PSI - Issue 22

178 L. Sena et al. / Procedia Structural Integrity 22 (2019) 171–180 L. Sena, R. C. Reis, A. A. Monteiro , C. Oliv ira, C. Reis , P. L. Silv , Analysis of iron FC 250 microstructure under the effect of phosphoric acid corrosion ,2019 In the graphite region, the emergence of hydrogen, and attenuation of oxygen, is the main mechanism, in aerated acids and neutrals, in the cathodic zone. Two reactions, 2 and 3 are pointed out (YARO, A et al, 2013).

(2) (3)

2H+ + 2e- → H 2

O 2 +2H 2 O + 4e

- → 4OH -

In Figure 6, the corrosion was lower due to the iron passivation, in a longer time immersion scale, so it was not possible to see so clearly the other phases.

(b)

(a)

(c) Fig. 6. SEM photomicrographs of the surface of the Gray Cast Iron samples, after immersion in phosphoric acid, at 298.15 K over a period of 424,800 s. (a) 1%; (b) 2%; (c) 3%. As temperature increases, oxidation is more advanced and in shorter times, as it may be shown in Figure 7. In these cases the longer period, from 21,600 to 43,200s also favors the process (Figures 7 and 8).