I
~
'
I
U.
S.
Depart:ment
of
Co:m:merce
National
Bureau
of
Standards
Research
Paper
RP
1725
Volu:me
37,
July
1946
Part
of
Journal
of
Research
of
the
National
Bureau
of
Standards
Nickel Plating on Steel by Chemical Reduction
By
Abner
Brenner
and
Grace
E.
Riddell
A process has been developed for
the
production
of
adherent
nickel deposits of good
quality
on
steel
without
the
uSe
of
an
electric
current.
The
deposition of nickel
is
brought
about
by
chemical
reduction
of a nickel
salt
with
hypophosphites
in a
hot
ammoniacal
solution.
The
reaction is
catalytic
and,
under
the
prescribed conditions of
concentration
and
pH,
no
reduction
occurs in
the
solution unless
certain
metals, such as
steel
or
nickel,
are
introduced
into
the
bath.
The
reduction
then
occurs only
at
the
surface
ot
the
immersed
metal
with
the
production
of a
coating
of
nickel of 96
to
97
percent
purity.
I.
Introduction
In
the
course of
an
investigation
on
a nickel
plating
bath
an
unusual chemical reaction was
accidentally encountered,
by
means of which
ni~kel
deposits of good quality can be produced
on a steel or nickel surface without the use of
an
electric current.
The
reaction involves reduction
by
hypophosphites in a heated nickel solution.
In
the nickel
bath
being investigated, trouble
was
had
with the oxidation of some constituents
of
the
bath
at
an
insoluble anode, and to obviate
this difficulty typical reducing agents were added.
In
one of the experiments
the
surprising result of
an
apparent
cathode current efficiency of 130
percent
was obtained, although during the plating
there was considerable gassing
at
the cathode,
which usually indicates a low current efficiency.
Plating
by
Che:mical
Reduction
693841-46-5
Furthermore, the object being plated, which was
a
tube
with
an
inside anode, was found
to
have
been completely plated
on
the outside, although
no external anodes
had
been used. Although this
process will
not
replace electrodeposition of nickel,
it
may
prove useful for special applications.
I
Contents
Pa~e
1.
Introduction
. .
31
II.
Literature.
. . 32
III.
General principles. 32
IV.
Bath
composition
and
operating
condition
. 33
V.
Other
methods
of chemical
reduction
. . . 34
31
~~----~--~~
~
-------------------------------------
II. Literature
The
foregoing experience
with
the
hypophos-
phite
reduction occurred before
it
was
noted
that
this reaction
had
been previously reported.
An
investigation of
the
literature
disclosed
that
the
reduction of nickel solutions with hypophosphite
to
produce metallic nickel
had
been observed
by
Wurtz.l
The
reaction was studied
in
detail
by
Bretau,
2
Paal
and
Fri
ederici, 3 Scholder
and
Heckel,4
and
Scholder
and
Haken.
6
These
workers used high concentrations of hypophos-
phite, usually several
hundred
grams
per
liter, to
effect
the
reduction.
The
solution containing
the
nickel
salt
was
heated
on a steam
bath
for several
hours, if necessary,
until
the reaction began.
The
reaction was usually quite vigorous, much hydro-
~
gen was evolved,
and
the mass bubbled
up
to
10
or 20 times
its
original volume.
The
nickel was
obtained mainly as a
dark
powder,
but
occasionally
it
deposited
on
the
walls of
the
flask as a mirror,
which gradually became detached
and
broke up
into
thin
flakel?
The
reaction,
by
which nickel is produced is
NiCb
+ N
aH
2
P0
2
+
H20~
Ni+2HCI+NaH
2
P0
3
(1)
or
Ni+++H2P02+H
2
0~NiO+2H++H2P03'
Concurrently, some of thehypophosphite is oxidized
by
the
water, particularly
in
the presence of certain
metals, to phosphite,
and
hy
.drogen is liberated:
NaH2P02+H20~NaH2P03+H2'
(2)
These equations show
that
the
reaction mixture
becomes more acid, as either
an
acid
salt
or
free
acid is produced.
The
reduction of nickel ion
(eq.
1)
is catalyzed by certain metals, including
nickel, and as nickel is produced
by
the reaction,
it
is therefore autocatalytic. This explains why
the
reaction, which is
rather
slow in
starting,
pro-
ceeds
with
so
much
vigor after
it
once begins.
Some of
the
above investigators obtained cobalt
by
a hypophosphite reduction similar to
that
used
for nickel. Only alkaline solutions could be used,
and
the reduction was slowf'r
than
that
of
nickel.
The
product which these workers (see footnotes
3
and
4) obtained from
an
initially neutral nickel
solution was
not
pure nickel
but
a mixture of
phosphides, probably as
Ni
2
P
and
Ni
s
P
2
(see foot-
note
3), with perhaps some uncombined nickel.
The
precipitate usually contained 85 percent of
nickel and about
12
percent of phosphorus,
with
small amounts of moisture and oxygen.
The
product obtained from
an
alkaline solution was
much purer.
It
contained about 97 percent of
nickel and
3 percent of phosphorus,
and
hence
consisted largely of uncombined nickel.
The
nickel phosphides obtained
by
the hypophosphite
reduction are more similar to an intermetallic
compound, in which the phosphorus behaves as
a metal,
than
to a phosphine derivative.
They
are soluble in acids only with difficulty
and
do
not
give
off
phosphine when
so
treated.
III. General Principles
In
an
extension of the
work
of the previous
investigators,
it
was found
that
the
reduction of
nickel compounds to nickel could be so controlled
as to cause the catalytic reduction to occur,
with
the virtual exclusion of the purely chemical reduc-
tion. Thus
an
adherent compact nickel coating
could be deposited on certain metal surfaces
immersed
in
the
bath
without
any
appreciable
precipitation of nickel occurring throughout
th
e
bulk of the solution or on
the
walls of the glass
vessel. This preferential deposition was made
possible
by
employing dilute solutions of hypo-
1
Compt
. rend.
21,
149
(1
8
45
).
•
Bul
.
so
c.
chim. [4]9, 518 (
1911)
.
3 B er. deut . che
rn
. Oeo. 84,1766
(19
3
1)
.
. • Z . anorg. all
ge
m. Chern .. :
198,
329
(1931).
I
Ber
. deut. ch cm. Oes.
64,
2870
(1931).
32
phosphite,
about
10
grams per liter instead of
several hundred grams per liter. Under these con-
ditions,
the
deposition of nickel occurs
on
the sur-
face of iron, nickel, gold, cobalt, palladium, and alu-
minum,
but
not
on platinum, copper, zinc, or lead.
The
method differs from the common methods
of chemically depositing metallic coatings from
aqueous solutions.
The
formation of coatings
by
chemical replacement, such as
the
formation of a
copper film on steel dipped in copper sulfate, in-
volves solution of the base metal.
The
formation of
metallic films, such as silver, copper, or gold
by
re-
duction, does
not
necessarily involve
any
catalytic
action, as the metal precipitates indiscriminately
over all articles immersed in it, as well as on
the
walls of the vessel.
Journal
of
Research
IV. Bath Composition and Operating Conditions
The
composition of
the
baths
may
vary
within
wide limits. Some typical formulas
are
shown in
table 1.
Th
e
baths
are operated
at
or
above 90° 0
(194°
F).
As
already noted,
they
produce a
deposit only on certain metals.
In
the
absence of
m
eta
ls
that
catalyze nickel deposition,
the
baths
are fairly stable. A
bath
containing hypophos-
phite
has been
kept
at
90° 0 for 5 hours
without
deteriorating very much.
At
room temperature, a
bath
is stable for several days.
The
nickel deposits contained
about
97 percent
of nickel,
and
therefore are similar to those
that
Paal
and
Friederici obtained from
an
alkaline
solution.
TABLE
I.
- Bath compositions
Bath
3
N ickel chloride,
NiOI,.6H,0
. . 30g/litcr . .
..
. 30g/liter
..
...
30g/liter.
Sodium
h
ypophosphi
te,
10
g/liter.
..
. .
10
g/
lit
er
.
....
10
g/liter.
NaH,pO,.
Ammonium
chloride,
NH,OJ.
50
g/lit
er.
_
..
.
100
g/litcr .
..
. .
..
. _
....
.
__
._
Sodium c
itr
ate,
Na,O,H,O,.
100
g/liter
..
. .
..
...
...
.
..
. _
..
100
g/liter.
5j6H,0.
Rate
of deposition{:nm/hr..
..
0.006
.
..
. .
....
0.012
..
....
...
0.005.
Ill./hr. _
...
0.00025._..... 0.
0005...
...
..
0.0002.
App
ea
ran
ce
of
depos
it.
.......
SemibrighL
.
Dull
.....
..
..
Bright.
Alkaliforn
eut
rali
zingbat
h .
__
NH,OH._
..
.
NH,OH.
__
. .
NaOH.
pH
...
. . . . . _
...
.•.
........
....
8 to 9
..
. _
....
. 8 to 9 .
........
8 to
9.
Steel objects to be plated are cleaned
by
any
of
the accepted procedures and are given
an
acid
dip before being placed in the bath.
The
parts
to
be plated are suspended in the
bath,
for example,
by
a string. Small objects
may
be held on cloth
stretched over a frame and should be agitated
occasionally, although there is no need of con-
stant
motion, as in barrel plating, because current
distribution is
not
involved. During the deposi-
tion a considerable amount of hydrogen is evolved
from
the
surface of the metal. Under
the
condi-
tions of operation outlined, nickel deposits only
on the steel. However, after a
bath
has operated
in a glass vessel for
about
5 hours, a small
amount
of a precipitate containing nickel
may
deposit
on the
bottom
of the vessel where the source of
heat
has
been applied.
If
this precipitate is
not
removed,
it
may
cause decomposition of some of
the hypophosphite,
just
as another metal surface
does.
The
thickness of nickel deposited in a given time
depends in
part
on the relation of the steel surface
Plating
by
Chemical
Reduction
to the volume of the bath.
The
values
ill
the
above table were obtained with
an
area of 1
square decimeter
(16
square inches) in 1 liter
(1.1
quart)
of solution.
Th
e
rate
of plating is less
than
that
usually employed in nickel electro-
plating in a tank,
but
is
about
the same as in
barrel plating. Because of the gradual exhaustion
of the hypophosphite, less nickel will deposit
during each succeeding interval of time.
Most
of
the
nickel is deposited during the first hour,
and
the
reaction
is
virtually complete in 2 hours,
unl
ess
more hypophosphite is added.
As the hypophosphite is gradually exhausted,
more
must
be added if thick deposits are required,
especially if the area to be plated is relatively
large compared to the volume of solution. Addi-
tions of 5 grams
of
hypophosphite per liter
may
be made
at
half-hour or I-hour intervals. Thereby,
deposits
0.002 inch (0.05 mm) thick have been
obtained in
about
6 hours. As pointed
out
in
equations 1
and
2,
the
hypophosphite becomes
converted into phosphite, which, however, does
not
interfere with the operation of the
bath.
When
the
bath
becomes too concentrated
with
this
compound,
it
would be more economical to discard
the
bath
than
to
try
to remove the phosphite.
Wh
en
the
bath
becom
es
depleted in nickel,
additional nickel
must
be
added in
the
form of
soluble nickel salts.
The
function of the am-
monium salts
or
the citrates
is
to
keep nickel
in
solution
at
a
pH
of 8
to
9.
Ammonia has
the
disadvantage of being volatile
at
the temperature
at
which
the
'
bath
is
operated,
but
it
is more
satisfactory
than
organic amines, which were
tried as substitutes.
The
citrate can
be
replaced
by
other
hydroxy-organic acids, such as tartaric,
without affecting the operation of
the
bath.
Bath
1 (see table 1) is the preferred composition.
Bath
2,
which does
not
contain any citrate, plates more
rapidly
than
bath
1,
but
the deposits are dull
and
more likely
to
be
rough,
and
the
bath
does
not
remain as clear during operation.
Bath
3,
with
no ammonium salts, gave
the
brightest de-
posits,
but
had
the
disadvantage
that
after
the
deposits
had
reached a thickness of
about
0.0002 inch,
they
ceased
to
become thicker even
after
several hours.
The
reason for using a low concentration of
hypophosphite, as already pointed out, is
to
ob-
tain
a deposition on
the
articles
without
producing
33
"nickel
black" throughout the
bath.
A similar
consideration governs the chosen concentration
of ammonium salts, too high a concentration of
which will cause the chemical reduction to
take
place with liberation of nickel on
the
walls of
the
vessel. A higher nickel concentration tends to
produce rough deposits.
The
temperature of operation of the
bath
is
important.
At
temperatures below 90° C.
the
rate
of deposition of nickel is slower, and
the
deposit is more likely to contain impurities.
For
example,
at
60° C. the
rate
of deposition
is
about
half
that
at
90° C. Nickel
may
be slowly deposited
on steel
at
room temperature from a
bath
that
has
a high concentration of hypophosphite.
The
pH
of
the
bath
should be
kept
between 8
and
9 to obtain the
best
deposits. On operation
of
the
bath
the
pH
drops because of the formation
of hydrogen ions, as shown in equations
1
and
2.
Also, the volatilization of the ammonia causes a
drop in the
pH.
The
pH
is readily
kept
at
the
required value
by
adding ammonia water occa-
sionally, so as to keep
the
bath
perceptibly
ammoniacal in odor.
The
efficiency of the reduction of nickel salts
to
nickel
is
not
very high because, as pointed
out
in
connection with reaction
2, some of the hypophos-
phite
is catalytically oxidized
by
water
with
the
liberation of hydrogen.
The
best
yield of nickel
that
has been obtained was
about
37 percent,
based on the conversion of
the
hypophosphite to
phosphite. This yield, obtained
with
a large area
of
metal
in
the
bath,
amounts
to
a reduction of
about
2 grams of nickel
by
10
grams of hypophos-
phite.
With
a small area of
metal
in the
bath,
the
efficiency of deposition is likely to be
about
20
percent.
The
adhesion of the nickel deposit to mild steel
is such
that
it
cannot be flaked off
by
bending.
However, the adhesion is less satisfactory
on
high-
carbon steel.
Salt-spray tests were made on steel coated with
0.0002, 0.0005, and 0.001 inch of this reduced
nickel, in comparison with similar panels coated
with electrodeposited nickel.
The
protective value
of the two types of nickel was virtually the same.
The
chemical method of plating nickel is
not
economical to operate because
the
hypophosphite
is expensive (about
70 cents a pound)
and
its
utilization is inefficient. On
the
other
hand, no
special equipment is required, such as generators,
rheostats,
and
racks.
The
utility of the method
will lie in special applications, for example, plating
small lots of small steel parts, which cannot
be
plated economically in a barrel. Another ad-
vantage of the method is
that
it
can
be
employed
to plate the inside of
an
article
that
cannot
be
plated with
an
inside anode.
For
example,
by
this
method
a nickel deposit was applied to the inside
of a
bent
steel tube one-fourth inch in diameter.
v.
Other Methods
of
Chemical Reduction
The
reduction of nickel
and
cobalt salts to
the
metal
is
a
rather
rare chemical phenomenon, of
which
we
have found no recorded examples
other
than
the method employing hypophosphites.
In
the course of some
other
work another system was
encountered in which nickel and cobalt salts can
be
reduced to metal,
but
the
reduction is
not
as
simple or practicable as the hypophosphite
method.
This
reaction involves
the
reduction of
nickel or cobalt in
an
alkaline solution with lower
valent
molybdenum compounds.
The
molyb-
denum, in the form of molybdate in a strongly acid
solution, is electrolytically reduced in a porous
cell, preferably with a lead cathode, to the reddish-
34
brown solution of the lower valent salt. When
this solution is added to
an
alkaline metal solution
containing hydroxyacetates (to keep molybdenum
and
cobalt compounds from precipitating), a thin
mirror of nickel or cobalt will form on the walls of
the
test
tube and some gas will be liberated.
Cobalt deposits have been obtained
by
another
reaction, which was
hot
conducted in aqueous solu-
tion. A cobalt salt was dissolved in molten potas-
sium formate or hydroxyacetate or a mixture of
the
two, which melts
at
about
150
0
C. When a
rod of copper or gold was left
in
the melt for some
hours, a thin, bright coating of cobalt was ob-
tained
as
a result of chemical reduction.
WASHINGTON,
Aprilll,
1946.
Journal
of
Research