3.1. The Effect of Aging Conditions on EIS Results
Firstly, the corrosion behaviour of patinated bronze was examined in artificial acid rain solution (pH = 5).
Figure 1a,b shows the evolution of the EIS spectra for CuSn12 bronze after continuous immersion in artificial acid rain solution (pH 5) for three weeks. An increase in impedance modulus with time was observed. On the first day, the value of impedance modulus at the lowest frequencies was less than 1 kΩ·cm
2. Such a low value indicates that the patinated bronze surface was very reactive. On the second day of immersion, a small increase in impedance modulus was observed, while after one week, it increased significantly. Thus, it can be concluded that the corrosion resistance of the patinated CuSn12 bronze is improved with the exposure to acid rain solution. The phase angle plot initially shows the presence of a maximum at low frequencies. Moreover, the EIS spectrum for the second day reveals the existence of a poorly resolved capacitive behaviour at high frequencies.
In order to obtain more detailed information from the EIS results, the spectra were fitted to selected equivalent electrical circuits. For the spectra obtained on the first and the second day of immersion, it was necessary to use the equivalent electrical circuit with two time constants in order to obtain a good fit at medium and low frequencies. For high frequencies, it was not possible to obtain an accurate fit as the phase angle maximum was not well resolved. The selected model is shown in
Figure 2a.
In this model,
Rel defines the electrolyte resistance between the working and reference electrode.
Rct describes the charge transfer resistance, while
Qdl is a constant phase element related to the double-layer capacitance, referring to the medium-frequency data. Lower-frequency data are represented by
RF—faradaic resistance associated with oxidation–reduction processes involving the reactive patina layer and
QF—constant phase element representing faradaic capacitance. After a week of exposure, the maximum in the phase angle plot at high frequencies was expressed more. The evolution of this new maximum requires the addition of a new loop, developing the equivalent circuit presented in
Figure 2b. The additional
Rox-
Qox loop represents the inner oxide layer (non-reactive patina layer) [
13,
24]. The presented model was previously used for describing EIS spectra of CuSn6 bronze covered with sulphate patina by Marušić et al. [
13]. They found that the spectra of samples with sulphate patina exhibited an additional peak at the highest frequencies, which was not observed on bare bronze, thus indicating the presence of a new layer: the non-reactive patina layer. Detailed explanations and mathematical descriptions of applied models can be found in the literature [
13].
The parameters of EIS modelling of CuSn12 bronze are listed in
Table 2. The parameters for the initial state show very low
Rct and high
Qdl values. Similar values were observed by Yuan et al. [
20] when examining copper–nickel (70/30) corrosion in sulphide-containing seawater. It is possible that in contact with an acid rain solution, sulphide patina releases some corrosive S
2− ions. It is also quite likely that the patination solution may have remained in the pores of patina layer. In general, it is known that copper and its alloys are sensitive to the presence of sulphides [
25,
26,
27,
28]. In our case, this would mean that the corrosion of the underlying bronze substrate is enhanced by the presence of sulphides. During exposure to rainwater, sulphides are removed from the surface, probably by diffusion into the solution or by forming stable corrosion products. The lower amount of aggressive compounds leads to an increase in charge transfer resistance and decrease in double layer capacitance values.
A high QF value, on the first day of measurement, indicates the presence of reactive patina compounds. Over time, a decrease in QF and increase in RF values was observed. This can be ascribed to the transformation of the patina, where the amount of reactive species decreases while the patina layer becomes more stable. In addition, after a week of exposure, the formation of an internal oxide layer was observed; its resistance (Rox) increased over time, showing that the oxide layer becomes more protective as well as thicker (decrease in Qox).
Similar behaviour was observed for the other two types of bronze. Their EIS spectra are shown in
Figure 3 (CuSn6) and
Figure 4 (RG7). The impedance modulus increased with immersion time for both alloys. Initial EIS spectra are quite similar for both alloys, whereas after the three weeks, the CuSn6 bronze exhibited higher impedance modulus values. The increase in impedance modulus is the result of a transformation and stabilization of the patina layer, which is also evident from the changes in the phase angle plots. For RG7, the transformation was slower, while the CuSn6 bronze had already shown clear maxima at medium and high frequencies on the seventh day of immersion.
The EIS spectra for RG7 and CuSn6 bronze were fitted to the same models used for the CuSn12 bronze shown in
Figure 2. The obtained impedance parameters are given in
Table 3 (CuSn6) and
Table 4 (RG7). In general, the time evolution of the resistive and capacitive values is similar to that of the CuSn12 bronze, namely, increases in
R values and decreases in
Q occurs. Initially, the RG7 bronze showed the highest
Rct value, which slightly decreased on the second day. The higher
Rct value could be considered to be a result of the higher resistance of RG7 bronze towards the corrosion attack of sulphide ions, but it is also possible that release of sulphides from patina was slower such that this effect was not visible during the first measurement. On the 21st day of the immersion, the highest
Rox and the lowest
Qox values were observed for CuSn12 bronze. This can be explained by the fact that this type of bronze contains the highest amount of Sn, which is known to easily form a stable oxide layer [
29,
30]. On the other hand, the resistance of the reactive patina layer (
RF) increased the most for CuSn6 bronze. Gianni et al. [
21] made similar observations when examining the corrosion of bronzes in an urban environment, in that bronze with lower tin content exhibited a higher corrosion rate compared to high-tin bronze, but the patina that formed on the lower-tin bronze was more stable.
The second type of patina aging was conducted by alternating wet and dry conditions, as in the case of exposure to outdoor conditions. The EIS spectra of measurements for sulphide patinated CuSn12 bronze obtained during alternating wet/dry cycles are shown in
Figure 5. An increase in impedance modulus in time is present, although with small changes between cycles. After six cycles, there are no significant changes in the shape of the phase angle curves, which are similar to those measured for short-term immersion during continuous exposure to acid rain solution. The equivalent electrical circuit in
Figure 2a was used to fit EIS spectra. The obtained impedance parameters are shown in
Table 5.
Here, the increase in
Rct and
RF values in time is visible but not as pronounced as for continuous immersion conditions. The comparison of
RF values with those obtained during continuous immersion shows that a longer wetting time leads to better stabilization and higher corrosion resistance. The high value of
QF indicates the presence of a large amount of reactive species (sulphides). It can be concluded that a longer wetting period enhances the removal of reactive species from the surface and transformation of the reactive patina into a more stable patina layer. These experiments were also performed on CuSn6 and RG7 bronze. The evolution of
Rct and
RF with number of aging cycles for these two bronzes is presented in
Figure 6. A small increase in resistance values was observed for RG7 bronze, while for CuSn6 bronze, only some fluctuations were observed. Therefore, for all studied bronzes, more significant improvement in corrosion properties occurred under continuous wetting conditions.
The following experiments were also conducted under continuous immersion conditions but at a higher pH (6.5), simulating less corrosive rainwater. The EIS spectra for studied CuSn12 bronze during continuous immersion in simulated urban rain (pH 6.5) are shown in
Figure 7, with those for CuSn6 bronze in
Figure 8 and those for RG7 in
Figure 9. As in the case of a more aggressive solution (pH 5), the impedance modulus increases over time for all samples. However, the increase between the first and the second day is more significant than in the pH 5 solution. This indicates that a less corrosive medium enhances the stabilization of sulphide patinated bronze. The evolution of phase angle plot is similar to that in the pH 5 solution, but with a more expressed phase angle maxima. For this reason, the same models were selected for fitting EIS data (
Figure 2). The parameters obtained for CuSn12 bronze are given in
Table 6, with those for CuSn6 in
Table 7 and those for RG7 in
Table 8. They indicate an increase in
Rct values compared to more acid medium, but a more important difference was observed in
RF values. For all studied bronzes, the
RF values are higher than in the pH 5 solution. On the other hand, the
QF values are similar to those in the pH 5 solution. This implies that the amount of reactive patina is similar, but its resistance is probably higher due to the lower aggressiveness of the pH 6.5 medium.
EIS spectra for alternating wet/dry cycles for CuSn6 bronze at pH 6.5 are shown in
Figure 10. This type of aging resulted in negligible changes in impedance modulus over time, but impedance values are higher than at pH 5 (
Table 6). In addition, sulphate patinated samples were exposed to the corrosion chamber with NO
2 for 14 days. The phase angle plots for such samples exhibit more expressed peaks at medium and high frequencies. All spectra were fitted with an electrical equivalent circuit in
Figure 2b; parameters are shown in
Table 9.
The patina exhibits higher RF values after exposure to a nitrogen dioxide atmosphere compared to alternating wet/dry cycles, indicating on higher patina resistance (RF). The lower QF value indicates a lower amount of reactive species at the end of exposure, while lower Qdl and higher Rct values are probably due to the lower amount of sulphide compounds on the surface. For cyclic exposure, a high Qdl value was obtained after the last cycle, indicating that a high amount of sulphides was still present. This leads to the conclusion that transformation and stabilization of patinated samples occurred in all studied cases, but in a manner dependent on the wetting time. The longer the wetting time, the greater stabilization and corrosion resistance. Similar observations could be drawn for other types of studied bronzes.
3.3. Patina Composition Modification Characterized by Fourier Transform Infrared Spectroscopy
In order to analyse modifications in patina composition, the samples were characterized by ATR-FTIR spectroscopy and SEM. For freshly prepared patinated samples, there are no clearly visible peaks in the FTIR spectra. The FTIR spectrum shown in
Figure 12 was obtained after continuous immersion of patinated samples in simulated acid rain (pH 5). The strongest peaks were observed for RG7 bronze. The band at 610 cm
−1 may be assigned with the presence of a bond between either copper or tin and oxygen [
9,
31], while the 1071 cm
−1 band can be attributed to SO
42− vibration [
32,
33]. Both peaks indicate the formation of copper and tin oxides as well as sulphate patina, as observed by Kosec at al. [
16]. They concluded that sulphate compounds are formed by sulphide patina transformation. However, in their work, parts of the surface exhibited spectra with bands at higher wavenumbers that could be clearly ascribed to brochanite. Still, their experiment lasted for 35 days, which is longer than in our case. The transformation was less pronounced for CuSn6, and especially so for CuSn12 bronze.
On the other hand, the FTIR spectrum for alternating wet/dry cycles, shown in
Figure 13, indicates the highest transformation for the CuSn12 bronze. One can clearly see a band at 608 cm
−1 as well as bands indicating sulphate patina at 1113, 1149, and 1179 cm
−1 or bands at 1362 and 1427 cm
−1, which could be ascribed to carbonate patina [
32]. The other two bronzes show bands at low frequencies, which represent oxides (629 cm
−1), while the bands that could be related to sulphate patina are not particularly distinguishable [
32]. The FTIR spectra for cyclic and continuous immersion conditions are relatively similar, although EIS indicates bigger differences between these samples. As the ATR-FTIR spectra better reflects the composition of upper than inner patina layer, it can be concluded that for cyclic exposure of patina, the main transformations mainly occur in the upper patina layer.
The FTIR spectra of samples exposed to continuous immersion in simulated urban rain (pH 6.5) are presented in
Figure 14. Patinated CuSn6 samples exhibited a band at 596 cm
−1 related to the presence of oxides. The CuSn12 and RG7 bronzes exhibited similar FTIR spectra. The peak at 826 cm
−1 is also assigned to the presence of oxides [
31]. Some wider bands can be observed at higher wavenumbers, which could possibly be the result of sulphate patina formation.
In the case of alternating wet/dry cycles performed in urban rain, a band at 610 cm
−1 is visible in the FTIR spectra of all bronzes. After exposure to the corrosion chamber with NO
2 atmosphere, the most intensive peak appears at 604 cm
−1. In addition, the formation of sulphate patina is possible based on the appearance of small peaks at 1000 and 1396 cm
−1 [
32].
The results of FTIR investigations show that in all studied aging conditions, there are some changes in patina layer composition, including the formation of oxides and/or sulphate compounds. However, the differences between examined aging conditions are not as clear as those observed by electrochemical measurements. In the case of the continuous immersion, the transformations are probably more intense in the inner patina layer and thus less visible in the FTIR spectra.
3.4. Patina Composition Modification Characterized by Scanning Electron Microscopy
SEM images of prepared sulphide patina samples are given in
Figure 15 (upper part). The patina layer on the RG7 bronze appears to be the most heterogeneous, while the most uniform and the smoothest layer was observed on the CuSn12 bronze. The complex morphology of the RG7 patina is probably the result of the fact that RG7 bronze has the most heterogeneous composition.
The EDS analysis of multiple points on the surface revealed the presence of Cu and S for all bronzes. A high amount of Cu was obtained for all bronzes (54 at.%–64 at.%), reflecting the composition of the patina but also that of the substrate. The S content is highest in the case of CuSn6 at 25 at.%–32 at.%, with RG7 at 24 at.%–26 at.% and CuSn12 bronze at 24 at.%–27 at.%. It is only for RG7 that some O was observed, while Sn was not detected in the case of RG7.
The SEM results depend on the measurement point due to some differences in the thickness of the prepared patina.
Figure 15 (below) presents patinated samples after continuous immersion in simulated urban rain (pH 6.5). The changes in morphology can be observed by the formation of precipitations and cracks. EDS results show that the amount of Cu is decreased (to around 40%) due the thickening and transformation of patina layer. An increase in O content and a decrease in S content was measured for all studied bronzes (an example for CuSn6 bronze is given in
Figure 16). These results are in accordance with FTIR spectra showing formation of oxides and sulphates.
For alternating wet/dry cycles at pH 6.5, the amount of S was lower than at the initial state, while more O was detected. For continuous immersion, the atomic ratio S:O was higher than for wet/dry cycles, which also confirms that in the former case, more significant formation of oxides and sulphates occurred. By comparing continuous immersion in acid rain (pH 5) with the initial state, once again, a decrease in S and increase in O content was observed. In comparison to the less aggressive medium, the S:O ratio was similar for CuSn12 bronze, while for the other two bronzes, the oxygen content was higher for pH 5, which is in accordance with EIS and FTIR results showing more intense transformation of patina in more acid solution.