114 C. Casavola et al. Fig. 18.2 Images in the frequency domain, respectively to 0 h (a) and after 20 h (b) and 110 h (c) of corrosion test Fig. 18.3 Detail of the FFT spectrum respectively after 20 h (a) and 110 h (b) of corrosion test Fig. 18.4 Trend of the contrast in correspondence of different analyzed frequencies Fig. 18.6. From a macroscopic point of view, both the surface of sample corroded 20 and 110 h in synthetic rain displays an irregular and inhomogeneous layer of corrosion products. In fact, a different color between the surface exposure to synthetic rain for 20 and 110 h was observed. Also the color on the same surface is not uniform. The color difference variation is due to the patina nature growth and/or dissolution but also to the inhomogeneity of the layers. In particular, the microstructures of the surfaces indicated by the arrows in Fig. 18.6a, b have been shown, at higher magnification, in Fig. 18.7a, b respectively. As
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