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Surface characterization of artificial corrosion layers on copper alloy reference materials
Applied Surface Science 189 (2002) 90±101
Surface characterization of arti®cial corrosion layers on copper alloy reference materials I. Constantinides, A. Adriaens*, F. Adams Department of Chemistry, University of Antwerp, Universiteitsplein 1, 2610 Antwerp, Belgium Received 28 July 2001; accepted 15 December 2001
Abstract This paper describes the surface characterization of arti®cial patina layers on ®ve different copper alloys. The chemical composition of the examined bronzes covers the major families of archaeological copper alloys from antiquity until the Roman period. The patina layers of the ®ve samples were formed under identical conditions by electrochemical means. Light microscopy, scanning electron microscopy with energy dispersive X-ray micro analysis (SEM±EDX) and Fourier transform infrared spectroscopy (FTIR) were used to describe the main properties of the patina layers. The results were interpreted and classi®ed according to an existing corrosion model for copper alloys. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Corrosion; Copper alloys; Reference materials; Archaeometry; SEM±EDX; FTIR; Light microscopy
1. Introduction The main constituents of alteration layers on bronze, reported in the literature, are copper-based compounds [1±3]. As a consequence of this, bronze patinas have often been thought to be very similar to pure copper patinas. In the case of a Cu±Sn alloy, e.g., it is generally accepted that the surface layer consists of Cu (II) salts, e.g., malachite (in soil), brochantite (in the atmosphere) or atacamite (in seawater), which covers a red cuprous oxide layer that is in contact with the metal core. Experiments however have shown that tin can be selectively dissolved into the outer medium (de-alloying or destanni®cation) [4,5]. Some studies on bronze corrosion have revealed the presence of relatively high tin concentrations. Initially observed in high tin * Corresponding author. Present address: Department of Analytical Chemistry, Ghent University, Krijgslaan 281-S12, 9000 Ghent, Belgium. Tel.: 32-9264-4826; fax: 32-9264-4960. E-mail address: [email protected] (A. Adriaens).
bronzes, they were also encountered on artifacts with lower tin contents originating from very different media (soil, sea, etc.) [6±9]. These results have been correlated to ``unusual phenomena'' with tin being present as a metallic tin±copper surface layer possibly caused by arti®cial patination, tinning or sweat segregation during casting [10]. Opposing hypotheses explain the observations as a consequence of copper migration within the corrosive environment, leaving an altered matrix enriched in hydrated stannic oxide [11±13]. In fact both types of patinas, i.e. the ones without tin compounds and the ones with tin enrichment can coexist on the same artifact. Hence, an uni®ed model was needed, which could take into account both of the corrosion patterns observed in atmospheric conditions as well as in soil or water. Robbiola [14] demonstrated that tin bronzes are less corrodible with increasing tin content, undertook one of the ®rst studies in this ®eld. Chase [15] added to these results by obtaining polarization curves for copper±tin bronze. Taking into account his results together with additional data
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 0 0 5 - 3
I. Constantinides et al. / Applied Surface Science 189 (2002) 90±101
obtained by Chase [15], Robbiola et al. [16] came to the conclusion that ion migration is the key to most of the observed effects in the corrosion layer. A phenomenological model was developed to explain the formation of bronze patinas on the basis of the selective dissolution of copper. In addition two classes of corrosion structures were de®ned. A Type I structure (`even' surface) is de®ned as a two-layer passivating deposit due to an internal tin oxidation accompanied by a selective dissolution of copper. AType II structure (`coarse' structure) corresponds to more severe attacks, such as pitting but also to a general uneven corrosion. It is characterized by a three-layer structure, the presence of cuprous oxide and an increase in the chloride content at the internal layer±alloy interface. In this work, a systematic study on the patina layers of ®ve different copper alloys was performed. The alloys in a previous study has been certi®ed as reference materials and are representative for the composition of archaeological bronzes from antiquity to the Roman period [17]. Arti®cial corrosion layers were formed by electrochemical means, whereby experimental conditions were optimized to resemble as closely as possible those of genuine archaeological artifacts [18]. The advantage of using arti®cial patina layers is that the corrosion environment is very well de®ned and similar for all alloys, which means that one can obtain an idea about the in¯uence of the metal composition upon the formation of the corrosion. The corrosion products in this work were characterized using light microscopy, scanning electron microscopy with energy dispersive X-ray micro analysis (SEM±EDX) and Fourier transform infrared spectroscopy (FTIR). Attention was mainly given to: (1) the general appearance of the corroded surface; (2) the identi®cation of distinctly different layers in the corrosion formation; (3) the determination of the corroded surface compounds and the changes in surTable 1 Certi®ed values and uncertainties for BCR 691 in g kg Element
As Pb Sn Zn
1
91
face structure of the alloy and the interface. A classi®cation was made using the model of Robbiola et al. [16], described above. 2. Experimental 2.1. Formation of arti®cial patinas Five different copper alloys were used in this study: a quaternary bronze, an arsenical copper, a tin bronze, a leaded tin bronze and a brass. All of them were characterized for their microstructure in a previous study [19]. The values and their uncertainties as determined in the certi®cation campaign are listed in Table 1 [17]. The formation of the arti®cial patinas was done by electrochemical means. The procedure was divided into two stages: the ®rst corresponded to the selective formation of cuprite (Cu2O) in a solution of sodium sulfate (0.1 M Na2SO4) and an anodic potential of 40 mV/SCE (duration: 7 days). The second stage allowed the formation of more complex compounds, including chlorides, carbonates and sulfates in an ASTM D1384 solution with added carbonates (0.01 M Na2SO4, 2:8 10 2 M NaCl and 16:1 10 2 M NaHCO3) [18]. The electrochemical procedure of this stage included an anodic potential of 350 mV/SCE for a duration of 8 days, followed by an anodic potential of 850 mV/SCE for another 3 days. 2.2. Analytical approach Light microscopy was used for the large-®eld examination of the corrosion surfaces. Under polarized light, corrosion products appear very colorful and give a ®rst indication concerning the chemical composition of the compounds. In this work an Olympus SZX-12 (Hamburg, Germany) was used.
[17]
Composition Quaternary bronze
Arsenical copper
Tin bronze
Leaded tin bronze
Brass
1.94 79 71.6 60.2
46.0 1.75 2.02 0.55
1.94 2.04 70 1.57
2.85 92 101 1.48
0.99 3.9 20.6 148
0.10 7 2.1 2.2
2.7 0.14 0.29 0.05
0.20 0.18 6 0.25
0.22 17 8 0.24
0.10 0.3 0.7 5
92
I. Constantinides et al. / Applied Surface Science 189 (2002) 90±101
SEM±EDX was employed to investigate the corrosion±alloy interface both by imaging and line spectra. A JEOL 6300 (Mitaka, Japan) was used in this study. It is equipped with an energy dispersive Si(Li) detector. Prior to analysis the samples were mounted in an epoxy resin, ground and polished mechanically according to standard metallurgical procedures. In addition the samples were carbon coated to avoid charging. Fourier transform infrared spectrometer measurements were carried out to characterize the functional groups of the corrosion products and their degree of crystallization as function of the bandwidths. A Nicolet 20 DXB (controlled by a Nicolet 120 microcomputer) was used in this study. The standard system has a spectral range from 400 to 4000 cm 1 with a maximum resolution of 4 cm 1. All spectra were recorded in the absorbance mode for 200 scans in the mid-IR range. Micro samples were taken from the corroded bronze samples mechanically by scraping the surface carefully with a ®ne needle. They were then mixed with KBr (95%) and pressed into small pellet. 3. Results and discussion 3.1. The quaternary bronze A backscattered electron image (BSE) of the alloy± corrosion products interface for the quaternary bronze
(Cu±Sn±Zn±Pb alloy) is shown in Fig. 1. The image is representative for practically the entire surface. The corrosion layer is clearly visible and well-separated from the alloy. Its appearance is compact with a low porosity and it is composed of two sub-layers (marked in Fig. 1). Several perpendicular cracks are visible. The thickness of the patina varies from 14 to 146 mm, with an average thickness of ca. 34 mm. Fig. 2 shows the results of the SEM±EDX line scan performed along the arrow in Fig. 1 and illustrates the corresponding distributions of Cu, Sn, Zn and Cl in the alloy±corrosion products interface. A selective depletion of Cu and an enrichment of Sn can be observed in the corrosion. The Cl signal increases strongly by crossing from the outer to the inner layer and further increases in the alloy±corrosion products interface, which is rather broad due to progressive pit corrosion. At a few places on the sample surface, pit corrosion can be observed, which is a very local type of corrosion. Fig. 3a shows an image taken with the optical microscope. The image gives a clear indication of cuprous chloride, probably paratacamite (Cu2Cl(OH)3, green) as the inner layer and cuprite (Cu2O, red) as the outer layer. The distribution of chlorine in this area shown in the X-ray map (Fig. 3b) con®rms this. These results show that for the quaternary bronze a Type I even corrosion layer surface is privileged, which is characterized by a two-layer system, showing
Fig. 1. BSE image of the corrosion layer in the quaternary bronze (magni®cation 800).
I. Constantinides et al. / Applied Surface Science 189 (2002) 90±101
93
Fig. 2. Line scan pro®le across the alloy±corrosion interface of the quaternary bronze.
in the outer layer a depletion of Cu and an enrichment of Sn. Due to mechanical stress or chemical reaction (e.g. dehydration), crack formation has occurred in some places, which results in pit corrosion. and hence the extreme dissolution of the alloy i.e. an increase of the Cu concentration. Because of the high Cu concentration and the simultaneous presence of Cl anions originating from the electrolyte, CuCl was formed building up an internal layer beneath the original one. The internal layer consists of CuCl or CuCl2.
average thickness of ca. 38 mm. Fig. 5 shows the results of the line scan performed along the arrow in Fig. 4 and shows the corresponding distributions of the elements Cu, As and Cl. The results show no signi®cant dissolution of copper, this in contrast to the quaternary bronze. Both As and Cl however show an enrichment in the inner layer. It can be concluded that the arsenical copper shows a Type II structure and the beginning of pit corrosion, which is indicated by the presence of cuprous chloride under the cuprite layer (Cu2O) [20].
3.2. The arsenical copper
3.3. The tin bronze
A photograph of the arsenical copper (Cu±As alloy) corrosion±alloy interface in Fig. 4 shows a well-separated three-layer corrosion structure. The outer layer (green) is rough with unequal precipitations, which tower from the surface. According to the FTIR measurements this layer is composed of a hydrated copper carbonate, malachite (Cu2CO3(OH)2H2O). The sandwiched cuprite layer (Cu2O, red) on the other hand has a compact and homogeneous-looking structure, while the inner nantokite layer (CuCl, pale green) reveals an inhomogeneous structure with globules. The thickness of the corrosion varies from 23 to 78 mm, with an
The corrosion of the tin bronze (Cu±Sn alloy) consists of a three-layered structure (Fig. 6). The outer layer is porous, whereas the middle and the inner layer are more compact. Contrary to the quaternary bronze and the arsenical copper, where the interface between corrosion and alloy can be clearly distinguished, it is clear that the interface is much less distinct. Additionally, corrosion can clearly be observed in places where the original surface is destroyed. Examinations of the cross-sections re¯ect the loss of matter due to an aggressive corrosion process visible in local pits or in crevices. The latter ones are a typical observation in
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Fig. 3. (a) Photograph of an example of pit corrosion (magni®cation 90); (b) X-ray map of Cl in the same area (magni®cation 400).
this bronze. The thickness of the patina varies from 40 to 138 mm, with an average thickness of ca. 67 mm. The line scan in Fig. 7 was measured on the tin bronze along the arrow in Fig. 6 and shows the distributions of the elements Cu, Sn and Cl in the two compact layers of the corrosion. The Cl signal increases from the middle to the inner layer reaching a maximum at the interface with the alloy. The Cl signal then drops sharply in the alloy. The Sn signal is slightly enriched
in the middle layer and depleted in the inner. Fig. 8 shows the FTIR spectrum of the outer corrosion layer and shows a good agreement with malachite. In general the tin bronze can be classi®ed as a Type II corrosion structure because of the typical coarse surface and the different layers in the corrosion zone. From the outer to the inner layer, the corrosion shows the presence of malachite, a cuprous oxide layer and a cuprous chloride layer.
I. Constantinides et al. / Applied Surface Science 189 (2002) 90±101
95
Fig. 4. Photograph of the corrosion layer in the arsenical copper (magni®cation 90).
3.4. The leaded tin bronze The corrosion layer of the leaded tin bronze (Cu± Sn±Pb alloy) is distinctly visible from the alloy (Fig. 9). Examinations of the cross-section show in general a compact two-layer structure. Perpendicular
cracks are visible due to the dehydration phenomena of the corrosion components in the passive layer after atmospheric exhibition, which can possibly be a starting point for localized corrosion. Lead is visible in bright inclusions both in the patina layer as well as in the alloy. The thickness of the corrosion
Fig. 5. Line scan pro®le across the alloy±corrosion interface of the arsenical copper.
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I. Constantinides et al. / Applied Surface Science 189 (2002) 90±101
Fig. 6. BSE image of the corrosion layer in the tin bronze (magni®cation 430).
varies from 18 to 60 mm, with an average thickness of 25 mm. At some places on the surface, laminated layer corrosion can be observed, in which alternating patina deposits are visible (Fig. 10). Also here huge lead inclusions can be observed in both the alloy and the
corrosion, indicating that no remarkable dissolution of lead took place. The elements Sn and Cu show similarities in behavior with the quaternary bronze and the tin bronze, i.e. Sn shows an enrichment and Cu a depletion in the corrosion layer. Lead is present in inclusions in both
Fig. 7. Line scan pro®le across the alloy±corrosion interface of the tin bronze. Only the two compact corrosion layers are pro®led.
I. Constantinides et al. / Applied Surface Science 189 (2002) 90±101
97
Fig. 8. FTIR spectrum of the corrosion surface of the tin bronze (full line) and a reference spectrum of malachite (broken line).
the metal as well as the corrosion layers. Within the laminated layer corrosion area, a line scan pro®le shows alternating signals of Cu and Sn, re¯ecting clearly the different deposits (Fig. 11). Similar as for the previous alloys, the chlorine signal is signi®cantly enhanced in the alloy±corrosion products interface. Also here FTIR analyses show the presence of malachite on the surface. The corrosion formation of the leaded tin bronze can hence be classi®ed as a Type I even structure, with an enrichment in Sn and a depletion in Cu in the corrosion. At a few places a Type II coarse structure can be observed. Additionally, laminated layer corrosion could be observed.
3.5. The brass The brass (Cu±Zn alloy) shows two main types of corrosion: a compact two-layer system and a sponge-type structure (Figs. 12 and 13). The thickness of the corrosion varies from 3 to 73 mm, with an average thickness of 24 mm. Line scans on the compact corrosion structure show a selective depletion of Cu and an enrichment of Zn and Sn compared to the alloy. The sponge-type structure however shows no dissolution of Cu in the corrosion zone (Fig. 15). The slight decrease of the Cu pro®le in the alloy is caused by a change of the absorption coef®cient, which is higher in the alloy than in the passive layer
Fig. 9. BSE image of the corrosion layer in the leaded tin bronze (magni®cation 1700). The bright areas are lead inclusions.
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I. Constantinides et al. / Applied Surface Science 189 (2002) 90±101
Fig. 10. BSE image of the laminated layer corrosion in the leaded tin bronze (magni®cation 600).
[20]. The steep enhancement of the Zn pro®le in the transition from the corrosion layer±alloy is an indication for localized inclusions. Fig. 14 presents an optical microscope image of the cross-section from the brass, where both types of corrosion are visible. From outer to inner layer, one can distin-
guish a cuprous chloride compound (CuCl or other soluble complexes such as CuCl2 or CuCl3 ) and cuprite. FTIR measurements con®rm the absence of malachite. From these results it can be concluded that the corrosion formation of brass under the present
Fig. 11. Line scan pro®le in the laminated layer corrosion of the leaded tin bronze.
I. Constantinides et al. / Applied Surface Science 189 (2002) 90±101
99
Fig. 12. BSE image of the compact corrosion in the brass (magni®cation 1400).
conditions can be classi®ed as a Type I structure. Furthermore, cracks are visible especially at domains, which were close to the interface. This leads to the assumption that, because of the anodic polarization procedure, the compact layer probably tended to crack after some critical degree of growth, obviously due to internal stress. At the bottom of these cracks, a renewed selective dissolution occurred i.e. cracking
went further on possibly causing this macroscopic ``granular'' structure. Presumably, the formation of the Type I corrosion layer was attributed to the local presence of Sn, whereas the sponge-like corrosion occurred at areas with a poor Sn content. The outer layer consisted of CuCl or other soluble complexes such as CuCl2 or CuCl3 , whereas the internal layer was formed by cuprite (Cu2O).
Fig. 13. BSE image of the sponge-type corrosion structure in the brass (magni®cation 900).
100
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Fig. 14. Line scan pro®le in the sponge-type corrosion structure of the brass.
Fig. 15. Photograph of the corrosion layers in the brass (magni®cation 400).
4. Conclusions In this work arti®cial patina layers were produced on ®ve different copper alloys by electrochemical means. The obtained patina layers were characterized using light microscopy, SEM±EDX and FTIR. The data were classi®ed according to the model by Robbiola et al. [16] and are listed in Table 2. Results showed that the morphology of the surfaces and the elemental compositions of the corrosion products depend strongly on the chemical composition of the alloys. A Type I even structure could be observed on the quaternary bronze and the brass; a Type II
`rough' structure was predominant for the arsenical copper and the tin bronze. The leaded bronze showed the presence of both structures. Table 2 Overview of the thickness and type of corrosion Composition
Range of thickness (mm)
Type
Quaternary bronze Arsenical copper Tin bronze Leaded tin bronze Brass
14±146 23±78 40±138 18±60 3±73
I/Pit II/Pit II I/II/Laminated layer I
I. Constantinides et al. / Applied Surface Science 189 (2002) 90±101
The ®gures of Table 2 indicate that the quaternary bronze and the tin bronze showed a similar thickness. The same conclusion can be drawn for the arsenical copper and the leaded tin bronze, though smaller than the previous two. The thinnest layer was observed for the brass. The line scans showed that the elements Cu and Sn were mainly responsible for the corrosion of the elements. In addition this work has also shown that the formed patina layers are very comparable to natural patinas on archaeological bronze [21]. Acknowledgements The authors wish to thank L. Robbiola for his valuable comments and P. Cool for the FTIR analyses. The European Commission, DG XII, is gratefully acknowledged for ®nancing the project SMT4CT96-2055 ``IMMACO''. AA is indebted to FWO, Belgium. References [1] R. Tylecote, The effect of soil conditions on the long-term corrosion of buried tin bronzes and copper, J. Archaeol. Sci. 6 (1997) 345. [2] E. Mattson, A.G. Nord, K. Tronner, M. Fjaestad, A. LagerloÈf, I. UlleÂn, G. Borg, Deterioration of Archaeological Material in Soil: Results of Bronze Artifacts, Konserveringstekniska Studier, RiksantikvarieaÈmbetet (RIK) 10, Stockholm, 1996. [3] N. Nielsen, Corrosion product characterization, in: B. Brown, H. Burnett, T. Chase, M. Goodway, J. Kruger, M. Pourbaix (Eds.), Corrosion and Metal Artifacts: A Dialogue Between Conservators, Archaeologists and Corrosion Scientists, US Bureau of Standards Special Publication, Washington, Vol. 479, 1977, p. 17. [4] C. Swann, S. Flemming, M. Jaksic, Recent application of PIXE spectrometry in archaeology I. Characterization of bronzes with special consideration of corrosion processes on data reliability, Nucl. Instrum. Meth. B 64 (1992) 499. [5] T. Weisser, The de-alloying of copper alloys, in: Conservation in Archaeology and the Applied Arts, International Institute of Conservation, Stockholm, 1975, p. 207. [6] L. Soto, J. Franey, T. Graedel, W. Kammlott, On the corrosion resistance of certain ancient Chinese bronze artifacts, Corr. Sci. 23 (1983) 241. [7] N. Meeks, Tin-rich surfaces on bronze: some experimental and archaeological considerations, Archaeometry 28 (1986) 133.
101
[8] L. Robbiola, L. Hurtel, Standard nature of the passive layers buried archaeological bronze: the example of two Roman half-length portraits, in metal 95, in: I. MacLeod, S. Pennec, L. Robbiola (Eds.), Proceedings of the International Conference on Metals Conservation, James and James, London, 1997, p. 109. [9] W. Oddy, N. Meeks, Unusual phenomena in the corrosion of ancient bronzes, in: Science and Technology in the Service of Conservation, IIC Washington Congress, 1982, p. 119. [10] R. Tylecote, The apparent tinning of bronze axes and other artifacts, J. Hist. Met. Soc. 19/2 (1985) 169. [11] T. Stambolov, The corrosion and conservation of metallic antiquities and works of art, A Preliminary Survey, Central Laboratory for Objects of Art and Science, Amsterdam, 1968. [12] R. Gettens, The corrosion products of ancient Chinese bronze, J. Chem. Educ. 2 (1951) 67. [13] W. Geilmann, Verwitterung von Bronzen im Sandboden. Ein Beitrag zur Korrosionsforschung, Angew. Chem. Int. Edit. 68 (1956) 201. [14] L. Robbiola, Caracterisation de L'Alteration de Bronzes Archaeologiques enfouis a partir d'un Corpus D'Objets de L'Age du Bronze, Mechanismes de Corrosion, Ph.D. Diss., L'Universite de Paris 6, Paris, 1990. [15] W. Chase, Chinese bronzes: casting, ®nishing, patination and corrosion, in: D. Scott, J. Podany, B. Considine (Eds.), Ancient and Historic Metals, The Getty Conservation Institution, 1994, p. 85. [16] L. Robbiola, J. Blengino, C. Fiaud, Morphology and mechanism of formation of natural patinas of archaeological Cu±Sn alloys, Corr. Sci. 40 (12) (1998) 2083. [17] C. Ingelbrecht, A. Adriaens, E. Maier, Certi®cation of Arsenic, Lead, Tin and Zinc (Mass Fractions) in Five Copper Alloys CRM 691, EUR 19778/1 EN, Of®ce for Of®cial Publications of the European Communities, Luxembourg, 2001. [18] T. Beldjoudi, F. Bardet, N. Lacoudre, S. Andrieu, A. Adriaens, I. Constantinides, P. Brunella, Surface modi®cation processes on European union bronze reference materials for analytical studies of cultural artifacts, Surf. Eng. 17 (3) (2001) 231±235. [19] I. Constantinides, M. Gritsch, A. Adriaens, H. Hutter, F. Adams, Microstructural characterization of ®ve simulated archaeological copper alloys using light microscopy, scanning electron microscopy-energy dispersive X-ray microanalysis and secondary ion mass spectrometry, Anal. Chim. Acta 440 (2) (2001) 189±198. [20] L. Robbiola, R. Portier, Electron and microscopy and EDX analysis in the investigation of the decupri®cation phenomena in Cu±Sn alloys: a comparison between archaeological and synthetic bronzes, in: H.A. Calderon, Yacaman (Eds.), Electron Microscopy, Vol. III, 1998, p. 289. [21] M. Wadsak, I. Constantinides, G. Vittiglio, A. Adriaens, K. Janssens, M. Schreiner, F. Adams, P. Brunella, M. Wuttmann, Multianalytical study of patina formed on archaeological metal objects from Bliesbruck-Reinheim, Mikrochim. Acta 133 (1±4) (2000) 159±164.
Surface characterization of arti®cial corrosion layers on copper alloy reference materials I. Constantinides, A. Adriaens*, F. Adams Department of Chemistry, University of Antwerp, Universiteitsplein 1, 2610 Antwerp, Belgium Received 28 July 2001; accepted 15 December 2001
Abstract This paper describes the surface characterization of arti®cial patina layers on ®ve different copper alloys. The chemical composition of the examined bronzes covers the major families of archaeological copper alloys from antiquity until the Roman period. The patina layers of the ®ve samples were formed under identical conditions by electrochemical means. Light microscopy, scanning electron microscopy with energy dispersive X-ray micro analysis (SEM±EDX) and Fourier transform infrared spectroscopy (FTIR) were used to describe the main properties of the patina layers. The results were interpreted and classi®ed according to an existing corrosion model for copper alloys. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Corrosion; Copper alloys; Reference materials; Archaeometry; SEM±EDX; FTIR; Light microscopy
1. Introduction The main constituents of alteration layers on bronze, reported in the literature, are copper-based compounds [1±3]. As a consequence of this, bronze patinas have often been thought to be very similar to pure copper patinas. In the case of a Cu±Sn alloy, e.g., it is generally accepted that the surface layer consists of Cu (II) salts, e.g., malachite (in soil), brochantite (in the atmosphere) or atacamite (in seawater), which covers a red cuprous oxide layer that is in contact with the metal core. Experiments however have shown that tin can be selectively dissolved into the outer medium (de-alloying or destanni®cation) [4,5]. Some studies on bronze corrosion have revealed the presence of relatively high tin concentrations. Initially observed in high tin * Corresponding author. Present address: Department of Analytical Chemistry, Ghent University, Krijgslaan 281-S12, 9000 Ghent, Belgium. Tel.: 32-9264-4826; fax: 32-9264-4960. E-mail address: [email protected] (A. Adriaens).
bronzes, they were also encountered on artifacts with lower tin contents originating from very different media (soil, sea, etc.) [6±9]. These results have been correlated to ``unusual phenomena'' with tin being present as a metallic tin±copper surface layer possibly caused by arti®cial patination, tinning or sweat segregation during casting [10]. Opposing hypotheses explain the observations as a consequence of copper migration within the corrosive environment, leaving an altered matrix enriched in hydrated stannic oxide [11±13]. In fact both types of patinas, i.e. the ones without tin compounds and the ones with tin enrichment can coexist on the same artifact. Hence, an uni®ed model was needed, which could take into account both of the corrosion patterns observed in atmospheric conditions as well as in soil or water. Robbiola [14] demonstrated that tin bronzes are less corrodible with increasing tin content, undertook one of the ®rst studies in this ®eld. Chase [15] added to these results by obtaining polarization curves for copper±tin bronze. Taking into account his results together with additional data
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 0 0 5 - 3
I. Constantinides et al. / Applied Surface Science 189 (2002) 90±101
obtained by Chase [15], Robbiola et al. [16] came to the conclusion that ion migration is the key to most of the observed effects in the corrosion layer. A phenomenological model was developed to explain the formation of bronze patinas on the basis of the selective dissolution of copper. In addition two classes of corrosion structures were de®ned. A Type I structure (`even' surface) is de®ned as a two-layer passivating deposit due to an internal tin oxidation accompanied by a selective dissolution of copper. AType II structure (`coarse' structure) corresponds to more severe attacks, such as pitting but also to a general uneven corrosion. It is characterized by a three-layer structure, the presence of cuprous oxide and an increase in the chloride content at the internal layer±alloy interface. In this work, a systematic study on the patina layers of ®ve different copper alloys was performed. The alloys in a previous study has been certi®ed as reference materials and are representative for the composition of archaeological bronzes from antiquity to the Roman period [17]. Arti®cial corrosion layers were formed by electrochemical means, whereby experimental conditions were optimized to resemble as closely as possible those of genuine archaeological artifacts [18]. The advantage of using arti®cial patina layers is that the corrosion environment is very well de®ned and similar for all alloys, which means that one can obtain an idea about the in¯uence of the metal composition upon the formation of the corrosion. The corrosion products in this work were characterized using light microscopy, scanning electron microscopy with energy dispersive X-ray micro analysis (SEM±EDX) and Fourier transform infrared spectroscopy (FTIR). Attention was mainly given to: (1) the general appearance of the corroded surface; (2) the identi®cation of distinctly different layers in the corrosion formation; (3) the determination of the corroded surface compounds and the changes in surTable 1 Certi®ed values and uncertainties for BCR 691 in g kg Element
As Pb Sn Zn
1
91
face structure of the alloy and the interface. A classi®cation was made using the model of Robbiola et al. [16], described above. 2. Experimental 2.1. Formation of arti®cial patinas Five different copper alloys were used in this study: a quaternary bronze, an arsenical copper, a tin bronze, a leaded tin bronze and a brass. All of them were characterized for their microstructure in a previous study [19]. The values and their uncertainties as determined in the certi®cation campaign are listed in Table 1 [17]. The formation of the arti®cial patinas was done by electrochemical means. The procedure was divided into two stages: the ®rst corresponded to the selective formation of cuprite (Cu2O) in a solution of sodium sulfate (0.1 M Na2SO4) and an anodic potential of 40 mV/SCE (duration: 7 days). The second stage allowed the formation of more complex compounds, including chlorides, carbonates and sulfates in an ASTM D1384 solution with added carbonates (0.01 M Na2SO4, 2:8 10 2 M NaCl and 16:1 10 2 M NaHCO3) [18]. The electrochemical procedure of this stage included an anodic potential of 350 mV/SCE for a duration of 8 days, followed by an anodic potential of 850 mV/SCE for another 3 days. 2.2. Analytical approach Light microscopy was used for the large-®eld examination of the corrosion surfaces. Under polarized light, corrosion products appear very colorful and give a ®rst indication concerning the chemical composition of the compounds. In this work an Olympus SZX-12 (Hamburg, Germany) was used.
[17]
Composition Quaternary bronze
Arsenical copper
Tin bronze
Leaded tin bronze
Brass
1.94 79 71.6 60.2
46.0 1.75 2.02 0.55
1.94 2.04 70 1.57
2.85 92 101 1.48
0.99 3.9 20.6 148
0.10 7 2.1 2.2
2.7 0.14 0.29 0.05
0.20 0.18 6 0.25
0.22 17 8 0.24
0.10 0.3 0.7 5
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SEM±EDX was employed to investigate the corrosion±alloy interface both by imaging and line spectra. A JEOL 6300 (Mitaka, Japan) was used in this study. It is equipped with an energy dispersive Si(Li) detector. Prior to analysis the samples were mounted in an epoxy resin, ground and polished mechanically according to standard metallurgical procedures. In addition the samples were carbon coated to avoid charging. Fourier transform infrared spectrometer measurements were carried out to characterize the functional groups of the corrosion products and their degree of crystallization as function of the bandwidths. A Nicolet 20 DXB (controlled by a Nicolet 120 microcomputer) was used in this study. The standard system has a spectral range from 400 to 4000 cm 1 with a maximum resolution of 4 cm 1. All spectra were recorded in the absorbance mode for 200 scans in the mid-IR range. Micro samples were taken from the corroded bronze samples mechanically by scraping the surface carefully with a ®ne needle. They were then mixed with KBr (95%) and pressed into small pellet. 3. Results and discussion 3.1. The quaternary bronze A backscattered electron image (BSE) of the alloy± corrosion products interface for the quaternary bronze
(Cu±Sn±Zn±Pb alloy) is shown in Fig. 1. The image is representative for practically the entire surface. The corrosion layer is clearly visible and well-separated from the alloy. Its appearance is compact with a low porosity and it is composed of two sub-layers (marked in Fig. 1). Several perpendicular cracks are visible. The thickness of the patina varies from 14 to 146 mm, with an average thickness of ca. 34 mm. Fig. 2 shows the results of the SEM±EDX line scan performed along the arrow in Fig. 1 and illustrates the corresponding distributions of Cu, Sn, Zn and Cl in the alloy±corrosion products interface. A selective depletion of Cu and an enrichment of Sn can be observed in the corrosion. The Cl signal increases strongly by crossing from the outer to the inner layer and further increases in the alloy±corrosion products interface, which is rather broad due to progressive pit corrosion. At a few places on the sample surface, pit corrosion can be observed, which is a very local type of corrosion. Fig. 3a shows an image taken with the optical microscope. The image gives a clear indication of cuprous chloride, probably paratacamite (Cu2Cl(OH)3, green) as the inner layer and cuprite (Cu2O, red) as the outer layer. The distribution of chlorine in this area shown in the X-ray map (Fig. 3b) con®rms this. These results show that for the quaternary bronze a Type I even corrosion layer surface is privileged, which is characterized by a two-layer system, showing
Fig. 1. BSE image of the corrosion layer in the quaternary bronze (magni®cation 800).
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Fig. 2. Line scan pro®le across the alloy±corrosion interface of the quaternary bronze.
in the outer layer a depletion of Cu and an enrichment of Sn. Due to mechanical stress or chemical reaction (e.g. dehydration), crack formation has occurred in some places, which results in pit corrosion. and hence the extreme dissolution of the alloy i.e. an increase of the Cu concentration. Because of the high Cu concentration and the simultaneous presence of Cl anions originating from the electrolyte, CuCl was formed building up an internal layer beneath the original one. The internal layer consists of CuCl or CuCl2.
average thickness of ca. 38 mm. Fig. 5 shows the results of the line scan performed along the arrow in Fig. 4 and shows the corresponding distributions of the elements Cu, As and Cl. The results show no signi®cant dissolution of copper, this in contrast to the quaternary bronze. Both As and Cl however show an enrichment in the inner layer. It can be concluded that the arsenical copper shows a Type II structure and the beginning of pit corrosion, which is indicated by the presence of cuprous chloride under the cuprite layer (Cu2O) [20].
3.2. The arsenical copper
3.3. The tin bronze
A photograph of the arsenical copper (Cu±As alloy) corrosion±alloy interface in Fig. 4 shows a well-separated three-layer corrosion structure. The outer layer (green) is rough with unequal precipitations, which tower from the surface. According to the FTIR measurements this layer is composed of a hydrated copper carbonate, malachite (Cu2CO3(OH)2H2O). The sandwiched cuprite layer (Cu2O, red) on the other hand has a compact and homogeneous-looking structure, while the inner nantokite layer (CuCl, pale green) reveals an inhomogeneous structure with globules. The thickness of the corrosion varies from 23 to 78 mm, with an
The corrosion of the tin bronze (Cu±Sn alloy) consists of a three-layered structure (Fig. 6). The outer layer is porous, whereas the middle and the inner layer are more compact. Contrary to the quaternary bronze and the arsenical copper, where the interface between corrosion and alloy can be clearly distinguished, it is clear that the interface is much less distinct. Additionally, corrosion can clearly be observed in places where the original surface is destroyed. Examinations of the cross-sections re¯ect the loss of matter due to an aggressive corrosion process visible in local pits or in crevices. The latter ones are a typical observation in
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Fig. 3. (a) Photograph of an example of pit corrosion (magni®cation 90); (b) X-ray map of Cl in the same area (magni®cation 400).
this bronze. The thickness of the patina varies from 40 to 138 mm, with an average thickness of ca. 67 mm. The line scan in Fig. 7 was measured on the tin bronze along the arrow in Fig. 6 and shows the distributions of the elements Cu, Sn and Cl in the two compact layers of the corrosion. The Cl signal increases from the middle to the inner layer reaching a maximum at the interface with the alloy. The Cl signal then drops sharply in the alloy. The Sn signal is slightly enriched
in the middle layer and depleted in the inner. Fig. 8 shows the FTIR spectrum of the outer corrosion layer and shows a good agreement with malachite. In general the tin bronze can be classi®ed as a Type II corrosion structure because of the typical coarse surface and the different layers in the corrosion zone. From the outer to the inner layer, the corrosion shows the presence of malachite, a cuprous oxide layer and a cuprous chloride layer.
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Fig. 4. Photograph of the corrosion layer in the arsenical copper (magni®cation 90).
3.4. The leaded tin bronze The corrosion layer of the leaded tin bronze (Cu± Sn±Pb alloy) is distinctly visible from the alloy (Fig. 9). Examinations of the cross-section show in general a compact two-layer structure. Perpendicular
cracks are visible due to the dehydration phenomena of the corrosion components in the passive layer after atmospheric exhibition, which can possibly be a starting point for localized corrosion. Lead is visible in bright inclusions both in the patina layer as well as in the alloy. The thickness of the corrosion
Fig. 5. Line scan pro®le across the alloy±corrosion interface of the arsenical copper.
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Fig. 6. BSE image of the corrosion layer in the tin bronze (magni®cation 430).
varies from 18 to 60 mm, with an average thickness of 25 mm. At some places on the surface, laminated layer corrosion can be observed, in which alternating patina deposits are visible (Fig. 10). Also here huge lead inclusions can be observed in both the alloy and the
corrosion, indicating that no remarkable dissolution of lead took place. The elements Sn and Cu show similarities in behavior with the quaternary bronze and the tin bronze, i.e. Sn shows an enrichment and Cu a depletion in the corrosion layer. Lead is present in inclusions in both
Fig. 7. Line scan pro®le across the alloy±corrosion interface of the tin bronze. Only the two compact corrosion layers are pro®led.
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Fig. 8. FTIR spectrum of the corrosion surface of the tin bronze (full line) and a reference spectrum of malachite (broken line).
the metal as well as the corrosion layers. Within the laminated layer corrosion area, a line scan pro®le shows alternating signals of Cu and Sn, re¯ecting clearly the different deposits (Fig. 11). Similar as for the previous alloys, the chlorine signal is signi®cantly enhanced in the alloy±corrosion products interface. Also here FTIR analyses show the presence of malachite on the surface. The corrosion formation of the leaded tin bronze can hence be classi®ed as a Type I even structure, with an enrichment in Sn and a depletion in Cu in the corrosion. At a few places a Type II coarse structure can be observed. Additionally, laminated layer corrosion could be observed.
3.5. The brass The brass (Cu±Zn alloy) shows two main types of corrosion: a compact two-layer system and a sponge-type structure (Figs. 12 and 13). The thickness of the corrosion varies from 3 to 73 mm, with an average thickness of 24 mm. Line scans on the compact corrosion structure show a selective depletion of Cu and an enrichment of Zn and Sn compared to the alloy. The sponge-type structure however shows no dissolution of Cu in the corrosion zone (Fig. 15). The slight decrease of the Cu pro®le in the alloy is caused by a change of the absorption coef®cient, which is higher in the alloy than in the passive layer
Fig. 9. BSE image of the corrosion layer in the leaded tin bronze (magni®cation 1700). The bright areas are lead inclusions.
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Fig. 10. BSE image of the laminated layer corrosion in the leaded tin bronze (magni®cation 600).
[20]. The steep enhancement of the Zn pro®le in the transition from the corrosion layer±alloy is an indication for localized inclusions. Fig. 14 presents an optical microscope image of the cross-section from the brass, where both types of corrosion are visible. From outer to inner layer, one can distin-
guish a cuprous chloride compound (CuCl or other soluble complexes such as CuCl2 or CuCl3 ) and cuprite. FTIR measurements con®rm the absence of malachite. From these results it can be concluded that the corrosion formation of brass under the present
Fig. 11. Line scan pro®le in the laminated layer corrosion of the leaded tin bronze.
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Fig. 12. BSE image of the compact corrosion in the brass (magni®cation 1400).
conditions can be classi®ed as a Type I structure. Furthermore, cracks are visible especially at domains, which were close to the interface. This leads to the assumption that, because of the anodic polarization procedure, the compact layer probably tended to crack after some critical degree of growth, obviously due to internal stress. At the bottom of these cracks, a renewed selective dissolution occurred i.e. cracking
went further on possibly causing this macroscopic ``granular'' structure. Presumably, the formation of the Type I corrosion layer was attributed to the local presence of Sn, whereas the sponge-like corrosion occurred at areas with a poor Sn content. The outer layer consisted of CuCl or other soluble complexes such as CuCl2 or CuCl3 , whereas the internal layer was formed by cuprite (Cu2O).
Fig. 13. BSE image of the sponge-type corrosion structure in the brass (magni®cation 900).
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Fig. 14. Line scan pro®le in the sponge-type corrosion structure of the brass.
Fig. 15. Photograph of the corrosion layers in the brass (magni®cation 400).
4. Conclusions In this work arti®cial patina layers were produced on ®ve different copper alloys by electrochemical means. The obtained patina layers were characterized using light microscopy, SEM±EDX and FTIR. The data were classi®ed according to the model by Robbiola et al. [16] and are listed in Table 2. Results showed that the morphology of the surfaces and the elemental compositions of the corrosion products depend strongly on the chemical composition of the alloys. A Type I even structure could be observed on the quaternary bronze and the brass; a Type II
`rough' structure was predominant for the arsenical copper and the tin bronze. The leaded bronze showed the presence of both structures. Table 2 Overview of the thickness and type of corrosion Composition
Range of thickness (mm)
Type
Quaternary bronze Arsenical copper Tin bronze Leaded tin bronze Brass
14±146 23±78 40±138 18±60 3±73
I/Pit II/Pit II I/II/Laminated layer I
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The ®gures of Table 2 indicate that the quaternary bronze and the tin bronze showed a similar thickness. The same conclusion can be drawn for the arsenical copper and the leaded tin bronze, though smaller than the previous two. The thinnest layer was observed for the brass. The line scans showed that the elements Cu and Sn were mainly responsible for the corrosion of the elements. In addition this work has also shown that the formed patina layers are very comparable to natural patinas on archaeological bronze [21]. Acknowledgements The authors wish to thank L. Robbiola for his valuable comments and P. Cool for the FTIR analyses. The European Commission, DG XII, is gratefully acknowledged for ®nancing the project SMT4CT96-2055 ``IMMACO''. AA is indebted to FWO, Belgium. References [1] R. Tylecote, The effect of soil conditions on the long-term corrosion of buried tin bronzes and copper, J. Archaeol. Sci. 6 (1997) 345. [2] E. Mattson, A.G. Nord, K. Tronner, M. Fjaestad, A. LagerloÈf, I. UlleÂn, G. Borg, Deterioration of Archaeological Material in Soil: Results of Bronze Artifacts, Konserveringstekniska Studier, RiksantikvarieaÈmbetet (RIK) 10, Stockholm, 1996. [3] N. Nielsen, Corrosion product characterization, in: B. Brown, H. Burnett, T. Chase, M. Goodway, J. Kruger, M. Pourbaix (Eds.), Corrosion and Metal Artifacts: A Dialogue Between Conservators, Archaeologists and Corrosion Scientists, US Bureau of Standards Special Publication, Washington, Vol. 479, 1977, p. 17. [4] C. Swann, S. Flemming, M. Jaksic, Recent application of PIXE spectrometry in archaeology I. Characterization of bronzes with special consideration of corrosion processes on data reliability, Nucl. Instrum. Meth. B 64 (1992) 499. [5] T. Weisser, The de-alloying of copper alloys, in: Conservation in Archaeology and the Applied Arts, International Institute of Conservation, Stockholm, 1975, p. 207. [6] L. Soto, J. Franey, T. Graedel, W. Kammlott, On the corrosion resistance of certain ancient Chinese bronze artifacts, Corr. Sci. 23 (1983) 241. [7] N. Meeks, Tin-rich surfaces on bronze: some experimental and archaeological considerations, Archaeometry 28 (1986) 133.
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[8] L. Robbiola, L. Hurtel, Standard nature of the passive layers buried archaeological bronze: the example of two Roman half-length portraits, in metal 95, in: I. MacLeod, S. Pennec, L. Robbiola (Eds.), Proceedings of the International Conference on Metals Conservation, James and James, London, 1997, p. 109. [9] W. Oddy, N. Meeks, Unusual phenomena in the corrosion of ancient bronzes, in: Science and Technology in the Service of Conservation, IIC Washington Congress, 1982, p. 119. [10] R. Tylecote, The apparent tinning of bronze axes and other artifacts, J. Hist. Met. Soc. 19/2 (1985) 169. [11] T. Stambolov, The corrosion and conservation of metallic antiquities and works of art, A Preliminary Survey, Central Laboratory for Objects of Art and Science, Amsterdam, 1968. [12] R. Gettens, The corrosion products of ancient Chinese bronze, J. Chem. Educ. 2 (1951) 67. [13] W. Geilmann, Verwitterung von Bronzen im Sandboden. Ein Beitrag zur Korrosionsforschung, Angew. Chem. Int. Edit. 68 (1956) 201. [14] L. Robbiola, Caracterisation de L'Alteration de Bronzes Archaeologiques enfouis a partir d'un Corpus D'Objets de L'Age du Bronze, Mechanismes de Corrosion, Ph.D. Diss., L'Universite de Paris 6, Paris, 1990. [15] W. Chase, Chinese bronzes: casting, ®nishing, patination and corrosion, in: D. Scott, J. Podany, B. Considine (Eds.), Ancient and Historic Metals, The Getty Conservation Institution, 1994, p. 85. [16] L. Robbiola, J. Blengino, C. Fiaud, Morphology and mechanism of formation of natural patinas of archaeological Cu±Sn alloys, Corr. Sci. 40 (12) (1998) 2083. [17] C. Ingelbrecht, A. Adriaens, E. Maier, Certi®cation of Arsenic, Lead, Tin and Zinc (Mass Fractions) in Five Copper Alloys CRM 691, EUR 19778/1 EN, Of®ce for Of®cial Publications of the European Communities, Luxembourg, 2001. [18] T. Beldjoudi, F. Bardet, N. Lacoudre, S. Andrieu, A. Adriaens, I. Constantinides, P. Brunella, Surface modi®cation processes on European union bronze reference materials for analytical studies of cultural artifacts, Surf. Eng. 17 (3) (2001) 231±235. [19] I. Constantinides, M. Gritsch, A. Adriaens, H. Hutter, F. Adams, Microstructural characterization of ®ve simulated archaeological copper alloys using light microscopy, scanning electron microscopy-energy dispersive X-ray microanalysis and secondary ion mass spectrometry, Anal. Chim. Acta 440 (2) (2001) 189±198. [20] L. Robbiola, R. Portier, Electron and microscopy and EDX analysis in the investigation of the decupri®cation phenomena in Cu±Sn alloys: a comparison between archaeological and synthetic bronzes, in: H.A. Calderon, Yacaman (Eds.), Electron Microscopy, Vol. III, 1998, p. 289. [21] M. Wadsak, I. Constantinides, G. Vittiglio, A. Adriaens, K. Janssens, M. Schreiner, F. Adams, P. Brunella, M. Wuttmann, Multianalytical study of patina formed on archaeological metal objects from Bliesbruck-Reinheim, Mikrochim. Acta 133 (1±4) (2000) 159±164.