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Evaluation of the anticorrosive properties of benzotriazole alkyl derivatives on 6% Sn bronze alloy
Surface & Coatings Technology 204 (2010) 2442–2446
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Evaluation of the anticorrosive properties of benzotriazole alkyl derivatives on 6% Sn bronze alloy G. Laguzzi ⁎, L. Luvidi Istituto di Metodologie Chimiche del CNR, Area della Ricerca di Roma 1, Via Salaria Km 29, 300 - 00016 Monterotondo Stazione (Roma), Italy
a r t i c l e
i n f o
Article history: Received 22 September 2009 Accepted in revised form 17 January 2010 Available online 25 January 2010 Keywords: BTA alkyl derivatives Bronze Artificial weathering Corrosion resistance Thin layer activation (TLA)
a b s t r a c t This paper focuses on the anticorrosive behaviour of surface films formed by different benzotriazole alkyl derivatives on a Cu6Sn bronze (B6), whose composition is similar to materials used for outdoor artefacts. Some alkyl derivatives of the 1,2,3-benzotriazole (BTA) as: 5-hexyl-1,2,3-benzotriazole; 5-dodecyl-1,2,3benzotriazole and [5-(1-undecyl)dodecyl]-1,2,3-benzotriazole were synthesized in order to investigate the influence exerted by the aliphatic chain on the inhibiting properties of the base molecule (BTA) toward bronze corrosion. The protective efficiency of the organic films, after a preliminary evaluation by electrochemical measurements, was determined by thin layer activation (TLA) and gravimetric techniques on the samples submitted to artificial weathering experiments in acid rain. For TLA measurements a γemitting radio nuclide 65Zn (t1/2 = 244 days), used as a corrosion tracer, was produced on the bronze surface by a high energy proton beam. At the end of artificial weathering exposures the radioactivity recovered on the bronze surface, once it was treated with a picking solution, allowed determination of the thickness loss. All the results arising from the different techniques, used in this study, show that BTA molecules bearing long aliphatic chain act as the best corrosion inhibitors. © 2010 Elsevier B.V. All rights reserved.
1. Introduction It is well known that outdoor bronze sculptures, generally situated in urban areas, undergo corrosion of the metallic surface with evident changes of the original appearance and reduction of their longevity [1–3]. Corrosion of outdoor bronzes is a complex phenomenon involving electrochemical reactions where alloy composition, atmospheric conditions, presence and concentration of different pollutants play a significant role in the degradation of the artefacts [4,5]. A perceptible result is the loss of the aesthetic characteristics of the statues. The attempt of conservators is mainly to preserve the artefacts while also maintaining the initial aspect, as decided by the artist. To reach this goal the techniques and the materials used have to result as least intrusive as possible. In particular the protective materials applied to the surfaces must satisfy specific requirements [6]: a) they have to be easily removable; b) no change of the colour below has to be produced by their application on surfaces; and c) the chemical interaction of the corrosion inhibitors with the metallic surface has to result as “soft” as possible. One of the most popular corrosion inhibitor, used worldwide to preserve outdoor bronze ⁎ Corresponding author. IMC-NCR Via Salaria km 29.300 - 00016 Monterotondo Stazione (Roma) Italy. Tel.: +39 06 90672515; fax: +39 06 90672519. E-mail addresses: [email protected] (G. Laguzzi), [email protected] (L. Luvidi). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.01.019
statues, is 1,2,3-benzotriazole (BTA). This is present in the composition of several commercially available protective materials [7] as for example Incralac, an acrylic based polymer and Soter, a microcrystalline wax. The chemical interaction of BTA with bronze alloy surfaces has been extensively studied by several authors [8–11]. The inhibition of bronze corrosion exerted by BTA consists in the possibility to form a complex [12] with Cu(I) and Cu(II) which gives rise to a very thin protective film. The growth rate and the thickness of the film respectively depend on the inhibitor concentration and on the time of immersion of the metallic surface into the BTA solution. Because of the suspicious toxicity of BTA, recent works [13,14] have been carried out in the attempt to replace BTA with bronze corrosion inhibitors considered innocuous molecules. In particular the anticorrosive properties of BTA versus different triazoles were examined [14] both on artificial patinas and on a patina covering ancient coins. In all the cases BTA showed the best performances in comparison with the other compounds. Moreover, the anticorrosive behaviour of imidazoles and thiadiazoles were investigated [13] in comparison with BTA. As reported by the Authors, on bronze specimens covered with patina, 1-(p-tolyl)4-methyl imidazole (TMI) and 2-mercapto-5R-acetylamino-1,3,4-thiadiazole (MAcT) were found to be efficient corrosion inhibitors despite their performances were significantly lower than BTA. In conclusion, it seems that BTA still remains one of the most powerful corrosion inhibitor for bronze surfaces. Concerning the
G. Laguzzi, L. Luvidi / Surface & Coatings Technology 204 (2010) 2442–2446
danger associated to the use of BTA, according to data arising from the CDS-NIOSH1, it is correct to classify the BTA as harmful rather than toxic molecule. Besides these considerations, during the past years several investigations have been carried out to develop BTA alkyl derivatives that can provide advanced anticorrosive performances with respect to the BTA base molecule. A contribution in understanding the interaction of several BTA methyl derivatives [15] on copper surfaces was obtained by spectroscopic and electrochemical investigations of several BTA methyl derivatives. In this study it was demonstrated that the capability of these compounds to be adsorbed on a cuprite surface strictly depends on the position of the methyl substituent in the BTA molecule. In particular 4-methyl BTA and 5-methyl BTA, where the alkyl substituent is not linked to the triazolic ring, showed the highest polarization resistance that resulted about twice with respect to BTA. Successive investigations [16–18] were carried out on 5-substituted BTA alkyl derivatives as: 5methyl-1,2,3-benzotriazole; 5-hexyl-1,2,3-benzotriazole; and 5-octyl1,2,3-benzotriazole. These studies showed that the chain length of the alkyl substitute influences the corrosion inhibiting properties of the BTA alkyl derivatives. The increased length of the alkyl chain corresponds to the increased anticorrosive property of the molecule. In the present paper the effect of the presence of long aliphatic chains (linear and branched) in the BTA molecule has been studied. A comparative determination of the anticorrosive behaviour of: 1,2,3 benzotriazole (BTA); 5-hexyl-1,2,3 benzotriazole (C6-BTA); 5-dodecyl-1,2,3 benzotriazole (C12-BTA) and [5-(1-undecyl)dodecyl]-1,2,3-benzotriazole (bisC11-BTA) has been carried out by TLA [19–21] and gravimetric techniques, after artificial ageing of B6 bronze specimens coated with the different inhibitors. A preliminary evaluation of the protective efficiency of the same BTA alkyl derivatives has also been performed through electrochemical measurements. For ageing tests, consisting in wet–dry cycles, as well as for the electrochemical experiments, artificial acid rain solutions have been used to reproduce the effects of an atmospheric corrosive environment. 2. Materials and methods 2.1. Bronze alloy The B6 bronze was provided by Goodfellow as hot rolled sheets. This is a pure mono-phase alloy whose detailed composition is the following: Sn 4.5–7.5%, Pb < 0.02%, P 0.02–0.4%, and Cu balance. Surfaces of B6 specimens (15 × 15 × 2 mm) were mechanically polished using various grinding papers (400, 800 and 1200) and finished with a Mecaprex polishing disc using 1 μm diamond paste. Specimens were rinsed with acetone and pickled in a non-aerated 10% H2SO4 solution for 5 s prior to treatment with corrosion inhibitors. 2.2. Corrosion inhibitors BTA was purchased by Aldrich while 5-alkyl-derivatives as: C6BTA, C12-BTA and bisC11-BTA were synthesized according to the procedure indicated by Krati et al. [22]. For each inhibitor a 10− 4 M solution in CHCl3 was prepared. B6 specimens were coated by double immersion in these solutions for 10 min at room temperature. 2.3. Electrochemical techniques A Solartron SI 1287 Electrochemical Interface was used for the following electrochemical tests: open circuit potential (OCP), polarization resistance (Rp) and polarization curves (PC). Open circuit measurements were performed on the untreated alloy and the alloy 1 CDC-NIOSH (Centers for Disease Control and Prevention-National Institute for Occupational Safety and Health) Atlanta, GA, USA.
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coated with the different inhibitors. The potential was measured for 60 min prior to polarization experiments. The Rp measurements were performed applying a small potential (±10 mV) with respect to the open circuit potential at a scan rate of 0.1 mV/s. Cathodic and anodic potentiodynamic polarization curves were carried out by polarization of the samples in the range of −300–+ 300 mV at the scan rate of 1 mV/sec. All electrochemical experiments were carried out using the artificial acid rain solution also utilized in artificial weathering tests. 2.4. Weathering experiments Artificial weathering of coated bronze specimens consisted in cyclic wet–dry exposures. Samples were dipped in an artificial acid rain solution, simulating the aggressive acid deposition in outdoor conditions. The composition of artificial acid rain was the following: sulphuric acid (96%) 31.85 mg dm− 3; ammonium sulphate 46.20 mg dm− 3; sodium sulphate 31.95 mg dm− 3; nitric acid (70%) 15.75 mg dm− 3; sodium nitrate 21.25 mg dm− 3; and sodium chloride 84.85 mg dm− 3; the pH was adjusted to 3.1. The artificial acid rain was prepared from analytical grade chemicals and distilled water. This solution is 10 times more concentrated with respect to annual average concentration of the rain fallen in the Manchester area in 1986 [23] and was previously used as reference in artificial weathering experiments [24]. The period of ageing was 19 days. Each wet–dry cycle was conducted by exposing the sample to an alternate condition of 40 min immersion in acid rain and 20 min of drying at 298 K and 60% RH. For cyclic wet–dry experiments the specimens were introduced into the vessels containing the acid rain solution and removed from them, by using an apparatus equipped with a proper timer and operated by a small cc electric motor. 2.5. Thin layer activation (TLA) technique 2.5.1. Specimens activation The corrosion tracer 65Zn (t1/2 = 244 days) radio nuclide was produced irradiating 15 × 5 × 2 mm B6 bronze specimens with an 11.5 MeV proton beam, generated by a Scanditronix MC-40 energy variable cyclotron. The proton beam, having an intensity of 170 nA, was focused on target with an irradiation angle of 30°. The irradiation time was 15 min. The nuclear reaction leading to 65Zn is the following: 65Cu (p,n)65Zn. Prior to calibration and corrosion experiments the samples remained isolated for at least 2 weeks until the activity dose decreased, due to the decay of short lived radioisotopes. These were produced in side nuclear reactions, starting from Cu and Sn isotopes composing the B6 bronze alloy. 2.5.2. Calibration curve The activity depth and the distribution of activity versus depth function were experimentally determined through step by step electrodissolution of an activated specimen. At the end of each electrodissolution step the thickness of the solid specimen was measured by using a mechanical micrometer while the 65Zn activity was measured in the electrolytic solutions. Accordingly, the error of the activity measurement and the depth determination remained less than 0.5% and 1 μm respectively. The calibration curve of the B6 bronze alloy is reported in Fig. 1. 2.5.3. Thickness loss determination method Through the calibration curve the thickness loss of a bronze specimen is calculated by γ-activity measurements. After artificial ageing the activated specimen were treated with a (1:1 HCl/H2O) pickling solution in order to remove the layer of the corrosion products from the metallic surface. After pickling, the specimens were dissolved by electrolysis. The γ-activity present in the pickling solution along with that recovered into the vessel used for ageing test, respectively corresponds to the corrosion products formed on the metallic surface and to those dissolved in the acidic rain solution. This
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G. Laguzzi, L. Luvidi / Surface & Coatings Technology 204 (2010) 2442–2446 Table 1 Polarization resistance and inhibition efficiency (%IE) of uncoated and coated B6 bronze specimens with different benzotriazole inhibitors after 1 h of immersion in acid rain solution.
Fig. 1. Calibration curve of the B6 bronze. Data obtained from two activated specimens.
amount of activity was then related to that recovered in the electrolytic bath, which corresponds to the residual activity present into the activated layer of the solid sample. The 65Zn γ-activity was quantitatively measured from its emission peak at 1115 KeV (Iγ = 0.50) by a Canberra multi channel analyser equipped with a NaI detector. 2.6. Surface analyses 2.6.1. Atomic force microscope (AFM) For AFM analyses a Digital Instrument Nanoscope IIIa, equipped with an E type scanner was used. Images were taken in Contact Mode with a non conductive SiN (NP-S1) Veeco tip. 2.6.2. Wetting test The determination of the contact angle of the different inhibitors was performed by using a Lorentz & Wettre instrument which allows direct reading of the height and diameter of the drops without taking photographs. The reading was always made 15 s after deposition of the drop on sample surface. For each inhibitor the obtained data were the mean of twenty measurements. 3. Results and discussion 3.1. Electrochemical tests A preliminary evaluation of the anticorrosive behaviour of the BTA 5alkyl derivatives has been evaluated by electrochemical techniques. After 1 h of immersion in acid rain the polarization resistance has been measured and the polarization curves have been recorded on uncoated bronze specimens and on specimens coated with the different inhibitors. The polarization resistance method permits determinations of the instantaneous corrosion rates of metals, by measurements of the changes of the current as a function of the applied potential. The polarization resistance (Rp), defined as the slope of the potentialcurrent density curve at the free corrosion potential (dE/di) E = Ecorr, is inversely related to the corrosion rate. Therefore increasing values of Rp testify an increasing capability of the organic films to act as corrosion inhibitors toward the bronze surface. In Table 1 the Rp values in artificial acid rain of uncoated and coated bronze specimens are reported. The inhibition efficiency of the benzotriazole derivatives is calculated using the equation Inhibition Efficiency ðIEÞ =
Samples
Rp (Ω cm2)
%IE
Uncoated BTA C6-BTA bisC11-BTA C12-BTA
502 1066 2943 5533 6400
– 53 83 91 92
It is also notable that the inhibition efficiency values of the longer chain inhibitors (C12-BTA and bisC11-BTA) are quite similar. Anodic and cathodic polarization curves are reported in Fig. 2. According to the literature [22,23], the anodic polarization curves show that oxidation reactions of bronze are controlled both by charge transfer and diffusion phenomena (Fig. 2). The copper dissolution in acid rain takes place, at low current densities, by charge transfer reaction with production of Cu+ads ions. These adsorbed cations interact with Xn− anions carried on bronze surface by diffusion (first diffusive step). Successively the obtained surface products diffuse in solution (second diffusive step). The diffusive processes are relatively slow and consequently they determine the rate of the anodic reaction. They are also responsible for the increasing curve slope as potential increases. In Fig. 2 we can observe, in the first part of the anodic curves of C12-BTA and bisC11-BTA, a slight increasing of the slope with respect to the other investigated samples. Consequently, these corrosion inhibitors, in the potential range between OCP and 0 V, show a better capability to reduce the anodic current with respect to BTA and C6-BTA. An analysis of the cathodic polarization curves points out, for the uncoated, BTA and C6-BTA samples the presence of a current peak followed by a flex which can arise from the oxygen reduction [24,25]. It is possible to note that in the polarization curves relevant to BTA derivatives bearing longer alkyl chain (C12-BTA and bisC11-BTA) this peak is absent. This seems to demonstrate that these molecules more efficiently hinder the corrosion of the metallic surface. Cathodic polarization curves of the samples treated with the different inhibitors are shifted to lower current density with respect to the blank. These inhibitors decrease the kinetics of the corrosion process especially in the cathodic reaction where is noticeable the disappearance of the uncoated reduction peak as shown in Fig. 2. The BTA derivatives, interacting with the bronze surface, influence the cathodic reduction processes hindering the formation of corrosion products. On the other hand, the BTA derivatives seem to have lower effect in the anodic oxidation reactions, for this reason they can be
RpðinhÞ −Rp = RpðinhÞ :
In Table 1 it is possible to observe that the %IE value increases following the sequence: uncoated < BTA < C6-BTA < bisC11-BTA ≤ C12BTA.
Fig. 2. Anodic and chatodic polarization curves recorded for uncoated and coated B6 bronze specimens after 1 h of immersion in acid rain.
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classified as cathodic inhibitors. The decreasing of the cathodic current in the polarization curves reflects the different protective behaviour of BTA derivatives on the bronze alloy, as shown in the following sequence: C12-BTA > bisC11-BTA > C6-BTA > BTA. The negative shift of the free corrosion potentials (Ecorr) observed in the presence of the inhibitors confirms the more effectiveness they have in the cathodic rather than in the anodic reaction. 3.2. TLA, gravimetric tests and surface analyses The corrosion inhibiting properties of C6-BTA, C12-BTA and bisC11-BTA toward a B6 bronze alloy has been also evaluated by the thin layer activation and gravimetric techniques on specimens exposed to an artificial acid rain. TLA, in particular, allows the determination of the thickness loss of activated specimens [26]. A homogeneous distribution of the radioactivity into the metallic material, as verified by the linear function of the calibration curve (Fig. 1), provides the simplest model for the subsequent thickness loss determination. The TLA results have been compared with those obtained by the gravimetric technique where non activated specimens have been submitted to the same artificial ageing tests (Table 2). From the inspection of Table 2 it is possible to notice that results arising from TLA measurements are confirmed by data obtained through the gravimetric technique, since specimens coated with the same inhibitors show a similar trend in terms of thickness loss and weight loss respectively. The behaviour of BTA and BTA alkyl derivatives was evaluated by cyclic dry/wet tests. This artificial weathering method may be intended as the more realistic respect to the traditional full immersion exposure, since the atmospheric exposures of outdoor artefacts usually include both wet and dry climatic conditions. The protective action of BTA 5alkyl derivatives against copper corrosion was studied by Tommesani et al. [16] by electrochemical experiments. The coating of the electrodes was obtained, in that work, by immersion of copper-based materials in diluted aqueous solutions of 5-alkyl derivatives of BTA. As declared by the Authors, the uncompleted solubility of C12-BTA in water resulted in a not homogeneous covering of the electrodes. For this reason C12-BTA showed a lower protective action respect to other BTA derivatives bearing a shorter aliphatic chain as 5-hexyl-BTA and 5-butyl-BTA. In the present work the coating of B6 bronze surfaces was realized dipping the metal specimens in a 10− 4 M solution of the inhibitors in CHCl3. Before proceeding with the experiments the covering homogeneity of the films, for each corrosion inhibitor, was controlled by AFM. As an example, in Fig. 3 the images of bronze surfaces, before and after the treatment with the C12-BTA inhibitor are reported. From the inspection of the AFM images it is possible to notice that the treatment of the bronze specimen with a CHCl3 solution of C12-BTA gives rise to an organic film, which homogeneously covers the metallic surface. As shown in Table 2, the amount of thickness lost from the artificially aged B6 bronze specimens, strictly depends on the specific inhibitor coating the surface. In particular, we can observe that the thickness loss decreases as the number of carbon atoms forming the alkyl chain of the BTA increases. However, C12-BTA and bisC11-BTA respectively bearing a linear and a branched structure of the alkyl chain show a similar effectiveness as corrosion inhibitors. This seems to
Table 2 Thickness loss and gravimetric results obtained after cyclic wet/dry tests. Inhibitors (10− 4 M in CHCl3)
Thickness loss, μm (TLA technique)
Weight loss, mg (gravimetric technique)
– BTA C6-BTA C12-BTA bisC11-BTA
1.13 ± 0.05 1.02 ± 0.06 0.71 ± 0.08 0.42 ± 0.02 0.41 ± 0.04
0.85 ± 0.3 0.76 ± 0.2 0.68 ± 0.2 0.58 ± 0.1 0.58 ± 0.4
Fig. 3. AFM imagines of uncoated (A) bronze surface and coated (B) with C12-BTA.
demonstrate that the structural difference of the alkyl chains does not affect the anticorrosive properties of these molecules. The analogy of the anticorrosive behaviour between C12-BTA and bisC11-BTA has to be consequently inquired into a different cause. In their work, where the effectiveness toward bronze corrosion of several 5-alkyl derivatives of BTA was investigated, Tommesani et al. [16] pointed out that the hydrophobic characteristics of the molecules were mainly responsible of their anticorrosive properties. In the light of these considerations, in the present work, bronze surfaces coated with the different inhibitors have been submitted to contact angle measurements to verify the hydrophobic characteristics of the BTA alkyl derivatives. Wetting tests results are reported in Table 3 where it is possible to observe that the contact angles of C12-BTA and bisC11-BTA are very similar. This indicates that these compounds have quite similar hydrophobic characteristics. This
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Table 3 Contact angles values of bronze specimens coated with the different inhibitors. Inhibitor
BTA
C6-BTA
C12-BTA
bisC11-BTA
Contact angle (α)
82 ± 6
93 ± 5
103 ± 6
104 ± 5
could reasonably explain their similar anticorrosive behaviour as verified by the different technique used in the present work. 4. Conclusions Despite the fact that the base BTA molecule is used in commercially available protective materials, literature data along with those obtained in the present work demonstrate that 5-alkyl derivatives of BTA show better anticorrosive properties with respect to BTA. In particular 5-alkyl derivatives of BTA having long alkyl chains, because of their significant hydrophobic characteristics, seem to act as the best corrosion inhibitors of bronze surfaces. In this study, the anticorrosive properties of the different BTA alkyl derivatives have been evaluated on laboratory scale. The best homogeneous coating, which was necessary for a comparative evaluation of the corrosion inhibitors, was obtained by using CHCl3 solutions. For a practical application in the field of conservation of bronze artefacts, if solutions of inhibitors have to be directly used on the surfaces, a specific search for a more appropriate solvent must be done. However, problems related to specific solvents can be excluded in case of the use of mixtures of the corrosion inhibitors with microcrystalline waxes, as those commercially available. For a current use in the bronze artworks conservation a toxicological investigation of the proposed protective molecules must also be carried out. It is reasonable to consider this important item once the advanced anticorrosive properties have been demonstrated as in the case of the present paper. Acknowledgments The Authors wish to express their gratitude to the Centro di Corrosione “Aldo Daccò” (Univ. of Ferrara) for the synthesis of C6-BTA and to Mr. Roberto Moscardelli (IMC-CNR) for his skilful technical support.
References [1] D.A. Scott, J. Podany, B.B. Considine (Eds.), Ancient and Historical Metals, Conservation and Scientific Research, Proceedings, The Getty Conservation Institute, 1994. [2] T.E. Graedel, K. Nassau, J.P. Franey, Corrosion Science 27 (1987) 639. [3] T.E. Graedel, Corrosion Science 27 (1987) 741. [4] T. Drayman-Weisser (Ed.), Proc. Dialogue/89 — The Conservation of Bronze Sculptures in the Outdoor Environment, National Association of Corrosion Engineers, Houston, Tx, USA, 1989. [5] M. Tullmin, P.R. Roberge (Eds.), Uhlig's Corrosion Handbook, 2000, p. 305. [6] AIC, Code of Ethics of the American Institute for Conservation of Historic and Artistic Works, 1994. [7] D. Scott, Copper and Bronze in Art Corrosion, Colorants, Conservation, 1st edGetty Publications, Los Angeles, CA, 2002. [8] I. Dugdale, J.B. Cotton, Corrosion Science 3 (1963) 69. [9] B.S. Fang, C. Olson, D.W. Lynch, Surface Science 176 (1986) 476. [10] D. Tromans, J.C. Silva, Corrosion 51 (1997) 16. [11] Y. Ling, Y. Guang, K.N. Han, Corrosion 18 (1978) 367. [12] J.B. Cotton, I.R. Scholes, British Corrosion Journal 2 (1967) 1. [13] L. Muresan, S. Varvara, E. Stupnisek-Lisac, H. Otmacic, K. Marusic, S. HorvatKurbegovic, L. Robbiola, K. Rahmouni, H. Takenouti, Electrochimica Acta 52 (2007) 7770. [14] K. Rahmouni, H. Takenouti, N. Hajjaji, A. Srhiri, L. Robbiola, Electrochimica Acta 54 (2009) 5206. [15] C. Tornkvist, D. Thierry, J. Bergman, B. Liedberg, C. Leygraf, Journal of the Electrochemical Society 136 (1989) 58. [16] L. Tommesani, G. Brunoro, A. Frignani, C. Monticelli, M. Dal Colle, Corrosion Science 39 (1997) 1221. [17] A. Frignani, M. Fonsanti, C. Monticelli, G. Brunoro, Corrosion Science 41 (1999) 1217. [18] Brunoro, A. Frignani, A. Colledan, C. Chiavari, Corrosion Science 45 (2003) 2219. [19] G. Laguzzi, L. Tommesani, L. Luvidi, R. Bucci, G. Brunoro, Corrosion Science 41 (1999) 197. [20] G. Laguzzi, L. Luvidi, G. Brunoro, Corrosion Science 43 (2001) 747. [21] F. Gallese, G. Laguzzi, L. Luvidi, V. Ferrari, S. Takacs, G. Venturi Pagani Cesa, Corrosion Science 50 (2008) 954. [22] N. Krati, D. Roizard, A. Brembilla, P. Lochon, Bulletin de la Societè Chimique de France 3 (1989) 443. [23] S.B. Lyon, J.D. Scantlebury, J.B. Johnson, In Cyclic cabinet corrosion testing, in: G.S. Haynes (Ed.), STP 1238 West Conshohocken, ASTM, PA, 1995, p. 3. [24] G. Brunoro, G. Laguzzi, L. Luvidi, C. Chiavari, British Corrosion Journal 36 (2001) 227. [25] S. Magaino, Electrochimica Acta 42 (1997) 377. [26] M.F. Stroosnijder, Thin layer activation in material technology, in: P. Misaelides (Ed.), Application of Particle and Laser Beams, Material Technology, Kluwer, Netherlands, 1995, p. 399.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Evaluation of the anticorrosive properties of benzotriazole alkyl derivatives on 6% Sn bronze alloy G. Laguzzi ⁎, L. Luvidi Istituto di Metodologie Chimiche del CNR, Area della Ricerca di Roma 1, Via Salaria Km 29, 300 - 00016 Monterotondo Stazione (Roma), Italy
a r t i c l e
i n f o
Article history: Received 22 September 2009 Accepted in revised form 17 January 2010 Available online 25 January 2010 Keywords: BTA alkyl derivatives Bronze Artificial weathering Corrosion resistance Thin layer activation (TLA)
a b s t r a c t This paper focuses on the anticorrosive behaviour of surface films formed by different benzotriazole alkyl derivatives on a Cu6Sn bronze (B6), whose composition is similar to materials used for outdoor artefacts. Some alkyl derivatives of the 1,2,3-benzotriazole (BTA) as: 5-hexyl-1,2,3-benzotriazole; 5-dodecyl-1,2,3benzotriazole and [5-(1-undecyl)dodecyl]-1,2,3-benzotriazole were synthesized in order to investigate the influence exerted by the aliphatic chain on the inhibiting properties of the base molecule (BTA) toward bronze corrosion. The protective efficiency of the organic films, after a preliminary evaluation by electrochemical measurements, was determined by thin layer activation (TLA) and gravimetric techniques on the samples submitted to artificial weathering experiments in acid rain. For TLA measurements a γemitting radio nuclide 65Zn (t1/2 = 244 days), used as a corrosion tracer, was produced on the bronze surface by a high energy proton beam. At the end of artificial weathering exposures the radioactivity recovered on the bronze surface, once it was treated with a picking solution, allowed determination of the thickness loss. All the results arising from the different techniques, used in this study, show that BTA molecules bearing long aliphatic chain act as the best corrosion inhibitors. © 2010 Elsevier B.V. All rights reserved.
1. Introduction It is well known that outdoor bronze sculptures, generally situated in urban areas, undergo corrosion of the metallic surface with evident changes of the original appearance and reduction of their longevity [1–3]. Corrosion of outdoor bronzes is a complex phenomenon involving electrochemical reactions where alloy composition, atmospheric conditions, presence and concentration of different pollutants play a significant role in the degradation of the artefacts [4,5]. A perceptible result is the loss of the aesthetic characteristics of the statues. The attempt of conservators is mainly to preserve the artefacts while also maintaining the initial aspect, as decided by the artist. To reach this goal the techniques and the materials used have to result as least intrusive as possible. In particular the protective materials applied to the surfaces must satisfy specific requirements [6]: a) they have to be easily removable; b) no change of the colour below has to be produced by their application on surfaces; and c) the chemical interaction of the corrosion inhibitors with the metallic surface has to result as “soft” as possible. One of the most popular corrosion inhibitor, used worldwide to preserve outdoor bronze ⁎ Corresponding author. IMC-NCR Via Salaria km 29.300 - 00016 Monterotondo Stazione (Roma) Italy. Tel.: +39 06 90672515; fax: +39 06 90672519. E-mail addresses: [email protected] (G. Laguzzi), [email protected] (L. Luvidi). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.01.019
statues, is 1,2,3-benzotriazole (BTA). This is present in the composition of several commercially available protective materials [7] as for example Incralac, an acrylic based polymer and Soter, a microcrystalline wax. The chemical interaction of BTA with bronze alloy surfaces has been extensively studied by several authors [8–11]. The inhibition of bronze corrosion exerted by BTA consists in the possibility to form a complex [12] with Cu(I) and Cu(II) which gives rise to a very thin protective film. The growth rate and the thickness of the film respectively depend on the inhibitor concentration and on the time of immersion of the metallic surface into the BTA solution. Because of the suspicious toxicity of BTA, recent works [13,14] have been carried out in the attempt to replace BTA with bronze corrosion inhibitors considered innocuous molecules. In particular the anticorrosive properties of BTA versus different triazoles were examined [14] both on artificial patinas and on a patina covering ancient coins. In all the cases BTA showed the best performances in comparison with the other compounds. Moreover, the anticorrosive behaviour of imidazoles and thiadiazoles were investigated [13] in comparison with BTA. As reported by the Authors, on bronze specimens covered with patina, 1-(p-tolyl)4-methyl imidazole (TMI) and 2-mercapto-5R-acetylamino-1,3,4-thiadiazole (MAcT) were found to be efficient corrosion inhibitors despite their performances were significantly lower than BTA. In conclusion, it seems that BTA still remains one of the most powerful corrosion inhibitor for bronze surfaces. Concerning the
G. Laguzzi, L. Luvidi / Surface & Coatings Technology 204 (2010) 2442–2446
danger associated to the use of BTA, according to data arising from the CDS-NIOSH1, it is correct to classify the BTA as harmful rather than toxic molecule. Besides these considerations, during the past years several investigations have been carried out to develop BTA alkyl derivatives that can provide advanced anticorrosive performances with respect to the BTA base molecule. A contribution in understanding the interaction of several BTA methyl derivatives [15] on copper surfaces was obtained by spectroscopic and electrochemical investigations of several BTA methyl derivatives. In this study it was demonstrated that the capability of these compounds to be adsorbed on a cuprite surface strictly depends on the position of the methyl substituent in the BTA molecule. In particular 4-methyl BTA and 5-methyl BTA, where the alkyl substituent is not linked to the triazolic ring, showed the highest polarization resistance that resulted about twice with respect to BTA. Successive investigations [16–18] were carried out on 5-substituted BTA alkyl derivatives as: 5methyl-1,2,3-benzotriazole; 5-hexyl-1,2,3-benzotriazole; and 5-octyl1,2,3-benzotriazole. These studies showed that the chain length of the alkyl substitute influences the corrosion inhibiting properties of the BTA alkyl derivatives. The increased length of the alkyl chain corresponds to the increased anticorrosive property of the molecule. In the present paper the effect of the presence of long aliphatic chains (linear and branched) in the BTA molecule has been studied. A comparative determination of the anticorrosive behaviour of: 1,2,3 benzotriazole (BTA); 5-hexyl-1,2,3 benzotriazole (C6-BTA); 5-dodecyl-1,2,3 benzotriazole (C12-BTA) and [5-(1-undecyl)dodecyl]-1,2,3-benzotriazole (bisC11-BTA) has been carried out by TLA [19–21] and gravimetric techniques, after artificial ageing of B6 bronze specimens coated with the different inhibitors. A preliminary evaluation of the protective efficiency of the same BTA alkyl derivatives has also been performed through electrochemical measurements. For ageing tests, consisting in wet–dry cycles, as well as for the electrochemical experiments, artificial acid rain solutions have been used to reproduce the effects of an atmospheric corrosive environment. 2. Materials and methods 2.1. Bronze alloy The B6 bronze was provided by Goodfellow as hot rolled sheets. This is a pure mono-phase alloy whose detailed composition is the following: Sn 4.5–7.5%, Pb < 0.02%, P 0.02–0.4%, and Cu balance. Surfaces of B6 specimens (15 × 15 × 2 mm) were mechanically polished using various grinding papers (400, 800 and 1200) and finished with a Mecaprex polishing disc using 1 μm diamond paste. Specimens were rinsed with acetone and pickled in a non-aerated 10% H2SO4 solution for 5 s prior to treatment with corrosion inhibitors. 2.2. Corrosion inhibitors BTA was purchased by Aldrich while 5-alkyl-derivatives as: C6BTA, C12-BTA and bisC11-BTA were synthesized according to the procedure indicated by Krati et al. [22]. For each inhibitor a 10− 4 M solution in CHCl3 was prepared. B6 specimens were coated by double immersion in these solutions for 10 min at room temperature. 2.3. Electrochemical techniques A Solartron SI 1287 Electrochemical Interface was used for the following electrochemical tests: open circuit potential (OCP), polarization resistance (Rp) and polarization curves (PC). Open circuit measurements were performed on the untreated alloy and the alloy 1 CDC-NIOSH (Centers for Disease Control and Prevention-National Institute for Occupational Safety and Health) Atlanta, GA, USA.
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coated with the different inhibitors. The potential was measured for 60 min prior to polarization experiments. The Rp measurements were performed applying a small potential (±10 mV) with respect to the open circuit potential at a scan rate of 0.1 mV/s. Cathodic and anodic potentiodynamic polarization curves were carried out by polarization of the samples in the range of −300–+ 300 mV at the scan rate of 1 mV/sec. All electrochemical experiments were carried out using the artificial acid rain solution also utilized in artificial weathering tests. 2.4. Weathering experiments Artificial weathering of coated bronze specimens consisted in cyclic wet–dry exposures. Samples were dipped in an artificial acid rain solution, simulating the aggressive acid deposition in outdoor conditions. The composition of artificial acid rain was the following: sulphuric acid (96%) 31.85 mg dm− 3; ammonium sulphate 46.20 mg dm− 3; sodium sulphate 31.95 mg dm− 3; nitric acid (70%) 15.75 mg dm− 3; sodium nitrate 21.25 mg dm− 3; and sodium chloride 84.85 mg dm− 3; the pH was adjusted to 3.1. The artificial acid rain was prepared from analytical grade chemicals and distilled water. This solution is 10 times more concentrated with respect to annual average concentration of the rain fallen in the Manchester area in 1986 [23] and was previously used as reference in artificial weathering experiments [24]. The period of ageing was 19 days. Each wet–dry cycle was conducted by exposing the sample to an alternate condition of 40 min immersion in acid rain and 20 min of drying at 298 K and 60% RH. For cyclic wet–dry experiments the specimens were introduced into the vessels containing the acid rain solution and removed from them, by using an apparatus equipped with a proper timer and operated by a small cc electric motor. 2.5. Thin layer activation (TLA) technique 2.5.1. Specimens activation The corrosion tracer 65Zn (t1/2 = 244 days) radio nuclide was produced irradiating 15 × 5 × 2 mm B6 bronze specimens with an 11.5 MeV proton beam, generated by a Scanditronix MC-40 energy variable cyclotron. The proton beam, having an intensity of 170 nA, was focused on target with an irradiation angle of 30°. The irradiation time was 15 min. The nuclear reaction leading to 65Zn is the following: 65Cu (p,n)65Zn. Prior to calibration and corrosion experiments the samples remained isolated for at least 2 weeks until the activity dose decreased, due to the decay of short lived radioisotopes. These were produced in side nuclear reactions, starting from Cu and Sn isotopes composing the B6 bronze alloy. 2.5.2. Calibration curve The activity depth and the distribution of activity versus depth function were experimentally determined through step by step electrodissolution of an activated specimen. At the end of each electrodissolution step the thickness of the solid specimen was measured by using a mechanical micrometer while the 65Zn activity was measured in the electrolytic solutions. Accordingly, the error of the activity measurement and the depth determination remained less than 0.5% and 1 μm respectively. The calibration curve of the B6 bronze alloy is reported in Fig. 1. 2.5.3. Thickness loss determination method Through the calibration curve the thickness loss of a bronze specimen is calculated by γ-activity measurements. After artificial ageing the activated specimen were treated with a (1:1 HCl/H2O) pickling solution in order to remove the layer of the corrosion products from the metallic surface. After pickling, the specimens were dissolved by electrolysis. The γ-activity present in the pickling solution along with that recovered into the vessel used for ageing test, respectively corresponds to the corrosion products formed on the metallic surface and to those dissolved in the acidic rain solution. This
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G. Laguzzi, L. Luvidi / Surface & Coatings Technology 204 (2010) 2442–2446 Table 1 Polarization resistance and inhibition efficiency (%IE) of uncoated and coated B6 bronze specimens with different benzotriazole inhibitors after 1 h of immersion in acid rain solution.
Fig. 1. Calibration curve of the B6 bronze. Data obtained from two activated specimens.
amount of activity was then related to that recovered in the electrolytic bath, which corresponds to the residual activity present into the activated layer of the solid sample. The 65Zn γ-activity was quantitatively measured from its emission peak at 1115 KeV (Iγ = 0.50) by a Canberra multi channel analyser equipped with a NaI detector. 2.6. Surface analyses 2.6.1. Atomic force microscope (AFM) For AFM analyses a Digital Instrument Nanoscope IIIa, equipped with an E type scanner was used. Images were taken in Contact Mode with a non conductive SiN (NP-S1) Veeco tip. 2.6.2. Wetting test The determination of the contact angle of the different inhibitors was performed by using a Lorentz & Wettre instrument which allows direct reading of the height and diameter of the drops without taking photographs. The reading was always made 15 s after deposition of the drop on sample surface. For each inhibitor the obtained data were the mean of twenty measurements. 3. Results and discussion 3.1. Electrochemical tests A preliminary evaluation of the anticorrosive behaviour of the BTA 5alkyl derivatives has been evaluated by electrochemical techniques. After 1 h of immersion in acid rain the polarization resistance has been measured and the polarization curves have been recorded on uncoated bronze specimens and on specimens coated with the different inhibitors. The polarization resistance method permits determinations of the instantaneous corrosion rates of metals, by measurements of the changes of the current as a function of the applied potential. The polarization resistance (Rp), defined as the slope of the potentialcurrent density curve at the free corrosion potential (dE/di) E = Ecorr, is inversely related to the corrosion rate. Therefore increasing values of Rp testify an increasing capability of the organic films to act as corrosion inhibitors toward the bronze surface. In Table 1 the Rp values in artificial acid rain of uncoated and coated bronze specimens are reported. The inhibition efficiency of the benzotriazole derivatives is calculated using the equation Inhibition Efficiency ðIEÞ =
Samples
Rp (Ω cm2)
%IE
Uncoated BTA C6-BTA bisC11-BTA C12-BTA
502 1066 2943 5533 6400
– 53 83 91 92
It is also notable that the inhibition efficiency values of the longer chain inhibitors (C12-BTA and bisC11-BTA) are quite similar. Anodic and cathodic polarization curves are reported in Fig. 2. According to the literature [22,23], the anodic polarization curves show that oxidation reactions of bronze are controlled both by charge transfer and diffusion phenomena (Fig. 2). The copper dissolution in acid rain takes place, at low current densities, by charge transfer reaction with production of Cu+ads ions. These adsorbed cations interact with Xn− anions carried on bronze surface by diffusion (first diffusive step). Successively the obtained surface products diffuse in solution (second diffusive step). The diffusive processes are relatively slow and consequently they determine the rate of the anodic reaction. They are also responsible for the increasing curve slope as potential increases. In Fig. 2 we can observe, in the first part of the anodic curves of C12-BTA and bisC11-BTA, a slight increasing of the slope with respect to the other investigated samples. Consequently, these corrosion inhibitors, in the potential range between OCP and 0 V, show a better capability to reduce the anodic current with respect to BTA and C6-BTA. An analysis of the cathodic polarization curves points out, for the uncoated, BTA and C6-BTA samples the presence of a current peak followed by a flex which can arise from the oxygen reduction [24,25]. It is possible to note that in the polarization curves relevant to BTA derivatives bearing longer alkyl chain (C12-BTA and bisC11-BTA) this peak is absent. This seems to demonstrate that these molecules more efficiently hinder the corrosion of the metallic surface. Cathodic polarization curves of the samples treated with the different inhibitors are shifted to lower current density with respect to the blank. These inhibitors decrease the kinetics of the corrosion process especially in the cathodic reaction where is noticeable the disappearance of the uncoated reduction peak as shown in Fig. 2. The BTA derivatives, interacting with the bronze surface, influence the cathodic reduction processes hindering the formation of corrosion products. On the other hand, the BTA derivatives seem to have lower effect in the anodic oxidation reactions, for this reason they can be
RpðinhÞ −Rp = RpðinhÞ :
In Table 1 it is possible to observe that the %IE value increases following the sequence: uncoated < BTA < C6-BTA < bisC11-BTA ≤ C12BTA.
Fig. 2. Anodic and chatodic polarization curves recorded for uncoated and coated B6 bronze specimens after 1 h of immersion in acid rain.
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classified as cathodic inhibitors. The decreasing of the cathodic current in the polarization curves reflects the different protective behaviour of BTA derivatives on the bronze alloy, as shown in the following sequence: C12-BTA > bisC11-BTA > C6-BTA > BTA. The negative shift of the free corrosion potentials (Ecorr) observed in the presence of the inhibitors confirms the more effectiveness they have in the cathodic rather than in the anodic reaction. 3.2. TLA, gravimetric tests and surface analyses The corrosion inhibiting properties of C6-BTA, C12-BTA and bisC11-BTA toward a B6 bronze alloy has been also evaluated by the thin layer activation and gravimetric techniques on specimens exposed to an artificial acid rain. TLA, in particular, allows the determination of the thickness loss of activated specimens [26]. A homogeneous distribution of the radioactivity into the metallic material, as verified by the linear function of the calibration curve (Fig. 1), provides the simplest model for the subsequent thickness loss determination. The TLA results have been compared with those obtained by the gravimetric technique where non activated specimens have been submitted to the same artificial ageing tests (Table 2). From the inspection of Table 2 it is possible to notice that results arising from TLA measurements are confirmed by data obtained through the gravimetric technique, since specimens coated with the same inhibitors show a similar trend in terms of thickness loss and weight loss respectively. The behaviour of BTA and BTA alkyl derivatives was evaluated by cyclic dry/wet tests. This artificial weathering method may be intended as the more realistic respect to the traditional full immersion exposure, since the atmospheric exposures of outdoor artefacts usually include both wet and dry climatic conditions. The protective action of BTA 5alkyl derivatives against copper corrosion was studied by Tommesani et al. [16] by electrochemical experiments. The coating of the electrodes was obtained, in that work, by immersion of copper-based materials in diluted aqueous solutions of 5-alkyl derivatives of BTA. As declared by the Authors, the uncompleted solubility of C12-BTA in water resulted in a not homogeneous covering of the electrodes. For this reason C12-BTA showed a lower protective action respect to other BTA derivatives bearing a shorter aliphatic chain as 5-hexyl-BTA and 5-butyl-BTA. In the present work the coating of B6 bronze surfaces was realized dipping the metal specimens in a 10− 4 M solution of the inhibitors in CHCl3. Before proceeding with the experiments the covering homogeneity of the films, for each corrosion inhibitor, was controlled by AFM. As an example, in Fig. 3 the images of bronze surfaces, before and after the treatment with the C12-BTA inhibitor are reported. From the inspection of the AFM images it is possible to notice that the treatment of the bronze specimen with a CHCl3 solution of C12-BTA gives rise to an organic film, which homogeneously covers the metallic surface. As shown in Table 2, the amount of thickness lost from the artificially aged B6 bronze specimens, strictly depends on the specific inhibitor coating the surface. In particular, we can observe that the thickness loss decreases as the number of carbon atoms forming the alkyl chain of the BTA increases. However, C12-BTA and bisC11-BTA respectively bearing a linear and a branched structure of the alkyl chain show a similar effectiveness as corrosion inhibitors. This seems to
Table 2 Thickness loss and gravimetric results obtained after cyclic wet/dry tests. Inhibitors (10− 4 M in CHCl3)
Thickness loss, μm (TLA technique)
Weight loss, mg (gravimetric technique)
– BTA C6-BTA C12-BTA bisC11-BTA
1.13 ± 0.05 1.02 ± 0.06 0.71 ± 0.08 0.42 ± 0.02 0.41 ± 0.04
0.85 ± 0.3 0.76 ± 0.2 0.68 ± 0.2 0.58 ± 0.1 0.58 ± 0.4
Fig. 3. AFM imagines of uncoated (A) bronze surface and coated (B) with C12-BTA.
demonstrate that the structural difference of the alkyl chains does not affect the anticorrosive properties of these molecules. The analogy of the anticorrosive behaviour between C12-BTA and bisC11-BTA has to be consequently inquired into a different cause. In their work, where the effectiveness toward bronze corrosion of several 5-alkyl derivatives of BTA was investigated, Tommesani et al. [16] pointed out that the hydrophobic characteristics of the molecules were mainly responsible of their anticorrosive properties. In the light of these considerations, in the present work, bronze surfaces coated with the different inhibitors have been submitted to contact angle measurements to verify the hydrophobic characteristics of the BTA alkyl derivatives. Wetting tests results are reported in Table 3 where it is possible to observe that the contact angles of C12-BTA and bisC11-BTA are very similar. This indicates that these compounds have quite similar hydrophobic characteristics. This
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Table 3 Contact angles values of bronze specimens coated with the different inhibitors. Inhibitor
BTA
C6-BTA
C12-BTA
bisC11-BTA
Contact angle (α)
82 ± 6
93 ± 5
103 ± 6
104 ± 5
could reasonably explain their similar anticorrosive behaviour as verified by the different technique used in the present work. 4. Conclusions Despite the fact that the base BTA molecule is used in commercially available protective materials, literature data along with those obtained in the present work demonstrate that 5-alkyl derivatives of BTA show better anticorrosive properties with respect to BTA. In particular 5-alkyl derivatives of BTA having long alkyl chains, because of their significant hydrophobic characteristics, seem to act as the best corrosion inhibitors of bronze surfaces. In this study, the anticorrosive properties of the different BTA alkyl derivatives have been evaluated on laboratory scale. The best homogeneous coating, which was necessary for a comparative evaluation of the corrosion inhibitors, was obtained by using CHCl3 solutions. For a practical application in the field of conservation of bronze artefacts, if solutions of inhibitors have to be directly used on the surfaces, a specific search for a more appropriate solvent must be done. However, problems related to specific solvents can be excluded in case of the use of mixtures of the corrosion inhibitors with microcrystalline waxes, as those commercially available. For a current use in the bronze artworks conservation a toxicological investigation of the proposed protective molecules must also be carried out. It is reasonable to consider this important item once the advanced anticorrosive properties have been demonstrated as in the case of the present paper. Acknowledgments The Authors wish to express their gratitude to the Centro di Corrosione “Aldo Daccò” (Univ. of Ferrara) for the synthesis of C6-BTA and to Mr. Roberto Moscardelli (IMC-CNR) for his skilful technical support.
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