Chemical and structural properties of ancient metallic artefacts: multitechnique approach to study of early bronzes

Abstract

The results are presented of a characterisation study of the microstructure and microchemistry of archaeological bronze (Cu–Sn) artefacts from the eighth to the sixth century bc. Metallographic examination, with optical and electron optical microscopy, has been performed on polished sections of early Iron Age studs and bracelets found in incineration tombs of the Necropolis of Chiavari in Italy. A heterogeneous microstructure of the bronze was observed, exhibiting a wide range of grain sizes, and a predominant α-phase solid solution containing α/δ and α/? eutectoid phases decorated with a high density of inclusions. The composition of grain boundary surfaces was determined, using scanning Auger microscopy (SAM), on specimens fractured in vacuo. The extent of tin segregation at the grain boundaries was 3–5 times greater than that in the grain interiors. Copper rich sulphides occasionally containing the oligoelements (iron and lead) were identified as the predominant type of inclusions formed at the grain interfaces and within the grains. Analysis of the corrosion patina at the surface of the bronze artefacts was conducted by combining X-ray diffraction, scanning electron microscopy with energy dispersive X-ray analysis, and SAM. The patina was found to exhibit a multilayered structure and a complex chemical composition forming various crystallographic phases including malachite, cuprite, and copper–tin oxide. Corrosion of the underlying bronze matrix proceeded along the grain boundaries, where the sacrificial corrosion of tin reacting with diffused oxygen and chlorine was identified. The results of this study have been used to clarify the metallurgy and manufacturing processes of the examined finds, and to evaluate the state of their degradation.     

Papers to be presented at Eurocor '77

Abstract

Despite numerous studies on atmospheric corrosion of copper and copper based alloys, the corrosion induced release processes of individual alloy constituents suffer from significant knowledge gaps. This investigation comprises metal release rate measurements of copper, zinc and tin from some copper based alloys including brass (20 wt-%Zn) and bronze (6 wt-%Sn), and their pure alloying metals, copper, zinc and tin. Data have been generated during a 2·5 year urban field exposure in Stockholm, Sweden and parallel laboratory investigations in a specially designed rain chamber using artificial rain. Brass shows significantly lower annual release rates of both copper and zinc compared to pure metal sheets of its alloy constituents. Zinc is preferentially released compared to copper. Dezincification of brass occurs both at field and laboratory conditions, a process influenced by rain characteristics. Alloying with tin does not largely reduce the release rate of copper from bronze compared to pure copper. No measurable amount of tin is released from the bronze surface.     

The atmospheric corrosion of quaternary bronzes: The action of stagnant rain water

The corrosion behaviour of a quaternary bronze UNS C83600 exposed to stagnant acid rain was examined through wet-dry tests. During the tests, parallel monitoring was performed to determine the evolution of both the bronze surface and the weathering solution composition. The results show that the kinetics of bronze oxidation is governed by diffusion through a two-layer patina: an inner Sn-rich layer and an external Cu and Pb-rich layer. The corrosion rate of the alloy decreases with time, but the dissolution of individual metals (Cu, Zn and Pb) in the environment increases with different trends, showing progressive patina destabilisation.     

Pedological stratification effect of corrosion and contamination products on Byzantine bronze artefacts

Abstract

The present paper deals with the identification of six ancient Byzantine bronze coins found in the same archaeological site of Nufarul (Tulcea County, Romania), by corrosion product and alloy analysis. The microstratigraphies (i.e.layers), together with microchemical tests including reflection colorimetry, IR spectroscopy, X-ray diffraction and scanning electron microscopy assisted by X-ray spectrometry have rendered evident the stratified morphology of three types of patina. They are the primary type resulting from redox processes, the secondary type determined by acid–base and related hydrolytic processes and the ternary type (or the contamination patina) from segregation, diffusion and osmosis processes. The stratigraphical distribution of the chemical components in the structure of the patina is caused by the pedological (soil) processes at the archaeological sites and can be the main factor used in the authentication of ancient bronze artefacts.     

The atmospheric corrosion of quaternary bronzes: The leaching action of acid rain

The effect of leaching rain on the corrosion behaviour of bronze UNSC83600 was investigated as to the influence of alloying elements (Cu, Sn, Zn, Pb) through dropping tests simulating a severe runoff condition with a solution reproducing natural acid rain. Corrosion was followed with time monitoring both samples and leaching solutions (up to 30 days) by SEM, EDS, Raman spectroscopy, XRD, AAS. The bronze patina behaves as a porous layer enriched in stable tin compounds allowing uniform dissolution of Cu, Zn and partly of Pb. Laboratory results are in good agreement with field studies of outdoor bronzes in unsheltered condition.     

Characterization of corroded bronze Ding from the Yin Ruins of China

This paper presents the result of scientific examinations carried out on the soil-buried archaeological bronzes Ding from Yin Ruins of China. Eight of typical fragments from different bronze Ding were selected as researched samples according to their deterioration characteristics. Optical microscopy (OM), scanning electron microscopy coupled with energy (SEM-EDX), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were used to understand the corrosive morphological characteristics, to clear the nature of patina and to analyze the elementary composition of bronze Ding. The results indicated that it is not possible to distinguish the original lustrous metallic surface in most samples because of the corrosive crust. The substrate of bronze Ding contains74-86% Cu, 1.1-4.6% Pb, and 10-18% Sn, which is in agreement with the historical investigation in the ritual vessels of Shang time. Copper-containing compounds were the main constituents of natural patina: Cu₂(OH)₃Cl existed as corrosion product in all the powdery or crack surface; Cu₂(OH)₂CO₃ was the main corrosive product in a compact and hard corrosive surface. This study provides useful information for the restoration and protection of bronze Ding in Yin Ruins.     

Microstructure and corrosion behaviour of ferritic steel–Zr-based metal waste form alloys in simulated ground water

ABSTRACT

The microstructure and corrosion behaviour of the metal waste form (MWF) alloys based on ferritic steel with Zr content in the range of 3–15?wt-% were investigated. The MWF alloys are composed of α-Fe and Fe-Zr phases and with the increase of Zr content, α-Fe phase gradually decreases and the relative content of Fe–Zr intermetallic phase also increases. TEM and XRD confirmed the presence of Fe₂Zr and Fe₂₃Zr₆ intermetallics. Potentiodynamic polarisation curves showed The MWF alloys exhibited passivation behaviour in the simulated Kalpakkam (KGW) and Rajasthan ground water (RGW) media. Electrochemical impedance spectra revealed improved passive film stability in RGW than in KGW which is related to the formation of more stable adherent insoluble passive film in RGW. Higher Zr containing MWF alloy exhibited higher corrosion resistance than lower Zr containing MWF alloys. The relative content of Fe–Zr intermetallics is attributed for the corrosion resistance of The MWF alloys.     

Comparative investigation into the corrosion of different bronze alloys suitable for outdoor sculptures

Several bronze alloys, suitable for production of outdoor sculptures, were developed in the frame of the European project “Eurocare-Bronzart”. The elemental composition of the alloys was searched to fulfil specific criteria such as: reduction of lead content, good resistance toward corrosion and aesthetic characteristics conformable to artistic purposes. After metallurgical characterization, the resistance toward corrosion was evaluated in artificial environments. Ageing experiments were performed in a salt spray cabinet and in a climatic chamber in the presence of a controlled concentration of SO₂. The Thin Layer Activation (TLA) method was applied to calculate the thickness loss of activated specimens exposed to artificial corrosive atmospheres. After artificial ageing experiments the surface of the materials was investigated by SEM-EDS techniques.The bronze alloys containing different percentage of nickel showed the best properties of resistance toward corrosion.     

Characterization of archaeological bronze and evaluation of the benzotriazole efficiency in alkali medium

An analytic study permits us to characterize the altered surface of an archaeological bronze coin and to determine the structure and the composition of the patina covering the whole surface of this artefact. This patina could be interpreted at first sight as a type I patina, with copper product deposits on it, with some punctual and enlarged localized type II corrosion. To achieve the purpose of this work, we investigate the behaviour of an archaeological bronze in the presence of benzotriazole (BTA) in alkali medium with the intention of getting a better passivity while favouring the formation of a polymeric film on the surface of the working electrode. The behaviour of the considered interface is investigated by electrochemical impedance spectroscopy, in the presence and absence of an oxide layer, according to the immersion time. At pH = 9, in the presence of 15 mmol/l of BTA, the optimum percentage of inhibition efficiency (IE%) is 67% obtained after 30 min of immersion. A pre‐polarization of the bronze working electrode is realized in order to accommodate the preservation technique used in museums and to improve the formation of the Cu(I)‐BTA polymeric film. The use of a pre‐polarized electrode for 30 min at ?35 mV/SCE carries an enhancement of the protection versus the non‐polarized electrode. While comparing the result of our investigation with that obtained using the traditional preservation method, we can establish that using a concentration of BTA 15 times lower, important inhibitor efficiencies (%) of 92 and 97.4%, respectively, for 30 min and 96 h of immersion are reached.     

Voltammetric behaviour of an archeaological bronze alloy in aqueous chloride media

The interface archaeological bronze alloy‐chloride media was characterized by using linear sweep rate cyclic voltammetry. The electrochemical behaviour of the Cu‐Sn alloy being different from pure copper and pure tin. In fact, the voltammograms show an anodic peak and two cathodic peaks. Surface examination revealed a compact layer which spread over the ancient material. The influence of different parameters such as scan rate, chloride concentration and repetitive cycling on the voltammetric response of the archaeological bronze was also studied. The results indicate that patina layer formation is a spreading process over the electrode surface leaving only small pores. Diffusion of the halide controls the growing rate of this layer. Two other anodic peaks are evidences when potential cycling increases. The oxidation became more difficult as a result of a constant layer thickness.     

Brazing alloys for marine service

Abstract

The microstructure of brazing alloys to B.S. 1845 and to certain proprietary specifications is reviewed and related to corrosion performanee in sea water on joints with copper, 70/30 cupro-nickel and 7% aluminium bronze. The factors influencing corrosion are illustrated and discussed and recommendations are made as to the suitability of various braze metal-parent metal combinaiions. Of seven brazing alloys tested with the three parent materialsi only two were completely satisfactory, two could be used within certain limitations and three were unsatisfactory.     

Assessment of the interphase behaviour of two bronze alloys in archaeological soil

The aim of this work is to evaluate the behaviour of bronze in archaeological soil by gravimetric study, electrochemical research and analytic survey. Archaeological soil samples were collected from Jama that date back to the second Punic war. Mineralogical, micro‐structural, micro‐morphological and chemical analyses of soil constituents were performed. The bronze samples correspond to Cu‐9.4Sn (B₁) and Cu‐7.7Sn with 1% of Pb (B₂). The open circuit potential value recorded for B₂/corrosion product/soil is stable after only 7 min due to its reactivity allowing to a rapid development of a protective corrosion layer. The B₂ interphase is more developed and stable than B₁. Under anodic polarisation, the B₂ alloy corrodes less than the B₁ alloy owing to its lower anodic current. Based on the EIS study, two evolution stages of the bronze alloy/corrosion product/soil interphase are detected. For the first stage, the interphase is equivalent to two R//C loops at high and medium frequencies, respectively related to the corrosion layer and charge transfer process, associated to mass transport phenomena. For the second stage, the resistance of corrosion layer decreases and the first loop vanishes. SEM‐EDS analysis demonstrate that the first stage (20 days of immersion) can be characterized by a well covered surface with a thin corrosion product, the external layer exhibits low tin content. However, after 310 days of immersion, the surface state is different with isolated particulates composed by Cu and soil compounds on a cracked layer with the presence of craters showing an internal layer.     

Corrosion evaluation and surface characterization of the corrosion product layer formed on Cu-6Sn bronze in aqueous Na2SO4 solution

The binary bronze alloy Cu-6Sn corrosion, and formation and properties of corrosion product layer (patinas) during 12 days of exposure to 15 mM Na₂SO₄ aqueous solution were investigated by a range of diverse experimental techniques. For the reasons of comparison, some techniques were applied, in parallel, to copper. Gravimetric measurements revealed lower corrosion rates of bronze than those of copper, probably caused by the presence of tin compounds in the corrosion product layer. Cyclic voltammetry results showed that the oxidation processes on bronze are affected by the formation of tin oxide species. Electrochemical impedance spectroscopy showed that, as opposed to copper which produced only two time constants, bronze corrosion resistance was dominated by the additional high-frequency time constant representing redox processes occurring at the corrosion product surface. SEM, ATR FTIR and PIXE results suggest that Cu-6Sn bronze corrosion in 15 mM Na₂SO₄ solution was impeded by the formation of two-layered structure of corrosion products that formed due to selective dissolution of copper at the layer/solution interface, leaving the outer layer enriched in highly corrosion resistant Sn oxi/hydrohide species.     

Effect of alloying on the structure of bronze with enhanced tin content

Bronzes with an enhanced (14 wt %) tin content which were alloyed with titanium, zirconium, and boron have been studied in the as-cast, homogenized, and deformed states by scanning and transmission electron microscopy and X-ray diffraction analysis. These alloys are of interest as a matrix material for superconducting Nb/Cu-Sn composites in which the high tin content and the alloying of the bronze matrix make it possible to improve superconducting characteristics at the expense of optimization of the structure and properties of layers of the Nb₃Sn compound formed at the niobium-bronze interface via reactive diffusion. The distribution of alloying elements in different states of the bronze has been investigated. It has been shown that Zr is uniformly distributed in the alloy and forms no coarse inclusions, whereas Ti forms in the as-cast state large platelike precipitates that can adversely affect technological characteristics of the bronze matrix and the composite as a whole.     

Corrosion of bronze by acetic and formic acid vapours,sulphur dioxide and sodium chloride particles

This paper studies the corrosion of patinated and unpatinated bronze by acetic and formic acid vapours, sulphur dioxide and sodium chloride salt particles, at 100% relative humidity. Weight loss, X-ray diffraction, infrared and scanning electron microscopy were the techniques used. Acetic and formic acid vapours, sulphur dioxide and sodium chloride produce a high corrosion rate on bronze. In general, no protective effect was found by the patina on bronze. The principal compounds identified were Cu₂O, Cu₂S, Cu₅(SO₄)₂(OH)₆ · 5H₂O, Cu(CH₃CO₂)₂ · XH₂O and Cu(HCO₂)2.     

Effect of Al2O3 reinforcement and precipitates on corrosion behaviour of 2618 and 6061 aluminium MMCs

ABSTRACT

The corrosion resistance features of two different Metal Matrix Composites based on 6061 and 2618 aluminium alloys reinforced by 20% Al₂O₃ particles by stir casting process, were studied in 3.5% NaCl and compared. The composites and their respective base alloys were characterised in terms of microstructure by optical and scanning electron microscopy and in terms of corrosion resistance by polarisation curves and electrochemical impedance. Results show how the presence of Al₂O₃ has different effects depending on matrix composition. In 6061 MMC, Al₂O₃ promotes the formation of Mg₂Si which has a cathodic behaviour altering the pitting susceptibility of the composite. In 2618 MMC, the high content of Cu induces a strong uniform attack. The subsequent increase in the Open Circuit Potential caused a pitting attack.     

已锈蚀青铜在大气环境中的腐蚀发展及其保护研究

用动电位扫描法和循环伏安实验对青铜文物在模拟大气环境介质0.028 mol/L NaCl+0.01 mol/L Na2SO4+0.016 mol/L NaHCO3中的电化学行为进行了研究,探讨了CuCl、Cu2O、碱式氯化铜、混合锈对裸青铜的保护作用.结果表明：在腐蚀电位较负时,Cu2O、CuCl对基体有一定的保护性,而且保护性能优于碱式氯化铜和混合锈;在腐蚀电位较正时,Cu2O或CuCl对青铜的保护性变差,而此时碱式氯化铜和混合锈对裸青铜的保护性优于Cu2O、CuCl.0.1％MBO(2-巯基苯并恶唑)酒精溶液对裸青铜或被Cu2O、CuCl覆盖的青铜阳极过程有一定的抑制作用,对被碱式氯化铜或混合锈覆盖的青铜阳极过程有优异的抑制作用.       

The Atmospheric corrosion of statue bronzes exposed to SO2 and NO2

We report on the atmospheric corrosion of different cast statue bronzes in humid air containing ppb levels of SO₂ and SO₂+NO₂. In addition, copper, tin, zinc, and lead samples were studied in these environments. The samples were exposed to synthetic atmospheres with careful control of pollutant concentrations, relative humidity and flow conditions. Deposition of SO₂ was studied using on-line gas analysis. The weight gain was registered after four weeks exposure, and corrosion products were analysed by Electron Probe Micro Analyser (EPMA), X-ray diffraction (XRD), and Fourier Transform Infrared spectroscopy (FTIR). The synergistic effect of SO₂ and NO₂ was remarkable on all bronze materials examined. The weight gain was correlated to alloy composition. Thus, high zinc and low lead content resulted in the greatest weight gain, while high tin content favoured a low weight increase. Lead exhibited a rapid deposition of SO₂ followed by zinc and copper, while tin was unreactive towards SO₂. There was no measurable indication that microstructure influenced corrosion. The corrosion product morphology found in SO₂+NO₂ environment indicated a localised type of attack. The anodic sites were covered by a tin-rich corrosion product close to the metal. Oxidation of soluble divalent tin by O₂ at the anodic sites to form insoluble SnO₂ – xH₂O, is suggested to explain the corrosion protection afforded by alloying with tin. Tin was enriched in the corrosion products while no lead was found. The zinc/copper relation was higher in the corrosion products than in the alloy composition.     

On the corrosion resistance of certain ancient chinese bronze artifacts

Certain bronze mirrors and weapons produced in China during the Chou and Han Periods (400 B.C.–200 A.D.) have successfully resisted corrosion for more than two millenia. These artifacts are characterized by lustrous black (sometimes brown, dark green, or grey) surface coatings that protect the high tin bronze substrate. To assess quantitatively the corrosion resistance of these objects, polished sections of the ancient bronze have been exposed to corrosive atmospheres containing H₂S and OCS in wet air. These gases were chosen for these experiments because they are emitted to the atmosphere by natural processes such as swamps and volcanoes and because they are known to rapidly sulfidize copper and its alloys. Experiments demonstrate that the uncoated bronze is relatively susceptible to sulfidation, being similar in this characteristic to modern bronzes previously studied. In a separate experiment, samples of a modern alloy closely matching the ancient bronze composition (70–75% Cu, 20–25% Sn, 1–5% Pb) were treated with a modern protective coating of known permeability and exposed along with a coated ancient bronze sample to a highly corrosive atmosphere. By comparing the resulting sulfidation, the H₂S permeation constant for the ancient coating is estimated to be < 2 × 10^?4 barrers. This is an extremely low figure even by modern standards and suggests the possibility that the coating, could it be duplicated, might prove useful for the protection of modern materials.     

Ternary Zn–Mn–Sn alloy electrodeposition from an ionic liquid based on choline chloride

Zinc–manganese–tin (Zn–Mn–Sn) ternary alloys have been successfully prepared, for the first time, by electrochemical deposition from a choline chloride-based ionic liquid. The influence of electrolyte composition and electrodeposition potential on the ternary Zn–Mn–Sn alloy surface morphology, composition and corrosion resistance were investigated and contrasted with the characteristics of binary Zn–Mn and Zn–Sn electrodeposits. It was found that the composition of the plating bath and deposition potential have a significant influence on the alloy content, morphology and corrosion properties of the Zn–Mn–Sn alloys. The content of Sn in the ternary alloy increased with a decrease in electrodeposition potential and an increase in concentration of Sn²⁺ in the electrolyte. The corrosion studies showed that the Zn–Mn–Sn alloys have a synergistic property combining the barrier behaviour afforded by Sn and the strong passive behaviour of Mn, while the sacrificial characteristics are still preserved.