112 C. Casavola et al. velocity) air quality, and the synergistic effects of pollutant mixtures. The conservation state of specific artifacts exposed to polluted urban environments is commonly evaluated by analyzing the patina properties and the degradation of the material [16]. The corrosion of materials occurs with oxidation and the development of an electric current, which goes through the metal to the cathode where the reduction reaction takes place. The result of this process is the consumption of metal and the oxide formation. Patina or thick corrosion crust formed on artifacts, may display complex products and structures. Some of those products or structural details may depend on the microstructure of the metal or the alloy, which is attacked by corrosive agents. Their formation may also be influenced by a variety of growth mechanisms relating to the development and morphology of the corrosion products themselves. The identification of the corrosion products or characterization of the patina on archaeological finds is an essential requisite to acquire a better knowledge about the condition of ancient objects, corrosion processes and conservation treatment or preventive procedures for long-term, stable preservation [17]. Typically, several instruments are used to simulate in the laboratory the action of corrosion on materials such as climate chambers, salt spray and diving alternate (wet and dry), where the materials are subjected to immersion-emersion cycles in a solution [18]. From the above considerations it appears evident that monitoring of the corrosion is a very important aspect for the evaluation of the degradation of cultural heritage artifacts [19]. A relevant aspect that should be considered in developing a proper monitoring system is the possibility to guarantee high sensibility without damaging the measurement object. This indicates that a proper approach should be found among non destructive techniques as well as those based on ultrasounds [20], thermography [21], magnetic noise [22], X-ray [23] etc. In addition, fastness of the analysis and easiness are aspects that should be considered as well [24]. Optical techniques appear to be a promising approach to be explored; in fact they are non-contact techniques that allows to get full-field information with high sensibility that they have been used in many field of experimental mechanics including residual stress measurement [25–30], hybrid testing [31] and slow motion detection [32]. Corrosion onset can be detected as a surface 3D variation so that contouring methods by optical techniques could be implemented to follow the corrosion damage mechanism. One of the most promising approaches for optical contouring is based on Fringe Projection (FP) which essentially consists in analyzing modulation of the fringe pattern projected on the surface to be analyzed [33–37]. The modulated pattern, in fact, contains the information about the object geometry. If compared with laser scanning FP is very effective in reducing the measurement time as it does not require the operation of scanning of the object. By demodulating the information in the recorded pattern it is possible to extract the shape of the examined object, creating a unique match between the image pixels and the points analyzed in the physical space [38]. The determination of the coordinates of the surface of an object is obtained through the Triangulation method [39]. 18.2 Materials and Methods In the present study, corrosion was studied on Copper-Tin alloy (Cu-Sn) samples. The chemical composition (wt %) of CuSn alloy is listed in Table 18.1. The material was supplied as plates 1 mm thick: 50 100 mm sample were cut from the plate. The specimen surface was mechanically polished by abrasive paper with decreasing granulometry (down to 4000 grit) and degreased with alcohol prior to corrosion test. Corrosion test was performed by immersing the sample for 20 and 110 h in an acid solution (synthetic rain); at the end of each step the sample was dried. The synthetic rain (solution) consists of: 95 ml distilled water, 2.5 ml nitric acid, 1.5 ml hydrochloric acid and 1 ml hydrofluoric acid HF. The surface morphology of the sample after the time exposure to synthetic rain was investigated by Optical Microscope (OM) Nikon Epiphot 200. The weight changes of the sample were measured to a precision of 0.1 mg by analytical balance. The average weight loss results of the sample were calculated using weight of sample measured before and after exposure to synthetic rain. Average weight loss values were applied to calculate the corrosion rate using Eq. (18.1) [40]: Corrosion Rate Œmpy D W 3:45 106 D T A (18.1) Where the corrosion rate is in mils per year, W is weight loss of the samples given in [g], D is the density of the metal in [g/cm3], T is exposure time in [h] and finally A is the exposure surface area given in [cm2]. Table 18.1 Chemical composition (wt %) of Cu-Sn alloy Sn Zn Pb Ni Fe Sb P Cu Cu-Sn alloy 18–21 0.5 1 0.5 0.3 0.2 0.05 Balance
RkJQdWJsaXNoZXIy MTMzNzEzMQ==