Issue 20

A. Borruto et alii, Frattura ed Integrità Strutturale, 20 (2012) 22-31; DOI: 10.3221/IGF-ESIS.20.03

Al

Si

S

Cr

Fe

Mo

3.90

1.65

9.16

10.78

62.43

12.09

pt1

Table 1 : EDS analysis (weight%) of areas shown in the Fig. 12.

A section of the pit was made (Fig. 7 – sample 1 section). The progress of the corrosive process under the macroscopic pit (Fig. 13) and the inter-granular damage (Fig. 14) were monitored.

Figure 13 : SEM analysis. Section view of the pit. Corrosive process. – 600 X. Scale The pipe section scale was examined (Fig. 7 – sample 2 section):

Figure 14 : SEM analysis. Section view of the pit. Intergranular damage. – 2200 X.

internal pipe section: Fig. 15 shows an internal area of the pipe where the protective scale layer is still present, even if very thin; the scale appears damaged and penetrated, so that a significant fracture is present. EDS analysis (Tab. 2) shows in point 1, where the scale is compacted, a greater chromium concentration (probably Cr 2 O 3 ) than in points 3, 4, where the oxidation is principally due to Fe 2 O 3 and Fe 3 O 4 oxides. Considering the external pipe section scale: Fig. 16 shows a very porous and infiltrated surface, with relevant penetration of grain boundaries (Fig. 17).

O

Si

Cr

Mn

Fe

Mo

1.30 20.39 2.63 72.72 2.97

Pt 1 Pt 2

4.60 95.40 Pt 3 95.29 0.04 1.38 0.13 3.17 Pt 4 95.98 0.03 1.01 0.12 2.86 Table 2 : EDS analysis (weight%) of areas shown in the Fig. 15.

Figure 15 : SEM analysis. Internal area of pipe where protective scale layer is still present, even if very thin; the scale appears damaged and penetrated, so that a relevant fracture is present. – 400 X.

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