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What effect does siderite crystal growth have on chlorides and potential information within the corrosion products of archaeological iron artefacts?

Ruben With

Résumés

Dans des environnements d'excavation anoxiques, carbonatés et légèrement acides, la formation de cristaux de sidérite sur du fer archéologique semble avoir un effet de passivation sur le processus de corrosion. L'objectif de ce projet a été d'observer de quelle façon la formation de cristaux de sidérite à l'intérieur de la croute de corrosion d'objet archéologiques en fer aurait un effet sur cette croute de corrosion et son contenu en chlorure. Pour ce faire, un environnement similaire à celui mentionné auparavant, mais simplifié, pour optimiser la formation de cristaux de sidérite, a été créé.   

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Texte intégral

Background for the project

1Archaeological iron objects from anoxic and carbonated sites are often in a better preserved state than excavated ironwork from other sites. This is attributed to the formation of the corrosion product siderite under specific environmental burial conditions. Research shows that the presence of chlorides accelerates the corrosion of iron, but also seems to increase the rate of siderite precipitation. Furthermore, the oxidation of siderite upon excavation leads to the formation of stable iron corrosion products if stored dry (Matthiesen, Hilbert and Gregory, 2003:185). The aim of this Master project is to gain understanding of the siderite formation process and its effect on the corrosion crust and its chloride content.

2Archaeological iron objects recovered from the site of Nydam Mose in Denmark have significant differences in condition. The better preserved iron objects are thought to have been directly incorporated into the reducing environment, whilst the more poorly preserved objects were buried in an initial oxidising environment that gradually changed to a reducing one. The corrosion product siderite only occurs on iron artefacts from anoxic and carbonated environments, indicating that the state of preservation is dependent on the precipitation rate and the density of the siderite layer formed. The initial corrosion products on the more poorly preserved objects would predominantly have been composed of iron oxides and iron hydroxide oxides, reduced upon incorporation into the anoxic and carbonated environment.

3The processes of formation of siderite and the reasons for its stability in the burial environment have become a field of study. Short-term laboratory tests suggest that an increase in chloride content of the electrolyte will cause a more rapid formation of a denser siderite layer on iron coupons (Jiang and Nešić, 2009:5). Given that siderite oxidizes to stable products suggest, in my opinion, that the corrosion process and precipitation of siderite may have an effect on the chlorides within the pores of the corrosion layers initially formed around the artefact during the first period of burial.

Experimental design

4A series of short-term laboratory experiments have been designed in collaboration with researchers specialised in mineral growth and dissolution from the Department of Geosciences (University of Oslo). The experiments were performed in a glass cell reactor, and tests were undertaken on both iron coupons and archaeological finds from both oxidising terrestrial and marine environments.

5During preliminary experiments clean and beforehand corroded iron coupons were kept for 4 days in electrolytes with different NaCl concentrations in a synthetic anoxic environment achieved by the bubbling of CO2 through it. Otherwise the parameters were set to optimize for siderite growth. The objective of these experiments was, among others, to establish at which NaCl concentration siderite growth were more efficient.

Preliminary results

6Results from the first preliminary test showed a rapid siderite crystal growth  at 74 °C and pH 6,4 ± 0,1 when the  electrolyte was saturated with Fe2+ ions, the CO2 partial pressure was at ca 0,54 bar, total pressure ca 1,0 atm and the NaCl concentration was ca 20 % (w/v). The siderite crystal growth was clearly denser at the surface of the clean coupon than on the surface of the already formed corrosion products (fig. 1).

7The bands at 1402/1409, 859/864 and 734/737 cm-1 present in the FTIR-spectra indicates the formation of siderite (Matthiesen, Hilbert and Gregory, 2003:185). These spectra also show a pronounced difference in purity of the two samples (fig. 2).

Upcoming experiments

8Upon completion of the preliminary experiments, archaeological samples will be kept in the most optimal electrolyte for siderite growth at different lengths of time. Comparative analysis of both treated and untreated cross-sections will be performed by SEM-EDS, thus providing data of the elemental distribution within the corrosion products. Furthermore, FTIR analysis will provide complementary analysis of the corrosion products formed. The performed analysis will then hopefully provide insight in siderite formation and its effect upon the chloride content of a given corrosion layer.

Fig. 1 Siderite crystals enlarged with scanning electron microscopy

Fig. 1 Siderite crystals enlarged with scanning electron microscopy

Siderite crystal growth on iron coupons after 4 days in the glass cell reactor: Top left: clean coupon 1000x, 15 kV, Bottom left: corroded coupon 1000x, 15kV, Top right: clean coupon 300x, 15kV, Bottom right: corroded coupon 300x, 15kV

 Credits: Ruben With

  

Fig. 2 Fourier transform infrared spectra of the newly precipitated corrosion products

Fig. 2 Fourier transform infrared spectra of the newly precipitated corrosion products

FTIR-spectra of the precipitated product on both a corroded and a clean iron coupon, top and bottom respectively.

Credits: Ruben With

Results

9During the period between the conference and time of printing the experiments were completed with encouraging results. The newly formed corrosion products apparently separated the chlorine from the metal surface, seemingly without affecting the corrosion crust of the object. However, more statistically valid research is needed to verify the obtained results.

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Bibliographie

Jiang, X. and Nešić, S., 2009, "Electrochemical Investigation of the Role of Cl- on Localized CO2 Corrosion of Mild Steel", in conference publications of the 17th International Corrosion Congress, paper #2414, Ohio, US, [online]. Available at: <http://www.corrosioncenter.ohiou.edu/documents/publications/8209.pdf>, [27.02.2014]

Matthiesen, H., Hilbert, L. R. and Gregory, D. J., 2003, "Siderite as a Corrosion Product on Archaeological Iron from a Waterlogged Environment" in Studies in Conservation, vol. 48, no. 3, pp. 183-194, Maney Publishing, UK, [online]. Available at: <http://www.jstor.org/discover/10.2307/1506891?uid=3738744&uid=2134&uid=2&uid=70&uid=4&sid=21103730349897>, [30.07.2013] 

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Document annexe

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Table des illustrations

Titre Fig. 1 Siderite crystals enlarged with scanning electron microscopy
Légende Siderite crystal growth on iron coupons after 4 days in the glass cell reactor: Top left: clean coupon 1000x, 15 kV, Bottom left: corroded coupon 1000x, 15kV, Top right: clean coupon 300x, 15kV, Bottom right: corroded coupon 300x, 15kV
Crédits  Credits: Ruben With
URL http://ceroart.revues.org/docannexe/image/4336/img-1.jpg
Fichier image/jpeg, 80k
Titre Fig. 2 Fourier transform infrared spectra of the newly precipitated corrosion products
Légende FTIR-spectra of the precipitated product on both a corroded and a clean iron coupon, top and bottom respectively.
Crédits Credits: Ruben With
URL http://ceroart.revues.org/docannexe/image/4336/img-2.jpg
Fichier image/jpeg, 30k
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Pour citer cet article

Référence électronique

Ruben With, « What effect does siderite crystal growth have on chlorides and potential information within the corrosion products of archaeological iron artefacts? », CeROArt [En ligne],  | 2014, mis en ligne le 01 octobre 2014, consulté le 29 mars 2017. URL : http://ceroart.revues.org/4336

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Auteur

Ruben With

Upon completion of the Bachelor’s Degree in Cultural Heritage and Conservation Knowledge, Ruben With proceeded to the Master’s Degree in Objects Conservation in 2012, both at the University of Oslo. He is currently undertaking an internship at the Archaeological museum of Stavanger, which marks the completion of the Master’s Degree (2014). E-mail: with_ruben@hotmail.com

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Droits d’auteur

Licence Creative Commons
CeROArt – Conservation, exposition, restauration d'objets d'arts est mis à disposition selon les termes de la licence Creative Commons Attribution - Pas d'Utilisation Commerciale - Pas de Modification 4.0 International.

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