Materials and Corrosion 2012. 63. No. 1 Electrochemical corrosion failure of leakage on titanium tubes Results and discussion 3.1 Matrix materials 3.1.1Ch 23456 9 ululuululuuustumllm Chemical compositions of the titanium tubes' matrix materials are listed in Table 1. which are in accordance with the requirements of the ASME SB-338 Gr 2 titanium specification (equals to the TA2 industrial purity titanium in GB/T 3620 1-2007 standard of China (15)), as well as their original values provided by manufacturer RMI. In fact, contents of some of the impurities are even nearly one order of magnitude less than their limitations 3. 1.2 Metallographic structures Figure 4(a)and(b )show the metallographic structures of the titanium tubes in, respectively, transverse and longitudinal directions. It is obvious that the structures in both directions display a similar morphology, i. e, equiaxed polygonal grains -the typical structure of a-titanium, and their average ASTM grain 912314.5.789.1h 3. 1.3 Mechanical test In order to examine whether performances deterioration had occurred on the titanium tubes various mechanical tests were onducted on the two leaked tubes, and the results were also compared with the original values after manufacture. As listed in Table 2, all the mechanical properties of the tubes were eligible according to their standards, and the yield strength and Figure 3. External appearances of the ruptures on the two leaked the tensile strength were even a bit increased than the original tubes: (a)tube a, (b)tube B alues Based on all the analysis results illustrated above, it can be oncluded now that the matrix materials of the titanium tubes tensile test, hardness survey, etc. were also applied to evaluate after service were still qualified, in other words, the failures could their mechanical properties. With respect to the second one, the not be ascribed to the inappropriate selection of tube materials chemical constituents of the desalinated water and the seawater were respectively detected by graphite furnace atomic absorption 3.2 Environmental media pectrometry(GFAAS), ion chromatography(IC), and ICP-AES In terms of the final one, besides further observation of the In this section, inspection will be mainly focused on the two kinds macroscopic morphologies of the ruptures on the two leaked of liquids, the desalinated water and the seawater. Since the tubes in Fig 3, scanning electron microscopy (SEM) and energy concentrations of the constituents in the desalinated water were dispersive spectrometry(EDS) were adopted to analyze their quite low, GFAAS was particularly adopted, and the results were microscopic morphologies along with micro-area compositions; listed in Table 3. It should be noted that the contents of iron and meanwhile, X-ray photoelectron spectroscopy(XPS), secondary copper elements were relatively high, which means that corrosion ion mass spectrometry (SIMS), and X-ray diffraction(XRD)were had exactly occurred in the RCW system. With respect to the also employed to characterize their near-surface features. seawater, by using ICP-AES and IC, its element constituents were Table 1. Chemical co e titanium tubes(wt%) Element C N Others nal values from RMI <0.3 <0.25 <0.40 GB/T3620.1-2007TA2 <0.30 <0.08 <0.0 <0.10 www.matcorr.com o 2012 WILEY-VCH Verlag GmbH& Co KGaA, Weinheimtensile test, hardness survey, etc. were also applied to evaluate their mechanical properties. With respect to the second one, the chemical constituents of the desalinated water and the seawater were respectively detected by graphite furnace atomic absorption spectrometry (GFAAS), ion chromatography (IC), and ICP-AES. In terms of the final one, besides further observation of the macroscopic morphologies of the ruptures on the two leaked tubes in Fig. 3, scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) were adopted to analyze their microscopic morphologies along with micro-area compositions; meanwhile, X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and X-ray diffraction (XRD) were also employed to characterize their near-surface features. 3 Results and discussion 3.1 Matrix materials 3.1.1 Chemical compositions Chemical compositions of the titanium tubes’ matrix materials are listed in Table 1, which are in accordance with the requirements of the ASME SB-338 Gr.2 titanium specification (equals to the TA2 industrial purity titanium in GB/T 3620.1-2007 standard of China [15]), as well as their original values provided by manufacturer RMI. In fact, contents of some of the impurities are even nearly one order of magnitude less than their limitations. 3.1.2 Metallographic structures Figure 4(a) and (b) show the metallographic structures of the titanium tubes in, respectively, transverse and longitudinal directions. It is obvious that the structures in both directions display a similar morphology, i.e., equiaxed polygonal grains – the typical structure of a-titanium, and their average ASTM grain size is about 8. It should be also pointed out that the sizes of these grains are basically even, without excessively coarsened or fined. 3.1.3 Mechanical test In order to examine whether performances deterioration had occurred on the titanium tubes, various mechanical tests were conducted on the two leaked tubes, and the results were also compared with the original values after manufacture. As listed in Table 2, all the mechanical properties of the tubes were still eligible according to their standards, and the yield strength and the tensile strength were even a bit increased than the original values. Based on all the analysis results illustrated above, it can be concluded now that the matrix materials of the titanium tubes after service were still qualified, in other words, the failures could not be ascribed to the inappropriate selection of tube materials. 3.2 Environmental media In this section, inspection will be mainly focused on the two kinds of liquids, the desalinated water and the seawater. Since the concentrations of the constituents in the desalinated water were quite low, GFAAS was particularly adopted, and the results were listed in Table 3. It should be noted that the contents of iron and copper elements were relatively high, which means that corrosion had exactly occurred in the RCW system. With respect to the seawater, by using ICP-AES and IC, its element constituents were Materials and Corrosion 2012, 63, No. 1 Electrochemical corrosion failure of leakage on titanium tubes 9 Figure 3. External appearances of the ruptures on the two leaked tubes: (a) tube A, (b) tube B Table 1. Chemical compositions of the titanium tubes (wt%) Element Fe C N H O Others Single Total tube in 4# of Unit II 0.069 0.012 0.006 0.0019 0.12 / / original values from RMI 0.07 0.01 0.008 0.0015 0.12 / / ASME SB-338 Gr.2 0.30 0.08 0.03 0.015 0.25 0.10 0.40 GB/T 3620.1-2007 TA2 0.30 0.08 0.03 0.015 0.25 0.10 0.40 www.matcorr.com
2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim