Note: Descriptions are shown in the official language in which they were submitted.
~135~
This invention xel~tes to a cyanide-free bath for ~ ~
the electrodeposition o~ gold and to the electrodeposition of ` ~-
gold in such baths.
Alkaline cyanidic baths for electrogilding are known.
They are distinguishèd by excellent stability and robustness,
and are frequently used. However, the disadvantage of such baths
lies in the extraordinary toxicity of the cyanides, as a result ~;~
of which they pose a health hazard to those working with them
and the disposal of their waste liquors gives rise to technical
problems.
Neutral gold electrolytes containing an aurocyanide,
trivalent arsenic and, as sulphur donor, an alkali metal thio~
sulphate, are also known (German Patent Publication No. 20 10 725).
However, such electrolytes also contain cyanide and have the
further disadvantage of being neither ~loss-forming nor gloss-
maintaining, and having no levelling effect.
Finally, cyanide-free gold baths containing gold in the
form of sulphite and gloss-increasing additives have been
proposed (German Patent Publication No. 16 21 180)~ Howe~er,
such gold sulphito-complexes are not sufficiently stable, and ~ ~
when the solution stands for a long time elementary gold is ~ -
formed, even with a very large excess of free sulphite ions, with
the result that the solution becomes unusable.
The present Lnvention provides a process for the
electrodeposition of gold, wherein electric current is passed ~;
through an electrodeposition bath free from cyanide ions and -`;
containing gold in the form of a thiosulphato-complex.
The present invention also provides a bath for the
electrodeposition of gold, wherein the bath is free from cyanide
ions and is an~aqueous solution of at least one conductiye salt
- and gold in the form of a thiosulphato-complex.
This bath is generally stable and substantially
-- 1 --
c.. ~ ,,, - - - ,
~3~
overcomes the disadvantages of the known gold baths. Thus, it ~ ;
is generally possible by a relatively non-toxic electrodeposition
process to obtain gold deposits having good technological and
decorative properties, and at the same time a high degree of
purity.
The gold thiosulphato-complexes are complexes of vari-
able composition containing gold as the central atom and at least
one thiosulphate ligand. The approximate composition of the comp-
lexes may correspond to the formula M3 12[Aul 2(S2O3)2 7~, in
which M represents one equivalent o~ a metal, for example a sodium
atom. The gold thiosulphato-complexes are known and may be made `;~
by methods in themselves known.
Sodium dithiosulphato-aurate (I) (Na3CAU(s23)2]~2H2o)
may be made, for example, by the reduction of sodium tetrachloro-
aurate (III) in neutral aqueous solution with thiosulphate, and
precipitation of the resulting complex with alcohol.
In a similar manner, the compound sodium heptathio-
sulphato-diaurate (I) (Nal2~Au2(S2O3)7]) may be made and isolated
~rom aqueous solution.
The gold thiosulphato-complexes to be used in accordance
with the invention are generally readily soluble in water. The
concentration of gold in the bath of the invention is advantageously
from 0~1 g ofgold per litre up to saturation, and preferably `
. ;: .,
from 3 to 30 g of gold per litre.
The gold thiosulphato-complexes may be added preformed
to the bath or may be produced in the bath itself.
The present invention further provides a mixture of
compounds suitable for making up a bath free from cyanide ions
for the electrodeposition of gold which comprises a gold thiosul-
phato-complex or its precursors and one or more ingredients suit-
able for incorporation into electrodeposition baths.
Advantageously, the electrodeposition bath or mixture ~ ~-
,''`'' '~; '
- 2 -
for its preparation may contain an excess of thiosulphate, for
` example, in a ratio by weight of metal:thiosulphate o~ up to
substan-tially 1:20. The concentration of thiosulphate in the
bath solution is advantageously at least 1 g/l and preferably
20 to 500 g/l.
The thiosulphate is an ammonium and/or alkali metal
salts, preferably a sodium or potassium salts, of thiosulphuric
acid or their adducts with basic compounds such as, for example,
amines or polyamines. - ~
When the mixture of compounds for making up the bath ~ `
contains excess thiosulphate, the mixture may include cyanide-
containing salts, provided the thiosulphate is sufficient to ~ ;
convert the cyanide into the less toxic thiocyanate in the bath.
Since the anodic solubility of gold in thiosulphate
solutions is low, insoluble anodes, for example, platinized
titanium, should generally be used and, if desired, reducing
agents, for example, nitrites, oxalates or sulphites, preferably
the alkali metal salts, for example, the sodium or potassium
salts, may be added to the bath.
The bath also contains one or more additi~es commonly
used in electrodeposition baths, including the conductive salt,
e.g., ammonium or alkali metal salts of inorganic or weak organic
acids such as, for example, sulphuric acid, sulphurous acid,
carbonic acid, boric acid, sulphamic acid, acetic acid and citric ~ ;~acid. The bath may aiso contain substances that regulate the pH,
, advantageously the usual organic and/or inorgainc buffer mixtures
such as, for example, disodium phosphate, alkali metal carbonate, , ~`
alkali metal borate r alkali metal acetate or citrate, alkali metal
metabisulphite or a mixture of boric acid and ethylene glycol.
The pH of the bath is in the range of from 4 to 13,
preferably 5 to 11. It is advantageous to work at a temperature
in the range of from 10 to 80C, preferably 20 to 55C, and at a
- 3 -
; "
7~ :
current density of from 0.1 to 2 amper~s per dm . ~ ;~
The bath of the invention is used in a known manner.
The gold coatings deposited from the bath of the inven-
tion are generally highly glossy and show outstanding ductility "~
in thick coatings. Moreover, the bath has a le~elling ef~ect
even from 3 ~m, and is suitable for the deposition of both thin
and thick gold layers. By using low current densities, for
example, 0.1 - 0.5 amperes per `dm , high purity gold coatings o~ :
excellent e]ectrical conductivity are obtained. When carrying
out electrolysis at 0.4 - 1.2 ampexes per dm2, gold coatings of
high hardness (120 - 160 Vickers) coupled with good resistance -
to abra~ion are obtained. Finally, the bath of the invention has
the special advantage of being cyanlde-free and thus relatively
non~toxic, whereby a reduction in health risks and a decrease in
expenditure in disposing of waste liquors are achieved.
The following examples illustrate the invention. `~
Example 1 ~-
Bath composition~
Gold in the form of sodium dithiosulphato-aurate (I) ~ `
Na3~Au(S2O3)2].2H2O : 0.04 molar = 7.9 g of gold/litre. , ~;
Sodium thiosulphate
Na2S23 5H2 : 0.5 molar = ll9 g/litre.
Sodium sulphite `
Na2S3 : 0.06 molar = 7.6 g/litre.
Sodium borate
Na4B4O7.1O~I2O : 0.015 molar = 6.4 g/litre.
Operating conditions~
pH (ad~usted with NaOH): 9.0 ~ ~
Temperature: 25C. ~--
Stirring the electrolyte and cathode mo~ement.
Current density: 0.1 to 1.5 A/dm
:,- .
Anode: platinized titanium. `~ ~
,,. :. ~ '
- 4 - ~ ~
~L~53~1~4 :~
A care~ully de~atted w~tch lunette of lead-containing
brass was immersed in the above solution and electrolyzed for
about 20 minutes under the above operating conditi~ns. A 6 to
7 ~m thick glossy gold layer free from pores was deposited on the
lunette.
Example 2
Bath composition~
Gold in the form of sodium heptathiosulphato-aurate (I)
Nal2cAu2(s2O3)7~ 1H2 : 0.04 molar = 15.7 g of gold/lltre.
Potassium thiosulphate ;~
K2S23 : 0.8 molar = 240 g/litre.
Sodium sulphite
Na2S3 : 0.1 molar = 12.6 g/litre.
Boric acid
B(OH)3 : 0.3 molar = 18.6 g/litre.
Ethylene glycol
HO-CH2-CH2-OH : 0.6 molar = 37.2 g/litre.
Operating conditions:
~ pH; 6.8
Temperature: 23C
Anode: platinized titanium.
Current density: 0.3 to 1.5 A/dm2. l ;-
A scratched copper plate having an area of about 1 dm2
was gilded in the above electrolyte under the above operating
conditions for 15 minutes. The fine grooves caused by the scratch- ;
ing were distinctly levelled. ~;
Example 3
Bath composition~
Gold in the form of sodium dithiosulphato-aurate (I)
Na3~AU(s2o3)2~ 2H2o : 0.05 molar = 10 g/litre.
Sodium thiosulphate
Na2S2O3 : 0.5 molar = 119 g/litre. --
~(~53~
Sodium sulphite ;~ `
Na2S3 : 0.05 molar = 6.3 g/litre.
Potassium dihydrogen phosphate
KH2PO4 : 0.02 molar = 2.7 g/litre. ~
Potassium metabisulphite -
K2S25 : 0.05 molar = 11.1 g/litre.
Operating conditions:
6.9
Temperature: 20C.
Anode: carbon. ;
Current density 0.1 to 1.2 A/dm .
A printed circuit having a total area of about 0.7 dm2
was gilded in the solution under the above conditions for 6
minutes. The resulting gold coating about 2.5 ~m thick was pore-
free and could be soldered extremely well.
Example 4
Bath composition:
Gold in the form of sodium dithiosulphato-aurate (I)
Na3~AU(s2o3)2] 2H2 : 0.05 molar~ = 9.85 g~1itre. -
Sodium thiosulphate
Na2S2O3 : 1.0 molar = 156 g/litre.
Sodium sulphite
Na2S3 : 0.1 molar = 12.6 g/litre. ; ~; ;
Potassium metabisulphite ` ~.
K2S2O5 : 0.01 molar = 2.2 g/litre. -~
Boric acid
B(OH)3 : 0.15 molar = 18.6 g/litre. ~.
Ethylene glycol
2 2 : 0.3 molar = 37.2 g/litre.
Operating conditions:
pH: 6.5
Temperature: 22C.
- 6 -
: . , - .. . . . . : ~. .. -, , :
~ 53~74~ :
node: Plantinized ti.tanium.
Diode leads having a kotal surface area of 1 m2 were'
gilded ln a drum at an average current density of 0.1 A/dm2 under ~
the above conditions. The resulting 2 ~m thick coatings were ~ :;
pore-free and could be soldered well even after a long storage
period of a few months.
` - '' ; ' ~''
'''` '~ '
, -:
,~
!
.' ,`,~
.' ~''~
`'" ~ '.
'~
'
' .
~:
, ~`
:;
_ 7 _ ~
, ' ' ' . '., ~' : ' , ' ' - ,
