artigo8_T1S-2012_Corrosão Arqueologica

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Mechanisms of long-term anaerobic corrosion of iron archaeological artefacts

in seawater

C. Rémazeilles

a

_{, D. Neff}

b

_{, F. Kergourlay}

b,c

_{, E. Foy}

b

_{, E. Conforto}

d

_{, E. Guilminot}

e

_{, S. Reguer}

c

_,

Ph. Refait

a

, Ph. Dillmann

b,*

a

Laboratoire d’Etude des Matériaux en Milieux Agressifs, EA3167, Université de La Rochelle, Bâtiment Marie Curie, Avenue Michel Crépeau, F-17042 La Rochelle cedex 01, France

b_{Laboratoire Archéomatériaux et Prévision de l’Altération LPS /SIS2M, CEA Saclay, LMC IRAMAT UMR5060 CNRS, France} c

Synchrotron SOLEIL, L’Orme des Merisiers Saint-Aubin, BP 48 91192 Gif-sur-Yvette Cedex, France

d

Centre Commun d’Analyse, Université de La Rochelle, 5 Perspective de l’Océan, F-17071 La Rochelle cedex 09, France

e

Arc’Antique, 26 rue de la haute forêt, 44300 Nantes, France

a r t i c l e

i n f o

Article history: Received 16 June 2009 Accepted 1 August 2009 Available online 11 August 2009 Keywords: A. Archaeological artefacts A. Steel C. Marine corrosion C. Sulphate-reducing bacteria C. Rust

a b s t r a c t

An iron ingot immersed during 2000 years at 12 m depth in the sea has been examined with the help of a combination of microscale techniques. This methodology allowed us to show that the main phase precip-itated during the immersion is an iron hydroxychloride (b-Fe2(OH)3Cl) that is characteristic of corrosion

in anoxic and chlorinated medium. Moreover locally on the external part of the corrosion products sul-phur containing phases have been identiﬁed as mackinawite (FeS) in nanocrystalline or slightly oxidised state. The presence of this phase could be explained by the activity of sulphate-reducing bacteria. The presence of b-Fe2(OH)3Cl could be interpreted via a thermodynamic modelling taking into account the

environmental conditions.

1. Introduction

As far as the conservation of ferrous archaeological artefacts recovered from marine environments is concerned, the presence of chloride induces serious problems. These species are involved in corrosion resumption phenomena after excavation that can pro-voke the rapid destruction of the archaeological objects[1], even when they are stored in stable climatic conditions. Turgoose [2]

demonstrated that a relative humidity as low as 20% was sufficient for the degradation to occur. For this reason, the objects are cur-rently treated to remove chloride ions. The treatments proved their efficiency in most cases but it is still difficult today to be absolutely sure that an object will not present any corrosion resumption after treatment. Moreover, these treatments are rela-tively long (several years for large artefacts) so that their optimisa-tion would concern restoraoptimisa-tion workshops and curators. This could be envisioned only once a complete characterisation of the corro-sion system formed by the corroded archaeological items is achieved. Very few data have been dedicated to the identification of marine corrosion systems and its layout at microscopic scale, specifically for very long periods. In the conservation field it is gen-erally admitted that corrosion products of iron artefacts immersed

in seawater are mainly composed of akaganeite, an iron oxyhy-droxide containing chlorine (b-FeOOH), goethite (

a

-FeOOH) or magnetite (Fe3O4) [3]. Some publications mention FeOCl[4] but

its presence has been asserted only by thermogravimetric methods and never by structural analyses. More recently, steel samples cor-roded several years in a harbour have been characterised thanks to X-ray diffraction and micro-Raman spectroscopy [5–7]. Authors observed a marbled corrosion layer mainly composed of magnetite localised in the inner part of the corrosion layer and goethite in the outer part. Moreover, in some zones of the corrosion layer the presence of iron hydroxychlorides was suspected thanks to ele-mentary composition analyses. Sulphated green rust was detected locally inside the corrosion layer[7]. In fact, it was recently shown that the formation of akaganeite required large dissolved Fe(II) species and chloride concentrations[8], so that its presence may rather be associated with localised corrosion phenomena, e.g. pit-ting, crevice corrosion, etc.

Complementary to these studies, interesting results has been published on contemporary steels corroded in marine environment and must be used to understand ancient systems. Indeed, it is gen-erally considered that for mild and low alloy steels, general (or ‘uniform’) corrosion is the most important form of corrosion. How-ever, marine immersion of such steels leads, after some years of exposure, to a somehow localised phenomenon [9–11], called ‘broad pitting’ [12] as it produces relatively large and shallow

*Corresponding author. Tel.: +33 1 69 08 14 69; fax: +33 1 69 08 69 23. E-mail address:philippe.dillmann@cea.fr(Ph. Dillmann).

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Corrosion Science

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depressions on the surface. This pitting can be very severe and proved to be associated with anaerobic conditions[13] that are met at the steel/corrosion product interface when the rust layer becomes sufﬁciently thick to hinder the access of dissolved O2to

the steel surface. Several laboratory studies demonstrated that pit-ting of mild steel could be associated with sulphate-reducing bac-teria (SRB) [14,15], micro-organisms that are active only under anaerobic conditions. It proved possible to replicate somehow the ‘broad pitting’ morphology in media inoculated with SRB, and this was assumed to result from localised SRB activity in the bio-ﬁlm covering the steel surface[16]. Detailed analyses of the rust layers formed on steel in various marine sites demonstrated the existence of interaction between bacteria and corrosion processes

[17,18]. It could be demonstrated by use of molecular techniques, that SRB were mainly present in the inner strata of the rust layer, and therefore associated with the sulphated green rust, GR(SO42),

and iron sulphide FeS[18], the compound that results from SRB activity. So it was proposed that the rate of the anaerobic phase of marine immersion corrosion of steel could be controlled by the transfer of the nutrients necessary for the development of the metabolic activity of SRB[19,20].

During the last past years, the long-term behaviour of low car-bon steel was investigated via the thorough study of iron archaeo-logical artefacts in various environments [21–24]. Information about corrosion mechanisms and reliable estimation of the rates could be obtained and should be used to predict the behaviour of carbon steel structures that are envisioned for the very long term

[25,26]. Civil engineering is one of these ﬁelds. Moreover, this ap-proach is in France of primary importance for the long term or in-terim storages of nuclear wastes in which a low alloy steel overpack will be used and will corrode in different potential media

[27,28]. In addition to this speciﬁc application, these results led to a better understanding of archaeological complex corrosion sys-tems, which should allow improving diagnosis and conservation treatment on archaeological and heritage artefacts. For this reason, the already used methodology, based on a multi scale and multi technique approach can be transposed to corrosion of archaeolog-ical artefacts in marine environment in order to enlighten the cor-rosion system linked to this speciﬁc environment. The discovery of iron ingots in two shipwrecks, dated from the Gallo-roman period, near Les Saintes Maries de la Mer in Mediterranean Sea[29,30], of-fered this opportunity.

Considering all these aspects, the present paper aims to charac-terise as ﬁnely as possible the corrosion system of a corpus of gal-lo-roman bars corroded in marine environment and propose a corrosion mechanisms based on the studies already presented both on archaeological artefacts and contemporary steels. Thus, in the following, the analysis of the thick rust layers covering the ingots of Les Saintes Maries de la Mer are presented. Cross sections were studied using a complete set of complementary microscopic, spec-troscopic and analytical techniques including scanning electron microscopy (SEM) with X-ray microanalysis (EDS), micro-Raman spectroscopy (

l

RS), micro-X-ray diffraction (

l

XRD) and Fourier transform infrared micro-spectroscopy (

l

FTIR). The thorough description of the morphology, composition, stratiﬁcation and nat-ure of the corrosion products indicated that the process mainly developed in anaerobic conditions and allowed us to propose a model describing the corrosion system.

2. Materials and methods 2.1. Archaeological corpus

The objects considered were iron ingots (Fig. 1) raised from two shipwrecks, dated from the Gallo-roman period, immersed at a

depth of 11 m and at 1.5 miles from the coast near Les Saintes Mar-ies de la Mer in Mediterranean Sea. They were discovered by A. Cha-baud in 1996[29]. Seven ingots were excavated and stored in tap water. A ﬁrst bath contained 4 ingots and the total amount of chlo-rine in water after 3 weeks of storage near the archaeological site was 470 mg/L. After this period and a transport in the laboratory, the ancient water was renewed and after a second 3 week storage period the total amount of chlorine in water was of 480 mg/L. Then ingots were taken for analyses. A second bath contained the three other ingots. After 5 weeks of storage the total chlorine quantity was of 185 mg/L. Then, after transporting in the laboratory, water renewal and 4 weeks of storage, the total Cl content in water was of 195 mg/L. The objects were never exposed to the atmosphere in order to avoid their modiﬁcation during their storage. One of these ingots has been dedicated to the study of the corrosion pat-terns formed during a long period of immersion in a marine envi-ronment. Its section was of about 6 3 cm2 ₍_{Fig. 1}_{) and it was}

partially covered by a concretion layer.

2.2. Physico-chemical characterisation protocol

In the frame of inter-laboratory experiments, similar and com-plementary techniques have been employed. SEM–EDS and

l

RS experiments were performed in several laboratories completed by

l

XRD at Pierre Süe laboratory, and Fourier transform infrared micro-spectroscopy at LEMMA laboratory. Each of them applied the analytical conditions it is used to, even for experiments carried out with similar techniques. Nevertheless, results proved to be absolutely consistent.

After being quickly dried, the ingot was mounted in epoxy resin and cut in order that cross sections could be analysed. Three sec-tions have been observed with the analytical protocol described in this paragraph so that the representativeness of the observation can be reinforced. Each cross section was prepared as follows: ﬁrst it was ground with SiC papers and polished with a 3

l

m diamond paste under ethanol or hexane. Then, the morphology was ob-served using Optical and Scanning Electron Microscopes (OM and SEM). Compositions were determined by Energy Dispersive Spec-troscopy (EDS) coupled to SEM (acceleration voltage: 15 kV or 20 kV). EDS detection was carried out with a Si(Li) detector equipped with a beryllium window allowing to quantify oxygen with an error of 2% and other elements under 0.5 mass% with 1% of error. Micro X-ray diffraction (

l

XRD)[31], micro-Raman Spec-troscopy (

l

RS) and micro-Fourier Transform Infrared microscopy (

l

FTIR) were used to identify the phases present in the corrosion layers.

l

XRD was performed on a rotating anode generator. The Mo anticathode delivered a monochromatic beam of 17.48 keV fo-cused to a 30 30

l

m2_{surface using a Xenocs}Ó_{FOX 2D MO 25_25P}

diffraction optic. Diffraction patterns were collected using a 2D detector (image plate) [32]. Data process was realised thanks to EVA software and the associated JCPDF database. Raman analyses were carried out with Labram HR spectrometers (JobinYvon Hori-ba) using an excitation wavelength of 514 nm or 632 nm. The spot size under the 50 objective was 3

l

m. Spectral resolution was of 2 cm1_{. In order to avoid the degradation of the corrosion products}

by the laser heating, power was ﬁltered down to 100

l

W. The iden-tiﬁcation of the phases was established thanks to comparison to spectra presented in the literature[33–36].

The

l

-IRTF analyses were performed with a Continuum micro-scope connected on a Nexus spectrometer (Thermo-Nicolet). The microscope was equipped with a 15 objective, 10 oculars and a MCT-A detector which limited the spectral window to 650– 4000 cm1_{. The background was acquired using a gold mirror.}

Spectra were obtained with the Omnic acquisition software at a resolution of 8 cm1 _{and by averaging at least 128 scans. After}

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acquisition, the specular reﬂectance spectra were treated by Kra-mers–Krönig Transform.

Experiments on cross sections showed that natural Fe(II) hydroxychloride could remain exposed to air during several hours without noticeable oxidation. The acquisition time being short en-ough, the experiments by

l

RS,

l

XRD and

l

-IRTF spectroscopy were carried out without speciﬁc protection against atmosphere.

After all chemical and structural observations made on corro-sion products, the metallic part of the transverse section was etched using Nital (4 ml nitric acid, 100 ml ethanol) in order to reveal the metallographic structure. This structure was then observed by OM.

3. Results

Three zones can be observed on the cross sections: metal, corro-sion product and concretion layers (Fig. 2). The concretion layer is intermittent and when present, it is mainly constituted of calcite (CaCO3). Moreover localised zones of about 10

l

m contain Si, Mg,

Al and O attesting that this layer corresponds to sediments (aluminosilicates).

3.1. The metallic substrate

Fig. 3a and b show typical microstructure of the metallic core of the analysed ingots. It is constituted of a heterogeneous hypoeu-tectoïd steel with various carbon contents (0.1–0.7%) as often ob-served in ancient ferrous alloys. These ingots were realised in solid state by hammering at high temperature. This heat treatment generates classically acicular structures with a very characteristic pro eutectoid cementite (Fe3C) feature at some location. Moreover,

because these gallo-roman ingots were realised by the former

bloomery process, a signiﬁcant number of non metallic compounds coming from the iron smelting stage (i.e. slag inclusions) was embedded in the metallic matrix. These inclusions are made of a polyphased mix of iron oxides, silicates and other phases as already observed in this kind of artefacts[37,38]. No signiﬁcant amounts (i.e. <100 ppm) of phosphorus or other elements were detected in the metallic matrix.

3.2. Corrosion patterns

The general layout of the corrosion system observed on the cross sections is described in the following part. A speciﬁc para-graph is dedicated to the sulphur-rich phases that have been iden-tiﬁed on the external part of the corrosion layer. The thickness of the corrosion layer varies between 200

l

m and 5 mm (Fig. 1b). On some part of the layout the corrosion products can penetrate several millimetres inside the metallic core along slag inclusion alignment (Fig. 2b). Chlorine content is of 18 wt.% inside the major part of the corrosion layer. Only in the outer part Cl content is less than 12 wt.%. These observations are consistent with the nature of the identiﬁed phases (Fig. 4). The main phase is an iron(II) hydroxychloride (Fe2(OH)3Cl) and is in contact with the metal.

The beta form and small peaks of gamma form of this phase have been detected thanks to

l

XRD and only the beta by

l

RS and FTIR. So b-Fe2(OH)3Cl is more likely predominant. In the outer part of the

layer thin layers of akaganeite (b-FeOOH, about 60

l

m) and mag-netite (Fe3O4, about 10

l

m) are successively observed. It is also

possible to follow the remaining cementite (Fe3C) from the former

pearlite zones at location of the corrosion product located in front of carburised zones of the substrate (Fig. 5). This cementite layout is only observed inside the iron hydroxychlorides zones and never in the akaganeite and magnetite layers as shown on Fig. 6.

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Cementite of the substrate does not seem to have been highly al-tered during the immersion period and so it constitutes a marker of the corrosion front progression. Consequently, it indicates that akaganeite and magnetite layers, containing no cementite, could have been formed by other phenomenon than those leading to the formation of the hydroxychlorides.

3.3. Sulphur-containing zones

EDS analyses of cross sections showed the local presence of sulphur in signiﬁcant amount (until 19 wt.%) with iron, oxygen, carbon and sometimes species coming from the environment like silicon, calcium, aluminium, magnesium, potassium and chlorine. Sulphur mappings (Fig. 2 c) revealed that in most cases the sulphur-rich zones were forming discontinuous thin strips (20–100

l

m thick) located between the rust layer and the calcium carbonate concretions covering the ingots.

Raman spectroscopy of these sulphur-containing zones allowed us to identify FeS mackinawite. Typical Raman spectra of the Fe and S containing compounds present in the rust layers of the ro-man ingots were recently compared to synthetic iron sulphides

[36].Fig. 7 presents one of the Raman spectra acquired on the archaeological sample. Two sharp lines, at 207 and 283 cm1_{, are}

typical of nanocrystalline mackinawite[36]. This FeS compound precipitates ﬁrst from Fe(II) and sulphide dissolved species. Aging of this precipitate should ﬁnally lead to well crystalline mackinaw-ite that is characterised by a slightly different spectrum[36]. So these spectral components seem to be associated with a compound formed rather recently. The other lines (125, 175, 252, 310, 318 and 350–370 cm1_{) are typical of a slightly oxidised phase, that}

is Fe(III)-containing mackinawite[36]. Mackinawite is very sensi-tive towards the oxidising action of oxygen and this slight oxida-tion may have occurred during storage and preparaoxida-tion of the sample.

Fig. 2. General layout observed on a cross section, (a) Backscattered electron picture and corresponding mapping images of (b) chlorine, (c) sulphur, (d) calcium.

Fig. 3. Nital etching on the metallic substrate showing various carbon content, optical microscope, (a) 0.7 wt.% carbon and (b) 0.3 wt.% carbon.

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Fig. 4. Fe(II) hydroxychlorides, the main components of the rust layer. (a)lXRD analysis showing the typical peaks of b-Fe2(OH)3Cl (b, JCPDF 00-034-0199),c-Fe2(OH)3Cl (c,

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Among all sulphur-containing zones, one in particular pre-sented atypical heterogeneities and the association of magnetite, FeCO3siderite and FeS mackinawite[36], all identiﬁed by Raman

spectroscopy. In this peculiar case, the Fe and S rich zones were scattered in a matrix mainly made of siderite. It must be noted that siderite could only be detected in this peculiar zone. The observa-tions are summarised in the schematic diagram ofFig. 8.

4. Discussion

4.1. E-pH equilibrium Pourbaix diagrams of iron in concentrated chloride solutions

The presence of iron hydroxychlorides (b and

c

-Fe2(OH)3Cl) and

akaganeite in the rust layer is typical of corrosion processes of iron in concentrated chloride solutions[8]. To facilitate the understand-ing and the description of the long-term behaviour of iron in such environments, the corresponding potential-pH equilibrium dia-gram was drawn (Fig. 9) at a temperature of 25 °C and a pressure of 1 atm. In the following, the inﬂuence of temperature and pres-sure is not discussed, as no information is available. The diagram was drawn considering only the b phase of Fe2(OH)3Cl, that seems

to predominate.

Standard Gibbs free energies of formation of the considered compounds and species are given inTable 1. The main concern re-lated to akaganeite: Its chemical composition is variable, and must be expressed as b-FeO1x(OH)1+xClx[39]. For instance, Laberty and

Navrotsky[40]gave an estimate of the standard enthalpy of forma-tion of b-FeOOH for x close to 0 (x = 0.032), and then did not con-sider the role of Cl_{. In contrast, Biedermann and Show} _[41]

determined experimentally the equilibrium conditions between akaganeite and solution to be:

3:04 0:05 ¼ 2:7pH þ log½Fe3þ ð1Þ This corresponds to a chemical composition of b-FeO0.7(OH)1.3Cl0.3

for akaganeite, and the reaction:

Fe3þþ 2H2O þ 0:3Cl() b FeO0:7ðOHÞ1:3Cl0:3 þ 2:7Hþ ð2Þ

is governed by the following equilibrium conditions:

pK ¼ 2:7pH þ log½Fe3þ þ 0:3 log½Cl ð3Þ or:

ðpK 0:3 log½ClÞ ¼ 2:7pH þ log½Fe3þ ð30_Þ

[41]worked with a constant chloride concentration of 0.5 mol L1_{that corresponds to an activity of 0.32, since the activity}

coef-ﬁcient of Cl_{in such solutions is about}

_c

Cl= 0.64[42]. The value of

pK can then be computed from equations (1) and (3’), which gives pK = 2.89. An estimation of the standard Gibbs free energy of for-mation of akaganeite with composition b-FeO0.7(OH)1.3Cl0.3 can Fig. 5. Optical microscope image of the corrosion layer in regard to a carburised

zone. Cementite lamellae are visible inside the corrosion products.

Fig. 6. Picture of the outer part of the corrosion layer, optical microscope.

Fig. 7. Typical Raman spectrum obtained in a sulphur-rich zone of the rust layer. Mnare the vibration bands of nanocrystalline mackinawite, M*, those of

Fe(III)-containing mackinawite[36]. k0= 632.18 nm.

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be deduced from this pK value. The computation leads to:DG°f

(b-FeO0.7(OH)1.3Cl0.3) = 514 kJ mol1. This value and the

correspond-ing composition for akaganeite were retained for the drawcorrespond-ing of the diagram of Fig. 9. The corresponding equilibrium equations are given inTable 2.

The diagram was drawn for an activity of chloride ions equiva-lent to that characteristic of seawater that is 0.35[42]. The frontier between the stability domains of Fe(OH)2and b-Fe2(OH)3Cl (line 6)

is located at pH 7.51. To illustrate the effects of an increase of the chloride concentration, the lines corresponding to an activity of Cl

equal to 1 were added to the diagram. They correspond to the dot-ted lines 5–8 and 13–15. It can be seen for instance that line 6 is shifted from pH 7.51 to pH 7.97.

According to this diagram, b-Fe2(OH)3Cl at 25 °C in a solution

with the same chloride activity as seawater, would be only stable for pH values lower than 7.51, would form when the activity of Fe2+_{reaches 0.01 and could persist for long periods only in anoxic}

conditions, since dissolved oxygen would oxidise it into akagane-ite. The formation of the iron hydroxychloride on the iron roman ingots would have then required speciﬁc conditions at the steel surface that are absence of oxygen, slight acidiﬁcation of the elec-trolyte since the pH value of seawater is rather close to 8.2, and/or increase of the chloride concentration.

4.2. Origin of the magnetite layer

First, the presence of a quasi continuous layer of magnetite in the external part of the corrosion products will be discussed. Two hypotheses can be evocated to explain its presence. It could be supposed that this magnetite is the result of the ﬁrst stages of the anoxic corrosion process, when the local concentration of chlo-rine in the water was not sufﬁciently high to provoke the precipi-tation of chlorinated phases as Fe2(OH)3Cl. Nevertheless, literature

demonstrates[43]that generally, the thickness of such magnetite layers formed in anoxic media never exceed several tens nanome-tres. That is not the case for the layer observed in this study that has an average thickness of about 10

l

m. For that reason it seems that a second formation hypothesis is more probable. We suggest that the presence of the magnetite layer is linked to the processes that happened before the marine corrosion period and are linked to hot forging process that generates a calamine layer. Because these bars are semi-products[30], this layer was not removed from the artefact after the treatment. These forging treatments happened around 1000 °C, thus, the hot corrosion layout that was obtained was probably constituted of successive layers of wüstite, magnetite

Fig. 8. Schematic diagram of the characterised corrosion layout.

Fig. 9. E-pH equilibrium Pourbaix diagram of iron in concentrated chloride aqueous solution at 25 °C, for an activity of chloride species of 0.35 (full lines) and 1.0 (dotted lines 5–8 to 13–15).

Table 1

Gibbs free energies of formation used for calculations in standard temperature and pressure conditions.

Species Average oxidation no of Fe DG°f/kJ mol1 References

Solid species

a-Fe 0 0

Fe(OH)2(s) +2 492 [48]

b-Fe2(OH)3Cl(s) +2 923.5 ± 1 [35]

b-FeO0.7(OH)1.3Cl0.3(s) +3 514 This work, computed from[41]

Liquid and dissolved species

H2O – 237.18 [48] Fe2+ aq +2 91.5 [48] FeOH+ +2 277.4 [48] Fe3+ aq +3 16.7 [48] Fe(OH)3 +3 660.0 [48] Cl _– 131.2 [49]

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and hematite. Depending of the cooling speed, wüstite, which is not stable at ambient temperature, could have been transformed in magnetite and hematite. It can be supposed that, during the artefact immersion in anoxic marine corrosion conditions, hema-tite dissolved and disappeared. It can then be assumed that the only remainder of the hot corrosion layer formed during hammer welding is the more or less continuous layer of magnetite around the artefacts. These observations and hypotheses were also made for other ferrous archaeological artefacts corroded in other envi-ronments[44]. In that case, this layer is a reliable marker of the ori-ginal surface[33,45].

4.3. Corrosion processes

Seawater is a biologically active medium. Many of the micro-organisms are observed in association with solid surfaces where they form the so-called biofilm, a composite film of bacteria and associated polymer. The first effect of the biofilm is to create a dif-fusion barrier that can support strong concentration gradients so that the chemistry at the metal/liquid interface may be different from that in the bulk seawater. First, the metabolic activity of aer-obic micro-organisms can consume dissolved oxygen and oxidise organic matter. The oxygen concentration decreases, the carbon dioxide concentration increases, and the water becomes more acidic. Then, the consumption of oxygen at the external part of the biofilm generates anoxic conditions closer to the metal, allow-ing the growth of anaerobic micro-organisms such as SRB. The activity of SRB consumes sulphate ions and produces hydrogen sul-phide, another cause for the acidification of the water at the interface.

The composition and morphology of the rust layers formed on the iron roman ingots can be interpreted according to this scenario.

The main compounds found inside these layers, the Fe(II) hydroxy-chlorides b

c

-Fe2(OH)3Cl, can only persist for long periods in

anaer-obic conditions. It is also the case for the Fe(II) sulphides that are moreover generated by the metabolic activity of anaerobic mi-cro-organisms, the SRB. This testiﬁes that the oxygen concentra-tion was close to zero at the metal surface and inside the rust layer. As a matter of fact, FeS compounds are found in the outer part of the rust layer, which indicates that the environment was al-ready anoxic at this place. In one particular zone, FeS compounds were associated with siderite FeCO3. The presence of FeCO3could

be the consequence of a locally intense bacterial activity that would have produced a large amount of carbon dioxide and conse-quently of carbonate species, via oxidation of organic matter. In the anaerobic part of the bioﬁlm, there were zones colonised by SRB, where the sulphide concentration was sufﬁcient for the precipita-tion of FeS. It is likely that FeS (and FeCO3), precipitated from the

Fe2+ _{ions that diffused from the inner b-Fe}

2(OH)3Cl layer. This

would explain why FeS was mainly found as a thin layer at the frontier between other corrosion products and concretions. More-over, this is consistent with the respective solubility of these com-pounds. As illustrated by the Pourbaix diagram of Fig. 9, b-Fe2(OH)3Cl is not so insoluble as FeCO3and FeS (the equilibrium

conditions at 25 °C are given by: [Fe2+_][HCO

3]/[H+] = 100.47, for

FeCO3[46], and [Fe2+][HS]/[H+] = 103.00for FeS[47]), and would

be in equilibrium with rather large (102 _{to 10}3_{mol L}1_{) Fe}2+

concentrations.

Another consequence of metabolic activity is the acidification of the electrolyte. This excess of protons is necessarily accompanied by an increase of the concentration of anionic species, and partic-ularly the mobile and abundant chloride ions. So both an acidificat-ion and an increase of chloride concentratacidificat-ion are likely to occur at the metal/liquid interface. As illustrated by the Pourbaix diagram ofFig. 9, this would explain the formation of b-Fe2(OH)3Cl on the

surface of the iron ingots. The presence of akaganeite in the outer part of the b-Fe2(OH)3Cl layer does not match with the proposed

scenario. It must however be reminded that the ingots were stored for sometimes after their excavation and before analysis in tanks full of tap water, with contact to the air between the different preparation steps. It can be forwarded that b-Fe2(OH)3Cl was

par-tially dissolved during the preparation, as a consequence of the modiﬁcations induced in the corrosion system by the transfer of the ingot from seawater to tap water. Fe2+cations, diffusing from the inner to the outer part of the rust layer, were oxidised into Fe(III) by oxygen, and some akaganeite should have precipitated in cracks present under the magnetite layer. This hypothesis stands on the basis that no cementite was detected in this part of the layer contrary to what was observed in the thick iron hydroxychloride layer.

It must finally be noted that the influence of micro-organisms, such as SRB, does not seem to have induced strong modifications on the kinetics of the corrosion process. The areas clearly associ-ated to their metabolic activity, where FeS is present, are not con-nected to local severe degradations of the metal. This should be due to the fact that the whole metal surface was in anoxic condi-tions, implying that galvanic coupling between oxygenated and an-oxic zones did not take place.

5. Conclusions

Corrosion layout formed during bimillenar immersion period in seawater of an iron bar has been identiﬁed thanks to a combination of microbeam analysis techniques. The new insight given by this study is the identiﬁcation of an iron(II) hydroxychloride b-Fe2(OH)3Cl as the main phase formed inside the corrosion layer.

Its presence testiﬁes that the metal surface was maintained in

Table 2

Equilibrium equations of E-pH equilibrium Pourbaix diagrams drawn inFig. 9. Water (a) H_{2} () 2H++ 2 e Eh= 0.000–0.059 pH Fe–H2O system (1’) Fe2+ + H2O () FeOH++ H+ 8.98 = log [Fe2+ ] – log [FeOH+ ] + pH (2’) FeOH+ + 2H2O () Fe(OH)3+ 2H++ e

Eh= 0.951–0.0591 log [FeOH+] + 0.0591 log [Fe(OH)3] 0.1183 pH

(3’) Fe2+

+ 3H2O () Fe(OH)3+ 3H++ e

Eh= 1.483–0.0591 log [Fe2+] + 0.0591 log [Fe(OH)3] 0.1774 pH

(4) Fe + 2 H2O () Fe(OH)2(s)+ 2 H++ 2 e

Eh= 0.091–0.0591 pH

(5) 2 Fe + 3 H2O + Cl() b-Fe2(OH)3Cl(s)+ 4 e+ 3 H+

Eh= 0.209 – 0.0148 log [Cl] – 0.0443 pH

(6) b-Fe2(OH)3Cl(s)+ H2O () Fe(OH)2(s)+ H++ Cl

7.97 = pH – log [Cl

]

(7) Fe(OH)2(s)+ 0.3 Cl () b-FeO0.7(OH)1.3Cl0.3(s)+ 0.7 H++ e

Eh= 0.180 – 0.0177 log [Cl] 0.0414 pH

(8) b-Fe2(OH)3Cl(s)+ H2O () 2 (b-FeO0.7(OH)1.3Cl0.3(s)) + 0.4 Cl+ 2.4 H+

+ 2 e Eh= 0.416 + 0.0118 log [Cl] – 0.0709 pH (9) Fe () Fe2+ + 2 e Eh= 0.474 + 0.0296 log [Fe2+] (10) Fe + H2O () FeOH++ H++ 2 e Eh= 0.21 + 0.0296 log [FeOH+] – 0.0296 pH (11) Fe2+ + 2 H2O () Fe(OH)2(s)+ 2 H+ 12.94 = log [Fe2+ ] + 2 pH (12) FeOH+ + H2O () Fe(OH)2(s)+ H+ 3.96 = log [FeOH+ ] + pH (13) 2 Fe2+_{+ Cl}_{+ 3 H} 2O () b-Fe2(OH)3Cl(s)+ 3 H+

17.91 = 2 log [Fe2+_{] + log [Cl}_{] + 3 pH}

(14) Fe2+

+ 2 H2O + 0.3 Cl() b-FeO0.7(OH)1.3Cl0.3(s)+ 2.7 H++ e

Eh= 0.95–0.0591 log [Fe2+] – 0.0177 log [Cl] 0.1596 pH

(15) FeOH+

+ H2O + 0.3 Cl () b-FeO0.7(OH)1.3Cl0.3(s)+ 1.7 H++ e

Eh= 0.41–0.0591 log [FeOH+] – 0.0177 log [Cl] 0.1005 pH

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anoxic conditions. FeS phases were observed on the outer part of the corrosion layer, in contact with the concretion mainly constituted of calcite. The presence of these sulphur containing phases is nec-essarily associated with the metabolic activity of SRB, since SO42

is the only S-containing species of seawater in the absence of these micro-organisms. The presence of these anaerobic micro-organ-isms in the outer part of the rust layer conﬁrms that anoxic condi-tions were met inside the whole rust layer. Considering the relatively high quantity of metal remaining in this ingot, it seems that in these conditions iron was rather preserved from corrosion. The process was evidently inﬂuenced by micro-organisms but re-mained rather uniform and accelerated localised degradations did not take place.

The original surface of the artefacts could be correlated to a thin magnetite layer visible on the external part of the rust layer that could have grown during the forging of the bar. This ﬁrst detailed description of the corrosion system characteristic of an iron arte-fact left for millenaries in seawater should give a better under-standing of the processes involved during the desalinisation treatments required for the restoration and preservation of these objects.

Acknowledgements

This study was realised in the frame of the ODEFA PNRC project of the French Ministry of Culture and in the ARCOR project of the French National Research Agency ANR. Archaeologists who al-lowed us to work on the gallo roman bars are gratefully thanked. Authors gratefully thank the DRASSM and all the archaeologists in-volved in the excavations of the Sainte Marie de la Mer shipwrecks for allowing them the present study on the archaeological material. References

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