Note: Descriptions are shown in the official language in which they were submitted.
1~32939
The present invention relates to rhodium plating
baths. Rhodium is an expensive material and it is thus
desirable to find a plating bath system in which the rhodium
content can be kept low whilst at the same time providing a
satisfactorily bright electro-deposit.
According to the present invention an aqueous
acidic rhodium plating bath comprises 0.1 to 30 g/l, espec-
ially 0.1 to less than 1.0 g/l, of rhodium ions e.g. as the
sulphate, 0.1 to 20 g/l, especially 0.1 to 5 g/l, of a
carboxylic acid containing at least one amino group, e.g.
glutamic acid, and 10 to 100 g/l, especially 20 to 70 g/l,
of orthophosphate radicals, the bath preferably having a pH
in the range 0.1 to 2Ø The sulphate is neither objectionable
nor essential to the plating system. The orthophosphate ion
concentration is preferably greater than the sulphate ion
concentration in the bath.
A preferred plating bath composition comprises 0.1
to 0.9 g/l of rhodium ions as the sulphate, 0.1 to 5 g/l of
glutamic acid, and 40 to 60 g/l of orthophosphate radical,
the pH of the bath being in the range 0.5 to 1.5.
The invention also provides a rhodium electro-
plating composition comprising rhodium sulphate and a poly-
carboxylic organic carbo~ylic acid containing at least one
amino group that is at least slightly soluble under plating
conditions. The amounts of the various components of the
composition will be such as to provide upon dissolution in
water, the amounts of the various components set out above.
The invention also provides a method of electro-
plating using a rhodium electroplating bath according to the
present invention. The bath is prepared and maintained at a
temperature of from 20 to 85, and an anode and a workpiece
having a metallic surface are immersed therein. A voltage
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which is controlled to provide a current density of from
0.5 to 30 amperes per square foot at the surface of the
workpiece is applied across the workpiece and anode member
to deposit rhodium from the bath upon the metallic surface.
The bath preferably has a pH of at least 1.0 and
is maintained at a temperature of from 40 to 55C and the
current density at the workpiece surface is from 5 to 20
amperes per square foot.
The baths of the invention may be employed for
rack and also for barrel plating and in both cases moderate
to vigorous agitation is beneflcial. In accordance with
conventional practice, insoluble anodes will normally be used,
and they may appropriately be of platinum or platinum-sur-
faced construction: with such insoluble anodes, the bubbling
of generated gas may be relied upon to provide the desired
agitation in the bath. The configuration of the anode will
vary depending upon the plating method and the associated
equipment used.
Rhodium deposits can be produced upon substantially
any of the various metallic surfaces that previously have
been so plated including silver, nickel or gold. The sur-
faces of the workpiece should be adequately cleaned to ensure
that a high quality, adherent deposit is produced, and when
the metal of the surface to be plated is soluble in the bath,
a strike of insoluble metal may be applied as a precaution
against contamination.
Continuous carbon treatment and filtration of the
bath may be used to remove impurities, and surfaces coming
into contact with the bath should be fabricated of an inert
material such as glass, polyethylene, unplasticized polyvinyl
chloride or acrylic polymers. The components of the bath
may be replenished on a predetermined schedule or as needed
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based upon bath analyses. It will, of course be appreciated
that additions of the rhodium salt will have to be made to
replenish the rhodium ion, due to the use of an insoluble
anode.
The invention may be put into practice in various
ways and one specific embodiment will be described by way of
example to illustrate the invention.
Example 1
A plating bath was made up consisting of 0.5 g/l of
rhodium ions as the sulphate, 2.0 g/l of glutamic acid, and
175 g/l of phosphoric acid (85% aqueous added at a rate of
100 ml/l) providing 50 g/l of orthophosphate radical. The
pH was 1.
A bright nickel plated brass panel 1 inch by 3.5
inches was plated as the cathode in 100 ml of this plating
bath at 20 ASF at 50C (120F) for 5 minutes, using an
insoluble platinum anode. The cathode to anode area ratio
was about 4:1. This plating produced a clear bright rhodium
deposit of about .0169 gram. The efficiency was approxi-
mately 4 mg per ampere minute.Example 2
The solution of Example 1 was replenished several
times by small increments each averaging 1 gram of rhodium
metal to replace the rhodium plated out during the plating
tests.
The current density used varied from 5 to 25 ASF
and the plating time varied from 2 minutes to 30 minutes.
In all cases the resulting plating was a typical bright
clear white deposit. The efficiency of the bath was found
to be in the range of 3 to 5 mg per ampere minute with an
average of 4 mg per ampere minute. The following solution
was used to replenish the solutions of Example 1.
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Rhodium ions as the sulfate 10 g/l
Glutamic acid 40 g/l
Orthophosphoric acid 975 g/l
; (Providing 250 g/l of orthophosphate radical)~
Whenever necessary the pH ofthe bath was adjusted to 1.0 with
a 50% orthophosphoric acid solution.
The bath disclosed in this invention is most suit-
able for flash deposits of rhodiums of the order of 3 to 15
micro-inches thick.
The main advantage of the bath in accordance with
the present invention is in reducing the drag out losses in
such flash plating. With conventional rhodium baths contain-
ing about 2 grams per litre of rhodium, the rhodium metal lost
by drag out is about 4 times greater than with the bath dis-
closed in the present~invention which typically contains about
0.5 grams per litre of rhodium, although the deposits produced
by the bath of the present invention are comparable to those
produced by conventional bright rhodium baths. whilst the
efficiency of the baths according to the present invention is
slightly higher than those of the conventional baths. The -
bath in accordance with Example 1 contains less than 2.5 g/l
of sulphate and the replenisher composition contains less
than 50 g/l of sulphate. Thus, the ratio of orthophosphate
; radical to sulphate radical in the bath is preferably at least
20:1 but may be as low as 2:1, whilst that in the replenisher
composition is at least S:l.
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