Corrosion inhibitors for the preservation of metallic heritage artefacts
E. Cano, D. Lafuente
Centro Nacional de Investigaciones Metalúrgicas (CENIM)-Consejo Superior de Investigaciones Científicas (CSIC)
Avda. Gregorio del Amo 8, 28040 Madrid
Publicado originalmente en: EFC book nº65 “Corrosion and conservation of cultural heritage artefacts” P.Dillmann, A. Adriens, E. Angelini and D. Watkinson (eds.) WoodHead Publishing. European Federation of Corrosion.2013. pp. 570-594
ISBN 978-1-78242-154-2(Print) 978-1-78242-157-3 (Online)
D.O.I. 10.1533/9781782421573
Introduction
With few exceptions, all metals are subject to degradation by chemical reaction of the metal with its environment, that is, corrosion. This includes, of course, metals that make up or are part of cultural heritage assets. While corrosion of metals in the industrial field can be, in many circumstances, expressed in economic terms, due to the economic losses caused by this process, due to the costs involved in the maintenance of metallic objects or their replacement, in the case of cultural heritage, every object is unique and therefore any loss is irreplaceable.
Corrosion is defined by the IUPAC as[1]:
“An irreversible interfacial reaction of a material (metal, ceramic, polymer) with its environment which results in consumption of the material or in dissolution into the material of a component of the environment. Often, but not necessarily, corrosion results in effects detrimental to the usage of the material considered. Exclusively physical or mechanical processes such as melting or evaporation, abrasion or mechanical fracture are not included in the term corrosion”.
ISO 8044 Standard defines it as [2]:
“Physicochemical interaction between a metal and its environment that results in changes in the properties of the metal, and which may lead to significant impairment of the function of the metal, the environment, or the technical system, of which these form a part”
The corrosion effects[1] in the case of artistic or historic artefacts can be seen as positive, for instance, producing a patina which is considered aesthetically pleasant. However, in most cases, it produces a damage. In the case of heritage artefacts, the impairment of the function produced by corrosion is related with the loss of some specific values (artistic, historic, scientific, social, etc.) of that object.
These definitions of corrosion give us some clues about different strategies that can be used to prevent or reduce corrosion. First of all, being a reaction of a material with its environment, the first choice could be to change either the material or the environment. While the first is not applicable to cultural heritage, since the physical nature of the object cannot be changed, the modification of the environment is probably the first choice: preventive conservation[2] strategies involve no action on the object itself, and are therefore preferable from the point of view of current conservation ethics. This strategy is more easily applied in indoor environments, such as museums, where the relative humidity and pollution can be controlled. For outdoor environments, this approach is more difficult to implement: atmospheric humidity cannot be controlled –although covering some artefacts to protect them from direct precipitation is sometimes feasible- and reducing pollution involves large scale actions, such as reducing the traffic around some monuments, usually with a limited impact. Even in indoor environments, in many cases it is not economically or practically feasible to act on the environment, but the interfacial character of the reaction, as pointed out the IUPAC’s definition, gives us the option for a different strategy: acting on the metal surface to avoid its contact with the environment or reduce the electrochemical reaction rates.
Many corrosion prevention treatments fall in this category, being the most usual organic coatings, such as paints and varnishes, were a polymeric material is applied on the metal; or coatings with inorganic materials, such as metals (usually nobler than the base metal) or ceramics (applied by sol-gel, PVD, CVD, etc.). Passivation by formation of a protective and homogeneous layer of corrosion products on the surface of the metal (either naturally or artificially) also produces the isolation of the metal from the environment. Many corrosion inhibitors can also be included in this category, since they form a protective layer (of molecular thickness) that avoids the reaction of the metal with the environment, as it will be shown later.
Some requirements should be considered when choosing a corrosion protection treatment for cultural heritage objects[3]: they should produce no or very little change in the surface appearance; should be as reversible as possible, that is, it should be possible to remove them and return the object to its original state; should not modify the material of the original artefact, including, in most cases, the modifications suffered during the history of the object, such as patinas or corrosion layers (as far as they do not threaten the object conservation and its legibility); they need to have long term efficiency, since heritage artefacts are intended to be preserved for a long time (as long as possible); and finally, it is desirable for them to have an easy maintenance, because any treatment will eventually need to be renewed.
Corrosion inhibitors fulfil to a large extent some of these requirements. Some of them reduce corrosion settling adsorbed layers of the inhibitor molecules on the surface of the metal. In most cases, the low thickness of the inhibitor protective layers makes them invisible (in other cases, however, the inhibitors produces visible changes). These layers are chemically stable in the environment in which they are formed. Due to their low thickness, they are not resistant to mechanical removal but if the inhibitor is present in the environment (and replenished when it is consumed), the layer will eventually be formed again. Another advantage of inhibitors is that they can be used in many cases –as it is common practice in metallic heritage conservation– in combination with protective coatings, increasing the protective function of the whole system.
Types and mechanisms of corrosion inhibitors
Corrosion inhibitors are defined by ISO 8044 as “a chemical substance that decreases the corrosion rate when present in the corrosion system at suitable concentration, without significantly changing the concentration of any other corrosion agent” [2]. As we will see later, the use of corrosion inhibitor for metallic heritage conservation is, in many cases, in the limits of this definition and closer to the coating or conversion coating ones, that is: “a substance layer that, on the metal surface, decreases corrosion rate”
Metals corrosion, specifically which affects cultural heritage, is in a vast majority of cases an electrochemical reaction, involving an anodic reaction, typically:
[1]
and a cathodic process,
[2]
[3]
The mechanism of inhibition involves the reduction of the anodic, the cathodic, or both reactions rates. Accordingly, a first typical classification of corrosion inhibitors is made in anodic inhibitors (for those inhibiting the anodic reaction), cathodic inhibitors (those inhibiting the cathodic reaction) or mixed type inhibitors (acting on both anodic and cathodic reactions). Depending on the type of inhibitor, the corrosion potential (Ecorr) of the system is modified in a positive (anodic inhibitor) or negative direction (cathodic inhibitor), or remains unaltered (mixed inhibitor).
Many other classifications can be found in the literature, attending to the chemical composition (organic, inorganic, surfactants…), the type of corrosive media in which they are effective (inhibitors for acid, neutral or alkaline solutions, for chloride-containing solutions, vapor phase inhibitors…), or the field of application (for cooling systems, for drinking water systems, for reinforced concrete or, as in our case, for cultural heritage).
Specific requirements and needs for corrosion inhibitors in conservation treatments
Some inhibitors have been and are currently being used extensively in conservation and restoration treatments. Under the European project PROMET, a survey was made amongst conservators-restorers of ten Mediterranean countries to determine the type of coatings and corrosion inhibitors used for conservation treatments of copper, iron and silver alloys [5]. The results showed that most of conservators used ethanol solutions of benzotriazole (BTA) for copper alloys, applied to the objects by brushing, immersion or spraying. For iron alloys, the use of corrosion inhibitors was not so popular, being tannic acid and BTA the preferred inhibitors. For silver, the use of inhibitors was scarce but again BTA was the selected product. A summary of corrosion inhibitors used for conservation-restoration treatments of different metals reviewed in this chapter is presented in Table 1.
As opposed to industrial applications, in the metal conservation field, the main way of using inhibitors is not adding the substance to the corrosive liquid media, since the majority of objects are exposed to atmospheric conditions. On the contrary, inhibitors are used to produce surface modifications or films by adsorption of the inhibitor on the metal surface, by means of the metal immersion on a non-corrosive inhibitor solution for a given time [6], followed by drying and, in many cases, a top layer with a varnish or wax coating [5]. This different way of use is significant for the researches on the use of inhibitors in the cultural heritage field. While in immersion tests the competitive adsorption of the ions of the solution, the water molecules and the inhibitor molecules have a key role in the inhibition process and its efficiency, in their application as films the physical and chemical resistance of the formed film is a key factor affecting the efficiency of the inhibitor. The different application methods might produce differences in the inhibition properties, as has been shown for instance by Mansfeld et al., who reported that the BTA was a good inhibitor for Cu immersed in 5% NaCl, but not when it was pre-coated by BTA and then exposed to the 5% NaCl solution [7]; or by Kosec et al, that showed that the inhibition properties of BTA and 1-(p-tolyl)-4-methyl imidazole were different when brushed onto patinated bronze and when the patinatedbronze was immersed in a solution containing the dissolved inhibitor[8].
Another key difference is that, while in basic and industrial-orientedresearches the inhibitors are applied to the clean metal, in heritage conservation they are applied in most cases over pre-existing corrosion products (or patinas) that have to be preserved. Therefore, the testing of these products for this application requires the use of a specific methodology adapted to the particular needs and conditions of their use. For instance, the PROMET project combined accelerated and electrochemical laboratory tests on artificially and naturally corroded coupons, simulating the condition of historic artefacts, with natural exposure tests in real conditions, both for coupons and real objects[9]. Most of the inhibitors’ studies are made on clean metal, but some recent papers have dedicated some attention to the recreation of surfaces similar to the ancient objects ones. Faltermeier, in 1999, pointed out that studies published on inhibitors did not dealt with heterogeneous corrosion layers and/or ternary alloys such as Cu-Zn-Pb commonly found in archaeological artefacts[10]. He proposed a standard methodology for testing inhibitors including the formation of a cupric chloride patina on the samples prior to inhibitor application, and the evaluation of the inhibitor efficiency using gravimetric methods. In recent years, many researchers have carried out studies of corrosion inhibitors using different types of patinas on bronze alloys, trying to simulate as close as possible the real conditions of inhibitors application in conservation-restoration treatments [9, 11-14]. Kosec et al. demonstrated the relevance of the patina composition in a recent paper, which showed significant differences in the inhibitor efficiency depending on the patina on which they are applied: they found out thatinvestigated inhibitors (BTA and 1-(p-tolyl)-4-methyl imidazol) inhibited the corrosion of both electrochemically formed and chloride-based patina, but were ineffective in the case of a nitrate-based patina[8].
Some papers have specifically studied the reaction of the inhibitors with the corrosion products. Brostoff studied the reaction of BTA with different Cu corrosion products and demonstrated that the presence of copper chloride have a great effect on the Cu-BTA reactions, predominating Cu(I)-BTA complexes in reactions with cuprite and copper powder, and Cu(II)-BTA complexes in reactions with chloride containing minerals (nantokite, atacamite and paratacamite)[15]. Rahmouni et al. studied the electrochemical behaviour of different natural and artificial patinas in presence of some inhibitors: BTA, amino-triazole (ATA) and bi-triazole, showing that the inhibitive properties of the compounds are diverse for each patinas[16]. The specific reaction of5-amino-2-mercapto-1,2,4-thiadiazole (AMT) with typical corrosion products in heritage artefacts, namely paratacamite, malachite and brochantite, has been recently studied by D’Ars et al., who concluded that AMT reacts with copper salts forming an AMT-Cu(II), and that brochantite suffers a partial alteration after its reaction with AMT [17].
In some cases, tests are carried out using real objects, to test them in actual-life conditions[9, 16, 18, 19]. The use of real objects have many disadvantages, mainly the low reproducibility, due to the reduced number of available samples and the huge variability in their composition, conditions, etc., but the historic materials behaviour could be in some cases very different to the modern ones. For instance, Bastidas and Otero demonstrated that the behaviour of copper from ancient chalcographic plates in acid cleaning baths with inhibitors was be very different to the modern copper samples, due to the presence of numerous inclusions in the ancient ones which can act as preferential sitesfor pitting[20, 21].
Inhibitors can also be used as vapour corrosion inhibitors (VCI), also known as vapour phase inhibitors (VPI). VCIs are substances with a low vapour pressure that have the ability to vaporise and condense on the metal surface, forming an adsorbed layer that protects the metal from the corrosive environment [22]. This makes them suitable for metal protection in enclosed spaces, such as display cases or packages, and the use of VCI for protection of metallic heritage has been proposed in some cases [23, 24].The need of closed spaces and, especially, the possible health hazards for the conservation professionals or visitors of the museum make conservators reluctant to use this kind of inhibitors[25], even though some recent works claim the safety of VCI in packaging materials [26].
inhibitors EVALUATION
The parameter commonly used to quantify the corrosion inhibition properties of a substance is the inhibitor efficiency (IE), defined as:
[4]
whereCRabs and CRpre are the metal corrosion rate in the absence and presence of inhibitor, respectively. Corrosion rates can be obtained by different ways, being the most used gravimetric and electrochemical techniques. Gravimetric measurements are usually carried out using coupons and measuring the weight before and after exposure to the corrosive environment, without the inhibitor and with the inhibitor added to the solution or the metal pre-treated. Electrochemical techniques, such as polarization resistance, calculation of Tafel slopes from voltametries and electrochemical impedance spectroscopy (EIS), allow for an indirect calculation of corrosion rates and have the advantage of providing information on the mechanisms of the corrosion and inhibition processes [27, 28].
It should be noticed that, in the case of heritage artefacts, the weight measurements (as a direct measure of the chemical reaction rate) might not reflect the damage suffered by the object, which is far more complex and it is related with the notion of “loss of value” (that could be aesthetic, symbolic, historic, socioeconomic, scientific, technologic, etc.) [4]. In some cases, an small corrosion effect might imply a significant loss of value, e.g. in the case of silver tarnishing; while in others, a stronger corrosion effect might be considered acceptable, such as in the formation of a patina in an outdoor sculpture. Some attempts have been made to quantify this loss of value in the case of damage caused by pollution, using concepts such as “non observed adverse effect level” (NOAEL) or “lowest observed effect level” (LOAEL) [29]. For this reason and due to the indefinite life expectancy of a heritage object, it is not feasible to establish a target efficiency value for a corrosion inhibitor for this application, which should, in principle, be “as high as possible”.
Since the adsorption of the inhibitor molecules on the metal surface is a fundamental step in the inhibition process, the study of the adsorption process can also provide useful information of the inhibition mechanisms. The study of the adsorption isotherms is a classical method for studying this process and they have the general form:
[5]
were k is the equilibrium binding constant of the adsorption reaction; c is the inhibitor concentration; g(,) is the configurational term parameter, in which is the number of water molecules replaced by one molecule of organic inhibitor and is the degree of coverage of the metallic surface; and f is the interaction term parameter (f> 0 lateral attraction, and f< 0 lateral repulsion between the adsorbed inhibitor molecules) [30-35]. These models assume that: (i) the adsorption sites on the metal surface are homogeneous, (ii) a mono-layerinhibitor adsorption is formed, and (iii) corrosionis uniform and no localised attack takes place[30], which is not always the case, especially in heritage objects. They also consider a thermodynamic equilibrium between the inhibitors in the environment and the adsorbed layer, thus in those cases where the concentration of the inhibitor changes or they are applied in solvents and then exposed to a different environment, these models are not useful.
The use of quantum chemical calculations for the evaluation of inhibition properties is not a new tool, but has gained a huge popularity in the last years due to the improvement of the calculation capabilities of personal computers. A recent review of the use of this techniques has been recently published by Gece[36]. This calculations allow to correlate inhibitor efficiencies with molecular properties such as orbital energies (mainly highest occupied molecular orbital energy, EHOMO, and lowest unoccupied molecular orbital energy, ELUMO), dipole moment, charge density, heat of formation and ionization potential [37]. Quantum chemistry calculations can be very helpful to study fundamental inhibition mechanisms and have shown a good correlation with experimental data in some cases, for simple corrosion systems. However, in others, the correlation is not so clear, since the assumptions and simplifications needed to allow the computing of the models might neglect important factors in the corrosion inhibition process [36]. This is especially true in the case of very complex systems such as metallic heritage artefacts, which typically have inhomogeneous surfaces, usually covered by corrosion products.