Materials and Corrosion 2009. 60. No. 1: Pitting corrosion on 316L pipes 901 Inlet region Output region (CTA filter cake, 15--20% humidity, 98-105 C) (CTA powder, <0.03% humidity, 135C) 21250mm camer gas N一 Ta dryer cylinder Φ=3100mm igure 2 Schematic diagram of the operating conditions of TA drye through the steam pipes to heat the wet TA cakes that are 2.2 Matrix material examination ansported outside the pipes from the inlet region of the dryer. Moreover, carrier gas(nitrogen) is sent in the opposite direction Chemical compositions of the matrix materials in the inlet and of the transportation of the cakes, i.e. from the output region of output region of the steam pipes are listed in Table 1, which are in the dryer to remove the vapors evaporated from the cakes. More accordance with the requirements of SUS 316L specification concrete details of the operating conditions of TA dryer can be The high content of Cr in 316L can facilitate superior resistance to referred from Fig. 2. In this paper, pitting dominantly took corrosion, while the existence of Mn, S, and Si may form MnS and place at the inlet region of the steam pipes. Compared with the silicon oxides, which are likely to act as the initiation sites for surface of the pitting-free pipes(Fig 3(a)), which was smooth and pitting [7 plane, the surface of the pitted pipes was covered with a relatively The metallographic structures of the matrix material large area of pits [Fig 3(b)). As shown in Fig 3(c) and (d), many etched in 4g CuSO4, 20 ml HCl and 20 ml ethanol, are corrosion pits were randomly distributed on the pipe surface, and displayed in Fig. 4. As is shown in Fig. 4(a), the material is the depth of some pits even reached 0.5 mm. typically austenitic in structure with the average ASTM grain size [(d) ire 3. Macroscopic morphologies of pitted steam pipes surface; (a) pitting-free surface, (b) pitted surface, (c),(d)magnification of pitting hology www.matcorr.com c 2009 WILEY-VCH Verlag GmbH & Co KGaA, Weinheimthrough the steam pipes to heat the wet TA cakes that are transported outside the pipes from the inlet region of the dryer. Moreover, carrier gas (nitrogen) is sent in the opposite direction of the transportation of the cakes, i.e. from the output region of the dryer to remove the vapors evaporated from the cakes. More concrete details of the operating conditions of TA dryer can be referred from Fig. 2. In this paper, pitting dominantly took place at the inlet region of the steam pipes. Compared with the surface of the pitting-free pipes (Fig. 3(a)), which was smooth and plane, the surface of the pitted pipes was covered with a relatively large area of pits [Fig. 3(b)]. As shown in Fig. 3(c) and (d), many corrosion pits were randomly distributed on the pipe surface, and the depth of some pits even reached 0.5 mm. 2.2 Matrix material examination Chemical compositions of the matrix materials in the inlet and output region of the steam pipes are listed in Table 1, which are in accordance with the requirements of SUS 316L specifications. The high content of Cr in 316L can facilitate superior resistance to corrosion, while the existence of Mn, S, and Si may form MnS and silicon oxides, which are likely to act as the initiation sites for pitting [7]. The metallographic structures of the matrix material etched in 4 g CuSO4, 20 ml HCl and 20 ml ethanol, are displayed in Fig. 4. As is shown in Fig. 4(a), the material is typically austenitic in structure with the average ASTM grain size Materials and Corrosion 2009, 60, No. 11 Pitting corrosion on 316L pipes 901 Figure 3. Macroscopic morphologies of pitted steam pipes surface; (a) pitting-free surface, (b) pitted surface, (c), (d) magnification of pitting morphology Figure 2. Schematic diagram of the operating conditions of TA dryer www.matcorr.com
2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim