16 Yang, Gong and Yuan Materials and Corrosion 2012. 63. No. 1 olate carbon ste the other degradation as he was also aroused by the presence of TiH, on the titanium tubes in this event. The value of x usually Fe" hydrolyzes anode:Fe→Fe+2e ranges from 1.5 to 2.0 with respect to the thickness of the titanium hydride, and the surface is commonly TiH2 [32]. TiH2 is an absolutely brittle compound in fcc(face-centered cubic H cathode: O+2H:O+4e structure [33, 34], and its thickness will keep growing with the increase of hydrogen concentration in it. Once the thickness exceeds a critical value, cracks always in brittle mode are brought about [32], i.e. the HE [35], which was distinctly observed in T+2H→TiH tube, titanium Fig 11 on the titanium tube with eligible strength and toughness. Now it can be briefly summarized that since there existed less than 0. 5mm gap between the titanium tubes and carbor Figure 13. Schematic diagram of the mechanisms of electrochemical plate due to unsymmetrically hydraulically expanding and and hac on tit inlet of tube sheet, galvanic corrosion and crevice corrosion naturally happened in the gap zone under penetration of seawater. Then, the HAC including HB and HE was induced on the outside wall of the tube, leading to a localized electron donors to produce hydrogen ions, the reverse reaction of bulge inwards to the inside wall of the tube. In the following, the Equation(8). However in our opinion, these hydrogen atoms will sustaining erosion from seawater containing sediment particles be mutually coupled and form hydrogen gases H2, Equation (10). in the tube side was exerted on the bulge. The bulge wall was thus This can be also ascribed to the presence of ferrous ions, which gradually thinned and finally ruptured. have already yielded relatively high content of hydrogen ions in the system to prohibit the reverse reaction of Equation(8), still the 5 Conclusions and recommendations Le Chatelier,'s principle H+To2→Tio(OH) (9) ()Matrix materials of the titanium tubes in the RCW heat exchangers were standard Gr2 pure titanium, qualified in chemical compositions, microstructures, and mechanical left some gaps between the titanium tubes and carbon steel What is more, other than mutually coupled to form hydroger plates because of the unsymmetrically hydraulically expand gases within TiO2, the excessive hydrogen atoms with diameter of 1.06A[29]can easily traverse through the TiO2 crystal structure ing. It was such kind of the gaps that resulted in occurrence whose smallest lattice parameter is as"wide"as 2.95A (301 of galvanic corrosion and crevice corrosion, and then (even larger than the diameter of hydrogen molecule, 2.72 A (31 ).(il) Under such electrochemical corrosions, the titanium tubes into and within the pure titanium under TiO2 film until reaching were localizedly bulged inwards to the inside wall due to HB e maximum solubility of 20-100 ppm [28. This will lead to and then thinned by erosion from seawater containing sediment particles in the tube side until being ruptured. two results, on one hand, these hydrogen atoms will also be (iv) The interaction between galvanic corrosion and crevice mutually coupled to produce hydrogen gases; on the othe corrosion decreased the initiation temperature from 70C to hand, the titanium hydride TiHx will be formed, seen in 35-40C for hac to occur on titanium which was a new Equation(11) phenomenon founded in the present study Ti+xH→TiHx Based on cause analysis for this failure, a simple, effective Now we can basically determine the sources ofhydrogen that countermeasure was proposed, i. e, special polymer resin finally resulted in the HAC in this event. With the increasing sealants must be coated on the inlet surface of the tube sheet amount of hydrogen gases within both the TiO2 and the pure to fill possible 0.5 mm gaps between the tubes and the plates. As pon the outside wall of the tubes. As a result, the passive film subsequent HAC were thus eliminated. Afterward, the bulges was destroyed and the tube wall was localizedly bulged inwards caused by electrochemical corrosion did not occur any more in the inside wall along the thickness, which was exactly the origin of the routine inspection of next few years the 'unknown force on the outside walls of both tubes a and b Actually, once the tube wall was bulged, the erosion effect from the seawater containing sediment particles in the tube side would Acknowledgements: This work was cooperated with Third be exerted on this bulge, gradually thinning its wall thickness Qinshan Nuclear Power Co. Ltd ( TQNPC)and was supported until perforation. Relevant mechanical mechanisms will be by Shanghai Leading Academic Discipline Project(Project proposed in Part II. Besides the HB induced by hydrogen gases, Number: B113) o 2012 WILEY-VCH Verlag Gmbh Co KGaA, Weinheim www.matcorr.comelectron donors to produce hydrogen ions, the reverse reaction of Equation (8). However in our opinion, these hydrogen atoms will be mutually coupled and form hydrogen gases H2, Equation (10). This can be also ascribed to the presence of ferrous ions, which have already yielded relatively high content of hydrogen ions in the system to prohibit the reverse reaction of Equation (8), still the Le Chatelier’s principle. H þ TiO2 ! TiOðOHÞ (9) H þ H ! H2 (10) What is more, other than mutually coupled to form hydrogen gases within TiO2, the excessive hydrogen atoms with diameter of 1.06 A˚ [29] can easily traverse through the TiO2 crystal structure whose smallest lattice parameter is as ‘‘wide’’ as 2.95 A˚ [30] (even larger than the diameter of hydrogen molecule, 2.72 A˚ [31]). As a consequence, the hydrogen atoms will quickly diffuse into and within the pure titanium under TiO2 film until reaching the maximum solubility of 20–100 ppm [28]. This will lead to two results, on one hand, these hydrogen atoms will also be mutually coupled to produce hydrogen gases; on the other hand, the titanium hydride TiHx will be formed, seen in Equation (11). Ti þ xH ! TiHx (11) Now we can basically determine the sources of hydrogen that finally resulted in the HAC in this event. With the increasing amount of hydrogen gases within both the TiO2 and the pure titanium (indeed detected by SIMS), the HB was engendered upon the outside wall of the tubes. As a result, the passive film was destroyed and the tube wall was localizedly bulged inwards the inside wall along the thickness, which was exactly the origin of the ‘unknown’ force on the outside walls of both tubes A and B. Actually, once the tube wall was bulged, the erosion effect from the seawater containing sediment particles in the tube side would be exerted on this bulge, gradually thinning its wall thickness until perforation. Relevant mechanical mechanisms will be proposed in Part II. Besides the HB induced by hydrogen gases, the other degradation as HE was also aroused by the presence of TiHx on the titanium tubes in this event. The value of x usually ranges from 1.5 to 2.0 with respect to the thickness of the titanium hydride, and the surface is commonly TiH2 [32]. TiH2 is an absolutely brittle compound in fcc (face-centered cubic) structure [33, 34], and its thickness will keep growing with the increase of hydrogen concentration in it. Once the thickness exceeds a critical value, cracks always in brittle mode are brought about [32], i.e. the HE [35], which was distinctly observed in Fig. 11 on the titanium tube with eligible strength and toughness. Now it can be briefly summarized that since there existed less than 0.5mm gap between the titanium tubes and carbon steel plate due to unsymmetrically hydraulically expanding and unsealed welding on the inlet of tube sheet, galvanic corrosion and crevice corrosion naturally happened in the gap zone under penetration of seawater. Then, the HAC including HB and HE was induced on the outside wall of the tube, leading to a localized bulge inwards to the inside wall of the tube. In the following, the sustaining erosion from seawater containing sediment particles in the tube side was exerted on the bulge. The bulge wall was thus gradually thinned and finally ruptured. 5 Conclusions and recommendations (i) Matrix materials of the titanium tubes in the RCW heat exchangers were standard Gr.2 pure titanium, qualified in chemical compositions, microstructures, and mechanical properties. (ii) The unsealed welding to the inlet of the tube sheet naturally left some gaps between the titanium tubes and carbon steel plates because of the unsymmetrically hydraulically expand￾ing. It was such kind of the gaps that resulted in occurrence of galvanic corrosion and crevice corrosion, and then induced the HAC including HB and HE. (iii) Under such electrochemical corrosions, the titanium tubes were localizedly bulged inwards to the inside wall due to HB and then thinned by erosion from seawater containing sediment particles in the tube side until being ruptured. (iv) The interaction between galvanic corrosion and crevice corrosion decreased the initiation temperature from 70 8C to 35–40 8C for HAC to occur on titanium, which was a new phenomenon founded in the present study. Based on cause analysis for this failure, a simple, effective countermeasure was proposed, i.e., special polymer resin sealants must be coated on the inlet surface of the tube sheet to fill possible 0.5 mm gaps between the tubes and the plates. As a result, the galvanic corrosion and crevice corrosion, as well as subsequent HAC were thus eliminated. Afterward, the bulges caused by electrochemical corrosion did not occur any more in the routine inspection of next few years. Acknowledgements: This work was cooperated with Third Qinshan Nuclear Power Co. Ltd. (TQNPC) and was supported by Shanghai Leading Academic Discipline Project (Project Number: B113). 16 Yang, Gong and Yuan Materials and Corrosion 2012, 63, No. 1 Figure 13. Schematic diagram of the mechanisms of electrochemical corrosions and HAC on titanium tubes
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