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PLASMA REDUCTION OF ACCELERATED SOIL CORROSION ON ANCIENT-LIKE BRONZES

Jelica Novakovic, Olga Papadopoulou, Michalis Delagrammatikas, Panayota Vassiliou Lab. of Physical Chemistry, School of Chemical Engineering, National Technical University of Athens

9, Iroon Polytechniou str., 15780, Athens, Greece Tel: +30-210-772-3063, E-mail: pvas@chemeng.ntua.gr

Eleni Filippaki, Constantine Xaplanteris, Yannis Bassiakos Lab. of Plasma Physics, Institute of Materials Science, NCSR ‘Demokritos’, Athens, Greece

Keywords: Cu-based alloys, bronze disease, soil corrosion, patina, Plasma reduction

ABSTRACT

Three Cu-based alloys, representative of major bronze families used in antiquity and manufactured with a technique that simulates ancient casting were buried for five years in NaCl enriched soil. This procedure leads to the formation of patinas similar to those of authentic archaeological objects. The alloy with the higher lead content seems to be the most corrosion resistant. All corroded alloys were characterized by a combination of analytical techniques (OM, SEM, EDS Analysis, X-ray Diffraction). Hydrogen glow discharge plasma was tested as a cleaning and stabilizing method. A gradual elimination of the dangerous chloride containing compounds in favor of the formation of more stable species, as a result of a reduction process, was observed. The progress of the treatment was monitored with the above-mentioned analyses.

INTRODUCTION

The conservation of archaeological Cu-based artifacts, due to their uniqueness and artistic value, comprises a great challenge and raises numerous difficulties. As ancient objects are witnesses of the past, they cannot be sampled to perform adequate characterization procedures for conservation, even after an extended pre-evaluation [1]. On the other hand, the use of commercially produced copper alloys as test samples for the above mentioned evaluation does not cover the peculiarities of the alloying elements, the metallurgical features attributed to the casting technique and the nature and microstructure of the corrosion products. The study of a large number of ancient bronzes led to the production of reference Cu-based alloys, whose chemical composition and micro-chemical structure are similar to that of ancient ones, in order to accomplish the simulation of an ancient patina [2]. A broad category of archaeological objects, are those which have been buried for centuries or even millennia. During a long burial period, extended redistribution of material takes place. The mechanism of these transformations is under the control of mass transport phenomena. Metal ions move outwards and chloride ions together with impurities of the soil diffuse inwards, resulting in the production of a variety of corrosion products, sometimes rather complex [1]. Previous studies revealed that the main corrosive agent in a burial environment is chloride containing species that induce the formation of dangerous copper chlorides and oxy-chlorides at the interface between the alloy and the external corrosion layers. Specifically, cuprous chloride (CuCl) is considered the principal agent for the outbreak of the “bronze disease”, the deterioration process occurring after the interaction of chloride-containing species within the bronze patina with moisture and air. This is a cyclic reaction which induces the formation of Cu2(OH)3Cl in various crystalline structures and accelerates the corrosion of the sound metal. These compounds react further with copper to form cuprous chloride and water. The final products are light green, powdery basic copper chlorides and the procedure is accompanied by an important volume expansion, which may result in the fragmentation of the object [3]. “Bronze disease” in antiquities can be prevented by a number of methods. The application of

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hydrogen plasma in the conservation and restoration of metallic archaeological objects is under research and development for the last twenty years [4]. The method is based on the reduction of the corrosion products on the metallic excavated objects by reactive reducing species such as hydrogen atoms in a H2 glow discharge plasma at low pressure and temperature. The phases containing chloride ions are destabilized and the chlorides can be removed to the gaseous phase of the discharge. The reduced corrosion products on the surface become brittle and can be easily removed [4-6]. The aim of this work is a further investigation of the nature and structure of artificial patinas formed on three different Cu-based alloys representative of the major bronze families used in antiquity. It has been shown [7] that ancient-like bronzes, when exposed in soil accelerated corrosion conditions can simulate, in a satisfying way, authentic ancient objects and can be described by using the phenomenological model proposed by Robbiola [8]. The studies carried out in various media (solutions, soil) and on different types of specimens (archaeological objects, reference alloys with composition similar to the ancient ones but produced with conventional techniques) by other researchers, provided useful guidelines for the identification and the surface characterization of the corrosion products [2, 8-18]. This work also attempts to investigate the potentials of the plasma reduction method in the conservation of bronze archaeological objects.

EXPERIMENTAL

Production of reference bronze alloys and experimental conditions

Reference Cu-Sn or Cu-Zn alloys, representative of ancient Greek and Roman ones, have been produced in the laboratory [19] in an electrically heated furnace at 1100 ˚C by using graphite crucible purposely designed in order to tailor the solidification and cooling behavior of the alloy and therefore its metallurgical features. Copper and copper-iron sulphides, which are typical impurities of ancient casting techniques, have been added, on purpose, to the melt before casting, in order to produce samples with micro-chemical structure and metallurgical features similar to those of ancient alloys. The chemical composition of the specimens is given in Table 1.

Table 1: Chemical composition of the specimens

Cu Sn Pb Zn

Bronze Alloy (A) 92.3 7.5 0.2 -

Zn-bronze(or Brass) Alloy (B) 82.3 3 0.5 14

Pb-bronze Alloy (C) 88 4 8 -

Table 2: Attica Soil analysis according to reference method ISO 11464

Cl-

(mg/g) SO4

2-

(mg/g) HCO3

-

(mg/g) TOC

(mg/g) Mg

(mg/g) Fe

(mg/g) Ca

(mg/g) pH Conductivity

(mS/cm)

0.2 5 0.5 0.3 28 14 80 8.7 275

Artificially long-term degradation tests have been carried out in the laboratory. Coin resembling samples (26mm diameter and 3mm height), were embedded in the soil of an Attica area (red with high pH-terra rossa) for five years. Soil samples were collected and treated according to the reference method ISO 11464 and then analyzed. The main soil properties are shown in Table 2. The particle size distribution was also determined in order to get a better insight to the burial conditions. The most important soil fraction (45%) corresponds to medium particle size (1mm), 35% of the particles have size under 0.2mm and only 20% corresponds to coarser soil agglomerates

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with size above 2mm. These results indicate that the particular type of soil provides moderate aeration and permeability in water and thus neither makes the corrosion of the buried metal objects extremely severe nor assists their better preservation. The study of Nord, Mattsson et al showed that soil conditions allowing access for air and water increase the corrosion rate. On the contrary, soils with grain size distribution from 0.25 to 0.50mm reduce the capillary rise of water and the hydraulic flow [16]. The soil was enriched after the burial with an additional quantity of chloride ions by pouring into the soil 50ml of aqueous solution of 3.5%NaCl per kg soil. The soil was kept wet by adding every month an estimated amount of water, in order to maintain a high percentage of humidity.

Plasma treatment

A radio frequency (RF) glow discharge plasma apparatus, similar to the Veprek’s prototype [4], was used. The reactor is a Pyrex-glass bell jar of 40cm inner diameter and 46 cm length. It is evacuated down to 10-2Torr and the H2 gas is introduced through a needle valve and a flow meter in the reactor. The temperature is monitored through a thermocouple inserted along the discharge axis in a Pyrex-glass sleeve. The power from a RF generator (27.12MHz, 2.8KWatt) is coupled through two copper electrodes 30 x 32 cm2, placed externally along the reactor. The samples are placed on Pyrex grids along the axis of the discharge. The samples were treated in 100 % H2 plasma for 1, 2 and 4 hours under 0.65 kWatt power, at 190-200 ºC and 0.8 Torr pressure.

Physico-chemical characterization

The physical and chemical characterization of the samples (and the progress of the elimination of the corrosion products) was performed by means of the combined use of optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). Both SEM and EDS characterizations were carried out by using a Quanta 200 FEI scanning electron microscope equipped with a tungsten filament and solid state back scattered electron detector. The microstructure of the samples has been determined by XRD (SIEMENS X-ray Diffractometer

5000) using Cu Kα x-ray source. The assignment of the crystalline phases was based on the JPDS powder diffraction file cards [20]. In this work, specifically for the study of the corrosion products, an additional utility of X-ray diffraction was used, the Glancing Angle X-ray Diffraction (GAXRD). In the x-ray diffraction pattern of thin films deposited on a substrate, contribution from substrate to the diffraction can sometimes overshadow the contributions from a thin film. GAXRD is used to record the diffraction pattern of thin films, with minimum contribution from the substrate. When the angle of incidence (αI) of X-ray beam decreases, so that the refractive index is less than unity, total external reflection of X-rays occurs below the critical angle of incidence αC. The diffracted and scattered signals at the angle 2θ arise mainly from the limited depth from the surface. The incident angle is fixed at a very small angle (2°, 5° and 10° for the particular experiments), hence penetration of x-rays into the specimen is reduced. X-rays are focused only in the top-most surface of the sample and the contribution of the substrate to the diffraction pattern can be minimized. GAXRD proved an efficient method for examining the corrosion layers located in various depths and the results show that some trace compounds were found only in the diffraction patterns of GAXRD 5° and GAXRD 10°.

RESULTS AND DISCUSSION

Characterization of the patinas and identification of corrosion products The examination of the corroded specimens after a 5 year burial period under the same

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experimental conditions pointed out different surface morphology depending on the chemical composition of each of them, as shown in Fig.1 The coupons are covered by ‘’earthy’’ crusts of different colors, which contain soil components. The patina of Sn rich (7.5 %) alloy A was more powdery and had areas with light green corrosion products. Pb rich (8.0%) alloy C was the one that maintained its original shape better than the other two types and less incorporated material was found on its surface. The X-ray diffraction patterns of reference alloys A, B and C are presented in Fig. 2.

(a) (b) (c) Figure 1: Reference bronze coupons extracted from the NaCl enriched Attica soil after 5 years of

burial – (a) bronze alloy (b) Zn-bronze alloy (c) Pb-bronze alloy

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Alloy A

Alloy B

Alloy C

Figure 2: X-ray diffraction patterns of reference Bronze alloy (A), Zn-bronze alloy (B) and

Pr-bronze alloy (C) Bronze Alloy (Specimen A): The X-ray diffractogram (Fig.3) shows the major corrosion products. Apart from earth minerals, such as calcite and quartz, a huge quantity of cuprite and a significant amount of paratacamite were detected. The three diffraction peaks of the alloy are also obvious. The presence of nantokite (CuCl) is suspected (GAXRD 10º), but it is not certain due to the extremely low intensity of its peaks. Malachite (in typical XRD) is also traced as micro-inclusions. Tin oxides were not detected in the spectrum, most probably because they exist in an amorphous form. This is in agreement with findings of other researchers [8, 16]. EDS analysis confirms the tin presence in the corrosion layers (Fig. 4, spot 2) The metallographic section of the coupon, which may be described as “coarse” surface and corresponds to Type II corrosion pattern according to Robbiola et al. [8] is shown in Fig.4. The corrosion products are well developed and are about 200µm thick. At high magnification, OM reveals a three-layered structure:

• An external porous zone of green Cu(II) compounds

• A red sandwiched layer of cuprite (Cu2O), disrupted and fragmented

• An internal layer characterized by relatively lower copper amounts and higher amounts of

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tin than in the alloy matrix. It consists of mixed amorphous and crystalline oxides. EDS analysis results detected significant chloride content in the vicinity of the alloy matrix, which indicates a serious corrosive attack and high amounts of copper species in the external layer. This phenomenon is explained by Lucey [14]. According to his theory, cuprite is considered as an electrolytic membrane, allowing transport of anions (O2-, Cl-) inward and cuprous ions (Cu+) outward. The amount of tin in the internal layer is higher than in the external layer.

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2 theta(degrees)

Intenity(counts)

paratacamite: p

cuprite: c

quartz: q

calcite: cl

alloy A: a

XRD

GAXRD 5°

GAXRD 10°

GAXRD 2°

pp

cl

q

c

p

c

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a , p , q

c , a

c

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cl

cl

cl

c

p

p

c , a

c , a

p

p

p

p

c

c

c

c

a , p , q

Figure 3: X-ray diffraction patterns of the corroded bronze alloy (sample A), typical and for

incidence angles 2º, 5º and 10 º

Internal layer External layer Element

Spot 1 Spot 2 Spot 3

C 28. 4 43.9 31.5

O 23.5 20.3 22.4

Mg - 0,8 0.4

Al - 0.3 0.5

Si 1.5 1.7 2.4

Cl 5.9 1.7 7.0

Sn 6.9 12.2 1.0

Ca - 1.3 0.9

Mn - 0.5 -

Fe 0.5 0.5 3.2

Cu 32.2 16.8 30.6

SEM EDS Analysis OM

Figure 4: Cross-section examination of the corroded bronze alloy (specimen A)

Zn-bronze Alloy (Specimen B): The main diffraction peaks are almost the same with coupon A (Fig.5). Considerable amounts of zincian paratacamite, copper oxides (both Cu2O and CuO) in abundance and the expected earth minerals are identified. Two of the alloy peaks are clearly observed. ZnO is detected in traces. Again, we can assume that amorphous SnO2 is also present. The OM and SEM images show corrosion products distributed in two layers, easily distinguished:

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• A thick outer layer of green Cu(II) and Zn compounds mixed with elements from the soil

• A much thinner red cuprite layer These layers are consistent and continuous and thus correspond to Type I structure according to Robbiola [8]. In the alloy-cuprite interface a “gold” flake appears at high magnification (Fig.6). It was not feasible to be identified. It might be a “ghost” structure of the initial alloy or nantokite which was not detected in the XRD spectrum due to thick corrosion layer above it. The corrosion products have an average thickness of 180µm. The elemental analysis points out to a high tin content and noticeable chloride content in the outer layer (in fact, higher than in bulk alloy). The internal layer is characterized by a copper content lower than in the alloy, and oxygen. The other soil agents are found in traces. The O/Sn atomic ratio decreases gradually from outer layers towards the alloy. On the other hand, the elimination of Cu from the alloy towards the surface indicates a decuprification process.

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XRD

GAXRD 10°

GAXRD 5°

GAXRD 2°

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o , c

a

a

ca

c

paratacamite: pz

cuprite: c

CuO: o

calcite: cl

quartz: q

alloy B: a

pz

Figure 5: X-ray diffraction patterns of the corroded Zn-bronze alloy (sample B), typical and for incidence angles 2º, 5 º and 10 º

Internal layer External layer Element

Spot 1 Spot 2 Spot 3

C 14.3 13.7 16.4

O 4.8 14.0 19.2

Al 0.8 0.5 0.6

Si 0.8 1.0 1.1

Cl 0.8 1.4 7.5

Sn 2.5 4.4 4.1

Ca 1.3 1.5 2.0

Fe - - -

Cu 64.4 57.3 40.8

Zn 11.3 6.3 8.3

SEM EDS Analysis OM

Figure 6: Cross-section examination of the corroded Zn-bronze alloy (specimen B)

Pb-bronze Alloy (Specimen C): This alloy, due to its significant lead content (8% w/w) exhibits

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completely different corrosion behaviour than the other two. The diffraction peaks that predominate in the spectrum are those of the alloy and cuprite. The “weaker” peaks belong to paratacamite, quartz and calcite (Fig.7). Other compounds identified as micro-inclusions are PbO and tin oxide (GAXRD 10º), PbCO3 (cerussite), PbOCO3 and anglesite (PbSO4). The corrosion products form a very cohesive two-layered structure, with an average thickness of 100µm. The outer layer is yellowish, irregular and contains soil components. The internal layer is a red zone of cuprite (Fig.8 OM). The SEM image shows an area with three layers, but despite that deviation in the same object, the specimen condition is characterized as Type I [8]. EDS analysis proves that the only environmental element detected in the patina in high percentage is oxygen and favors the oxides formation of copper, lead and tin (obvious in the cross-section as “milky” compounds). Chloride ions are present in traces and are captured inside the passive layer of the protective oxides. Alloy C is the most corrosion resistant, property attributed to the Pb content. This result is in good agreement with other studies [13].

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c

c

c

c

ccl a , qa

a , c

a , cq

paratacamite: p

cuprite: c

calcite: cl , quartz: q

alloy C: a

Figure 7: X-ray diffraction patterns of the corroded Pb-bronze alloy (sample C), typical and for

incidence angles 2º, 5º and 10 º

Element Internal Layer External

layer

Spot 1 (black layer)

Spot 2 (grey

layer)

Spot 3 (white line)

Spot 4 (external

layer)

C 35.3 23.6 24.2 33.5

O 15.8 15.7 17.5 34.5

Al - - 0.3 0.3

Pb 2.3 14.6 9.7 1.5

Cl 0.6 0.6 0.5 0.2

Sn 1.9 2.6 2.7 0.8

Ca 2.5 2.9 3.1 19.1

Cu 41.9 40.2 42.1 10.1

SEM EDS Analysis OM

Figure 8: Cross-section examination of the corroded Pb-bronze alloy (specimen C)

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Generally, the chemical composition of each alloy can be regarded as the most crucial parameter, which determines the nature of corrosion products. Constantinides et al. in their work made similar observations for each bronze family (tin bronze, brass, leaded bronze) despite the differences in manufacturing technique and although they dealt with corrosion induced by electrochemical means [10]. Hydrogen plasma treatment Hydrogen glow discharge plasma technique has been employed for the cleaning and stabilization of the long-term artificially corroded coupons. The experimental conditions, especially tempera-ture, were carefully chosen in order to ensure that no phase transformation will take place. Previ-ous works [21] have pointed out the safety limits for bronze treatment.

(a) (b) (c)

Figure 9: Bronze alloy (a), Zn bronze alloy (b), and Pb bronze alloy C) (c), before(left half) and after 1 h plasma treatment(right half) – The surface darkening is evident.

Bronze Alloy (Specimen A): XRD patterns of plasma treated coupon A are shown in Fig.10. The amount of paratacamite decreases after 1 hour. Cu2O and CuO peaks are enhanced and copper appears as a result of the reduction process. Initial plasma treatment is followed by mechanical cleaning, which uncovered a new green corrosion layer under the soil crust. This explains the slight increase of the intensity of paratacamite peaks. The reduction products also increase. After 4 hours of plasma treatment the amount of paratacamite shrinks again and CuO starts to decrease because cupric ions are transformed into cuprous ions and copper. The EDS analysis confirms the chloride ion removal, which in the vicinity with the alloy is complete. OM images (Fig.11) verify the cuprite layer creation and the deposition of metallic copper above it. Cuprite coexists with the remnants of paratacamite and soil compounds in other areas of the same object.

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paratacamite : p

cuprite : c , CuO : o

calcite: cl , quartz: q

alloy A: a , copper : Cu

corroded

2 hours plasma

1 hour plasma

4 hours plasma

p q

clp

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c , o

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Cu , a

q

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q

Figure 10: X-ray diffraction patterns of bronze alloy A after 1, 2 and 4 hours of plasma treatment

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EDS Analysis

Internal layer (Photo 1)

External layer

(Photo 2)

Element

Spot 1

Spot 2

Spot 3

Spot 1

C 31.1 52.7 44.5 31.8

O 3.7 5.3 19.3 22.3

Mg - - 2.6 0.8

Al - - - -

Si - - - -

Cl - - 0.3 0.5

Sn 3.6 3.5 3.9 2.4

Ca 0.2 0.5 2.0 20.1

Mn - - - -

Fe - 0.8 - -

Figure 11: Cross-section examination of bronze alloy A after 4 hours of plasma treatment (images from SEM and optical microscope)

Zn-bronze Alloy (Specimen B): XRD analysis detected CuO as a reduction product and less zincian paratacamite after the first hour of treatment. After mechanical cleaning, a new green corrosion layer under the soil crust was uncovered. Hydrogen plasma treatment for another hour results in increase of the reduction products peaks (CuO, Cu2O), although paratacamite peaks are again increased, most probably due to the removal of the previous soil crust. The presence of Zn as an additional reduction product could not be excluded. Additional plasma treatment (four hours), results in further zincian paratacamite shrinkage and accumulation of reduction species. It is possible that, in the external layers, cuprite CuO and Zn diffraction peaks continue to increase at the expense of zincian paratacamite. This is obvious in the GAXRD 10º spectrum (Fig.13).

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paratacamite zincian : pz

cuprite : c , CuO : o

calcite: cl , quartz : q

zinc: Zn , alloy B: a

corroded

1 hour plasma

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4 hours plasma

c , opz

q

pzcl

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pz

qpz ac

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pz

pz

pz q c , opz

o , c

o , c, Zn

o , c , Znc , o, Zn

pz pz

cl

cl

pz

c , o, Zn

Figure 12: X-ray diffraction patterns of Zn-bronze alloy after 1, 2 and 4 hours of plasma treatment The EDS analysis (Fig.15) confirms the chloride ion removal, which in the vicinity with the alloy is

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almost complete. OM images (Fig.14) verify the cuprite layer creation. In other areas of the same object cuprite coexists with the remnants of zincian paratacamite or metallic Zn and soil compounds. Some of the described transformations are depicted in Fig. 14.

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4 hours plasma(GAXRD)pz

c, o, Zn

c, o, Zn

paratacamite zincian : pz

cuprite : c

CuO : o

zinc: Zn

Figure 13: GAXRD10° diffraction pattern of Zn bronze alloy after 4 hours of plasma treatment

(a) (b) (c) (d) Figure 14: Structures shown on the cross-section of Zn bronze alloy after 4h plasma treatment (OM

images)

Photo 1

Photo 2

EDS Analysis

Photo 1 Photo 2

Element Spot 1 Spot 2 Spot 3 Spot 1 Spot 2

C 25.1 33.8 32.0 25.7 35.0

O 17.2 23.2 25.3 11.0 18.2

Mg 0 1.3 0.6 0 0.3

Al 0.3 2.1 1.1 0.5 0.5

Si 1.1 6.2 3.4 1.0 1.4

Cl 0.3 0.5 0.4 0.5 3.9

Sn 3.4 - - 6.1 1.0

Ca 0.4 1.1 17.5 1.1 2.2

Cu 45.4 26.6 15.3 46.8 31.1

Zn 6.9 3.4 3.7 7.0 5.8

Fe - 1.2 0.8 0.3 0.8

Pb - - - - -

Figure 15: SEM images of Zn-bronze alloy after 4 hours of plasma treatment

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Pb-bronze alloy (Specimen C): The XRD spectra via which the progress of the reduction was monitored are shown in Fig.16. It can be concluded that during the treatment paratacamite was gradually eliminated, while reduction species (Cu2O, Cu and Pb) continued to form. Despite the chloride removal, no other rapid alteration was observed.

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alloy C: a , copper: Cu , cerrusite(PbCO3) : ce

lead: Pb

corrodedp

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Figure 16: X-ray diffraction patterns of Pb-bronze alloy after 1, 2 and 4 hours of plasma treatment

Photo 1

Photo 2

EDS Analysis

Photo 1 Photo 2

Element Spot 1 white

Spot 2 (grey) Spot 3 Spot 1 Spot 2 Spot 3 Spot 4

C 22.3 28.0 31.6 29.8 27.9 33.4 33.0

O 5.2 12.9 21.5 10.9 14.6 17.3 25.8

Al 0.3 0.1 0.1 - - - -

Si 0.4 0.5 0.3 - - - -

Cl 0 0.1 0.2 0.9 2.1 3.8 0.7

Ca 0.1 0.5 0.2 0.6 1.4 1.5 19.5

Cu 24.5 16.5 34.2 52.1 47.9 39.9 18.5

Pb - 39.3 7.1 3.1 4.5 3.1 1.4

Na - 2.3 2.6 - - - -

Sn - - - 2.6 1.7 1.0 1.1

Figure 17: SEM images of Pb-bronze alloy after 4 hours of plasma treatment

EDS analysis (Fig.17) reveals the complete removal of chloride species from some areas of the cross-section (first SEM image), but elsewhere, inside more complex corrosion structures noticeable quantities are still detected. Figure 18 shows the stabilized structures of oxides, which appear irregular and have replaced initial corrosion products even inside pits. Metallic copper is deposited on the surface of the coupon or it is dispersed in the reduced structures.

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(a) (b) (c) (d) Figure 18: Structures present on the cross-section of Pb-bronze alloy after 4h plasma treatment

(OM images)

The above results point out that hydrogen plasma treatment might thermally destabilize the basic copper chlorides with the consequent release of chloride ions in the gaseous phase and formation of cupric oxide, which can be regarded as an intermediate product prior to its subsequent reduction to cuprite and furthermore to metallic copper. Direct reduction of the basic copper chlorides to cuprite is also a possibility. The Sn, Zn and Pb corrosion products probably undergo direct reduction in metallic Sn, Zn and Pb respectively. It is worth mentioning that no traces of deposited Sn were detected.

CONCLUSIONS

Three different reference alloys have been employed for carrying out artificial long-term degradation tests to simulate as best as possible, the specific corrosion conditions of the archaeological bronze objects. The use of combined analytical techniques provides good insight into the nature of the created corrosion structures. The Robbiola phenomenological model has been employed to explain the morphology of the observed corrosion layers and their composition. The identified compounds on the particular coupons were indeed similar to those encountered on bronze archaeological objects. Generally, the chemical composition of each alloy can be regarded as one of the most crucial parameters, determining the nature of corrosion products. In case of sample A (bronze) and B (Zinc-bronze) the corrosion process could be mainly attributed to decuprification i.e. selective dissolution of copper from the copper solid solution. Alloy A seems to be the most susceptible in the outbreak of “bronze disease”, since it has undergone the most severe attack by chloride ions. Alloy C (lead-bronze) was the most corrosion resistant, property attributed to the compact patina enriched with Pb compounds.

The treatment with 100% hydrogen plasma at temperatures range 190 °C to 200 °C for 1 h to 4 h duration can be characterized as mild having as priority the preservation of the metallographic characteristics of the metal itself. The gradual elimination of the chlorine containing corrosion products in favor of the formation of more stable species and sometimes even the complete reduction back to copper metal is proportional to the duration of the plasma treatment. It should be taken into account that a partial reduction is preferable to a complete reduction, as part of the patina might be aesthetically desired. The newly formed metal surface produced by the reduction can be very reactive and easily corroded; also the consistency with the bulk metal is an issue to be investigated. Plasma treatment up to four hours apparently removes efficiently chloride content from lightly and mildly corroded bronze surfaces. The knowledge gained from the above described examination can be very useful in determining the optimum procedure to restore and conserve authentic archaeological objects.

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REFERENCES

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