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Rhodium Electroplating Systems

Rhodium is a platinum group metal with a good white colour and is hard and tarnish
resistant. For jewellery purposes, we desire a bright deposit, defect-free and hard and
there are several suitable rhodium plating systems on the market. These are sulphate
type baths and are very acidic.
Usually, deposit thickness of about 0.5 microns, but up to 2-3 microns, is plated on gold
jewellery to give the required surface characteristics. There is a tendency for internal
stress to build up in the deposit as thickness increases, resulting in cracking eventually.
For the high carat golds, a thicker layer of rhodium is plated directly on the substrate, but
for low carat golds, a nickel interlayer is plated first, allowing a thinner, cheaper rhodium
deposit without losing colour and providing good corrosion resistance.
As with gold, good surface preparation is required to provide a clean surface for quality
electroplating. The following practice is recommended:
 Electrolytic cleaning
 Rinse in demineralised water
 Check surface wettability (no formation of droplets)
 Dip piece in aqueous solution of sodium cyanide (35 g/l)
 Rinse in running water or demineralised water
 Dip in electrolyte with power on (do not touch) & electroplate
 Rinse and dry.
Plating should take from 30 seconds to 2 minutes, depending on thickness desired. Inert
anodes of platinum are used at 4-5 cm distance with a surface area at least as big as the
cathode. The bath should be well agitated or stirred.
Periodic replenishment of the rhodium in the bath is necessary and this is done with
special rhodium replenishment solutions which have a high rhodium concentration and
low acidity. It is important to avoid contamination of the electrolyte by other metals, so
good rinsing and use of non-metallic tanks is recommended.
A typical rhodium electroplating system has the characteristics shown in Table 3.
The extremely high hardness of the deposit is notable. This is an advantage in rhodium
plating master models in silver for investment casting and electroforming as it enables a
high degree of polish to be obtained on the model, with benefit down the line to the casting
or electroform.
Electrolyte additives such as magnesium sulphate, selenic acid and sulphites are often
used to control internal stress build-up.
Rhodium content 1.5 – 2.5 g/l
Bath temperature 40 – 50°C
pH < 1
Plating rate 2 mg/A/min
Current density 1.5 – 5.0 A/dm2
Salts/acids Sulphuric acid
Deposit purity, rhodium % 99.9
Time to deposit 1 micron 30 secs.
Deposit appearance Bright
Hardness of deposit HV 950
Table 3 – Typical bright rhodium electroplating system