Note: Descriptions are shown in the official language in which they were submitted.
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Functional chromium layer with improved corrosion resistance
Field of the Invention
The present invention relates to plating bath compositions and a process for
depositing functional chromium layers by electroplating.
Backs:1round of the Invention
Functional chromium layers deposited by electroplating are used to improve
wear and corrosion resistance of products such as shock absorbers, hydraulic
pistons and the like.
The plating bath compositions used comprise chromic acid, sulfate ions, water
and an alkyl-sulfonic acid or salt thereof.
Alkyl-sulfonic acid catalysts having a molar ratio S : C 1 :
3 are disclosed in
EP 0 196 053 B1. Examples of suitable alkyl-sulfonic acids are methyl-sulfonic
acid, ethyl-sulfonic acid, propyl-sulfonic acid, methane-disulfonic acid and
1,2-
ethane-disulfonic acid. Said alkyl-sulfonic acids improve the cathodic current
efficiency during plating.
The use of alkyl-polysulfonic acids, halogenated alkyl-polysulfonic acids and
corresponding salts such as methane-disulfonic acid for reducing the corrosion
of lead anodes during plating is disclosed in EP 0 452 471 B1.
Aromatic-trisulfionic acids as an additive in plating bath compositions for
depos-
iting functional chromium layers are disclosed in US 2,195,409. The chromium
layers obtained from such plating bath compositions are bright and uniform.
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A plating bath composition for depositing a functional chromium layer with an
improved cathodic current efficiency comprising propane-1,2,3-trisulfonic acid
is
disclosed in DE 43 05 732 A1.
Obiective of the present Invention
The objective of the present invention is to provide a plating bath
composition
and a process utilizing said plating bath composition for depositing
functional
chromium layers having an improved corrosion resistance.
Summary of the Invention
This objective is solved with an aqueous electroplating bath for depositing a
functional chromium layer, comprising
(i) a source for chromium(VI) ions,
(ii) a source for sulfate ions and
(iii) methane-trisulfonic acid or a salt thereof.
This objective is further solved with a process for depositing a functional
chro-
mium layer onto a metallic substrate, comprising, in this order, the steps of
(i) providing a metallic substrate,
(ii) contacting said substrate with an aqueous electroplating bath
comprising a source for chromium(VI) ions, a source for sulfate
ions and methane-trisulfonic acid or a salt thereof and
(iii) applying an
external current to said substrate as the cathode and
thereby depositing a functional chromium layer onto said sub-
strate.
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The functional chromium layers deposited from the aqueous plating bath and by
the process according to the present invention have an increased corrosion re-
sistance compared to functional chromium layers deposited from conventional
electroplating bath compositions comprising known alkyl-sulfonic acid.
Detailed Description of the Invention
The aqueous electroplating bath according to the present invention comprises a
source for chromium(VI) ions, sulfate ions, methane-trisulfonic acid, or a
salt
thereof, and optionally a surface active agent.
The source for chromium(VI) ions is preferably a chromium(VI) compound solu-
ble in the plating bath such as Cr03, Na2Cr207 and K2Cr207, most preferably
Cr03. The concentration of chromium(VI) ions in the electroplating bath accord-
ing to the present invention preferably ranges from 80 to 600 g/I, more
prefera-
bly from 100 to 200 g/I.
Sulfate ions present in the electroplating bath are preferably added in form
of
sulfuric acid or a plating bath soluble sulfate salt such as Na2SO4. The
concen-
tration of sulfate ions in the electroplating bath preferably ranges from 1 to
15 g/I, more preferably from 2 to 6 g/I.
The ratio of the concentration in wt.-% of chromic acid to sulfate preferably
ranges from 25 to 200, more preferably from 60 to 150.
The alkyl-sulfonic acid in the electroplating bath is either methane-
trisulfonic
acid (HC(S020H)3) or a mixture of methane-trisulfonic acid and one or more
other alkyl-sulfonic acids. Suitable other alkyl-sulfonic acids in mixtures
with
methane-trisulfonic acid comprise methane-sulfonic acid, methane-disulfonic
acid, ethane-sulfonic acid, 1,2-ethane-disulfonic acid, propyl sulfonic acid,
1,2-
propane-disulfonic acid, 1,3-propane-disulfonic acid and 1,2,3-propane-
trisulfonic acid. Corresponding salts such as sodium, potassium and ammonium
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salts of the aforementioned sulfonic acids can also be employed instead of or
as a mixture with the free alkyl-sulfonic acids.
A precursor of methane-trisulfonic acid or a salt thereof which is oxidized in
the
electroplating bath according to the present invention to methane-trisulfonic
acid
or a salt thereof may be used as part or sole source of methane-trisulfonic
acid
or a salt thereof.
The concentration of methane-trisulfonic acid or a salt thereof in the plating
bath
according to the present invention preferably ranges from 2 to 80 mmo1/1, more
preferably from 4 to 60 mmo1/1.
The overall concentration of methane-trisulfonic acid and other alkyl-sulfonic
acids or salts of the aforementioned in case a mixture of methane-trisulfonic
acid with other alkyl-sulfonic acids is employed preferably ranges from 4 to
160 mmo1/1, more preferably from 12 to 120 mmo1/1.
A high number of micro-cracks inside the functional chromium layer deposited
is
desired because thereby a high corrosion resistance and desirable mechanical
properties such as a reduced internal stress are achieved. Micro-cracks in con-
trast to macro-cracks within a functional chromium layer do not extend to the
surface of the underlying substrate and thus do not result in corrosion of the
underlying substrate material, which usually is steel.
Methane-trisulfonic acid or a salt thereof or constituent of a mixture with
other
alkyl-sulfonic acid(s) enables a high number of desired micro-cracks in the
range of 200 to 1000, more preferably 450 to 750 micro-cracks per cm of the
functional chromium layer surface as determined after etching in an aqueous
solution containing sodium hydroxide and K3[Fe(CN)6] with an optical micro-
scope. The number of micro-cracks along lines is counted and the number of
micro-cracks per cm is then calculated with the formula
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Micro-cracks per cm = (average number of cracks per line) : (length if line in
cm)
The number of micro-cracks and the corrosion resistance is increased with me-
thane-trisulfonic acid or a salt thereof as the catalyst compared with methane-
disulfonic acid sodium salt or propane-1,2,3-trisulfonic acid sodium salt as
the
5 sole alkyl-sulfonic acid. This is shown in Examples 1 to 3.
Furthermore, an increased number of desired micro-cracks is also obtained at
higher current densities (Example 3) whereas the number of micro-cracks is
decreasing at higher current densities in case of known alkyl-sulfonic acids
such
as methane-disulfonic acid (Example 1). Higher current density values during
plating are desired because the plating speed is increased thereby.
The electroplating bath according to the present invention optionally further
comprises a surface active agent which reduces formation of undesired foam on
top of the plating liquid. The surface active additive is selected from the
group
comprising perfluorinated sulfonate tenisdes, perfluorinated phosphate ten-
sides, perfluorinated phosphonate tensides, partially fluorinated sulfonate
ten-
sides, partially fluorinated phosphate tensides, partially fluorinated
phosphonate
tensides and mixtures thereof.
The concentration of the optional surface active agent preferably ranges from
0.05 to 4 g/I, more preferably from 0.1 to 2.5 g/I.
The current density applied during plating preferably ranges from 10 to
250 A/dm2, more preferably from 40 to 200 A/dm2. The substrate to be plated
with a functional chromium layer serves as the cathode during electroplating.
Cathodic current efficiency is the percentage of current, which is actually
used
for the deposition of the metal (chromium) at the cathode during
electroplating
of the functional chromium layer.
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The preferred current efficiency of the process according to the present inven-
tion is 22 % at a current density of 50 A/dm2.
The temperature of the electroplating bath according to the present invention
is
held during plating preferably in a range of 10 to 80 C, more preferably in a
range of 45 to 70 C and most preferably from 50 to 60 C.
Inert anodes are preferably applied in the process according to the present in-
vention.
Suitable inert anodes are for example made of titanium or a titanium alloy
coat-
ed with one or more platinum group metal, alloys thereof and/or oxides
thereof.
The coating preferably consists of platinum metal, iridium oxide or a mixture
thereof. Such inert anodes enable higher current densities during
electroplating
and thereby a higher plating rate compared to lead anodes.
The plating bath according to the present invention can also be operated with
conventional lead anodes.
Chromium(III) ions are formed when using such inert anodes. Methane-
trisulfonic acid and/or a salt thereof as the alkyl-sulfonic acid in a
chromium(VI)
ion based functional chromium electroplating bath is very sensitive to chromi-
um(III) ions.
In a preferred embodiment of the present invention, cations of a further metal
such as silver ions, lead ions and mixtures thereof are added to the
electroplat-
ing bath. Thereby, the negative impact of chromium(III) ions can be minimized.
The concentration of ions of a further metal preferably ranges from 0.005 to
5 g/I, more preferably from 0.01 to 3 g/I.
The present invention provides a functional chromium electroplating bath and a
process for depositing a functional chromium layer onto a substrate having an
increased corrosion resistance which is also obtained at high current
densities.
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Examples
The invention will now be illustrated by reference to the following non-
limiting
examples.
The number of micro-cracks was determined with an optical microscope after
etching the surface of the chromium layer in an aqueous solution containing
sodium hydroxide and K3[Fe(CN)6]. The number of micro-cracks along several
lines having the same length is determined from which the average number of
micro-cracks is calculated and then divided by the line length given in cm to
provide the "average number of micro-cracks" in cracks/cm.
The corrosion resistance of the functional chromium layers was determined ac-
cording ISO 9227 NSS (neutral salt spray test).
An aqueous electroplating bath stock solution containing 250 g/I Cr03, 3.2 g/I
sulfate ions and 2 m1/I of a surface active agent was used throughout examples
1 to 3. Different amounts of alkyl-sulfonic acids were added to this stock
solu-
tion prior to depositing the functional chromium layers.
Example 1 (comparative)
The alkyl-sulfonic acid was methane-disulfonic acid disodium salt added to the
stock solution in a concentration of 2 to 12 g/I (7.6 to 45.4 mmo1/1). This
alkyl-
sulfonic acid is disclosed in EP 0 452 471 B1.
Table 1 summarizes the average number of micro-cracks determined at differ-
ent concentrations of methane-disulfonic acid disodium salt as the sole alkyl-
sulfonic acid (plating bath temperature: 58 C, current density: 50 A/dm2).
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Catalyst concentration Current efficiency (%) Average number of mi-
(mmo1/1) cro-cracks (crack/cm)
7.6 24.6 600
15.2 24.3 820
22.7 22.2 820
30.3 19.7 660
37.9 20.3 530
45.4 18.9 440
A high number of desired micro-cracks is only obtained when a narrow concen-
tration range of the catalyst methane-disulfonic acid disodium salt is used in
the
stock solution.
Table 2 summarizes the average number of micro-cracks determined at differ-
ent current densities for an electroplating bath composition with 18.9 mmo1/1
(5 g/1) methane-disulfonic acid disodium salt as the sole alkyl-sulfonic acid.
Current density Current efficiency Average number of micro-cracks
(A/d m2) (%) (crack/cm)
30 21.7 730
40 23.7 660
50 24.4 630
60 25.3 620
70 25.9 580
The number of desired micro-cracks is declining with increased current
density.
Formation of undesired red rust was determined after 192 h of neutral salt
spray
test according to ISO 9227 NSS (>0.1 % of the surface area covered with red
rust after 192 h).
Example 2 (comparative)
The alkyl-sulfonic acid was propane-1,2,3-trisulfonic acid trisodium salt
added to
the stock solution in a concentration of 14.3 mmo1/1 (5 g/l). This alkyl-
disulfonic
acid is disclosed in DE 43 05 732 A1.
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The current efficiency at 50 A/dm2 and a plating bath temperature of 55 C is
17.4 % and the number of micro-cracks in the chromium layer deposited under
these conditions is 160 cracks/cm.
Formation of undesired red rust was already determined after 24 h of neutral
salt spray test according to ISO 9227 NSS (>0.1 % of the surface area covered
with red rust after 24 h).
Example 3 (invention)
The alkyl-sulfonic acid was methane-trisulfonic acid trisodium salt added to
the
stock solution in concentrations of 6.2 to 37.2 mmo1/1 (2 to 12 g/l).
Table 3 summarizes the average number of micro-cracks determined at differ-
ent concentrations of methane-trisulfonic acid trisodium salt as the sole
alkyl-
sulfonic acid (plating bath temperature: 58 C, current density: 50 A/dm2).
Catalyst concentrationAverage number of mi-
Current efficiency (%)
(mmo1/1) cro-cracks (crack/cm)
6.2 23.6 270
12.4 24.5 670
18.6 24.3 950
24.8 23.7 1020
31.0 23.8 920
37.2 22.9 1020
Table 4 summarizes the average number of micro-cracks determined at differ-
ent current densities for an electroplating bath composition with 24.8 mmo1/1
(8 g/l) methane-trisulfonic acid trisodium salt as the sole alkyl-sulfonic
acid.
Current density Current efficiency Average number of micro-cracks
(A/dm2) (%) (crack/cm)
30 19.4 810
40 23.2 780
50 24.8 860
60 26.0 850
70 27.2 840
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A high number of desired micro-cracks was obtained in the whole current densi-
ty range applied.
Formation of undesired red rust was not determined until 552 h of neutral salt
spray test according to ISO 9227 NSS.
