AND CORROSION PROTECTION OF METALS
Russian Journal of Applied Chemistry, 2012, Vol. 85, No. 1, pp. 57−61.
Pleiades Publishing, Ltd., 2012.
Original Russian Text © D.B. Kal’nyi, V.V. Kokovkin, I.V. Mironov, 2012, published in Zhurnal Prikladnoi Khimii, 2012, Vol. 85, No. 1, pp. 60−64.
On Anodic Dissolution of Silver Coatings Deposited
on Base Metals
D. B. Kal’nyi
, V. V. Kokovkin
, and I. V. Mironov
Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia
Novosibirsk State University, Novosibirsk, Russia
Received May 27, 2011
Abstract—Potentiodynamic polarization and potentiostatic electrolysis techniques were employed to study
the anodic oxidation of silver and a number of other metals (copper, iron, nickel, and tin) in sulﬁ te media, both
individually and in Ag–M binary electrodes. The conditions in which silver is dissolved with a nearly 100% current
efﬁ ciency were found.
It is known  that a considerable part of metallic
silver used for technical and decorative purposes is in the
form of surface ﬁ lms deposited onto various substrates.
The role of substrates is played by various metals and
metal alloys, including copper, iron, nickel, brass, and
tin. The optimal process for recovery of silver from
scrap articles of this kind is that in which the substrate
metal is not dissolved.
The conventional methods for recovery of silver
[2–5] are based on its dissolution in the presence of
oxidants in solutions of acids or complexing agents,
with the subsequent recovery of silver in the form of
insoluble salts or its reduction to the metal. The main
shortcomings of these methods are the gross expenditure
of reagents and the dissolution nonselectivity: substrate
metals pass into solution together with silver.
A substantially lower expenditure is characteristic
of techniques based on the anodic oxidation of silver.
In processing of materials containing considerable
amounts of base metals, selective transfer of silver into
solution is possible in an alkaline medium in which
many transition metals form insoluble hydroxides.
An additional advantage of the electrolytic method is
that the oxidation of silver may be accompanied by its
deposition onto a cathode. The oxidation of silver in
alkaline media in the presence of chlorides, nitrates,
sulfates, and borates has been extensively studied [6–12].
However, the formation of passivating ﬁ lms composed
of silver oxides on the surface of the metallic phase has
been frequently observed. Silver is commonly dissolved
in special electrolytes , including those containing
such complexing agents as thiourea , rhodanide [15,
16], and amino acids .
A promising complexing agent for these purposes is
sodium sulﬁ te, because sulﬁ te complexes of silver(I) are
rather stable . Compared with the cyanide widely
used in hydrometallurgy of silver, the sulﬁ te ion forms
stable complexes with a substantially smaller number of
metal ions . In addition, the sulﬁ te by itself creates
an alkaline medium (pH ≈ 9–10) in solution. All this
gives reason to expect a high selectivity in recovery of
silver(I) into solutions in the presence of other metals.
It is known that sulﬁ te ions are unstable against
atmospheric oxygen. The ﬁ nal oxidation product is the
sulfate ion (SO
– 2e = SO
= –0.90 V
). Probably, just the widely accepted opinion about
fast oxidation by oxygen has hindered development
of methods using sulﬁ te ions as complexing agents,
including those for recovery of silver. The kinetics of
oxidation in aqueous solutions was studied in [20,
21], but the process mechanism remains unclear.
The goal of our study was to examine the oxidation
of metallic silver and substrate metals, including copper,
iron, nickel, and tin, both individually and in Ag–M
binary electrodes in media containing sodium sulﬁ te and
to determine the stability of sodium sulﬁ te solutions in