F- Chen et aL/ Engineering Failure Analysis 37(2014)42-52 a Fig. 7. Appearance of tubes 052003-17, 18 imaged by stereo microscope(a)wavy defect zone(b)pits(c) boundary of defect zone and non-defect zone. 2.2. Failure analysis of titanium tubes 052003-17, 18 2.2.1. Visual inspection Titanium tube 052003-17, 18 belong to unit 2 and the failure position is on the upper part of the tube wall instead of near the contact part with the support plates commonly seen in Part I 1. Fig 6 depicts the appearance of titanium tube 052003-17, 18. From the picture we can see a lot of pitting defects distributed in a strip in the longitudinal direction on the top of the tube at 45(Fig. 6(a)). The pitting defects only appear on the top of the tube and the bottom is flawless( Fig. 6(b)). The boundary of the defect strip is very clear. Since the media on the shell side is pure water steam, no foreign ions are present. So the pits must be created by cavitation corrosion instead of pitting corrosion. 2.2.2. Microscopic morphology observation by stereo microsc Fig 7 is the surface morphology of titanium tube 052003-17, 18 imaged by stereo microscope. In the defect zone, wave structure and pits are seen(Fig. 7(a and b)) And the boundary between the defected part and the normal part where no defect is found is very clear(Fig. 7(c)). The wave structure matches the typical pattern of erosion.2.2. Failure analysis of titanium tubes 052003-17, 18 2.2.1. Visual inspection Titanium tube 052003-17, 18 belong to unit 2 and the failure position is on the upper part of the tube wall instead of near the contact part with the support plates commonly seen in Part I [1]. Fig. 6 depicts the appearance of titanium tube 052003-17, 18. From the picture we can see a lot of pitting defects distributed in a strip in the longitudinal direction on the top of the tube at 45 (Fig. 6(a)). The pitting defects only appear on the top of the tube and the bottom is flawless (Fig. 6(b)). The boundary of the defect strip is very clear. Since the media on the shell side is pure water steam, no foreign ions are present. So the pits must be created by cavitation corrosion instead of pitting corrosion. 2.2.2. Microscopic morphology observation by stereo microscope Fig. 7 is the surface morphology of titanium tube 052003-17, 18 imaged by stereo microscope. In the defect zone, wave structure and pits are seen (Fig. 7(a and b)). And the boundary between the defected part and the normal part where no defect is found is very clear (Fig. 7(c)). The wave structure matches the typical pattern of erosion. Fig. 7. Appearance of tubes 052003-17, 18 imaged by stereo microscope (a) wavy defect zone (b) pits (c) boundary of defect zone and non-defect zone. F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 42–52 47