Use of electrochemical impedance spectroscopy on outdoor bronze monuments. Paola Letardi CNR - ISMAR - Genova

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1 Use of electrochemical impedance spectroscopy on outdoor bronze monuments Paola Letardi CNR - ISMAR - Genova

2 Introduction Outdoor bronze monuments The EIS techniques EIS measurements on bronze statues Instruments and methods Case studies Conclusions

3 Bronze Monuments age, composition, finishing weathering conditions and exposure environmental pollution past conservation treatments Patinas - crusts Aesthetic appearance Stability Cleaning procedure Protective treatments Maintenance The number and the range of all the parameters involved produces a very wide range of possible situations

4 The EIS technique (Electrochemical Impedance Spectroscopy) Perturbation: E(t)=E 0 cos(ω t) System response : I(t)=I 0 cos(ω t-φ ) Response function: Z (ω) = E/I

5 The EIS technique and interface properties EIS spectrum can provide information about electrochemical processes... C.Gabrielli Identification of Electrochemical Processes by Frequency Response Analysis, (1998) A.Amirudin, D.Thierry Application of electrochemical impedance spectroscopy to study the degradation of polymer-coated metals, Prog. Org. Coat. 26 (1995) 1-28

6 Instruments and methods Potentiostat E I FRA WE REF CE Contact probe REF CE WE Personal Computer Electrolyte Cloth soaked with mineral water a new measurement method suitable for cultural heritage application is under development

7 Instruments and methods CE: REF: ST Contact Probe Stainless Steel AISI 316 Stainless Steel AISI 316 inserted in PTFE CE = 1.5 cm A= 1.77 cm 2 REF CE cable REF cable Letardi P., in: P.A.Vigato (Ed.), The Science of Art, Outdoor bronze protective coatings: characterisation by a new contact-probe Electrochemical Impedance measurements technique, Libreria Progetto di Padova, 2002, pp

8 Instruments and methods Transportable instrumentation has been used Gamry FAS1 Femtostat Measurements conditions: frequency range: applied potential: acquisition: 100Khz-10mHz 10 mv rms 10 points/decade EIS spectra acquisition time 20-45min Gamry Ref600

9 EIS measurements on bronze statues Case studies

10 EIS on bronze statues Monumento ai Mille (E.Baroni( E.Baroni,, 1915) Genova, Scoglio di Quarto Letardi P., in: J. Ashton, D. Hallam (Eds.), Proceedings of Metal04, National Museum of Australia, 2004, pp

11 Monumento ai Mille [A] - Soter 201LC [B] - R21 [C] - Tromm TeCe 3534F [D] - Incralac [E] - Incralac+Soter [F] - Incralac+R21 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

12 Monumento ai Mille Z ν=10mhz [Ωcm 2 ] C M - Area III [A] C t exposure [months] M - Area III [B] C t exposure [months] M - Area III [C] [D] C M - Area III [E] C M - Area III C M - Area III [F] 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 Z [Ωcm 2 ] Coupons 11m Coupons 23m Monument Area III - 11m Monument Area III - 23m frequency [Hz] [F] t exposure [months] Letardi P., in: J. Ashton, D. Hallam (Eds.), Proceedings of Metal04, National Museum of Australia, 2004, pp

13 EIS on bronze statues Cristo degli Abissi (G.Galletti,, 1954) Baia di S.Fruttuoso di Camogli,, (Genova) Letardi P., in: F. Simonetti (Ed.), Il Cristo degli Abissi 50 anni di storia, Tormena 2004, pp

14 Cristo degli Abissi C D F E A B

15 Cristo degli Abissi Nyquist Plot Zim [Ω] Even though a deeper analysis would be required, the shape of Impedance spectra on the measured cleaned areas suggests that the various corrosion processes, which may occur, have slight different relevance on the different kinds of patina. The small spread in the low frequency values of Impedance modulus indicates that the corresponding corrosion rates are quite similar. Corrosion rate is not varying very much among the measured areas, even though it is a little smaller in areas A1,A2, C2. Data on encrusted area B1 should have required further controls, in order to better evaluate the effects of the high surface roughness, water uptake by the highly porous calcareous encrustation, etc. Z [Ω] Φ [degree] Z C2 re [Ω] C1 Bode Plot A2 A B2 B frequency [Hz]

16 EIS on bronze statues Monumento a Bartolomeo Colleoni (A.Verrocchio,, 1496) Venezia,, Campo Santi Giovanni e Paolo CSA, EAS06, Restorer Somerset, Giovanni November Morigi 13-14, 2006

17 Monumento a Colleoni 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.

18 Monumento a Colleoni Area 1 Horse; neck right side. Rain washed area. Grey-green patina Area 2 Horse; back. Area hit by rain. Green patina. Area 3 Area 3 Horse; left thigh. Almost sheltered area. Black patina.

19 Monumento a Colleoni Area 5 Area 5 Horseman; left leg. Sheltered area. Black patina Area 4 Horseman; left shoulder. Rain washed area. Green patina

20 Monumento a Colleoni Z [Ω] Area C11 AreaC22 Area C33 Area C44 Area C55 A B C A B C A B C A B C A B C Z [Ω] Φ [degree] frequency [Hz] frequency [Hz] frequency [Hz] frequency [Hz] frequency [Hz] Φ [degree] Z ν=10mhz [Ω] untreated 10 8 washed washed + coated Area For zones (A) C5>C3>C2>C4>C1 For zones (B) C5>(C3 C2)>(C4 C1) For zones (C) C3>C4>(C2 C1 C5) untreated washed washed + Incralac + wax

21 Monumento a Colleoni Z [Ω] Φ [degree] frequency [Hz] Z im [Ω] -250x10 0 Z im [Ω] -2,5x10 3 Z [Ω] Area 1 1B (0.5 h) 1B II (1.5 h) 1B III (11 h) x x x10 0 0,0 0,0 2,5x10 3 5,0x10 3 7,5x Z re [Ω] Green-grey patina Evolution with time on selected patinas Area 3 3B (0.5 h) 3B II (1.5 h) 3B III (10.5 h) Z [Ω] Area 5 5B (0.5 h) 5B II (1.6 h) 5B III (11 h) Black patina Black patina Φ [degree] frequency [Hz] -1x x x x10 3 Z re [Ω] Zim [Ω] Zim [Ω] 0 0 1x10 3 2x10 3 Φ [degree] ,5x ,0-60 0,0 2,5x10 3 5,0x10 3 7,5x x frequency [Hz] Z im [Ω] Z im [Ω] 0 25x x x10 3 Z re [Ω]

22 EU-ARTECH / JR1 evaluation of new treatments for outdoor bronze monuments Comparison of coatings on natural patina Z 10mHz [MΩ] none T9 T6 T5 T3b T3c T3d T1 T2 T4 Sceptre Coupons none T9 T6 T5 T3b T3c T3d T1 T2 T4 Coating Improved protection Negligible effect Vcorr increase Vcorr decrease Tromm TeCe 3534F Incralac

23 Conclusions During the last five years in situ EIS measurements on bronze monuments have been improved Measurements on monuments are prone to several parameters which are out of control, thus statistical analysis of results is recommended Field of interest for EIS application to outdoor bronze conservation are: - electrochemical properties of patinas - evaluation of cleaning procedure - testing coating system effectiveness further work is needed to better exploit applications of this technique to metal conservation

24 Conclusions #Patinas on a single monument could have different properties and corrosion speed # The same coating system could have different effectiveness according to the underlying patina

25 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)..many thanks to the organizers of this meeting!