Removal of Chloride and Iron Ions from Archaeological Wrought Iron with Sodium Hydroxide and Ethylenediamine Solutions




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НазваниеRemoval of Chloride and Iron Ions from Archaeological Wrought Iron with Sodium Hydroxide and Ethylenediamine Solutions
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Table 3 Experimental results from ICP-AES analysis. a





Notes


a. Table gives the weight and percentage of the elements phosphorus (P), ron (Fej. aluminium (AI), calcium (Caj. and potassium (K) removed in NaOH or EN; n.d. indicates that the element was not detected. b. Objects are from Group 1 13346, 4824, 6569), Group 2 (94419. 99429. 99441). and Group 3 (2279, 3004a. 3004b).


the pores of a corrosion layer, the magnitude of the diffusion constant reflects the rate at which Cl- ions move through this solution, which in turn depends on the pore size, channel size, and their connectivity- within the solid [27]. Diffusion constants are smaller for ions diffusing through a solution within the pores of a solid (e.g., a corrosion layer) than for ions diffusing through an open solution.

In our Cl- ion data, the authors often observed a delay before any Cl- ions were detected in solution, and many curves of Cl- ion versus square root of time were S-shaped instead of linear. Figure 5 contains several examples of our data, showing that the extrapolated lines did not pass through the origin, but instead crossed the x-axis at a specific time. For most objects from Groups 1 and 2 treated first in E.Y, approximately linear results were observed. In contrast, however, tor most objects from Groups 1 and 2 treated first in NaOH, and Group 3 objects treated in EN, non-linear curves were ob­served. Elsewhere, however, the authors have provided another diffusion model, different from that of North and Pearson, that can be used, at least qualitatively, to

describe the S-shaped behaviour and the delay time observed m some of our data [43]. This other model is based on the solution to the diffusion equation for the limiting case where the Cl- ions are initially concen­trated at the interface between the iron and the corro­sion layer. This abrupt starting distribution can be viewed as the opposite to the uniform starting distribu­tion used by North and Pearson. These diffusion models were not use to interpret the results because of the wide variation in the Cl- ion data as a function of time for the different artifacts. Another reason was that many of the objects in this study underwent a significant physical change (e.g., pieces broke open) during immersion, and diffusion models can only be applied when the condi­tions under which they are derived (e.g., fixed diffusion distance) remain unchanged.


Other dissolved ions


High levels of phosphorus were detected in the treat­ment solutions and these are most likely due to twentieth-century contamination by agricultural activity.





Figure 4 Weight of iron removed from the objects against time for six objects: (a) objects treated first in NaOH; (b) objects treated first in EN. Error bars as Figure 3





Figure 5 Weight of chloride ions removed from the objects against the square root of time: (a) results for Renews objects, two from Group 1 (6563, 6741) and one from Group 3 (2995); (b) resuits for two Group 2 Ferryland objects. Error bars as Figure 3.


The phosphorus is probably present in solution as phosphate (PO43-) ions which are known to be adsorbed onto soil particles, especially acidic soils, and are not easily removed by leaching with rainwater [44. 45]. However, exposure to high levels of hydroxyl ions during treatment in NaOH or EN probably displaces the adsorbed phosphate ions, leading to the high levels in solution. Relatively high levels of aluminium were also detected; these are probably the result of the dissolution of soil minerals associated with the outer corrosion layers; muscovite was identified on Group 1 and 3 objects, and albite was detected on Group 2 objects. The higher levels of aluminium from Group 2 objects may reflect a higher solubility of albite than muscovite in alkaline solutions. Higher levels ot magnesium, calcium and potassium were detected and are probably from seawater contamination of the burial environment. It is not known whether the other trace metal ions detected in sample solutions are from the burial environment or from the metal itselt. More analvsis of the metal content of the objects and a more detailed study of the burial environment would be needed for a better understand­ing of the ICP-AES results.


Effect of temperature


The solubility of iron oxides and oxyhydroxides in­creases with increasing temperature in alkaline solutions (because of the increased solubility of anionic species such as Fe(OH)3- [23, 46]) and so the use of higher treatment temperatures may dissolve more material from the corrosion products and increase the porosity of the remaining material. Higher treatment temperatures will also cause the metal and corrosion products to expand and this may also increase the porosity of corrosion layers. In our experiments, the use of EN treatment solutions at ~50°C rather than at room temperature may


have expanded the corrosion layers and dissolved additional iron corrosion products. The etfects ot using a higher treatment temperature are probably responsible for additional chloride being removed from objects placed in EN at ~50°C after a long immersion in NaOH at room temperature.


Role of Fe(II) ions


Turgoose predicted that placing archaeological iron into an alkaline solution would cause Fe2+ ions to precipitate within the corrosion layer, thereby restricting the diffusion of Cl- ions out of the object [22, 26]. More recently, Turgoose et al. have shown that Cl- ions diffuse out more easily after archaeological iron has been immersed in a NaOH solution [47. 48], They attributed this beneficial effect to rapid electrochemical reactions occurring at the iron/corrosion interlace which cause fissures and cracks to develop in the corrosion layer on a microscopic scale. It is likely that the cracks and fissures are caused by the precipitation of small amounts of solid corrosion products within the corrosion layer. These rapid processes are thought to be oxidation-reduction reactions involving iron metal, magnetite. and iron(II) species. The reactions occur on electron­ically conducting surfaces (e.g., magnetite) and cause irreversible changes and softening of the corrosion layer [48].

Recent electrochemical studies of iron in NaOH solutions (pH 11-13) have detected electrochemical activity in passive iron oxide films and confirmed that oxidation-reduction reactions occur easily between the iron(II) and iron(III) oxidation states [49-53], These studies have also shown that there is little or no dissolu­tion ot the corrosion products in the passive layer (e.g.. FeO(OH), Fe3O4 and Fe(OH)2) in these NaOH solu­tions under oxidizing or reducing conditions. Under reducing conditions, the passive film does not dissolve but instead undergoes a solid-state conversion from Fe(III) compounds to lower oxidation-state compounds (per­haps including Fe(OH)2) which have a porous structure.

The advantage ot these irreversible changes and softening of the corrosion layer is the increased ease with which Cl- ions are removed from archaeological iron. But there are disadvantages, too. An increase in porosity tends to decrease the mechanical integrity of the corrosion crust, making it more likely that the corrosion products can break off. The authors observed this prob­lem with artifacts placed first in NaOH treatment solu­tions, especially the extensively mineralized Group 1 artifacts.


Effect of pH


One reason that alkaline solutions have proved more effective than near-neutral solutions for removing Cl-ions from archaeological iron is an increased dissolution of material at higher pH. Alkaline solutions tend to be good at dissolving both inorganic and organic material. The solubility of quartz, for example, increases markedly above pH 9 [54, 55]. Greasy dirts, fatty compounds, oils, and other organic material (e.g., cellulose and protein) are broken down in alkaline solutions by saponification into water-soluble compounds such as soaps and alcohols [56].

Another reason tor the effectiveness of alkaline solutions is their ability to passivate an iron surface. The corrosion rate of iron slows significantly if the pH at the metal surface is high enough to precipitate Fe2+ ions as Fe(OH)2 (which has a minimum solubility at pH 11 [24]) and, once formed, is easily oxidized and hydro-lysed to Fe(OH)3 [57]. In general, the iron corrosion rate slows as the pH is increased above nine and drops to a negligible rate above 12 [58]. As long as archaeological iron is corroding during immersion, the Cl- ions are prevented from diffusing out because they are attracted to the Fe2+ ions being generated by the corrosion process. If the corrosion can be stopped, for example by passivating the iron at high pH, then the potential gradient generated by Fe2+ ions is removed. The Cl- ions no longer act as counterions and are able to diffuse out ot the corrosion layer into the treatment solution [43]. In this study, archaeological iron was immersed in an alkaline solution of either NaOH (pH 13.5) or EN (pH 11.5). Considering only the influence of pH, the NaOH solution should be more effective than the EN solution in passivating the iron and allowing the Cl- ions to diffuse out.

The pH of the solution also affects whether or not Cl- ions are adsorbed. Under acid conditions, Cl- ions are adsorbed onto iron oxide surfaces which have a net positive charge because of the excess H+ ions in solution [24]. The adsorption of Cl- ions onto iron oxide surfaces decreases in neutral and alkaline solutions because the rising pH shifts the net surface charge on the iron oxide surface to be negative [59, 60]. Therefore, when archae­ological iron covered with iron oxides or oxyhydroxides is placed in highly alkaline solutions (e.g.. NaOH or EN solutions), the number ot Cl- ions that remain adsorbed on an oxide surface is expected to below.

Duprat et al. have compared the effectiveness of NaOH and EN solutions (each solution at pH 11 and containing 3% w/v NaCl) for inhibiting iron corrosion


[61. 62]. Thev observed that the EN solution was more effective than the NaOH solution in slowing iron corro­sion. They attributed the overall effectiveness of these two solutions to their alkalinity and their ability to form a passivanng film on iron. They attributed the increased effectiveness of EN over NaOH (at the same pH and Cl- ion concentration) to the EN molecule contributing an additional inhibiting effect by being adsorbed onto the metal surface [63, 64].


Adsorption of EN


Ethylenediamine can act as a corrosion inhibitor for iron (slowing the corrosion rate of iron) because EN can be adsorbed onto oxide-free [65] and oxide-covered [66] iron surfaces. Ethylenediamine slows corrosion by form­ing a new layer when adsorbed onto bare iron or by rein­forcing an existing oxide film when adsorbed onto oxide-coated iron. The ability of EN to act as a corrosion inhibitor has been demonstrated for iron in strong acid solutions where the iron metal is oxide-free [65, 67-71]. EN has also been shown to be an effective corrosion inhibitor for iron in alkaline solutions where the surface is covered with a corrosion layer [61-64, 72-75].

Chemisorption is thought to be the main mechanism responsible for the adsorption of EN onto iron. Chemisorption (also called specific adsorption) involves the formation of coordinate bonds between neutral EN molecules and the iron surface [76, 77]. These bonds, also called donor-acceptor bonds, involve the sharing of the unpaired electrons on the nitrogen atoms in the neutral EN molecule with empty d-orbitals on iron. The bonds are relatively strong and chemisorbed molecules are not easily removed by rinsing. McCafferty and Hackerman studied the effectiveness of diamines for slowing iron corrosion in 6 M hydrochloric acid (HC1) [67, 68]. For EN, they suggested that both nitrogens were adsorbed onto iron, with the molecule lying parallel to the surface. Of the diamines studied. EN was one of the least effective corrosion inhibitors. In neutral or alkaline conditions, with an oxide film present. EN is thought to be chemisorbed onto the surface (either onto the metal at flaws in the existing oxide film, or onto iron oxyhydroxide corrosion products); this incorporation into the film serves to reinforce it [61, 62, 66).

Another mechanism, physisorption, may be respons­ible for additional EN adsorption. Physisorption (also called physical adsorption) involves adsorption of charged inhibitor ions by the formation of relatively weak electrostatic bonds between inhibitor ions and an electrically charged iron surface [76, 77]. In certain

acids, especially those containing halogens, such as HCl, the halogen ions chemisorb onto the metal and these negative ions attract and electrostatically bond positive ions (e.g., protonated EN) [68]. In general, organic amines tend to be more effective corrosion inhibitors in acids if halide ions are present because of this joint adsorption process [78, 79], Under alkaline conditions, where chloride ions are much less likely to be adsorbed, it is possible that a few protonated EN species (minor constituents in alkaline solutions) may be electrostatically bonded to the oxide-covered surface with its negative surface charge [63, 66].

The effectiveness of EN as an iron corrosion inhibitor depends on concentration and temperature. Zaritskii studied iron corrosion in EN solutions using weight-loss measurements [73]. He observed that the iron corroded rapidly for the first few days and then the corrosion rate slowed to a steady rate. He found that, at room tempera­ture, the corrosion rate of the iron decreased with increasing concentration of EN between 1-20% EN. Zaritskii also studied the effect of temperature on the iron corrosion rate in EN solutions and noted (for the same EN concentration) a higher corrosion rate at 80°C compared to room temperature. Zaritskii's result suggests that the effect of temperature should be included in any future studies of EN as a treatment for archaeological iron. Sokolova et al. studied the effect­iveness of EN as a corrosion inhibitor tor iron using weight-loss measurements [72]. It took about 10 days for their iron samples to reach a steady-state corrosion rate. They varied the concentration of EN from 10-5 M to 1.5 M; the corrosion rate started to slow significantly at 0.1 M EN and became negligible at 1 M EN (The authors used 5% v/'v (0.75 M) EN in the present study.)


Soluble [Fe(EN) ]2+ complexes


As Duprat et al. have shown, ethylenediamine can act as a corrosion inhibitor for iron because of its alkalinity and its ability to be adsorbed onto an iron surface [61, 63]. Unfortunately. EN also has the ability to form soluble complexes with Fe2+ ions and this property can make its use dangerous. Neutral EN molecules interact with Fe2+ ions to form iron-ligand complexes; the EN ligand replaces some or all of the water molecules that normally surround iron ions in solution [80. 81]. Bonds are formed between the unpaired electrons on the two nitrogens in EN and the d-orbitals of the iron ions. Both nitrogen atoms on an EN molecule are capable of interacting with the same metal ion simultaneously, acting as a bidentate ('two-toothed') ligand, replacing two water molecules and forming a five-membered ring. This process is known as chelation and the ligand involved is called a chelating agent [81].

Ethylenediamine is well known for its ability to form complexes with transition metal ions in the divalent (+2) oxidation state [10, 80, 82]. The stability constants for these complexes are often determined using acid solutions (to avoid precipitation of metal hydroxides) and inert atmospheres (to avoid oxidation of divalent ions to higher oxidation states) [80]. Ethylenediamine is also known to form complexes with certain monovalent ions (e.g., Ag+) and trivalent ions (e.g.. Co3+), as well as a range of other divalent ions (e.g., Sn2+. Cd2+, Pb2+, Pd2+ and Pt2+) [80, 83], In aqueous solutions, transition metal ions typically have six water molecules associated with the central metal atom (i.e., a coordination number of six), giving it octahedral symmetry, and these six water molecules can be replaced with up to three EN molecules. Iron(II) ions, for example, can form three different complexes with ethylenediamine: [Fe(EN)x]2+ with x = 1, 2 or 3 [80]. A schematic diagram showing an Fe2+ ion surrounded by three EN molecules is shown in Figure 6. Information about the reactions and forma­tion constants for these three complexes is summarized elsewhere [10].

Iron(III) ions can form complexes with various ligands, but they are usually stable only under acid conditions or other controlled conditions. Iron(III) ions can react with EN to form [Fe(EN)x]3+ complexes but they are difficult to isolate. The complex [Fe(EN3]Cl3, for example, is made by adding EN to anhydrous FeCl. in absolute ethanol [84]. In general, iron(III) complexes are unstable under alkaline conditions because the iron(III) ions react with hydroxyl ions and precipitate as insoluble iron hydroxides and oxyhydroxides [85, 86], For example,





Figure 6 Schematic diagram of the compiex ion [Fe(EN)3]2+ showing each ethylenediamine molecule occupying two coordination positions on Fe2+.


the iron(II)-EDTA complex (where EDTA is ethylene-diaminetetraacetic acid) is stable under alkaline condi­tions, but the iron(III)-EDTA complex is not [85].

In this study, relatively high levels of iron were detected in EN treatment solutions compared to NaOH treatment solutions. This behaviour is attributed to the reaction of EN with Fe2+ ions within the corrosion layer to form soluble [Fe(ENx)]2+ complexes which then diffused into the treatment solution. The Fe2+ ions inside the corrosion layer may have been present at excavation, may be the result ot ongoing corrosion of iron metal, or may arise from the dissolution of corrosion products (e.g., dissolution of Fe(OH), or magnetite). As the soluble [Fe(EN)]2+ complexes diffused into the treatment solution, they encountered higher levels of dissolved oxygen. The authors believe that the Fe2+ ions in the complexes are oxidized to Fe3+ ions which then precipitate as insoluble iron hydroxides and oxy­hydroxides [87]. The intense colour observed in EN treatment solutions compared to the paler NaOH treat­ment solutions may be due to either the soluble [Fe(EN)x ]2+ complexes or their oxidation to colloidal Fe3+ material.

For many objects, it was observed that if the outer iron(III) oxyhydroxide layer had fallen off and the inner black layer (presumably magnetite) was exposed to EN, then this black layer became easy to remove and some­times collected as black sludge at the bottom of the containers. The exact mechanism for this process is not known. It may be that if magnetite is exposed to an EN solution, then the EN molecule can adsorb on its surf­ace, interact with Fe2+ ions to form soluble [Fe(EN)x]2+ complexes, and destabilize the magnetite by making it less cohesive and less adherent to the underlying metal. It may also be that EN is stimulating iron corrosion beneath the magnetite. If the iron is corroding in the presence of EN then newly formed Fe2+ ions will react with it to torm soluble [Fe(EN)x]2+ complexes and prevent the iron from passivating. This will undermine the magnetite and it will fall off. The ability of EN to stimulate iron corrosion has been studied by Sakakibara et al. in anhydrous methanol containing 0.1 M LiClO4 [88, 89]. They showed that bare iron passivated in this electrolyte in the absence of EN but continued to corrode in the presence of EN (10-4 M) because of the formation of soluble [Fe(EN)x]2+ complexes [88, 89].


Passivation versus corrosion of iron in EN solutions


When archaeological iron is placed in an EN solution, there are competing factors at work because EN can stimulate as well as inhibit corrosion. The main advantage of using an EN solution to treat archaeological iron is its ability to act as a corrosion inhibitor, either by being adsorbed onto the metal surface or by reinforcing a pre­existing iron(III) oxyhydroxide film. When freshly excavated archaeological iron is placed in an EN solution, it is expected to be corroding because of the presence of an acidic FeCl, solution trapped next to the metal surface. Over time, the iron surface is expected to passivate in EN solutions, mainly because of the increase in pH but also because of the adsorption of EN molecules on the metal surface. The limited results from ICP-AES analysis (Figure 4) suggest that, at least in some cases, the iron eventually passivates because the level ot iron in EN solutions usually stops increasing.

The main disadvantage of using an EN solution to treat archaeological iron is its ability to stimulate corrosion by reacting with Fe2+ ions to form soluble [Fe(EN)x]2+ complexes. The results from the ICP-AES analysis of a few of the treatment solutions clearly indicated an increase in dissolved iron in EN solutions compared to NaOH solutions. Rapid corrosion of iron metal in archaeological objects treated in EN solutions has been reported in the conservation literature [8, 9, 11]. The authors also noted instances where exposure of the magnetite layer on an artifact to EN resulted in what appeared to be the dissolution ot magnetite. We interpret this as a reaction between EN and Fe2+ ions (from magnetite or from corroding iron metal beneath the magnetite) to form soluble [Fe(EN)x ]2+ complexes.

Our results suggest that heavily mineralized archaeo­logical iron (Group 1 objects) is seriously damaged by immersion in NaOH (2% w/v. pH 13.5) because of the high pH and the cracking caused by rapid oxidation-reduction reactions occurring inside the corrosion layers. Our results also suggest that heavily mineralized archae­ological iron does not experience such serious damage when immersed in EN (5% v/v, pH 11.5) prior to immersion in NaOH. The neutral EN molecule may be adsorbed onto the existing iron(III) oxyhydroxide corrosion layer where it acts as a corrosion inhibitor by helping to reinforce the corrosion layer. The EN mole­cule may also be removing Fe2+ ions as soluble [Fe(EN)x ]2+ complexes from within the corrosion layers, thereby removing their contribution to the rapid oxidation-reduction reactions that cause cracking within the corrosion layer. If the Fe2+ ions are removed during immersion in EN, then they can no longer participate in electrochemical reactions when the artifact is transferred into NaOH. Unfortunately, with less cracking, Cl- ions are more likely to remain trapped within the corrosion layer.


CONCLUSIONS


Results have been presented from a systematic assess­ment of a treatment approach for archaeological iron developed at the Canadian Conservation Institute in the early 1980s. Thirty-two archaeological wrought iron objects from Ferryland and Renews, Newfoundland, were treated by immersion in individual treatment solutions containing an aqueous solution of either NaOH (2% w/v, pH 13.5) or EN (5% v/v, pH 11.5). Treatment solutions for all 32 objects were analysed quantitatively for dissolved Cl- ions. Treatment solutions for nine objects were analysed by ICP-AES for 26 additional dissolved elements. This systematic approach provided specific information about the quantity and time-dependence of Cl- ion and other dissolved ele­ments as they diffused out of each object into a given treatment solution.

The results demonstrate that immersion of archaeo­logical iron in an aqueous sodium hydroxide solution is an effective way to treat archaeological iron with a substantial metal core where removal of chloride ions is important; the softening of the corrosion layers and the passivation of iron metal are contributing factors. The results also demonstrate that immersion of archaeological iron in an aqueous ethylenediamine solution is not particularly effective at removing Cl- ions although it is effective in preserving the corrosion layer on heavily mineralized iron; removal of dissolved iron(II) ions and minimal softening are contributing factors. The use of EN solutions in conjunction with NaOH solutions may provide an effective way to treat archaeological iron without a substantial metal core where maintaining the outer corrosion layers is important.

Unfortunately, immersing archaeological iron with a substantial amount of remaining iron metal in EN solutions can be dangerous because of the ability of EN to form soluble complexes with iron(II) ions. If the iron metal is still corroding and forming new iron(II) ions at anodic sites, then the iron may continue to corrode (because of the reaction ot Fe2+ ions with EN to form soluble complexes) rather than passivate (through the reaction of Fe2+ ions with hydroxyl ions to precipitate iron(II) hydroxide). Finally, ICP-AES analysis of treatment solutions provided a better understanding of what other elements were being removed from the archaeological iron by treatment in alkaline solutions.


ACKNOWLEDGEMENTS


The authors thank Cathy Mathias and Dr James Tuck from Memorial University of Newfoundland and Steve Mills from Parks Canada for supplying the iron artifacts tor treatment. We also thank statt at the Canadian Conservation Institute (CCI) for their help, particularly Kimberly Figures and Judy Logan for help with the treatment, Nancy Binnie for help with chloride ion analysis, Jane Sirois for X-ray diffraction analysis and Carl Bigras for photography. Comments on the final draft from Jane Down. David Grattan, Cathy Mathias. Gavie McIntyre and Susanne Sutherland were also greatly appreciated. Finally, funding for Vasilike Argyropoulos by the CCI Fellowship Program is gratefully acknowledged.


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