Le prochain séminaire de l’équipe MCMC de l’ICMPE aura lieu
Le mardi 26 juin 2011 à 11h
Salle de réunion du 1er étage du bâtiment F
Electrochemical synthesis and characterization of patina on CuxSn bronzes
(x14) in sulphate medium.
Par Aline Petitmangin
A.T.E.R., Institut de Chimie et des Matériaux Paris-Est,
ICMPE, UMR 7182 CNRS-UPEC,
2 à 8 rue Henry Dunant, F-94320 Thiais, France
Formation and growth mechanisms of oxides films on alpha-copper-tin bronzes are still in
discussion. In the literature two different patterns of corrosion are exposed. The first one claims
that the bronze corrosion is similar to that of copper whereas the second one asserts that the
copper-tin alloys oxidation lead to a double layer structure. The inner layer is enriched in tin
consequently to copper selective dissolution. The outer layer is essentially composed of copper
oxides.
The present study deals with the chemical-physical and electrochemical characterizations
of artificial patinas, thin layer of corrosion products electrochemically formed at the surface of
high purity home-made copper-tin alloys. A bronze containing from 3.5 to 14 wt% of tin is
oxidized in a 10-2 mol.L-1 oxygen-free sulphate solution, buffered at pH = 6.8. The synthesized
species are cathodically reduced. As the successive steps of the potential vs. time curves are
characteristic of the electrochemical reactions occurring at the electrode surface, they can be
assigned to different reference oxides. Among them, the reduction step, taking place towards
0.3 V/SCE, corresponds to the reduction of the copper species.
For different tin contents in bronzes and for different oxidation step durations, applying
the Faraday law allows to deduce the quantity of copper oxides present in the patinas from the
length of the copper products reduction step. The results are reported in Figure 1. They illustrate
that, whatever the tin content, the copper oxidation kinetic follows a k.t1/3 law. The longer the
oxidation time is, the less rapidly the patina grows. This behaviour is indeed characteristic of a
protective layer growth. After each oxidation the corrosive solution is analysed by ICP-AES in
order to determine the amount of Cu2+ originated from dissolution. Similarly to copper oxidation,
the copper dissolution kinetic follows the same law.
The determination of speed constant k values for different tin amounts, namely 3.5, 7, 11
and 14 wt%, points out the tin content influence on the patina protective effect. The higher the tin
content is, the lower the speed constant. This result is in total agreement with the beneficial effect
of tin on bronze corrosion protection.
In order to understand the protective role of tin, depth profiles of the corrosion products
layers are studied by X-Ray Photoelectron Spectroscopy (XPS). For each sample, different Cu
and Sn spectra are obtained by abrading progressively the oxidized surface. The deconvoluted
XPS data lead to depth profile as presented in Figure 3, that clearly speak in favour of an
“CuxSn/SnO2/Cu2O” layered structure, schematized in Figure 4, similar to that reported in the
literature.
-8
3 10
-8
2 10
-8
1 10
-8
-1/3
4 10
6.4
-2
-8
k / 10 mol.cm .min
Cu Sn
3.5
Cu Sn
-9
-2
Copper oxides amount / mol.cm
5 10
7
Cu Sn
11
0
0
6
5.6
Cu Sn
14
50
100
150
200
250
300
350
5.2
400
2
6
8
10
12
14
tin content / wt%
oxidation time / min
Figure 1: Influence of tin content on the copper oxide
1
4
SnO
2
Figure 2: Influence of tin content on the speed constant k
Cu
Intensity / a.u.
0.8
0.6
0.4
Sn
0.2
0
Cu O
2
0
20
40
60
80
100
120
Abrading time / min
Figure 3: Depth profile of a Cu11Sn sample oxidized
during 70 minutes at -0.02 V/SCE
Figure 4: Layered structure schematic representation
of the patina
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