Nanoscale investigations of the corrosion of metallic artworks by X-ray Photoemission Spectroscopy Maria Pia Casaletto, Antonella Privitera, Viviana Figà Istituto per lo Studio dei Materiali Nanostrutturati, Consiglio Nazionale delle Ricerche, CNR-ISMN Palermo, Italy mariapia.casaletto@cnr.it Abstract— X-ray Photoemission Spectroscopy (XPS) is a surface-sensitive analytical technique commonly used in the field of materials science at the nanoscale level. Historically applied to the industrial field, it is also successfully used in the investigation of Cultural Heritage, both for diagnostics and for conservation purposes. The nature and the state of conservation of artefacts can be assessed by XPS surface analysis, evidencing the presence of efflorescence, patinas, degradation/corrosion products, coatings and protective layers applied in restoration treatments. The feasibility of probing the surface reactivity in the nanometric range and understanding the chemical processes, occurring in different aggressive environments, makes XPS a very powerful tool also in the field of Conservation of Cultural Heritage. This analytical technique can be very fruitfully employed for the validation of new eco-compatible conservation products (e.g. corrosion inhibitors for metallic artworks) and for the choice of the suitable conservation and restoration protocols. In this work, some case-studies related to the application of XPS to the conservation of copper-based and iron-based alloys artefacts are reported. In particular, the focus is on the evaluation of the conservation state and the identification of degradation phenomena of archaeological bronzes and on the assessment of naturally-derived corrosion inhibitors for a sustainable conservation of iron-based artworks. Keywords — Metal corrosion; X-ray Photoemission Spectroscopy; Cultural Heritage. I. INTRODUCTION Technology-driven solutions in the field of art and archaeological studies are nowadays a mandatory demand that need to be fulfilled. Scientific researches are addressed to investigate the possible applications of a variety of non- conventional and innovative analytical techniques and to develop compact, non-invasive and portable equipment of traditional diagnostic systems. The correlation between Surface Science and Heritage Science, even if so far only occasionally strenghtened, should be consolidated and widely spread for mutual advantages. X-ray photoemission spectroscopy (XPS) is a suitable and powerful tool for the identification of elements present onto the surface and for the determination of the relative quantitative surface composition of a specimen. Furthermore, it is able to discriminate the oxidation state of an element in the nanoscale, finding extended application in the field of metals and alloys protection from corrosion. The advantages of XPS are related to different analytical aspects. It is a non-destructive multi-elemental analysis (all elements from the Periodic Table are detected, except H and He), requiring very small amount of specimen, due to its very low sensitivity limit (0.1 at. %). It allows a surface-sensitive (1-5 nm) chemical analysis of a sample, with the possibility to perform also depth profiling analysis in the case of multilayered structures. Furthermore, also surface chemical imaging (XPS imaging) can be provided. The constrains of XPS consist in the requirement of ultra-high vacuum (UHV) conditions for the experiment, in a suitable sampling and in a sophisticated and expensive equipment that, unfortunately, is unmovable and, therefore, can not be used for ‘in situ’ measurements. In order to be applied to Cultural Heritage materials, the development of new sample-handling and sample-preparation techniques are required for XPS analysis. According to these reasons, XPS can be fruitful applied in the field of Cultural Heritage for studying and evaluating the conservation and the restoration of ancient materials, because it provides information on the chemical nature of artefacts, the state of degradation and the efficiency of conservation protocols. Due to its characteristics, XPS can not be used as a routine analytical techniques for applications to the Science of Cultural Heritage, as conventionally occurring in the field of Materials Science. Its poignant and incomparable contribution concerns the capability to highlight and understand the mechanisms of chemical and physical degradation, in order to propose, apply and validate materials and methods for mitigation, remediation and inhibition of degradation and/or corrosion phenomena. The application of XPS to the diagnostics of metallic archaeological findings started around the 1976 when Lambert and McLaughlin investigated the oxidation state of an ancient bronze artefact [1] and later on provided an overview of the use of XPS in archaeology [2]. Afterwards, some research groups adopted this approach. For example, Paparazzo et al. employed this technique for studying bronzes from the collections of the Vatican Museums evidencing the role of different burial sites on the chemical compounds present at the surface [3]. More recently, it was used for the evaluation of corrosion products on ornamental objects from the necropolis of Colle Badetta- Tortoreto [4] and for the identification of the corrosion products of “the Dancing Satyr” [5]. Several applications of X- ray photoelectron spectroscopy to Cultural Heritage Materials in the last ten years were reported in the literature in order to