corrosion inhibitors for the preservation of metallic...
TRANSCRIPT
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 effects1 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 conservation2 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
1 See ISO 8044 [2] ISO 8044:1999, 'Corrosion of metals and alloys -- Basic terms and definitions'.for the definition of “corrosion effect” and “corrosion damage” .2Preventive conservation is defined as “all measures and actions aimed at avoiding and minimizingfuture deterioration or loss. They are carried out within the context or on the surroundingsof an item, but more often a group of items, whatever their age and condition. Thesemeasures and actions are indirect – they do not interfere with the materials and structuresof the items. They do not modify their appearance.” [3]
ICOM-CC: 'Terminology to characterize the conservation of tangible cultural heritage', Resolution adopted by the ICOM-CC membership at the 15th Triennial Conference, New Delhi, 2008.
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 objects3: 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 3 For terms and definitions related with conservation of cultural heritage, refer to EN 15898 Standard. [4]
EN 15898:2011, 'Conservation of cultural property. Main general terms and definitions'.
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.
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 TREATMENTS
Inhibitors for Copper and its alloys
BTA 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 FinšgarandMilošev[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 60º 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 Marušić 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 alloys
While 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 inhibitors4
were tested as additives for traditional coatings (Paraloid B-72 and Rennaissance wax)
and innovative coatings (Poligen ES910095)[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
4 The additives were commercial products by Cortec Corporation: M435 (a blend of triazoles), M370 (ammonium salt of tricarboxylic acid) and M109 (calcium sulphonate); and by Dow Chemical: Alkaterge-T (bis-oxazoline). 5Poligen ES91009 is a synthetic polyethylene wax manufactured by BASF presented as a dispersion in water (ready-to-use).
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 metals
The 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 project6, 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
6 COLLAPSE “Corrosion of Lead and Lead-Tin Alloys of Organ Pipes in Europe” EC 5 FP: Energy, Environment and Sustainable Development. EVK4-CT-2002-00088
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].
CONCLUSIONS
Corrosion 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.
Finally, it is worth noting the great success obtained with carboxylates treatments for
different metals (copper, iron and mainly lead), which also fulfil the reversibility
requirements of modern conservation-restoration ethics. Nevertheless, there is still a
vast field to research on this topic in the future.
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TABLE
Table 1: Summary of inhibitors used for conservation-restoration treatments of different
metals.
Inhibitor Metal/alloy References
Benzotriazole (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-aminopyrimidine Cu [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-methylimidazole Bronze [8, 13]
Tributylamine Steel [84]
Ethylenediamine (EN) Fe [85, 86]
Hexylamine Steel [33]
Triphenylmethane derivatives Cu [31, 35]
Dodecylamine Steel [33]
Carboxylates
Fe [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/VPI
Cu [22]
Bronze [23]
Ag [67]
Fe [22, 25]
Zn [22]
TanninsFe [5, 67, 75, 76]
Steel [74, 77]
Chromate based Fe [25]
Thiourea Fe [25]
Siderophores Fe [25]
PEGFe [87, 89, 90, 92, 93]
Zn [119]
Hostacor KS1 Fe [87, 89, 90]
(triethanolamine salt of an
arylsulphonamido carboxylic acid)
Hostacor IT
(triethanolamine salt of an
acrylamido carboxylic acid)
Fe [89, 91]
Phosphates Fe [25, 81, 92, 93]
Morpholine Ag [99]
Chlorophyl Ag [98]
Pyridine Ag [98]
Cysteamine Ag [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]