D0:101002mac0201106190 Materials and Corrosion 2012. 63. No. 1 ailure analysis of leakage on titanium tubes within heat exchangers in a nuclear power plant. Part ll: Mechanical degradation Y Gong, Z -G Yang and- -Z. Yuan Serious failure incidents like clogging, quick thinning, and leakage frequently occurred on lots of titanium tubes of heat exchangers in a nuclear power plant in China. In the part I of the whole failure analysis study with totally two parts factors mainly involving three kinds of electrochemical corrosions were investigated, including galvanic corrosion, crevice corrosion, and hydroge assisted corrosion. In the current Part ll, through microscopically analyzing the dispersive spectrometry(EDS), another four causes dominantly lying in the g2 ruptures on the leaked tubes by scanning electron microscopy(SEM)and energ aspect of mechanical degradation were determined-clogging, erosion, mechanical damaging, and fretting. Among them, the erosion effect was the primary one, thus the stresses it exerted on the tube wall were also supplementarily evaluated by finite element method( FEM). Based on the analysis results, the different degradation extents and morphologies by erosion on the tubes when they were clogged by different substances such as seashell, rubber debris, and sediments were compared and relevant mechanisms were discussed. Finally, countermeasures were put forward as well. 1 Introduction tubes(the tube side) to cool the desalinated water outside the tubes(the shell side) that has been just utilized in advance to cool Since China now has the largest numbers of nuclear power units the power and the steam equipments in, respectively, the nuclear that are under construction, designed, and planned in the world and the conventional islands, hence immediate determination of [1, 2 ] safety evaluation of the 13 units currently under operation the causes of these premature failures were extremely required to really has an instructive value for those upcoming ones. Indeed, avoid further economic losses and safety problems in the first 3 years after starting commercial operation( from 2003 Consequently, based on our past successful failure analysis to 2006) of the two 728MWe CANDU 6 nuclear power experiences[3-6), systematical study was carried out in series of imported from Atomic Energy of Canada Limited(AECL)) of two parts. The previous Part I[7 primarily focused on scope of Qinshan Phase Ill in Qinshan Nuclear Power Plant, the shell and electrochemical corrosion, such as galvanic corrosion, crevice tube RCW (recirculating cooling water) heat exchangers installed corrosion, and their interaction effect on the initiation of in the conventional island were frequently encountered with hydrogen-assisted corrosion including hydrogen blistering and failure incidents including clogging, quick thinning, and even hydrogen embrittlement. In the current Part II, analysis will leakage on their titanium heat exchange tubes, substantially less mainly orient to the aspect of mechanical degradation, especially than their design lifetime 40 years. Hereby, these RCw heat the typical representative erosion, on basis of microscopically changers are employed to use the natural seawater inside the analyzing the ruptures on the leaked tubes. As a result, different erosion effects on the titanium tubes when being clogged by different substances were comparatively discussed. And finall Y Gong, Z-G. Yang the prevention methods were proposed. Department of Materials Science, Fudan University, Shanghai 200433 Actually, such an engineering practical study of mechanical degradations on titanium tubes that are applied in the E-mail:zgyang@fudan.edu.cn conventional island of a nuclear power unit has been rarely J-Z Yuan reported. Bermudez et al. [8]observed the surface morphologies Third Qinshan Nuclear Power Co Ltd, Haiyan 314300, Zhejiang variation of pure titanium under a simply simulated erosi Province, (P. R. China) corrosion environment; Neville and McDougall [9] detaile o 2012 WILEY-VCH Verlag Gmbh Co KGaA, Weinheim wileyonlinelibrary.com www.matcorr.comFailure analysis of leakage on titanium tubes within heat exchangers in a nuclear power plant. Part II: Mechanical degradation Y. Gong, Z.-G. Yang* and J.-Z. Yuan Serious failure incidents like clogging, quick thinning, and leakage frequently occurred on lots of titanium tubes of heat exchangers in a nuclear power plant in China. In the Part I of the whole failure analysis study with totally two parts, factors mainly involving three kinds of electrochemical corrosions were investigated, including galvanic corrosion, crevice corrosion, and hydrogen￾assisted corrosion. In the current Part II, through microscopically analyzing the ruptures on the leaked tubes by scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS), another four causes dominantly lying in the aspect of mechanical degradation were determined – clogging, erosion, mechanical damaging, and fretting. Among them, the erosion effect was the primary one, thus the stresses it exerted on the tube wall were also supplementarily evaluated by finite element method (FEM). Based on the analysis results, the different degradation extents and morphologies by erosion on the tubes when they were clogged by different substances such as seashell, rubber debris, and sediments were compared, and relevant mechanisms were discussed. Finally, countermeasures were put forward as well. 1 Introduction Since China now has the largest numbers of nuclear power units that are under construction, designed, and planned in the world [1, 2], safety evaluation of the 13 units currently under operation really has an instructive value for those upcoming ones. Indeed, in the first 3 years after starting commercial operation (from 2003 to 2006) of the two 728MWe CANDU 6 nuclear power units (imported from Atomic Energy of Canada Limited (AECL)) of Qinshan Phase III in Qinshan Nuclear Power Plant, the shell and tube RCW (recirculating cooling water) heat exchangers installed in the conventional island were frequently encountered with failure incidents including clogging, quick thinning, and even leakage on their titanium heat exchange tubes, substantially less than their design lifetime 40 years. Hereby, these RCW heat exchangers are employed to use the natural seawater inside the tubes (the tube side) to cool the desalinated water outside the tubes (the shell side) that has been just utilized in advance to cool the power and the steam equipments in, respectively, the nuclear and the conventional islands, hence immediate determination of the causes of these premature failures were extremely required to avoid further economic losses and safety problems. Consequently, based on our past successful failure analysis experiences [3–6], systematical study was carried out in series of two parts. The previous Part I [7] primarily focused on scope of electrochemical corrosion, such as galvanic corrosion, crevice corrosion, and their interaction effect on the initiation of hydrogen-assisted corrosion including hydrogen blistering and hydrogen embrittlement. In the current Part II, analysis will mainly orient to the aspect of mechanical degradation, especially the typical representative erosion, on basis of microscopically analyzing the ruptures on the leaked tubes. As a result, different erosion effects on the titanium tubes when being clogged by different substances were comparatively discussed. And finally, the prevention methods were proposed. Actually, such an engineering practical study of mechanical degradations on titanium tubes that are applied in the conventional island of a nuclear power unit has been rarely reported. Bermu´dez et al. [8] observed the surface morphologies variation of pure titanium under a simply simulated erosion– corrosion environment; Neville and McDougall [9] detailedly 18 DOI: 10.1002/maco.201106190 Materials and Corrosion 2012, 63, No. 1 Y. Gong, Z.-G. Yang Department of Materials Science, Fudan University, Shanghai 200433, (P. R. China) E-mail: zgyang@fudan.edu.cn J.-Z. Yuan Third Qinshan Nuclear Power Co. Ltd., Haiyan 314300, Zhejiang Province, (P. R. China)
2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com www.matcorr.com