Materials and Corrosion 2012. 63. No. 1 D:101002maco.201106189 Failure analysis of leakage on titanium tubes within heat exchangers in a nuclear power plant. Part l: electrochemical corrosion Z.-G. Yang,Y Gong and ) .-Z.Yuan Titanium tubes generally exhibit superior resistance against electrochemical corrosions amid seawater for their passive films Tio2. However, hydrogen- assisted corrosion(HAC) is actually the Achilles'heel to titanium materials when he temperature exceeds near 70C. In this event, severe degradations like quick thinning and leakage were frequently detected on a large number of titani tubes exposed to natural seawater environment within heat exchangers in a nuclear power plant, which caused serious safety problems. This paper is the Part I of totally two parts conducted for the whole failure analysis study, mainly focusing on electrochemical aspect of failure causes and their behaviors By means of over ten kinds of characterization methods, the analysis results identified that the hAc induced by the interaction effects between galvanic corrosion and crevice corrosion led to local bulges of the inner walls of some titanium tubes, and then the bulges were quickly thinned and eventually uptured under the eddy erosion from the seawater containing sediment particles. Finally, relevant mechanisms were addressed in detail and prevention methods were proposed as well 1 Introduction tube [5 rcw heat exchangers, within which the desalinated water is conveyed outside the tubes(also called the shell side), and mong all the thirteen nuclear power units presently under the seawater is transported inside the tubes(also called the tube peration in China[1, 2 ] the two 728MWe CANDU 6 units in the side). However, natural seawater usually contains high contents Phase Ill of Qinshan Nuclear Power Plant, which were imported of salts, chloride ions, and even sediment particles, various from Atomic Energy of Canada Limited (AECL), are the first and selective corrosions as well as mechanical degradations are the only two pressurized heavy water reactor(PHWR) units in consequently prone to emerge on these heat exchanger tubes with China, and started commercial operation on 31 December 2002 matrix of titanium, greatly reducing their service lifetimes and 24 July 2003, respectively, with design lifetime of 40 years [ 3) In this event, during about 3 years after actual operatio In a Candu 6 style unit, the recirculating cooling water(2003-2006), failure incidents including clogging, quick thin- (RCW)system consists of two heat exchange loops -the first one ning, and even leakage, etc. frequently occurred on a great cools the power equipments in the nuclear island and the steam number of titanium tubes within the rCW heat exchangers of the equipments in the conventional island by means of desalinated two CAndU 6 nuclear power units in Qinshan Phase Ill, causing water, while the second one cools such warmed desalinated water substantial economic losses as well as potential safety problems by using natural seawater within a specific kind of equipment [6). Materials quality, equipment operation, service environment, called RCW heat exchanger [4]. Hereby, each of the two CANDU routine maintenance, or other factors, which were the main 6 units is equipped in the conventional island with four shell and causes for inducing these premature failures in these cooling equipment, were urgently investigated. Consequently, in order to immediately identify the causes of such failures, investigation Z-G. Yang, Y Gong into four aspects were carried out by referencing our previous Department of Materials Science, Fudan University, Shanghai 200433 successful failure analysis experiences 7-9), including matrix R. Ch materials, environmental media, operation conditions, installa- E-mail:ziyang@fudan.edu.cn tion and maintenance. based on the leaked tubes and the 1-Z Yuan environmental media as seawater, desalinated water. etc.. over 10 Third Qinshan Nuclear Power Co Ltd, Haiyan 314300, Zhejiang kinds of characterization methods were conducted for failure Province(P R China) analysis in series of totally two parts, and totally seven kinds of www.matcorr.com wileyonlinelibrary.com o 2012 WILEY-VCH Verlag GmbH& Co KGaA, WeinheimFailure analysis of leakage on titanium tubes within heat exchangers in a nuclear power plant. Part I: Electrochemical corrosion Z.-G. Yang*, Y. Gong and J.-Z. Yuan Titanium tubes generally exhibit superior resistance against electrochemical corrosions amid seawater for their passive films TiO2. However, hydrogen￾assisted corrosion (HAC) is actually the Achilles’ heel to titanium materials when the temperature exceeds near 70 8C. In this event, severe degradations like quick thinning and leakage were frequently detected on a large number of titanium tubes exposed to natural seawater environment within heat exchangers in a nuclear power plant, which caused serious safety problems. This paper is the Part I of totally two parts conducted for the whole failure analysis study, mainly focusing on electrochemical aspect of failure causes and their behaviors. By means of over ten kinds of characterization methods, the analysis results identified that the HAC induced by the interaction effects between galvanic corrosion and crevice corrosion led to local bulges of the inner walls of some titanium tubes, and then the bulges were quickly thinned and eventually ruptured under the eddy erosion from the seawater containing sediment particles. Finally, relevant mechanisms were addressed in detail and prevention methods were proposed as well. 1 Introduction Among all the thirteen nuclear power units presently under operation in China [1, 2], the two 728MWe CANDU 6 units in the Phase III of Qinshan Nuclear Power Plant, which were imported from Atomic Energy of Canada Limited (AECL), are the first and the only two pressurized heavy water reactor (PHWR) units in China, and started commercial operation on 31 December 2002 and 24 July 2003, respectively, with design lifetime of 40 years [3]. In a CANDU 6 style unit, the recirculating cooling water (RCW) system consists of two heat exchange loops – the first one cools the power equipments in the nuclear island and the steam equipments in the conventional island by means of desalinated water, while the second one cools such warmed desalinated water by using natural seawater within a specific kind of equipment called RCW heat exchanger [4]. Hereby, each of the two CANDU 6 units is equipped in the conventional island with four shell and tube [5] RCW heat exchangers, within which the desalinated water is conveyed outside the tubes (also called the shell side), and the seawater is transported inside the tubes (also called the tube side). However, natural seawater usually contains high contents of salts, chloride ions, and even sediment particles, various selective corrosions as well as mechanical degradations are consequently prone to emerge on these heat exchanger tubes with matrix of titanium, greatly reducing their service lifetimes. In this event, during about 3 years after actual operation (2003–2006), failure incidents including clogging, quick thin￾ning, and even leakage, etc. frequently occurred on a great number of titanium tubes within the RCW heat exchangers of the two CANDU 6 nuclear power units in Qinshan Phase III, causing substantial economic losses as well as potential safety problems [6]. Materials quality, equipment operation, service environment, routine maintenance, or other factors, which were the main causes for inducing these premature failures in these cooling equipment, were urgently investigated. Consequently, in order to immediately identify the causes of such failures, investigations into four aspects were carried out by referencing our previous successful failure analysis experiences [7–9], including matrix materials, environmental media, operation conditions, installa￾tion, and maintenance. Based on the leaked tubes and the environmental media as seawater, desalinated water, etc., over 10 kinds of characterization methods were conducted for failure analysis in series of totally two parts, and totally seven kinds of Materials and Corrosion 2012, 63, No. 1 DOI: 10.1002/maco.201106189 7 Z.-G. Yang, Y. Gong 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) www.matcorr.com wileyonlinelibrary.com
2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim