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Corrosion Science 51 (2009) 159–170
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Corrosion Science
journal homepage: www.elsevier .com/locate /corsc i
The atmospheric corrosion of quaternary bronzes: The leaching action of acid rain
E. Bernardi a, C. Chiavari b,*, B. Lenza b, C. Martini b, L. Morselli a, F. Ospitali c, L. Robbiola d
a Dipartimento di Chimica Industriale e dei Materiali, Università di Bologna – Via Risorgimento 4, 40136 Bologna, Italyb Dipartimento di Scienze dei Metalli, Elettrochimica e Tecniche Chimiche, Università di Bologna – Via Risorgimento 4, 40136 Bologna, Italyc Dipartimento di Chimica Fisica ed Inorganica, Università di Bologna – Via Risorgimento 4, 40136, Bologna, Italyd Service des Microscopies Electroniques, LECA, UMR 7575 CNRS, ENSCP, Université Paris 6, 11 rue P&M Curie, F-75231 Paris Cedex 05, France
a r t i c l e i n f o
Article history:Received 17 July 2008Accepted 8 October 2008Available online 1 November 2008
Keywords:A. AlloyA. CopperB. SEMB. Raman spectroscopyC. Atmospheric corrosion
0010-938X/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.corsci.2008.10.008
* Corresponding author. Tel.: +39 0512093462; fax:E-mail address: [email protected] (C. Chiav
a b s t r a c t
The effect of leaching rain on the corrosion behaviour of bronze UNSC83600 was investigated as to theinfluence of alloying elements (Cu, Sn, Zn, Pb) through dropping tests simulating a severe runoff conditionwith a solution reproducing natural acid rain. Corrosion was followed with time monitoring both samplesand leaching solutions (up to 30 days) by SEM, EDS, Raman spectroscopy, XRD, AAS. The bronze patinabehaves as a porous layer enriched in stable tin compounds allowing uniform dissolution of Cu, Znand partly of Pb. Laboratory results are in good agreement with field studies of outdoor bronzes inunsheltered condition.
� 2008 Elsevier Ltd. All rights reserved.
1. Introduction
Outdoor bronzes naturally form corrosion layers called patina,partially protecting the metallic substrate. During the last century,the phenomenon of acid rain induced a strong change in the natureand properties of copper based patinas [1–7]. In urban atmosphere,these patinas are unstable and partially leached away by rainwa-ter, as demonstrated by the green streaking on bronze and coppermonuments in metropolitan areas [2,3,8]. The action of the envi-ronment on outdoor bronzes has been the subject of several stud-ies. However, the corrosion mechanism is still not completelyclarified [1,2,7,9–12]. The corrosion mechanism of bronze has beenusually assimilated to that of copper, even if the role of other alloy-ing elements has been increasingly considered [7,9,10,13–17]. Inparticular, several studies focused on the role of tin and its insolu-ble oxides in the mechanism of formation of patinas, clearly indi-cating that alloys and pure metals behave very differently whenexposed to the environment [13–21]. Furthermore, the influenceof the geometry of exposure on the corrosion behaviour has beentaken into account [6,14,22,23]. In particular, the cyclic action ofacid rain was found responsible for the different patinas formedin ‘‘sheltered” or ‘‘unsheltered” areas on a given bronze monument.Patinas grown on sheltered and on unsheltered regions stronglydiffer in electrochemical, morphological and compositional fea-tures [6,7,14,15]. Actually, unsheltered areas undergo the leachingaction of the rain (runoff). For example, in unsheltered areas on
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+39 0512093467.ari).
Rodin’s bronzes exposed in outdoor conditions, a significant lossof thickness has been observed sometimes reaching several hun-dreds of lm [6].
In the last decade, the action of runoff conditions on metals hasbeen extensively studied, through laboratory and field studies[20–27], with particular attention to those metals (copper andzinc) widely used for roofing. Concerning bronze alloys, the metalrunoff also appears to play a determinant role as commonly ob-served in the bronze statuary exposed to the direct action of therain. Accordingly, the characterisation of patinas, i.e. the supportwhere to apply corrosion inhibitors or protective coatings, be-comes of primary importance [28,29]: a clear comprehension ofthe mechanism of formation of patinas on outdoor bronzes is nec-essary in order to define a proper conservation strategy. Thus, it isof utmost importance to complete our understanding of the influ-ence of acid rain on the corrosion of quaternary bronze in unshel-tered condition.
Within this context, the present work focused on the study ofthe cause-effect correlation between the corrosion evolution andrunoff condition, as well as on the influence of each alloying ele-ment on corrosion behaviour.
To this aim, a multidisciplinary work, inclusive of several mon-itoring techniques, has been set up. This work has been carried outwithin a research activity on atmospheric corrosion of the quater-nary bronze alloy UNS C83600. In the first steps of this research,this alloy was submitted to cyclic wet–dry tests, in both naturaland synthetic rain solution [15,16]. The comparison of results ob-tained with the different weathering solutions gave useful indica-tions for the formulation of artificial rains. In order to complete the
Table 1Characteristics of the weathering solution (artificial rain).
Artificial rain
Conductivity (20 �C) 37.92 ± 0.09 lS/cmpH 4.27 ± 0.01Cl� 1.24 ± 0.04 mg/lN � NO�3 1.02 ± 0.02 mg/lSO2�
4 1.94 ± 0.02 mg/lC2O2
4� <d.l.HCOO� 0.04 ± 0.02 mg/lCH3COO� 0.23 ± 0.02 mg/lN � NHþ4 0.86 ± 0.03 mg/lNa+ 1.75 ± 0.03 mg/lCa2+ 0.80 ± 0.02 mg/l
Fig. 1. (a) Rain dropping device and (b) comparison of the bronze surface at 4 h and30 days of exposure.
160 E. Bernardi et al. / Corrosion Science 51 (2009) 159–170
picture of corrosion, further investigations on the same alloy havebeen made by exposure tests reproducing a cyclic exposition tostagnant deposition (i.e. simulating sheltered areas) and theresults will be compared in a subsequent publication to those pre-sented here.
In this work, a specific device has been designed in order toreproduce the leaching action of the rain and to investigate theinfluence of the alloying elements on the alloy dissolution. The pre-cipitation runoff was then simulated through a dropping devicewhere a single drop of artificial rain was periodically guttered onthe specimens. The evolution of the corrosion process with increas-ing time of exposure was determined monitoring both the amountof dissolved metal ions by AAS (from the collected leaching water)and the characteristics of the bronze samples (surface and cross-section by SEM/EDS, Raman spectroscopy and XRD). The leachingrate of each alloying element was then calculated. As referencematerial, the quaternary bronze (UNS C83600) was selected asone of the typical alloys used for artistic casting, while the refer-ence leaching solution was formulated starting from a real rain col-lected in the urban area of Bologna (Italy). The results fromlaboratory studies were compared to the results on real patinasof historical outdoor bronzes.
2. Experimental
2.1. Bronze
The alloy is a quaternary bronze (G85 or ‘‘85 metal”, UNSC83600) often used for artistic casting. Its composition, determinedby flame atomic absorption spectrophotometry (FAAS), is 88.76 Cu,4.4 Sn, 3.9 Pb and 2.4 Zn (in wt%) also containing minor and traceselements <1 wt% (Ni and Fe mainly) which will not be consideredin this work. Bronze sheets, sized 2.5 � 5 � 0.5 cm3, were cut fromsand cast alloy, surface polished by abrasive papers up to 1000 grit,degreased by acetone and rinsed with distilled water. The as-castalloy microstructure is identical to the one detailed in [15]. Beforeweathering tests, each specimen was weighed and the surfacequality was inspected by optical microscope (OM) and scanningelectron microscope (SEM/EDS).
2.2. Artificial rain
The leaching solution is an artificial rain. Its composition isbased on the typical rainfall characteristics of a mixture of naturalrain with pH < 4.5, weekly collected during the winter months in amonitoring station of wet and dry atmospheric depositions, inBologna (Italy). The main steps of the experimental procedureadopted for deposition sampling and analysis are reported in pre-vious works [15,30,31]. In order to obtain the same pH (pH 4.3) andthe same concentration of the major inorganic and organic ions(NHþ4 , NO�3 , Cl�, SO2�
4 , HCOO�, CH3COO�) of the natural mixture,the artificial rain was synthesised with analytical-reagents and ul-tra-pure deionized water (18 MX). The composition of the artificialrain used for the laboratory rain simulation is given in Table 1. Ithas very similar characteristics to the one used in [25] for simulat-ing continuous rain events.
2.3. Weathering method
The dropping device specifically designed for the experiment, isillustrated in Fig. 1.
The synthetic rain is periodically guttered, in single drops, onthe specimens mounted on a Plexiglas fixture inclined 45�, witha dropping rate of 57 cm3/h and a drop fall height of 2.5 cm. Theartificial solution, after leaching the bronze surface, is periodicallycollected and analysed for Cu, Sn, Pb, Zn by atomic absorption
spectrometry – Perkin Elmer 2100 (Zn by F-AAS, the othersthrough GF-AAS). Immediately after collection the leaching solu-tion is acidified at pH < 2 with HNO3 65% suprapur, in order toavoid absorption of metals on the container surfaces and dissolvepossible metal complexes [31].
The tests were performed at 20 �C for a maximum period of 30days. Each test was carried out on the same sample and duplicateexperiments have been performed.
The development of the corrosion process was followed bymonitoring both the solution (every hour during the first two days,then every 3 days) and the formation of corrosion products (at 1, 7,15, 24, 48 h and then at 7, 14 and 30 days). It must be noted that,during the first two days, the solution was hourly collected duringthe day while during the night a cumulative fraction was sampled.
E. Bernardi et al. / Corrosion Science 51 (2009) 159–170 161
2.4. Characterisation of corroded bronze specimens
The exposed specimens have been characterised by surface,depth loss and mass variation measurements. Transversal cross-sections were also performed and conventionally prepared as formetallographic specimens.
Surface examination was carried out using optical and scanningelectron microscopes. The elemental and phase composition of cor-rosion products have been studied by a combined energy disper-sive spectrometer (EDS) and Raman spectrometer (RS) system,integrated in a variable pressure scanning electron microscope(VP-SEM) Zeiss EP EVO 50 with secondary and back scatteredelectrons detectors. The images have been collected in variablepressure mode (80 Pa). The EDS X-ray detector is an Oxford Instru-ments INCA ENERGY 350 [z > 4 (Be)]. The accelerating voltage usedfor analyses was 20 keV. The system is also equipped with a Reni-shaw Raman SCA (Structural and chemical analyser for SEM). TheSCA probe is inserted by retractable optics in the SEM chamber,between the sample and the SEM column. The electron beam andthe laser beam (k = 514.5 nm) are confocal, allowing to acquiresequential SEM images, EDS and Raman spectra on the same zonewith micrometric precision [laser spot size < 2 lm FWHM]. TheSCA probe is linked by optical fibre to the Raman spectrometer.
Complementary Raman analyses have been also performed outof SEM, with a Renishaw Raman Invia spectrometer configuredwith a Leica DMLM microscope, an Ar+ laser (514.5 nm) wasemployed as exciting source. In a typical Raman-SCA experiment,the radiation power was set at 0.3 mW to avoid structuralmodification of the sample due to overheating. The time of eachscan was 30 s, and the total number of scans ranged from 2 to 8.In a typical Raman Invia experiment, the laser power wasmaintained at 0.3 mW, whereas the time of each scan was 10 sand the total number of scans was 4.
The phase composition has been also checked at high exposuretimes by X-ray diffractometry (XRD) in the Bragg-Brentano geom-etry, using Cu Ka radiation (k = 0.154060 nm).
The mass measurements were carried out by a digital balanceKERN AGB210_4 with a repeatability of ±0.1 mg. The mass varia-tion is obtained by the difference between the initial weight ofthe sample and the weight of the corroded sample at differenttimes of exposure. At the end of the test (30 days), each specimenwas weighed also after removing the corrosion layer (mass lossmeasurement). The removal of corrosion layer was performed bylocalised pickling: 30 cm3 of deaerated solution of H2SO4 10% v/vwere guttered on the corroded area that was subsequently washedby distilled water. The pickling was performed also on an unex-posed surface in order to correct the results obtained from exposedsamples. Due to the fact that the corroded surface is only the arealeached by the droplets, the uncorroded surface was protected byan acid resistant adhesive film of PET. The area of the corroded sur-face Acor has been calculated tracing the figure of this area on apaper of known density (0.0069 ± 0.0001 g/cm2), then weighingthe papery form. The procedure results are within a relative preci-sion of 2%. A slight increase of the corroded area with time hasbeen observed up to 48 hours, so the values of Acor were measuredat each exposure time.
At the end of the test, before and after pickling, several rough-ness profiles along the leached areas were performed by stylusprofilometry (tip radius: 5 lm, precision ± 0.2 lm). The averageprofile determined after pickling gives information on the thick-ness d (depth loss) of total volume involved in corrosion process.
An attempt to determine the metal content in the patina(mM,cor) has been performed from analysis of pickling solution afterdissolution of the corrosion layer. Unfortunately, pickling solutionsusually applied for copper alloys [9,15] were not satisfactorilyeffective. Actually, tin compounds were not completely dissolved
by pickling, as also reported in [32], where EDS analyses revealeda very thin layer rich in tin still remaining on the pickled surface(pickling solution according to ISO/DIS 8407.3). In our case, thearea corresponding to this layer is the whole corroded surface:therefore, due to the significant undissolved mass fraction whichcannot be taken into account, the mass loss measurements areaffected by an important error (calculated as �20% underestimate).In fact, AAS analyses of the pickling solution suggest a generalunderestimation of the amounts of cations of the patina that isdue to their different reactivities to pickling solution. Accordingly,this procedure was not effective for determining the correct frac-tion of each metal in the patina (mM,cor) and in particular the tincontent mSn,cor. Other pickling solutions are currently beinginvestigated.
Therefore, in this work, mM,cor has been calculated from thedepth loss measurement (d) and not from mass loss measurement.Actually, the layer insoluble to the pickling remains very thin (esti-mated as <1 lm) and the resulting error in the depth loss measure-ment is minimized.
2.5. Calculation of the metal fraction in the patina (mM,cor)
The total mass loss at the end of the test (after 30 days) wasdetermined as follows:
mtot ¼ qdAcor ð1Þ
where Acor is the corroded surface (cm2), d is the depth loss mea-sured by roughness profiles (cm) and q is the density of the bronze(q = 8954 ± 193 mg cm�3).
The total amount of metal M (M = Cu, Sn, Pb, Zn) transformedfrom its metallic state to metal cation during bronze exposure,mM (g), is derived from the total mass loss mtot multiplied by themass fraction of M in the bronze, vM (according to the compositiongiven in Section 2.1):
mM ¼ mtotvM ð2Þ
As the AAS analyses of the leached solution give information onthe amount of metal released in the environment mM,sol (g), theamount of oxidised metal in the patina mM,cor (g) is derived asfollows:
mM;cor ¼ mM �mM;sol ð3Þ
2.6. Calculation of the corrosion rate (mcor)
The corrosion rate vcor of bronze exposed during period t (hereexpressed in lm year�1) is defined as follows:
mm ¼dt
ð4Þ
where d is the depth loss (lm) and t is the time of exposure (year).
2.7. Calculation of the dissolution factor
In order to quantify the general tendency of each alloyingelement M to dissolve in the corrosive environment (and not toremain in the patina), a dissolution factor fM was defined by anal-ogy to [18,19,24] as
FM ¼mM;sol
mMð5Þ
where mM,sol (g) is the mass of metal cations dissolved in the leach-ing solution and mM (g) is the total amount of metal M transformedfrom its metallic state to metal cation, as previously defined.
The dissolution factor fM will range from 0 to 1, according to thetendency of metal M, initially present in the alloy, to be respectively
-16-14-12-10-8-6-4-202
0 2 4 6 8 10 12 14
distance, mm
heig
ht ,
µm
Fig. 2. Roughness profile of the sample at 30 days of exposure, carried out in the central area of the leached surface. The non corroded area has an height equal to zero.
162 E. Bernardi et al. / Corrosion Science 51 (2009) 159–170
fully transformed into corrosion products or totally dissolved in theenvironment.
2.8. Calculation of the leaching rate (Lr)
The leaching solution periodically collected was analysed bymeans of AAS. The concentration of metal cations mM,sol of eachsampling was thus determined as specified in Section 2.3. Hourlyleaching rate was then calculated in lg cm�2 h�1 as follows:
Lr ¼ mM;sol
Acor:tð6Þ
where, mM,sol (lg) is the amount of metal cations M dissolved in theleaching solution during each n-th hour (t=1 h), and Acor is the cor-responding corroded area (cm2).
Fig. 3. (a) SEM and EDS maps of the surface of the samples at 30 days of exposure and relSn (nano-crystals of SnOx(OH)y).
3. Results and discussion
The results from surface examination of the corroded specimenswill be firstly shown. Afterwards, the results from analyses of theleaching solutions at different times will be presented anddiscussed.
3.1. Surface characterisation of bronze specimens
Visual investigation of the specimen with time reveals that therepetitive impingement of the drops progressively induces surfacecorrosion, which extends down forming a grey whitish area asshown in Fig. 1b. For longer time (after several days), on the lowerpart of the surface where leaching rainwater drop stagnates, a thinbrownish deposit is also locally observed. After 30 days of dropping
ative Raman spectra of (b) lower areas rich in Cu (Cu2O) and (c) areas in relief rich in
Fig. 4. SEM and X-ray maps of bronze specimen at 30 days of exposure (cross-section).
E. Bernardi et al. / Corrosion Science 51 (2009) 159–170 163
exposure, the bronze exhibits significant corrosion. As confirmedby roughness profile measurements (Fig. 2), the surface leachedby the artificial rain has an average depth loss of �13 lm, withincreased surface roughness.
As shown in Fig. 3a, this surface roughness is related to a topo-graphical inhomogeneity of the corrosion layer linked to the as-cast microstructure of the bronze. It is observed from the X-raymaps (Fig. 3a) that the areas in relief are rich in tin products whilethe others are mainly Cu-containing species. Raman spectra col-lected in these different areas reveal furthermore that, in tin-richareas, the compounds are poorly crystallised (Fig. 3c) while, forcopper-rich area, cuprous oxide is present (Fig. 3b).
Examination of the cross-section of the samples exposed 30days was also performed. SEM image and X-ray maps are reportedin Fig. 4, providing complementary information both on the bulkalloy (the original microstructure) and on the adhering corrosionlayer.
The bulk bronze shows a typical cored dendritic microstructurewith a-Cu matrix (Cu–(Sn,Zn) solid solution). The outer rims ofdendrites and the interdendritic spaces are Sn-enriched becauseof remarkable microsegregation. Pb globules (with a diameter of10–20 lm), insoluble in the a-Cu matrix, are present in the inter-dendritic spaces and limited amounts of Zn sulphides are observed(S map not shown in Fig. 4), as also confirmed by Raman analyses.
The metal/patina interface reveals the influence of the micro-structure on the proceeding of corrosion: the corrosive attackdevelops preferentially between the copper-rich core and the tin-rich areas of dendrites, where localised pits form, then involvesthe whole Sn-rich area. The resulting patina is poorly regular andwith a thickness of about (3–5 lm), consisting of a thin oxidisedcopper layer over the copper-rich core, whereas thicker islets, richin Sn and O, form over Sn-rich interdendritic areas. Thus, in thecorrosion layer, tin species are mainly formed over the externalpart of the dendrites, while the centre of dendrites is richer in cop-per products.
So, the continuous dropping of synthetic acid rain on the bronzesurface for 30 days makes the leaching prevail on the growth ofcorrosion products. Nevertheless, an adhering patina layer (fewlm) is present and characterised by a relative enrichment ofSn-containing species. Under a continuous water flux exposure, acorrosion layer is always observed. This layer is unable to avoidthe dissolution of the other metal cations. Accordingly, duringthe entire test, a progressive decrease of mass (up to a loss of8 mg cm�2) is recorded. This is evident in Fig. 5, where the massvariations per unit of corroded area (mg cm�2) as a function ofexposure time are reported.
The composition of the corrosion products in the patina evolveswith time. Table 2 reports the concentrations of elements mea-sured by EDS on the corroded surface at different weathering timeas well as the metal/Sn ratio. The results confirm a relative super-ficial enrichment of Sn linked to Cu and Zn depletion.
Results of Table 2 also indicate that with increasing time the Cu/Sn and Zn/Sn ratios decrease. Besides, Pb/Sn is initially very highbut after an important decrease it fluctuates, probably related tothe fact that Pb are not miscible in the bronze matrix and not uni-formly distributed onto the surface. It must be noticed that, forshorter times of exposure, the layer thickness is thin and EDS anal-yses are probably influenced by the underlying matrixcomposition.
At the same time, besides EDS spectra, also Raman spectra wererecorded in SEM on the same areas. The detected products arelisted in Table 3, which also includes XRD results on samples ex-posed for 30 days.
As far as Cu is concerned, the first crystallised product identifiedon the surface is cuprous oxide at 7 h of weathering. Cuprite is themain corrosion product detected during the whole exposure. Only
at 30 days weak reflections attributed to nantokite (CuCl) were de-tected by XRD. On the contrary (Table 3), Pb cations form differentcorrosion products with time. In particular, oxide and basic car-bonates develop in the first stages, progressively accompanied bysulphates, more insoluble so more stable under the dropping expo-sure (well crystallised anglesite is detected only at 30 days).
It has to be noted that in the first 30 days of exposure, the cop-per sulphates, typically found on outdoor patina, have not beendetected. This could be due to the preferential formation of lead
Log (time, hours)
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
00.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
Mas
s va
riatio
n, m
g cm
-2
Fig. 5. Mass variations (mg cm�2) as a function of exposure time.
Table 2Weight percent of the elements and metal/Sn ratios on the corroded surface at different weathering time, measured by EDS analyses.
Weight (%)
t (h) Cu Sn Pb Zn C O Ni Si Cl P S N Cu/Sn Pb/Sn Zn/Sn
0 88.7 4.4 3.9 2.4 20.2 0.89 0.551 83.9 4.3 1.9 2.6 3.2 2.5 0.7 _ _ _ _ _ 19.5 0.45 0.614 86.7 4.9 1.5 2.9 2.8 0.9 0.8 _ _ _ _ _ 17.7 0.31 0.597 84.5 4.7 0.8 2.7 3.4 3.4 0.8 _ _ _ _ _ 18.0 0.17 0.57
15 76.0 5.7 0.7 2.6 4.4 9.7 0.7 _ _ 0.1 0.1 _ 13.3 0.12 0.4624 69.7 6.7 1.3 2.3 5.2 14.1 0.7 _ _ _ _ _ 10.4 0.19 0.3448 62.9 7.9 1.5 2.1 8.7 15.9 0.6 0.2 0.1 0.2 0.2 _ 7.96 0.19 0.27
169 56.1 11 2.3 1.9 5.6 21.7 0.5 _ 0.3 0.2 0.4 _ 5.10 0.21 0.17336 41.9 25.2 4.5 1.3 _ 25.7 0.3 0.1 _ 0.3 0.4 _ 1.66 0.18 0.05720 17.4 41.4 3.6 0.9 2.3 34.0 _ 0.2 _ 0.4 0.4 2.4 0.42 0.09 0.02
164 E. Bernardi et al. / Corrosion Science 51 (2009) 159–170
sulphates, which are thermodynamically more stable with respectto copper sulphates considered by different authors [5,23] as pre-cursor of basic copper sulphate (DGf (kJ/mol): CuSO4 = �622.2,PbSO4 = �813.0 [33]). Furthermore, the brochantite stability do-main in the Pourbaix E-pH diagram [3] shows that this copperhydroxysulphate is not stable at the conditions of pH and [SO2�
4 ]of the rain used for the experiment, so the most soluble phasesof the patina are removed by runoff. FitzGerald et al. also pointedout that the production of brochantite occurs in stagnant waterfilm after the rain and is enhanced by its evaporation. Therefore,continuous rain events, as in this case, hinder the formation of bro-chantite on copper/copper alloys surfaces [34]. This is also inagreement with Odnevall Wallinder et al. [22], in a study of runoff
Table 3Corrosion products detected on the surface at different weathering time, through EDS, Ra
Cuprite Cu2O Nantokite CuCl
1 h _ _4 h _ _7 h X (thin film) _15 h X (variable crystallinity) _24 h X _48 h X _7 days(169 h) X _14 days (336 h) X _30 days (720 h) X X
phenomena occurring on copper exposed in urban atmosphere,who observed that the formation of posnjakite (Cu4SO4(OH)6 �H2O), precursor of brochantite, takes place after a month in shel-tered copper and after more than one year in unsheltered copper.Actually, the chemical reactions involved in the exposure to con-tinuous rain dropping, take place in a time scale smaller than thecondition of equilibrium requires [22].
As far as Sn is concerned, even if EDS spectra detected anincreasing presence of tin, always correlated with oxygen elementsince the first hours (Table 3), Raman analyses show nanocrystal-line tin products only since 48 h. Fig. 6a shows a representativespectrum with a broad peak centred at 578 cm�1 (see alsoFig. 3c) which can be related to tin species as discussed hereafter.
man and XRD analyses.
SnOx PbO Lead sulphates Hydrocerussite
_ _ _ __ _ _ __ X _ __ X _ __ X _ X (traces)X (amorphous) X XX (amorphous) X X XX (amorphous) X X XSnO2 X PbSO4 _
Fig. 6. Raman spectra of (a) a whitish zone where nano-crystals of SnOx(OH)y arepresent and (b) a well-formed crystal of cassiterite (after 30 days of exposure).
E. Bernardi et al. / Corrosion Science 51 (2009) 159–170 165
Only at 30 days and in only one spectrum, crystallised cassiteritehas been detected (Fig. 6b). No Sn products have been identifiedby XRD analyses.
Concerning the detection of tin products, the Raman spectrumof microcrystalline tin dioxyde is characterised by bands at 476(w), 638 (s), 782 (w) cm�1, corresponding, respectively, to Eg, A1g,B2g Raman-active vibration modes (in Fig. 6b only the more intenseA1g band is plainly visible). Nevertheless, for nanometric crystals,the surface atoms correspond to a not-negligible fraction of the to-tal atoms, giving rise to important changes in the Raman spectrum,which could explain results in Fig. 6a. Several Raman works reportthe relationship between the size and the spectral changes innanocrystalline SnO2 [35–37]. A first common interpretation[38–39] associates the reduction of particles size (from micromet-ric to nanometric scale) with a broadening, an intensity decreasingand a red-shifting (downshifting) of Raman bands, due to a relax-ation of the selection rules. However, Abello et al. [40], supportedalso by a recent work [41], suggest that these changes are due tothe reduction of volume modes (responsible of A1g, Eg, B2g vibrationmodes) and to the increase of surface modes (567, 589 cm�1)related to the decrease of the crystals size. The surface modes are
purely observable linked to a size-particle effect, and the resultingspectrum may express a single broad band at �580 cm�1. Instead,Sharygin and Vovk [42] suggest that the wide band at�580 cm�1 isnot only due to surface modes, but also to the presence of tin oxi-hydroxide {SnO2}Sn(OH)4.
Therefore, the Raman spectra collected here are more probablycorresponding to nano-crystals of tin oxide and/or hydroxide.
Finally, it must be pointed out that tin products are mainly con-centrated in the contact area of the drop with the alloy surfacewhere the leaching effect is stronger, whereas at the bottom ofthe specimens where the drop stagnates, tin products are coveredby lead and copper compounds.
3.2. Leaching solutions
3.2.1. Accumulated leached metalsThe analyses of the metallic elements dissolved in the leaching
solutions give complementary data to the EDS analyses of the sur-faces, allowing a more complete picture of the mechanism of thealloy dissolution. First of all, analysis reveals that the tin concentra-tion is always under the detection limit (<1.2 lg/l). Thus, Sn be-haves differently from other metallic alloying elements: it is notleached by the rain in significant quantities, according to previousresults [20,21,27].
For the other elements, the accumulated amounts of the lea-ched metals (lg cm�2) as a function of the exposure period are re-ported in Fig. 7.
As shown in Fig. 7, all metal alloying elements (Cu, Zn, Pb) followa linear trend increasing with time. A linear behaviour with highcorrelation coefficients (R2) but different slopes (Cu >> Pb > Zn) isconfirmed by linear regression analysis. However, the slope of Cuand Pb curves for long-term exposure slightly differs than that forshort-term exposure (increasing for Cu and decreasing for Pb),while for Zn a real linear behaviour is evidenced. This could be re-lated to the fact that Zn dissolves without forming deposit on thebronze surface in opposition to Cu and Pb. Specifically, up to 5 hof dropping, Pb is the most dissolved element in the solution, evenif it has a low alloy content (3.9 wt%). From now onwards, Cu be-comes the most concentrated element in the leaching solution. Atthe same time, the first corrosion products (Cu2O and PbO, Table3) are detected on the surface in the direct dropping area.
Considering the total time of exposure, the global linear regres-sion curve for Cu runoff amount as a function of time is:
Curunoff ¼ 309T � 72ðR2 ¼ 0:998Þ ð7Þ
with Curunoff in lg cm�2, t in days.We can estimate a factor of acceleration of this severe labora-
tory test by comparing with analogous curves obtained in realcases. Considering both the linear regression curves and the Curunoff in the bronze Cu–3Sn–9Zn (Curunoff = 136 lg cm�2 afterone year of exposure) obtained by Sougrati et al. [20], it is possibleto estimate how many days of rain dropping in this system are nec-essary to obtain the same amount of Cu released in one year of nat-ural exposure. Therefore, the estimated acceleration factor of thistest is 1.5 years/day. Accordingly, 30 days of exposure in our test-ing conditions approximately corresponds to �45 years of naturalexposure.
In this dropping test, the bronze surface suffers a continuousrain event, i.e. a permanent chemical–mechanical action. Hence,the accumulated leached metal (Curunoff) is very high by comparingwith real data, due to the strongly accelerated conditions of thetest. In real conditions, the time of wetness is the sum of the wetperiods alternated to the dry periods, where the patina has timeto grow before being partially removed by the leaching of the fol-lowing rain event.
y = 11.3 x - 14.3R2 = 0.999
y = 1.10 x + 39.1R2 = 0.974
y = 0.421 x + 1.16R2 = 0.999
0.0E+00
5.0E+01
1.0E+02
1.5E+02
2.0E+02
2.5E+02
3.0E+02
3.5E+02
4.0E+02
4.5E+02
5.0E+02
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48
time, hours
Acc
umul
ated
leac
hed
met
als,
µg
cm-2
Cu Zn Pb
y = 319.2 x - 282R2 = 0.997
y = 15.4 x + 93.9R2 = 0.983
y = 10.2 x - 6.49R2 = 0.998
0.0E+00
1.0E+03
2.0E+03
3.0E+03
4.0E+03
5.0E+03
6.0E+03
7.0E+03
8.0E+03
9.0E+03
1.0E+04
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
time, days
Acc
umul
ated
leac
hed
met
als,
µ µg
cm-2
Cu Zn Pb
Fig. 7. Accumulated values (lg cm�2) of the leached metals as a function of exposure time: (a) short-term test and (b) long-term test.
166 E. Bernardi et al. / Corrosion Science 51 (2009) 159–170
3.2.2. Leaching rate LrIn addition to the accumulated leached metals determination,
the hourly leaching rate Lr (lg h�1 cm�2) was determined accord-ing to Eq. (6), by analysing the metal cation contents released inthe rain hourly collected in the first 2 days, as reported in Section2.3. The evolution of Lr with exposure time is reported on Fig. 8.
During the first hour of ageing, Pb is the alloying element withthe highest leaching rate (33 lg h�1 cm�2). From the very nexthour, its Lr rapidly decreases down to a value of 1 lg h�1 cm�2 at48 h. Successively, it stabilises around a value of �0.6 lg h�1 cm�2.
In regards to Cu and Zn, in the first hour the leaching rate isnoticeably lower than in the case of Pb (�4 lg h�1 cm�2 for Cuand 1.6 for Zn). For Cu, Lr rapidly increases during the first fewhours, reaching a plateau with a nearly constant value of�12 lg h�1 cm�2 until the end of the test. For Zn, a nearly constanthourly leaching rate is observed ranging between �1.6 lg h�1 cm�2 and �0.5 lg h�1 cm�2 after 30 days.
According to our previous results, evidencing a discontinuouscorrosion layer, the relatively constant hourly leaching rates sug-gest that a constant fraction of patina formed and dissolved during
the exposure (as also suggested by [20]). The amount of oxidisedalloy is compensated by the cation dissolution due to leachingaction. As a matter of fact, this implies that a stationary regimeof formation/dissolution of the corrosion layer is reached afterabout one day under this exposure condition.
Even if Cu displays the highest leaching rate, it is evidenced thatPb and Zn suffer a preferential corrosion by comparison to Cu. Asillustrated in Fig. 9, the mass proportion of Pb and Zn elementsin the leaching solution is higher than that in the initial alloy.The mass ratio Pb/Cu of the leaching solution is always higher thanthat of the alloy, while Zn/Cu in solution is only remarkably highduring the first hour, but afterwards it decreases remaining closeto the nominal bulk proportion.
3.3. Corrosion rate vcor and dissolution factor fM
From previous results, it is evident that the alloy dissolutionquickly reaches a linear behaviour as expressed by the constantleaching rate of Cu, Zn and Pb. Thus, in order to quantify the con-tribution of each single metal to the dissolution of the alloy and
02468
10121416182022242628303234363840
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32time, hours
Hou
rly le
achi
ng ra
te,
µg h
-1cm
-2
Cu Zn Pb
Fig. 8. Hourly leaching rate (lg h�1 cm�2) at short-term.
E. Bernardi et al. / Corrosion Science 51 (2009) 159–170 167
their tendency to dissolve in the environment, the metal fraction inthe patina mm,cor as well as the dissolution factors fM have beendetermined applying respectively Eqs. (3) and (5). These data arereported for the specimens exposed 30 days (t = 720 h) in Table4, as well as the two following parameters: (i) the metal amountsmeasured in the leaching solution (mM,sol), see Section 3.2, and (ii)the mM,cor values determined from Eq. (3).
The negative value of the Pb fraction in the patina, mPb,cor, is dueto a superficial enrichment of Pb: the excess of Pb ions in the solu-tion derives from the lead rich layer smeared on the bronze surfaceduring polishing, which alters the lead percentage on the surface.This result is in agreement with observations already reported inliterature on similar bronzes [7,15]. Consequently, the dissolutionfactor fPb cannot be calculated with accuracy. As regard Cu andZn, the dissolution factors followed this order: fZn > fCu. Therefore,Zn dissolves preferentially by comparison to Cu.
As no Sn was detected in the leaching solutions (mSn,sol = 0), theamount of Sn in the patina, mSn,cor, is equal to the initial content ofSn in the whole corroded volume (mSn = mSn,cor = 2.0 mg in Table4). As a result, the value of fSn is zero. After 30 days, the totalamount of oxidised Sn remains in the patina. This is also confirmedby EDS analyses at 30 days, which reveal a Sn content in the patinaequal to 41 wt%. In the hypothesis of a simplified patina only con-taining Cu2O (q � 6 g cm�3) and SnO2 (q � 7 g cm�3), an averagedensity of �6.4 g cm�3 for the patina can be extrapolated. Then,being known the patina thickness and area Acor (�3 lm and4 cm2, respectively), the calculation of the total amount of Sn inthe patina gives �3.1 mg. This value is in good agreement withthe previous value of mSn,cor, confirming that Sn remains in thepatina.
Thus, the Sn enrichment in the patina, revealed by EDS analysesin Table 2, can be explained by considering that: (i) the originalvolume of the alloy decreases in correspondence with the surfaceleached by the rain (as previously shown by roughness measure-ments) and (ii) Sn is not leached by the rain differently from theother alloying elements.
Consequently, the corrosion process of bronze affects differ-ently the alloying elements: Cu, Zn and Pb follow a linear dissolu-tion trend, whereas Sn quickly passivates by forming insoluble Sncompounds remaining in the patina. This thin and Sn-enriched
patina is however not protective for the alloy, which keeps dissolv-ing with the same kinetics until the end of the test. Actually, in thissevere corrosive condition simulating a very strong permanentrainfall event, tin compounds do not hinder a rapid alloy oxidationand dissolution. The layer can not build up and stabilize. It remainsporous and fully permeable even if it appears that, in real condi-tions according to [20], the presence of this tin-rich layer on bronzecould slightly decrease the leaching rates of Cu (by comparison topure Cu metal). The mechanisms of formation involves, at thebeginning of the exposure, the preferential oxidation and leachingof Pb, then of Zn and Cu.
Finally, the corrosion rate of the alloy vcor at 30 days is ob-tained from Eq. (4) considering an average depth loss d = 0.0013 ±0.0001 cm. The calculated value of vcorr is 158 ± 1 lm year�1,which, divided by the acceleration factor estimated by compari-son with real cases [20] (see Section 3.2), gives a vcor of �0.3 lmyear�1. A comparable value, extrapolated by the graph of theatmospheric corrosion rate for copper exposed in natural condi-tion, has been reported by FitzGerald et al. in [34]. Moreover,Odnevall Wallinder and Leygraf [23] found a vcor decreasing withtime to less than 1 lm year�1 after 2 years of exposure in urbanatmosphere. A good agreement is here achieved between theresults of this test and those performed in natural exposureconditions.
3.4. Comparison with real patinas
Dissolution factors calculated in this present work are com-pared with those obtained on pale green patinas sampled fromquaternary outdoor bronzes, correspondently to areas exposed todirect rain action [13,14]. These bronzes (mainly from XIX century,from France) mainly consist of single-phase solid solution a-Cu(Sn,Zn) with immiscible Pb globules. Sn, Zn and Pb contents inthe alloys are respectively in the range of 2–5, 2–16 and 0.5–7 wt%. The dissolution factors, fCu and fZn, reported on these worksshow almost constant values (0.91–0.95 for Cu and 0.96–0.99 forZn) independently of the composition of the original alloys. As inthe case of Cu sheets, where, after long exposure (several decades),the ratio between Cu runoff and copper mass loss converges [4,22],also in the case of quaternary bronzes exposed to direct rains a
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48
time, hours
Met
al/C
u
Zn/Cu solution Zn/Cu alloy
Pb/Cu solution Pb/Cu alloy
0.00
0.05
0.10
0.15
0.20
0.25
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
time, days
Met
al/C
u
Zn/Cu solution Zn/Cu alloyPb/Cu solution Pb/Cu alloy
Fig. 9. Ratio of Pb and Zn leached in the solution with respect to copper as a function of exposure time: (a) short-term test, (b) long-term test. Horizontal lines show thevalues of the metal/Cu ratios in the bulk alloy. In the graph (a) the value of Pb/Cu at 1 h is out of range (=8.5) and has not been reported because it is out of range.
168 E. Bernardi et al. / Corrosion Science 51 (2009) 159–170
similar behaviour is observed: as evidenced in Table 4, the ratiobetween malloy and malloy,sol at the end of the test is near to one.
Consequently, the main phenomenon responsible for the forma-tion of pale green patina is a selective dissolution of the alloy,involving more than 90% of the initial Cu and Zn, which are leachedby the rains [14]. The oxidised metal fraction not leached by therains, mm,cor, remains in the patina as corrosion products, scatteredin a matrix rich in nanocrystalline tin oxides and hydroxides.Mechanism proceeds by migration/dissolution of Cu, Zn, Pb cationsthrough the thin patina towards the interface with the environ-ment, where the leaching action of the rain prevails against anyother phenomena. Thus, the patina is markedly depleted in copperwith regards to the original alloy. In this work, it is pointed outthat, in the case of bronze, the amount of leached Cu, Zn and Pbgrows linearly, whereas Sn, from the very first moments, formsinsoluble compounds in the form of amorphous or nanocrystallineoxides/hydroxides [13,14].
It is clearly shown that bronze patinas, here artificiallyaged in conditions close to those of unsheltered areas of
outdoor sculpture, mainly consist of tin compounds as alreadyfound in real patinas [6,13,14]. Consequently, the corrosionbehaviour of bronze cannot be assimilated to that of Cu alsoregarding the choice of corrosion inhibitor. In fact, in the caseof outdoor bronze statues, also after restoration, the applica-tion of corrosion inhibitors is done on the already corrodedsurface. Therefore, the support where the inhibitors shouldbe chemisorbed is not the original alloy, but the previouslyformed patina. Due to the different reactivity of alloying metaloxides towards inhibitors, corrosion inhibitors offer differentperformances [28,29]. Tin oxides, for examples, in oppositionto copper (I) oxides, show a very weak reactivity to BTAbased inhibitors, typically used for the conservation ofbronzes: this explains the minor efficiency of these inhibitorson bronzes with respect to copper [28,29]. The remarkabledepletion of copper and enrichment of tin observed in unshel-tered areas of sculpture requires the use of specifically tai-lored inhibitors.
Table 4Corrosion parameters after 30 days of exposure.
mM (mg) mM,sol (mg) mM,cor (mg) fM
Alloy 47 ± 4 41.8 ± 0.9Cu 41 ± 3 38.6 ± 0.5 3 ± 3 0.93 ± 0.08Pb 1.8 ± 0.2 2.2 ± 0.8 �0.3 ± 0.8 *
Zn 1.1 ± 0.1 1.1 ± 0.1 0.0 ± 0.1 0.98 ± 0.12Sn 2.0 ± 0.2 <d.l. 2.1 ± 0.2 0
* fPb cannot be calculated by Eq. (5) due to the Pb enrichment on the bronze surface.
E. Bernardi et al. / Corrosion Sc
4. Conclusion
The following conclusions can be drawn:
(1) Under a severe permanent leaching drop effect, a progres-sive dissolution of the bronze is clearly evidenced. However,a porous patina is always observed.
(2) Sn cations remain in the patina, forming poorly crystallisedstable species related to the presence of SnOx(OH)y. Thus, theSn content in the patina increases with the time of exposure.
(3) As pointed out by the constant leaching rates, the dissolu-tion of the other alloying elements Cu, Zn and Pb takes placewith a linear trend after several hours, revealing a quasi sta-tionary state of the system: the alloy oxidation and thepatina dissolution rates becoming comparable.
(4) With respect to Cu, a preferential corrosion of Zn and Pboccurs, more pronounced at short-term than at long-termexposure. Even if the rain flux is continuous, precipitationof Pb and Cu compounds can take place, preferentially wherethe drops are stagnant.
(5) The incorporation of anionic species others than O2� andOH� in the patina remains very low. The main compoundsidentified are cuprous oxide and poorly crystallised tin(hydroxyl) oxide.
By comparison with real patinas directly exposed to rainfallevents, the severe dropping simulation here tested could be consid-ered as an effective ageing method with an acceleration factor equiv-alent to 1.5 years of exposure in natural conditions per day of test.
Appendix A
Symbol M
easure UnitAcor C
orrosion area cm2 M easured d C orrosion depth cm M easured q D ensity of the alloy g cm�3 M easured vM M ass fraction of metal Min the alloy
– M easuredmtot T
otal mass loss at the endof the testg m
tot = Acor. � d � qmM T
otal mass of metal Moxidised in the corrodedvolume (Acor � d)g m
M= mtot � vMmM,sol M
ass of metal Mdissolved into theleaching solutiong M
easuredmM,cor M
ass of metal M in thepatinag m
M,cor = mM �mM,solvcor C
orrosion rate of the alloyat exposure time tlm year�1 v
cor = d/tLr L
eaching rate of metal Mat exposure time tg cm�2 h�1 L
r = mM,sol/(Acor � t)fM D
issolution factor ofmetal M– f
M = mM,sol/mMReferences
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