Composition and electrochemical properties of natural patinas of outdoor bronze monuments

The present study concerns the chemical-physical and electrochemical characterizations of the corrosion compounds formed on outdoor bronzes exposed in urban conditions. Here, we focused on the pale green patinas of surfaces directly exposed to the rainfall leaching. The investigation has been performed from scraped-off patina products of a well representative outdoor bronze monument (Monument to Francis Garnier, 1898) situated in Paris (France). The composition of the patina was analysed by energy dispersive spectrometry (EDS), coupled with SEM, and Raman spectroscopy. Structural characterization was conducted through high-resolution TEM (EDS coupled). The electrochemical reactivity of patina was investigated by cyclic voltammetry and EIS with a microcavity electrode in sulphate solution at pH 2 and 5.6 with time of immersion (up to 10 h). The bronze patina corresponds to a two-layer structure. The external layer is characterised by a marked selective dissolution of Cu and Zn of the alloy, which has been determined from the calculation of dissolution factors. The same phenomenon with the same amplitude has been evidenced on other bronze monuments used for comparison. The pale green patina is a complex mixture of crystalline copper hydroxisulphate and of amorphous/nanocrystal Sn-containing compounds. This patina is stable at pH 5.6 and reactive at pH 2 in relation with the dissolution of copper sulphate compounds in the acidic solution.     

Electrochemical evaluation of patinas formed on nineteenth century bronze bells

The cathedral of Cuernavaca City, Mexico is one of the earliest monuments built in the American Continent by the Spanish Conquerors. The cathedral lodges a collection of historic artifacts including eight bells. The aim of this study was to provide preliminary information for the conservation diagnosis of the historic bells. A set of three bells dating back to the nineteenth century was studied. Two of the bells present a crack and the third and oldest remains complete. Metallurgical characterization and electrochemical evaluation were used to describe the behavior of the patinas formed on the surfaces of the bells. Studies were done “in situ” over the surface of the artifacts. Preliminary experiments were carried out using a modern cast bronze with equivalent chemical composition. Besides the useful information for conservation purposes, the work depicts the behavior of an ancient material exposed to the atmosphere for a long period of time. The obtained data suggest a relationship between alloy composition, especially tin and lead content and electrochemical properties of the materials. Techniques used include potential measurements as function of time, potentiodynamic polarization curves, electrochemical impedance spectroscopy, electrochemical noise as well as metallography.     

Comparative studies of chemical and electrochemical preparation of artificial bronze patinas and their protection by corrosion inhibitor

The bronze artefacts of cultural heritage are often covered with patina, a layer of corrosion products, which confers their aesthetic and also protects the substrate bronze. Due to the increasing atmospheric pollution these layers are often dissolving when exposed in urban environment. In this work we propose the use of an innoxious imidazole compound as a corrosion inhibitor for patinated bronze. On a Cu-6Sn (wt%) bronze, three types of patinas were synthesized: two by chemical methods (in a sulphate solution and a chloride one) and one by an electrochemical process (in a sulphate/carbonate solution). A blue-green patina was obtained in all three cases, and their morphological and structural characterization was performed by SEM, EDS and Raman spectroscopy. It was found that the sulphate patina is composed essentially of brochantite, the chloride patina of atacamite, and the electrochemical patina of malachite. All three patinas have also a smooth part of surface consisted of cuprite. As corrosion inhibitor 4-methyl-1-(p-tolyl) imidazole was used on all patinas, in a solution of 0.2 g L⁻¹ Na₂SO₄ + 0.2 g L⁻¹ NaHCO₃ acidified to pH 5 which simulates acid rain in urban environment. The results have shown that the inhibitor improves the stability of all three kinds of patinas and can be recommended for protection of works of art.     

Electrochemical and spectroscopic evidences of corrosion inhibition of bronze by a triazole derivative

The electrochemical behavior of the bronze (Cu-8Sn in wt%) was investigated in 3% NaCl aqueous solution, in presence and in absence of a corrosion inhibitor, the 3-phenyl-1,2,4-triazole-5-thione (PTS). The inhibiting effect of the PTS was evidenced for concentrations higher than 1 mM for the cathodic process whereas its effect was clearly seen with a concentration as low as 0.1 mM for the anodic process. A significant positive shift of the corrosion potential was also observed, and its inhibiting effect increased with both its concentration and the immersion time of the sample. From voltammetry and electrochemical impedance spectroscopy experiments, the inhibiting efficiency of the PTS was found to be in the 94-99% range for 1 mM concentration. Scanning electron microscopy and X-ray energy dispersion analysis of the specimen surface show the presence of sulphur on the surface. Raman micro-spectrometry study confirms the protective effect of the PTS in aqueous solution through three types of interactions with the electrode, namely the adsorption of the inhibitor in a flat configuration, the formation of copper-thiol molecules, and when copper is released, the formation of a polymeric complex.     

Electrochemical characterizations of carbon nanomaterials by the cavity microelectrode technique

The cavity microelectrode (CME) technique was used to screen carbide-derived carbons (CDC) and carbon onions and investigate their electrochemical performance for electrical double-layer capacitors (EDLC) in an H₂SO₄ electrolyte. The results show that the CME is very effective in terms of screening large numbers of powder samples rapidly, reliably, and efficiently, compared with conventional based electrochemical techniques using large surface electrodes. The results obtained from CME for CDC agree well with the conventional technique. High rate response was obtained with carbon onions because of their high graphitization degree. Therefore, the CME technique could also differentiate microstructure variation in powder materials, especially under high scan rate (as high as 10 V/s). These results show that, though the CME cannot completely replace conventional methods for electrochemical characterization of EDLCs, it is a complementary tool that can be used to quickly evaluate the performance of materials (combinatorial electrochemistry study) and highlight possible trends to explore using conventional techniques.     

Electrochemical and structural characterizations of an experimental track-etched membrane in KCl solutions

Characteristic electrochemical transport parameters for an experimental poly(ethylene)terephtalate (PET) track-etched membrane with well-defined structure and low porosity (Θ=0.13%) were determined with the membrane in contact with KCl solutions at different concentrations. Membrane potential, Δφ_m, measurements were performed to investigate the effective fixed charge concentration, X_f, and transport number of the ions, t_i, in the membrane using two different procedures: keeping the concentration ratio constant, or keeping one concentration constant and changing the other one. Results show the membrane presents a weak cation-exchanger character, since the following values were obtained: X_f=−(2.5±0.2)×10⁻² M, t_K =(0.56±0.06), t_Cl⁻=(0.44±0.05); taking into account these values, concentration dependence of membrane potential was predicted. Membrane electrical resistance, R_m, was obtained from Impedance Spectroscopy (IS) measurements using equivalent circuits as models, and the membrane porosity Θ=(0.11±0.02)% was also obtained from resistance values, which agrees very well with the value determined from geometrical parameters. From R_m, Δφ_m and Θ values, the diffusion coefficient of the ions in the membrane pores can be calculated, and the following average values were obtained: D_K⁺=(1.9±0.4)×10⁻⁹ m²/s and D_Cl⁻=(0.8±0.2)×10⁻⁹ m²/s, but for an average concentration higher than 0.06 M, their values do not differ practically from solution in agreement with the small negative charge previously indicated.     

Investigation of the corrosion protection of chemically and electrochemically formed patinas on recent bronze

In humid atmospheres, copper and bronze passivate, with the formation of an oxidized layer (patina). Patinas may also be created by chemical patination procedures, which are frequently used to achieve special visual effects. However, unless these patinas are effectively protected, corrosion may be initiated when they are exposed to a polluted atmosphere. In the present study the stability of the green nitrate and the green chloride type of patinas, and of electrochemically formed patina, was investigated, as well as suitable protection methods. Two inhibitors, dissolved in different solutions, were used: benzotriazole and the less hazardous imidazole type of inhibitor, and waxes. The dissolving of untreated and pre-treated surfaces, in test solutions representing urban rain (pH 5.0), was investigated by means of potentiodynamic techniques. The microstructure of the patinas and the corrosion products was investigated by SEM/EDX. It was found that both investigated inhibitors inhibited the corrosion of electrochemically formed patina, and of the green chloride type of patina, but were ineffective in the case of the green nitrate type of patina. The difference between the performance of the investigated inhibitors when brushed onto patinated bronze, and when the bronze is immersed in a solution containing the dissolved inhibitor, was determined.     

Inhibiting effect of 4-methyl-1-p-tolylimidazole to the corrosion of bronze patinated in sulphate medium

Bronze statues exposed to an urban area suffer corrosion processes induced by acid rainwater. Use of corrosion inhibitors is one of the most convenient and cost-effective techniques to mitigate it. In a previous work, it was found that 4-methyl-1-p-tolylimidazole (MTI) is non-toxic and efficient inhibitor of copper, and thus we will examine the anticorrosive effect of this substance to copper-tin bronze. First, with bare Cu–6Sn (wt%) bronze, inhibiting efficiency in artificial acid rainwater in urban environment was evaluated by Tafel extrapolation, polarization resistance, and electrochemical impedance spectroscopy (EIS) methods. This study showed that all three methods supply similar inhibiting efficiency validating thus the use of EIS, the least invasive method. This study was followed by the evaluation of inhibiting effect on patinated bronze in sulphate medium. A quntitative study about the inhibiting effect on patinated bronze is scarcely reported in the literature. EIS data revealed three capacitive loops. Comparison with EIS obtained in bare specimens allowed attributing the additional capacitive loop located at the highest frequency domain to the dielectric property of the patina layer. The impedance modulus determined in presence of MTI increased markedly with immersion time, revealing a protective effect of this non-toxic corrosion inhibitor.     

Protection of ancient and historic bronzes by triazole derivatives

Ancient and historic tin bronzes can be substantially affected by corrosion when submitted to non adequate storage conditions or more aggressive environments such as, for example, the marked increase of air pollution and of acidity of rainwater in urban medium recently affecting outdoor monuments. To protect them, some triazole compounds were tested as corrosion inhibitors of bronze covered with patina layer. As in the case of cultural artefacts, each specimen is unique, an accurate comparison of inhibiting effect of different molecules is therefore hard to realise. To overcome this difficulty, on the basis of a global understanding of the nature of natural bronze patinas, artificial patinas were synthesised on a contemporary bronze (Cu-Sn-Pb) having a similar composition to that of bronze coins dating of the post Roman Empire (5th-6th century AD). Surface characterization and electrochemical experiments were carried out both on contemporary bronze covered with artificial patina and on three ancient bronze coins discovered in Morocco. All triazole derivatives used, benzo-triazole (BTA), amino-triazole (ATA) and bi-triazole (BiTA), exhibited fairly good protective properties on the synthetic bronze. The BTA is efficient in an artificial patina isolated of the substrate bronze and an old bronze specimen covered with patina layer, but this substance is considered as toxic. The BiTA has shown only a slight inhibiting effect on the ancient bronze coin. The ATA is the most effective at 0.1 mM concentration, and the initial treatment of bronze at a higher concentration makes inhibiting effect lower. The ATA is therefore considered the most promising candidate to be applied to protect antique bronze artefacts covered with natural patina layer.     

Infra-red and Raman spectroscopic study on the thermal stability and high temperature transformation of hydroazafullerene C59HN

Hydroazafullerene C₅₉HN was studied by vibrational infra-red and Raman spectroscopy and its thermal stability was examined. Fingerprints modes were identified and unambiguously differentiate it from bisazafullerene. At 700 K full transformation to bisazafullerene occurred, while an intermediate metastable phase was identified at 540 K showing different spectra where the splitting of most of the lines is strongly reduced.     

A comparison between Raman spectroscopy and surface characterizations of multiwall carbon nanotubes

The distribution of graphene units with an axial symmetry gives rise to different types of carbon filaments: nanotubes, nanofilaments and classical fibers. In this work the surfaces of different multiwalled nanotubes are characterized by two complementary techniques: chemical ones based on Total Surface Area and Active Surface Area measurements, associated with a physical approach the Raman scattering spectroscopy. From analysis of Raman data we deduce the values of the in-plane coherence lengths, identified as L₁ the planar projection of graphene sheets, and we propose an analysis for the observed line-width behavior related to the graphitization step. From the surface chemical properties we establish a general relationship between the density of functional surface groups and the in plane coherence length L₁ for all types of MWNT. This analysis allows us to show the influence of both, the structural organization and the different treatments on the interfacial characteristics of these nanocarbons.     

Electrochemical and spectroscopic characterization of lithium titanate spinel Li4Ti5O12

Herein we describe electrochemical and spectroscopic properties of lithium titanate spinel as well as an easy method based on colorimetry to determine the lithium content of electrodes containing lithium titanate spinel as active material. Raman microspectrometry measurements have been performed to follow lithium insertion into and extraction from the active material, respectively. The Raman signals display a pronounced fading of intensity already at low levels of lithium intercalation and disappear at a SOC higher than ∼10%. However, the colorimetric method can be used up to a SOC of 50%.     

Raman spectroscopic studies on the structural changes of electrosynthesized polypyrrole films during heating and cooling processes

Polypyrrole films were electrochemically synthesized by direct oxidative polymerization of pyrrole in acetonitrile containing β‐naphthalene sulfonic acid or tetrabutylammonium tetrafluoroborate as an supporting electrolyte. We characterized the as‐grown polypyrrole films by resonance Raman spectroscopy in the temperature region of ?195 to 275°C. During the heating process, the Raman bands related to the oxidized species decreased gradually, due mainly to the affect of oxygen and moisture in the air, and, finally, the neutral polymer chains decomposed into disordered carbons. During the cooling process, polymer chains changed from disordered (coil‐like) to ordered (rodlike) structures and caused the elongation of the conjugated chain length. This results in a red shift of the absorption of the electron spectra of the polymer and the enhancements of the Raman bands related to the oxidized species. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3390–3395, 2003     

LR spectroscopic study of chemisorbed dinitrogen species on ammonia synthesis iron catalysts

The results of the LRS study of N-containing chemisorbed species on the promoted iron catalyst for ammonia synthesis have further substantiated the existence of two species of N₂(a) as the dominant N-containing chemisorbed species under the functioning catalyst conditions. A model of active site, as 3-Fe cluster on (111) or (211) surface of -Fe, and two modes of multinuclear coordination activation for the observed two species of N₂(a) were proposed. It was further illustrated from reaction energetics that the mechanism of the dominant reaction pathway for ammonia synthesis/decomposition may be associative, rather than dissociative.The work was supported by a grant from the National Natural Science Foundation of China; parts of the work were presented at 24th ICCC (Athens, 1986).     

Electrochemical degradation of polyfuran in wet acetonitrile and aqueous solutions

The degradation of polyfuran in a wet acetonitrile solution and in an aqueous solution has been investigated with a cyclic voltammetry technique, along with Fourier transform infrared and Fourier transform Raman spectroscopy techniques. Infrared spectroscopy shows that the main defects that exist in polyfuran after cycling in dried acetonitrile are mainly saturated C? H structures, whereas those after cycling in an aqueous solution are mainly carbonyl groups in the polymer chain. This may be because polyfuran can undergo degradation through a crosslinking mechanism in a dried acetonitrile solution, whereas in an aqueous solution, it undergoes degradation through a nucleophilic attack mechanism. Raman spectroscopy shows that not all the furan rings are involved in the degradation process, although the electrochemical activity of polyfuran is totally lost in an aqueous solution after only one cycle. The sites that are sensitive to the degradation process may be the electrochemically active sites, that is, the positively charged sites in polyfuran chains. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3160–3165, 2002     

Electrochemical impedance spectroscopy of lead(II) ion-selective solid-state membranes

Silver sulphide/lead sulphide membranes were studied using electrochemical impedance spectroscopy. The influence of the electrolyte concentration and the membrane thickness were evaluated. The complex impedance plots have shown two capacitive loops: one at high frequency range, related with the charge transfer resistance at the interface membrane-Ag and a second one at low frequency range, associated with the diffusion process through the membrane. A simple model was used to take into account the experimental results: the changes of the potential with the time and the electrolyte concentration; the changes of the charge transfer resistance and the diffusion resistance with the electrolyte concentration. An empirical equation was used to calculate the diffusion coefficient of Ag⁺ inside the membrane.     

Electrochemical and spectroscopic characterization of poly(1,8-diaminocarbazole): Part II. Electrochemical, in situ vis/NIR and Raman studies of redox reaction of PDACz in protic and aprotic media

Electrochemical redox reactions of poly(1,8-diaminocarbazole) (PDACz) films in aqueous (0.1 M HClO₄) and nonaqueous (0.1 M LiClO₄ in acetonitrile) solutions were studied by cyclic voltammetry, in situ vis/NIR and Raman spectroscopy. It has been demonstrated that spectroelectrochemical behavior of the polymer is strongly dependent on the nature of the solution used for doping-undoping but not on the medium used for electropolymerization. A redox couples Fe²⁺/Fe³⁺, Fe(CN)₆^4−/3− and tertrathiafulvalene were used as the probes for the studies of electroactivity of the oxidized polymer films. The results were discussed in terms of different mechanism of deprotonation process of the polymer in aqueous solution of 0.1 M HClO₄ and in 0.1 M LiClO₄ solution in aprotic acetonitrile and the reaction schemes in the two media are proposed.     

Electrochemical and anticorrosion performances of zinc-rich and polyaniline powder coatings

In this work, hydrochloride polyaniline (PANI-Cl) powder was incorporated as a conductive pigment into powder zinc-rich primer (ZRP) formulations in order to enhance the electronic conduction paths between zinc particles inside the coating and the steel substrate (i.e. percolation). Coatings were applied onto steel substrates and immersed in a 3% NaCl solution at ambient temperature.The protective properties and electrochemical behaviour of coatings were investigated by monitoring the free corrosion potential versus time and by using EIS. It was found that corrosion potential remains cathodic and constant for a long time up to 100 days of immersion. From EIS results, it was shown that the coatings exhibit larger impedance values than those observed with liquid or other zinc-rich powder formulations containing carbon black. From Raman spectroscopy results, it may be proposed that zinc particles in contact with PANI-Cl pigments were passivated. Other zinc particles remain still active which ensures the cathodic protection of the substrate. Moreover, coatings exhibit good barrier properties.     

Electrochemical and spectroscopic properties of dendritic cobalto-salicylaldiimine DNA biosensor

A metallodendrimer-based electrochemical DNA biosensor was constructed by a layer-by-layer assembly of cobalt(II) salicylaldiimine metallodendrimer (SDD-Co(II)) and a 21 bases oligonucleotide NH₂-5′-GAGGAGTTGGGGGAGCACATT-3′ (pDNA) on a gold electrode. The complementary oligonucleotide was 5′-AATGTGCTCCCCCAACTCCTC-3′ (tDNA). UV-visible spectra of SDD-Co(II) in 1:1 (v/v) acetone-ethanol solution showed absorption bands at 325 nm and 365-420 nm related to π-π* intra-dendrimer transitions and d-π* metal-dendrimer charge transfer transitions, respectively. Square-wave voltammetry (SWV) characterisation of the Au|SDD-Co(II)|pDNA biosensor system in phosphate buffer saline solution of pH 7.4, indicated a reversible one-electron electrochemical process with a formal potential, E°′, value of +210 mV. Electrochemical impedance spectroscopy (EIS) data confirmed that the hybridisation of the biosensor's pDNA with the tDNA to form double-stranded DNA (dsDNA) resulted in an increase of the impedimetric charge transfer resistance, R_ct, value from 6.52 to 12.85 kΩ. The limit of detection (LOD), calculated as 3σ of the background noise, and sensitivity of the sensor were 1.29 kΩ/nmol L⁻¹ and 0.34 pmol L⁻¹, respectively.     

Sensitive Detection of SARS-CoV-2 Variants Using an Electrochemical Impedance Spectroscopy Based Aptasensor

The global pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) caused a threat to public health and a worldwide crisis. This raised the need for quick, effective, and sensitive detection tools to prevent the rapid transmission rate of the infection. Therefore, this study aimed to develop an electrochemical impedance spectroscopy (EIS)-based aptasensor employing an interdigitated gold electrode (IDE) to detect SARS-CoV-2 Spike (S) glycoprotein and viral particles. This allowed us to sensitively detect SARS-CoV-2 S glycoprotein with a limit of detection (LOD) of 0.4 pg/mL in a buffer solution and to obtain a linear increase for concentrations between 0.2 to 0.8 pg/mL with high specificity. The proposed aptasensor also showed a good sensitivity towards the heat-inactivated SARS-CoV-2 variants in a buffer solution, where the Delta, Wuhan, and Alpha variants were captured at a viral titer of 6.45 ± 0.16 × 10³ TCID₅₀/mL, 6.20 × 10⁴ TCID₅₀/mL, and 5.32 ± 0.13 × 10² TCID₅₀/mL, respectively. Furthermore, the detection of SARS-CoV-2 performed in a spiked human nasal fluid provided an LOD of 6.45 ± 0.16 × 10³ TCID₅₀/mL for the Delta variant in a 50 µL sample and a detection time of less than 25 min. Atomic force microscopy images complemented the EIS results in this study, revealing that the surface roughness of the IDE after each modification step increased, which indicates that the target was successfully captured. This label-free EIS-based aptasensor has promising potential for the rapid detection of SARS-CoV-2 in complex clinical samples.