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Published on November 1, 2007

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In-situ characterisation of patina and coatings on outdoor bronze with electrochemical methods :  In-situ characterisation of patina and coatings on outdoor bronze with electrochemical methods Paola Letardi CNR- ISMAR, via DeMarini 6, I-16149 Genova e-mail: paola.letardi@ismar.cnr.it Use of Electrochemical Techniques in metal Conservation Kick-off meeting – 13 march 2006 - Ghent Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University Slide2:  Introduction age, composition, finishing weathering conditions and exposure environmental pollution past conservation treatments Patinas - crusts Cleaning procedure Protective treatments The number and the range of all the parameters involved produces a very wide range of possible situations Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University Introduction:  Introduction e e e e e Corrosion is an electrochemical process involving the oxidation of a metal (the anodic reaction) and the corresponding reduction of another material (the cathodic reaction) oxidation 4Cu ® 4Cu+ + 4e- anodic reaction reduction O2 + 2H2O + 4e- ® 4OH- cathodic reaction 4Cu+ + 4OH- ® 2Cu2O + 2H2O Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University Slide4:  Energy level (Potential) Ref Charge flow (Current) Introduction Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University Electrochemical NdT’s on outdoor bronze statues:  Electrochemical NdT’s on outdoor bronze statues Polarisation Resistance Rp Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University EIS Electrochemical Potential Eoc:  Electrochemical Potential Eoc The corrosion potential is determined by: the chemically active compounds (chemical potential and concentration) the number of electrons involved in the redox reaction the temperature. In the case of steel without incoherent corrosion products, Pourbaix showed the free corrosion potential Eoc (in mVSCE) is proportional to the instantaneous corrosion rate vc (in mm/year) : log vc = 1.183 – 0.004416 Eoc Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University Electrochemical Potential Eoc: “La Recoleta” cemetery:  Electrochemical Potential Eoc: “La Recoleta” cemetery Cicileo and co-workers had tested the Eoc measurement on some bronze statues in “La Recoleta” cemetery in Buenos Aires . The portable arrangement used for in situ potential measurements consists of: a glass tube with a reference saturated calomel electrode (Esce = 242 mV/H) in one of its entrances; the other entrance is obtruded with a cotton piece, which becomes damp with distilled water and is applied to the zone whose potential is to be measured. A tester connected the reference electrode to a small bare area of the structure for potential measurements. Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University Electrochemical Potential Eoc: “La Recoleta” cemetery :  Crespo M., Cicileo G., Rosales B., in: J. Ashton, D. Hallam (Eds.), Proceedings of Metal04, National Museum of Australia, 2004, pp. 185-194 An increase in Eoc or a stable potential with time is suggested to characterise protective patinas. Unstable potentials or its decrease with time is ascribed to patinas with no protecting ability. A maximum 30mV variation in Eoc on a two years time is indicated for the bronze patina to be considered as stable Caution is suggested in the evaluation of patina protectiveness by electrochemical methods because of heterogeneous alloying elements distribution (Corr.Sci. 46 (2004), 929-953) Electrochemical Potential Eoc: “La Recoleta” cemetery Cicileo et al, Corr.Sci 46 (2004), p. 931 Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University The Rp technique:  The Rp technique With this widely used technique in corrosion monitoring, the polarisation resistance of a material is defined as the slope of the potential-current density (DE/Di) curve at the free corrosion potential, yielding the polarisation resistance Rp that can be related (for reactions under activation control) to the corrosion current by the Stern-Geary equation: where: Rp is the polarisation resistance icorr the corrosion current The proportionality constant , B, for a particular system can he determined empirically (calibrated from separate weight loss measurements) or, as shown by Stern and Geary, can be calculated from ba and bc, the slopes of the anodic and cathodic Tafel slopes, i.e.: Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University The Rp technique:  The Rp technique Rp has been applied by ICR and others in some campaigns on bronzes in outdoor exposure, as the Marcus Aurelius monument in Rome and Perseo and Medusa in Florence, the monument “Ai Mille” near the sea side in Genoa, as well as on archaeological finds from the sea, as the Warriors of Riace and the Dancing Satyr from Mazara del Vallo. This procedure has been adapted to the study and control of ancient metals from the industrial and engineering fields, in which this methodology is largely applied. Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University The Rp technique : The dying horse:  The Rp technique : The dying horse Rp measurements before the restoration treatment showed a very high corrosion rate. After the restoration, the sculpture has been treated with a double layer protective system: the inner one based on synthetic resins while the outer one is a microcrystalline wax used as a “sacrificial layer”, which should be controlled and renewed at regular intervals. Rp measurements enlightened a sharp decrease in the corrosion rate after that treatment. G.D’Ercoli et al, ICR - http://www.icr.arti.beniculturali.it/Restauri/CavalloRai/Rai13122000.htm Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University The Rp technique : Monumento ai Mille:  The Rp technique : Monumento ai Mille A monitoring by Rp measurements has been performed by ICR on 6 areas of the monument. For each area, measurements have been performed on untreated/cleaned/cleaned and coated with 6 different protective systems zones. After 2.5 years the overall best performance in the aggressive urban-marine environments is provided by the double layer system Incralac+Soter. The work is still in progress. D’Ercoli G., Marabelli M., in “Monumenti in bronzo all'aperto. Esperienze di conservazione a confronto”, Nardini, Firenze, 2004, pp. 113-117 Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University The Rp technique : Guggenheim collection in Venice:  The Rp technique : Guggenheim collection in Venice Angelucci S., Bartuli C., in “Monumenti in bronzo all'aperto. Esperienze di conservazione a confronto”, Nardini, Firenze, 2004, pp. 195-200 Marino Marini, The Angel of the City (1948). A natural compact patina is present, with the exception of the sections more exposed to the continuous touching of visitors (horse muzzle and tail, man’s penis). The average corrosion rate is not very high, apart in the area more exposed to the rain washing and the touching. The alloy composition has a leading influence on the corrosion rate, even though the conservation history my play even a more relevant role. Different part of the same statue can be characterised by very different corrosion rate, according to the exposition and the patina present. The statues coated with the double layer acrylic+wax show very low corrosion rates just after treatment. The coating barrier effect persist on the surfaces less exposed to aggressive weathering effect, while it is completely removed after two years on the other surfaces. Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University The EIS technique:  The EIS technique Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University The EIS technique:  The EIS technique EIS spectrum can provide information about electrochemical processes... …and interface properties C.Gabrielli “Identification of Electrochemical Processes by Frequency Response Analysis”, (1998) http://accessimpedance.iusi.bas.bg/vlab2/e-school/tech04.pdf A.Amirudin, D.Thierry “Application of electrochemical impedance spectroscopy to study the degradation of polymer-coated metals”, Prog. Org. Coat. 26 (1995) 1-28 Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University The EIS technique:  a new measurement method suitable for cultural heritage application is under development WE CE REF Cloth soaked with electrolyte Contact probe The EIS technique WE Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University Slide17:  Comparative study of Protective Coating Systems for OUTDOOR bronze Sculpture Parallel exposure start: June 2001 The EIS technique Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University Slide18:  Cast Bronze (Cu 90 -Sn 8 -Pb 2) Coupon 3x3 cm and 6x6 Polished with metallographic papers (up to grit 1200) Degreased in methanol (ultrasonic bath) Rinsed with de-ionised water Dried in air Exposure angle 45°, facing south Comparative study of Protective Coating Systems for OUTDOOR bronze Sculpture The EIS technique [A] - Soter 201LC [B] - R21 [C] - Tromm TeCe 3534F [D] - Incralac [E] - Incralac+Soter [F] - Incralac+R21 Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University Slide19:  Comparative study of Protective Coating Systems for OUTDOOR bronze Sculpture The EIS technique Letardi P., in: J. Ashton, D. Hallam (Eds.), Proceedings of Metal04, National Museum of Australia, 2004, pp. 379-387 The results obtained show that the coating [D] offer no advantage, after only 1 year, with respect to the waxy coatings [A], [B] and [C], which have the advantage of a better accepted aesthetic appearance and are known to give less problem concerning reversibility and re-coating. The wax [B], both alone and in the double layer system [F], is the product showing the more different performances on coupons and on the monument area III. The different trends observed for the selected coatings on the polished bronze coupons and on the monument area III should be considered in more detail. The patina, which is present between the metal and the coatings applied on the monument, is supposed to be the main source for the differences observed on coating behavior with respect to the polished bronze coupons Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University EIS on the Monument to Bartolomeo Colleoni (Andrea del Verrocchio, 1496):  EIS on the Monument to Bartolomeo Colleoni (Andrea del Verrocchio, 1496) During the pre restoration work, five homogeneous Areas have been identified. In each Area three 5x5 cm zones have been delimited: one (A) has been left untouched; one (B) has been washed; one (C) has been washed and coated with a double layer system Incralac+Wax. The EIS technique Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University The EIS technique:  The EIS technique EIS on the Monument to Bartolomeo Colleoni (Andrea del Verrocchio, 1496) For zones (A) 5A>3A>2A>4A>1A For zones (B) 5B>(3B» 2B)>(4B» 1B) For zones (C) 3C>4C>(2C» 1C» 5C) untreated washed washed + Incralac + wax Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University The EIS technique:  The EIS technique EIS on the Monument to Bartolomeo Colleoni (Andrea del Verrocchio, 1496) Black patina Black patina Green-grey patina Evolution with time on selected patinas Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University The EIS technique:  The EIS technique EU-ARTECH / JR1 evaluation of new treatments for outdoor bronze monuments Incralac Tromm TeCe 3534F Comparison of coatings on natural patina Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University Conclusions:  Conclusions Polarisation Resistance Rp medium cost instrumentation. Quick enough measurements. More standardised methods and evaluation of applicability limits would be worthwhile . Electrochemical Potential Eoc cheap and easily available instrumentation. More detailed analysis on interpretation (inhomogeneous alloys and patinas) would be advisable. Unsuitable for coated surfaces. Electrochemical Impedance Spectroscopy expensive instrumentation. Medium-long acquisition time. Suitable for in-situ analysis of coatings and electrochemical properties of patinas. Interesting in-situ application have been reported. None has still become widely applied. …further work is needed to better exploit applications of these techniques to metal conservation Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University Acknowledgements:  Acknowledgements M.Montini (CNR-ISMAR) G.Gaggero (CNR-ICB) G.Cutugno (CNR-ISMAR) G.Morigi (restorer) M.Marabelli (ICR) G.D’Ercoli (ICR) M.F. Giubilei (Settore Musei – Comune di Genova) P.Donati (Soprintendenza PSAE della Liguria) R.Mazzeo (Bologna University) C.Bartuli (Rome University) Meeting of the ETIC Sub-WG – 13 March 2006 – Ghent University

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