Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to the electroplating of
chromium and its alloys.
Conventionally, chromium has been plated from aqueous
chromic acid baths prepared from chromic oxide (CrO3) and
sulphuric acid.
Such baths, in which the chromium is in hexavalent
form, are characterized particularly by low current efficiency.
The chromic acid fumes emitted as a result of hydrogen evolution
also present a considerable health hazard.
According to the present invention, there is provided
; 10 a chromium or chromium alloy electroplating solution in which
the souxce of chromium comprises a chromium (III)- thiocyanate
complex.
The invention also provides a process of plating chromium
or a chromium containing alloy comprising passing an electric
plating current between an anode and a cathode in a plating
solution having a source of chromium comprising a chromium
(III)- thiocyanate complex.
In a preferred embodiment the invention provides such a
solution in which the chromium (III) concentration is in the ;~
range of 0.03 to 0.5 M and in which the thiocyanate concen-
tration is in the range of 0.05 to 1.0 M and in which the pH
lies in the range of 2 - 3.5 and which includes boric acid
- in saturation concentration for plating colbalt chrome a~loy
in which the source of the cobalt comprises cobalt sulphate
(CoSO4- 7H2O)
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In another embodiment, the invention provides a
composition of matter for use in the electrodeposition of -
chromium containing deposits consisting essentially of ~n
aqueous solution of aquo chromium (III) thiocyanato complex
as a source of trivalent chromium, the concentration of the
Cr (III) being in the range of 0.03 to 0.5M.
In still a further embodiment, the invention provides
a method of electrodepositing chromium containing material
on a cathode, which comprises passing an electroplating
current between an anode and said cathode in a plating solution
consisting essentially of an aqueous solution of an aquo
chromium (III) thiocyanato complex as a source of trivalent
chromium.
The preferred complexes for the purposes of the
present invention are aquo chromium (III) thiocyanato complexes.
These may be prepared from chromium perchlorate and sodium
thiocyanate in aqueous solution in a manner to be described
in more detail below. The complexes so formed are of the
general formula:
(H2O)6 nCr (NCS)n( ) where n= 1,6.
Note subscripts are always positive but superscripts may be
positive or negative, e.g., if n equals 6 the ionic chromium
; species is Cr(NCS) subscript 6 superscript 3-. Complexes of
this type are well known. See Inorganic Chemistry 9,1023,(1970).
~, .
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In the plating solution, the several forms of complex
are present in an equilibrium mixture.
The mechanism of the plating process is believed to be
as follows. Plating current is carried predominantly by the -
sodium ions present in the solution from the dissociation of
the original sodium thiocyanate. Since the thiocyanate is a
bridging ligand the complexes are readily absorbed onto the
cathode as a result of which chromium, not sodium, is
preferentially deposited. Transport of the complexes to the
region of the cathode is by diffusion rather than under the
action of the plating field. Consequently, chromium is
desposited not only from the positvely charged but also the
negatively charged and neutral complexes. Substantially
all the thiocyanate is liberated to the solution as the
anion, NCS , though a small amount may be incorporated in
the deposit.
Although only aquo complexes have been investigated, there
seems to be no theoretical reason why other chromium (III)
thiocyanate complexes, such as amine complexes, should not
plate satisfactorily by the same ligand bridging mechanism.
Chromium has been plated from solutions according to
the invention having the following range of composition:
Substance Concentration
Chromium (III) 0.03 - 0.5 M
Thiocyanate 0.05 - 1.0 M
Boric acid Saturation (SOgm/litre)
The plating solution in each case was aqueous and pre-
pared from chromium perchlorate and sodium thiocyanate.
Process conditions employed when plating from the
~ .
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above range of solutions were as follows:'
Current den~ity 20 - 120 mA/cm2 of cathode surface ~ ~-
Cell voltage 7 - 15 volts
Temperature 20 - 25 c
pH 2 - 3. 5
The experimental plating runs from which the above
ranges were derived generally employed constant current conditions.
However some results were obtained under potentiostatic conditions
; in which the potential of the cathode was kept constant relative to a
standard calomel electrode arranged to sample the solution in the
vicinity of the cathode.
The plating apparatus in which the plating runs were
carried out included a platinised titanium anode. It was found to be
necessary for long term operation of the bath to isolate this anode,
by means of a semi permeable barrier, in a sodium perchlorate
anolyte solution. If this was not done the electrolyte pH fell steadily
and plating ceased at a pH value of ] . 5. In the course of the exper-
imental plating runs chromium was plated onto a brass cathode in a
Hull cell and onto copper and metallised glass cathodes in different
plating cell arrangements. Thicknesses of up to 0. 001 inches were
deposited.
The chromium plated by the above processes was sub-
stantially pure though detection of sulphur indicated that a small
amount of thiocyanate had been incorporated in the deposit. The
effect of this may be beneficial in reducing stress in the deposit.
The deposited chromium was observed to be bright and was found
to be relatively hard (700 Vickers Hardness No). The deposits
were uncracked and, presumably because of this had excellent
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corrosion resistance.
In these experimental plating runs, the plating process
had good throwing power and operated over a wide range of
conditions. The current efficiency was greater than in the
convential chromic acid bath where, typically, current densities
of 200 mA/cm2 are required.
In addition to pure chromium plating, solutions containing
aquo-chromium (III) - thiocyanato complexes have also been
used to plate alloys of chromium. In particular, an alloy
of cobalt and chromium has been plated from a solution
containing cobalt sulphate as the source of cobalt. Also an
alloy of nickel and chromium has been plated from a solution
containing nickel sulphate as the source of nickel.
The invention will now be more particularly described
with reference to the following examples of preferred plating
solutions and processes according to the present invention.
EXAMPLE 1
Preparation of a plating solution according to the
invention comprised the initial step of preparing a solution
of chromium perchlorate in water. This solution was prepared
by adding 150 grams of sodium dichromate (NA2Cr207) to
485 ml of perchloric acid (~C104) and 525 ml of water. ~ydrogen
peroxide was added in dropwise fashion until the solution
became deep blue. When this state was reached the solution
was boiled down to half its volume driving off hydrogen
peroxide and leaving the required solution of chromium
perchlorate Cr~C104)3. This solution was further diluted
to 0.15 M. concentration to provide a source of trivalent
chromium for plating.
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1064~24 :~
To prepare the plating solution, 150 ml of the diluted
chromium perchlorate solution was saturated with boric acid. Then
sodium hydroxide was added in dropwise fashion to adjust the pH of
the solution to 1 - 2. 2 grams of sodium thiocyanate (NaNCS) were
added to the solution and the resultant mixture was subsequently
heated at 80 c for one hour to produce a plating solution comprising
an equilibriunl mixture of aquo-chromium (III) thiocyanata complexes.
Production of the complexs in this manner involves the progressive
replacement with SCN groups of the H2O groups in the hydrated
chromium ion Cr(H2O)6 3- present in the chromium perchlorate
solution. The pH of the solution was finally adjusted to 2. 5.
The concentration of the various constituents of this
plating solution is as follows:
Cr (III) 0.1 M
NCS 0. 2 M
H3BO 3 50 gm/ltr
Na 2 M
C104 0. 5 M
A plating process according to the present invention and
employing the plating solution as prepared above was carried out as
f ollows:
The plating solution was introduced into a plating cell
` having a platinised titanium mode and a flat surfaced brass cathode.
The anode was isolated from the plating solution proper by a cationic
selective ion exchange membrane or barrier and was surrounded by
an anolyte of sodium perchlorate at 0. 5 M concentration. The pH
of the anolyte was 2.
- A plating current of density 2S mA/cm of cathode sur-
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~ ~064424
face was passed between anode and cathode for a time of 15
minutes. The plating current was kept constant throughout
this time. The termperature of the solution during plating
was 20c. A total weight of 0.021 gm of chromium was deposited.
The deposited chromium appeared bright to the eye and
uncracked when examined under a microscope. Its hardness was
measured to be 700 VHN. Its resistance to corrosion in a
high humidity, high sulphur dioxide atmosphere was excellent.
EXAMPLE II
An alloy of cobalt and chromium was plated onto a
copper cathode in a cell similar to that employed in Example 1.
The plating solution had the following composition:
CoSO4 0.025 M
Cr (III) 0.1 M
NCS 0.2M
H3BO3 50 gm/ltr
Na 2 M
~; C104 0.5M
Plating was carried out under constant current conditions
and a current density of 150 MA/cm2 of cathode surface was
employed for a time of 2 minutes. The solution temperature
was 20c during plating. A weight of 3 mgm of cobalt
chromium alloy was deposited. The composition of the alloy
was 20 (At)~ Co. This alloy is magnetic and the coercivity of
the deposit was measured as 30 Oe. Corrosion resistance
as measured electrochemically was excellent.
EXAMPLE III
An alloy of nickel and chromium was also plated onto a cathode
in a cell similar to that employed in Example 1. The plating
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solution had the following composition:
NiS04. 6H2o o. 25 M
Cr (III) 0.1 M
NCS 0. 2 M
H3B03 50 gm/ltr
Na 2 M
C104 0. 5 M
Plating was carried out under constant current conditions
at a current density of 150 mA/cm2 of cathode surface. The solution
temperature was 20 c during plating. The plating current was
applied for a time of 2 minutes. The deposit was observed to be
magnetic indicating that a nickel chromium alloy had been plated.
The specific magnetic properties were not measured in this example.
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