Issue 20

A. Borruto et alii, Frattura ed Integrità Strutturale, 20 (2012) 22-31 ; DOI: 10.3221/IGF-ESIS.20.03 25 R ESULTS he samples were taken from pieces of pipe removed from the furnace in the atmospheric distillation unit. The specimens were tested with macrographic and micrographic analysis (M.O.), electronic microscopy (SEM) and EDS analysis. Macrographic analysis The macrographic analysis shows two types of damage inside the pipe: pitting corrosion and areas with thickly oxidized scale presence (Fig. 4). Fig. 5 is a magnification of a part of figure 4 and shows the presence of a large surface pit. Fig. 6 is also a magnification of a part of figure 4: it highlights the deposits of the products of macroscopic corrosion, which are very coherent and thick. Areas with different colours are recognizable on the internal surface of the pipes, seemingly due to the presence of corrosion products such as  -Fe 2 O 3 (hematite) and  -Fe 2 O 3 (maghemite) respectively reddish brown and dark yellowish brown [8-9], besides Fe 3 O 4 and Cr 2 O 3 oxides. Each Fe oxide shows a colour that is a function of several factors including the electron transitions, the particle size and shape, crystal defects, adsorbed impurities, and the degree of particle packing [9]. Particularly, a reduction in particle size of Fe oxides results in paler colours, one exception is goethite [9-10]. Figure 4 : Macrographic analysis. View of the internal surface of a pipe take from a pre-heating fornace. Figure 5 : Macrographic analysis. Magnification of a detail from Fig. 4. Pitting corrosion. Figure 6 : Macrographic analysis. Magnification of a detail from Fig. 4. Corrosion scale. Micrographic analysis (M.O.) Surfaces, showing pit and scale, were examined. Micrographic analysis was carried out, comparing the areas under the pit showed in Fig. 5, with those under the scales showed in Fig. 6. For this purpose two samples were taken from the investigated corroded pipe as shown in Fig. 7. Figure 7 : Areas of sampling; sample1: pit zone; sample2: scale zone. T