Increase of pollution in recent years, requires monitoring of the corrosion behavior of artistic bronzes coated with syntetic patina, in urban atmosphere.
This present work aims to investigate the corrosion rezistance of various artificial patina currently used in bronze sculpture. Electrochemical and microscopic methods were used to investigate the protective effect of several chemically produced patinas on artistic bronze

J. D. SABĂU (CHELARU)

L. M. MUREŞAN

O. NEMEŞ

T. KOLOZSI

V. F. SOPORAN

The Annals of “Dunarea de Jos” University of Galati Fascicle IX Metallurgy and Materials Science

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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 2 – 2012, ISSN 1453 – 083X 
 
 
 
 
INVESTIGATION OF CORROSION PROTECTION BY CHEMICALLY 
APPLIED PATINA ON ARTISTIC BRONZES 
 
J.D. SABĂU (CHELARU)1, L. M. MUREŞAN2, V.F. SOPORAN1, 
O. NEMEŞ1, T KOLOZSI3 
1Technical University of Cluj-Napoca, Faculty of Materials and Enviromental Engineering,  
Department of Environmental Engineering,  
2Babes-Bolyai University, Physical Chemistry Department,  
3University of Art and Design, Cluj Napoca 
email: julieta_dana@yahoo.com 
 
ABSTRACT 
 
Increase of pollution in recent years, requires monitoring of the corrosion 
behavior of artistic bronzes coated with syntetic patina, in urban atmosphere. 
This present work aims to investigate the corrosion rezistance of various 
artificial patina currently used in bronze sculpture. Electrochemical and 
microscopic methods were used to investigate the protective effect of several 
chemically produced patinas on artistic bronze. 
 
KEYWORDS: bronze, corrosion, patina, polarization curves 
 
1. Introduction 
 
On a bronze sculpture, the color of the surface 
has the ability to enrich the aesthetic. 
The art of patination can be defined as a slip 
coloring metal surface oxidation, caused by 
prolonged exposure in the surrounding atmosphere. 
This definition is actually describing the two patina 
categories used in the case of bronzes: natural and 
artificial. 
Natural patina is the result of compounds 
formation after corrosive chemical reactions that 
occur on the surface of bronze. There is no way to 
achieve a true natural patina except with time itself. 
This is very important to natural patina but very 
impossible for the sculptor, because it requires years 
to accomplish an end result. Artificial patinas, on the 
other hand, have the advantage not only of creating 
similar results of natural patina, they also have the 
ability to incorporate a wider range of color hue and 
depth not found in natural patinas, and this is 
accomplished almost immediately [1]. 
Making a special color on the surface of bronze 
art objects requires a mastery of the artist. This work 
is almost unknown until recently, the secrets being 
kept by each artist. 
The importance of producing artificial patinas 
on bronze comes from the well known ability of this 
alloy to react against atmospheric agents by forming 
different coloured corrosion products of pleasant 
aspect, which protect the bronze objects from further 
corrosion [2]. 
Many factors influence the initial development 
of the patina, some of them being active throughout 
the life of a work of art. 
The final quality of the patina depends on the 
alloy composition and on environmental conditions it 
develops in. From a chemical point of view the patina 
layer can be described as a hydrated copper oxide 
stabilized on a copper oxide substrate [2, 7 - 9]. 
However, the poluants present in urban atmosphere 
gives birth to various corrosion products on bronze 
surface. The most damaging are chlorides (especially 
sodium chloride, which encourages the development 
of copper chloride), sulfates and oxides, particularly 
sulfur dioxide [9 - 13]. 
Increase of pollution in recent years requires 
monitoring of the patination process that was 
formerly left to the action of nature.  
Artificially produced patina gives to the surface 
protection against corrosion and aesthetic value and 
can be formed on bronze surfaces by using chemical 
reagents or by electrochemical methods [14, 15]. For 
this reason, finding new artificial patinas to protect 
works of art cast in bronze is a current concern. 
In this context, the main objective of this paper 
is to study the protective effect of various artificial 
patinas deliberately produced by sculptors on bronze. 
The study was conducted under conditions that 
simulate acid rain. 
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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 2 – 2012, ISSN 1453 – 083X 
 
 
2. Experimental data 
 
The bronze used in the experiments was chosen 
to be similar to the bronze monument of Matthias 
Corvinus in Cluj Napoca which dates back to 1902. 
The statues of this monument were made of bronze 
on a metal frame and according to the chemical 
analysis, the composition of the alloy varies in 
different parts of the sculptural group; there are 
variations in the percentage of tin, zinc, lead, but the 
content of copper exceeds 90% of the alloy in most 
cases (Table 1). 
 
Table 1. The chemical composition of the alloy used in the casting  
of the King Matthias the Ist sculptural group 
 
Sn  Pb Zn Cu  
[%] 
Sample 1 7.17 0.08 0.36 rest 
Sample 2 8.00 0.52 1.30 rest 
Sample 3 7.72 0.41 0.21 rest 
 
Consequently the alloy used in electrochemical 
studies, according to current standards, was chosen 
CuSn8, whose chemical composition is displayed in 
Table 2. 
 
Table 2. The chemical composition of the used alloy 
 
Cu  Sn  Impurities, [%] max Alloy [%] Zn Pb Sb Fe Al S Bi Mg As Mn Ni 
CuSn8 rest 7-9 0.8 1.0 0.1 0.2 0.02 0.1 0.01 0.01 0.15 0.2 1.0 
 
For microscopic study, the electrodes were 
polished on the polishing machine with alumina 
paste, after which the surface was washed with 
ammoniacal cupric chloride. The study of the surface 
was conducted through optical microscopy 
(OLIMPUS GS 51). 
The metallographic analysis revealed the 
existence of a homogeneous structure, the crystals 
having polyhedral appearance with macles (Fig 1). 
Crystalline grain size is between 50-200 µm, 
which gives the alloy a high resistance to corrosion 
[16]. 
 
 
a) b) c) 
Fig. 1. Microscopic structure of alloy use in the casting of King Matthias I sculptural group: 
a – x100; b – x200; c – x500 
 
 
a) b) c) 
Fig. 2. Microscopic structure of bronze used for electrochemical studies: a – x100; b – x200; c – x500 
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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 2 – 2012, ISSN 1453 – 083X 
 
 
Figure 2 presents the microscopic structures of 
electrodes, magnified at x100, x200, x500, which 
were subjected to accelerated corrosion in an 
environment that simulated acid rain. 
The images in Figure 2 display the dendritic 
segregation of the α solid solution and interdendritic 
segregations of eutectoid α + δ in small quantities. 
The eutectoid α + δ is formed in a Cu - Sn alloy and 
results from the decomposition of phase γ at a 
temperature of 520 ° C (see the equilibrium diagram 
Cu - Sn). According to the picture displayed in the 
Metallurgic Atlas, the structure is the specific one for 
a cast bronze. The electrochemical corrosion 
measurements were performed on a PC – controlled 
electrochemical analyzer AUTOLAB – PGSTAT 10 
(Eco Chemie BV, Utrecht, The Netherlands) using a 
three electrodes cell containing a working electrode, a 
saturated calomel electrode (ECS) as reference 
electrode and a platinum counter electrode. 
The working electrodes made of bronze CuSn8, 
cylindrical shaped, were placed in a PVC tube, while 
the sealing was assured with epoxy resin. In this way, 
the surface of the electrode exposed to the solution 
was disk - shaped, with a surface S = 1.76cm2. For 
electrical contact a metal rod was attached (Figure 3). 
The electrolyte solution for corrosion 
measurements contained 0.2g/L Na2SO4 + 0.2g/L 
NaHCO3 (pH = 5) and simulate acid rain. 
 
 
 
Fig. 3. The electrodes used during the 
experiments 
 
The study was conducted on three electrodes 
covered with an artificial patina: electrode (1) was 
treated with flowers of sulfur; electrode (2) was 
treated with a solution containing copper sulfate and 
potassium permanganate and electrode (3), with 
sodium thiosulfate and ferric nitrate. 
 
3. Results and discussion 
 
The experiments started with measuring the 
potential of the working electrode in an open circuit 
for a period of 3600s, the records being presented in 
Figure 4. 
As shown in figure 4, for all studied electrodes, in the 
first minutes of immersion in the corrosive solution, 
the potential in open circuit evolves towards more 
positive values, reaching a stationary value after 
approx. 35 minutes. This behavior can be attributed to 
the chemisorption of oxygen on the surface of the 
dissolved bronze, along with the formation of surface 
oxide layers, hydroxisulfate and/or hydroxycarbonate 
[19]. 
-500 0 500 1000 1500 2000 2500 3000 3500 4000
-0,20
-0,15
-0,10
-0,05
0,00
0,05
0,10
0,15
0,20
E/
V 
vs
 A
g/
A
gC
l/K
C
l sa
t.
time/s
  patina 1
  patina 2
  patina 3
 bare bronze
Fig. 4. Time variations of the open circuit 
potential of the bronze electrodes immersed in 
0.2g/L Na2SO4 + 0.2g/L NaHCO3 (pH = 5) 
 
It can be noticed that the open circuit potentials 
of the coated electrodes 1 and 3 are more positive 
than that of bare bronze, suggesting an interaction of 
the patina with the anodic process (metal dissolution), 
while the open circuit potential of electrode 2 is the 
same with that of bare bronze suggesting a weak 
protection effect of the corresponding patina. 
-0,06 -0,04 -0,02 0,00 0,02 0,04 0,06 0,08 0,10 0,12 0,14 0,16 0,18 0,20-6,0x10
-5
-5,0x10-5
-4,0x10-5
-3,0x10-5
-2,0x10-5
-1,0x10-5
0,0
1,0x10-5
2,0x10-5
3,0x10-5
4,0x10-5
5,0x10-5
I/A
E/V
 patina 1
 patina 2
 bare bronze
 patina 3
 
Fig. 5. Linear polarization curves (± 20 mV vs. 
ocp) for the studied electrodes immersed in  
0.2g/ L Na2SO4 + 0.2 g/L NaHCO3 (pH = 5) 
 
To determine the polarization resistance of the 
electrodes, linear polarization curves were recorded, 
in the potential domain of ± 20 mV around the value 
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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 2 – 2012, ISSN 1453 – 083X 
 
 
of the open circuit potential (ocp) (Figure 5). The 
polarization resistance values for each electrode, 
calculated as the inverse of the slope of each curve, 
are shown in Table 3. 
 
Tabel 3. The value of polarization resistance (Rp = 1/p, p is the slope of the curve) 
 
Electrode Slope (p) Rp=1/p [Ω] 
Patina 1 7.25x10-4 1380 
Patina 2 2.25x10-4 4444 
Patina 3 1.87x10-4 5348 
Bare bronze 3.24x10-4 2060 
 
As can be seen from Table 3, the highest value 
for Rp was noticed in the case of the electrode (3), 
treated with a solution containing sodium thiosulfate 
and ferric nitrate. 
To determine the kinetic parameters of the 
corrosion process, polarization curves were recorded 
in the potential range of ± 200mV vs. ocp (Figure 6). 
-0,3 -0,2 -0,1 0,0 0,1 0,2 0,3-8,5
-8,0
-7,5
-7,0
-6,5
-6,0
-5,5
-5,0
-4,5
-4,0
-3,5
-3,0
lo
gI
[A
/c
m
2 ]
E/V
 patina 1
 patina 2
 patina 3
 bare bronze
 
Fig. 6. The polarization curves (± 200 mV vs. ocp) for the studied electrodes immersed  
in 0.2g/L Na2SO4 + 0.2g/L NaHCO3 (pH = 5) 
 
The kinetic parameters of the corrosion process 
were determined from the Tafel interpretation of the 
polarization curves and the results are presented in 
Table 4. 
 
Table 4. Corrosion process parameters for the examined samples 
 
Electrode Ocp Ecor icor βa βc 
 [mV vs SCE] [µA/cm2] [mV] 
Patina 1 -48 -101 3.25 91 77 
Patina 
2(brown) 95 65 1.87 85 134 
Patina 3(green) 41 17 1.33 55 164 
Bare bronze 51 46 5.36 46 284 
  βa and βc are the Tafel coefficients 
 
As shown in Table 4, all the patinas determine a 
decrease of corrosion current density of the bronze 
sample. Among them the coated electrode 1 is the 
less resistant to corrosion and it presents accelerated 
corrosion in comparison with the other two synthetic 
patinas. Treatment 3 seems to be the best among the 
treatments tested; however, the amelioration of 
corrosion rate is not significant. Moreover, no definite 
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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 2 – 2012, ISSN 1453 – 083X 
 
 
trend was observed in the shift of βa and βc values in 
the case of different patinas. A comparison between 
the corrosion current density and of the polarization 
resistance values of investigated electrodes is shown 
in Figure 7. The electrodes surfaces after the 
accelerated corrosion tests are presented in Figure 8. 
 
 
 
Fig. 7. Variation of corrosion current density and of polarization resistance  
for the investigated electrodes 
 
   
a) b) c) 
 
Fig. 8. The electrode surfaces after the corrosion test performed in a solution containing  
0.2g/L Na2SO4 + 0.2g/L NaHCO3 (pH = 5), a) patina 1, b) patina 2, c) patina 3 
 
4. Conclusions 
 
In this paper we compare the corrosion 
resistance of the patina obtained by using traditional 
flowers of sulfur with new synthetic, chemically 
obtained patinas on bronze. Corrosion tests were 
conducted in an environment that simulates acid rain. 
Bronze coated with new synthetic patinas have 
a higher corrosion resistance than the traditional one 
prepared with sulfur. 
In future we intend to investigate some new 
recipes and new methods of corrosion protection of 
artistic bronzes. 
Acknowledgments 
 
The “PRODOC” POSDRU/6/1.5/S/5, ID 7676 
project is acknowledged for financial support of this 
work. 
 
References 
 
[1]. P. Kipper - Patina for Silicon Bronze, Regal Printing, Hong 
Kong, 2003. 
[2]. B. Rosales, R. Vera, G. Moriena - Evaluation of the 
protective properties of natural and artificial patinas on copper. 
Part I. Patinas formed by immersion, Corrosion Science, vol. 41, 
1999, pg. 625-651 
[3]. R. Picciochi, A.C. Ramos s.a - Influence of the environment 
on the atmospheric corrosion of bronze, Journal of Applied 
Electrochemistry, vol. 34, 2004, pg. 989-995. 
[4]. L. Robbiola, J.M. Blengino - C. Fiaud, Morphology and 
mechanisms of formation of natural patinas on archaeological Cu-
Sn alloys, Corrosion Science, vol. 40, 1998, pg. 2083 – 2111. 
[5]. L. Robbiola, R. Portier - A global approach to the 
authentication of ancient bronzes based on the characterization of 
the alloy – patina – environment system, Journal of Cultural 
Heritage, vol. 7, 2006, pg. 1-12. 
[6]. K.P. FitzGerald, J. Nairn, s.a. - Atmospheric corrosion of 
cooper and the colour,structure and composition of natural patinas 
on copper, Corrosion Science, Vol. 48, pg. 2480-2509, 2006 
-  93  -
FAC
UL
TA
TE
A
D
E
M
E
TA
LU
RG
IE, ŞTIINŢA MATER
IA
L
EL
O
R
ŞIM
EDIU
FONDATĂ
1976
 
 
THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 2 – 2012, ISSN 1453 – 083X 
 
 
[7]. F.J.R. De Oliveira, D.C.B. Lago, s.a. - Study of patina 
formation on bronze specimens, Materials Chemistry and 
Physics,vol. 115, 2009, pg. 761–770. 
[8]. C. Chiavari, A. Colledan, s.a. - Corrosion evaluation of 
traditional and new bronzes for artistic castings, Materials 
Chemistry and Physics, vol.95, 2006, pg. 252-259. 
[9]. G.M. Ingo, T. De Caro, Riccucci C., Khosroff S. - 
Uncommon corrosion phenomena of archaeological bronze alloys, 
Applied Physics A, Material Science & Processing, Vol. 83. 2006, 
pg. 581-588. 
[10]. A. Domenic-Carbo, M.T. Domenic-Carbo, I. Martinez-
Lazaro - Electrochemical identification of corrosion products in 
archaeological artefacts. A case study, Microchimica Acta, 
Vol.162, 2007, pg. 351-359. 
[11]. J.D. Chelaru, V.F. Soporan, O. Nemeş - Time analysis of 
King Matthias The I Sculptural Group, International journal of 
conservation science, Vol.1, 2010, pg. 69-74. 
[12]. M.P. Casaletto, G.M. Ingo, M. Albini, A. Lapenna, I. 
Pierige, C. Riccucci, F. Faraldi - An integrated analytical 
characterization of corrosion products on ornamental objects from 
the necropolis of Colle Badetta-Tortoreto (Teramo, Italy), Applied 
physics a: materials science & processing, Vol 100, 2010, pg. 801-
808. 
[13]. I. Sandu, A. Dima, I.-G. Sandu - Restauration and 
conservation of metal objects, Corson Publishing House, Iaşi, 
2002. 
[14]. I. Sandu - Deterioration and degradation of the cultural 
patrimony treasures. Supply of inorganic materials, vol. I, 
„Alexandru Ioan Cuza” University Publishing House, Iaşi, 2008. 
[15]. L. Mureşan, S. Varvara, s.a. - Protection of bronze 
covered with patina by innoxiouus organic substances, 
Electrochimica Acta, vol.52, 2007, pg. 7770-7779. 
[16]. V., Cândea, C. Popa, Iniţiere în ştiinţa metalelor, Editura 
Vega, ISBN 973-951-70-3-0, Bucureşti, 1995 
[17]. A.Vlasa, S. Varvara, L. Mureşan - Electrochemical 
investigation of the influence of two thiadiazole derivatives on the 
patina of an artchaeological bronze artefact using a carbon paste 
electrode, Studia Universitatis BABEŞ – BOLYAI, vol.2, 2007, 
pg. 63 -71. 
[18]. J.D. Chelaru, V.F. Soporan, O. Nemes, T. Koloszi, V. 
Duca - The influence of the enviromental factors on the King 
Matthias Sculptural Group, „International Conference 16th 
Building Services, Mechanical and Building Industry Days”, 2010, 
pg. 265-272. 
[19]. N. Souissi, L. Bousselmi, s.a. - Electrochemical behavior of 
an archaeological bronze alloy in various aqueous media: New 
method for understanding artifacts preservation, Material and 
Corrsion, vol. 54, 2003, pg. 318-325. 
[20]. T. Koloszi, A. Pasat, R. Moraru, O.M. Gui, J.D. Chelaru 
- Digital techniques in reinforcement and conservation of a large 
sculptural group. Case study: the sculptural group of Mathias I in 
Cluj Napoca, Romania, 5th International Conference on 
Interdisciplinarity in Education - ICIE'10, Tallinn, 2010, Estonia. 
 
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Investigation of Corrosion Protection by Chemically Applied Patina on Artistic Bronzes

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Investigation of Corrosion Protection by Chemically Applied Patina on Artistic Bronzes

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