Note: Descriptions are shown in the official language in which they were submitted.
3'7~ '
Method an~ Composition for Electroplating
Chromium and its Alloys and the Method of
Manufacture of the Composition Using
Chromium Thiocyanate Complex
Related Applicatlons
This application is related to our U.K. speclfication
1,431,639 naming Dona}d John ~arclay and William Morris Morgan
as inventors and complete specification of which was published
on April 14, 1976. This is also related to our copending
Canadian application Serial No. 292,187, filed December 1,
1977, naming Donald John Barclay and William Morris Morgan as
inventors, and entitled "Electroplating Chromium and Its Alloys".
Background of the Invention
The above U.K. specification teaches a chromium, or
chromium alloy electroplating solution, in which th~ source of
chromium comprises an aqueous solution of a chromium (III)
thiocyanate complex. Said specification further describes
a process of plating chromium, or a chromium containing alloy,
which process comprises passing an electroplating current
between an anode and a cathode in said electroplating solution.
In a preferred form the chromium ~III) thioc~anate
complex consists essentially of an aqueous solution of
a chromium (III) aquo thiocyanate complex or mixture of
complexes having the general formula:
~; .
~lZ33'7L)
1 [(H~0)6 nCrIII (NCS) n]3 n; where n = a positive integer
2 from 1 to 6 (~ote: that the subscripts are always positive but
3 the superscript may be positive, negative or zero). Complexes o
4 this type are well known, see Inorganic Chemistry 9, 1024, (19707.
The plating from the solutions described in the above-.
6 mentioned speci~ications has many advantages over the conventional
7 methods of plating chromium from the highly toxic chromic acid
8 baths or from baths using toxic organic solvents. Most important
9 of these is the removal of the serious health hazard present
10 during plating and the fact that the effluent is easier and safer
11 to dispose of. This process is less expensive, is rll~re
12 efficient electrically, and the useful life of the processing
13 apparatus is much l~nger due to less corrosion. Significantly
14 the deposited chromium is micro-crack free and is capable of
15 being bent without cracking. H~wever, it has been found that the
16 appearance of the chromium deposits at low current densities is a
l7 strong function of the ratio of the total concentration of
18 chromium III to total thiocyanate in the plating solution. For
19 example, at a total chromium III to total thiocyanate ratio of 1:2,
20 while bright chromium is being deposited at current densities in
21 the range 20mA/cm2 to 120mA/cm2, this quality falls off rapidly
22 below 20mA/cm2; and in fàct a black deposit is obtained at current
23 densities less than about 15mA/cm2. This can be deleterious when
24 plating complex shapes on which there is a wide range of current
25 densities. It is believed that reason for the black deposit is
2~ the incomplete deposition of chrom~um fro~ Cr(~2ol6 ~ at l~ current
27 densities. Previous attempts to plate chromium from this ion
28 resulted in nonmetallic deposits.
3,1Z33'70
Summary of the InVention
The present invention provides a chromium or chromium
alloy electroplating solution in which the source of chromium
comprises an aqueous equilibrated hexathiocyanatochromium (III)
salt complex, the molar ratio of total chromium III to total
thiocyanate being about 1:6, so that the concentration of
Cr(H2O)6 is maintained low. The solution may optionally
be further characterized by containing a highly ionizable salt
to increase the electrical conductivity of the solution. As
used herein the ratio of chromium III to thiocyanate in the
solutions means the total chromium, both free ions and
complexed ions, to the total thiocyanate, both free ions and
complexed ions.
The present invention also provides a method of
electroplating chromium comprising the steps of providing an
aqueous equilibrated solution of hexathiocyanatochromium III
salt complex, the molar ratio of total chromium III to total
thiocyanate being about 1:6, so that the concentration of
Cr(H2O)63+ is maintained low, and passing an electric current
between an anode and a cathode in said solution.
In one embodiment such a method is provided wherein
the salt solution was equilibrated prior to use by being
heated to an elevated temperature. Preferably the elevated
temperature is about 80C and the solution is maintained at
such temperature for about three hours. In some cases, an
embodiment is provided further characterized by addition of a
highly ionizable salt to increase the electrical conductivity
of the solution.
This solution and method generally provides good
plating at low current densities. This will allow good plating
- 1~2337~
results with current densities even as low as 5mA/cm2 and up
to 320 mA/cm2.
Description of the Preferred Embodiments
According to the present invention a chromium III
thiocyanate complex or mixture of complexes preferably of
the general formula [(H2O)6 CrIII(NCS)n]3 n wherein n is a
positive intPger from one to six is employed in aqueous
solution. The solution is so formed and equilibrated that
the ratio of the total chromium III (both free and complexed
ions) to the total thiocyanate (both free and complexed ions)
is 1:6. This is accomplished by providing thiocyanate in
sufficiently large excess whereby the equilibrium of the
equation:
- 3(a) -
33~tO
1 Cr(H2O)6 + n~NC.S) + [Cr(H2O)6 n(NCS)n] ~ n(H2O)
2 tends to the right so that the concentration of Cr(H2O)63+ is low
and the deposition of black deposits is minimized. By this means
4 the range of practical current densities can be increased from
5 ~s low as 5mA/cm and up to 320mA/cm .
6 The advantage of a l:6 ratio of chromium III to thio-
7 cyanate is not only that it increases the practical range of
8 current densities, but also that the ion Cr(NCS)63 is stable and
9 is commercially available from hexathiocyanatochromium III salts,
l0 such as K3 Cr(NCS)6; Na3 Cr(NCS)6; or (NH4)3 Cr(NCS)6. By
ll equilibrating this ion, for example at 80C for three hours, the
12 required [Cr (H2O)6 n ~NCS)n]3 n species may be formed. It will be
~3 clear that the preparation of the ~hromium III aquo thiocyanate
14 ions by this technique is significantly easier t~han by the method
15 describ~d in the above-mentioned patent specification.
16
17 Another significant advantage of plating chromium from
trivalent chromium thiocyanate electrolytes is the possibility of
l9 adding organic species such as wetting agents. This is not
20 possible with chromic acid electrolytes since the organic material
21 is spontaneously oxidized. It has been found that adding wetting
22 agents such as TRITON-X (TRITON is a Trademark of Rohm and Haas
23 Company) or FC-98 (a trademark of the 3M Corporation), to the
24 aqueous chromium III thiocyanate plating solution improves the
25 appearance of the deposited chromium and extends the maxim~m ~urrent
26 density at which deposition can occur before gas streaming prevents
27 plating. Hull cell tests have shown that with the addition of
28 these wetting agents the range of current density for bright
29 deposits can be increased to between 5mA/cm and 170mA/cm2.
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1 It has also been found that the conductivity of
2 the platint.~ solution can be increased by adding salts such
3 as sodium perchlorate (NaC10~), sodium sulphate (Na2SO~) or
4 potassium sulphate (K2SO4).
Increasing the total chromium concentration in the
6-solution up to about 0.1~, while maintaining the 1`:6 ratio of
7 chromium to thiocyanate, improves the plating efficiency of.the
8 bath and.increases the maximum current. density at which bright
9 deposition occurs to 320m~/cm2.
Nickel or cobalt sulphate can be added to the solution
11 for plating chromium/nickel or chromium/cobalt alloys, as des-
12 crihed in the above-mentioned U.~. specification.
13 The above-mentioned U.K. speci~ication
14 describes the use of an ion exchange membrane to separate the
15 anolyte from the bulk plating solution'.~ The purpose of this
16 membrane is to prevent the fall of solution pll during plating.
17 ~part from this pll change no dele-terious anode reactions have
18 been observed. ~ disadvantage.associated with the use of the
19 membrane is the hi~her volta~e required to overcome its resis-
20 ~ance to currcnt flow and the resulting rise in bath tempera-
21 ture. ~ccordingly ion exchange membranes have not been used in
22 thc plating ~rocess described'herèin.
23 Using a normal ratio of solution volume to electrode
24 surface area (no more than 100 cm2 electrode area per litre of
25 solution) and when plating parts with a normal bright chromium
26 thickness (say 0.3 to 0.5~m), the pH change during:a plating
27 cycle is not sufficient to affect plating. The p~l can be ad-
~8 justed, periodically during use, by addition of small quantities
- *M = moles per liter
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13LZ3~70
1 of sodium hydroxide solution. ~lternatively the p~l can be
2 maintained within the required limi-ts by automatic additions oE
3 sodium hydroxide solution.
4 The invention will now be described with reference -to
5 the following examples:
~ EX~MPLE I
7 A 0.05M aqueolls solution of chromium III aquo thio-
8 cyanate complex ions prepared as described in the above-mentioned
9 U.K. patent specification, i.e., by heating sodium thiocyanate
10 together with a chromium perchlorate solution. However, sufficient
11 excess sodium thiocyanate ~as used to result in a ratio of
12 chromium III to thiocyanate of 1:6. Sodium perchlorate ~.as added
13 to increase in the conductivity of the solution. A wetting agent,
14 TRITON-X was also added to the solution.
lS The composition of the platin~ solution comprised:
16 Chromium III....................... 0.05M
17 Thiocyanate........................ 0.30M
18 Boric ~cid......................... 50g~1itre ~sat~ration)
19 Sodium Perchlorate......... ,....... 100g/litre
rrl~lToN-x....... ~ o~l~l/litre
~1 ~ brass plate was plated from the solution described above
22 usin~ a standard 267 mm Hull cell under the followin~ conditions:
23 ~node............ ..................Platinized titanium mesh
24 Cat~ocle........ ...................Brass Hull cell plate
2~ p~l. ............ ...................2.6
26 Temperature...... ...................20C
27 Total Current.... ...................4~
28 Cell Voltage..... ...................8.5V
29 Plating Time..... ...................2 minutes
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1 The cathode was found to be plated with a bright
2 c~lromium deposit from the 4m~/cm2 to the 170mA/cm2 positions on
3 a ~lull cell scale.
4 Below the 4mA/cm2 level there were interference colors
5 due to very thin chromium deposits followed by a bare brass area.
6 Above the 170m~/cm2 level deposition had been prevented b~ excessive
7 hydrogen ~as streaming. The pH of the solution after plating was
8 1.80. Thus ~right chromium of rich attractive color was obtained
9 over the whole of the plating current density ran`ge and no black
10 deposit was produced at low current densities.
11 EX~MPLE II
12 ~ solution was prepared as in Example I with 0.lM
13 chromium tIII)
14 The composition of the plating solutio~ comprised:
15 . Chromium (III)..................... 0.1
L6 Thiocyanate........................ 0.6M
17 Boric ~cid......................... 50g/litre (saturation)
18 Soclium Perchlorate..... ,.......... 100g/litre
19 TRITON-`X............... ~.......... 0.1ml/litre
~ brass ~lull cell cathode was plated ~rom this
21 solution in a standard 267mm Hull cell under the following
22 conditions:
23 ~node.............................. Platinized titanium mesh
24 Catl-ode........................... Brass ~ull cell pla-te
p~....... ,........................ 2.6
26 Telnperature....................... 20C
27 Total Current...................... 5~
28 CFll Voltage....................... llV
29 Plating Time....................... 2 minutes
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1 The cathode was found to be plated with a bright
2 chromium deposit from the IOmA~cm to the 320mA/cm2 positions.
3 Below lOmA/cm2 and above 320mA/cm2 a band o inter-
4 ~erence colors faded out to bare brass. The deposit was slightly
5 darker in color than deposits plated ~rom 0.05M chromium solu-
6 tions. The high current density limit was calculated from R. O.
7 Hull's formula given in the book, "Nickel and Chromium Plating",
8 (page 23) by Dennis & Such, published by Newnes-Butterworths.
9 Examples III to VI describe alternative methods of
10 preparing a solution containing Cr(NCS)6 ions.
11 EXAMPLE III
12 A 0.05M aqueous solution of sodium hexathiocyanato-
13 chromium tIII) (Na3Cr(NCS)6) was saturated with boric acid
14 (H3B03). The solution was then heated at 80C for three hours.
15 The solution was then cooled and lOOg/~tre NaC104 was added
16 to increase the conductivity of the solution. The wetting
17 agent TRITON-X was added.
18 The composition of the plating solution comprised:
19 Chromium (III)..................... 0.05M
Thiocyanate........................ 0.30M
21 Boric Acid......................... 50g/litre (saturation)
22 Sodium Perchlorate................. lOOg/litre
23 TRITON-X........................... O.lml/litre
24 EXAMPLE IV
A 0.05M aqueous solution of sodium hexathiocyanato-
26 chromium (III) (Na3Cr~NCS)6) was saturated with boric acid.
27 The solution was then reduced electrochemically on a mercury
28 electrode at a potential of -lOOOmV vs SCE ~Standard Calomel
Electrode) to produce a
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1 solution of chromium (II) thiocyanate complexes. This solution
2 w~s reoxidiz~d ~t -300mV vs SCE to p~oduc~ a mi~ture of
3 chromium (III) aquo thiocyanate complexes. (See Inorganic
4 Chemistry 9, 1028, 1970.) 100cJ/litre sodium perchlorate was
5 added to increase the conductivity of the solution. ~ _
6 wetting agent TRITON-X was also added.
7 EXAMPLE V
8 ~ 0.05M aqueous solution of potassium he~athio-
9 cyanatochromium (III) (K3Cr(NCS)6) was saturated with boric
10 acid. The solution was then heated at 80~C for three hours.
11 The solution was then cooled and 100g/litre sodium sulphate
12 was added to increase the conductivity oE the solution. The
13 wetting agent TRITON-X was added.
14 The composition of the bath comprised:
Chromium (III)..................... ~0.05M
-
lfi Thiocyanate........................ Ø30
17 Boric ~cid........................ .50g/litre (saturation~ ~
18 Sodium Sulphate.................... .100g/litre ~ ~,
1~ TRI~'ON-X................ O......... Ølml/litre
EX~MPLE ~
21 ~ convellient way of providincl the electrolytes is a
22 concentrate o~ an aqueous solution of the chromium (III) thio-
23 cyanate comple~ having the 1:6 chromium to thiocyanate ratio, ~_
which can be diluted to give the desired concentration of the
25 various ions.
26 While the invention has been particularly shown and
27 described with reference to preferred embodiments thereof, it
28 will be understood by those skilled in the art that various ~
29 changes in form and details may be made therein without departing ~2es
30 from the spirit and scope of the invention.
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