7.8 7.8 
Mo concentration 
(wt %) 
37.5 45.3 34.7 66.5 
in alloy layer 
X-ray diffraction main peak 
Position 43.7.degree. 
43.7.degree. 
43.9.degree. 
43.7.degree. 
Half-width 6.0.degree. 
5.2.degree. 
0.3.degree. 
6.5.degree. 
Hydrogen overvoltage 
(mV) 130 121 288 257 
__________________________________________________________________________ 
TABLE 9 
__________________________________________________________________________ 
Effects of pH of Plating Bath 
Example Comparative Example 
20 21 22 9 10 
__________________________________________________________________________ 
Plating bath composition 
Ni ion (mol/l) 
0.228 
0.228 
0.228 
0.228 
0.228 
Mo ion (mol/l) 
0.019 
0.019 
0.019 
0.019 
0.019 
Citrate ion (mol/l) 
0.344 
0.344 
0.344 
0.344 
0.344 
Mo/(Ni + Mo) (mol %) 
7.7 7.7 7.7 7.7 7.7 
Ni + Mo concentration 
(mol/l) 
0.25 
0.25 
0.25 
0.25 0.25 
Plating conditions 
Current density 
(A/dm.sup.2) 
5 5 5 5 5 
Temperature (.degree.C.) 
50 50 50 50 50 
Plating time (min) 
24 24 24 24 24 
pH 7.0 8.5 9.0 10.5 5.0 
Mo concentration 
(wt %) 
38.5 
39.6 
36.0 
9.0 8.5 
in alloy layer 
X-ray diffraction main peak 
Position 44.0.degree. 
44.0.degree. 
44.0.degree. 
44.0.degree. 
44.0.degree. 
Half-width 5.5.degree. 
6.0.degree. 
5.8.degree. 
0.4.degree. 
0.3.degree. 
Hydrogen overvoltage 
(mV) 
107 111 109 197 238 
__________________________________________________________________________ 
TABLE 10 
______________________________________ 
Effects of Heat Treatment at 150.degree. C. after Plating 
Comparative Example 
11 12 13 
______________________________________ 
Plating bath composition 
Ni ion (mol/l) 0.228 0.228 0.228 
Mo ion (mol/l) 0.019 0.057 0.019 
Citrate ion (mol/l) 0.344 0.344 0.344 
Mo/(Ni + Mo) (mol %) 7.7 20.0 7.7 
Plating conditions 
Current density (A/dm.sup.2) 
5 5 5 
Temperature (.degree.C.) 
50 50 50 
Plating time (min) 24 24 24 
pH 7.8 8.0 9.0 
Mo concentration 
(wt %) 40.2 45.0 36.5 
in alloy layer 
Heat treatment temperature 
(.degree.C.) 
150 150 150 
after plating 
X-ray diffraction main peak 
Position 44.5.degree. 
44.5.degree. 
44.5.degree. 
Half-width 0.3.degree. 
0.3.degree. 
0.3.degree. 
Alloy layer crystal Ni.sub.4 Mo 
Ni.sub.4 Mo 
Ni.sub.4 Mo 
after heat treatment 
Hydrogen overvoltage 
(mV) 108 120 127 
______________________________________