METALS AND MATERIALS International, Vol. 14, No. 1 (2008), pp. 71~75 Repassivation Kinetics of Zirconium Alloys Investigated by a Scratching Electrode Technique Bo-Young Kim 1 , Chan-Jin Park 2, * , and Hyuk-Sang Kwon 1 1 Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea 2 School of Materials Science and Engineering, Chonnam National University, 300, Yongbong-dong, Buk-gu, Gwangju 500-757, Korea The repassivation behavior of zirconium alloys in a solution containing 0.1 M H BO and 0.1 M LiOH was examined by means of a rapid scratching electrode technique. The repassivation behavior of the scratched surface of the alloys varied with time. The place exchange model reveals that at the initial stage of repassivation (within 10 ms) log i (t) is linearly proportional to q (t) with a slope of 1/K. The high field ion conduction model reveals that after 25 ms log i (t) is linearly proportional to 1/q(t) with a slope of cBV. The cBV value, in particular, which is a measure of the protectiveness and erosion resistance of an alloy, increases when there is an increase in the Nb content of the alloys, as well as in the applied potential and the solution temperature. This result suggests that these parameters can affect the repassivation rate of zirconium alloy. Keywords: Zirconium alloy, niobium, passive film, repassivation 1. INTRODUCTION Zirconium (Zr) alloys have been used extensively as clad- ding materials for fuel rods in nuclear reactor systems on account of their low thermal neutron absorption cross sec- tion, excellent corrosion resistance, and good mechanical properties at high temperatures [1-3]. Nevertheless, many reports have shown that zirconium alloy tubes tend to fail as a result of the fretting corrosion that occurs at the tube-grid contact. Such failure is due to flow-induced assembly vibra- tions and to erosion (debris-induced fretting) caused by the flow and movement of debris such as Fe 3 O 4 and Fe 2 O 3 in the coolant [4,5]. These corrosion failures reduce the lifetime of the fuel cladding tube and increase the contamination by radioactive materials. Fretting corrosion and erosion are induced by the localized breakdown of a passive film on the alloy and by the expo- sure of a bare metal surface to corrosive environments. These alloys can nevertheless become resistant to fretting corrosion and erosion when the repassivation rate is suffi- ciently fast. Accordingly, the resistance to fretting corrosion and erosion can be simply estimated by analyzing the current density that arises from a scratched surface and the corre- sponding repassivation rate. The repassivation behavior of metals and alloys has been investigated with various abrading and scratching electrode techniques [6-13]. In particular, Kwon et al. used a rapid scratching electrode technique to analyze the repassivation kinetics of passive alloys [7,8,11]. Their report, which is based on a high field ion conduction model, indicates that the cBV value, which is the slope obtained from the linear region in the plot of log i(t) versus 1/q(t) during repassiva- tion, can be used as an effective measure of both the repassi- vation rate and the protectiveness of the reformed film. The initial stage of fretting corrosion and erosion is closely associated with the film breakdown and the following repas- sivation. A faster repassivation rate indicates a higher resis- tance to the initiation of fretting corrosion and erosion. Thus, the initiation of fretting corrosion and erosion in zirconium alloys can be roughly understood by examining the repassi- vation kinetics after film breakdown. Our aim is to examine the repassivation kinetics of zirconium alloys using a rapid scratching electrode technique. 2. EXPERIMENTAL PROCEDURE Sheets of pure Zr alloy and Zr -2.5 wt.% Nb alloy were used in this experiment. After heating the sheets to 1050 o C for 30 min, we water-quenched the specimens and annealed them at 750 o C. We then hot rolled and annealed the speci- *Corresponding author: parkcj@chonnam.ac.kr doi: 10.3365/met.mat.2008.02.071 Published 26 February 2008