Materials and Corrosion 2012. 63. No. 1 Electrochemical corrosion failure of leakage on titanium tubes 711.80eV, i.e., the FeO(OH)(Fig 8): ere was only one titanium spectrum here, and its binding energy was 458.53eV, i.e., the TiOz. Now it can be confirmed that some kind of corrosion which would bring about an irregular chemical alence on titanium only occurred on the outside wall of the tube and was probably related to the unknown force exerting on the tub 3.3.3SMS Considering the well-known fact that titanium exhibits strong nteraction with hydrogen [18]. thus the titanium with irregular chemical covalence on the outside wall near the rupture was possibly related to the hydrogen element, which can only be sensitively detected by surface analysis technique SIMS [19).As a displayed in Fig 9(a), hydrogen element in form of H2 gas was indeed present, while it did not exist on the inside wall in contrast, seen in Fig. 9(b). This result verified the conclusion we put forward above that corrosion only took place on the outside wall of the tube, and it was very probably the HAC. However, it still needs further method to determine the actual chemical compositions of the titanium hydrides 3.4 XRD It is clearly shown in Fig. 10 that a kind of crystalline with irregular stoichiometric number as TiH1924 was present other than pure titanium near the rupture. Then, the actual chemical omposition of the titanium hydride can be finally ascertained. In fact, the peak ofTiH1924 can be regarded as TiH2(20), the product of hydrogen absorption reaction. 3. 4 Rupture analysis of tube B 3.4.1 Macroscopic morphologies of the rupture on tube A: (a) As shown in Fig. 11(a), a crack with length of 3 mm was engendered from the rupture along the axial direction of the tube and exhibited brittle fracture morphology, while the inside wall seawater,whose mechanism will be detailedly discussed in Part II near the crack was even seriously deformed, seen in Fig. 11(b) of this study Similar to that on tube A, the rupture fringes here were also bent inwards to the inside wall but with severer extent. So it can be 3.3.2XPS udged that the two ruptures on both tubes a and b were aroused For the purpose of characterizing the surface features of both the by the same cause, but the degradation extent on tube b was outside wall and the inside wall near the rupture, XPS was utilized severer than that on tube A. to determine the chemical valences of relevant elements. As Consequently, similar microscopic analysis and near-surface shown in Fig. 7(a), totally four kinds of main elements were characterization were not needed to conduct again on tube B present on the outside wall, among them the two metal elements Instead, cross-sections of the rupture were observed. Figure 12( should be paid special attention to since they were from the and(b)illustrated that the surface of the outside wall was matrix materials of the tube and the plate. The electron binding deteriorated so severe that both delamination and descaling energy of iron was 711.80eV, which corresponded to the occurred on it, which were the evidence of crevice corrosion. On ompound FeO(OH)(17]. However for titanium, it had two the contrary, the inside wall of the tube was really smooth, seen in close spectra, that is to say two species of chemical valences Fig. 12(a). However, the fringe of the rupture was seriously existed. After locally magnifying(Fig. 7(b)), the binding energy of thinned due to the erosion effect from the seawater containing the left one was 458.48ev, while that of the right one was sediment particles, see Fig. 12(c) 455.74eV. Referring to the handbook [17 the former one represented TiO2, however the latter one could not correspond to 4 Failure analysis any compound. In other words, an irregular chemical valence was introduced on the titanium due to corrosion, and needed Based on the analysis results presented above, it can be concluded subsequent analysis from the electrochemical point of view that serious corrosion had is In order for comparison, the inside wall near the rupture was occurred on the titanium tubes of the 4# RCW heat exchanger in detected by XPS. The binding energy of iron was still Unit Il. Furthermore, since almost all the rupture locations on the www.matcorr.com o 2012 WILEY-VCH Verlag GmbH& Co KGaA, Weinheimseawater, whose mechanism will be detailedly discussed in Part II of this study. 3.3.2 XPS For the purpose of characterizing the surface features of both the outside wall and the inside wall near the rupture, XPS was utilized to determine the chemical valences of relevant elements. As shown in Fig. 7(a), totally four kinds of main elements were present on the outside wall, among them the two metal elements should be paid special attention to since they were from the matrix materials of the tube and the plate. The electron binding energy of iron was 711.80 eV, which corresponded to the compound FeO(OH) [17]. However for titanium, it had two close spectra, that is to say two species of chemical valences existed. After locally magnifying [Fig. 7(b)], the binding energy of the left one was 458.48 eV, while that of the right one was 455.74 eV. Referring to the handbook [17], the former one represented TiO2, however the latter one could not correspond to any compound. In other words, an irregular chemical valence was introduced on the titanium due to corrosion, and needed subsequent analysis. In order for comparison, the inside wall near the rupture was also detected by XPS. The binding energy of iron was still 711.80 eV, i.e., the FeO(OH) (Fig. 8); as for the titanium, there was only one titanium spectrum here, and its binding energy was 458.53 eV, i.e., the TiO2. Now it can be confirmed that some kind of corrosion which would bring about an irregular chemical valence on titanium only occurred on the outside wall of the tube, and was probably related to the unknown force exerting on the tube. 3.3.3 SIMS Considering the well-known fact that titanium exhibits strong interaction with hydrogen [18], thus the titanium with irregular chemical covalence on the outside wall near the rupture was possibly related to the hydrogen element, which can only be sensitively detected by surface analysis technique SIMS [19]. As displayed in Fig. 9(a), hydrogen element in form of H2 gas was indeed present, while it did not exist on the inside wall in contrast, seen in Fig. 9(b). This result verified the conclusion we put forward above that corrosion only took place on the outside wall of the tube, and it was very probably the HAC. However, it still needs further method to determine the actual chemical compositions of the titanium hydrides. 3.3.4 XRD It is clearly shown in Fig. 10 that a kind of crystalline with irregular stoichiometric number as TiH1.924 was present other than pure titanium near the rupture. Then, the actual chemical composition of the titanium hydride can be finally ascertained. In fact, the peak of TiH1.924 can be regarded as TiH2 [20], the product of hydrogen absorption reaction. 3.4 Rupture analysis of tube B 3.4.1 Macroscopic morphologies As shown in Fig. 11(a), a crack with length of 3 mm was engendered from the rupture along the axial direction of the tube and exhibited brittle fracture morphology, while the inside wall near the crack was even seriously deformed, seen in Fig. 11(b). Similar to that on tube A, the rupture fringes here were also bent inwards to the inside wall but with severer extent. So it can be judged that the two ruptures on both tubes A and B were aroused by the same cause, but the degradation extent on tube B was severer than that on tube A. Consequently, similar microscopic analysis and near-surface characterization were not needed to conduct again on tube B. Instead, cross-sections of the rupture were observed. Figure 12(a) and (b) illustrated that the surface of the outside wall was deteriorated so severe that both delamination and descaling occurred on it, which were the evidence of crevice corrosion. On the contrary, the inside wall of the tube was really smooth, seen in Fig. 12(a). However, the fringe of the rupture was seriously thinned due to the erosion effect from the seawater containing sediment particles, see Fig. 12(c). 4 Failure analysis Based on the analysis results presented above, it can be concluded from the electrochemical point of view that serious corrosion had occurred on the titanium tubes of the 4# RCW heat exchanger in Unit II. Furthermore, since almost all the rupture locations on the Materials and Corrosion 2012, 63, No. 1 Electrochemical corrosion failure of leakage on titanium tubes 11 Figure 5. Macroscopic morphologies of the rupture on tube A: (a) outside wall, (b) inside wall www.matcorr.com
2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim