PSI - Issue 22

174 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 For the second phase, FSATOM Model 752A shaking plates were used to raise the solution temperature, and a 3% solution concentration was used immersing CPs at 323,15 K and 373,15 K for 21,600 s and 43,200 s. After soaking they were washed and dried to remove corrosion products. Corrosion was evaluated by weighing, before and after immersion, using a scale of SHIMADZU model AU220 with an accuracy of 0.1 mg. The surface attack was observed by Scanning Electron Microscopy (SEM), using a Hitachi TMBOBOPlus model.

Table 1 - Chemical composition of gray cast iron, according to technical report. Source - ARAUJO, 2017.

C

Si

Mn

P

S

Cr

Mo

Ni

Cu

Mg

3,29

2,01

0,72

0,073

0,041

0,13

0,010

0,028

0,51

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3. Results and discussion The study of the obtained corrosion products can provide indispensable information about the corrosion mechanism and also the products formed on the PCs (MOHEBBI, 2011). The results obtained in the immersion tests reinforced previous studies in which, with a higher concentration, the corrosion rate increases, as occurred in aluminum 6063 in solutions of phosphoric acid and sodium hydroxide (KRUIZENGA, GILL, 2013). It was observed that the longer the test duration, the lower the rate, making the material more resistant. This is occurs due to the passivation state, the formation of a thin oxide film on the surface of the cast iron or by an oxidized state (KRUIZENGA, GILL, 2013). It was also verified the release of oxygen, ions or molecules that were adsorbed, blocking the contact of the corrosive medium with the material (KRUIZENGA, GILL, 2013). For freely gasified cast iron in stagnant seawater, the wear is shown to increase with the immersion time, but there is a decrease of corrosion by the creation of the oxide layer (SHERIF, ABDO, ALMAJID, 2015). The percentage of 1% (shown in Figure 1) represents this decrease, in mass loss over time, and the ability of iron to form the protective film in mild media where the layer cannot be unobstructed, is evident. Figure 1 refers to the values found for the three percentages of acid, at room temperature, during 86,400 and 424,800 seconds. The increase in temperature interferes with the corrosion rate mainly in the percentage of 3%, in which higher values of mass loss for 373,15 K and duration of 21,600 s were obtained, again in shorter time, as shown in Figure 2. Kruizenga (KRUIZENGA, A., et al 2014) reported that every 283.15 K, the corrosion rate doubled, for both solutions. For the concentration of 3% the greatest mass losses occurred, and the temperature increase of 323,15 K and 373,15 K influenced the oxidation of the metal. For CPs at 298.15 K the longer test permanence (424,800 seconds) resulted in lower corrosion rates (Figure 3).