Traditional and innovative protective coatings for outdoor bronze: Application and performance comparison Barbara Salvadori , 1 Andrea Cagnini, 2 Monica Galeotti, 1,2 Simone Porcinai, 2 Sara Goidanich, 3 Antonello Vicenzo, 3 Claudio Celi, 2 Piero Frediani, 4 Luca Rosi, 4 Marco Frediani, 4 Giulia Giuntoli, 4 Laura Brambilla, 5 Ruben Beltrami, 3 Stefano Trasatti 6 1 Institute for the Conservation and Valorization of Cultural Heritage, National Research Council, Via Madonna del Piano 10, Sesto Fiorentino I-50019, Italy 2 Opificio delle Pietre Dure, Via degli Alfani, 78, Florence I-50121, Italy 3 Department of Chemistry, Materials and Chemical Engineering “Giulio Natta”, Politecnico di Milano, Piazza Leonardo da Vinci 32, Milano I-20133, Italy 4 Department of Chemistry “Ugo Schiff”, University of Florence, via della Lastruccia 3-13, Sesto Fiorentino (FI) I-50019, Italy 5 Haute-Ecole Arc Conservation-Restauration, Espace de l’Europe 11, Neuch^ atel 2000, Switzerland 6 Department of Chemistry, University of Milan, Via Camillo Golgi 19, Milan I-20133, Italy Correspondence to: B. Salvadori (E - mail: salvadori@icvbc.cnr.it) ABSTRACT: A comparative evaluation of traditional and innovative coatings for outdoor bronze surfaces was carried out, including blending or pre-treatment with non-toxic corrosion inhibitors on bare and patinated bronze surfaces to simulate both as cast and aged surfaces. Coatings recently applied in bronze conservation practice, commercially available coatings and an innovative end- capped poly(lactic acid) with a benzotriazole (BTA) moiety have been tested, as well as a triple-layer system (wax/acrylic polymer/ wax). BTA, single-layer Soter wax, and double layer Incral44/Soter wax were used as a reference. The products were evaluated in terms of aesthetic features, molecular structure, and corrosion protection by means of colorimetric, spectroscopic, and electrochemical tests. All inhibitors showed similar or significantly better results compared to BTA. In particular, sodium oleate and tolyltriazole could be considered as promising lower-toxicity alternatives to BTA, while the triple-layer coating applied without pre-filming inhibitors proved to give a very high protection against corrosion. V C 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46011. KEYWORDS: applications; coatings; electrochemistry; microscopy; spectroscopy Received 26 June 2017; accepted 2 November 2017 DOI: 10.1002/app.46011 INTRODUCTION Bronze is one of the most commonly used materials for out- door statuary. In the last decades, the state of conservation of artworks exposed in urban polluted sites has been greatly affected, as a result of accelerating corrosion processes and pos- sible soiling, at times up to the point of compromising the aes- thetic features and value through the formation and spreading of diffuse alteration products. Preventive conservation measures such as storage in a controlled environment could ensure long- term preservation for most of the objects. However, it is a com- mon practice to keep the objects in their original place and pro- tect bronze artefacts against corrosion instead of moving them indoors. Indeed, art historians prefer to keep the sculptures in the context where the artist realized them, but also technical and economic issues may influence this decision. Coatings are sometimes associated with inhibitors having the specific role to slow down the corrosion processes avoiding damages. Common products used for protection of outdoor bronzes are waxes and acrylic resins-based coatings containing the classical corrosion inhibitor benzotriazole (BTA). This product presents some drawbacks related to not completely satisfactory aesthetic fea- tures, 1 lack of stability 2–5 and, above all, suspected toxicity. 6–8 In addition, the scarce permanence of BTA on the surface has been hypothesized 2,9 with health hazard for operators, visitors and environment. In the last years, the general trend has been directed towards the substitution of these toxic protective sys- tems by environmental friendly and low-toxicity products. Another possible solution to make the use of BTA safer is the immobilization of BTA through chemical bonding with suitable polymers [poly(lactic acid)], in order to avoid the leakage of the anticorrosive but toxic agent. 10 The literature offers a lot of studies on protective coatings against corrosion for industrial V C 2017 Wiley Periodicals, Inc. J. APPL. POLYM. SCI. 2018, DOI: 10.1002/APP.46011 46011 (1 of 12)