METHOD OF INCREASING CORROSION RESISTANCE IN GALVANICALLY DEPOSITED PALLADIUM/NICKEL COATINGS

METHODE POUR AUGMENTER LA RESISTANCE A LA CORROSION DE REVETEMENTS GALVANIQUES A BASE DE PALLADIUM- NICKEL

English Abstract

ABSTRACT OF THE DISCLOSURE
In a method of electrodepositing a palladium/nickel alloy coating
upon a substrate wherein the coating is plated onto the substrate by
electrodeposition from a bath containing 5 to 30 g per liter of palladium and 5
to 30 g per liter of nickel and having a palladium-nickel ratio such that the
coating contains 30 to 90% by weight of palladium, the improvement which
comprises providing in the bath during the electrodeposition of the coating at
least one sulfonyl urea compound capable of improving the corrosion resistance
of the coating.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In a method of depositing a palladium/nickel alloy coating upon a
substrate wherein the coating is galvanically deposited from an aqueous bath
containing substantially 5 to 30 g/liter of palladium in the form of a
palladium ammine, substantially 5 to 30 g/liter of nickel as nickel ammine,
palladium and nickel in a ratio substantially equal to that of the coating to
be formed and such that the coating contains 30 to 90% by weight palladium, the
improvement which comprises increasing the corrosion resistance of said coating
by incorporating therein at least one sulfonyl urea compound having the formula
<IMG>
wherein R is an aromatic group and R1 and R2 may be the same or different and
each can be selected from the group which consists of hydrogen, lower alkyl,
lower alkyl substituted with amino, lower alkene or lower alkyne in an amount
of 0.1 to 10 g/liter.
2. The method defined in claim 1 wherein R of said sulfonyl urea
compound is phenyl, naphthyl or a nitrogen-containing heterocycle and R1 and R2
are the same or different and each is hydrogen or lower alkyl.
3. The method defined in claim 2 wherein the sulfonyl urea is
benzene sulfonyl urea.
4. The method defined in claim 3 wherein said sulfonyl urea is
present in an amount of 1 to 10 g/liter in said bath.
5. The method defined in claim 4 wherein said coating is
electrodeposited to a thickness of 1 to 5µ.
6. The method defined in claim 5 wherein said thickness is about
2.5u.

7. The method defined in claim 1 wherein R is a benzyl or phenyl
group and R1 and R2 are each hydrogen or methyl or ethyl.
8. In an electroplating bath for the electrodeposition of a
palladium/nickel coating which comprises an aqueous solution containing 5 to 30
g/liter each of palladium and nickel in the form of respective ammines with the
palladium and nickel in a ratio corresponding to the composition of the coating
deposited and such that the coating contains 30 to 90% by weight palladium, the
improvement which comprises 0.1 to 10 g/liter of a sulfonyl urea in said
solution, said sulfonyl urea having the formula:
<IMG>
wherein R is an aromatic group and R1 and R2 may be the same or different and
each can be selected from the group which consists of hydrogen, lower alkyl,
lower alkyl substituted with amino, lower alkene or lower alkyne in an amount
of 0.1 to 10 g/liter.
9. The bath defined in claim 7 wherein the sulfonyl urea is a
benzene sulfonyl urea.
10. The bath defined in claim 8 wherein said R1 and R2 are the same
or are different and each is selected from the group which consists of hydrogen
and lower alkyl.
11. The bath defined in claim 9 wherein R1 and R2 are each hydrogen.
12. The bath defined in claim 8 wherein R is a benzyl or phenyl
group and R1 and R2 are each hydrogen or methyl or ethyl.

Description

I ~7S38~ '
METHOD OP INCREASING CORROSION RESISTANCE IN
GALV~NICALLY DEPOSITED PALLADIUM/NICKEL COATINGS
.
FIELD OF THE INVENTION
.
Our present invention relates to a method of increasing the corrosion
resistance of galvanically deposited palladium/nickel alloy coatings and to
palladium/nickel electrodeposition baths or plating compositions for use in the
formation of such coatings.
BACKGROUND OF THE INVENTION
Palladium/nickel alloy coatings may be applied to conductive
substrates for decorative and/or technological purposes. For example, such
coatings are useful because they can be employed as a substitute for gold
coatings and have an appearance similar to that of gold and corrosion
resistance which can be significantn
As described in Britlsh patent 1,143,178, such coatings are generally
deposited from a bath which is an aqeuous solution of palladium and nickel.
The palladium content of the bath is usually around 5 to 30 grams per liter and
the nickel content is substantially 5 to 30 grams per liter as well~ the bath
;~ containing sulfonic acid salts among other additives and the palladium/nickel
-ratio in the solution being selected so that the galvanically deposited or
electroplated coating will contain 30 to 90~ by weight palladium.
The resulting coating can be used, as noted, as a replacement for
gold coatings since it has an appearance and decorative effect similar to that
of gold and various properties, e.g. as a contact material for electrical
contacts, which are also similar to those of gold. Thus, such coatings have an
important role in electrotechnology.
The most important characteristic apart from high conductivity that a
material thus suitable for use in electrotechnology must possess is a high
resistance to corrosion of all types. In some cases the earlier
' ,,; - 1 -
.~

I 1 7538~ ~
palladium/nickel coatings did not have sufficient corrosion resistance.
In the electrodeposition of such coatings, moreover, it is known to
add brighteners to the bath. Such brighteners have been aromatic sulfonic
acids and their salts or other derivatives.
Typical of such brighteners are naphthalene sulfonic acid salts and
aromatic sulfonamides such as the sodium salt of naphthalene-1,5-disulfonic
acid, the sodium salt of naphthalene-1,3,-6-trisulfonic acid, saccharine
(o-sulfobenzoic acid imide) and p-toluene-sulfonamide.
Reference may also be had in this connection to U.S. Patents
4,010,084 and 4,102,755.
In the earlier palladium systems, there is occasionally a detrimental
spontaneous salting out of palladium in the form of an insoluble salt,
especially when the sodium salt of naphthalene-1,5-disulfonic acid is used,
with a result that the coating has an unsatisfactory appearance and
technological quality, especially when p-toluenesulfonamide is used as a
brightener as well. Although the same problems do not also arise when the
electrolyte contains the sodium salt of naphthalene-1,3-6-trisulfonic acid
andjor saccharine as brighteners~ sufficient corrosion resistance may be a
problem in these cases.
Corrosion resistance can be conveniently measured, for the purposes
of the present invention, by the immersion of test strips for 60 seconds at
room temperature in a dilute acid solution consisting of equal parts of
concentrated nitric acid and water.
When corrosion resistance is mentioned herein, therefore, such test
conditions are employed.
German patent 1,028,407 describes the use of specific brighteners for
the galvanic deposition of bright nickel coarings, the brighteners being added
to the bath in an amount of 0.1 to 1 gram per liter, preferably 0.5 grams per

I 1 7538~
liter. The brightener compound, which has the same general formula as is given
below, functions exclusively as a brightener, making no noticeable contribution
to improvements in corrosion resistance. The brightening effect appears to be
related to the urea group of this molecule which operates in a manner similar
to earlier urea brighteners, the imine group of the molecule also having a
brightening effect.
As far as we have been able to ascertain, such compounds have never
been proposed as corrosion resistance promoters for palladium/nickel coatings
or analogous deposits.
Thus the present invention is concerned with the improvement of the
corrosion resistance of electroplated palladium/nickel alloy coatings, a method
of depositing such coatings on a substrate which then has improved corrosion
resistance. Also included is a plating bath for producing such a coating.
DESCRIPTION OF T~E INVENTION
We have now found, most surprisingly, that a brightener of the
formula:
. ~R
R-S02-N-C-~
O R2
hitherto used exclusively as a brightener for a deposition of bright nickel
coatings, when added in an amount o~ 0.1 to 10 g/liter of a bath for the
deposition of palladium/nickel alloy coatings as previously described will
greatly improve the corrosion resistance of the resulting coating. hhile any
of the compounds of this formula described in German patent 1,028,407 can
effectively be used, in general, R should be an aromatic group, e.g. phenyl or
naphthalene or substituted phenyl whose substituents are lower alkyl, eg. methylor amino. Nitrogen-containing heterocyclical moieties such as pyridino and morpho-
lino may also be used for the moiety R. Rl and R2 may be the same or different
and each can be selected from the group which consists of hydrogen, lower alkyl,
~r ,~

I 1 753~6
e.g. methyl or ethyl, lower alkyl substituted with amine and lower alkene or
lower alkyne. When the term "lower alkyl" is used herein it is intended thereby
to include alkyl groups containing from 1 to 6 carbon atoms and in a straight
or branched chain configuration. The lower alkenes and lower alkynes can be 2
to ~ carbon atoms. Preferably Rl and R2 are each hydrogen and R is phenyl. More
preferably, Rl and R2 are each hydrogen or methyl or ethyl and ~ is benzyl or
phenyl. The bath to which thls brightener is added can be any of the known
baths for galvanically depositing (electroplating) palladium/nickel alloy coat-
ings. In particular, the Bath should contain an aqueous solution of palladium
c~ S
and nickel a~i~es with a palladium content of about 5 to 30g/liter, a nickel
content of 5 to 30 g/liter, one or more sulfonic acids salts in an amount of,
say, 0.01 to 20 g/liter, conductivity promoting salts, e.g. in an amount of 10
to 200 g/liter, 0.01 to, say 5 g/liter of a wetting agent and one or more sul-
fonyl ureas of the aforementioned formula in a total amount oE 0.1 to 10 g/liter.
The palladium/nickel ratio is set so that the electrodeposited coating contains
30 to 90% by weight of palladium.
For optimum corrosion resistance, the palladi-lm/nickel coating should
be applied in a thickness of 1 to 5~, preferably 2.5~ and the corrosion
resistance promoting additives should be used in an amount of 1 to 10 g/liter
of the plating bath.
Surprisingly, the sulfonyl urea of the formula given, which functions
only as a brightening additive in nickel-plating baths and which does not have
a noticeable effect upon im~rovement of the corrosion resistance1 in the
special palladium/nickel bath of the invention functions as a corrosion
resistance promoter leading to high-durability decorative and/or
electrotechnical coating hav~ing unusually high corrosion resistance.
In fact, the coatings which result from the use of these compounds
alone or in combination with aliphatic unsaturated and heterocyclic sulfonic

1 ~ 7~3~6
acids and/or in combination with acetylene alcohol and/ot acetylene amine
and/or amino alcohols, results in palladium/nickel coatings which are
practically free from long-term and short-term corrosion and which show no
noticeable corrosion in the aforementioned tests.
The aliphatically unsaturated and heterocyclic sulfonic acids can be
one or more members of the group selected from sodium vinyl sulfonates, sodium
allyl sulfonate, sodium propyne sulfonate, sodium methallyl sulfonate,
N-pyridinium propyl sulfobetain, N-pyridinium methyl sulfobetain and the sodium
salt of N-benzyl pyridinium-2-ethyl sulfonic acid.
The effect of the system of the invention appears to be quite
different from the effect of the sulfonyl urea in nickel baths. In nickel
baths the brighteners increase the ductility of the coating and reduce the
tensile stresses in the bright nickel coatings which, as a rule, are thicker by
a factor of 10 than the preferred coating of the invention. The invention
operates with additives in the bath far greater in number and in concentration
than the additives in a bright nickel bath.
It appears that the improvement is a result of an electrochemical
passivation at the coating surface. Naturally, care should be taken to prevent
interference by metallic impurities.
Apart from the introduction of the additives of the invention, the
palladium/nickel coating can be formed using the techniques described in the
British Patent 1,i43,178.
The bath temperature is preferably room temperature (20C + 20C)
while the current density may range between 0.1 to 10 ~/dm2 and preferably is
about 0.5 to 2 A/dm2.
SPECIFIC E~AMPLES
The bath has the following composition:
20 g Pd as [Pd~NH3)4]C12,

I .11 753~ ,
9 g Ni as [Ni(~l3)6]S04~
50 g Conductivity - promoting salt as (NH4)2S04, N~140H sufficient to
give a pH of ~.5,
3 g Na allysulfonate,
2 g Ben~enesulfonylurea ( ~ -S02NHCONH2),
0.5 g Wetting agent (phosphoric acid ester), and
Water sufficent for 1 liter.
The palladium/nickel coating was deposited upon a copper substrate at
a bath temperature of 35C with a current density of 1 A/dm~ with agitation of
the bath to form electrical contact having a thickness of ~u.
When the coating was subjected to the above-described corrosion
tests, no long-term corrosion was visible and even after such long-term
testing, no noticeable change in contact resistance of the coating was
observed.
When the same bath was used under the same conditions but without the
sulfonyl urea, the contact element was found to corrode within a short time and
to almost immediately develop a high contact resistance when sub~ected to the
corrosion test.
Similar results were obtained when the bath of the present inventlon
contained 5 to 20 g/liter of palladium as the palladium ami*e Eluoride, 5 to 15
I t~ æ
g/liter of nickel as the nickel-s~e sulfate or as the nickel sulfa~ate
[Ni(S03~12)2], 50 to 100 g of the conductivity promoting salt in the form of
~ - ammonium sulfate or ammonium hydroxide to provide a pH of 8.0 to 9.0, 1 to 10
; g/liter of sodium allyl sulfonate, and 0.1 to 1 g of the wetting agent per
liter.
--6--