How to make a rhodium bath last longer.
The life of a rhodium bath is not related exclusively to the consumption of the rhodium, but
there are some important tips and parameters that can be kept under control and replenished
if necessary. Generally, a rhodium process consists of:
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Rhodium
Inorganic acids (sulphuric acid, phosphoric acid, etc.)
Brightener additives
Wetting Agent additives
Water
Each of these parameters is of fundamentally importance in obtaining a constant white and
shiny deposit. During processing, all of the components of this solution are consumed and
correct replenishment is fundamental in order to maintain the solution in the best condition for
processing. In cases where the replenisher solutions of rhodium provided by your supplier are
well balanced, the addition of the replenisher should be sufficient to restore the rhodium and
the quantities of Brightener agents consumed. Consequently, in cases where the rhodium
concentration is optimal and the deposit still doesn’t seem acceptable, you should at least
resort to a quantitative analysis of the inorganic acid, which is responsible for the conductibility
of the bath and it is the primary agent responsible for its penetrating power. If all of the
parameters are at optimal concentrations, but the deposit it is still not satisfactory, an active
carbon filtration* could be necessary, which is capable of removing most organic polluting
agents. It should be noted that active carbon filtering often removes many Brightener
additives, which will then have to be restored, through additions as suggested by the supplier.
In cases where the rhodium is still unusable after all of the above-mentioned analyses, it is
highly possible that there are some “inorganic” pollutants present, which are mostly tied to the
heavy metals dissolved in the solution (the copper of wires, nickel, etc.). In this case the
rhodium solution has unfortunately to be substituted.
*Active carbon filtering: active carbon filtering is a simple filtering process that is performed
by adding 2-3 g/l of active vegetal carbon (which has no heavy metals) to the rhodium
solution. When this has been added to the powder, shake the solution and let it act for 1 hour,
at most, and then filter the entire solution through filtering paper, to extract the suspended
particles of carbon. The filtered solution will necessarily be a limpid yellow/orange colour.
Pre-plating treatment for rhodium.
Many companies have been resorting to pre-plating treatment with rhodium for several years
now. There are different types, which are used for different purposes. The most common are
related primarily to three reasons:
1. Improved coverage of the successive rhodium deposit
2. Reduction of rhodium plating costs
3. To protect the rhodium bath from pollution
1 – Improved coverage of the successive rhodium deposit
Not everyone knows that rhodium looks like a spider web under an electronic microscope
during deposit, which is slowly completed, closing the gaps and covering the underlying metal
like a shield. It takes several minutes of rhodium plating for this shield to be completely
closed; that’s why spots and points of oxidation often appear after just a few weeks, even
when the rhodium has been deposited on low-carat alloys or silver. Customers are generally
surprised because the rhodium should provide 100% protection against oxidation of the
underlying metal. There are basically two procedures that can be used to prevent this
inconvenience from occurring:
1. Increase the rhodium plating time to above one minute;
2. Use a pre-plating process to deposit other noble metals under the rhodium.
I will not go into detail on the first point for obvious reasons of the costs involved;
nevertheless, the second point is a fundamental part of our current theme. There are many
types of pre-plating procedures, with different metals (gold, palladium, etc.); considering the
white deposit of rhodium, the deposit of a hard, compact metal, possibly a white metal, which
has a moderate cost, is preferable for the pre-rhodium plating procedure. All of these
properties are present in palladium, which is precisely why it is the most widely used metal for
pre-plating treatments in the gold sector. The best final coverage of rhodium can be obtained
inasmuch as palladium, in addition to guaranteeing an extremely compact barrier with the
underlying metal, is more compatible with rhodium from the electro-chemical point of view,
because they are both metals that belong to the platinum group. (PGM).
2 – Reduction of rhodium plating costs
As mentioned in point 1, palladium is capable of developing a more compact deposit than
rhodium. The result of this is that in order to obtain a high quality deposit, it is not necessary
to deposit one or more minutes of rhodium, but it will be sufficient to maintain the limit of 20
seconds, providing there is a deposit of palladium of at least one minute underneath. In
economic terms, this all translates into a real savings that varies between 20 and 40% in the
cost of rhodium and a considerable increase in the quality of the total final deposit. Attached
hereto is a table that shows the economic benefits of this procedure. (see comparison sheet
Pd+Rh).
3 - Protecting the rhodium bath from pollution
As mentioned in the introductory paragraph, the life of a rhodium bath does not depend only
on factors of consumption of the components of the solution, but often on fundamental
strategies that prevent external factors from polluting the bath. The primary agents
responsible for this type of pollution are residues of organic pollutants (i.e. foam from the
degreasing vat) or inorganic pollutants (the most dangerous of which are the heavy metals
from copper wires used to tie large pieces for previous washing processes, performed poorly
before entry into the rhodium bath). Therefore, by using a pre-rhodium plating bath, most of
these pollutants are withheld in the pre-rhodium bath, therefore maintaining the bath for final
finishing. The most widely used methods for this purpose are:
1. Pre-rhodium treatment made up of a bath of low-concentration rhodium
2. Pre-rhodium treatment in a bath of palladium
If the purpose is only to preserve the final bath of rhodium, the first method can be used,
formulating two rhodium baths, a pre-rhodium treatment with low concentration (0.8 – 1.0 g/l;
treatment time: 15”) and a second bath at 2 g/l (treatment time: 60”). In this manner, the
second bath will be protected from polluting substances, which are withheld in the first bath.
Generally this type of pre-rhodium plating is used to produce high value goods where the cost
of the rhodium plating does not influence the final value of the object to a great extent. If the
purpose of using the pre-rhodium treatment is to guarantee a high standard of quality, in the
attempt to decrease the cost of rhodium plating, the use of palladium as a pre-rhodium plating
treatment is the best choice, for all of the reasons mentioned previously.