1. Introduction
Weathering steel (WS), also known by its trade name COR-TEN, is a widely used material in civil engineering for buildings such as bridges, power poles or facades [
1]. They are generally defined as low alloy steels with a total content of alloying elements (Cr, Ni, Cu and P) not higher than 5% weight [
2], with an increased mechanical resistance and the protective effect that rust develops against atmospheric corrosion. Over time a stratified rust forms with two sublayers: an outer layer composed mainly of lepidocrocite, which is porous and not very compact, and an inner layer consisting of chromium-rich goethite of nanophasic size with sufficient compactness to exert a barrier effect and hinder the access of electrolyte to the steel/rust interface [
3].
The requirements for the formation of a protective rust on conventional WS are well known in the scientific literature related to civil structures. Wet/dry cycles and moderate aggressivity atmospheres with low time of wetness and low levels of chlorides (<6 mg Cl
−/m
2d) and SO
2 (<20 mg/m
2d) are necessary to obtain allowable corrosion rates for bare WS [
4]. In addition, the presence of sheltered zones and cavities that allow for the accumulation of water should be avoided in the structural design stage [
5].
Technological advances have influenced artistic movements in the search for new materials for the creation of artworks. The use of WS in sculpture began in the early 1960s. Artists were looking for new materials with a durability similar or even greater than copper alloys from the atmospheric corrosion point of view [
6]. The bright and lively colors of the rust make this steel a very aesthetic material with shades ranging from orange to violet, passing through yellow or brown [
7]. Some artists have made WS a very recognized material in contemporary art, as is the case with Richard Serra, with art installations and public art all over the world.
However, engineering criteria are not always taken into consideration when WS is used in cultural heritage, as is the case of sculptural work. In these cases, the main criterion should be the creative vision of the artist. Both the exposure conditions and the design of the artwork are conditioned by artistic reasons rather than material durability. Thus, there are WS works in places not recommended for applications in civil structures, such as
Peine del Viento XV by Eduardo Chillida, located offshore, just a few meters from the coast [
8], or
East-West/West-East by Richard Serra installed in the middle of the desert in Qatar [
9]. There can also be interstices and cavities designed in the artwork that may hinder proper drying of the metal surface. This is the case of the artwork
Head nº2 by Naum Gabo [
10] or
Cometh the Sun by Curtis Paterson [
11], which suffered premature deterioration requiring conservation-restoration interventions.
Another difference in the artistic application of WS with respect to its use in civil structures is the creation of artificial patinas by chemical treatments as part of the creative process of the work. Artists investigate different methods of creation with chemicals, testing salts and acids to obtain the colors and aesthetics that best fit their artistic intention before atmospheric exposure [
12]. However, the subsequent effect of these treatments on the formation of protective rust is unknown. The conditions of application of these treatments are very personal since each artist develops their own methods of patination according to the aesthetics of their work.
Due to all the above mentioned issues, the conservation of artworks using WS has become a major concern among conservators-restorers. A growing number of research groups are delving into existing problems for the safeguard of WS artworks. For instance, Raffo et al. focused on rainwater runoff and dissolved ions from corroding WS as a potential source of pollution [
13]. Travassos et al. quantified patina thicknesses in the steady state of WS using non-destructive methods (magnetic induction); they suggested that thickening rates of patinas lower than 2 μm/year could be used as an evaluation method of WS sculptures in good conservation status [
14]. Grassini et al. [
15] and Angelini et al. [
16] analyzed electrochemical properties of patinas using electrochemical impedance spectroscopy (EIS) for the same purpose. Aramendia et al. [
17,
18,
19] studied the influence of marine and urban pollutants in the conservation of several artworks exposed to Bilbao atmosphere (Spain).
However, none of these studies has dealt with the effect of chemical treatments for artificial patination applied to WS artworks. Advance in this knowledge is necessary to ensure proper conservation and to highlight potential dangers to safeguard their future. Therefore, the aim of this paper is to study the consequences in terms of atmospheric corrosion arising from applying one of the most common chemical treatments in contemporary art used by sculptors to obtain artificial patinas on WS. After consulting a set of Spanish sculptors with WS artworks, application of dilute sulfuric acid (10%) was identified as one of the most common methods and selected as artificial treatment of patination for this study [
20]. Once applied on WS specimens, a two-year atmospheric exposure test was performed in Madrid. An evolution of the artificial patina was assessed in terms of corrosion rate and compared with respect to a patina formed without chemical treatment. Their composition was characterized using scanning electron microscopy (SEM-EDX), X-ray diffraction (XRD) and Raman spectroscopy, paying special attention to the presence of ferrihydrite, a poorly crystalline oxyhydroxide of difficult characterization typically related to the initial stage of atmospheric corrosion. Finally, color measurements of the patinas were performed to evaluate their aesthetic appearance.
4. Discussion
The application of chemical treatments to generate artificial patinas on WS is a procedure used by sculptors to simulate patina colors formed during long-term atmospheric exposures. The artificial patina color formed with 10% H2SO4 hardly differs from the color that the natural patina acquires from practically the beginning of its formation in the atmosphere of Madrid. The chromaticity and luminosity values of the samples with artificial and natural patina are almost identical after three months of exposure (first measurement performed). Sculptors quickly achieve the aesthetics that nature takes months to generate. However, the use of sulfuric acid that leads to the artificial patina formation initially implies additional corrosion. Specifically, it caused a thickness loss of WS of 2.39 µm prior to atmospheric exposure. This initial corrosion would be similar to that experienced by bare WS exposed for a year to a rural atmosphere. Although it is a patination treatment of low corrosivity, the ions in which the sulfuric acid dissociates (H+ and SO42−) could affect the subsequent corrosive process and, therefore, the long-term development of a protective rust in the atmosphere.
There are several criteria to assess the state of conservation of WS exposed to the atmosphere. One of them is to use the engineering criteria [
2]. The atmospheric corrosion rate of WS must be ≤ 6 µm/year for its use without paint coatings to be advised. After two years of atmospheric exposure, the corrosion rate of artificially patinated WS with sulfuric acid was 4.7 µm/year. This was slightly higher than the corrosion rate experienced by the WS without artificial patina, 4.22 µm/year. Therefore, the patination of WS with 10% H
2SO
4 does not seem to compromise the material durability when exposed to the atmosphere of Madrid. In just two years of exposure, it reached a tolerable rate of atmospheric corrosion that would allow optimal conservation of WS. The atmosphere of Madrid through successive rain-washes allows the removal of possible traces of the acid patination treatment. In addition, there was no corrosive source of pollution nearby. It is an urban atmosphere with low levels of SO
2 (0.8 mg/m
2d) located 360 Km from the nearest coast.
Another alternative is to use protective ability indexes that evaluate how protective a patina formed in the atmosphere is on WS based on its composition determined using XRD. Yamashita et al. [
32] found that the corrosion rate of WS exposed to rural and industrial atmospheres decreased as the mass ratio of goethite to lepidocrocite (α/γ) increased. They considered α/γ > 2 to be an indication of obtaining a protective patina after decades of atmospheric exposure [
33]. Later, other research groups modified the protective ability index proposed by Yamashita based on the thermodynamic stability of newly identified phases of iron oxides and oxihydroxides. For instance, Dillmann et al. proposed adding the magnetite content to the numerator of the previous index from old rusts and considering this as protective despite the fact that is a conducting phase [
34]. Both protective ability indexes were calculated for the artificial and natural patina with respect to the exposure time in the atmosphere of Madrid. However, since the amount of quantified spinel in the different patinas is relatively small, the protective ability indexes were very similar. Hence,
Figure 9 shows only the results of the α/γ index. None of the patinas had a goethite content high enough to be greater than twice the lepidocrocite content (α/γ > 2). They need more exposure time to achieve that situation since the corrosion rate of WS has not yet reached the steady state. However, nothing suggests that the artificial patination precludes the above from happening.
The artificial patination treatment of 10% H2SO4 has an α/γ index close to the same as for artificial patination after 24 months of atmospheric exposure. It decreased after 12 months of atmospheric exposure but the α/γ ratio continued above that of the natural patina. During the initial atmospheric corrosion mainly lepidocrocite is formed, therefore the index decreases. Later, after 24 months of atmospheric exposure the α/γ ratio increased as expected. This was the behavior shown by the natural patina from the beginning of exposure. Lepidocrocite is progressively transformed into goethite via dissolution and precipitation during wet/dry cycles in the atmosphere.
In a certain way, the sulfuric acid treatment increases the protective ability of the patina because its α/γ ratio after 12 months of atmospheric exposure is similar to that of the natural patina after 24 months of exposure.
A necessary but not sufficient condition for the formation of a protective patina on WS is its stratification into two sublayers. The additional condition is that the inner layer must be rich in goethite while the outer layer in lepidocrocite. From the optical images in
Figure 6, the artificial patina just after its formation (zero time) does not seem to be stratified since its thickness is very thin, less than 10 µm; it is a patina with little quantity of corrosion products to stratify. Nevertheless, it is possible to distinguish two different parts (outer and inner) according to the location of the phases. As for its composition,
Table 6 shows a summary of it within the patina.
The goethite phase found is not pure in the inner part of the patina but mixed with ferrihydrite and lepidocrite phases. Pure ferrihydrite is also located in the inner part of the patina. Finally, pure lepidocrocite is located in the outer part of the patina, while it is mixed with ferrihydrite both externally and internally in the patina. The rapid oxidation of the patination treatment on WS generates mixed phases, although mostly goethite is located in the inner part of the patina and lepidocrocite in the external part. After 12 months of exposure to the atmosphere of Madrid, the patina thickness increases (around 28 µm) and stratifies. Lepidocrocite is mainly concentrated in the middle and outer part of the patina, while goethite and ferrihydrite are alternated within the inner layer [
1]. It is in this part where chromium has accumulated, which is associated with the formation of nanophasic goethite, that a greater protective effect is exerted [
32,
35]. Therefore, it can be affirmed that the 10% H
2SO
4 treatment of patination applied on WS exposed to the urban atmosphere of Madrid generates a patina that evolves normally towards a protective patina and that does not compromise the WS preservation.
On the other hand, ferrihydrite was detected using Raman spectroscopy along with the different phases of iron commonly identified using XRD. It is usually found on the inner part of the artificial and natural patina and mostly in direct contact with the WS metal substrate. It is a fact to highlight because it is a difficult phase to identify when it naturally forms in the atmosphere within the set of corrosion products. It has a low particle size, which makes it go unnoticed in XRD analysis, being appreciated as amorphous. In addition, its high thermodynamic instability allows it to transform into more stable and crystalline oxides, mainly goethite and hematite. Cornell and Schwertmann describe the competitive mechanism that leads to goethite or hematite formation [
36]. At acidic pH and moderated temperature, the dissolution of ferrihydrite occurs and precipitates to goethite. However, hematite requires neutral pHs and high temperature. Since the presence of hematite in atmospheric corrosion products is minor, ferrihydrite could be an excellent precursor of nanophasic goethite, which is mostly concentrated in the inner layer of a protective patina. Another fact to keep in mind is that chromium is also associated with the presence of ferrihydrite as well as being nanophasic of a protective patina formed during years of atmospheric exposure.
Author Contributions
Conceptualization, A.C., I.D., D.N. and E.C.; methodology, A.C., I.D. and E.C.; validation, D.N., I.L. and S.M.-R.; formal analysis, A.C., I.D., I.L. and S.M.-R.; investigation, A.C. and I.D.; resources, I.D., D.N., I.L., S.M.-R. and E.C.; data curation, A.C., I.D., D.N., I.L. and S.M.-R.; writing—original draft preparation, A.C. and I.D.; writing—review and editing, A.C., I.D., D.N., I.L., S.M.-R. and E.C.; supervision, I.D., D.N., S.M.-R. and E.C.; project administration, I.D. and E.C.; funding acquisition, D.N., I.L., S.M.-R. and E.C. All authors have read and agreed to the published version of the manuscript.
Funding
This research was funded by Agencia Estatal de Investigación, grant number BES-2015-071472, and by Comunidad de Madrid and European Structural and Investment Funds, project TOP-HERITAGE CM (S2018/NMT-4372) and project GEOMATERIALES 2 (S2013/MIT-2914).
Acknowledgments
Authors would like to thank Mickael Bouhier and Robin Le Penglau for the development of the Multicorr software and their help in the analysis and Ana Ibáñez for her help with English corrections and are thankful for the professional support of the CSIC Interdisciplinary Thematic Platform from: Open Heritage Research and Society (PTI-PAIS).
Conflicts of Interest
The authors declare no conflict of interest.
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Figure 1.
Atmospheric corrosion station located in Madrid (Spain).
Figure 2.
Monthly data of Madrid on relative humidity and temperature supplied by the National Meteorological Institute (AEMET) of Spain.
Figure 3.
Values of the CIEL*a*b* color space. (a) Chromaticity a* and b*; (b) lightness L*.
Figure 4.
Corrosion versus time plot for both patinated WS with 10% H2SO4 and bare WS without artificial patina exposed to the Madrid atmosphere. Figures in orange represent quantified corrosion for patinated WS, while the blue ones represent those of non-patinated WS.
Figure 5.
Raman spectra of artificial (10% H2SO4) and natural patinas formed on WS for 2 years at Madrid atmosphere, showing (a) characteristic peaks of lepidocrocite together with (b) less representative spectra with an additional signal at 720 cm−1.
Figure 6.
Optical microscope images showing evolution of the cross section of (a) the artificial patina (10% H2SO4) from its initial formation on WS (t = 0) and (b) after 12 months of exposure to the Madrid atmosphere. (c) Natural patina is shown for comparative purposes.
Figure 7.
(a) Image of the phase distribution obtained using principal component analysis (PCA) data treatment from a Raman map acquired on the scanned cross section for the artificial patina (10% H2SO4) before atmospheric exposure; (b) Raman spectra of the different phases detected in the Raman map.
Figure 8.
Raman map of the scanned cross sections after being exposed to the atmosphere of Madrid for 12 months with the corresponding Raman spectra of the principal components identified in the Raman map for (a) artificial patina and (b) natural patina.
Figure 9.
Evolution with exposure time of the α/γ ratio for the artificial and natural patina exposed to the atmosphere of Madrid for 24 months.
Table 1.
Chemical composition (wt. %) of the used weathering steel (WS).
Weathering Steel | C | Si | Mn | P | S | Cu | Cr | Ni | Al |
---|
ASTM A242 | 0.091 | 0.60 | 0.42 | 0.109 | 0.0123 | 0.302 | 0.807 | 0.181 | 0.025 |
Table 2.
Values of ΔE calculated for the artificial and the natural patina.
Time of Exposition | ΔE Value |
---|
3 months | 2.01 |
6 months | 5.05 |
12 months | 1.67 |
24 months | 1.08 |
Table 3.
Characteristic wavelength shifts (cm−1) of the Raman peaks corresponding to lepidocrocite, feroxhyte, maghmite and ferrihydrite found in different bibliographic sources. Main signal in bold.
Phases | Signals (cm−1) | References |
---|
Lepidocrocite (γ-FeOOH) | 250, 306, 380, 525, 650 | Actual data of our study |
250, 302, 343, 379, 525, 650 | [25] |
250, 380, 650 | [26] |
220, 250, 309, 350, 377, 527, 648 | [27] |
250, 300, 380, 525, 650 | [28] |
250, 348, 379, 528, 650 | [29] |
Feroxhyte (δ-FeOOH) | 400, 680 | [25] |
680, 1350 | [26] |
Maghemite (γ-Fe2O3) | 380, 460, 510, 670, 720 | [25] |
377, 510, 670, 715 | [27] |
670–720, 1400 | [26] |
380, 460, 510, 670, 720, 1160, 1400 | [28] |
350, 512, 665, 730 | [29] |
Ferrihydrite (Fe5HO8·4H2O) | 370, 510, 710, 1350 | [25] |
361, 508, 707, 1045 | [30] |
370, 510, 710 | [29] |
370, 510, 710 | [31] |
Table 4.
Thicknesses of patinas (average ± SD) obtained after exposure to the atmosphere of Madrid.
Artificial Patina (t = 0) | Artificial Patina (t = 12 months) | Natural Patina (t = 12 months) |
---|
6 ± 1 µm | 28 ± 12 µm | 20 ± 9 µm |
Table 5.
Percentage of the identified phases determined using X-ray diffraction (XRD) results (Rietveld refinement) obtained from the artificial patina (10% H2SO4) and the natural patina exposed for two years to the atmosphere of Madrid.
Specimen | Lepidocrocite (%) | Goethite (%) | Spinel (%) |
---|
Artificial patina (t = 0) | 70.32 | 17.58 | 12.39 |
Artificial patina (t = 12 months) | 82.10 | 15.23 | 2.67 |
Artificial patina (t = 24 months) | 76.90 | 19.31 | 3.80 |
Natural patina (t = 12 months) | 89.41 | 8.30 | 2.40 |
Natural patina (t = 24 months) | 82.37 | 14.15 | 3.28 |
Table 6.
Summary of the localization of the different identified iron phases within the artificial patina at zero time. L means lepidocrocite, F, ferrihydrite and G, goethite.
Part of the Patina | L | F | L + F | G + F | G + F + L |
---|
Outer part | Yes | - | Yes | - | - |
Inner part | - | Yes | Yes | Yes | Yes |
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