Corrosion inhibitors for the preservation of metallic ...



Corrosion inhibitors for the preservation of metallic heritage artefacts E. Cano, D. LafuenteCentro Nacional de Investigaciones Metalúrgicas (CENIM)-Consejo Superior de Investigaciones Científicas (CSIC)Avda. Gregorio del Amo 8, 28040 Madridecano@cenim.csic.esPublicado originalmente en: EFC book n?65 “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. 570-594ISBN 978-1-78242-154-2(Print) 978-1-78242-157-3 (Online)D.O.I. 10.1533/9781782421573IntroductionWith 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 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 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: 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 inhibitorsCorrosion 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: QUOTE [1]and a cathodic process, QUOTE [2] QUOTE [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 treatmentsSome 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 EVALUATIONThe parameter commonly used to quantify the corrosion inhibition properties of a substance is the inhibitor efficiency (IE), defined as: QUOTE [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: QUOTE [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. Surface analysis techniques have also been extensively used for the characterization of the inhibitor layers formed on the metals [38-43]. The use of this techniques allows the study of the layer composition formed on metals following the procedures used by conservators-restorers, and the study after exposure of the coated metals to the atmospheric environment, closer to the real life of the objectsthan the immersion tests necessary for electrochemical measurements [42]. The main disadvantage is that sometimes the efficiencies observed by electrochemical or gravimetric measurements are difficult to correlate with the surface characterization results [44]. For readers interested in further details on evaluation methods for corrosion inhibitors, a comprehensive review can be found in the book by V.S. Sastri “Corosion Inhibitors. Principles and Applications” [45] or the recent one by the same author on “Green Corrosion Inhibitors: Theory and Practice” [46].corrosion inhibitors USED in conservation treatmentsInhibitors for Copper and its alloysBTA is by far the most used and most studied corrosion inhibitor for copper and its alloys, both for industrial applications and for heritage conservation uses. Dugdale and Cotton’s pioneering work in 1963 reported that BTA was at that time already in use for industrial applications since many years, but this is considered to be the first scientific study of its inhibition mechanism [47]. This first work demonstrated that BTA forms a polymeric complex with copper that acts as a barrier for corrosion, and that this film is very thin and chemically resistant. It was also proposed that BTA protects copper in aqueous and gaseous environments polluted with sulphur dioxide, hydrogen sulphide and salt mist [24]. Cotton and Scholes also suggested different forms for BTA application, such as its addition to an aqueous solution, its incorporation into lacquers and polishes, or its use as VCI by exposing the metal to benzotriazole-impregnated paper [24].Scientific literature regarding the use, applications and mechanisms of BTA as a copper corrosion inhibitor is huge, in all types of corrosive solutions: acid, neutral and alkaline, oxidizing or reducing, containing different ions (chlorides, sulphates, ammonia…), etc. BTA is known to be chemically adsorbed on the metal, displacing the adsorbed water and forming different complexes with Cu(I) and Cu(II) [30, 32, 34, 42]. The composition, structure and orientation of the adsorbed layer have also been matter of different studies, but a complete agreement has not been reached. The review of all this topics is out of the scope of this text, but interested readers can find a detailed review on a recent publication by Fin?garandMilo?ev[43].BTA first application for heritage conservation was contemporary to Dugdale and Cotton’s research. Madsen [48] proposed BTA as a treatment for bronze disease in 1967, and suggested its application by objects immersion in a 3% BTA solution in ethanol in a vacuum chamber at room temperature; or, if the vacuum was not available, in a 3% water solution at 60? C. Ethanol solution was considered to be better because the low surface tension of the solution allows it to reach deep pores in the corrosion layer. BTA is also less soluble in water and treatment with the water solution might also produce an undesirable white deposit on the surface of the patina [49]. The current standard practise in conservation follows the original recommendation by Madsen, most of the times without vacuum, and for immersion times of up to 24 h [50].Madsen also suggested using BTA treatments as a first layer, with an additional coating with Incralac -an acrylic based varnish containing BTA itself-. Even though BTA was originally included in the Incralac formulation as an UV stabilizer, not as a corrosion inhibitor [50], Madsen suggested that it might migrate to the metal surface and contribute to its inhibition.BTA became a very popular treatment for antiquities in the following years. Sease, in a review made in 1978about the BTA use for antiquities, recognized this treatment as a popular one, and pointed out that the exact mechanism of inhibition was not completely understood, since the formation of a polymeric layer on the copper surface would not satisfactorily explain the inhibition mechanism in heavily corroded objects such as antiquities [51]. She also raised some concerns about its efficiency to arrest bronze disease, since the cupric chloride-BTA complex layer formed upon treatment would only be superficial and therefore subject to eventual disruption and reactivation of the corrosion process [51]. Indeed, reactivation of bronze disease after BTA treatments has been reported in many occasions [52]. The low pH present in pits in active bronze disease might be responsible for the lower efficiency of inhibition in these cases [53], since at low pH the adsorption of molecules, rather than complex formation, is favoured [43].The efficiency of BTA for treatment of copper alloys seems to be lower than for pure copper. Using electrochemical techniques, Brunoro et al. demonstrated that the inhibition of some BTA derivatives was lower in complex multiphasic bronze alloys, which was attributed to the alloying elements (Sn, Zn and Pb), forming a weaker metal–triazole bond[54]. However, results obtained by Sharma et al. using leaded bronzes treated with a neutral BTA aqueous solution, showed the formation of a lead complex film, so this treatment was proposed for leaded bronzes[55]. More recent studies by Galtayries et al, have tried to elucidate these differences using surface analysis techniques. Their results, however, did not demonstrate a straightforward relationship between the inhibitor efficiency and the amount of adsorbed inhibitor. The discrepancies found with Sharma’s work were attributed to differences in both the concentrations of the inhibitor used and the samples pre-treatments [44].Quite early in its application some concerns were raised about the toxicity of the BTA. Oddy and Sease recommended some cautions in its use [51, 56]. The environmental and health hazards of this compound is still a controversial issue: while some authors claim that it is carcinogenic and causes toxic effects on flora and fauna [6], others classify it only as slightly toxic [43]. Some recent toxicologic studies reported BTA as having a low acute toxicity, and non presenting evidence of antiestrogenic activity in the in vivo assays [57]. However, suspects about its safety remain and the search for “safe” alternatives to BTA has been a constant in corrosion inhibitor studies in the last 30 years, both in corrosion science in general and in the metal conservation literature. Since the first BTA researches, alternatives to this molecule have been sought. Cotton and Schonlesstudied other triazoles: indazole, benzimidazole, indole and methyl-benzotriazole, and found that just indazoleinduced resistance to tarnishing on copper strips exposed to salt spray, but the layer was less resistant to organic solvents than BTA [24]. Ganorkar et al. proposed the use of AMT as a complexing agent which is able to remove cuprous chloride from corroded bronzes and forming a polymeric layer on the surface capable of inhibiting the corrosion process [58].Faltermeier studied several nitrogen based and sulphur based alternatives: BTA and AMT, 2-aminopyrimidine, 5,6-dimethylbenzimidazole, 2-mercaptobenzimidadole, 2-mercaptobenzoxazole, 2-mercaptobenzothiazole and 2-mercaptopyrimidine.Nevertheless, none of them yielded a better efficiency than BTA. Besides, some of them caused unacceptable colour changes on the coupons surface,and thereforethe conclusion of his work was that none of them could be recommended for conservation treatments of chloride containing archaeological artefacts[59]. AMT and BTA were also compared as inhibitors for their use in acid cleaning of historic chalcographic plates, resulting in a better efficiency of AMT, especially in citric acid [20, 21].Balbo et al. also tested some thiadiazole and imidazole derivatives and 3-mercaptopropyl-trimetoxysilane (PropS-SH) as alternatives to BTA for inhibition of cast bronze exposed to concentrated acid rain and in 3.5?wt.%NaCl solution. The best results were yielded by AMT and especially by PropS-SHwhen enough curing times were used[6]. As an alternative to BTA, Thachli et al. proposed the use of electropolimerizedATA for copper protection, reporting an efficiency of 99% and remaining efficient even after one month of immersion in 0.5 M NaCl solution [60]. These tests were made on pure clean copper, but Rahnouni and co-workers tested the same compound and bi-triazole in artificial patinas -simulating ancient ones- and real coins. In this work, ATA yielded good results, but not as good as BTA [16]. Other researchers have also reached good results with other azoles: 4-methyl-1-(p-tolyl)-imidazole (TMI), 1-phenyl 4-methyl-imidazole (PMI), 2-mercapto 5-R-acetylamino-1,3,4-thiadiazole (MAcT), 2-mercapto 5-R-amino-1,3,4-thiadiazole (MAT), were studied by Muresan et al. for their use on the inhibition ofartificially patinated bronze. Again, results showed that TMI and MAcT were efficient inhibitors but with a lower performance than BTA [12]. TMI has also been studied by Maru?i? et al. on three different artificial patinas on bronze [11].Brunoro and co-workers tested 5-methyl-1,2,3-benzotriazole; 5-hexyl-1,2,3-benzotriazole; 5-octyl-1,2,3-benzotriazole; 5-methoxy-1,2,3-benzotriazole; 5-(piridinethoxycarbonyl)-1,2,3-benzotriazole chloride; 2-chloroethyl-1,2,3-benzotriazol-5-carboxylate; and 5-mercapto-1-phenyltetrazole as inhibitors for different cast bronzes, containing Sn, Zn and Pb[54].Dermaj et al. proposed the use of 3-phenyl-1,2,4-triazole-5-thione (PTS) as inhibitor for bronzes [61, 62]. This compound has also been tested on pre-corroded bronze artifacts, simulating archaeological ones, with promising results[18]; and on real objects, providing a good protection except in Ag containing objects, in which the sulphur in the inhibitor caused a darkening on theobjects surface [9].A very active area of research in corrosion inhibitors in last years has been the use of the so-called “green inhibitors”, i.e., inhibitors obtained from natural plant extracts [63]. These products present the advantage of their biodegradability, easy availability and non-toxic nature, but also disadvantages such as the very complex nature and high variability of the extracts (depending on the exact origin of the plants). Their application to cultural heritage is, so far, quite limited, even though efforts are being made to use this kind of products as substitutes for unhealthy compounds. Hammouch et al. have done some tests of Opuntiaficusindica extract (OTH) for bronze and iron based artefacts[64], but in the case of bronze no long-term or real object tests have been made [9].Great attention has been given in the last years to the application of saturated linear carboxylic acids and their sodium salts to the protection of heritage metals. Sodium heptanoate (CH3(CH2)5COONa) was first tested as inhibitor for copper in immersion, showing a good inhibition of the corrosion attributable to the formation of a copper heptanoate layer [65, 66]. Tests in real objects using sodium heptanoate have also given satisfactory results [67]. Sodium decanoate and other carboxylation treatments (a solution containing the carboxylic acid and an oxidant, either sodium perborate or hydrogen peroxide) have also been studied [14, 68]. While the carboxylation solutions formed a thicker layer when applied on copper [14], they were not suitable for application on brass since they caused the appearance of white stains on the surface of the metal[68]. Alternative methods for the layer deposition have been evaluated with different results: Elia et al. tested the deposition fromethanolic solutions of heptanoic, decanoic and docecanoic acids, in an attempt to improve the resistance of the coating and to avoid the use of water (as authors consider that it can promote corrosion), but the results have not been satisfactory[69]; on the other side, the deposition of the same compounds using cyclic voltammetry showed promising results, saving treatment time and allowing a good control of the deposited layer [70].Since inhibition treatments are not completely effective in some cases, even with BTA which is usually the best inhibitor, a very interesting approach is to take advantage of the synergetic effects of the combination of various inhibitors. Golfomitsouet al. have carried out a very interesting research on the topic, evaluating the efficiency and mechanisms of the combination of BTA with other inhibitors and additives: AMT, 1-phenyl-5-mercapto-tetrazole, ethanolamine, benzylamine, potassium ethyl xantate and potassium iodide [53, 71, 72]. She found that the combination of AMT 0.01M and BTA 0.1 M in distilled water increased the efficiency of the treatment, with the advantageof requiring a lower concentration of the inhibitor. The lower concentrations of the inhibitors and the use of water as solvent also result –according to the author– in both a safer and cheaper application of the treatment, and less alteration of the visual aspect of the objects. On the other side, it was found that some combinations were ineffective, in some cases even increasing the corrosion rate[53].Inhibitors for Iron and its alloysWhile inhibitors for iron and steel are widely used in industrial applications, their use for conservation purposes is not so widely spread as in the case of copper alloys. Interestingly, references to their use in the conservation of cultural heritage are earlier than for copper. Plenderleith, in his book “The Conservation of Antiquities and Works of Art: Treatment, Repair and Restoration” mentioned the use of commercial rust cleaning products with the additional advantage of working as corrosion inhibitors[73]. Evidences have been found of the use of products including chromate based inhibitors for iron conservation at the beginning of the XX century [25].The most popular corrosion inhibitors for conservation of iron and steel heritage, nowadays, are tannins [5, 67]. The name tannin is applied to a wide group of polyphenolic compounds extracted from leaves, bark or fruits from different plants, with an exact composition depending on their origin. The main advantages of tannins as corrosion inhibitors are that, being natural extract from plants, they are non-toxic, inexpensive and can be applied on pre-rusted metal, without the need of a cleanun-corroded surface [74]. Tannins protect the metal by formation of a conversion layer of iron (III) tannate complex which protects the metal, but produces a significant and, in many cases, unacceptable change in the colour of the rust from reddish to blue-black [74, 75]. While in some cases very good results are reported[76] the efficiency attained in other cases is not very high [74] and they can even act as corrosion accelerators for bare steel in neutral solutions [77].As it has already been mentioned, the application of OTH has also been studied for protection of historic steel exposed to atmospheric conditions, obtaining good results with pre-corroded coupons, especially when the formulation was applied by brushing [64, 78]. PTS has also been tested for this application, showing also promising results in tests using artificially corroded coupons [79]. However, the behaviour of these protection systems in long-term and real object tests was not so good, and PTS and OTH provided less protection than other protection systems studied, being only recommendable as short term protection [9, 80]. Long-chain carboxylic acids and sodium carboxylate solutions have also been applied to protect iron and steel for conservation purposes. Sodium decanoate provided a slightly better protection than sodium heptanoate when applied on iron [67]. In the case of iron, the carboxylation layer formed by immersion in sodium decanoate is very thin -nanometric scale-, but even so it provides better protection than other traditional inhibitors used for iron such as tannins or phosphates[81]. When the carboxylation solutions (composed of decanoic acid with the addition of an oxidant) are used, thicker layers can be obtained[14, 82], but their performance is controversial: while Hollner et al. have obtained better results than with sodium decanoate[82, 83], Rapp et al. obtained poor results [68].As opposed to the main use of inhibitors as final protection layer for copper based artefacts, inhibitors for iron are used in many other steps of the conservation process. One of these applications is their use to arrest iron corrosion in desalinization processes of archaeological iron artefacts. Amines are traditional inhibitors for iron [33, 84], and some of this compounds have also been used for these purposes. Immersion of iron objects in a 5% ethylenediamine (EN) solution has been used as part of the stabilization treatment for archaeological iron. While effective in many cases, it was proven to stimulate corrosion in some other cases due to its ability to form soluble iron (II) complexes, what summed to its toxicity turned out to reduce its applicability [85, 86].A major problem for conservation of waterlogged iron-wood composite objects is the iron corrosion in the traditional impregnation treatment for wood using polyethylene glycol (PEG) solutions [25]. To prevent such problem, some commercial corrosion inhibitors have been proposed. Hostacor KS1 (a triethanolamine salt of an arylsulphonamido carboxylic acid) has been studied as an additive to the PEG 400 solution to supress the corrosion of iron [87, 88]. It works by reacting with the dissolved oxygen of the solution and forming a passivating layer on the iron surface [89]. Bobichon et al. demonstrated that in 20% PEG 400 solutions it was more efficient than triethanolamine[90]. The application of this product faced one of the main challenges of using commercial products: its production was discontinued and was substituted by Hostacor IT (a triethanolamine salt of an acrylamido carboxylic acid). Consequently, new studies were made to assess the applicability of the new product to the iron corrosion inhibition in PEG solutions, both in laboratory tests using electrochemical techniques [89] and in real objects treatments [91], showing that Hostacor IT is effective in slowing down the corrosion of exposed iron in PEG solutions. Phosphates have also been tested as inhibitors for PEG solutions:Gourbeyre et al. studied the layers formed on iron samples exposed to a 20% PEG solution with Na2HPO4 added as a corrosion inhibitor and found that, for concentrations below 5 × 10–3 M, the layer was composed of iron oxides and phosphates, while for higher concentrations corrosion was inhibited by a layer of PEG and phosphates resulting from segregation of the former one [92]. A subsequent paper demonstrated that phosphates acted as an anodic inhibitor, since the PEG/phosphate complex was preferentially deposited on metal’s anodic sites, inhibiting the iron dissolution[93].Inhibitors have also been tested as additives to traditional and innovative coatings for iron protection. Under the PROMET project, several commercial corrosion inhibitors were tested as additives for traditional coatings (Paraloid B-72 and Rennaissance wax) and innovative coatings (Poligen ES91009)[9]. These inhibitors were already in use for protecting industrial objects but had never been tested in the conservation field. They were tested on clean and pre-corroded steel analogues, using accelerated ageing in climate chambers[94],electrochemical tests (Rp and EIS) [95], as well as in real objects [96]. Results showed no clear improvement in the coating’s behaviour with the corrosion inhibitor additives and, in some cases, the effect was clearly deleterious. The reasons for the failure of these inhibitors were not investigated, but authors considered that might be related with the interaction of the additives with the coating itself, affecting the crosslinking of the polymer or the wettingproperties of the binder emulsion with respect to the substrate, hence impairing the barrier effect of the coating. Industrial heritage conservation has a significant space for the application of corrosion inhibitors. Most of this heritage includes a significant part made of iron or steel, and conservators face unusual problems that can, in some cases, be solved by the application of corrosion inhibitors. The requirements of industrial heritage are not the same of the industrial machines: even when kept in operating conditions, these artefacts are only occasionally operated, so the corrosion problems might be different. One example is the conservation of collections of vehicles, in which corrosion of the hydraulic braking systems can be a problem. While in operating vehicles the main reason for decay of the braking fluid –and loss of its anticorrosion properties– is thermal cycling, in these infrequently used systems the degradation is mainly attributed to water uptake by hygroscopic braking fluids. Hedditch et al. studied the inhibition efficiency of different commercial hydraulic braking fluids and the effect of the addition of dodecanedioic acid, finding a good performance in different fluids up to 5% water content[97]. Commercial tannate based inhibitors were also tested for the conservation of a boiler from an operational paddle steamer, using gravimetric and electrochemical techniques, and it was showed that the addition of these inhibitors would reduce corrosion to negligible rates even under the boat’s operation [76]. In this case, it was important to evaluate the corrosion under the exact operation conditions, such as the specific water used for the steam system.Other metalsThe use of corrosion inhibitors for silver conservation treatments is by far less usual than for copper or iron. Some compounds such as morpholine, BTA, chlorophyl, pyridine or cysteamine can be found in the literature as corrosion inhibitor for silver, [98, 99]but they have not been studied in detail and their application is not widespread in the conservation community .Notoyaet al. studied the structure of the polymeric layer formed on Cu, Ag, Au, Cr, Ni, Fe and Zn using time-of-flight secondary ion mass spectroscopy (ToF SIMS) of BTA-pretreated metals [100]. They found that the corrosion inhibition in a NaCl solution was closely related with the degree of polymerization, following the order Cu>>Ag>>Zn>Ni, Fe. Comparing these results with the survey made by Argyropoulos[5], it seems that the use of BTA for silver might make any sense, but not for iron. Self-assembled monolayers (SAMs) have in recent years attracted some attention as corrosion protection systems, and some studies have been made on their application on silver objects with conservation purposes. Burleigh et al. described the procedure to produce a SAM of alkanethioles on silver by immersion, applicable to coins or jewellery [101]. They found tetradecanethiol (C14) and hexadecanethiol to be the most effective in preventing silver tarnishing. Evesque et al. studied the corrosion protection using electrochemical techniques and electrochemical quartz crystal microbalance (EQCM), concluding that the protective film was composed of an inner self-assembled layer of one or two monolayers, plus an outer one of about 10 monolayers [102]. Bernard et al. compared an electrodeposited film of poly(amino-triazole) and a SAM of hexadecane-thiol, and found the later to be more effective in preventing silver tarnishing exposed to a sulphur containing solution[103]. Recently, Liang et al. have tested octadecanethiol from aqueous solution (OSA), octadecanethiol from organic solvent (OSO), phytic acid (IP6) and silicon tungstemic acid (STA) monolayers for protection of silver coins, and found that OSA was the most effective treatment, which is especially interesting for conservation treatments since it avoids the use of organic solvents[104].The use of inhibitors for lead is even scarcer, since lead is usually considered to be resistant to atmospheric corrosion. However, when exposed to acetic or formic acid vapours it undergoes a catastrophic corrosion process [105, 106]. These pollutants can reach very high concentrations in display cases and storage boxes in museums and inside organs pipes; for that reason, lead protection treatments (including inhibitors) had focused on acetic acid environments. The use of BTA as corrosion inhibitor for lead was studied by Sharmaet al. [107]. BTA has been reported to form a Pb-BTA complex, creating a layered structure Pb-BTA/PbO/Pb capable of preventing lead corrosion, so it was proposed as an inhibitor for lead objects and leaded bronzes. The proposed treatment involved immersion or brushing with an aqueous solution of BTA neutralized with 1 g of pure calcium carbonate to obtain a pH ~ 7. Sankarapapavinasam et al. studied the inhibitive properties of hydrazine (Hy) and substituted hydrazines (phenyl hydrazine (PHy), 2,4dinitrophenyl hydrazine (2,4-DNPHy), 4-nitrobenzoyl hydrazine (4-NBHy), and tosyl hydrazine (THy)) in acetic acid solutions. THy and 4-NBHy were found to be the most efficient inhibitors, and their mechanism of inhibition was explained by the blockage of anodic dissolution sites [108].The most studied type of lead inhibitor in acetic acid environments has been the group of carboxylates. A first paper by Rocca and Steinmetz studied carboxylates with different carbon chain length (from C7 to C11) and found the best inhibition performance for long-chain molecules, being attributable the protection to a metallic soap formation on the lead surface [109]. Sodium decanoate was selected for the best compromise between protection and solubility, and it was tested in atmospheric conditions with presence of acetic acid vapours, yielding good results, as well as in real heritage objects [67, 110]. The most comprehensive study of corrosion inhibitors for lead exposed to acetic acid environments was made under the COLLAPSE project, applying these protection systems to lead alloy organ pipes [111]. Sodium dodecanoate and undecanoate were tested and compared with thiourea, phosphatising and sulphatising treatments, exhibiting the best efficiency of all tested treatments [112]. Carboxylates also performed very well in comparison with Paraloid B72 and microcrystalline wax, but none of the protective treatments was completely effective in the long term; therefore, these surface protection systems were not recommended for the treatment of organ pipes, and only the change in the environment (reducing the concentration of acetic acid vapours) was considered to be a good system[111, 113].Dowsett et al. studied the formation of lead decanoate layers using in-situ spectroelectrochemical techniques[28]. They showed that there were significant differences in the formation and protective properties of decanoate layers that strongly depend on the solution’s preparation conditions, and that solution’s pH alone is not a good indicator for the preparation of good quality layers. Electrochemical deposition of lead dodecanoate has also been explored by De Wael et al., having the advantage of a shorter treatment time [114]. A later work by same authors have demonstrated that immersion treatments produce a better quality layer than electrochemically assisted ones, especially if an electrochemical pre-reduction conditioning treatment is applied. [115]Regarding zinc (and also brass), the benefits of BTA as an inhibitor have been demonstrated as well [116, 117]. Carboxylation treatments have also been tested on it, giving very good results and turning out the longer the C- chain length, the better corrosion resistant [118], as well as the application of PEG, in alkaline media, where PEG400 had fair effective performance but the research on this field is still developing[119]. ConclusionsCorrosion inhibitors are one of the different methods that conservation-restoration professionals have available to protect and prolong the life of metallic cultural heritage. The scientific literature on corrosion inhibitors is huge, but the vast majority of it deals with fundamental studies of corrosion inhibition or industrial applications. Protection of heritage metals has specific needs and requirements, therefore, scientific studies on corrosion inhibitors for this application should address and follow these specificities. Since the first papers dealing with the BTA application for copper in the late sixties of last century, the number and quality of scientific studies on the application of corrosion inhibitors for metallic heritage conservation have been increasing, especially in the last 15 years. Inhibitors for copper and their alloys, mainly bronze, have attracted most of the attention; on the other hand, those for iron, silver and lead have been less studied.While many alternatives have been sought, BTA is probably still the best inhibitor for copper and its alloys. The combination of different inhibitors, in order to take advantage of their synergetic effects seems a promising way to increase the protection while reducing the dosage of inhibitors and, therefore, the possible health or environmental risks. Most studies have pursued not only better inhibitors, but also safer (for the people and environment) and easier to apply ones. In this respect, the use of natural plant extracts as corrosion inhibitors is a very up-to-date trend. They are easily available and are usually innocuous, but they have a complex nature, which added to the complexity of the heritage metals, make in many cases difficult to understand the mechanisms of protection or the reasons for their failure.SAMs are probably the state-of-the-art system for silver, showing a good protection against tarnishing in sulphur-containing environments, but they are not easy to apply so their applicability in real life conservation practice is not simple. 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Biallozor: Corrosion Science, 2001, 43, (7), 1309-1319.TableTable 1: Summary of inhibitors used for conservation-restoration treatments of different metals.InhibitorMetal/alloyReferencesBenzotriazole (BTA)Cu[5, 7-10, 15, 19-21, 24, 30, 32, 34, 38-43, 47, 49-51, 53, 67, 72, 100]Ag[5, 9, 99, 100]Fe[5, 9, 100]Bronze[6, 9, 16, 44, 48, 50, 52, 54, 55]Lead[55]Zn[100, 117]2-aminopyrimidineCu[10]2-amino-5-mercapto-1,3,4-thiadiazole (AMT)Cu[10, 19-21, 50, 53, 58, 67, 72]Bronze[17, 50, 52]Benzylamine (BZA)Cu[53, 72]Ethanolamine (ETH)Cu[53, 72]1-Phenyl-5-Mercapto-Tetrazole (PMT)Cu[53, 72]Potassium Ethyl Zanthate (KEX)Cu[53, 72]Potassium Iodide (KI)Cu[53, 72]5,6-dimethylbenzimidazole (DB)Cu[10, 67]2-mercaptobenzimidazole (MBI)Cu[10, 67]2-mercaptobenzoxazole (MBO)Cu[10, 67]2-mercaptopyrimidine (MP)Cu[10, 67]2-mercaptobenzothiazole (MBTS)Cu[10, 19, 67]4-methyl-1-(p-tolyl) imidazole (TMI)Bronze[11, 12]1-phenyl 4-methyl-imidazole (PMI)Bronze[12, 13]2-mercapto 5-R-acetylamino-1,3,4-thiadiazole (MAcT)Bronze[12]2-mercapto 5-R-amino-1,3,4-thiadiazole (MAT)Bronze[12]1-(p-tolyl)-4-methylimidazoleBronze[8, 13]TributylamineSteel[84]Ethylenediamine (EN)Fe[85, 86]HexylamineSteel[33]Triphenylmethane derivativesCu[31, 35]DodecylamineSteel[33]CarboxylatesFe[14, 25, 67, 68, 82]Cu[14, 65-70, 82]Zn[67, 118]Lead[28, 67, 109-115]3-phenyl 1,2,4-triazole-5 thione (PTS)Bronze[18, 61, 62]Steel[79]Amino-triazole (ATA)Bronze[16]Poly-amino 1,2,4-triazole (pATA)Cu[60]Bi-triazole (BiTA)Bronze[16]VCI/VPICu[22]Bronze[23]Ag[67]Fe[22, 25]Zn[22]TanninsFe[5, 67, 75, 76]Steel[74, 77]Chromate basedFe[25]ThioureaFe[25]SiderophoresFe[25]PEGFe[87, 89, 90, 92, 93]Zn[119]Hostacor KS1(triethanolamine salt of an arylsulphonamido carboxylic acid)Fe[87, 89, 90]Hostacor IT(triethanolamine salt of an acrylamido carboxylic acid)Fe[89, 91]PhosphatesFe[25, 81, 92, 93]MorpholineAg[99]ChlorophylAg[98]PyridineAg[98]CysteamineAg[98]Opuntiaficusindica (OTH) extractBronze[64, 80]Fe[64, 78, 80, 81]Self-assembled monolayers (SAMs)Ag[101-104]Hydrazine (Hy)Pb[108]Phenyl hydrazine (PHy)Pb[108]2,4 dinitrophenyl hydrazine (2,4-DNPHy)Pb[108]4-nitrobenzoyl hydrazine (4-NBHy)Pb[108]Tosyl hydrazine (THy)Pb[108] ................
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