F-A. Chen et aL/ Engineering Failure Analysis 37(2014)42-52 cavities solid wall 777777 Fig. 11. Illustration of the microjet mechanism of cavitation corrosion. 3.2. Cavitation corrosion In the flowing fluid, when the pressure in a local region suddenly drops below the steam pressure corresponding to the local temperature for some certain reason, some of the fluid vaporizes. Then the gas dissolved in the fluid escapes and forms cavities. This process is called cavitation. On the one hand, the boiling point of water declines with the fall of pressure. On the other hand, the pressure of the fluid decreases with the increase of its flowing velocity, which can be explained by the b oulli equation in hydromechanics where P, P, D, C stand for the pressure, density, flowing velocity of the fluid and constant respectively. The moment the high pressure steam rushed out of the bypass pipes, its pressure dropped dramatically, leading to the increase of its velocity and piling point also declined. in this tation corrosion to the tubes. No consensus on the mechanism of cavitation corrosion has been reached yet. Rayleigh 11 proposed the theory of cavitation systematically in 1917 and nowadays more researchers tend to explain the mechanism as the combination of microjet mechanism and impact wave mechanism. According to the former one [12-14. when the cavities burst under the action of pressure gradient or near the boundary they deform into oblate spheres or shoe-shaped spheres and then divide and finally burst. Right before burst, a microjet forms and passes through the divided cavity and lashes the tube wall, causing cavitation corrosion, see Fig. 11. According to the latter one 15-17. cavities will burst when they reach high pressure regions in the fluid, which turns their potential energy into kinetic energy that flows within a small region. In this way, impact waves form in the fluid. When they contact the tubes, they will cause stress pulses and pulsing regional plastic deformation and even work hardening to the tube wall Plastic deformation and pits will form on the tube wall by repeating fluid impact waves. By the interaction of these two mechanisms, cavitation corrosion of the tube wall oc- curs due to the release of high pressure steam. There's also one favorable condition for cavitation corrosion in our case. That is the oxygen-deficient environment and the fast flow. It is commonly known that there exists a compact oxide layer on the surface of titanium which makes it corrosion- resistant. In oxygen-sufficient environment, once the passivation layer is damaged, it is re-formed immediately, preventing from further corrosion or degradation. But in the oxygen deficient condenser and with fast steam flow continual away the passivation layer damaged by the mechanical action of erosion in our case, it is not easy to form again. So all will undergo continuously cavitation corrosion 3.3. Synergetic effect of erosion and cavitation corrosion In in Part ll, erosion and cavitaion corrosion occured at the same time and both of them caused mechanical deg- radation. Apart from the factors we have mentioned that benefited erosion and cavitaion corrosion, the interaction between them was also one ruling cause. On the one hand cavitation corrosion left pits on the tube wall, where the passivation oxide layer had already been removed and the surface structure ruined. Then the pits served as vulnerable points of erosion, the result of which was the mechanical degradation of tube wall. In this way, erosion damage was made easier. On the other hand, erosion destroyed the surface structure of tube wall and created wavy pattern, making the tube wall structure loose and irregular. So the mechanical performance of the tube wall deteriorated and it was more vulnerable to deformation and consequently more vulnerable to cavitation corrosion By the interaction between these two processes, the pits and wavy zone were enlarged and deepened. As a result, serious wall thinning was observed 4. Conclusions 1. The titanium tubes in the condenser are TAl level industrial a-phase pure titanium tubes. their chemical composition, metallographic structure, microscopic morphology and mechanical properties all meet national and international standards. So the material is qualified.3.2. Cavitation corrosion In the flowing fluid, when the pressure in a local region suddenly drops below the steam pressure corresponding to the local temperature for some certain reason, some of the fluid vaporizes. Then the gas dissolved in the fluid escapes and forms cavities. This process is called cavitation. On the one hand, the boiling point of water declines with the fall of pressure. On the other hand, the pressure of the fluid decreases with the increase of its flowing velocity, which can be explained by the Ber￾noulli equation in hydromechanics: P þ qm2=2 ¼ C; where P, q, v, C stand for the pressure, density, flowing velocity of the fluid and constant respectively. The moment the high pressure steam rushed out of the bypass pipes, its pressure dropped dramatically, leading to the increase of its velocity and the boiling point of water also declined. In this way, cavitation happened and the high velocity steam caused serious cavi￾tation corrosion to the tubes. No consensus on the mechanism of cavitation corrosion has been reached yet. Rayleigh [11] proposed the theory of cavitation systematically in 1917 and nowadays more researchers tend to explain the mechanism as the combination of microjet mechanism and impact wave mechanism. According to the former one [12–14], when the cavities burst under the action of pressure gradient or near the boundary, they deform into oblate spheres or shoe-shaped spheres and then divide and finally burst. Right before burst, a microjet forms and passes through the divided cavity and lashes the tube wall, causing cavitation corrosion, see Fig. 11. According to the latter one [15–17], cavities will burst when they reach high pressure regions in the fluid, which turns their potential energy into kinetic energy that flows within a small region. In this way, impact waves form in the fluid. When they contact the tubes, they will cause stress pulses and pulsing regional plastic deformation and even work hardening to the tube wall. Plastic deformation and pits will form on the tube wall by repeating fluid impact waves. By the interaction of these two mechanisms, cavitation corrosion of the tube wall oc￾curs due to the release of high pressure steam. There’s also one favorable condition for cavitation corrosion in our case. That is the oxygen-deficient environment and the fast flow. It is commonly known that there exists a compact oxide layer on the surface of titanium which makes it corrosion￾resistant. In oxygen-sufficient environment, once the passivation layer is damaged, it is re-formed immediately, preventing titanium from further corrosion or degradation. But in the oxygen deficient condenser and with fast steam flow continually carrying away the passivation layer damaged by the mechanical action of erosion in our case, it is not easy to form again. So the tube wall will undergo continuously cavitation corrosion. 3.3. Synergetic effect of erosion and cavitation corrosion In our case in Part II, erosion and cavitaion corrosion occured at the same time and both of them caused mechanical deg￾radation. Apart from the factors we have mentioned that benefited erosion and cavitaion corrosion, the interaction between them was also one ruling cause. On the one hand, cavitation corrosion left pits on the tube wall, where the passivation oxide layer had already been removed and the surface structure ruined. Then the pits served as vulnerable points of erosion, the result of which was the mechanical degradation of tube wall. In this way, erosion damage was made easier. On the other hand, erosion destroyed the surface structure of tube wall and created wavy pattern, making the tube wall structure loose and irregular. So the mechanical performance of the tube wall deteriorated and it was more vulnerable to deformation and consequently more vulnerable to cavitation corrosion. By the interaction between these two processes, the pits and wavy zone were enlarged and deepened. As a result, serious wall thinning was observed. 4. Conclusions 1. The titanium tubes in the condenser are TA1 level industrial a-phase pure titanium tubes. Their chemical composition, metallographic structure, microscopic morphology and mechanical properties all meet national and international standards. So the material is qualified. Fig. 11. Illustration of the microjet mechanism of cavitation corrosion. F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 42–52 51