8 Yang, Gong and Yuan Materials and Corrosion 2012. 63. No. 1 different failure mechanisms were simultaneously detected. As a hydraulically expanded (expansion ratio 80%)with carbon steel matter of fact, such a comprehensive failure analysis study from tube sheets cladded by titanium(ASME SA-515 Gr 65 carbon steel engineering practice for titanium tubes that are applied in nuclear [13 78mm-thick, ASME SB-265 Gr 1 Ti[14]3 mm-thick), without power units has been rarely reported. In the Part I of this whole seal welding. Therein, the diameter of supporting holes on all the study, research mainly focused on the electrochemical corrosion plates was 19.25+0.51mm. The schematic diagram of the RCW on the titanium tubes, while in the Part II [10], analysis heat exchanger, as well as the working parameters including flow dominantly concentrated in the mechanical degradation on the directions and input/output temperatures of the desalinated tubes. Concretely in this paper addressed for the Part I, the water and seawater are all listed in Fig. 1(b) hydrogen-assisted corrosion(HAC) including both hydrogen The two target leaked tubes analyzed in the present study blistering(HB)and hydrogen embrittlement(HE), which is were both sampled from the 4#f RCW heat exchanger of unit II generally thought to emerge on titanium only above temperature which suffered the severest degradations on its tubes among all of 70C[11], was actually found occurring around 35-40C when the eight heat exchangers in the two CANDU 6 units-113 tubes it was induced by the interaction effect between galvanic corrosion were found to be heavily thinned, and another 24 tubes were even and crevice corrosion under complicated service conditions. leaked. Furthermore, the ruptures on over 90% of the leaked Relevant mechanisms of these electrochemical corrosions, tubes were located between the first baffle plate and the 78 mm- especially their interaction effect were discussed in detail. Finally, thick tube sheet, and even buried inside this tube sheet, also seen countermeasures and suggestions were also put forward. in Fig. 1(b). Meanwhile, as indicated with the circles in Fig. 2, obvious brown corrosion phenomenon was observed on the 2 Experimenta surface of the tube sheet itself as well Figure 3 presents the external appearances of the two lea 2.1 Visual observation tubes named A and B from this heat exchanger. As shown in Fig 3(a), the rupture on tube a was in shape of 5 x 3 mm ellipse As shown in Fig. 1(a), the RCW heat exchanger is a horizontal and located about 30 mm off the inlet. While for the rupture on der with dimension of about 2400 x 15 000 mm, in which tube B, it seemed relatively round with a diameter of nearly 6 mm, there exist totally 4932 tubes (ASME SB-338 Gr2 titanium[12) and was 70 mm off the inlet, seen in Fig 3(b). Hereby, it must be $ x 14630 x 0.71 mm) in 57 rows and 93 columns. All the noted that both the distances of the two ruptures off the tube tubes are sustained by 23 16 mm-thick carbon steel baffle plates inlets were less than 78 mm, i.e., they were both formed buried ith interval distance of 603 mm. and their two ends were inside the tube sheet. 2.2 Characterization methods hen investigations from three were successively carr out on the two leaked tubes including the evaluation of their matrix materials, the inspection of the two media they contacted e, desalinated water and seawater), and the microscopic analysis of the ruptures on them. As for the first one, oxygen nitrogen hydrogen(ONH)analyzer, carbon sulfur analyzer(CSA) and inductively coupled plasma atomic emission spectroscopy (ICP-AES), were used to inspect their chemical composition optical microscopy (OM) was utilized to observe their metallo- graphic structures; and series of mechanical tests including unit: m desalinated water baffle plies corroSIo seawater305°C tube bundle Figure 1. Illustration of the RCW heat nger:(a)external appearance,( b)scheme and operation Figure 2. Corrosion on the inlet of the tube sheet o 2012 WILEY-VCH Verlag Gmbh Co KGaA, Weinheim www.matcorr.comdifferent failure mechanisms were simultaneously detected. As a matter of fact, such a comprehensive failure analysis study from engineering practice for titanium tubes that are applied in nuclear power units has been rarely reported. In the Part I of this whole study, research mainly focused on the electrochemical corrosion on the titanium tubes, while in the Part II [10], analysis dominantly concentrated in the mechanical degradation on the tubes. Concretely in this paper addressed for the Part I, the hydrogen-assisted corrosion (HAC) including both hydrogen blistering (HB) and hydrogen embrittlement (HE), which is generally thought to emerge on titanium only above temperature of 70 8C [11], was actually found occurring around 35–40 8C when it was induced by the interaction effect between galvanic corrosion and crevice corrosion under complicated service conditions. Relevant mechanisms of these electrochemical corrosions, especially their interaction effect were discussed in detail. Finally, countermeasures and suggestions were also put forward. 2 Experimental 2.1 Visual observation As shown in Fig. 1(a), the RCW heat exchanger is a horizontal cylinder with dimension of about w2400
15 000 mm, in which there exist totally 4932 tubes (ASME SB-338 Gr.2 titanium [12], w19
14 630
0.71 mm) in 57 rows and 93 columns. All the tubes are sustained by 23 16 mm-thick carbon steel baffle plates with interval distance of 603 mm, and their two ends were hydraulically expanded (expansion ratio 80%) with carbon steel tube sheets cladded by titanium (ASME SA-515 Gr.65 carbon steel [13] 78 mm-thick, ASME SB-265 Gr.1 Ti [14] 3 mm-thick), without seal welding. Therein, the diameter of supporting holes on all the plates was 19.25 0.51 mm. The schematic diagram of the RCW heat exchanger, as well as the working parameters including flow directions and input/output temperatures of the desalinated water and seawater are all listed in Fig. 1(b). The two target leaked tubes analyzed in the present study were both sampled from the 4# RCW heat exchanger of unit II, which suffered the severest degradations on its tubes among all the eight heat exchangers in the two CANDU 6 units – 113 tubes were found to be heavily thinned, and another 24 tubes were even leaked. Furthermore, the ruptures on over 90% of the leaked tubes were located between the first baffle plate and the 78 mm￾thick tube sheet, and even buried inside this tube sheet, also seen in Fig. 1(b). Meanwhile, as indicated with the circles in Fig. 2, obvious brown corrosion phenomenon was observed on the surface of the tube sheet itself as well. Figure 3 presents the external appearances of the two leaked tubes named A and B from this heat exchanger. As shown in Fig. 3(a), the rupture on tube A was in shape of 5
3 mm ellipse and located about 30 mm off the inlet. While for the rupture on tube B, it seemed relatively round with a diameter of nearly 6 mm, and was 70 mm off the inlet, seen in Fig. 3(b). Hereby, it must be noted that both the distances of the two ruptures off the tube inlets were less than 78 mm, i.e., they were both formed buried inside the tube sheet. 2.2 Characterization methods Then investigations from three aspects were successively carried out on the two leaked tubes, including the evaluation of their matrix materials, the inspection of the two media they contacted (i.e., desalinated water and seawater), and the microscopic analysis of the ruptures on them. As for the first one, oxygen nitrogen hydrogen (ONH) analyzer, carbon sulfur analyzer (CSA), and inductively coupled plasma atomic emission spectroscopy (ICP-AES), were used to inspect their chemical compositions; optical microscopy (OM) was utilized to observe their metallo￾graphic structures; and series of mechanical tests including 8 Yang, Gong and Yuan Materials and Corrosion 2012, 63, No. 1 Figure 1. Illustration of the RCW heat exchanger: (a) external appearance, (b) scheme and operation parameters Figure 2. Corrosion on the inlet of the tube sheet
2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.matcorr.com