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Less is More: Measure of Chloride Removal Rate from Wrought Iron Artifacts During Electrolysis

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  • Ships of Discovery

Abstract

Electrolysis performs three useful functions for the conservator: it mechanically cleans artifact surfaces, removes salt, and (theoretically reduces corrosion products. Because each of these functions is fundamentally different, they must be separated for study. Mechanical cleaning and the reduction of corrosion products during electrolysis are recognized as relatively minor effects; therefore, from the perspective of the archaeological conservator, the most important function is chloride removal. The authors implemented a study to discover what current density most efficiently promotes this function with respect to wrought-iron artifacts recovered from marine environments. Complete data are presented for three experiments performed on archaeological specimens in which current was controlled an the quantity of chloride removed was measured. Over the course of four experiments, the lower current density of 50 muA.cm(-2) removed an average of 4.9 times more chloride (expressed in mg A(-1)) than a higher current density of 200A.cm(-2.)
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... Conservation scientists have thus carried out stabilization treatments based on chlorine extraction from the corrosion system such as electrolysis [3][4][5], hydrogen plasma [6][7][8], subcritical fluid [9][10][11][12], or the most common, alkaline solutions [4,[13][14][15][16][17][18][19]. A large number of studies aiming to improve the efficiency of these stabilization treatments have been conducted using macroscopic [18] or microscopic scale approaches [20][21][22]. ...
... The corrosion system of several similar bars was previously characterized by Rémazeilles et al. [24]. These authors showed that the corrosion layers mainly contain ferrous hydroxychloride β-Fe 2 (OH) 3 Cl, a phase characteristic of iron corroded in a deep marine environment. Due to the reactivity of β-Fe 2 (OH) 3 Cl during exposure to air, the iron bars for the present study were stored immediately after excavation in a wet environment (tap water) before being transferred to the laboratory. ...
... These authors showed that the corrosion layers mainly contain ferrous hydroxychloride β-Fe 2 (OH) 3 Cl, a phase characteristic of iron corroded in a deep marine environment. Due to the reactivity of β-Fe 2 (OH) 3 Cl during exposure to air, the iron bars for the present study were stored immediately after excavation in a wet environment (tap water) before being transferred to the laboratory. ...
... Artifacts from the 1554 Padre Island Plate Fleet Wrecks are not only on display, but are also the subject of continuous historical and conservation research efforts (Arnold et al. 1995;Carlin et al. 2001;Carlin and Keith 1996). Originally conserved in the early 2002 SAA Meeting Symposium Paper Denver, CO Drolet and Keith 5 1970's, the wrought iron artifacts from these two shipwreck sites are some of the first to be treated using electrolytic reduction. ...
... The results of both tests have been published in the International Journal of Nautical Archaeology and Studies in Conservation. (Carlin and Keith 1996;Carlin et al. 2001) ...
... Moreover, the spent solution requires specific waste neutralization, and the amount of chlorine extracted from the object is hard to assess (7). Another method, electrolytic reduction, exposes the surface of the original artifact which removes salts and reduces corrosion products (8). However, this treatment leads to a significant loss of matter, which is a concern in the context of archaeological artifacts, particularly, of small objects. ...
... After the treatment, new minerals were detected on all the coupons. On the coupons of the abiotic control, small aggregates were observed with SEM and identified as a mixture of mineral sulfur (␣-S8 ) and poorly crystallized mackinawite [Fe(II)/Fe(III)S] (44). The additional large Raman shift between 980 and 1,100 cm Ϫ1 and the presence of carbon as a major element in the EDS spectrum suggested the formation of amorphous siderite. ...
Article
Iron artifacts are common among the findings of archaeological excavations. The corrosion layer formed on these objects requires stabilization after their recovery, without which the destruction of the item due to physicochemical damage is likely. Current technologies for stabilizing the corrosion layer are lengthy and generate hazardous waste products. Therefore, there is a pressing need for an alternative method for stabilizing the corrosion layer on iron objects. The aim of this study was to evaluate an alternative conservation-restoration method using bacteria. For this, anaerobic iron reduction leading to the formation of stable iron minerals in the presence of chlorine was investigated for two strains of Desulfitobacterium hafniense (strains TCE1 and LBE). Iron reduction was observed for soluble Fe(III) phases as well as for akaganeite, the most troublesome iron compound in the corrosion layer of archaeological iron objects. In terms of biogenic mineral production, differential efficiencies were observed in assays performed on corroded iron coupons. Strain TCE1 produced a homogeneous layer of vivianite covering 80% of the corroded surface, while on the coupons treated with strain LBE, only 10% of the surface was covered by the same mineral. Finally, an attempt to reduce iron on archaeological objects was performed with strain TCE1, which led to the formation of both biogenic vivianite and magnetite on the surface of the artifacts. These results demonstrate the potential of this biological treatment for stabilizing archaeological iron as a promising alternative to traditional conservation-restoration methods.
... Active corrosion is generally more localized although it risks undermining the overall conservation state of the object. Over many years, dechlorination treatments have been developed to effectively stabilize ferrous or copper alloy objects: these treatments are generally chemical [7][8][9][10][11] or electrochemical [12][13][14][15][16]. They require the object to be totally immersed in an alkaline solution. ...
Article
Full-text available
Gels were used to perform localized dechlorination treatments on ferrous or copper alloy archaeological objects. Agar gel (3%w) was used as a medium for the electrolyte, a 1%w KNO3 solution. Localized electrolysis with gel was carried out using the same parameters as immersion electrolysis. To determine the end of treatment, two tools were validated: determining the quantity of chlorides present in the gels by XRF and monitoring the oxygen consumption of an object before and after treatment. This study shows that the technique results in the efficient extraction of chlorides. In the case of the stabilization of composite objects or for the localized treatment of copper objects, the use of a localized electrolytic gel treatment is a new effective solution proposed to conservators.
... Also, a large quantity of generated waste needs to be processed afterward for safe disposal. Second, electrolytic reduction allows an increase in the porosity of the corrosion layer and thus enhances the diffusion of harmful salts from the objects [13]. However, this method is restricted to large marine finds, as there is a significant loss of the surface and a lack of control over the amount of salts extracted and the corrosion products reduced during hydrogen bubbling [3]. ...
Article
Full-text available
This study evaluates mechanisms of biogenic mineral formation induced by bacterial iron reduction for the stabilization of corroded iron. As an example, the Desulfitobacterium hafniense strain TCE1 was employed to treat corroded coupons presenting urban natural atmospheric corrosion, and spectroscopic investigations were performed on the samples’ cross-sections to evaluate the corrosion stratigraphy. The treated samples presented a protective continuous layer of iron phosphates (vivianite Fe2+3(PO4)2·8H2O and barbosalite Fe2+Fe3+2(PO4)2(OH)2), which covered 92% of the surface and was associated with a decrease in the thickness of the original corrosion layer. The results allow us to better understand the conversion of reactive corrosion products into stable biogenic minerals, as well as to identify important criteria for the design of a green alternative treatment for the stabilization of corroded iron.
... In order to enhance the extraction of harmful chloride ions, two key factors have to be considered: to slow down or stop the corrosion of the object and to increase the porosity of its corrosion crusts [50]. Slowing down the corrosion can be achieved by passivating the iron surface with an alkaline treatment solution [57], by removing dissolved oxygen from the treatment solution [58], or by electrochemical methods [59]. In addition, the porosity of the corrosion products may be increased by heating the treatment solutions, dissolving extraneous material in alkaline solutions or reducing some of the iron oxyhydroxides FeO(OH) to magnetite Fe 3 O 4 (or some other lower oxidation state iron oxides or iron hydroxides) [50,60]. ...
Article
While often considered as harmful for cultural heritage, microorganisms can also be used for its safeguarding. The methods used so far for the conservation-restoration of cultural heritage are often unsatisfactory in terms of efficiency and durability. Inhibitors or complexing agents are also toxic and pose potential threats to human health and to the environment. Microbial-based technologies can provide sustainable solutions for heritage conservation-restoration using ecologically friendly biological treatments. Over the last decades, the development of biological methods and materials has become a significant alternative for the preservation of ancient heritage. Of particular note, microbial metabolisms are exploited to consolidate, clean, stabilize or even protect surfaces of cultural items. Taking advantage of unique properties of microorganisms, reactive corrosion products are extracted or converted into biogenic minerals that provide the treated surfaces with long-term stability. Examples of the techniques proposed include the formation of passivating biogenic layers that can be applied for preservation of metal-based heritage, as well as the development of methods for the preventative removal of iron species from waterlogged wood. This review presents the current advance made in research aiming to preserve copper- and iron-based artefacts, in particular sculptures but also archaeological objects, as well as in the development of a method for the extraction of iron species from waterlogged wood.
... In order to enhance the extraction of chloride ions, two key factors have to be considered: whether to slow down or stop the corrosion of the object or whether to increase the porosity of its corrosion crusts (Scott and Eggert 2009). The former can be achieved by passivating the iron surface with an alkaline treatment solution (Selwyn and Argyropoulos 2005), by removing dissolved oxygen from the treatment solution (Watkinson 1996) or by using electrochemical methods (Carlin, Keith and Rodriguez 2001). The porosity of the corrosion products may be increased by heating the treatment solutions, dissolving extraneous material in alkaline solutions or reducing some of the iron oxyhydroxides FeO(OH) to magnetite Fe 3 O 4 or some other lower oxidation state iron oxide or hydroxide (Schmidt-Ott andBoissonnas 2002, Scott and). ...
Conference Paper
Archaeological iron artefacts encounter serious post-excavation problems when contaminated with salts. In fact, once excavated, the exposure to a higher oxygen concentration and lower relative humidity renders the corrosion crust formed during burial no longer stable. In particular, the process is induced by chloride ions and flakes, cracks and finally loose of shape on the object are observed. To this aim, the MAIA project (Microbes for Archaeological Iron Artefacts) studied microbial metabolisms dealing with iron and explored their potential for the development of innovative and sustainable methods for the stabilisation of corroded iron archaeological objects. Two different approaches were investigated; first, bacterial reduction of iron solid-phases and biogenic mineral formation were studied as a way to replace unstable corrosion products. Several bacterial strains were compared. Spectroscopic investigations with Scanning Electron Microscopy (SEM) and Fourier Transform Infrared (FTIR) spectroscopy on iron coupons and nails’ surfaces and cross-sections demonstrated the conversion of the outermost part of the corrosion layer into more stable biogenic minerals, such as vivianite and siderite; the second approach was to study fungi and their metabolic ability with iron. In particular alkaliphile fungi that tolerate chlorine were studied for their ability to produce biogenic minerals and to adsorb metals in their biomass. Colorimetric investigation and evaluation of the thickness of the corrosion layer, demonstrated that fungi are good candidates to develop bio-cleaning methods for corroded iron, permitting the selective removal of the unstable and powdery corrosion layer without damaging the original metal surface. This communication details these approaches and explores the possibilities of their exploitation for development of innovative and more sustainable treatments for the conservation-restoration of corroded iron.
... The application of low intensities of current minimizes the evolution of hydrogen on the cathode, which increases the rate of extraction of Cl − by increasing the area available for their diffusion [14]. Additionally, the employment of small currents enables the graphitized zone to be consolidated, due to the reduction of the iron oxyhydroxides of which it is composed, principally akaganeite, (Fe 3+ O(OH)(Cl x )), that is initially transformed into goethite (␣-FeO(OH)), and later into magnetite (Fe 3 O 4 or FeO·Fe 2 O 3 ) [15]. ...
Article
With the purpose of optimising a suitable methodology for the conservation of an archaeological object of iron, a low current intensities electrolytic treatment has been applied, to a piece of cast iron, proving to be effective in the extraction of chloride ions from the structure of akaganeite, principal corrosion product of iron in the marine medium. The monitoring of the electrolytic treatment has been proven by applying the "Rietveld " method to the patterns XRD of samples extracted from the corroded surface before and after the treatment. This method has permitted the unequivocal determination of the akaganeite and its chemical composition. This identification has been corroborated by means of SEM and EDS. After the electrolytic treatment, akaganeite was not present in the sample.
Article
Los tratamientos de estabilización electrolítica están basados en la extracción de los iones cloruros, principales causantes del deterioro de los objetos metálicos en ambientes marinos, mediante la aplicación de una diferencia de potencial con el que se consigue aumentar la difusión de dichos iones mediante la reducción de los productos de corrosión formados en el objeto. Además, durante el tratamiento electroquímico, en el objeto metálico va a variar su potencial de oxidación favoreciendo su pasivación frente a los procesos de corrosión que sufren tanto en el yacimiento como en la post-excavación
Conference Paper
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The underwater archaeological objects made of metal and organic materials present serious difficulties for preservation. The treatments of both materials are often incompatible and on these occasions it is necessary to opt for consensus solutions or separate treatment. A conveyor cilynder for anchor from the wreck Fougueux has been involved in a process that has included preventive conservation measures as sacrifice anodes and alkaline baths. The definitive treatment has led to the temporary removal of a stabilization system for reliable wood (plastination) and steel elements (cathodic polarization) have finally been restored to its original position.
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