Luc Robbiola

Christian Fiaud

Stéphane Pennec

Stéphane Pennec Lp

New model of outdoor bronze corrosion and its implications for conservation

Keywords: Atmospheric corrosion, bronze, conservation, corrosion products, outdoor sculpture Figure

A new model that takes into account both the tin and t h e copper in the alloy is proposed to improve the understanding of deterioration processes of outdoor bronze sculptures. The corrosion mechanism relates to the selective dissolution of copper. Two main processes occur: the process of tin oxidation with the formation of insoluble tin compounds which remain in the corrosion film, and the process of copper oxidation with the migration of copper ions to the surface. Considering this mechanism, mechanical action, and the acidity of the precipitations, two different evolutions at the surfaces are suggested. On the surfaces directly exposed to the rain, copper ions are washed out leaving a tin-rich external layer. A cyclic corrosion process is engaged. In the unexposed surfaces, the outermost layer is formed with well-known copper corrosion products. This model can be used to establish the degradation state of outdoor bronze sculptures for a new conservation approach.

Introduction

The purpose of this present paper is to outline a general framework for the understanding of the corroded surfaces of outdoor bronzes. Due to the increase in the pollution level of the atmosphere, the patinas on outdoor bronzes are now generally unstable. This can be observed by green trails on the stone basements due to the dissolution of copper. Numerous papers explain the bronze corrosion process by applying the copper corrosion model which leads to the following corrosion structure: a green outermost layer of copper II compounds (generally hydroxysulfates) and an internal layer of copper oxide located on the unattacked alloy [1-3] .

The model described here was established from several investigations of outdoor bronze sculptures such as Rodin's bronzes, a Stanislas statue in Nancy, France, and other bronzes exposed to urban atmospheres [4-6] . In these studies, different analyses were performed to correlate the nature of the corrosion products and their location with exposure conditions. The nature of patinas and their chemical characteristics will be reviewed first. Then, this information will be used to propose a qualitative mechanism for the growth of corrosion layers. Some aspects of the problem cannot be presently specified with ri gor. The semiquantitative assessment of the formation of corroded surfaces represents a step towards a more complete understanding of the corrosion processes. This paper will make frequent reference to published works on copper patina formation [START_REF]Copper Patina Formation[END_REF] .

The nature of corrosion products

Analysis of corrosion products on outdoor bronzes (Cu-Sn alloys) revealed numerous copper compounds which have their own characteristics, depending on surface orientation and the conditions of atmospheric exposure. Characterization of the corrosion products' thermodynamic stabilities and research on their formation kinetics have already been published [8-11] . Little is known about the corrosion products of tin. They are generally not identified in the bronze patinas. Nonetheless, recent results [START_REF] Robbiola | Nouvelle contribution à l'étude des mécanismes de corrosion des bronzes de plein air: caractérisation de l'altération de bronzes de Rodin[END_REF][START_REF] Ghali | Etude de la protection des monuments en bronze de la ville de Québec[END_REF][START_REF] Amarger | Restauration du groupe en bronze de J. Dalou, le Triomphe de la République, place de la Nation à Paris[END_REF] show that the corroded surfaces may be classified in relation with their aspect (colour, morphology, hardness) and composition.

The different surface categories are related to the rainfall exposure conditions. The results are summarised in Table 1.

Two types of surfaces may be distinguished: those exposed to rainfall, and those protected from this effect. The first category in the table lists results for surfaces exposed to rainfall. They are made of pale green products and black areas which stand as islands surrounded by the pale green products (Figure 1). As already mentioned by Weil et al., the pale green areas act as anodes and the black areas act as cathodes [START_REF] Weil | The Corrosive Deterioration of Outdoor Bronze Sculpture[END_REF] . Elementary analysis reveals that the surface compounds have a high tin element content. X-ray diffraction or infrared spectrometry analysis show that the copper compounds are mainly brochantite and seldom antlerite.

The second category in the table outlines results for the surfaces that are not exposed to the rain. They are generally green deposits and/or opaque black crusts [START_REF] Alunno-Rossetti | Analyses of the Patinas of a Gilded Horse of St. Mark's Basilica in Venice: Corrosion Mechanisms and Conservation Problems[END_REF] . These crusts (Figure 2) contain various incorporated atmospheric particles and are often found above the green deposits. Compared to the first group, these corrosion compounds have a low or very low tin elemental content, and X-ray or IR spectrometry reveal that they contain both antlerite and brochantite.

Due to their amorphous nature, tin compounds are very difficult to identify with X-ray diffraction or infrared spectrometry. Variations in the colour or local composition of the compounds have not been considered in this work. They are not characteristics of the corrosion mechanism we will describe. Therefore, the red-brown or "quasi-metallic" grey surfaces or cupric oxide and organic compounds observed or detected will not be considered here [START_REF] Robbiola | Nouvelle contribution à l'étude des mécanismes de corrosion des bronzes de plein air: caractérisation de l'altération de bronzes de Rodin[END_REF] .

Results given in Table I confirm that there is apparently no relationship between the exposure time and the nature of outermost corrosion layers. This can be correlated to a quasi-stationary state reached by the metallic materials in atmospheric conditions. The range of time to achieve this condition is decreasing: a bronze obtained its patina in several decades at the beginning of the century, while ten years seems to be actually sufficient in the urban areas of industrial countries [START_REF] Fiaud | La corrosion atmosphérique du cuivre[END_REF] . The results also confirm that although metallic structure relates directly to the alloy content, the Zn and Pb contents of the base alloy appear to have a minor influence on the main characteristics of the corrosion products [START_REF] Dolske | Acid Deposition Impacts on Historic Bronze and Marble Statuary and Monuments[END_REF] .

These remarkably constant features reveal that outdoor bronzes have a general corrosion behaviour which is strongly dependent on the washing effect of rain-water. Moreover, our understanding of corrosion phenomena must take into account the influence of the tin in the alloy. The nature of the outdoor bronze patinas cannot simply be regarded as that of the pure copper exposed to the atmospheric environment.

Qualitative model for patina formation

The observed physical and chemical characteristics of the bronze patinas have shown that some constant effects are observed. In order to explain these results, a qualitative model is proposed. This model is based on the formation process model of the corrosion structures observed for buried archaeological bronzes, according to the specific conditions of the atmospheric corrosion [START_REF] Robbiola | Corrosion Structures of Long-term Burial Cu-Sn Alloys: Influence of the Selective Dissolution of Copper[END_REF][START_REF] Robbiola | Apport de l'analyse statistique de la composition des produits de corrosion à la compréhension des processus de dégradation des bronzes archéologiques[END_REF] .

The corrosion process is essentially caused by a presence of water on the bronze surface. The water on the surface results from precipitation (fog, rain, snow, etc.) or condensation at the metal surface [10, p. 756] . In the absence of rainfall, the surface water can be assimilated into an electrolyte film whose composition and pH depend on the atmospheric compounds and the soluble corrosion products [9, p. 724] . Atmospheric corrosion can be regarded as a corrosion process under an electrolyte film. So, by comparison with bronzes partially immersed in an aqueous electrolyte solution, two particularities must be recalled. First, the anodic dissolution products that cannot be evacuated from the surface can easily induce a deposit on the metal surface. This deposit may have either a protective effect if the products are insoluble and homogeneous, or may have an aggressive character if they are soluble and hygroscopic (i.e. porous). Secondly, the oxygen reduction rate is greater than that of the usual aqueous electrolyte at the initial stage of corrosion [START_REF] Fiaud | La corrosion atmosphérique du cuivre[END_REF] . Therefore, it appears that the presence of porous deposits has no protective effect. Moreover, these deposits can enhance the dissolution rate of the base alloy. This important point will be considered later in this paper.

Significant amounts of tin products are systematically detected in corrosion products on different bronzes. This result underlines the fact that the Cu-Sn alloy behaviour is more complex than that of the pure copper. Although copper is electrochemically more noble than tin, recent works have shown that for corrosive conditions, such as those of natural environments, the Cu-Sn corrosion behaviour is characterized by selective dissolution of the copper from the alpha phase of the alloy [START_REF] Robbiola | Corrosion Structures of Long-term Burial Cu-Sn Alloys: Influence of the Selective Dissolution of Copper[END_REF][START_REF] Robbiola | Apport de l'analyse statistique de la composition des produits de corrosion à la compréhension des processus de dégradation des bronzes archéologiques[END_REF] . Copper species can form deposits on the surface, as previously mentioned. In relation to the effect of the rainfall, two types of surface evolutions must be distinguished.

Figure 3 is a schematic diagram of the corrosion process which results in the types of corrosion products described in the previous section. Three main steps are outlined.

The first step is the basic mechanism of the selective dissolution of copper. In contact with the surface water (electrolyte), the oxidation of the alloy can be represented as:

{1} {2} Sn + O 2 + n H 2 O  SnO 2 .nH 2 O
Cu  Cu + + e-and Cu +  Cu 2+ + e-

The "tin oxide," stable in aerated conditions and over a large pH range, is cathodic in comparison to the copper. The copper ions produced during the process may build a layer of copper compounds such as cuprous oxide on the top of the surface, while tin compounds remain in the internal part of the corrosion layer.

The second step of the process augments the thickness of the copper deposit by the formation of several copper(II) minerals, the nature of which is greatly dependent on the local atmospheric compositions, i.e. mainly brochantite in urban areas or chlorinecontaining species in a marine environment. This process is similar to the formation of copper patinas discussed by Graedel [9, p. 736-738] . For bronzes, the controlling step of the process is the migration of copper ions from the alloy to the atmosphere through the deposit.

The third step is related to the surface exposure conditions. For actual environments, the acidity of rain and smog can both solubilise the outermost copper corrosion compounds and promote the transformation of minerals. For example, brochantite is transformed into antlerite (this process will be discussed later in the paper). We can distinguish between surfaces directly washed by rainwater (step 3.I) and surfaces which are not exposed to rainfalls (step 3.II).

Soluble copper compounds are leached from the rain-exposed surface. The deposit described in the second step is progressively destroyed. The surface is "active" to the main corrosion process (i.e. selective dissolution of copper and internal oxidation of tin of the first step). The corrosion proceeds from a cyclic mechanism that is strongly correlated with the precipitation frequency. Then, the heterogeneities of the metallurgical structure locally enhance the formation of electrochemical coupling. The surface can be either anodic (pale green areas) or cathodic (islands of black compounds); this is probably related to the microsegregations in the alloy. However, the antlerite formation needs an acidic water layer for a sufficiently long period of time [10, p. 765] . In the case of short acidity periods, which do not allow the growth of antlerite, brochantite can reform on the surface. It is therefore expected that antlerite forms preferentially on the unexposed areas of sculptures that have lower evaporation rates than exposed surfaces.

The scenario developed here explains in a qualitative way nearly all of the results obtained on numerous outdoor bronzes. Nevertheless, this model does not permit one to solve all the problems. One of these problems is that the model is based on a phenomenological approach which can only be used to diagnose the de-graded state but cannot predict the evolution of local patinas on outdoor monuments. The second point is that the exact process of the transition from Step 2 to

Step 3.I is not clearly established. Finally, as mentioned by Graedel, the nature and role of organic species could be very important, but these factors are not yet well understood [9, p. 739] .

Application to the conservation of outdoor sculptures

Two main observations can be made based on the model described above. Both aged and recent bronze patinas are now less protective due to air pollution conditions in the environment. Also, the corrosion process is directly related to the presence of water on the metal surface.

Therefore, from the conservation point of view, two conclusions can be drawn. First, it is not relevant to remove the old patina to apply a new one, except to change the aesthetic appearance. Secondly, the conservation of outdoor bronze sculpture involves ensuring a good protection against exposure to water. To this end, conservation treatments may consist of cleaning, passivation, and coating.

Cleaning methods

What should we remove? Dust or "opaque" black surfaces, located on the non exposed areas, must be removed first. These materials form a layer (sometimes more than 5 mm thick) that is very hygroscopic. Not only can these surfaces maintain a high humidity level on the alloy surface, but its black appearance is unaesthetic. Non-compact, pale green corrosion products located on the exposed area are very porous and hygroscopic, and can compromise coating adhesion by their pulverulence.

On the other hand, we can conserve black spots located on the exposed area that delineate the limit of the original surface. However, it is often necessary to remove them for their unaesthetic appearance. Green unexposed areas can be found under the "opaque" black layer. They are not always stable, as is revealed by the transformation of brochantite to antlerite, but they can be considered a good substrate for an applied protective coating.

Several methods have been used for cleaning outdoor bronze sculpture including manual cleaning (scalpel, metal brushes, etc.), electrically operated metal brushes, water pressure tools, water-abrasive, and air-abrasive techniques. Each method can often be used properly, but the most efficient in removal of dust and noncompact corrosion products is certainly the air abrasive method [START_REF] Chase | Some Considerations in Surface Treatment of Outdoor Metal Sculpture[END_REF] . By varying air pressure, the diameter of the nozzle, and the nature of the abrasive (softness, size, geometry), it is possible to obtain a very good small-and large-scale cleaning. The walnut shell air-abrasive technique (See Figure 4) is widely used for bronze cleaning without stripping [START_REF] Lins | Techniques for Corrosion Product Removal[END_REF] .

Passivation

There are few corrosion inhibitors available for bronze. Authors have only considered the role of copper and have mentioned the utilisation of benzotriazole (BTA), a copper corrosion inhibitor for outdoor bronze. Is benzotriazole, a good inhibitor for bronze?

There is much to support the use of BTA in the literature. Benzotriazole may build up a three dimensional film with copper, protecting the alloy from outside attack. Turgoose shows that the formation of this film on ancient artifacts happened after a week of immersion in an alcohol bath; obviously, this is not possible for outdoor bronzes [START_REF] Turgoose | Techniques for Metal Sculpture Corrosion Inhibitors[END_REF] .

Benzotriazole has also been added to such coating systems as microcrystalline wax formulations to increase their quality, such as in microcrystalline wax. For some time, BTA was also added to the famous acrylic resin called Incralac [START_REF] Lafontaine | The Use of a Stabilizing Wax to Protect Brass and Bronze Artifacts[END_REF][START_REF] Weil | The Approximate Two-year Lifetime of Incralac on Outdoor Bronze Sculpture[END_REF] .

Coatings

Acrylic, polyester, polyurethane, natural wax, and mineral wax have been used as coatings for outdoor bronzes. The characteristics to consider when selecting as coating are life-time, reversibility, aesthetic appearance, and facility of main-tenance.

Each chemical family has very different products and it is very difficult to give general recommendations. However, polyurethane can be considered as the best for its life-time (10 years), but it is also the worst for reversibility and facility of maintenance, as it requires complete removal by air-abrasive methods and reapplication for adequate protection. Mineral waxes (microcrystalline) have short lifetimes (2 to 5 years), but have excellent reversibility and maintenance properties. For instance, maintenance consists of a simple water cleaning and application of a new layer. Acrylic has intermediate properties, with a relatively good lifetime, and quite good solventreversibility properties [START_REF] Weil | The Approximate Two-year Lifetime of Incralac on Outdoor Bronze Sculpture[END_REF] . It is noteworthy that natural wax can generate acid radicals which increase corrosivity at the surface. None of the coatings are fully convenient, but our choice is oriented to the use of a microcristalline wax (See Figure 5) with a high melting point because of its facility of maintenance. A two years' maintenance is also good, because it forces people to maintain and to clean their sculptures. Whatever cleaning technique and protection system is chosen, we must be able to understand what should we remove, what should we protect, how much time the coating can be protective, and what is the maintenance process.

Conclusion

The corrosion behaviour of Cu-Sn alloys in outdoor atmospheric conditions is notably more complex than for the pure copper. Though copper is intrinsically a more noble metal than tin, the analysis of corrosion products revealed a tin enrichment; this relates to the formation of tin oxide, which is cathodic to copper. This phenomenon is always observed for bronze corrosion and is attributed to a selective dissolution of copper from the Cu-Sn alloy. If the copper compounds (mainly cuprous oxide and hydroxysulfates) can build a protective and adherent deposit on the surface, the dissolution of the alloy will be stopped.

If such a deposit cannot be built up, i.e. for areas washed by rainwater, the copper compounds will be leached and the corrosion will proceed by a cyclic mechanism strongly related to the frequency of rainfall.

A new framework is given in this work for understanding the formation of natural patinas. It enables future research to determine the precise mechanism of the destabilisation of the copper compound layers and to improve conservation treatments.

Figure 2 .

 2 Figure 2. View of a bronze surface not exposed directly to the rain. The green compounds are partially recovered by "opaque" black crusts.

Figure 3 .

 3 Figure 3. Schematic representation of the processes involved in the outdoor bronze corrosion.

Figure 4 .

 4 Figure 4. Cleaning of a bronze statue by walnuts shells with an air abrasive technique.

Figure 5 .

 5 Figure 5. Coating of a bronze statue by applying microsrystalline wax on the cleaned surface.

Table 1 .

 1 Comparison between main types of corroded surfaces of the outdoor bronzes exposed in urban areas. The compositions are given in weight percent.

			Main types of corroded surfaces
		Areas washed by the rainwater	//	Areas not washed by the rainwater
	Composition	Pale green to blue	Black
	of the alloy	(porous)	(compact,

very hard) Green (compact, ± hard) "Opaque" black (hard) Rodin's statues Cu

		(base) 3.6Sn	mainly brochantite	brochantite, organic	brochantite, gypsum,	antlerite, gypsum,
	Paris [4]	1.3Zn	and cuprite	compounds (?)	often antlerite,	organic compounds (?),
	(1880-1920)		(Sn/Cu = 0.8 ±0.3)	(Sn/Cu = 0.2 ±0.1)	sometimes atacamite	atacamite, brochantite
					(Sn/Cu = 0.08 ± 0.07)	(Sn/Cu = 0.02 ± 0.02)
	Des Boers	__	__	__	mainly brochantite, some	brochantite, antlerite,
	Quebec [5]				atacamite and cuprite	atacamite, nantokite,
	(1905)				(Sn/Cu = 0.22)	cuprite, cassiterite
						(Sn/Cu= 0.014)
	G.E. Cartier				mainly brochantite,	antlerite, brochantite,
	Quebec [5]	__	__	__	antlerite, cuprite	atacamite, nantokite,
	(1920)				and some atacamite	cuprite, cassiterite
					(Sn/Cu=0.25)
	Mgr de Laval		brochantite and		brochantite, cuprite
	Quebec [5]	__	cuprite	__	(Sn/Cu = 0.24)	__
	(1908)		(Sn/Cu = 1.56)
	Le Triomphe de 88.4Cu, 4.3Sn,	mainly brochantite		antlerite, brochantite,
	la Republique 5.3 Zn, 1.4Pb	(Sn/Cu = 0.8 ±0.3)	__	sometimes atacamite	__
	[6] Paris
	(1889)
	Stanislas	61Cu, 4Sn, 14.5Zn,	brochantite, cuprite	__	antlerite, quartz, gypsum brochantite, quartz,
	Nancy	20Pb	(Sn/Cu = 0.7 ±0.1)		organic compounds?	organic compounds?
	(1831)				(Sn/Cu=0.04± 0.03)	(Sn/Cu=0.01)

I ) Surfaces washed by the rainwater (leaching out of copper II ions) ATMOSPHERE corrosion products bronze (Cu-Sn alloy) outermost layer :

  mainly copper species Cu 4 SO 4 .(OH) 6 , Cu 3 SO 4 (OH) 4

	(1)
	water film
	soluble species (H , O 2 , SO 4 ...)	Basic mechanism oxidation of the alloy : reduction of soluble species .
		-formation of insoluble tin
		compounds (SnO 2 .nH 2 O), and migration of copper ions to
		the surface.
		-formation of copper species
	(2)	Increase in the corrosion
		layer:
	corrosion	formation of copper II
	products	minerals (outermost layer) on
		a inner corrosion layer rich in
		tin compounds
	(3 . outermost layer : Cu , Cu
	mainly Cu 4 SO 4 .(OH) 6 , and
	tin compounds ( SnO 2. nH 2 O)

Surfaces not washed by the rainwater (3 . II ) corrosion products corrosion products

  The deposit of copper species on surfaces protected from rainfall may be stable, but acidic waters can promote the transformation of brochantite into antlerite (See fig.3.II). Increasing the acidity of the water film (pH higher than 3.5) leads to the dissolution of brochantite (Cu 4 SO 4 (OH) 6 ) followed by antlerite precipitation (Cu 3 SO 4 (OH) 4 ) in accordance with the reactions:

		+	--
			Cu , Cu +	++
	{3}	Cu 4 SO 4 (OH) 6 + 6H +  4Cu 2+ + SO 4 2-+ 6H 2 O
	{4}	3Cu 2+ + SO 4 2-+ 4H 2 O  Cu 3 SO 4 (OH) 4 + 4H +
		Cu , Cu + ++
			Cu , Cu +	++
	+	++


L Robbiola

C Fiaud

Basic Structure of Passive Layers of Cu-Sn Alloys Applied to Patinas of Arch a eol ogi cal Bronzes

INTRODUCTION

The passive bronze corrosion stru ctu r es have been characterized on numerous s i n gl ephase archaeological bronzes from Late t o Early Bronze Age period (1500-900 BC), from metallographical examinations of cross-sections of artifacts. This paper focuses on the microstructure and the elementary composition of the passive patinas by SEM and EDX analysis (ultrathin window). A basic structure of the protective patinas is first described, then, deviations to the standard model are given. This model of passive structure and its deviations well agree with observations of archaeological bronzes stemming from other historical an d geographical periods (Swann et al. (1992), Meeks (1993)).

BASIC STRUCTURE OF THE PASSIVE PATINA

In natural aqueous corrosive environments (pH between 3.5 to 9, redox potential in the range of -200mV to +400mV/ENH), Cu-Sn alloys can form a passive corrosion structure which preserves the limits of the original bronze surface (Robbiola et al. (1993)). The external aspect of such a protective patina is a shiny lustrous surface with different possible colours (blue, grey, dark-green...). From cross-section examinations, the structure is a two-layer one, whose thickness may vary between 5 and 70 µm (Fig. 1): -an outermost layer, characterized by high Sn/Cu ratios and by the presence of elements from the corrosive environment such as 0, Si, Al, P, Fe, Cl and Ca, -an internal layer in contact with the alloy, irregular in shape and thickness, which has lower Sn/Cu ratios than in the outermost layer, but higher ratio than in the alloy and on1y oxygen element. From microstructural examinations, it appears that the passive structure has been formed without apparent volume change. In accordance with previous results (Robbiola et al. (1994)), the process of formation can be attributed to internal oxidation of tin in relation with copper selective dissolution. Furthermore, tin atomic fraction in the outermost layer seems to depend on the aggressive character of the corrosive environment and on the Sn/Cu ratio of the alloy .

DEVIATIONS FROM THE BASIC STRUCTURE

From metallographical examinations, sorne discrepancies between t h e b asic mo d e l a n d observations may be encountered (Fig. 2). These deviations can be classified into three groups which are summarized in Figure 3.

a° The first group of deviation is characterized by the presence of an outer crust or deposit (often more than 300 µm thick) on the outermost layer. Analysis of deposits mainly revealed copper corrosion products such as cuprous oxide (cuprite) and/or cupric compounds (such as hydroxycarbonates or hydroxysilicates). The deposit formation can be rela ted to th e ICEM 13 stabilization of copper products on the surface following the leaching of the copper ions from the alloy, in relation with the environment composition. b° The second type of deviation corresponds t o a modification of the corrosion layers due to localised extension of internal layer into the alloy or internal cracks. The extension of the internal layer is accompanied by the presence of corrosive species, n o tab l y chloride anion which ar e known t o be important in localised corrosion. The cracks c o u l d b e related t o numerous factors such as internal stresses inside the corrosion layers, ageing of the corrosion compounds ... Furthermore, they may, in some cases, go in contact with the corrosive environment (aqueous electrolyte) giving rise to localised breakdown of the outermost layer. c° The third type of deviation corresponds to an attack at the layer/alloy interface, due to an internal or transgranular attack strongly related to an internal oxidation process (diffusion short-circuits).

7Figure 1 (

 1 Figure 1 (Left): Bronze Age needle, structure of a protective patina (etched cross-section -SEM examination) -the alloy is at the bottom, the outermost layer above) Figure 2 (right): Cross-section of a Bronze Age pin, internal attack inside a protective patina.

Figure 3 :

 3 Figure 3: Schematic representation of the passive structure of bronze artifacts with the three groups of structural deviations: a) crusts or deposit on the outermost layer, b) internal localised attacks within the passive layers and c) attack at the corrosion layers /alloy interface. Meeks N. (1993) Metal plating and patination (Butterworths-Heinemann, Oxford) pp 247-275. Robbiola L., Fiaud C. (1993) Control of Copper oxidation (Ed. de Revue Métallurgie, Paris) pp 157-162. Robbiola L., Fiaud C., Harch A. (1994) Modifications of passive films (Institute of Materials, London) pp 150-154. Swann C., Fleming S., Jaksic M. (1992) Nucl. Instr. and Meth. B64, 499-504.


L Robbiola

C Fiaud

A Harch

Characterisation of Passive Layers of Bronze Patinas (Cu-Sn Alloys) in Relation with the Tin Content of the Alloy

Jn order to develop our knowledge of the relations between the high degree of protectiveness of passive layers and their composition, corroded archaeological bronzes of Bronze Age (1500-950 BC) have been characterised by different methods of analysis. Results reveal that the passive layers have a bilayer structure and are due to the selective dissolution of copper from the copper solid solution of the alloy to the electrolyte. For all of the objects, the ratio [(Sn/Cu)p / (Sn/ Cu)a], with p for the outermost passive layer and a the alloy, has about the same value. Sn concentration within outermost passive layers is discussed in relation with the theoretical implications of the Kirchheim model. X Sn = 1 1 + M Sn M Cu Y Cu Y Sn

Introduction

Corrosion studies on passive films are mostly concerned with processes occurring during short periods of time, i.e. often less than a hundred of hours. This paper is focused on the study of ancient metallurgical materials which have reached a stationary state after several hundreds of years.

Often, antique bronzes (Cu-Sn alloys) reveal 'uniform' surfaces which preserve the original surfaces, i.e. decor or polishing traces are still visible. This case of passive surface, known in the literature as 'noble patina' [START_REF] Gettens | Patina noble and vile' in Art and Technology:a symposium on classical bronzes[END_REF][START_REF] Soto | [END_REF], is a typical example of the protection of an alloy for more than several millennia.

Little is known about the nature of these passive surfaces and the processes of their formation. External aspect, composition and microstructure of passive layers in natural conditions will be discussed in this paper from results obtained on a corpus of Bronze Age objects (1500-950 BC). Then the results will be related to the Kirchheim model describing passive films formed on homogeneous alloys in the quasi stationary condition.

Experimental Method

Description of the samples

The samples are 13 archaeological artefacts -p i n s , bars, needles, rings dated from about 1500 to 950 BC and discovered in an important Bronze Age metallurgical centre (Fort-Harrouard, France). They have been described in detail elsewhere [3]. The archaeological soil is representative of a moderately aggressive oxygenated soil [START_REF] Romanoff | Underground Corrosion[END_REF]. All these objects were not restored. The objects are Cu-Sn alloys with a Sn content from 4 to 14% by weight. They contain minor elements such as Ni (from 0.1to1% by weight), As (0.1-0.35%), Fe and Ag and Sb (0.03-0.25%), Zn and Co (0.003-0.01). Three objects have a lead content of more than 1%. However, lead is not miscible in the alloy and does not modify the nature of the copper solid solution.

Characterisation of the surfaces

The passive surfaces were analysed by energy dispersive spectrometry (EDS) on areas of ca. 0.l to 0.01 mm 2 • Each artefact was introduced directly into the chamber of a scanning electron microscope (SEM) without any preparation to keep it intact. In another part of the work, corrosion products were scrapedoff from the surface before being analysed by X-ray diffraction and by infrared spectrometry.

Three objects, considered as representative of the whole corpus, with different tin contents, were sectioned transversally and longitudinally for metallographic examinations and characterisation of the internal microstructure. They were homogeneous single phase alloys with an annealing structure and contained numerous copper sulphide inclusions due to the ores used [START_REF] Hanson | Chill Cast Tin Bronze[END_REF].

Principal Component Analysis (PCA)

The PCA method was applied in order to interpret the EDS analyses of the patinas. The method allows [START_REF] Minkowski | Factor Analysis in Chemistry[END_REF], first, a description of the variations of the p analysed chemical elements (here Si, P, Cl, Fe, Ni, Cu and As) and,secondly, a representation of the whole analytical results in a different reference system with p-coordinates defined as the main axes. Each new Di axis (i=1,…, p) is defined by an eigenvector Ui which corresponds to the eigenvalue λi of the diagonalised correlation matrix between the p chemical elements. So the p eigenvectors (principal components) are expressed as a linear combination of the previous variables (Si, P ...). The eigenvectors Ui are classified by decreasing order of the eigenvalues to obtain the best representation of the results for the first principal axes.

In our case, the analytical results were pretreated first by dividing each analytical point by its Sn value to have independent values, and then by scaling each variable by the standard deviation of the analysed points. This procedure allows giving an equal weighing for each value in the data matrix.

Results

The passive 'surface' of the bronzes appears as a shiny lustrous surface (Fig. 1) keeping the limit of the original surface. It is characterised by a two-layer structure (Fig. 2):

-An homogeneous outermost layer, around 5 to 40 µm deep, about the size of a metallic grain, and with differrent possible colours (green, dark grey, blue or bright grey). Copper sulphide inclusions from the alloy are still visible in the outermost layer. Outer crusts, sometimes observed on this layer, result from the precipitation of copper cations in the soil. -An internal grey-brown layer, whose thickness may vary between 1 and 30 µm. Intergranular attack is occasionally encountered at the boundary of the alloy.

From the microstructural examinations, it can be concluded that the passive structure has grown from the original surface to the alloy without apparent volume change.

A typical example of the distribution of the chemical elements is given in the Fig. 3.

The outermost layer is rich in tin, oxygen and soil elements (mainly Si, Al, P, Ca, Fe and Cl). The soil elements show significant variations in composition for the same object, and also from one object to another. Only the Sn-content in the outermost layer was found roughly constant in a given patina, yet it varied from one sample to another. Ratios (Sn/Cu) were then defined for the outermost layer (Sn/Cu)p and for the bulk alloy (Sn/Cu)a. The ratio between the two quantities,

β = � Sn Cu � 𝑝 � Sn Cu � 𝑎 (1)
was found to be β = 18 ± 4 for all of the samples. Similar values had been obtained in the literature [START_REF] Sawada | Bull. ofthe 30th anniversary[END_REF][START_REF] Geilmann | [END_REF][9]. So it appears that a general behaviour exists for objects buried in moderate aggressive soils (loam or dry or calcareous soil) for different metallurgical and historical periods (from 1000 BC [START_REF] Geilmann | [END_REF]9] to 1000 AD [START_REF] Sawada | Bull. ofthe 30th anniversary[END_REF]). The internal layer contains lower tin amounts than the outermost layer and no soil components. In this layer, the Sn/Cu varies strongly from one point to another.

Little information could be obtained from X-ray diffraction and infrared spectrometry results. The out-Fig. 2 Transversal microstructure of the passive layers. Inclusions within the passive layers and the alloy are copper sulphide inclusions (SEI). Bulk alloy is in bottom. ermost layer was amorphous and contained princi pally phosphate, silicate and hydroxyl groups under a non identified form. The products of the internal layer were difficult to isolate and no significant result could be interpreted. For both layers, it could only be assumed that the tin compounds are mainly amorphous hydrated tin oxides. It is known that tin (IV) oxide is thermodynamically stable over a large range of pH and potential values [10]. The copper c o mpou nd s could not be identified as well classified products, such as malachite, chrysocolle, libethenite..., as often encountered for corrosion copper products in bronzes [START_REF] Gettens | Patina noble and vile' in Art and Technology:a symposium on classical bronzes[END_REF][START_REF] Soto | [END_REF][START_REF] Sawada | Bull. ofthe 30th anniversary[END_REF][START_REF] Geilmann | [END_REF].

The PCA on the outermost layers analysed by EDS is given in Fig. 4 for the two first principal components (72% of the PCA representation). PCA allows to at tribute a relative weight to each participant (Si, P ...) to the growth of the layer. If the chemical species are in the same region of the diagram, they can be supposed to have the same contribution to the growth of the passive layer. The presentation of the results in the Fig. 4 shows different classes of analytical points which correspond more or less to the amounts of soil elements (Si, P, Cl, Fe) in the patinas. We can remark that the classes are related to the different colours of the outermost layer. The largest amounts of soil elements are encountered in the blue patinas, whereas bright grey surfaces have a higher copper content than the others. The exact nature of the species in which the soil elements are incorporated has not been investigated and remains an open question.

Consequently, there exist very important varia tions in soil species from one kind of patina to another, The soil compounds maintain the electroneutrality but anions and minor contents or traces of metallic cations do not directly interfere in the relative enrichment of the tin cations in the passive layers. The reaction between the soil products and the metallic cations is not the rate determining step in the attainment of the passive state.

Discussion

The excellent corrosion resistance of tin bronzes in moderately aggressive natural conditions can be attributed to the formation of highly resistant tin compounds. Hydrated tin oxides (SnO 2 .xH 2 O) are known to form amorphous gel like compounds. Furthermore, three important points appeared from the results:

(1) The pseudomorphic replacement of the surface alloy by the passive layers,

(2) The same β value (eq. 1) for all the patinas, and

(3) The large variation of soil components in the passive outermost layers.

2Cu + H 2 O = Cu 2 O + 2H + + 2e - (3)
Cu 2 O + 2H 2 O + ½ O 2 = 2 Cu(OH) 2 (3')
The process involved is in fact an internal oxidation of tin and copper: the relative Sn enrichment in the outermost passive layer could be due to both the higher mobility of copper cations and the greater stability of the tin oxide in the layer.

For homogeneous binary alloys in the stationary state, the approach of Kirchheim offers to explain the atomic fraction of the components in the passive layer and in the alloy [11], assuming that the composition of the subsequent layers depends on the diffusivities of the cations. In the passive state the average atomic fractions of the components can be estimated from the following formula for a binary alloy A x B 1-x [12]: Consequently, the high Sn amount in the passive layers can only be explained by Cu selective dissolution (decuprification). The soil components (mainly silicates and phosphates) do not seem to affect directly the process of the selective dissolution of the copper and the relative tin enrichment: their incorporation into the patina could be a second step in the formation of the passive surface. It can be assumed that the process leading to the oxidation of tin and copper is:

Sn + 4H 2 O = Sn(OH) 4 + 4H + + 4e - (2)
Sn(OH) 4 = SnO 2 .xH 2 O + (2-x)H 2 O (2') ( 4
)
where Xp is the atomic fraction of the component in the passive layer and Xa, in the alloy, D and Z are respecttively the diffusivity and the valency of the cation components A or B. α is the transfer coefficient, s the jump distance of the cation and E' the electric field within the film.

In the case of bronzes, A= Sn and B= Cu. . s . F . E ')

) XSn,a using data from this work and from [START_REF] Sawada | Bull. ofthe 30th anniversary[END_REF][START_REF] Geilmann | [END_REF][9]. Assuming that the soil and minor metallic elements do not play a major role in the selective dissolution of copper, the atomic fraction XSn has been calculated as: enrichment in tin cations within the passive layers. The process is similar to an isomorphic one with internal oxidation. Tin atomic fraction within the outermost passive layer of archaeological bronzes was found to agree with the theoretical prediction of the Kirchheim model describing passive films formed on homogeneous alloys. [START_REF] Hanson | Chill Cast Tin Bronze[END_REF] where YCu/YSn is the ratio of the mass percentages and MSn and MCu respectively the atomic mass of tin and copper.

- (Z Z A 1 -X a X R . T a X 1 - D D X a a 1 -p B A X
The full lines in Fig. 5 give the theoretical curves corresponding to β= 14 and β=22. The large relative Sn enrichment in the passive outermost layers is consistent and may agree with the Kirchheim's model for single phase alloys (XSn,a < 0.085).

In order to approximate the DCu/DSn ratio in the stationary state, we used ZCu= +2 and ZSn= +4, α =0.5, s = 10 -8 cm and E'= 5.10 6 V/cm which are in a range of reasonable values [12]. Equation (4) gives: 105 < D Cu / D Sn < 170, confirming a much higher diffusivity of copper cations than that of tin cations.

Conclusion

The excellent resistance of Cu-Sn alloys in a moderately aggressive soil is based on the beneficial role of tin in the alloy. The structure of the passive 'surface' is of the bilayer type. The microstroctural characterisation and the elementary analysis indicated that the main corrosion effect is the selective dissolution of copper into the soil with a relative

Fig. 1

 1 Fig. 1 Macroscopic view of a passive patina with repassivated pits. Bronze Age bar (1250-1050 BC).

Fig. 3 X

 3 Fig.3 X-ray maps of the centre of the Fig. 2 (EDS: TRACOR TM -Ultrathin window)

Figure 4

 4 reveals different groups of chemical elements. The group of the incorporated soil elements (Si, P, Fe and Cl) are in the same region of the diagram and are strongly correlated, i.e. these ele 152 ments have a similar contribution to the elaboration of the outermost layer. For the constituents of the alloy, copper is unrelated as well as As and Ni, showing that the basic element of the alloy does not have the same contribution as the minor or traces elements in the formation of the protective layer.

  Fig. 4 Representation of the chemical variables and of the analysed points of the outermost passive layers. Principal components analysis.

  Figure 5 shows the results of measurements of the average composition of the tin atomic fraction in the passivating patina XSn,p as a function of that in the alloy

B

  Fig.5 Fraction of tin cation in passivating fllm (X Sn,p) as a function of the tin fraction in the alloy (X Sn,a) for various studied bronzes. The solid lines are calculated using eqn (4) with β = 14 and β = 22.