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.