10 Yang, Gong and Yuan Materials and Corrosion 2012. 63. No. 1 a)ent revealed in Table 4, which conformed to the normal compositions of natural seawater- a high content of chloride ions 3. 3 Rupture analysis of tube a 3.3.1 SEM and EDs After magnifying the rupture in Fig. 3(a), it can be obviously learnt from Fig. 5(a) that the fringes of this rupture were bent inwards to the inside wall, which may be attributed to a relatively large force exerting on the outside wall of the tube. Meanwhile, rown colored rust covered around this rupture, which should be the corrosion products of iron oxides originated from the Ti/ carbon steel tube sheet. The inside wall around the rupture was pretty smooth, and the extent of rusting was not as severe as that on the outside wall too [Fig. 5(b)]. Under SEM, microscopic morphology of the area marked with rectangular in Fig. 5(b) was presented in Fig. 6(a). Besides the smooth fringe(site 001), some corrosion substances(site 002) were also scaling on the inside wall of the rupture. By means of EDS, the former one was exactly the titanium matrix material of the tube[Fig. 6(c)), while the latter one actually also contained iron and oxygen elements other than titanium [Fig. 6(d). It can be inferred that such corrosion substances were introduced from the outside wall of the tube after the rupture was formed. Furthermore, it must be particularly noted that the corner of the inside wall exhibited an unusual morphology seeming like oriented eddy erosion, seen in Fig. 6(b). Just due to this erosion effect, almost no corrosion substances were scaling around the corner, completely contrast to the site 002 in Fig. 6(a). Also, Figure 4. Metallographic structures of the titanium tubes: (a) transverse,(b)longitudinal another significant evidence also revealed that densely distributed pits existed near the corner with eddy erosion trace. This fact means that such kind of erosion was even accompanied with impact because of the sediment particles contained in the natural Table 2. Mechanical properties of the titanium tubes(wt%) Yield strength Uo?(MPa) Tensile strength o,(MPa) Elongation 8s (% Hardness HvI 35.0 riginal values from RMI 296-3 435-494 ASME SB-338 Gr 2 275-450 ≥20 GB/T3624-199516 370-530 165-225 Table 3. GFAAS results of the desalinated water (ppb, equals to ug/L) Elemen Fe Cu T desalinated water Table 4. Element constituents of the seawater(ppm, equals to mg/L) Cu Seawater 5.19×103 1.16×102 0.13 <0.002 <0002 0.015 o 2012 WILEY-VCH Verlag Gmbh Co KGaA, Weinheim www.matcorr.comrevealed in Table 4, which conformed to the normal compositions of natural seawater – a high content of chloride ions. 3.3 Rupture analysis of tube A 3.3.1 SEM and EDS After magnifying the rupture in Fig. 3(a), it can be obviously learnt from Fig. 5(a) that the fringes of this rupture were bent inwards to the inside wall, which may be attributed to a relatively large force exerting on the outside wall of the tube. Meanwhile, brown colored rust covered around this rupture, which should be the corrosion products of iron oxides originated from the Ti/ carbon steel tube sheet. The inside wall around the rupture was pretty smooth, and the extent of rusting was not as severe as that on the outside wall too [Fig. 5(b)]. Under SEM, microscopic morphology of the area marked with rectangular in Fig. 5(b) was presented in Fig. 6(a). Besides the smooth fringe (site 001), some corrosion substances (site 002) were also scaling on the inside wall of the rupture. By means of EDS, the former one was exactly the titanium matrix material of the tube [Fig. 6(c)], while the latter one actually also contained iron and oxygen elements other than titanium [Fig. 6(d)]. It can be inferred that such corrosion substances were introduced from the outside wall of the tube after the rupture was formed. Furthermore, it must be particularly noted that the corner of the inside wall exhibited an unusual morphology seeming like oriented eddy erosion, seen in Fig. 6(b). Just due to this erosion effect, almost no corrosion substances were scaling around the corner, completely contrast to the site 002 in Fig. 6(a). Also, another significant evidence also revealed that densely distributed pits existed near the corner with eddy erosion trace. This fact means that such kind of erosion was even accompanied with impact because of the sediment particles contained in the natural 10 Yang, Gong and Yuan Materials and Corrosion 2012, 63, No. 1 Figure 4. Metallographic structures of the titanium tubes: (a) transverse, (b) longitudinal Table 2. Mechanical properties of the titanium tubes (wt%) Yield strength s0.2 (MPa) Tensile strength sb (MPa) Elongation d5 (%) Hardness HV1 Sample 1 435 535 35.0 174 Sample 2 427 545 31.5 166 Average 431 540 33.3 170 original values from RMI 296–351 435–494 32–39 / ASME SB-338 Gr.2 275–450  345  20 / GB/T 3624-1995[16] 250 370–530 20 165–225 Table 3. GFAAS results of the desalinated water (ppb, equals to mg/L) Element Fe Cu Ti Mn Ni Cr desalinated water 137 39.2 <20 2.5 <5 0.7 Table 4. Element constituents of the seawater (ppm, equals to mg/L) Element Cl Mg Al Cu Fe Ti Mn Seawater 5.19
103 1.16
102 0.13 <0.002 0.088 <0.002 0.015
2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.matcorr.com