Electrochemical reduction of corrosion products
found on lead artefacts
Redha Amri, Hénia Mousser*
Département de Chimie Industrielle, Faculté des Sciences de l’Ingénieur, Université Mentouri Constantine,
25000 Constantine, Algérie.
bouzidi_henia@yahoo.fr
Mohamed Fakhri Bencharif
Laboratoire des Mesures Electrochimiques, Département de Physique, Faculté des Sciences Exactes, Université Mentouri
Constantine,
25000 Constantine, Algérie
André Darchen
UMR CNRS n°6226 Sciences Chimiques de Rennes, ENSCR, Avenue du Général Leclerc, CS 50837,
35708 Rennes cedex 7, France
Abdelhamid Mousser
Département de Chimie, Faculté des Sciences Exactes, Université Mentouri Constantine,
25000 Constantine, Algérie
Abstract—Electrochemical restoration of corroded lead artefacts
should be able to consolidate the objects and reverses the
corrosion products in to initial metallic state. Cerussite (PbCO 3),
anglesite (PbSO4), cotunnite (PbCl2) and lead oxides are the most
responded corrosion products found on the lead artefacts. The
knowledge of the reduction potential of the lead corrosion
products is primordial for an appropriate action in the
restoration process. The electrochemical reduction of the
cerussite and the anglesite into metallic lead on platinum
substrate, in the sodium sulphate and the sodium carbonate
solutions (0.5M) was investigated. This work is a part of a project
aiming to perform the restoration of numerous archaeological
coins. The reduction’s potentials are determined by linear sweep
voltammetry and corroborate by potentiostatic conditions. The
transformation of lead corrosion products into metallic lead was
carried out both in the sulphate and the carbonate medium.
Keywords: Electroreduction; cerussite; anglesite; lead; reduction.
Introduction
Many ancient lead artefacts are nowadays conserved in
museums, many of them are badly corroded that it becomes
extremely fragile and all surface are just a mass of corrosion
products. Depending on the characteristic of the archaeological
site in which the lead artefacts are found and the environment
where they are exposed or stored, the lead objects are
transformed to Anglesite (PbSO4), hydroxycarbonate
compounds such as cerussite (PbCO3), hydrocerussite
Pb(CO3)2(OH)2), cotunnite (PbCl2) and lead oxides (PbO,
PbO2). These corrosion’s products were currently found on
several archaeological objects dating from the Iron Age [1-8].
Many analytical methods have been used to identify the
chemical composition of archaeological artefacts [3, 5-9].
Electrochemical treatments of severely corroded objects have
to transform the corrosion’s products into the metal mass.
Some studies were carried out to determine the potential
reduction of lead corrosion products [3, 10-12]. According to
the Pourbaix diagram [13], the lead is stable in the neutral or
the alkaline solutions which are free from oxidizing agents. In
the soil, the carbonate ions are usually present so basic lead
carbonates and lead oxides were formed with most
archaeological objects when these last are made of lead. The
gray lead carbonate and lead oxide often form a protective
layer on the artefact that it prevents further oxidation [1]. Both
these corrosion’s compounds are found on lead from a marine
environment, lead chloride, lead sulphide and lead sulphate [1].
This work is the second part of a project aiming to perform the
restoration of numerous archaeological Numidian coins [5].
The research is carried out to provide more data on the
electrochemical restoration process of lead archaeological
objects and to determine the conditions of electrochemical
reduction of some lead corrosion’s products found on the
Numidian archaeological artefacts [5].
I.
EXPERIMENTAL
All the solutions were prepared using distilled water and
reagent grade chemicals. The cerussite and the anglesite were
prepared according to the literature [14-16].
All products were identified by X-ray diffraction and InfraRed beam.
The electrochemical experiments were carried out in a threeelectrode thermostated cell with a PAR 373A potentiostat
coupled with a Kipp and Zonen XY recorder. A rectangular
platinum plate (2.5 cm2) and a saturated calomel electrode
(SCE) were used as working and reference electrodes,
respectively.
Before each experience, the Pt working electrode was
mechanically polished with successively finer grades of SiC
emery papers and was rinsed with distilled water. In order to
eliminate residual oxides formed on the working electrode a
fixed reduction current density was applied (0.05mA/cm2) for
10 min [17].
In each electrochemical measurement a paste of lead
compound was prepared. A mixture of the corresponding
compound and distillate water was spread over the surface of
the working electrode, so that the entire electrode surface was
covered completely.
The reduction potential of each compound was investigated by
linear sweep voltammetry between the potential at open circuit
and -1.4V with 1 mV/s scan rate. The potentiostatic data were
carried out for each determined potential value. All
experiments were realized at room temperature.
XRD was performed with an X’Pert PRO MRD diffractometer
(CRD, SONATRACH of Boumerdes, Algeria) equipped with
a PANalytical's XRD Data Collector systems and a Panalytical
HighScore Plus software. The spectrometer is provided with a
copper anticathode (λ= 1.5401A˚, I = 30mA and V = 40 kV).
Sweeping was made between 2 and 70◦ (2θ) with a count-time
of 46 s for each step of 0.017◦ (2θ).
II.
RESULTS AND DISCUSSION
II.1. DRX analysis
Cerussite and anglesite prepared [14-16] were identified by Xray diffraction. The XRD difractogramms of these products
are presented in Fig. 1 and Fig. 2. In Fig. 1, the most intense
peaks are observed at: dhkl = 3.59; 3.49; 2.48 and 2.08 Å.
According to the ASTM file No 47-1734 [18], these peaks
indicate that the formed product is the cerussite.
Fig. 2: XRD spectrum of the anglesite.
II.2. Reduction of the cerussite
The figure 3 shows voltammetric curve of cerussite on
platinum substrate in sulfate and carbonate electrolyte
solutions. In the sodium sulfate electrolyte (solid line) a small
cathodic peak can be seen (approximately at -0.68V versus
SCE) with a small current (153µA). This peak indicates the
beginning of cerussite reduction. Beyond this peak the current
fall a little and take a constant value (~ 80µA) in this region
the cathodic process is controlled by diffusion [17].
Immediately after this region the current rise again, due to the
hydrogen evolution reaction occurring in parallel with
cathodic reaction of cerussite.
In sodium carbonate electrolyte (dash line) a cathodic reaction
occurs at more negative potential. The peak is observed
approximately at -1.18V versusSCE,with 3.85mA and
indicates occurring of the reduction of the cerussite. After this
point the current fall and increase at more negative potential
due to the hydrogen evolution. The reduction of the cerussite
was affected kinetically by the nature of the bath, since de
peak current value is more important in the carbonate bath
than in the sulfate one. Moreover the nature of the bath affects
the reduction of cerussite thermodynamically and the potential
of the peak in the carbonate bath shifts to more negative
values.
Fig. 1: XRD spectrum of the cerussite.
Fig. 2 shows the XRD diagramm of the anglesite phase which
is identified by the peaks observed at: dhkl = 3.01; 3.34; 3.22
and 4.27Å from ASTM file No 36-1461 [18] (dhkl = 3.007;
3.33; 3.21 and 4.265Å).
Fig. 3: Voltammetry of cerussite. (a) In Na2SO4 (0.5M).
(b) In Na2CO3 (0.5M).
To corroborate these potentials the cerussite was treated at 1.18V in carbonate solution (0.5M), (Fig. 4, a) and at -0.68V
and at -1.08V in sulfate solution (0.5M), (Fig. 4, b). The figure
4 shows that, in carbonate medium, a high current is measured
in the first hour of treatment. The electrochemical reduction of
the cerussite occurred at -1.18V (Fig. 4, b). At the end of the
treatment, the current density tends to zero and the white
cerussite was transformed to a grey metallic lead. In the
sulfate solution, the end of the cathodic reaction was observed,
with a low rate, at -0.68V. At -1.08V, the electrochemical
reduction of the cerussite is fast and the measured current is
more important than that measured at -0.68V.
The potentiostatic treatments of anglesite in the carbonate bath
at -0.73V and -0.87V were carried out and the results are
presented in the figure 6. At -0.73V no current is measured
and no change occurs in the nature or the appearance of the
anglesite. At -0.87V, a high current is observed and at the end
of the treatment (Fig. 6, (a)), the anglesite is transformed into
metallic lead. The anglesite shows the same behavior in
sulfate solution (0.5M) at -0.93V (Fig. 6, (b)).
Fig. 6: Electrochemical reduction of anglesite. (a) In Na2SO4 (0.5M).
(b) In Na2CO3 (0.5M).
Fig. 4: Electrochemical reduction of cerussite.
(a) In Na2SO4 (0.5M). (b) In Na2CO3 (0.5M).
II.3. Reduction of the anglesite
The figure 5 presents the voltammetric curve of anglesite.
Two cathodic peaks appear in sulfate solution. The anglesite
reduction (Fig. 5, (a)) occurs at -0.73V versus SCE. The
second reduction is observed at -0.93V versus SCE and is
probably due to the bulk reduction. In the carbonate
electrolyte, the electroreduction’s peak appears at 0.83Vversus SCE (Fig. 5, (b)). The reduction of anglesite is
affected by the nature of the bath. The potential reduction of
the anglesite shifts to more negative potential values in the
carbonate medium.
Conclusion
The electrochemical studies of the two corrosion products
cerussite and anglesite which were formed on the lead surface,
in the sulfate and the carbonate electrolytic baths, revealed the
reduction of cerussite and anglesite into metallic lead. In the
sodium carbonate solution (0.5M), the potential -1.18V can
reduce all corrosion products and the potential -1.1V is
sufficient for the same reduction in the sodium sulfate bath
(0.5M). The results show that the nature of the electrolytic
bath controls the electrochemical process of the reduction
thermodynamically and kinetically. The electrochemical
reduction of the cerussite and the anglesite, in Na2SO4 or in
Na2CO3 is characterized by a first peak (thin lead film) and by
a second one (bulk reduction), where the rate of the reduction
was high in the region of the second peak and controlled by
diffusion. However in Na2CO3, the reduction takes place at
more negative potentials with a single cathodic peak.
ACKNOWLEDGMENT
The authors express their appreciation to Mentouri
University of Constantine for its financial support necessary to
carry out this study, and their thanks to sirs Mustapha Belkadi
and Khaddja Gellil, Engineers at the Laboratory CRD,
SONATRACH of Boumerdes, Algeria for XRD measurments.
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Fig. 5: Voltammetry of anglesite. (a) In Na2SO4 (0.5M).
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