3.1. Sculpture Nedovršena igra (Author Gabrijela Kolar—1965)
The sculpture Nedovršena igra (NI) is made of quaternary Cu–Sn–Zn–Pb bronze. Optical microscopy revealed that the color and the surface morphology of patina at each area was quite different (
Figure 3). XRF measurements were conducted at all areas of interest and the obtained compositions are shown in
Table 1. Studies conducted by Robotti et al. [
28] and Šatović et al. [
29] showed that composition determined by XRF measurements on corroded bronze samples are influenced by the composition and thickness of corrosion products layer. For very thick patina layers, XRF results mainly show the composition of patina rather than composition of underlying bronze substrate. In this work, thickness of the patina layer was only qualitatively assessed during the patina scraping. The thickest patina layer was observed on NI1, followed by NI3, while the patina at NI2 and NI4 was much thinner.
Analyzed areas on bronze surface revealed the presence of Sn, Pb, and Zn, as well as some Fe. At several areas, significant amount of Al was also found. It is not uncommon that small amounts of Al are added during bronze casting [
30], but such high Al content may also be a result of alumina deposition on bronze [
10]. For many years, a steel production plant was operating in Sisak; such plants typically generate fly ashes that can contain up to 40% of alumina. In favor of this assumption is the fact that at point 1, which is a vertical surface, slightly sheltered from the wind, no Al was detected, while the highest amount of Al was observed at point 3, which is horizontal. At this rough surface, the deposition of airborne particles is more likely to have occurred. On the other hand, lack of Al at NI1, where patina layer appeared to be the thickest and XRF results were more influenced by patina composition than on other spots, may indicate that Al is present in a bulk alloy.
EDS examination of patina collected at point 1 of NI was performed in several spots and in principle two different compositions were observed. One with 53 wt % of Cu, 24 wt % Sn, 11 wt % Pb, 9 wt % of C, 3 wt % of S, and almost no oxygen content, and the other with a high amount of oxygen (29 wt %) and carbon (5 wt %), and a small amount of Sn (1.3 wt %), Ca (4.7 wt %), and Si (1.4 wt %). Aluminum was also present in some spots (max. 2 wt %), although it was not observed with XRF.
In order to explain the observed differences in patina appearance, we sampled a small amount of patina powder and characterized it by Raman and ATR-FTIR spectroscopy. Raman spectra of corrosion products collected at point 1 showed the presence of several bands positioned at lower wavenumbers 148, 203, 702, and 1083 cm
−1 (
Figure 4 and
Table 2). Similar spectra were recorded for different carbonate minerals [
31]. For copper carbonate minerals, Raman spectrum exhibits bands at 144, 215, 739, and 1096 cm
−1 (azurite) and 142, 205, 717, and 1096 cm
−1 (malachite) [
32,
33]. Thus, Raman spectrum obtained for NI1 patina indicates that mixture of carbonate compounds was probably present. Taking into account the complex composition obtained by EDS, we found it was also possible that patina contained carbonates not only of copper but of other elements too.
ATR-FTIR spectrum of this patina sample is given in
Figure 5. Broad bend in 3200–3500 cm
−1 region can be attributed to OH stretching. Two bands were observed at 1456 and 1415 cm
−1 that were probably related to the presence of carbonates [
34].
Spectroscopic analysis showed that carbonates were present in the analyzed patina, while from EDS analysis, the strong enrichment in Sn was detected, which can point at the possible selective dissolution of copper commonly observed in corrosion of bronze at non-sheltered areas [
35].
Table 2.
Raman bands (cm
−1), identified on patinas, analyzed at different positions of the statue NI (
Figure 4) and literature data [
32,
36,
37,
38,
39]. Letters next to a number denote strength of the band: vw (very weak), w (weak), s (strong), and vs (very strong).
Table 2.
Raman bands (cm
−1), identified on patinas, analyzed at different positions of the statue NI (
Figure 4) and literature data [
32,
36,
37,
38,
39]. Letters next to a number denote strength of the band: vw (very weak), w (weak), s (strong), and vs (very strong).
NI1 | NI2 | NI3 | NI4 | Cu2O [8,37,38] | Cu2S [37,38] | Malachite [32,37] | Azurite [32] | Brochantite [36] | Atacamite [39] |
---|
| | 87 | 88 (s) | | | | | | |
148 | 144 | | | 153 | | 142 | 144 | 141 | |
203 | | | | | | 205 | 215 | | 194 |
| | 226 | | 220 | | | | | 236 |
| 276 (s) | | 280 (s) | | 281 | | 281 | | 271 |
| | 306 | | 309 | | | | | |
| | 398 (w) | 397 (w) | | | 398 | | | 363 |
| 410 (w) | | | | | | 414 | 415 | 422 |
| | | 465 (w) | | 472 | | | 467 | |
| 523 | | 523 (s) | 523 | | 531 | 540 | 517 | |
| 608 (s) | | 608 (s) | 628 | 603 | | | 608 | |
702 | | | | | | 717 | | | |
| | 986 | | | | | | 990 | 976 |
1083 (s) | | | | | | 1096 | 1095 | | |
| | 1344 (s) | | | | 1364 | 1415 | | |
| | 1582 (s) | | | | 1576 | | | |
| | 3615 (vw) | | | | | | | |
| | 3622 (vw) | | | | | | | |
The second examined position on NI had a well-adhered green patina layer (
Figure 4b). EDS analysis showed some variations in patina composition; in some spots, an increase of Sn contents was observed, while the oxygen content was between 57 atom % and 61 atom %. Spots with lower Sn content had higher carbon and sulfur content. Raman spectra (
Figure 4b) revealed bands at 144 cm
−1 related to O–Cu–O bending, and at 523 and 608 cm
−1, characteristic of M–O stretching vibrations, to Cu but also to Sn oxides [
8]. Strong band positioned at 276 cm
−1 points at possible presence of Cu
2S, which also exhibits bands at 603 cm
−1. The presence of sulphate compounds cannot be excluded as for brochantite and antlerite Raman spectrum exhibits high intensity band at 415 cm
−1, as well as the bands at 141 and 608 cm
−1 [
36].
FTIR spectra (
Figure 5) revealed a broad peak at 992 cm
−1 that can be assigned to sulphate compounds, as well as the peak at 1098 cm
−1 [
40]. The peak at 1375 cm
−1 indicated that some organic compounds may have been present too.
EDS analysis of the bluish patina from spot 3 on NI showed (
Figure 1, spot 3) the presence of significant amounts of O and S (23 atom % of Cu, 58 atom % of O, and 16 atom % of S) in some spots, while in other spots high C content instead of S was observed. This could be either due to the presence of organic contaminants or carbonates in patina. Additionally, various amounts of Al (up to 4 atom %), Si, and Ca (up to 3 atom %) were observed.
Raman spectrum exhibited bands at 226, 380, and 479, and hindered band at 610 cm
−1 as well as characteristic band at 978 cm
−1 are similar to those observed in bronchantite and langite [
36]. The bands above 3600 cm
−1 are not commonly observed in Raman spectra of patina samples but have been found in complex silicate minerals such as chrysocolla [
41]. Additional bands at 1334 and 1582 cm
−1 could be due to the presence of graphite ashes. These bands could also originate from carbonates, which are likely to be present as 1415 cm
−1 band was observed in FTIR spectrum. Still, taking in account that fountain water contained a small amount of chloride compounds, the presence of chloride patina cannot be excluded, since the bands at 226, 398, and 986 cm
−1 could also have originated from chloride compounds such as atacamite.
Patina sampled in point 4 of NI also exhibited complex composition. Part of the powder contained up to 59 atom % of Sn and 38 atom % of O, while the other part was mainly composed of Cu (26 atom %), O (54 atom %), C (12 atom %), and S (3 atom %). Al was observed at all spots, but higher amounts were measured in spots containing mainly Cu and O, which would correspond to outer patina layer where weight percentage ratio was Cu/Al = 29:5.9. Such high Al content, compared to that observed by XRF, can be considered as a confirmation that Al was deposited from airborne particles, besides being present in a bulk alloy. EDS analysis of patina sampled from NI did not reveal Fe, and thus Fe content observed by XRF was related to its presence in alloy. Raman spectra (
Figure 4b) recorded on several samples from the same point also showed some differences, but observed bands at 465, 523 and 608 cm
−1 can be ascribed to presence of copper and tin oxides, whereas strong band at 280 cm
−1 points to the presence of Cu
2S [
37,
41]. On the other hand, FTIR spectrum (
Figure 4) was similar to that obtained in point 2, which was ascribed to copper carbonates and sulphates.
Analysis of the patina samples from NI showed that natural patina form on the sculpture was very complex and contained various compounds that formed during 55 years of exposure to polluted atmosphere and splashing with fountain water. Moreover, each examined area exhibited different color and morphology due to the difference in composition. Patina in point 1 was more typical of patina found on outdoor bronzes in sheltered areas, with inner patina layer rich in Sn compounds and outer layer rich in copper corrosion products, mainly carbonates forming in contact with fountain water-leaking upper parts of the sculpture. On the other hand, points 2 and 4 represent unsheltered areas that come in contact with rainwater as well as fountain water. In these spots, enrichments of patina with tin oxides was observed. In point 3, bluish, easily removable patina was studied, being composed of copper sulfates and carbonates as well as silicates. Such patina is probably the result of dissolution of bronze in the upper parts of the sculpture and precipitation of corrosion products on the bottom of the statue.
3.2. Sculpture at Memorial Child Cemetery (MC) (Author Milena Lah—1974)
The second studied sculpture is made of quaternary Cu–Sn–Zn–Pb bronze (
Table 3) with similar Sn content, lower Pb, and higher Fe content when compared to the first bronze sculpture. The presence of Fe usually means that bronze was casted from recycled alloys [
30]. As in the case of Al, it was not clear if Fe content was only due to the initial content in alloy or if it was influenced by the pollution from the nearby steel factory. Studies were conducted at three points, as shown in
Figure 2.
Optical micrographs (
Figure 6) showed relatively similar appearance of bronze surface in points 1 and 3 with outer layer of green patina and inner dark patina, while the surface in point 2 was covered by red and blue patina layers as observed by the naked eye and optical microscopy.
In point 1, EDS revealed two distinctive patina compositions, one mainly containing copper (27 atom %), oxygen (59 atom %), sulfur (3 atom %), and carbon (10 atom %) with some remaining Al and Si. The second kind of patina composition observed had low oxygen content but high Sn content and some sulfur. In Raman spectrum shown in
Figure 7a, bands at 191 cm
−1; 246 cm
−1 triplet band at 386, 479, and 610 cm
−1; strong band at 974 cm
−1; and broad band at 1102 cm
−1, accompanied by characteristic bands at 3402 and 3586 cm
−1, point to the presence of posnjakite or langite (
Table 4). FTIR spectrum (
Figure 8) also exhibited absorption maxima typical of copper sulphates [
42,
43].
The appearance of a second examined spot was much more different. It was placed on horizontal part of the sculpture and one must take in account possibility of mechanical removal of the loosely adhering patina from the surface if someone stepped on the sculpture. As observed in studies simulating bronze outdoor corrosion [
8], Cu- and Zn-selective dissolution accompanied to the formation of a tin-species network within the corroded structure occur on quaternary bronzes and may result in formation of structures similar to those in
Figure 6(2). Raman spectrum (
Figure 7b) exhibited very few peaks, mainly related to M–O stretching vibrations. From the FTIR spectrum (
Figure 8), we see that existence of some sulphate patina seems likely.
Both Raman (
Figure 7c) and FTIR spectra (
Figure 8) on patina from point 3 were similar to those from point 1, which leads to the conclusion of almost identical patina composition with inner layer mainly composed of sulfide compounds and with increased tin content, as well as outer layer with sulphate patina.
Cuprous sulfide found on analyzed patina could be due to initial artistic patination with liver of sulfur (K2S) or due to the presence of H2S in the atmosphere. The source of Cu2S presence could not be revealed. EDS analysis was conducted at several spots of each sampled patina powder, and various amounts of Fe and Al were observed for the same position. Sometimes there was Cu/Al or Cu/Fe ratio exceeding that observed by XRF, which would be in favor of their airborne deposition, and sometimes there was no Al and Fe, which would be in favor of its main source from the bulk alloy. Thus, it may be assumed that Al and Fe, observed by XRF, originated from both airborne particle deposition and the alloy itself.
3.3. Electrochemical Impedance Spectroscopy Studies
EIS measurements were conducted using an agar cell. NI studies were conducted in points 1, 2 and 4, while point 3 did not have a sufficiently flat area to achieve good contact between the agar and sculpture surface. Obtained impedance spectra are given in
Figure 9. Impedance modulus values at lowest measured frequency (usually 10 mHz) is often examined as a measure of material corrosion resistance, especially in the case of complicated EIS spectra obtained on patinated samples that are not easy to model with equivalent electrical circuits models. Survey of EIS measurements on copper and bronze patinas, conducted using contact probe setup [
44], revealed that in the studied set of bronze objects, surfaces exposed to marine environment or those in rain-washed areas with bronchantite exhibit impedance modulus lower than 17 kΩ cm
2, while the surfaces with the highest corrosion resistance exhibited impedance modulus between 0.3 and 1 MΩ·cm
2. The composition of patina on these surfaces was similar to those with lower corrosion resistance. Higher impedance values were observed only on surfaces with remains of protective coating. Ramirez et al. [
45] used agar-based electrolyte for studies of bronze sphinxes, wherein they examined two surface areas, one with dark patina with impedance moduls around 10–20 kΩ·cm
2 and another with green patina layer exhibiting impedance modulus around 30–50 kΩ·cm
2. In our study, spectrum obtained for NI1 exhibited the highest impedance modulus (0.14 M Ω·cm
2) compared to other two surface areas. Indeed, such value of impedance modulus puts the examined surface in a group of patinated surfaces with higher corrosion resistance, i.e., the analyzed surface, with outer carbonate layer and inner tin enriched layer, which provides protection to underlying bronze substrate.
For the other two examined spots, lower impedance modulus values were observed. All spectra exhibited two-phase angle maxima in Bode plot. Various EEC have been applied in the literature to describe EIS spectra obtained on patinated metals [
19]. For spectra with two-phase angle maxima, usually a model with nested (R-Q) circuit is used, such as the circuit shown in
Figure 10. In this model, R
el represents electrolyte resistance between working and reference electrode; high frequency data are described by R
1, which represents corrosion product layer resistance, and Q
1, which is constant phase element describing capacitive property of corrosion products layer; and the medium frequency part of the EIS spectra is represented by R
2—charge transfer resistance, and Q
2—constant phase element, describing double-layer capacitance, where n
1 and n
2 are coefficients describing the non-ideal capacitive behavior. Such a model is applicable for patina layers with pores through which an electrolyte can reach the metal surface. A similar model has been applied in other studies examining copper samples upon 3 years of exposure to atmospheric corrosion [
46]. Sometimes the second R-Q couple is considered to be the response of the inner patina layer, rather than the response of bare bronze [
47]. Considering the fact that EDS data point towards the existence of an inner tin oxide layer, such interpretation cannot be completely excluded, although low n
2 values for NI1 and NI3 are in favor of the first interpretation.
Obtained impedance parameters are given in
Table 5. Patina layer resistance is the highest for the NI1 spot, in sheltered area where composition is dominated by the presence of carbonates, while in unsheltered areas with dominantly sulphate (NI2) or sulfide patina, lower
R1 value was observed. The charge transfer resistance value was also the highest for NI1 with the lowest
Q2 value, which confirmed that this type of patina provided the best protection to underlying substrate. However, the optical microscopy of NI1 showed that the surface at this spot was not homogenous and that some defects in the structure existed. It should be taken in account that measurements were conducted with agar gel, and in a case of deeper craters in the surface, the wetting of the bottom of the craters might be limited. Thus, the observed resistance values were more representative of the area with compact patina layer. At two other examined spots, such defects were not observed.
Impedance spectra obtained on MC are given in
Figure 11. It can be immediately observed that the values of impedance modulus at the lowest frequencies at all examined spots were lower than on NI. Additionally, these EIS spectra exhibited three phase angle maxima in Bode diagrams, more or less clearly resolved. Spectra were modeled with electrical equivalent circuit shown in
Figure 12. It has additional time constant compared to the one in
Figure 9 (R
3-Q
3). In our previous studies on chemical and electrochemical artificial patina, these two elements were allocated to the Faradaic resistance and Faradaic capacitance, implying the oxidation–reduction processes of the corrosion products (i.e., the outer patina layer) [
48,
49]. As can be seen from the data given in
Table 4, n
3 values obtained were quite low, around 0.5, indicating that this time constant was more likely related to diffusion processes inside the patina layer.
EIS parameters given in
Table 6 showed that the resistance of patina layer on all studied spots at MC was lower compared to that on NI, as well as the charge transfer resistance. Patina layer resistance was the lowest for MC2 with the highest Q
1 value, indicating the thinnest patina layer. The impedance at all MC spots was dominated by diffusion impedance, indicating the presence of narrow pores in the layer. For MC1, EIS parameters were not provided as it was not possible to obtain good fit between experimental data and model in lower frequency range.
The EIS results revealed that electrochemical behavior of patinas observed on NI and MC were different, which can be attributed to the difference in patina morphology and composition. Obtained impedance parameters pointed towards the presence of fine pores that limited the diffusion of O2 in MC patina layers, while the structure of patina on NI would be more compact but with the presence of bigger pores. Part of the explanation on such differences is surely related to the influence of fountain water on NI, as calcite and other carbonate minerals may induce plugging of fine pores. It is also interesting to notice that MC is closer to the oil refinery than NI, which is in the city center. Thus, MC was exposed to higher H2S concentrations, which may have also been the cause of lower corrosion resistance of MC. From the obtained results, it appears that areas with dominant carbonate patina exhibited higher corrosion resistance than those with sulfate or sulfide patina.
It is important to notice that similar chemical composition of patina at two different spots does not necessarily result in the same electrochemical response, as was observed for MC1 and MC3. Raman and FTIR spectra of patina sampled at these two points were almost identical, but EIS spectra were quite different. For MC1, impedance modulus values at all frequencies were several times higher than that of the MC3, and thus the corrosion resistance of the surface at point MC1 was higher. Differences were also observed in a shape of the phase angle plot. The possible reason for such difference could be patina layer thickness, as point 3 lies in an unsheltered area where dissolution of patina is more intense than on sheltered MC1 area where thicker patina layer may build. Increased corrosion resistance of bronze is often associated with the formation of the tin oxide layer. However, in our study, at all studied points, tin oxides were found by EDS, while XRF measurements did not reveal significant difference in Sn content between examined spots.