Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
-~; 10~ 35a
The present invention relates to trivalent chromium
plating baths and in particular to plating baths containing
weak complexing agents such as hypopho---phite and glycine.
It is known to electroplate chromium from aqueous
baths containing trivalent chromium ions and an organic
buffer, preferably an aprotic buffer such as dimethylformamide
(DMF). Such techniques are described in British Patent
Specification No. 1,114,913. In electroplating from
electrolytes buffered with e.g. DMF, it is advantageous
to ensuxe, as far possible, that the electrolyte has a
single anion, usually sulphate or chloride. It is preferred
not to use mixed anion electrolytes (see in this regard
British Specification Nos. 1, 194,913 and 1, 333,714). More
recently a variety of trivalent chromium electrolytes have
been developed which use weak complexing agents instead of
or, optionally but not usually preferably, with an organic -
buffer. Typical weak complexing agents are hypophosphite,
usually as the sodium salt, glycine and mixtures of these. -
Such systems are described in U.S. Patent No. 3, 917,517 and
British Patent Application No. 38320/74, published as U.K.
Specification No. 1, 488,381. The term "weak complexing ; -
agent for trivalent chromium ions" is used and defined herein
as meaning a complexing agent for trivalent chromium ions
which does not bind trivalent chromium so strongly as to ;~
prevent electrodeposition of chromium from aqueous trivalent
chromium solutions containing it.
One advantage of electrolytes using weak complexing
agents is that they are more tolerant towards mixed anions
than electrolytes using organic buffers such as DMY. However,
electrolytes using weak complexing agents based on sulphate
'
. ~:
- '. ~L06~3~j39 : ~ ~
as the anion, have a disadvantage in that if the electrolyte is
cooled it deteriorates and bath constituents can crystallize
out.
Once they have crystallised out these materials are
difficult to get back into solution and it may be necessary
to heat the electrolyte well above its normal operating
temperature to complete re-dissolution. In the laboratory
this is a minor inconvenience, but in large scale plant
operation such cooling, which can easily occur when the
electrolyte is not in use such as overnight or over a weekend,
particularly when the weather is cool, can precipitate
sufficient material that the delay and expenditure of energy
- ~
; necessary to re-dissolve the materials may make such electro-
lytes uneconomic to operate despite their other advantages.
It is an object of the present invention to improve
the low temperature stability of sulphate based trivalent
chromium electrolytes.
Accordingly, the present invention principally provides
a trivalent chromium plating solution comprising water,
trivalent chromium ions, sulphate ions, a weak complexing
agent for said trivalent chromium ions, and halogen ions
selected from the group consisting of fluoride~ions, chloride
ions and mixtures thereof.
~he present invention in one aspect provides an
aqueous trivalent chromium plating bath electrolyte based on
sulphate as the anion which comprises trivalent chromium ions
in a concentration of at least 0.2 lar, sulphate ions in ;~
a concentration of from 1 to 6 molar, a weak complexing agent
for trivalent chromium ions in a concentration of at least
0.1 molar, and chloride ions in a concentration of from 0.1
:
~.068639
to 5.0 molar;the molar ratio of chloride ions to sulphate ions
being from 1:60 to 5:1 but preferably being in the range from
1:7 to 5:1.
In another aspect the invention provides a method for
electrodepositing chromium on a substrate which comprises
immersing said substrate as the cathode in an electrolyte
solution comprising water, trivalent chromium ions in a
concentration of at least 0.2M, sulphate ions in a concent-
ration of at least 0.2M, sulphate ions in a concentration
of from 1 to 6M, a weak complexing agent in a concentration
of at least O.lM, and chloride ions in a concentration of
from O.lM to 5.OM, and passing an electric current through
said solution thereby to deposit said trivalent chromium
ions on said substrate.
In a preerred embodiment of the plating solution and
of the method of using tKe solution there are present both
fluoride and chloride ions.
The concentration of trivalent chromium ions above
0.2 molar is typical of trivalent chromium baths but is
usually limited to a maximum of 2 molar by the solubility of
chromic sulphate. For decorative plating the optimum ~ -
concentration is about 1 molar. ;~
'~
:~ ,
.~ .
.~,~, .
lo68639
~he concentration of sulphate ions is typical for tri-
valent chromium baths and the preferred range is from 2 to 4 molar.
The concentration of chlorlde ions and the molar
ratio of chloride ions to sulphate ions are not critical
~o tlle invention. However, less than o.i molar chloride
or a lower molar ratio than 1:60 produces no slgnificant
efect. ~ concentration of chloride higher than l.5 molar
. . ' . , '' ~ .
or a molar ratio higher than i:2.5 produces no further ~
;.;
increase in stability. The use of higher concentrations `~
of chloride does however lead to significant improve-
ments in solution conductivity providing a distinct
commercial advantage. Concentrations of 2 to 4 molar
chloride are therefore preferred.
The concentration and precise nature of the
weak complexing agent are not critical to the useful
, :
effect of chloride ions in sulphate based electrolytes. -~
Hypophosphite and/or glyclne are the preferred weak
complexing agents and will typi~alIy be used at a con- .~
centration of from 0.1 to 6 mola.r and preferably from ~ ~;
0.25 to 3 molar, the uppex limit being largely a -
function of solubility.
The eiectrolyte may option~'ly include a variety
o other materials such as are typically used in trivalent
chromium electrolytes. Thus boric acid can be included as
.
a current efficiency enhancing agent at concentrations up
to saturation (about 1 molar) and typically at 20-60 gl~l.
Ammonium ions can be added to increase the conductivity of ,
the bath typically at a concentration, when used, of from
1 to 7 molar and preferably greater than 5 molar for
optimum effect. Conveniently the ammonium ion or a part
,
~'
- 4 -
, . . _~ .. ..... .. ~
~ 06~3~39
~f it can be added as ammonium chloride, thus also acting
as the source of chloride ions. Larger concentrations of
ammonium ions will usually be added as sulphate or as
ammonia and free acid. The electrolyte may also include
one or more surface active agents such as cetyl trimethyl-
ammonium bromide in concentrations up to 50 ppm. Such
additives are normal in trivalent chromi~lm electrolytes. l`
The baths o~ the in~ention typically operate
at temperatures from amblent temperature to 50C and
preferably from 25 to 35C.
Th~ additions of chloride ions to tri~alent
. .
ch~omium sulphate baths has the effect of increasing
the low temperature solubility of the less soluble ;
bath constituents. Indeed, addition of a suitable
quantity of chloxide ion to a bath which already has
a precipitate in it can greatly ease re-dissoluLion ` -
of the precipitated constituents to more xeadily
. : .
establish the complexed state most suitable for plating.
If a sulphate bath containing chloride develops a pre- ~
cipitate on cooling then simply re-warming to its normal ~ ;
~` operating tem~erature can suffice to re-dissolve the
~ precipitated material ir a rorm s~itabie for eiectro- - -
`' reduction.
~; A further advantage of.adding or including -~
chloride ions in trivalent chromium sulphate baths is -
that the overall current and practical plating range
,
efficiency of electrodeposition is improved signifi-
;: .:
cantly. With optimum ele~trolytes the current
- , . . . .
efficiency of mixed chloride/sulphate trivalent chromium
electrolytes can be as high as 170~ of that of a similar
bath omitting the chloride. The plating range of an
optimised chloridejsulphate bath is typically from 30 to
104 Am~2. However, in commercial plant operation the
range is somewhat narrower and is typica]ly 70 to 104 ~m 2,
.. . .
.
~6~3639 ~ ~
The present lnvention in another aspect provides an
aqeuous trivalent chromium plating bath electrolyte based
on sulphate as anion which comprises:
.
Trivalent chromium ions in a concentration of at
least 0.2 molar, sulphate ions in a concentration of at
least 0.3 molar, a weak cGmplexing agent for txivalent
chromium ions in a concentration of at least 0.1 molar,
and luoride ions in a concentration of at least 0.025
molar. ~
- ~he concentrati-on of trivalènt chromium ions above
.: , , ~ ;
0.2 molar is typical of trivalent chromium baths but is
nsually limited to a maximum of 2 molar by the solubility
o~ chromic sulphate. For decorative plating the optimum
concentration is about 1 molar.
;. :":
~ The minimum concentration of sulphate ions given
i ,
is a practical minimum figure corresponding to the minimum
~ level of trivalent chromium. The pre~erred range is from
; 1 to G molar optimally from 2 to 4 molar.,
.~ , . - - - .~ .
The particular concentration and precise natu~e
of the weak complexing agent are not critical to the useful ;
effect of fluoride ions in sulphate ~ased electrolytes.
~ Hypophosphitè and/or glycine are the preferred weak com- ' ;
; ~ plexing agents and will typicalIy be used at a concentra- ~ -
tion of from 0~1 to 6 molar pre~erably 0.25 to 3 molar,
. , ,
the upper limit being largely a function of solubility.
Glycine is additionally advantageous, because the chromium
deposit usually has a lighter color.
The concentration of fluori~e must be at least
0.025 molar in order to obtain any appreciable effect.
The maximum concentration is limited by solubility and
diminishing returns to 1.5 molar. Preferably the concen-
.: 6
, - , . :.. ,-: . . . . .
; , . . ~ . "
. . . . . ..
~(:)6~363~ `
. ~:
~ration is up to 1.25 molar, optLmally from 0.1 to 0.7
molar. ConvenientlY the fluoride can be added as sodium
fluoride and the minimum level corresponds to about l gl~
of NaF and the optimum from about 5 to 25 gl~l. Other
~}uoride containing salts and materials can be used as
~luoride ion sources,
In order to ensure a relatively high electrolyte
conductivity it is preferred to include ammonium ion in
the electrolyte. When used the concentration of ammonium ;
~inn will typically be ~rom 1 to 7 molar and preferably
greater than 5 M for optimum effect. The ammonium ion can
conveniently be added as the sulphate (but see below
concerning mixed sulphate/chloride systems). The electro-
lyte may optionally include a variety of other materials
such as are typically used in trivalent chromium electrolytes.
~.,
For example, boric acid can bé included~as a current effi- ;-
.
ciency enhancing agent at concentrations up to saturation
(about 1 molar) typicaIly at 20 to 60 gl~l. The electrolyte ~-
may also include one or more surface active agents such as
cetyltrimethylammonium bromide in concentrations up to 50
:. .. .
ppm. Such additives are normal in trivalent chromium
electrolytes.
The effect of adding fluoride to chromic sulphate
baths is to impair the production of particles of difficultly
electroreducible chromium complexes which tend to form at low
tsmperature. Fluoride ion has the ability to break up those , -- `
moieties enabling the optimum equilibrium to be established ^ ~
more readily. The baths of the invention typically operate ~-
~t tsmperatures from ambient temperature to 50~C and pre-
ferabiy from 25 to 35~iC. Restarting ~lating then only -
requires warming to operating temperature - extensive ~-
' '.. `,: " ' '.:
; - 7 -
-- ~068639
warming a~ elevated temperatures being unnecessary. In
similar baths not containing fluoride it would probably be
necessary to heat the bath at a relatively high temperature,
typically 50C or higher, for a prolonged period to achieve
dissolution of the precipitate.
A further effect of fluoride in that it acts as a
platin~ exhaltant. The average plating eficiency o a
~luoride containing, sulphate based trivalent chromium `
plating bath can be double that of a similar bath without ;-
the fluoride. A further advantage is that the colour of
the chromium deposited, which in trivalént chromium systems `
tends to be rather dar~,is lighter when deposited in the
presence o~ fluoride and more nearly matches the color of
plate produced from hexavalent chromium plating baths.
.. .... ~ - - .
As is indicated above fluoride ion can be included
in mixed sulphate/chloride baths. The inclusion of chloride
in sulphate trivalent chromium baths is itself, advantageous,
but the effect of chloride on its own is less than the effect
. ~ . . . .
of fluoride. The concentration of chloride ion, when present, ;~
will usuall~ be in th range 0.1 to 5 molar preferably 0.5 ~-
to 5 molar. The molar ratio of chloride to sulphate in
such baths should be in the range 1:60 to 5:1. The chloride
~ ion can conveniently be added as ammonium chloride.
.' . :
The plating range of an optimised fluoride containing
sulphate based bath is typically from 30 to 10~ Am~2. However
- in commercial plant operations the range is somewhat narrower
` and is typically 50 to 104 Am . Thus, fluoride acts to
reduce the loss of efficiency of: sulphate based baths at low
.,.
current densities. Because of the increased efficiency given
to trivalent chromium plating baths the average rate of plating
.-
. ~ .
; 8 ~
... . . .
~?6~363~
.~ ' ' - .
.,.
from baths of the invention can be as high as 0.15
~m min 1. Higher rates of deposition can be achieved
by raising the temperature or reducing the pH.
In addition to the electrolyte described above the
invention also includes a method of elec~roplating comprising
providing an anode and a cathode in an electrolyte of the
invention and passing an electric current through the electro-
lyte whereby chromium is electrodeposited on the cathode.
The method of the invention can be carried out at a
pH of from 0.5 to 7. However, in order to maintain a wide
plating range-the preferred pH is from 1.5 to 4. The make-
up pH of an electrolyte comprising the necessary components
as a solution of the stoichiometrically neutral salts in
water is usually within this range. However the pH can
be readily adjusted by adding suitable small quantities of
~ acid or alkali as necessary.
-~ :
The electrolyte may be made up as in British Specification
No. 1, 488,381, by a PH changing technique which can ensure
formation of the desired complex between the trivalent chromium
and the weak complexing agent.
!
The anodes used in the process of the invention are not
critical to the process of the invention. Carbon anodes will
.~ i ,
~ in general be used because of their cheapness and convenience.
;'- The following Examples illustrate the invention:
, EXAMPLE 1
.; . . .
' An aqueous electrolyte having the following composition
was made up:
1 M trivalent chromlum as sulphate
4 M NH4 as sulphate
0.75 M Boric Acid
1 M Sodium hypophosphite
`'~
_ g_
68t;3.9
.
Using a portion of the electrolyte a Hull Cell
panel was plated under.the following conditions~
pH .- 3.1 .:
: Temperature 25C
S Hull Cell current lOA . .. . . .:
~ull Cell voltage 14V
The rësults of plating for 60 seconds were: ..
Current Density Am-2 500 1000 2000 3500 5000 -~ :
~Thick~es.s.~m 0......... 10 Ø. 11 0.1.0 0.... 11 0... 12 ~ ~
. . . , , ,,`: .
.: 10 A further portion of the electrolyte was cooled :
.. : . .. , . , ": ..
` to 10C for 12 hours and re-warmed to 25DC. A Hull Cell
:. . : .
-; panel was plated under the same conditions as used above. . .
The results of plating for 60 seconds were~
.` Current.Density Am-2 500 1000 2000 3500 5000 :
lS . Thickness ~m 0.09 0.05 0.05 0.07 0.10 `~
.,
`~ ~s can be seen a considerable loss in plating -.
.~. : Speed occurred.
.. : , -
EXAMPLE 2
. . .
An aqueous electrolyte having the following
20compos:ition~was made up: , ~ :
~ 1 M trivalent chromiu~ as suiphate ~.
;~ 3 M NH4~ as sulphate ::
1 M NH4~ as chloride
~;~ . 0.75 M ~oric acid
1 ~ Sodium hypophosphite
. ~
' ~
.~ .
. : ' '; ~
.
.~ .
-- 10 --
. ' .
~ . , : :, . . .. . j.... .
~068~;~9
. The electrolyte was divided into two portions : ~
and Hull Cell panels were plated-a~ set out in Example 1. :
~::
The results were: . , . .. ::;
' , .,, , ' . ' . , '`'~' '~ ',' .
Without cooling: :~
Current Density Am ~ 500 1000 2000 3500 5000
.. .
Thickness ~m 0.15 0.16 0.16 0.16 0.17 .
:,
After cooling and re-warming: ~
Cuxrent Density Am~2 500 1000 2000 3500 5000 ::
. Thickness ~m 0.13 0.15 0.15- 0;15 0.15 ~: :
10 . ThP loss in plating speed after cooling and re~warming is . ~ .
. vixtually negligible. . . :.
.. ,. ., - ',' '- ~.
EX~lPLE 3
Ex~.ples l and 2 were repeated ~ut using glycine:~
, ,~... .
.. as the weak complexing agent. The observed efect of .
. 15 including chloride ion in the electrolyte was similar to
.
that observed in Example 2.
~, . . .... -~ ...
` . EXA~LE 4 .
An aqueous electrolyte having the following compo~
sition was made up: . . ~. .................. .. ~. : .
... . .
1 M trivalent chromium as sulphate .~: :
3 M NH4f as sulphate
. 3 M NH4~ as chloride
0.75 M Boric acid
: . .. . : ..
. 1 M sodium hypophosphite ~.`
. . .
: 25 The performance of this electrolyte ~as identical to : -
th~t of Example 2 except the ~lull Cell was in this case 11 volts
, '"' . ,
-' ' , ' '. :''.
,
!. ~ ,
- 11 - ,'. -.:,,
'' . '' '''
1061~63~
as compared to 13 volts for Example 2 indicating a ~`
superior conductivity in presence o~ higher chloride
ion and ammonium ion contents.
. ':.' .
'' ' ~
EXAMPLE
An aqueous electrolyte was made up having the
following composition~
1 molar trivalent chromium (as sulphate)
4 molar NH4~ (as sulphate)
l molar boric acid ~:
1 molar sodium hypophosphite
A Hull Cell panel was plated for 60 seconds under
,, thè following conditions: -
p~ .3.0 Hull Cell current lOA
Temperature 25C Hull Cell voltage 16V `'
.
The results were as follows:
/ Curxent Density (Am~2) 300 ;600 1~00 3000 5000.
`A . Thickness (~m~ 0.06 0.08 0.07 0.08 0.06
.:20 ~- .
~ EXAMPLE 6
.~ ~ Example 5 was repeated exc~pt that 6 M NH4~ ion
;~ was provided as a mixture of the sulphate t3 Mj and the
. . . ~ .
chloride ~3 M). The Hull Cell panel results were: .
Current Density (Am-2) 300 500 1000 2500 5000
; .. :.
ThicXness (~m~ 0.07 0.08 0.09 0.10 0.10 ~
- '.~
,. . ~'
- ~.
~ 12 --
Q76~63g ~ ; ~
-. . ~MPLE 7 ~
Example 5 was répeated except that 20 g~
(ca 0.5 M) sodium fluoride was included in the electrolyte. :`
The Hull Cell panel results were~
Current Density (Am 2) 300 500 1000 2500 5000 ~ .
Thickness (~m) 0.710 0.14 0.14.. 0.13 0.15
. There was a marked exhaltation of plating rate in .
.the presence of the fluoride.
:., .. . ' ~. '
ExAMpLE 8
Example 6 was repeated except that 20 gl~l NaF . ~
`; was included in the electrolyte. The Hull Cell panel
results were: , :
Current Density (Am-2) 300 600 1000 3000 5000 .
. :
Thickness ~m) 0.12 0.14 0.15 0.14 0.14
: . Again there was a significant improvement in :
plating rate. .................................................... ~. :;.
EXAMPLE 9
~n a~ueous electrolyte was made up having the
ollo~ing composition~
I Molar trivalent chromium (as sulphate) ..
4 ~olar NH4~ (as sulphate) `; ~ :
~.~ 1 Molax boric acid .-.
''. 7 1 Molar ~lycine
~,, ;
;~ A Elull Cell panel was plated for 60 seconds ~. ~
under the following condltion: : ~ .
p~ 2.8 ~ull Cell current 10A
:. . Temp. 25C Hull Cell voltage 16V
` 30 Th.e results were as follows~
:; Current Density (Am 2) 300 600 1000 2500 4500 ;
. Thickness (~m) 0.02 0.08 0.10 0.14 0016
- 13 - :.
, -, , .: , .,: . .. .
:` 10{;8639 ~ .
. EXAMPLE l~
Example 9 was repeated except that the 4 ~ NH
ion was provided as a mixture of the sulphate ~3 M) and
the chloride (1 M) and that 20 gl 1 NaF was included. The
results were as follows~
Çurrent Density (Am 2) 300 500 1000 2500 5000
.Thickness (~m) 0.05 0.10 0.11 0.15 0.16
EXAMPLE ll
`lO The following aqueous electrolyte was made up: :-
: l Molar trivalent chromium (as sulphate)
.
3 Molar NH~+ ~as sulphate) --
l Molar NH4~ (as chloride)
l ~olar ~oric acid
0O5 Molar glycine
0.6 M sodium hypophosphite i;
20 g/l sodium fluoride !
A Hull Cell panel was plated for 60 seconds
under the following c~nditions: l, `
. p~ 2.75 . Hull Cell current lOA - :
Temp. 26C Hull Cell voltage lOV.
: ~he results were as follo:rs:
; ' '' ' .:
.
.
;': ' .
- 14 -
:,
~; ~ 106~3639
Current Density (Am 2) 300 sno lO9O 2500 5000 ~ -
Thickness (~m) 0.07 0.12 0.12 0.16 0~15 -
.. . . . . .
EX~AMPLE 12
.. ... . .
Chromium was plated from a solution comprised `~
of~
Chrome tan 260 g/l
.~ ~mmonium sulphate 180 ~
Ammonium chloride 15~ g/l
Sodium flua~i~e 15 g~
Boric acid 40 g/l
Sodium hypophosphite 100 g/l
This solution showed good Hull Cell characteristics
~, ~ and was ultimately shown to work well on the gallon and sixty
gallon sc~le~. The color of the chromium dep~sit~was slightly
darker than that achieved~with conventional hexavalent chromium ;~
solutions but gave the impression of increased color depth and ~ ;
was~consid~ered to be attractlve.
The throwing power was significàntly improved and ~`
20 ~ high current density burning was substantially eliminated ~ -
since the plating~rate was more or less constant regardless
of the c~rrent density applied.
: ' ;:: ` i,' . .. . .
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