Protection of electrochemical and chemical patina in
aggressive environments containing chlorides
K. Marušić, T. Kosec*, H. Otmačić Ćurković
Faculty of Chemical Engineering and Technology, University of Zagreb, Croatia
*Slovenian National Building and Civil Engineering Institute, Ljubljana, Slovenia
Summary
Numerous cultural and historical artefacts are made of bronze. In humid and
unpolluted air bronze is protected by oxide films, called patina. However, in urban
atmospheres aggressive pollutants concentrate in acid rain which dissolves patina
and the bronze itself. This leads to enhanced deterioration of bronze artefacts.
In protection of historical and cultural artefacts, only methods that do not change the
visual appearance of the artefact itself can be applied. Such methods include
protection of bare metal as well as the patina covering it. One of the possible
methods is the use of corrosion inhibitors. Recent research has shown that imidazole
based inhibitors can be succesfully used to protect patinated bronze.
The aim of this study is to investigate the use of imidazole and benzotriazole type
inhibitors in protection of patinated bronze. The possibility of prolonged service-life
was evaluated for the corrosion inhibitors tested.
Since the studies can not be performed using cultural and hystorical objects, patina
similar to patina naturally formed in marine environments was synthesized
electrochemically. Also, chemical patina was applied to bronze surface. Both patinas
were studied in solutions that simulate agressive marine environment. The protective
properties of the investigated corrosion inhibitors were evaluated on differently
patinated bronze samples.
1
Introduction
Copper is a metal with characteristic light reddish colour, ductile and well recyclable.
This metal exhibits excellent electrical and heat conductivities, high resistance towards corrosion and good mechanical properties. To improve copper properties it is
alloyed, very often with tin. This copper tin alloy is known as bronze [1]. Bronzes are
a very popular material used for sculptures. Sculptures made of bronze are often
covered with a layer of corrosion products with a composition and colour depending
on the composition of the atmosphere they are exposed to [2-15]. This layer can be
formed spontaneously by exposing bronze to atmosphere for a long time, or by accelerated laboratory procedures. Electrochemical procedures are used to produce a
patina similar to one that forms during long time exposure of bronze to its environment [3, 16-19].
Sculptures are often exposed in urban areas close to sea or ocean. Atmospheres in
such areas contain chlorides in addition to substances typical for the urban atmosphere. The presence of chlorides induces corrosion of metals significantly. Thus, additional protection of metals exposed to chloride environments is needed.
1
Corrosion inhibitors are often used for protection of bronze statues, mainly benzotriazole. Since benzotriazole is toxic, lately many inhibitors are studied as possible replacement in protection of bronze [20-24].
2
Experimental
2.1 Preparation of electrodes
The investigations were performed on Cu-6Sn-6Zn bronze which consisted of 88%
by weight of copper, 6% tin and 6% zinc. A chemical and an electrochemical patina
were prepared.
Samples for chemical patination were sectioned from 5 mm plates in the form of
discs of 15 mm diameter and abraded with 800 and 1000-grid SiC paper. A mixture
of 5 % oxalic acid and India pumice powder was used for bright satin finish. Finally,
the samples were cleaned in distilled water and then well dried. A sulfide patina was
achieved on the bronze by brushing the hot surface with a 6 % solution of K 2S.
Test specimens with dimensions of 10 mm × 10 mm × 5 mm were used for preparation of the electrochemical patina. One side of the bronze plate was connected to an
electrical wire and the bronze sheet was embedded in epoxy resin. The bronze electrodes were abraded to a metallographic finish of 2000 grit and degreased with ethanol. The electrodes were then carefully rinsed with bi-distilled water. Six electrodes
were connected in parallel. They were placed in an aerated solution consisting of 0.2
g L-1 NaHCO3 + 0.6 g L−1 NaCl at 30 °C. A nickel wire was used as a counter electrode, and the reference electrode was a saturated calomel electrode. After a stable
open circuit potential (OCP) had been reached, the patina was synthesized under
potential control: 60 s at −0.20 V vs. OCP and 48 h at +0.22 V vs. OCP.
For the investigations involving inhibitors the specimens were prepared by dipping
patinated electrodes in a 3% solution of inhibitor dissolved in ethanol for 24 hours.
The electrodes were carefully rinsed with distilled water afterwards and dried in air.
The investigated inhibitors are 4-methyl-1-p-tolylimidazole (TMI) and benzotriazole
(BTA).
2.1 Electrochemical measurements
Electrochemical measurements on all specimens were performed in a test solution
containing 0.2 g/l Na2SO4 + 0.2 g/l NaHCO3 + 0.2 g/l NaNO3 + 0.2 g/l NaCl. The pH
value of the test solution was adjusted to pH 5 by addition of a small amount of dissolved H2SO4. This solution simulates rainwater in urban environments close to the
sea. Electrochemical impedance measurements were performed after 1, 6 and 24
hours. After 24 hours linear polarization and Tafel extrapolation methods were performed.
Electrochemical impedance spectroscopy was performed at Ec in the frequency
range 100 kHz-5 mHz and with an amplitude of 10 mV. Linear polarization was
measured in the range ± 20 mV from Eoc, using a potential scan rate of 0.1 mV/s, to
determine the value of polarization resistance, Rp. Anodic polarization curves were
then recorded, starting at −0.25 V versus Eoc and increasing up to 1.1 V versus SCE
using a potential scan rate of 1 mV/s. Electrochemical instrumentation consisted of a
PAR EG&G Model 263 potentiostat/galvanostat and frequency response detector
PAR 1025. Carbon rods or platinum plates were used as counter electrode and a
2
saturated calomel electrode (SCE) served as reference electrode. Potentials in the
text refer to the SCE scale.
3
Results and Discussion
The potentiodynamic curves corresponding to chemical and electrochemical patina
untreated and treated with inhibitors are shown in Fig. 1. For both types of patina application of studied inhibitors significantly decreases cathodic current densities. For
electrochemical patina BTA also decreases anodic current densities, while TMI is
efficient only up to 150 mV. Similar behavior was observed for chemical patina. TMI
slows down anodic dissolution of patina and underlying bronze up to the potential of
cca 200 mV, while the break down potential of sample protected by BTA is almost
100 mV higher than that of unprotected sample.
Chemical patina
Electrochemical patina
-2
-1
-2
-3
-3
-4
-2
log j (A cm )
-2
log j (A cm )
-4
-5
-6
Electrochemical patina
No inh
TMI
BTA
-7
-5
-6
-7
-8
Chemical patina
No inh
TMI
BTA
-9
-10
-8
-200
0
200
400
600
800
-11
1000
E (mV)
-400
-200
0
200
400
600
E (mV)
Figure 1: Potentiodynamic curves for different patinas.
Corrosion current density (jcorr) and corrosion potential (Ecorr) determined from polarization curves are presented in Table 1.
Table 1: Electrochemical parameters for different patinas, obtained from potentiodynamic
polarization curves (Ecorr and jcorr) and linear polarization curves (Rp) in simulated urban rain.
Ecorr (mV)
jcorr (μA cm-2)
Rp (kΩ cm2)
untreated
-5
7.42
6.59
Electrochemical
TMI
1
1.55
22.67
patina
BTA
-11
1.30
44.73
untreated
-171
2.93
10.28
Chemical patina
TMI
-134
0.34
158.57
BTA
-155
0.18
205.67
As studied corrosion inhibitors decrease rate of both anodic and cathodic process
this has resulted in significantly lower corrosion current density of the treated samples. Efficient corrosion inhibition may be also observed from polarization resistance
values, determined from linear polarization measurements. In the case of electrochemical patina polarization resistance of the sample treated with TMI is 3 times
higher than that of the untreated sample. BTA increases Rp by a factor of 7. Chemical
3
patina seams to be less reactive than electrochemical patina which can be seen from
higher corrosion current densities and lower polarization resistance. Both corrosion
inhibitors are even more efficient in protection of chemical than electrochemical patina. BTA and TMI increase polarization resistance of chemically patinated bronze by
factor of 15 and 20 respectively.
Electrochemical patina
Chemical patina
7000
6000
6000
No inhibitor
1h
6h
24h
4000
2
4000
-Zimag ( cm )
2
Zimag ( cm )
No inhibitor
5000
No inhibitor
1h
6h
24h
5000
3000
2000
3000
2000
1000
1000
0
0
0
2000
4000
6000
8000
10000
0
2000
4000
2
17500
200
1h
6h
24h
TMI
1h
6h
24h
150
2
2
-Zimag (k cm )
12500
Zimag ( cm )
8000
Zreal ( cm )
TMI
15000
TMI
6000
2
Zreal ( cm )
10000
7500
5000
100
50
2500
0
0
5000
10000
15000
20000
0
25000
0
2
50
100
Zreal ( cm )
250
300
80
100
120
90
BTA
80
1h
6h
24h
25000
20000
70
BTA
1h
6h
24h
60
2
-Zimag (k cm )
2
Zimag ( cm )
200
2
30000
BTA
150
Zreal (k cm )
15000
10000
50
40
30
20
5000
10
0
0
10000
20000
30000
40000
0
2
0
Zreal ( cm )
20
40
60
2
Zreal (k cm )
Figure 2: Nyquist diagrams for different patinas.
In order to follow electrochemical behavior of treated and untreated samples in time,
electrochemical impedance measurements were performed after different immersion
times as presented in Figure 2.EIS measurements performed on electrochemical patina show that its impedance decreases in time, which suggests that obtained chloride
4
patina is quite reactive and doesn’t stabilize in time. On contrary impedance of chemical patina increases in time which indicates its stabilization. As previously observed
from polarization measurements, treatment with inhibitors significantly slows down
the corrosion process on both types of patina. However protection of electrochemical
patina slightly weakens in time while chemical patina treated with corrosion inhibitors
improves in time.
4
Conclusions
In this work protection of two types of bronze patina by corrosion inhibitors was examined in the solution simulating urban rainwater in the vicinity of sea. Studies have
shown that electrochemically obtained chloride patina is more reactive than chemically obtained sulphide patina. However both kinds of patina can be protected by examined corrosion inhibitors: BTA and TMI. These inhibitors decrease both cathodic
and anodic corrosion reactions but TMI is efficient only at lower anodic overpotentials. In the case of chemical patina degree of protection increases in time, while that
of electrochemical patina slightly decreases.
5
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