Early Stages of Metal Corrosion in Coastal Archaeological Sites: Effects of Chemical Composition in Silver and Copper Alloys.

In this study, metal disks with different chemical composition (two Ag-based alloys and three Cu-based alloys) were buried in the soil of coastal archaeological sites for a period of 15 years. The aim was to naturally induce the growth of corrosion patinas to obtain a deeper insight into the role of alloying elements in the formation of the patinas and into the degradation mechanisms occurring in the very early stages of burial. To reach the aim, the morphological, compositional and structural features of the patinas grown over 15 years were extensively characterized by optical microscopy, field emission scanning electron microscopy coupled with energy dispersive spectrometry, X-ray diffraction and micro-Raman spectroscopy. Results showed that the Cu amount in Ag-based alloys strongly affected the final appearance, as well as the composition and structure of the patinas. Corrosion mechanisms typical of archaeological finds, such as the selective dissolution of Cu, Pb and Zn and internal oxidation of Sn, occurred in the Cu-based alloys, even if areas enriched in Zn and Pb compounds were also detected and attributed to an early stage of degradation. In addition, some unusual and rare compounds were detected in the patinas developed on the Cu-based disks.

Reproducing bronze archaeological patinas through intentional burial: A comparison between short- and long-term interactions with soil.

The reproduction of archaeological corrosion patinas is a key issue for the reliable validation of conservation materials before their use on cultural objects. In this study, bronze disks were intentionally buried for 15 years in the soil of the archaeological site of Tharros, both in laboratory and in situ, with the aim of reproducing corrosion patinas typical of archaeological artifacts to be used as representative surfaces for testing novel cleaning gels. The microstructural, microchemical and mineralogical features of the patinas were analyzed by a multianalytical approach, based on optical microscopy (OM), field emission scanning electron microscopy coupled with energy dispersive spectrometry (FE-SEM-EDS) and X-ray diffraction (XRD). The patinas developed in 15 years were compared with an archaeological bronze recovered from the same site after about two thousand years of burial (referred as short-term and long-term interaction, respectively). Results revealed a similar corrosion behavior, especially in terms of chemical composition and corrosion mechanisms. XRD detected the ubiquitous presence of cuprite, copper hydroxychlorides and terrigenous minerals, while OM and FE-SEM-EDS analyses of cross-sections evidenced similar patinas' stratigraphy, identifying decuprification as driving corrosion mechanism. However, some differences related to the type of local environment and to the time spent in soil were evidenced. In particular, patinas developed in situ are more heterogeneous and rougher, while the archaeological one is thicker and presents a major amount of cuprite, terrigenous deposits and uncommon corrosion compounds. Based on our findings, the disks buried in situ were selected and used as disposable substrates to study the cleaning effect of a novel polyvinyl alcohol (PVA)-based gel loaded with a chelating agent (Na2EDTA · 2H2O). Results show that the gel is effective in removing disfiguring degradation compounds and preserving the stable and protective patina. Based on the conservation needs, the time of application can be properly tuned. It is worth noticing that after a few minutes the green corrosion products can be selectively removed. The EDS analysis performed on the gels after cleaning reveals that they are highly selective for the removal of copper(II) compounds rather than Cu(I) oxide or Cu(0) from bronze substrates.

Smart Inhibition Action of Amino Acid-Modified Layered Double Hydroxide and Its Application on Carbon Steel.

Surface impregnation of concrete structures with a migrating corrosion inhibitor is a promising and non-invasive technique for increasing the lifetime of existing structures that already show signs of corrosion attack. The main requirement for inhibitors is their ability to diffuse the rebar at a sufficient rate to protect steel. The use of smart nanocontainers such as layered double hydroxides (LDH) to store corrosion inhibitors significantly increases efficiency by providing an active protection from chloride-induced corrosion. The addition of LDH to reinforced mortar can also improve the compactness and mechanical properties of this matrix. Here, we report the synthesis of a magnesium-aluminum LDH storing glutamine amino acid as a green inhibitor (labeled as Mg-Al-Gln), which can be used as a migrating inhibitor on mortar specimens. The corrosion behavior of the specimens was determined via electrochemical techniques based on measurements of corrosion potential and electrochemical impedance spectroscopy. A cell containing a 3.5% NaCl solution was applied to the mortar surface to promote the corrosion of embedded rebars. The specimens treated with Mg-Al-Gln presented an improved corrosion protection performance, exhibiting an increase in polarization resistance (Rp) compared to the reference specimens without an inhibitor (NO INH). This effect is a consequence of a double mechanism of protection/stimuli-responsive release of glutamine and the removal of corrosive chloride species from the medium.

Toward a Green and Sustainable Silver Conservation: Development and Validation of Chitosan-Based Protective Coatings.

When exposed to air, silver artifacts undergo an unpleasant darkening and shiny loss, commonly known as tarnishing. At the present, the development of protective coatings by using eco-friendly and biocompatible materials, able to ensure high transparency and to hinder the degradation of silver objects, remains a huge challenge. In this study, chitosan was used for the first time to realize sustainable coatings for silver protection. Both pure and benzotriazole-containing chitosan coatings were prepared and applied on sterling silver disks. A commercial product based on acrylic resin was used as a reference. The aesthetic features and protective properties of these coatings were evaluated by performing two different types of aging treatments. In particular, the assessment of the protective efficacy was carried out by reproducing both highly aggressive polluted environments and real-like museums' storage conditions. In the first case, chitosan-based coatings with benzotriazole performed better, whereas in storage conditions all the chitosan films showed comparable efficacy. Compositional, morphological and structural analyses were used to evaluate the protective properties of the coatings and to detect any physical or chemical modifications after the aging treatments. Our findings reveal that the two different testing methods provide complementary information. Moreover, chitosan coatings can achieve protective efficacy comparable with that of the commercial product but using non-toxic solvents and a renewable biopolymer. Chitosan coatings, designed for cultural heritage conservation, are thus promising for the protection of common sterling silver objects.

The vehicle braking systems as main source of inhalable airborne magnetite particles in trafficked areas.

Magnetite (Fe3O4) nano-particles (MNPs) have been found in human tissues and causally linked to serious illnesses. The possible negative role of MNPs has been not still fully ascertained even though MNPs might cause health effects due to their magnetic property, redox activity and surface charge. The origin of MNPs in human tissues still remains to be unambiguously identified since biological processes, natural phenomena and anthropogenic production have been proposed. According to this latter increasingly convincing hypothesis, anthropogenic MNPs might enter mainly in the human body via inhalation, penetrate deeply into the lungs and in the alveoli and also migrate into the blood circulation and gather in the extrapulmonary organs and central nervous system. In order to identify the releasing source of the potentially inhalable MNPs, we pioneered an innovative approach to rapidly investigate elemental profile and morphology of a large number of airborne micron and sub-micron-sized Fe-bearing particles (FePs). The study was performed by collecting a large amount of micron and sub-micron sized inhalable airborne FePs in trafficked and densely frequented areas of Rome (Italy). Then, we have investigated individually the elemental profile and morphology of the collected particles by means of high-spatial resolution scanning electron microscopy, energy dispersive spectroscopy and an automated software purposely developed for the metal-bearing particles analysis. On the basis of specific elemental tracing features, the investigation reveals that almost the total amount of the airborne FePs is released by the vehicle braking systems mainly in the form of magnetite. Furthermore, we point out that our approach might be more generally used to identify the releasing sources of different inorganic airborne particles and to contribute to establish more accurately the impact of specific natural or anthropogenic particles on the environment and human health.

Synergistic Inhibition Effect of Chitosan and L-Cysteine for the Protection of Copper-Based Alloys against Atmospheric Chloride-Induced Indoor Corrosion.

The protection of metals from atmospheric corrosion is a task of primary importance for many applications and many different products have been used, sometimes being toxic and harmful for health and the environment. In order to overcome drawbacks due to toxicity of the corrosion inhibitors and harmful organic solvents and to ensure long-lasting protection, new organic compounds have been proposed and their corrosion inhibition properties have been investigated. In this work, we describe the use of a new environment-friendly anticorrosive coating that takes advantage of the synergism between an eco-friendly bio-polymer matrix and an amino acid. The corrosion inhibition of a largely used Copper-based (Cu-based) alloy against the chloride-induced indoor atmospheric attack was studied using chitosan (CH) as a biopolymer and l-Cysteine (Cy) as an amino acid. To evaluate the protective efficacy of the coatings, tailored accelerated corrosion tests were carried out on bare and coated Cu-based alloys, further, the nature of the protective film formed on the Cu-based alloy surface was analyzed by Fourier-transformed infrared spectroscopy (FTIR) while the surface modifications due to the corrosion treatments were investigated by optical microscopy (OM). The evaluation tests reveal that the Chitosan/l-Cysteine (CH/Cy) coatings exhibit good anti-corrosion properties against chloride attack whose efficiency increases with a minimum amount of Cy of 0.25 mg/mL.

On-Demand Release of Protective Agents Triggered by Environmental Stimuli.

The aim of this study was to develop smart materials with stimuli-responsive properties for the long-term protection of steel. The idea was to obtain a tailored and controlled release of protective agents in response to the environment stimuli. First, the protective efficacy of three inhibitors containing a carboxylic moiety, such as p-aminobenzoic (pAB), succinic (SA), and caffeic (CA) acids, was investigated in alkaline chloride solutions. The results revealed that pAB is the most effective protective agent, significantly better than SA and CA. It is surprising that the steel surface in the pAB solution remains unchanged even after 5 months of corrosion treatment, whereas the formation of degradation products in the SA and CA solutions was observed after only 6 days. Based on these findings, pAB was selected and used for the functionalization of silica nanoparticles and layered double hydroxides (LDHs) that can act as delivery vehicles and as an inhibitor reservoir. Specifically, pAB was chemisorbed on silica amino groups via an amide bond, and this makes possible a gradual inhibitor release induced by an alkaline environment. The intercalation of pAB in its anionic form into the LDHs structure is responsible for a completely different behavior since the release is induced by chloride ions and occurs by an anionic exchange reaction. Thus, these materials play a dual role by acting as an inhibitor reservoir and by capturing chlorides. These findings reveal that it is possible to create a reservoir of corrosion inhibitors gradually released on demand based on the chemical environment. The stimuli-responsive properties and the complementary protective action of inhibitor-loaded silica and LDHs make them attractive for the long-term protection of steel and open the way for innovative solutions in the preservation of concrete cultural heritage.

Multi-Spectroscopic Approach for the Non-invasive Characterization of Paintings on Metal Surfaces.

The aim of this study is to propose a non-invasive multi-spectroscopic approach for the characterization of oil painting artworks that use a copper plates as substrate in place of a canvas. Indeed, in the last decade, many studies have been conducted on artworks made of single materials (e.g., paintings, stones, metals). However, the characterization and conservation of composite artifacts has never be fully investigated even though many masterpieces were created using this particular technique. In this work, several spectroscopic techniques such as Infrared Spectroscopy (FTIR), Energy-Dispersive X-Ray Fluorescence spectroscopy (EDXRF), and high spatial resolution Field Emission Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy (EDS), and Optical Microscopy (OM) were performed. The obtained results allowed to fully characterize the micro-chemical and microstructural features of the painted surfaces and of the metal plate. Particularly effective was the use of MA-XRF, resulting in the chemical map of the painted surfaces. Furthermore, traces of the mechanical preparation of the plate were found under the painted layers. Finally, the interface area between the paint film and the metallic support was characterized at a micro scale. This was particularly important in order to identify the degradation products formed by the interaction between the fatty acids of the binder and copper-based substrates.

Differential X-Ray Attenuation in MA-XRF Analysis for a Non-invasive Determination of Gilding Thickness.

When investigating gilded artifacts or works of art, the determination of the gilding thickness plays a significant role in establishing restoration protocols or conservation strategies. Unfortunately, this is done by cross-sectioning the object, a destructive approach not always feasible. A non-destructive alternative, based on the differential attenuation of fluorescence radiation from the sample, has been developed in the past years, but due to the intrinsic random nature of X-rays, the study of single or few spots of an objects surface may yield biased information. Furthermore, considering the effects of both porosity and sample inhomogeneities is a practice commonly overlooked, which may introduce systematic errors. In order to overcome these matters, here we propose the extrapolation of the differential-attenuation method from single-spot X-ray fluorescence (XRF) measurements to macro-XRF (MA-XRF) scanning. In this work, an innovative algorithm was developed for evaluating the large amount of data coming from MA-XRF datasets and evaluate the thickness of a given overlapping layer over an area. This approach was adopted to study a gilded copper-based buckle from the sixteenth to seventeenth century found in Rome. The gilded object under investigation was also studied by other analytical techniques including scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS). Previous results obtained from SEM-EDS were used to confront the data obtained with the proposed methodology and validate it. MA-XRF elemental distribution maps were fundamental in identifying and choosing sampling areas to calculate the thickness of the gilding layer, avoiding lead islands present in the sample that could negatively influence the results. Albeit the large relative standard deviation, the mean thickness values fell within those found in literature and those obtained from previous studies with SEM-EDS. Surface fissure has been found to deeply affect the results obtained, an aspect that is often disregarded.

Rebuilding of the Burial Environment from the Chemical Biography of Archeological Copper-Based Artifacts.

The long-term interaction between Cu-based alloys and environmental species gives rise to the formation of different and sometimes unusual compounds (i.e., the patina) with distinctive chemical and structural features as a function of the peculiar characteristics of the context. In this paper, we describe some representative case studies concerning degraded bronze Roman valuable or common use objects, and we show that an attentive study at a microscale level, as for forensic analysis, allows one to understand the chemical processes that underlie the formation of the surface alteration products. The achieved information describe fragments of chemical life and disclose the complex chemical changes suffered by the artifacts, allowing one to write their chemical biography. This challenging approach expands the panorama of available information and demonstrates that it is possible to reconstruct the different modes through which the bronze surfaces and interfaces have interacted with environmental species, organic matter, and microorganisms, opening up a new possibility to describe complex environmental chemical stories, in certain cases interrelated.

Double-Wall Nanotubes and Graphene Nanoplatelets for Hybrid Conductive Adhesives with Enhanced Thermal and Electrical Conductivity.

Improving the electrical and thermal properties of conductive adhesives is essential for the fabrication of compact microelectronic and optoelectronic power devices. Here we report on the addition of a commercially available conductive resin with double-wall carbon nanotubes and graphene nanoplatelets that yields simultaneously improved thermal and electrical conductivity. Using isopropanol as a common solvent for the debundling of nanotubes, exfoliation of graphene, and dispersion of the carbon nanostructures in the epoxy resin, we obtain a nanostructured conducting adhesive with thermal conductivity of ∼12 W/mK and resistivity down to 30 µΩ cm at very small loadings (1% w/w for nanotubes and 0.01% w/w for graphene). The low filler content allows one to keep almost unchanged the glass-transition temperature, the viscosity, and the curing parameters. Die shear measurements show that the nanostructured resins fulfill the MIL-STD-883 requirements when bonding gold-metalized SMD components, even after repeated thermal cycling. The same procedure has been validated on a high-conductivity resin characterized by a higher viscosity, on which we have doubled the thermal conductivity and quadrupled the electrical conductivity. Graphene yields better performances with respect to nanotubes in terms of conductivity and filler quantity needed to improve the resin. We have finally applied the nanostructured resins to bond GaN-based high-electron-mobility transistors in power-amplifier circuits. We observe a decrease of the GaN peak and average temperatures of, respectively, ∼30 °C and ∼10 °C, with respect to the pristine resin. The obtained results are important for the fabrication of advanced packaging materials in power electronic and microwave applications and fit the technological roadmap for CNTs, graphene, and hybrid systems.

ß-Cyclodextrin-grafted on multiwalled carbon nanotubes as versatile nanoplatform for entrapment of guanine-based drugs.

The design of ß-cyclodextrin/multiwalled carbon nanotubes hybrid (ß-CD-MWCNT) as nanoplatform for the entrapment and delivery of guanine based drugs is described here. The functionalized carbon nanomaterials have been characterized by XPS spectroscopy, electron microscopy (FEG-SEM and TEM), AFM, TGA, and FT-IR to achieve insights on structure, morphology and chemical composition. The drug binding abilities of nanocarrier towards the guanine (G) and Acyclovir (Acy) were proved by UV-vis and DSC experiments. Host-guest equilibrium association constants and drug loading have been evaluated for G/ß-CD-MWCNT and Acy/ß-CD-MWCNT complexes. The release studies showed a sustained delivery of Acy without initial burst effect confirming a strong interaction of drug with the nanoplatform sites. The preliminary antiviral data indicated that the Acyclovir loaded into the ß-CD-MWCNT platform interferes with HSV-1 replication and the antireplicative effect was higher than the free drug.

Highly efficient photoanodes for dye solar cells with a hierarchical meso-ordered structure.

An engineered bi-layered photoelectrode for dye solar cells has been developed which profitably employs two synergistic meso-ordered components, namely a thin meso-ordered TiO2 film and a main microparticles-based photoelectrode. The former has been deposited as an interfacial layer at the FTO-coated substrate and suppresses the back-transport reaction by blocking direct contact between the electrolyte and conductive oxide. The latter is made of hierarchical micro- and nano-structured building blocks prepared by template synthesis, which permits efficient light scattering without sacrificing the internal surface area. The optimization of light harvesting and charge recombination dynamics allowed us to achieve as high energy conversion efficiency as 9.7%.