TRIBOCORROSION MECHANISM STUDY OF STELLITE-6 AND ZIRCALOY-4 A COMPARISON IN LiOH-H 3 BO 3 SOLUTIONS

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1 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI 35 TRIBOCORROSION MECHANISM STUDY OF STELLITE-6 AND ZIRCALOY-4 A COMPARISON IN LiOH-H 3 BO 3 SOLUTIONS Lidia BENEA 1, Viorel-Eugen IORDACHE 2, François WENGER 2, Pierre PONTHIAUX 2, Jean PEYBERNES 3, Joëlle VALLORY 3 1 Dunarea de Jos University of Galati, Dept. of Metallurgy and Materials Science, Romania 2 LGPM, Ecole Centrale Paris, France, 3 DEN/CAD-DTP, CEA Cadarache, France Lidia.Benea@ugal.ro ABSTRACT Parts of nuclear pressurized water reactors (PWR), immersed in the coolant water, are submitted to both mechanical efforts and corrosion. Our paper presents a comparison of tribocorrosion mechanisms of two PWR materials, Stellite-6 and Zircaloy-4 respectively. Tribocorrosion experiments were performed by using a pinon-disc tribometer under controlled electrochemical conditions in LiOH-H 3 BO 3 solutions similar to coolant water. For Stellite-6 the wear rate is controlled by the kinetics of the mechanical removal of the passive layer and by the kinetics of the repassivation during latency time, which determines the amount of the restored passive layer. For Zircaloy-4, the effect of the latency time is not clearly evidenced, but the wear is strongly dependent on the electrochemical potential. KEYWORDS: tribocorrosion mechanism, Stellite-6, Zircaloy-4, coolant water. 1. INTRODUCTION In a typical pressurized water reactor (PWR), the reactor core creates heat, pressurized-water in the primary coolant loop carries the heat to the steam generator, and the steam generator vaporizes the water in a secondary loop to drive the turbine, which produces electricity. The cooling water in the primary cooling circuit reaches a temperature between 280 C and 330 C and is kept under about 150 times atmospheric pressure to prevent it boiling. To moderate the fission reactions, 1000 ppm boron B is added as boron water and 2 ppm Li as LiOH is also added to maintain the ph close to 7.2. This water is a corrosive medium; mechanical parts which are immersed in it are submitted to both mechanical efforts and corrosion. Passivable alloys with high mechanical properties are therefore used for their manufacture. The gripper latch arms (GLAs) of the control rod drive mechanisms are protected against wear and corrosion by a thick Stellite-6 (cobalt-base superalloy, mainly with 28 wt. % chromium) layer, deposited by plasma spraying. Stellite-6 alloy contains ~13 wt. % carbides that are of the chromium rich type. These carbides provide high hardness level of the alloy. Higher carbide level is associated with lower corrosion resistance, since, in forming the carbides, carbon ties up a portion of the chromium in the alloy [1], so the effective chromium content is much lower than is evident from the nominal composition. The passive properties of Stellite-6 are of great importance in PWR environments, because the nature of the oxide films governs the corrosion release rate and the oxides are involved in plant dosimetry. Chromium enhances passivation of the cobalt alloys in the presence of oxygen and is the key ingredient with regard to corrosion resistance in oxidising media. Oxide films formed on Stellite-6 are similar to those formed on 304 L stainless steel with a two layer structure [1]. The inner layer is chromium rich layer with an X-ray diffraction structure close to CoCr 2 O 4. The outer layer formed by precipitation, mainly of iron, has the structure of magnetite M 3 O 4. The thickness of the inner layer follows a parabolic growth low. The film thickness is estimated to be around 20 nm after 100 hours of immersion. Consequently, Stellite-6 is a material that exhibits passivity and its electrochemical characteristics in PWR primary water can be assimilated with any other passivating media. Typically, when a passive metal or alloy is subjected to sliding wear in a corrosive environment, the total material removal rate differs from those calculated by corrosion rate or mechanical wear. Based on field experiences, Lemaire and Le

2 36 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI Calvar [2] have found that the wear loss W increases with the number N of friction contacts between GLA and control rod, and also, for a given N, with the latency time τ between two successive steps, following the empirical law: 1 n ( / ) W = N wo τ t o (1) where n is This law was interpreted by a tribocorrosion mechanism consisting in a periodical destruction by friction of the Cr 2 O 3 oxide film formed during τ. The previous relationship could be obtained if one considers that the oxidation current i t of the bare metal after mechanical depassivation decreases with time t following the power law: it io ( t / t ) n = (2) Due to low neutron-capture cross-section, high mechanical strength, high thermal conductivity and low corrosion rate, the nuclear fuel claddings in PWR are typically made of Zircaloy-4, especially for the French reactors. The friction between the cladding tubes and the support, caused by turbulences of the cooling water, leads to severe damages of the claddings, difficult to predict on the basis of simple wear laws. The aim of this study is therefore to analyse in detail, in laboratory conditions, the tribocorrosion mechanisms of Stellite-6 and Zircaloy-4. Firstly, wear-latency time dependence was examined. Potential jump experiments are then used to characterise the passivation kinetics of Stellite-6. Because of the high stability of the ZrO 2 oxide film, potential jump tests were difficult to carry out on Zircaloy-4; however, a wear- potential dependence was found out by performing friction tests under potentiostatic conditions. o 2. MATERIALS AND EXPERIMENTAL DETAILS 2.1. Materials Characterisation The micrographs of the Stellite-6 on surface and cross section have been obtained after polishing till diamond grade 1 µm. This type of polishing was used only for SEM observation of Stellite-6 deposit before wear corrosion experiments. The initial state of Stellite-6 surface before experiments was preserved at 600 grade polishing abrasive paper, state that correspond to real work of this material in PWR gripper latch arms system. Scanning electron microscopy (SEM) examinations with quantitative EDS analysis have been carried out on unused samples as reference. Surface and cross section SEM investigations were performed, the composition results being similar. In figure 1(a, b) the micrographs of cross section with some local phase analysis are presented. Stellite-6 has a two-phase microstructure as shown in previous figures. The results of average (general) micro-analyses at areas identified as light and dark in the figures are given in Table 1 (cross section analysis). The chromium rich phase (dark on the SEM images) is generally known to be carbide, being the constituent of Stellite-6 which confers on it the hardness and wear resistance. The more ductile phase, a solid solution called matrix (light grey on the SEM images), was found to be higher in cobalt and slightly denuded in chromium compared with average (general) analysis. Wolfram-rich small phases were found (completely white phases on SEM images with 19 wt. % of W) congregated around the carbides but it was evident that there was also wolfram contained in solid solution in the matrix. (a) (b) Cr=22.62% Co=62.36% Cr=66.34% Co=15.49% Fig. 1. SEM surface micrographs of polished Stellite-6: (a) general view; (b) details of central part with local phase analysis.

3 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI 37 Element, wt. % Table 1. Microanalysis of Stellite-6 in the polished conditions. C Co Cr W O Fe Ni Si Total Region General analysis Dark (carbide) Light (matrix) Specified conditions Balance From Table 1 it is clear that there is a good correlation between the measured general analysis and that specified by manufacturer. Zircaloy-4 is a zirconium-base alloy containing 1.5 Sn, 0.2 Fe and 0.1 Cr wt.%; it comprises an α-zr phase matrix with Sn in solid solution and a β-phase consisting in Zr(Fe,Cr) 2 intermetallic precipitates of about 50 to 200 nm of diameter. The corrosion of Zircaloy-4 by water produces the formation of a thick ZrO 2 oxide (zirconia) film. The corrosion kinetics divides the process into two regimes: an initial pretransition region approximately parabolic with respect to time, followed by a post-transition region of more accelerated kinetics with an approximately linear dependence on time. The nature of the oxide film shows a mixture of a stable monoclinic phase and a small amount of a tetragonal phase, the latter stabilized by the local conditions, such as stresses, small grain size, dissolved alloying elements. The outer part of this oxide is composed of less dense grains; during its growth, the oxide film develops cracking and porosity [3] Wear Corrosion Tests Tribocorrosion experiments were performed by using an appropriate device fully described elsewhere [4, 5], permitting classical pin-on-disc tribological tests in an aqueous solution under controlled electrochemical conditions to be done. A 25 mm diameter cylinder-shape specimen is installed in a simple electrolytic cell and then mounted on a pin-ondisc tribometer. The Stellite-6 specimens were prepared by depositing the superalloy by plasma spraying on stainless steel substrate on a thick (few millimetres) layer. An inert cylinder-shape corundum pin was chosen as counterbody, drawing a circular wear track of 16 mm diameter on the working surface of the sample. For Stellite-6 samples the pin had a 2 mm diameter flat end, while for Zircaloy-4 the pin had a hemispherical working contact surface of 100 mm radius. Two normal forces were used for the sliding friction tests, corresponding to the service life start and end conditions of the GLAs and of the fuel cladding in the PWR: 377 N and 94 N in the case of the Stellite-6 specimens (120 MPa and 30 MPa initial average contact pressure, respectively), 30 N and 6 N for the Zircaloy-4 specimens (110 MPa and 65 MPa, respectively). The friction tests performed at 120 rotations per minute (rpm) were stopped after laps (502.4 m of total sliding distance), and the worn volume was estimated by direct weight loss measurements with an electronic scale (0.2 mg accuracy) and by 3-D topographic characterisation of the wear scare with a STIL High-Resolution Optical Measurements Station (1 µm lateral and 30 nm vertical resolutions). For Stellite-6, the tests were conducted at 85 C in a 7.7 ph solution of boric acid H 3 BO 3 (1000 ppm of boron B) and Lithia LiOH (12 ppm of lithium Li). For Zircaloy-4, the tests were conducted at room temperature in a similar solution (1000 ppm boron and 130 ppm lithium, ph = 8.7). The concentrations of B and Li are chosen to maintain the same ratio between the ph of the solutions and the pke constant, ensuring therefore a good similarity with the properties of the cooling water in PWR. A three-electrode set-up was employed for the electrochemical measurements. The disc-shaped end of the specimens acts as working electrode and a circular platinized titanium grid was used as counterelectrode. An Ag-AgCl reference electrode (Ag/AgCl/saturated KCl solution, + 360mV/NHE at 85 C) was chosen for the measurements at 85 C on Stellite-6, and a SSE (Hg/Hg 2 SO 4 /saturated K 2 SO 4 solution, + 670mV/NHE) was chosen for the room temperature measurements on Zircaloy-4. The electrodes were connected to a PAR 273A potentiostat and the measurements were conducted by using a PC with CorrWare software. The details of electrochemical set-up are presented schematically on figure 2. Fig. 2. Schematic set-up of the electrochemical cell.

4 38 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI 3. RESULTS AND DISCUSSION 3.1. Wear Laws Intermittent Friction Tests The relationship linking up the total wear W t to the latency time τ between two consecutive friction contacts was determined by using intermittent friction tests in the open-circuit potential (OCP) conditions. Firstly, the continuous friction test at 120 rpm is assimilated to an intermittent friction one of 0.5 s latency period (indeed, each point of the wear track enters into contact with the pin every 0.5 s). Then, strictly speaking, intermittent tests were conducted by alternating friction periods of 2 s (corresponding to 4 rotations of the pin) with latency periods of 20 s and/or 200 s; such a sequence was iterated 2500 times to reach the total friction rotations. Each test was repeated at least three times and the average values of the total wear W t of the specimens versus latency time τ are represented in figure 3. It is worth noticing that, for both materials, the difference between the wear values estimated by weighing and by topography was less than 5%. It means that corrosion does not develop out of the wear track, result confirmed as well by microscopical observations. The results obtained for Stellite-6 present a good reproducibility of approximately 10 %. A welldefined linear relationship was found between the logarithm of the total wear W t and the logarithm of the latency time τ. For both normal loads, the parameter n (the slope of the line) has the same value, of 0.7±0.05 (calculated for the average values of W t ). This indicates that the kinetic law of the oxidation reaction involved in the repassivation of the surface after friction is the same for both contact pressures. Moreover, to note that the value of the parameter n is close to the one of 0.65 declared for the wear of the GLAs in PWR environment. Fig. 3. Total wear W t versus latency time τ (intermittent friction tests). The continuous lines (for Zircaloy-4) and the dashed lines (for Stellite-6) represent linear regressions. The reproducibility was estimated to 10 % for Stellite-6 and to 60 % for Zircaloy-4. Not the same clear relationship between W t and τ is observed for Zircaloy-4. On the basis of the results presented in figure 3, it seems that under 6 N of normal load τ has practically no effect, while under a normal load of 30N a very slight increase of the total wear W t with τ is detected (the parameter n calculated for the average values of W t is 0.93, close to 1). The high value of the dispersion of the results, approximately 60%, could be also an indication of the complexity of the tribocorrosive behaviour of Zircaloy-4, inciting to and motivating further investigations Complementary Tribocorrosive Characterisations Electrochemical Behaviour of the Stellite-6 Alloy Potentiodynamic polarization curves I(E) of Stellite-6 measured when friction is not applied and during friction are shown in figure 4a. Without friction force applied, the superalloy presents a wide passivation plateau (0.8 V) starting immediately after the zero-current potential, limited at the cathodic end by the hydrogen evolution and by the transpassive dissolution domain at the anodic end. The anodic current on the passivation plateau is in the order of 10 µa. When friction is applied, a continuous increase of the anodic current with the potential is observed; this potential-dependent dissolution reaction seems to be the result of a low oxide growth rate compared to the frequency of the mechanical depassivation caused by the friction. To note also that in the potential domain where the passivation occurs, most of the anodic current measured during friction is flowing from the depassivated areas of the wear track (out of the wear track, the alloy remains in a passive state). Additional information can be obtained from potential jump experiments. The procedure, detailed elsewhere [4], essentially consists in applying a potential jump from the -0.7 V/ref potential to a anodic value in the range of -0.4 V/ref to +0.4 V/ref, after a cathodic treatment which reduces the superficial oxide film. The current transient is then recorded for at least 300 s with an acquisition frequency of 4 khz. Figure 4b shows the current transient corresponding to a potential jump to 0 V/ref, typical for all the other potential jumps. It is observed that the linear relationship between the logarithm of j and the logarithm of time (see relation 2) is found over only a decade of current.

5 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI 39 For example, after 0.5 s, h is approximately 0.12 nm; thus, the passive film is not completely built up. This could explain the potential-dependent anodic current observed during the tribocorrosion tests at 120 rpm (fig. 4a). (a) (b) (c) Fig. 4. Stellite-6: potentiodynamic polarization curves I(E) at 100 mv/min scan rate (a); current density transient j measured after a potential jump from -0.7 V/ref to 0 V/ref. (b), and the thickness h of the corresponding oxide film - the dashed line represents a linear regression (c). If assuming that all of the anodic current during the transient is used for the growth of the Cr 2 O 3 oxide film (Cr Cr 3+ +3e - anodic reaction), the thickness h of the oxide film can be calculated from the Faraday s law. As observed in figure 4c, after 300 s, h does not exceed few nanometres, which is a very low value Wear Electrochemical Potential Relationship for Zircaloy-4 As for Stellite-6, potentiodynamic polarization curves I(E) were measured for Zircaloy-4 under no friction condition and during friction. Then, continuous friction tests at 120 rpm for rotations were done in electrochemical potentiostatic state, i.e. imposing a constant value of the potential during all the friction test time, the current being measured in parallel. Potentiodynamic characteristics of Zircaloy-4 (fig. 5a) are similar to those of Stellite-6; a wide passivation plateau (50 µa current) is observed when no friction is applied, while during friction the anodic current linearly increases with the potential. An interesting result is given by the potentiostatic friction tests. The imposed potentials go from 1.7 V/ref (cathodic domain) to 1 V/ref, a high anodic value. The average value of the current measured during friction increases with the potential (fig. 5b), agreeing with the under-friction potentiodynamic characteristics. By integrating with time the current shown in figure 5b, Faraday s law gives the quantity of dissolute metal W F, assimilated to an electrochemical component of the total wear W t. These wear values are presented in figure 3c and figure 3d. Firstly, the total wear W t strongly increases with the value of the applied potential; at 6 N normal load, W t increases with almost 100% when the potential goes from the cathodic end to anodic end, while at 30 N W t increases with approximately 60%. In the mean time, the electrochemical wear increases too (as a consequence of the current increase). However, W F is very low comparing to W t (ratio higher than 10), and, which is important to note, the increase of W F only does not justify the increase of W t. Indeed, the total wear for a potential in the anodic domain is much higher then the sum of the total wear in the cathodic domain (where practically no oxidation reaction takes place) and the corresponding electrochemical wear. A synergetic term of the tribocorrosive wear of Zircaloy-4 is therefore evidenced. Our research team has just started to study in great detail the nature of this term; a first hypothesis takes into consideration an abrasive effect of the oxide particles acting as a third-body in the friction contact.

6 40 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI Fig. 5. Zircaloy-4: I(E) potentiodynamic characteristics at 100 mv/min (a); typical current recordings during sliding at different potential (b), total wear W t and electrochemical wear W F versus the imposed potential E (c, d) corresponding to the friction tests under potentiostatic conditions. Continuous lines are only for guidance. Reproducibility of the results on (c) and (d) is 10%. 4. CONCLUSIONS The potentiodynamic characteristics measured in LiOH-H 3 BO 3 solutions show that, for both alloys, Stellite-6 and Zircaloy-4, the friction at 120 rpm destroys (partially) the passive oxide film, the dissolution anodic current increasing with the potential. When immersed in solution in OCP conditions, the wear of Stellite-6 clearly depends on the latency time. The wear rate is therefore controlled by the kinetics of the mechanical removal of the passive layer and by the kinetics of the repassivation during latency time, which determines the amount of the restored passive layer. For Zircaloy-4, the effect of the latency time is not clearly evidenced, but the wear is strongly dependent on the electrochemical potential. A synergy between mechanical wear and electrochemical reactions has been found out; further studies are directed to the comprehension of the nature of this synergy. REFERENCES 1. Crook P., Silence W.L., 2000, Cobalt Alloys, PUhlig s Corrosion Handbook, Second Edition, Edited by R. Winston Revie, ISBN , pp Lemaire E, Le Calvar M, 2001, Evidence of tribocorrosion wear in pressurized water reactors, Wear, 249, pp Gebhardt O., Hermann A., 1996, Microscopic and electrochemical impedance spectroscopy analyses of Zircaloy oxide films formed in highly concentrated LiOH solution, Electrochimica Acta, 41 (1996), pp Benea L., Ponthiaux P., Wenger F., Galland J., Hertz D., Mallo J.Y., 2004, Tribocorrosion of stellite 6 in sulphuric acid medium: electrochemical behaviour and wear, Wear, 256 (2004) pp Wenger F., Ponthiaux P., Benea L., Peybernès J., 2004, Tribocorrosion of Stellite 6 alloy: mechanism of the electrochemical reactions, Proceeding EUROCORR 2004 Long Term Prediction & Modeling of Corrosion, Nice, France.