F- Chen et aL/ Engineering Failure Analysis 37(2014)42-52 Fig 9. Microscopic morphology of defects on tubes 052003-17, 18(a)wavy erosion zone(b)a large pit with thin, raised edge. 2.2.3. Microscopic morphology and composition analysis Fig. 9 is the microscopic morphology of the titanium tube imaged by SEM. From this figure it can be learned that the wavy races within the defect zone are somewhat directional (Fig. g(a)), and the pits are quite deep with raised thin edges (Fig 9(b)). To measure the element composition within the pits, EDS was conducted with the results shown in Fig. 10 and Table 4. The main constituent within the erosion and cavitation corrosion zone is titanium. The rest are iron and oxygen which come from the corrosion products of carbon steel support plates. Therefore, no foreign ions or substance are found and the defect must be created by the high pressure steam released from the bypass pipe. cording to the arrangement of tubes in Fig. 2, tube 052003-17, 18 are located at the shoulder of the tower structure. The fact that the defect zone is situated at the top of the tube at 45 and has clear boundary indicates that it is formed by impact of high pressure steam at a certain angle. 3. Discussion of failure mechanisms Erosion is caused by the lashing of solid or fluid in the form of loose particles against the surface of a material at a certain ngel and velocity. In this case, when liquid drops in the high pressure steam lashed against the titanium tubes right under the bypass pipes at a certain velocity, energy exchange happened. The energy of these liquid drops which they got from the team medium carrying them transfered to the outer wall of the tubes. when the energy was high enough or the time was long enough for plastic deformation of the tube wall to occur, indentation appeared. The material originally in the ndentation was pushed around the indentation to form a raised edge where thin peaks could be observed, as presented2.2.3. Microscopic morphology and composition analysis Fig. 9 is the microscopic morphology of the titanium tube imaged by SEM. From this figure it can be learned that the wavy traces within the defect zone are somewhat directional (Fig. 9(a)), and the pits are quite deep with raised thin edges (Fig. 9(b)). To measure the element composition within the pits, EDS was conducted with the results shown in Fig. 10 and Table 4. The main constituent within the erosion and cavitation corrosion zone is titanium. The rest are iron and oxygen which come from the corrosion products of carbon steel support plates. Therefore, no foreign ions or substance are found and the defect must be created by the high pressure steam released from the bypass pipe. According to the arrangement of tubes in Fig. 2, tube 052003-17, 18 are located at the shoulder of the tower structure. The fact that the defect zone is situated at the top of the tube at 45 and has clear boundary indicates that it is formed by impact of high pressure steam at a certain angle. 3. Discussion of failure mechanisms 3.1. Erosion Erosion is caused by the lashing of solid or fluid in the form of loose particles against the surface of a material at a certain angel and velocity. In this case, when liquid drops in the high pressure steam lashed against the titanium tubes right under the bypass pipes at a certain velocity, energy exchange happened. The energy of these liquid drops which they got from the steam medium carrying them transfered to the outer wall of the tubes. When the energy was high enough or the time was long enough for plastic deformation of the tube wall to occur, indentation appeared. The material originally in the indentation was pushed around the indentation to form a raised edge where thin peaks could be observed, as presented Fig. 9. Microscopic morphology of defects on tubes 052003-17, 18 (a) wavy erosion zone (b) a large pit with thin, raised edge. F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 42–52 49