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Characterization of the oxides formed on Zr-2.5wt.% Nb during high temperature corrosion
Journal of Alloys and Compounds, 179 (1992) 207-217 JAL 0082
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Characterization of the oxides formed on Zr-2.Swt.% Nb during high temperature corrosion Y u q u a n Ding and D e r e k O. N o r t h w o o d Engineering Materials Group, Department of Mechanical Engineering, University of Windsor, Windsor, NPB 3P4 Ontario (Canada)
(Received July 15, 1991)
Abstract Zr-2.5wt.% Nb alloy specimens were exposed to various high temperature environments and the oxides formed were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) using specimen preparation techniques specifically designed to examine the oxide-metal interface. The high temperature environments included oxygen at 300 °C, air at 490 °C, steam at 490 °C and pressurized water (containing 4.8 g LiOH/1 H20) at 300 °C. The oxidation rate in 300 °C oxygen gas was lower than 490 °C steam or 300 °C pressurized water. The oxidation rate was highest for the 490 °C air environment. Examination of the oxide at or near the oxide-metal interface showed that specimens with the lowest oxidation rate (300 °C oxygen) had a compact oxide film with a fine grain size (= 10 nm), whereas the specimens with the higher oxidation rates had an oxide film with interconnected porosity.
1. I n t r o d u c t i o n Z i r c o n i u m alloy c o r r o s i o n and the a s s o c i a t e d p h e n o m e n o n of h y d r o g e n i n g r e s s are o f t e n c o n s i d e r e d in t e r m s o f the effects o f a " b a r r i e r l a y e r " at the o x i d e - m e t a l interface w h i c h significantly r e d u c e s h y d r o g e n ingress, a n d d e c r e a s e s o x i d a t i o n r a t e s with time, so long as it, i.e. the barrier layer, r e m a i n s intact a n d protective. The oxidation kinetics o f z i r c o n i u m a n d its alloys o f t e n exhibit a transition, usually r e f e r r e d to as b r e a k a w a y , f r o m an initial p e r i o d o f a p p r o x i m a t e p a r a b o l i c o r c u b i c oxidation kinetics to a p e r i o d o f an a p p r o x i m a t e linear rate. This " b r e a k a w a y " is attributed to the b r e a k d o w n o f the barrier layer. The high t e m p e r a t u r e oxidation o f z i r c o n i u m a n d its alloys has b e e n investigated in a variety o f e n v i r o n m e n t s [ 1 ]. H o w e v e r , the high t e m p e r a t u r e d a t a are f o r the m o s t p a r t o f s h o r t d u r a t i o n a n d exhibit p o o r r e p r o d u c i b i l i t y f r o m s p e c i m e n to s p e c i m e n [2]. In o r d e r to b e t t e r u n d e r s t a n d the c o r r o s i o n a n d h y d r i d i n g b e h a v i o u r it is i m p o r t a n t to obtain i n f o r m a t i o n on the n a t u r e o f the o x i d e film, particularly at o r n e a r the oxide--metal interface. There h a v e b e e n a limited n u m b e r o f e l e c t r o n m i c r o s c o p y studies of the s t r u c t u r e o f the oxide n e a r the o x i d e - m e t a l interface in c o r r o d e d s p e c i m e n s o f z i r c o n i u m a n d its alloys [ 3 - 5 ] . B o t h t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y (TEM) a n d s c a n n i n g e l e c t r o n m i c r o s c o p y (SEM) have b e e n u s e d t o g e t h e r with a variety
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of specimen preparation methods. However, very few useflal results have been obtained, primarily because of the difficulties in preparing suitable specimens containing the oxide--metal interface. In the present work, we have used SEM and TEM techniques to characterize the oxide-metal interface in Zr-2.5wt.% Nb alloys (hereafter referred to as Zr-2.5Nb) specimens exposed to a number of different environments. Zr-2.5Nb is presently used for the manufacture of nuclear reactor pressure tubing because of its low neutron absorption cross-section, high strength and high corrosion resistance under reactor operating conditions. The microstructural observations are then correlated to the corrosion (oxidation) behaviour as measured by t h e thickness of the oxide film formed during the corrosion exposure.
2. E x p e r i m e n t a l details 2.1. Details o f corrosion tests a n d s p e c i m e n s The details of all the corrosion test specimens are given in Table 1. All the material tested was commercial grade Zr-2.5 Nb pressure tubing in the cold-worked and stress-relieved condition. Specimens designated as 22A, 22E and 1H14 were flat sections 6 0 x 10X 1 mm 3 which had been machined from the pressure tubing. The corrosion tests for these specimens were conducted at the Chalk River Nuclear Laboratories of Atomic Energy of Canada Limited. Specimens designated as L-11 or L-12 were sections cut directly from pressure tubing with dimensions of approximately 2 0 x 1 5 m m ~ X thickness of tubing ( = 4 nun). The high temperature environments included: (1) oxygen at 300 °C, (2) air at 490 °C, (3) steam at 490 °C, and (4) pressurized (8.65 MPa), lithiated (4.8 g LiOHA deionized water) water at 300 °C. Full details of the pressurized lithiated water tests can be found in ref. 6.
TABLE 1 Details of corrosion specimensa Specimen designation
Corrosion environments
Exposure time (days)
Oxide thickness (tzm)
22A 22A-1 22E 1H14 L-12 L-11 1,-12-1
300 490 490 490 490 300 490
100.7 7.6 7.4 5.0 1.7 8.4 7.6
5.6 18.0 5.08 5.63 2.70 10.3 11.2
°C °C °C °C °C °C °C
oxygen air steam steam lithiated water lithiated water air
aFe content: 1100 p p m for 22A, 22A-1 and 22E; 600 p p m for 1H14; 390 p p m for 1,-11, L12 and L-12-1 [6].
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2.2. Preparation of specimens f o r S E M and TEM e x a m i n a t i o n Full details of the SEM specimen preparation technique can be found in ref. 7. The SEM examination was pe r form ed on a Nanolab7 scanning electron microscope. The TEM specimens containing the o x i d e - m e t a l interface were prepared using an ion milling technique that was modified from one originally developed by Moseley and H uds on [8]. The ion beam thinning was initially conducted from the metal side only. The thinning was then continued alternately from the metal side or the oxide side until the foil specimen was perforated at the edges [6]. The thin foil specimens were examined at 100 kV in a JEM100CX m i c r o s c o p e with a scanning attachment.
3. R e s u l t s
3.1. Oxidation kinetics The oxide films f or m ed in steam at 490 °C were dense and adherent and black-grey in colour. The oxide films formed either after 100.7 days in oxygen at 300 °C, 7.6 days in air at 300 °C, or after 1.7 days in pressurized lithiated water at 300 °C, were all white in appearance. As shown in Table 1, the oxidation rate, as reflected the oxide film thickness, was lower in 300 °C oxygen gas than in 490 °C steam or 300 °C pressurized lithiated water. The oxidation rate was highest for the 490 °C air environment. By observing the differences in oxide structure it was hoped to identify possible reasons for the differences in corrosion behaviour.
3.2. Metallography o f oxides f o r m e d during high temperature corrosion 3.2.1. Effect of temperature The effects of t e m p e r a t u r e will be illustrated by comparing the oxide structure and m o r p h o l o g y on specimens e x p o s e d to either oxygen at 300 °C or air at 490 °C. Oxidation of Zr-2.5Nb pressure tubing in oxygen at 300 °C basically p r o c e e d s along grain boundaries at which filaments of the /3-Zr phase are present. This is illustrated in Fig. l ( a ) for specimen 22A which is a SEM micrograph of a tapered section which includes the oxi de-m et al interface region, i.e. the " u n d e r s i d e " of the oxide which is closest to the metal. The structure of the metal consists of long elongated a-grains (black phase in SEM micrograph) with filaments of fl-Zr (white phase in SEM micrograph) at the grain boundaries. The metal at or near the o x i d e - m e t a l interface has " l ar ge r " a-Zr grains than an u n c o r r o d e d metal specimen (see Fig. l(b)). The oxide at or n e a r the o x i d e - m e t a l interface "m i rrors" the structure of the metal. The specimen corroded in air at 490 °C shows a v er y different behaviour (see SEM micrograph in Fig. 2(a) for specimen 22A1). The metal structure, which is similar to that in cold-worked and Zr-2.5Nb heat-treated at 560 °C for 24 h [91, has changed and the fl-phase filaments have broken up m or e into discrete particles (see Figs. 2(b) and (c)). The
210
(a)
(b)
Fig. 1. (a) Scanning electron m i c r o g r a p h of the o x i d e - m e t a l interface formed on Zr-2.5Nb in a 300 °C oxygen e n v i r o n m e n t ( S p e c i m e n 22A); (b) SEM m i c r o g r a p h of uncorroded Zr-2.5Nb s p e c i m e n (metal surface parallel to axial direction of pressure tubing).
oxide has a " n o d u l a r " a p p e a r a n c e r a t he r than the elongated appearance in the 300 °C o x y g e n specimen. The oxide is also characterized by interconnected porosity, which was not p r e s e n t in the oxide on the 300 °C oxygen specimen. The specimen c o r r o d e d in steam at 490 °C shows a very similar microstructure of the oxide close to the o x i d e - m e t a l interface to that in the specimen c o r r o d e d in air at 490 °C, i.e. it is different from the specimen c o r r o d e d in oxygen at 300 °C. TEM examination of thin foils of the oxide at the o x i d e - m e t a l interface in the specimen e x p o s e d to 300 °C oxygen, see Fig. 3, showed the oxide to be dense and a d h e r e n t and to consist of spherical clusters of "cauliflowerlike" granules (mosaic structure), with sizes in the range 3 0 - 5 0 nm. The fine granules within the cluster were a bout 10 nm in size. Mismatch between the oxide lattice and the metal (a-Zr) substrate gives rise to stresses which m ay be relieved by the generation of the spherical clusters of "cauliflowerlike" granules (or mosaic structure).
3.2.2. Effect o f corrosion e n v i r o n m e n t s The effect of corrosion e nvi r onm e nt will be illustrated by first comparing the oxide structures in specimens e x p o s e d to either air or steam at 490 °C, and th en comparing the oxides f o r m e d at 300 °C in either oxygen or pressurized lithiated water. The oxide structures f o r m e d in 490 °C steam (see Fig. 4 for specimen 22E) are quite similar t o t hos e obs er ve d in 490 °C air, e.g. Fig. 2(a). Both oxides have a nodular a p p e a r a n c e and bot h oxide films contain interconnected porosity. The one difference n o t e d was that the oxide undersides, i.e. the surface of the oxide--metal interface, was flatter for the 490 °C air specimen than for the 490 °C steam s peci m e n (see Fig. 5 for specimen 22A-1, Fig. 6 for s p ecimen 22E and Fig. 7 for s pe c i men 1H14). Figures 6 and 7 show some small differences in the oxide structures of specimens 22E and 1H14
211
(a)
(b)
? "<: :
(c) Fig. 2. Scanning electron micrograph of the o x i d e - m e t a l interface formed on Zr-2.5Nb in 490 °C air environment (Specimen 22A-1); (b) SEM micrograph of u n c o r r o d e d but heat-treated (at 560 °C for 24 h) Zr-2.5Nb specimen; (c) SEM micrograph of uncorroded but heat-treated (at 490 °C for 7.6 days) Zr-2.5Nb specimen.
Fig. 3. Transmission electron micrograph of the oxide at the o x i d e - m e t a l interface in Zr-2.5Nb exposed to 300 °C oxygen (Specimen 22A).
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Fig. 4. Scanning electron micrograph of the oxide-metal interface formed in Zr-2.5Nb exposed to 490 °C steam environment (Specimen 22E). Fig. 5. Scanning electron micrograph of the oxide at the interface formed in Zr-2.5Nb exposed to 490 °C air environment (Specimen 22A-1).
Fig. 6. Scanning electron micrograph of the oxide at the interface formed in Zr-2.5Nb exposed to 490 °C steam environment (Specimen 22E). Fig. 7. Scanning electron micrograph of the oxide at the interface formed in Zr-2.5Nb (low iron content) exposed to 490 °C steam environment (Specimen 1H14). after 4 9 0 °C s t e a m c o r r o s i o n . T h e s e differences m a y result f r o m the different iron c o n t e n t s . The relationship b e t w e e n the u n d e r l y i n g m e t a l s t r u c t u r e a n d the o x i d a t i o n is well illustrated f o r the s p e c i m e n s e x p o s e d to 3 0 0 °C p r e s s u r i z e d lithiated water. This is s h o w n in Figs. 8 a n d 9 in w h i c h the oxidation has p r o c e e d e d along the grain b o u n d a r i e s at w h i c h filaments o f fi-Zr are p r e s e n t . The filaments o f oxide, w h i c h c o n s i s t o f m a n y fine ZrO2 grains, are o f a l e n g t h that is c o n s i s t e n t with t h e l e n g t h o f t h e grain b o u n d a r i e s a n d fi-Zr filaments in the metal. The fine ZrO2 g r a i n s m a k i n g u p the oxide film are relatively u n i f o r m in size r a n g i n g in d i a m e t e r f r o m 30 to 5 0 n m (see Fig. 10). H o w e v e r , the oxide, ZrO2, f o r m e d at or n e a r t h e ~-Zr h a s a variable grain size with s o m e grains b e i n g as small as 10 n m (see Fig. 11 for typical TEM m i c r o g r a p h ) .
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Fig. 8. Scanning electron micrograph of the oxide at the oxide-metal interface in Zr-2.5Nb exposed to a 300 °C, pressurized lithiated water environment (Specimen L-12, metal surface parallel to axial direction of pressure tubing). Fig. 9. Scanning electron micrograph of the oxide at the oxide-metal interface in Zr-2.SNb exposed to a 300 °C, pressurized lithiated water environment (Specimen L-12, metal surface perpendicular to axial direction of pressure tubing).
Fig. 10. Transmission electron micrograph of the oxide formed at the oxide--metal interface in specimen exposed in pressurized lithiated water for 1.7 days (40 h) Specimen L-12, metal surface parallel to axial direction of pressure tubing). Fig. 11. Transmission electron micrograph of the oxide formed at the oxide-metal interface in specimen exposed in pressurized lithiated water for 1.7 days (40 h). (Specimen L-12, metal surface perpendicular to axial direction of pressure tubing).
If w e n o w c o m p a r e t h e o x i d e s t r u c t u r e f o r m e d i n p r e s s u r i z e d l i t h i a t e d w a t e r a t 3 0 0 °C ( F i g s . 8 a n d 9) w i t h t h a t f o r m e d i n o x y g e n a t 3 0 0 °C, Fig. l(a), we note that the specimen exposed to oxygen does not show such a clear r e l a t i o n s h i p b e t w e e n the s t r u c t u r e of the m e t a l a n d the s t r u c t u r e of the oxide. In the oxygen atmosphere the oxidation process does not appear t o p r o c e e d a s p r e d o m i n a n t l y a l o n g t h e g r a i n b o u n d a r i e s , i.e. t h e f l - p h a s e a n d its d e c o m p o s i t i o n p r o d u c t s , a s i n t h e p r e s s u r i z e d l i t h i a t e d w a t e r , t h u s suggesting different corrosion mechanisms are operative.
214 4. D i s c u s s i o n
4.1. Effect of g r a i n b o u n d a r y structure a n d c h e m i s t r y on the corrosion o f Zr-2.5Nb at high t e m p e r a t u r e s The corrosion resistance of Zr-2.5Nb is known to be sensitive to the metallurgical history and the microstructure that is developed [11 ]. In the commercial pressure tubing a satisfactory corrosion resistance is obtained in tubing which is made by extrusion at 850 °C (in a-Zr+fl-Zr region), followed by air cooling and cold-working. The resulting structure consists of elongated a-Zr grains and a grain boundary network of metastable fl-Zr that contains approximately 20 wt.% Nb [12]. Upon heating at temperatures below 610 °C (the monotectoid temperature) metastable fl-Zr will slowly decompose through a number of intermediate phases, e.g. fl-enriched and w-phase, to an equilibrium structure consisting of a-Zr and fl-Nb (approximately 85 wt.%Nb) [13]. The grain boundaries in cold-worked Zr-2.5Nb pressure tubing can thus contain fl-Zr and/or its decomposition products together with impurities and other precipitates. There are thus many kinds of boundaries formed between these phases. At the beginning of the corrosion process at 300 °C, oxidation has been shown to proceed rapidly along the grain boundaries [7 ]. Therefore, oxidation at grain boundaries is faster than at the bulk a-Zr grains. Microstructural changes can, however, occur during exposure to high temperature air and steam and these changes can cause changes in corrosion behaviour. For example, after corrosion for longer times at 490 °C, the metastable flZr will have mostly decomposed to an equilibrium structure consisting of a-Zr and fl-Nb. During 300 °C corrosion, even for long times, the structures and chemical composition (phase make-up) of the grain boundaries change only slightly. The changes occurring at 490 °C, i.e. the breakup of the filaments and the change from fl-Zr to fl-Nb, should lead to a reduction in corrosion rates. However, the rate of metal oxidation increases rapidly with increase in temperature [14]. Thus, in the case of long-term corrosion testing at 490 °C, the kinetics of corrosion are more complex than at low temperatures because the competing effects of change in metal structure and change in oxidation kinetics, and mechanisms, which increase in temperature. Given the break-up of the fl-filaments, occuring at 490 °C, and the fact that the fl-filaments are the main sites for oxidation at the lower temperature, it is not surprising that the oxidation did not appear to proceed along the grain boundaries for the 490 °C exposures. 4.2. Effect o f corrosion e n v i r o n m e n t s on the o x i d e structure at or n e a r the o x i d e - m e t a l interface The oxidation rate in the 300 °C oxygen environment was much lower than that in the 300 °C pressurized lithiated water. Looking at the mechanisms of corrosion, we can distinguish two types of processes, namely chemical corrosion and electrochemical corrosion [14]. Oxidation in a 300 °C oxygen environment is an example of chemical corrosion, whereas oxidation in a
215 300 °C pressurized lithiated water environment is an example of electrochemical corrosion. Complex grain boundary structures affect electrochemical corrosion much m or e than chemical corrosion by promoting grain boundary attack. This was seen for the specimen e x posed to 300 °C oxygen where the grain b o u n d ar y oxidation was not as obvious as in the specimen exposed to 300 °C pressurized lithiated water. Since LiOH is added to the primary coolant of most pressurized water reactors (PWRs) to maintain an alkaline p H so as to control the transport of corrosion p r o d u c t s from the other (than zirconium alloy) structural materials of the r e a c t o r coolant system, the effect of LiOH on the corrosion resistance of the zirconium alloys used for fuel cladding, pressure tubing, etc. is of considerable technological interest. Several studies have been made of the effect of the LiOH concentration in aqueous solutions on the corrosion resistance of zirconium alloys, in particular the Zircaloys [ 15-21 ]. The studies of Zr - 2 .5 Nb pressure tubing clearly show that the oxidation rates in pressurized lithiated water are highly d e p e n d e n t on the LiOH concentration. Manolescu e t al. [21 [ have studied the effect of LiOH on the acceleration in corrosion of Zircaloy-2 and Z r - 2. 5N b for 0.0005 M to 1 M LiOH solutions. They found that the acceleration in corrosion owing to LiOH was greater for the Zr-2.SNb alloy. The p r es e nt authors have shown [7 and this study[ that the oxidation of cold-worked Zr-2.5Nb pressure tubing in pressurized lithiated water (4.8 g LiOH/1 deionized water) pr oc e e ds first along grain boundaries at which there are fl-Zr and its decomposition products, and then continues on a-Zr grains. The oxides f or m e d were mainly ext ended along grain boundaries, and were characterized by long filaments, which are c o m p o s e d of many fine ZrO2 grains. Short-circuit diffusion at grain boundaries is mainly related to the oxidation of grain bounda r y phases, flaws arising from fi-Zr filaments, and microcracking arising from phase transformation in ZrO2. The presence of LiOH may e n c o u r a g e the corrosion at the grain boundaries due to incorporation of lithium in the growing oxide, which generates additional anion vacancies and causes a more rapid transition from the tetragonal oxide phase to the monoclinic phase, and hence p roduces an increase in corrosion rates [22, 23]. Differences in the grain b oundary structures in Zircaloy-2 and Zr-2.5Nb, in particular the pr e s ence of /3-Zr and its decomposition p ro d u cts in Zr-2.5Nb m a y explain the difference found by Manolescu et al. [21 ] for Zircaloy-2 and Zr-2.5Nb in terms of the acceleration of corrosion due to LiOH. The oxidation rate was highest for the 490 °C air environment. Nitrogen, p res en t in air, is known to increase the corrosion rates of zirconium alloys [24]. When N ~- displaces 0 2 - in ZrO2, additional vacancies are p r o d u c e d in ZrOe lattice, which results in increased corrosion rates. The oxide at or ne a r the o x i d e - m e t a l interface in specimens e x p o s e d to air or steam at 490 °C contained interconnected porosity. Similar porosity has been seen by W a r r e t a l . [25] in their studies of steam (400 °C) prefilmed s p ecimen s o f Z r - 2 . 5 N b that had subsequently been e x p o s e d to either a v a c u u m at 500 °C or an in-reactor at 300 °C. Their explanation for the
216 p o r o s i t y w a s t h a t it r e s u l t e d f r o m o x i d e dissolution into the b a s e m e t a l u n d e r c o n d i t i o n s o f h i g h t e m p e r a t u r e ( 5 0 0 °C) a n d a w e a k l y oxidizing e n v i r o n m e n t ( i n - r e a c t o r a t 3 0 0 °C). S u c h an e x p l a n a t i o n m a y also b e a p p r o p r i a t e for the p r e s e n t tests. T h e m i c r o s t r u c t u r e at the o x i d e - m e t a l i n t e r f a c e in o u r c o u p o n s 2 2 E a n d 1H14, w h i c h h a d b e e n e x p o s e d to 4 9 0 °C s t e a m , w a s quite similar t o t h a t o b s e r v e d b y W a r t e t a l . [25] in the s t e a m p r e - f i l m e d s p e c i m e n s e x p o s e d to v a c u u m .
5. Conclusions D e t a i l e d SEM s t u d i e s s h o w t h a t the o x i d a t i o n of c o l d - w o r k e d Z r - 2 . 5 N b p r e s s u r e t u b i n g in b o t h o x y g e n a n d p r e s s u r i z e d lithiated w a t e r at 3 0 0 °C p r o c e e d s m a i n l y a l o n g t h e g r a i n b o u n d a r i e s . H o w e v e r , in t h e 3 0 0 °C o x y g e n e n v i r o n m e n t , t h e o x i d a t i o n a l o n g t h e g r a i n b o u n d a r i e s is n o t as p r o n o u n c e d as in t h e p r e s s u r i z e d lithiated water. P r e f e r e n t i a l o x i d a t i o n at t h e grain b o u n d a r i e s w a s n o t o b s e r v e d w h e n t h e Z r - 2 . 5 N b w a s e x p o s e d to e i t h e r an air or s t e a m e n v i r o n m e n t at 4 9 0 °C. T h e s e r e s u l t s are r a t i o n a l i z e d in t e r m s o f e i t h e r a c h e m i c a l or e l e c t r o c h e m i c a l c o r r o s i o n m e c h a n i s m b e i n g o p e r a t i v e . In t h e 4 9 0 °C air a n d s t e a m e n v i r o n m e n t s , t h e o x i d e s t r u c t u r e at the o x i d e - m e t a l i n t e r f a c e w e r e c h a r a c t e r i z e d b y the p r e s e n c e of i n t e r c o n n e c t e d p o r o s i t y . T h e h i g h o x i d a t i o n r a t e f o r s p e c i m e n s e x p o s e d to 4 9 0 °C air is c o n s i d e r e d to a r i s e f r o m t h e high t e m p e r a t u r e a n d t h e p r e s e n c e o f nitrogen.
Acknowledgments This work was supported by the Natural Sciences and Engineering R e s e a r c h Council o f C a n a d a ( O p e r a t i n g G r a n t A 4 3 9 1 ) a n d the C a n a d a D e u t e r i u m U r a n i u m (CANDU) O w n e r s G r o u p (COG) t h r o u g h W o r k i n g P a r t y No. 35. Mr. J o h n W. R o b i n s o n a s s i s t e d with t h e c o r r o s i o n t e s t s a n d with t h e SEM e x a m i n a t i o n s . S e l e c t e d s p e c i m e n s w e r e p r o v i d e d b y t h e Chalk River N u c l e a r L a b o r a t o r i e s o f A t o m i c E n e r g y o f C a n a d a Limited.
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Y. Ding and D. O. Northwood, J. Mater. Sci., in the press. P. T. Moseley and B. Hudson, J. Nucl. Mater., 99 (1981) 340. D. O. Northwood and W. L. Fong, M e t a l l o g r a p h y , 13 (1980) 97. L. L. Shreir, Corrosion, Vol. 1, M e t a l / E n v i r o n m e n t s Reaction, Newnes-Butterworths, London, 1976, p. 1:24. V. F. Urbanic and R. W. Gilbert, Effect of microstructure on the corrosion of Zr-2.5Nb alloy, Proc. Int. A t o m i c E n e r g y A g e n c y (IAEA) T e c h n i c a l C o m m i t t e e M e e t i n g on Fund a m e n t a l A s p e c t s o f Corrosion o f Z i r c o n i u m - B a s e Alloys f o r W a t e r R e a c t o r E n v i r o n ments, P o r t l a n d , OR, S e p t e m b e r , 1989. R. G. Fleck, E. G. Price and B. A. Cheadle, Pressure tube development for CANDU reactors, in D. G. Franklin and R. B. Adamson (eds.), Z i r c o n i u m i n the N u c l e a r I n d u s t r y : 6th Int. S y m p . , A S T M S T P 824, American Society for Testing and Materials, 1984, Philadelphia, PA, p. 88. B. A. Cheadle and S. A. Aldrige, J. Nucl. Mater., 47 (1973) 47. N. D. Tomashov, T h e o r y o f Corrosion a n d Protection o f M e t a l -- The S c i e n c e o f Corrosion, Macmillan, New York, 1965, pp. l l , 57. S. Ismail and D. O. Northwood, in A. Atrens, M. S. Pennisi and D. P. Schweinsberg (eds.), Effect of Material and Environmental Conditions on the High Temperature Aqueous Corrosion of a Zirconium alloy, Proc. "Step into the 90's,' J o i n t Conf., Gold Coast, Qld, A u s t r a l i a , Aug. 27-31, 1989, Vol. 3 Australasian Corrosion Assoc. Inc./Australasian Institute of Metal Finishing/Institute of Metals and Materials Australasia Ltd., 1989, pp. 693-701. S. G. McDonald, G. P. Sabol and K. D. Sheppard, in D. G. Franklin and R. B. Adamson (eds.), Effect of Lithium Hydroxide oil the Corrosion Behaviour of Zircaloy-4, Z i r c o n i u m i n the N u c l e a r I n d u s t r y : 6th Int. Syrup., A S T M S T P 824, 2waerican Society for Testing and Materials, Philadelphia, PA, 1984, p. 51. R. A. Perkins and R. A. Busch, Corrosion of Zircaloy in the presence of LiOH, 9th Int. Syrup. on Z i r c o n i u m i n the N u c l e a r I n d u s t r y , Kobe, J a p a n , N o v e m b e r , 1990, American Society for Testing and Materials, Philadelphia, PA, to be published. N. Ramasubramanian, Lithium uptake and the corrosion of zirconium alloys in aqueous lithium hydroxide solutions, 9th Int. S y m p . on Z i r c o n i u m i n the N u c l e a r I n d u s t r y , Kobe, J a p a n , N o v e m b e r , 1990, American Society for Testing and Materials, Philadelphia, PA, to be published. I. L. Bramwell, D. R. Tice and P. D. Parson, Corrosion of Zircaloy-4 PWR fueUcladding in lithiated and borated water environments, 9th Int. Syrup. on Z i r c o n i u m i n the N u c l e a r I n d u s t r y , Kobe, J a p a n , N o v e m b e r , 1990, American Society for Testing and Materials, Philadelphia, PA, to be published. B. Cox and Y. M. Wong, Effects of LiOH on pretransition zirconium oxide films, 9th Int. Syrup. o n Z i r c o n i u m i n the N u c l e a r I n d u s t r y , Kobe, J a p a n , N o v e m b e r , 1990, American Society for Testing and Materials, Philadelphia, PA, to be published. A. V. Manolescu, P. Mayer and C. J. Simpson, Corrosion, 38 (1982) 23. E. Hillner and J. N. Chirogos, The Effect o f L i t h i u m H y d r o x i d e a n d R e l a t e d S o l u t i o n s on the Corrosion R a t e o f Z i r c a l o y i n 680 °F W a t e r , U.S. Rep. WAPD-TM-307, Westinghouse Electric Corp., Bettis Atomic Laboratory, Pittsburgh, PA, 1962. F. Garzarolli, J. Pohlmeyer, S. Trap-Pretsching and H. G. Weidinger, Proc. Int. A t o m i c E n e r g y A g e n c y (IAEA) T e c c h n i c a l C o m m i t t e e M e e t i n g o n F u n d a m e n t a l Aspects o f Corrosion on Z i r c o n i u m B a s e A l l o y s i n W a t e r R e a c t o r E n v i r o n m e n t s , Portland, OR, September 1989. Chen He-min, Ma Chun-lai and Bai Xin-de, The Corrosion a n d Protection o f N u c l e a r R e a c t o r Materials, Atomic Energy press, China, 1984, p. 205. B. D. Wart, E. M. Rasile, A. M. Brennenstulfl, M. B. Elmoselhi, N. S. McIntyre, S. B. Newcomb and W. M. Stobbs, Microstructure and compositional analysis of the oxides on Zr-2.5wt.% Nb in relation to their role as permeation barriers to deuterium uptake in reactor application, M i c r o s c o p y o f O x i d a t i o n Conf., C a m b r i d g e , UK, March, 1990.
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Characterization of the oxides formed on Zr-2.Swt.% Nb during high temperature corrosion Y u q u a n Ding and D e r e k O. N o r t h w o o d Engineering Materials Group, Department of Mechanical Engineering, University of Windsor, Windsor, NPB 3P4 Ontario (Canada)
(Received July 15, 1991)
Abstract Zr-2.5wt.% Nb alloy specimens were exposed to various high temperature environments and the oxides formed were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) using specimen preparation techniques specifically designed to examine the oxide-metal interface. The high temperature environments included oxygen at 300 °C, air at 490 °C, steam at 490 °C and pressurized water (containing 4.8 g LiOH/1 H20) at 300 °C. The oxidation rate in 300 °C oxygen gas was lower than 490 °C steam or 300 °C pressurized water. The oxidation rate was highest for the 490 °C air environment. Examination of the oxide at or near the oxide-metal interface showed that specimens with the lowest oxidation rate (300 °C oxygen) had a compact oxide film with a fine grain size (= 10 nm), whereas the specimens with the higher oxidation rates had an oxide film with interconnected porosity.
1. I n t r o d u c t i o n Z i r c o n i u m alloy c o r r o s i o n and the a s s o c i a t e d p h e n o m e n o n of h y d r o g e n i n g r e s s are o f t e n c o n s i d e r e d in t e r m s o f the effects o f a " b a r r i e r l a y e r " at the o x i d e - m e t a l interface w h i c h significantly r e d u c e s h y d r o g e n ingress, a n d d e c r e a s e s o x i d a t i o n r a t e s with time, so long as it, i.e. the barrier layer, r e m a i n s intact a n d protective. The oxidation kinetics o f z i r c o n i u m a n d its alloys o f t e n exhibit a transition, usually r e f e r r e d to as b r e a k a w a y , f r o m an initial p e r i o d o f a p p r o x i m a t e p a r a b o l i c o r c u b i c oxidation kinetics to a p e r i o d o f an a p p r o x i m a t e linear rate. This " b r e a k a w a y " is attributed to the b r e a k d o w n o f the barrier layer. The high t e m p e r a t u r e oxidation o f z i r c o n i u m a n d its alloys has b e e n investigated in a variety o f e n v i r o n m e n t s [ 1 ]. H o w e v e r , the high t e m p e r a t u r e d a t a are f o r the m o s t p a r t o f s h o r t d u r a t i o n a n d exhibit p o o r r e p r o d u c i b i l i t y f r o m s p e c i m e n to s p e c i m e n [2]. In o r d e r to b e t t e r u n d e r s t a n d the c o r r o s i o n a n d h y d r i d i n g b e h a v i o u r it is i m p o r t a n t to obtain i n f o r m a t i o n on the n a t u r e o f the o x i d e film, particularly at o r n e a r the oxide--metal interface. There h a v e b e e n a limited n u m b e r o f e l e c t r o n m i c r o s c o p y studies of the s t r u c t u r e o f the oxide n e a r the o x i d e - m e t a l interface in c o r r o d e d s p e c i m e n s o f z i r c o n i u m a n d its alloys [ 3 - 5 ] . B o t h t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y (TEM) a n d s c a n n i n g e l e c t r o n m i c r o s c o p y (SEM) have b e e n u s e d t o g e t h e r with a variety
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of specimen preparation methods. However, very few useflal results have been obtained, primarily because of the difficulties in preparing suitable specimens containing the oxide--metal interface. In the present work, we have used SEM and TEM techniques to characterize the oxide-metal interface in Zr-2.5wt.% Nb alloys (hereafter referred to as Zr-2.5Nb) specimens exposed to a number of different environments. Zr-2.5Nb is presently used for the manufacture of nuclear reactor pressure tubing because of its low neutron absorption cross-section, high strength and high corrosion resistance under reactor operating conditions. The microstructural observations are then correlated to the corrosion (oxidation) behaviour as measured by t h e thickness of the oxide film formed during the corrosion exposure.
2. E x p e r i m e n t a l details 2.1. Details o f corrosion tests a n d s p e c i m e n s The details of all the corrosion test specimens are given in Table 1. All the material tested was commercial grade Zr-2.5 Nb pressure tubing in the cold-worked and stress-relieved condition. Specimens designated as 22A, 22E and 1H14 were flat sections 6 0 x 10X 1 mm 3 which had been machined from the pressure tubing. The corrosion tests for these specimens were conducted at the Chalk River Nuclear Laboratories of Atomic Energy of Canada Limited. Specimens designated as L-11 or L-12 were sections cut directly from pressure tubing with dimensions of approximately 2 0 x 1 5 m m ~ X thickness of tubing ( = 4 nun). The high temperature environments included: (1) oxygen at 300 °C, (2) air at 490 °C, (3) steam at 490 °C, and (4) pressurized (8.65 MPa), lithiated (4.8 g LiOHA deionized water) water at 300 °C. Full details of the pressurized lithiated water tests can be found in ref. 6.
TABLE 1 Details of corrosion specimensa Specimen designation
Corrosion environments
Exposure time (days)
Oxide thickness (tzm)
22A 22A-1 22E 1H14 L-12 L-11 1,-12-1
300 490 490 490 490 300 490
100.7 7.6 7.4 5.0 1.7 8.4 7.6
5.6 18.0 5.08 5.63 2.70 10.3 11.2
°C °C °C °C °C °C °C
oxygen air steam steam lithiated water lithiated water air
aFe content: 1100 p p m for 22A, 22A-1 and 22E; 600 p p m for 1H14; 390 p p m for 1,-11, L12 and L-12-1 [6].
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2.2. Preparation of specimens f o r S E M and TEM e x a m i n a t i o n Full details of the SEM specimen preparation technique can be found in ref. 7. The SEM examination was pe r form ed on a Nanolab7 scanning electron microscope. The TEM specimens containing the o x i d e - m e t a l interface were prepared using an ion milling technique that was modified from one originally developed by Moseley and H uds on [8]. The ion beam thinning was initially conducted from the metal side only. The thinning was then continued alternately from the metal side or the oxide side until the foil specimen was perforated at the edges [6]. The thin foil specimens were examined at 100 kV in a JEM100CX m i c r o s c o p e with a scanning attachment.
3. R e s u l t s
3.1. Oxidation kinetics The oxide films f or m ed in steam at 490 °C were dense and adherent and black-grey in colour. The oxide films formed either after 100.7 days in oxygen at 300 °C, 7.6 days in air at 300 °C, or after 1.7 days in pressurized lithiated water at 300 °C, were all white in appearance. As shown in Table 1, the oxidation rate, as reflected the oxide film thickness, was lower in 300 °C oxygen gas than in 490 °C steam or 300 °C pressurized lithiated water. The oxidation rate was highest for the 490 °C air environment. By observing the differences in oxide structure it was hoped to identify possible reasons for the differences in corrosion behaviour.
3.2. Metallography o f oxides f o r m e d during high temperature corrosion 3.2.1. Effect of temperature The effects of t e m p e r a t u r e will be illustrated by comparing the oxide structure and m o r p h o l o g y on specimens e x p o s e d to either oxygen at 300 °C or air at 490 °C. Oxidation of Zr-2.5Nb pressure tubing in oxygen at 300 °C basically p r o c e e d s along grain boundaries at which filaments of the /3-Zr phase are present. This is illustrated in Fig. l ( a ) for specimen 22A which is a SEM micrograph of a tapered section which includes the oxi de-m et al interface region, i.e. the " u n d e r s i d e " of the oxide which is closest to the metal. The structure of the metal consists of long elongated a-grains (black phase in SEM micrograph) with filaments of fl-Zr (white phase in SEM micrograph) at the grain boundaries. The metal at or near the o x i d e - m e t a l interface has " l ar ge r " a-Zr grains than an u n c o r r o d e d metal specimen (see Fig. l(b)). The oxide at or n e a r the o x i d e - m e t a l interface "m i rrors" the structure of the metal. The specimen corroded in air at 490 °C shows a v er y different behaviour (see SEM micrograph in Fig. 2(a) for specimen 22A1). The metal structure, which is similar to that in cold-worked and Zr-2.5Nb heat-treated at 560 °C for 24 h [91, has changed and the fl-phase filaments have broken up m or e into discrete particles (see Figs. 2(b) and (c)). The
210
(a)
(b)
Fig. 1. (a) Scanning electron m i c r o g r a p h of the o x i d e - m e t a l interface formed on Zr-2.5Nb in a 300 °C oxygen e n v i r o n m e n t ( S p e c i m e n 22A); (b) SEM m i c r o g r a p h of uncorroded Zr-2.5Nb s p e c i m e n (metal surface parallel to axial direction of pressure tubing).
oxide has a " n o d u l a r " a p p e a r a n c e r a t he r than the elongated appearance in the 300 °C o x y g e n specimen. The oxide is also characterized by interconnected porosity, which was not p r e s e n t in the oxide on the 300 °C oxygen specimen. The specimen c o r r o d e d in steam at 490 °C shows a very similar microstructure of the oxide close to the o x i d e - m e t a l interface to that in the specimen c o r r o d e d in air at 490 °C, i.e. it is different from the specimen c o r r o d e d in oxygen at 300 °C. TEM examination of thin foils of the oxide at the o x i d e - m e t a l interface in the specimen e x p o s e d to 300 °C oxygen, see Fig. 3, showed the oxide to be dense and a d h e r e n t and to consist of spherical clusters of "cauliflowerlike" granules (mosaic structure), with sizes in the range 3 0 - 5 0 nm. The fine granules within the cluster were a bout 10 nm in size. Mismatch between the oxide lattice and the metal (a-Zr) substrate gives rise to stresses which m ay be relieved by the generation of the spherical clusters of "cauliflowerlike" granules (or mosaic structure).
3.2.2. Effect o f corrosion e n v i r o n m e n t s The effect of corrosion e nvi r onm e nt will be illustrated by first comparing the oxide structures in specimens e x p o s e d to either air or steam at 490 °C, and th en comparing the oxides f o r m e d at 300 °C in either oxygen or pressurized lithiated water. The oxide structures f o r m e d in 490 °C steam (see Fig. 4 for specimen 22E) are quite similar t o t hos e obs er ve d in 490 °C air, e.g. Fig. 2(a). Both oxides have a nodular a p p e a r a n c e and bot h oxide films contain interconnected porosity. The one difference n o t e d was that the oxide undersides, i.e. the surface of the oxide--metal interface, was flatter for the 490 °C air specimen than for the 490 °C steam s peci m e n (see Fig. 5 for specimen 22A-1, Fig. 6 for s p ecimen 22E and Fig. 7 for s pe c i men 1H14). Figures 6 and 7 show some small differences in the oxide structures of specimens 22E and 1H14
211
(a)
(b)
? "<: :
(c) Fig. 2. Scanning electron micrograph of the o x i d e - m e t a l interface formed on Zr-2.5Nb in 490 °C air environment (Specimen 22A-1); (b) SEM micrograph of u n c o r r o d e d but heat-treated (at 560 °C for 24 h) Zr-2.5Nb specimen; (c) SEM micrograph of uncorroded but heat-treated (at 490 °C for 7.6 days) Zr-2.5Nb specimen.
Fig. 3. Transmission electron micrograph of the oxide at the o x i d e - m e t a l interface in Zr-2.5Nb exposed to 300 °C oxygen (Specimen 22A).
212
Fig. 4. Scanning electron micrograph of the oxide-metal interface formed in Zr-2.5Nb exposed to 490 °C steam environment (Specimen 22E). Fig. 5. Scanning electron micrograph of the oxide at the interface formed in Zr-2.5Nb exposed to 490 °C air environment (Specimen 22A-1).
Fig. 6. Scanning electron micrograph of the oxide at the interface formed in Zr-2.5Nb exposed to 490 °C steam environment (Specimen 22E). Fig. 7. Scanning electron micrograph of the oxide at the interface formed in Zr-2.5Nb (low iron content) exposed to 490 °C steam environment (Specimen 1H14). after 4 9 0 °C s t e a m c o r r o s i o n . T h e s e differences m a y result f r o m the different iron c o n t e n t s . The relationship b e t w e e n the u n d e r l y i n g m e t a l s t r u c t u r e a n d the o x i d a t i o n is well illustrated f o r the s p e c i m e n s e x p o s e d to 3 0 0 °C p r e s s u r i z e d lithiated water. This is s h o w n in Figs. 8 a n d 9 in w h i c h the oxidation has p r o c e e d e d along the grain b o u n d a r i e s at w h i c h filaments o f fi-Zr are p r e s e n t . The filaments o f oxide, w h i c h c o n s i s t o f m a n y fine ZrO2 grains, are o f a l e n g t h that is c o n s i s t e n t with t h e l e n g t h o f t h e grain b o u n d a r i e s a n d fi-Zr filaments in the metal. The fine ZrO2 g r a i n s m a k i n g u p the oxide film are relatively u n i f o r m in size r a n g i n g in d i a m e t e r f r o m 30 to 5 0 n m (see Fig. 10). H o w e v e r , the oxide, ZrO2, f o r m e d at or n e a r t h e ~-Zr h a s a variable grain size with s o m e grains b e i n g as small as 10 n m (see Fig. 11 for typical TEM m i c r o g r a p h ) .
213
Fig. 8. Scanning electron micrograph of the oxide at the oxide-metal interface in Zr-2.5Nb exposed to a 300 °C, pressurized lithiated water environment (Specimen L-12, metal surface parallel to axial direction of pressure tubing). Fig. 9. Scanning electron micrograph of the oxide at the oxide-metal interface in Zr-2.SNb exposed to a 300 °C, pressurized lithiated water environment (Specimen L-12, metal surface perpendicular to axial direction of pressure tubing).
Fig. 10. Transmission electron micrograph of the oxide formed at the oxide--metal interface in specimen exposed in pressurized lithiated water for 1.7 days (40 h) Specimen L-12, metal surface parallel to axial direction of pressure tubing). Fig. 11. Transmission electron micrograph of the oxide formed at the oxide-metal interface in specimen exposed in pressurized lithiated water for 1.7 days (40 h). (Specimen L-12, metal surface perpendicular to axial direction of pressure tubing).
If w e n o w c o m p a r e t h e o x i d e s t r u c t u r e f o r m e d i n p r e s s u r i z e d l i t h i a t e d w a t e r a t 3 0 0 °C ( F i g s . 8 a n d 9) w i t h t h a t f o r m e d i n o x y g e n a t 3 0 0 °C, Fig. l(a), we note that the specimen exposed to oxygen does not show such a clear r e l a t i o n s h i p b e t w e e n the s t r u c t u r e of the m e t a l a n d the s t r u c t u r e of the oxide. In the oxygen atmosphere the oxidation process does not appear t o p r o c e e d a s p r e d o m i n a n t l y a l o n g t h e g r a i n b o u n d a r i e s , i.e. t h e f l - p h a s e a n d its d e c o m p o s i t i o n p r o d u c t s , a s i n t h e p r e s s u r i z e d l i t h i a t e d w a t e r , t h u s suggesting different corrosion mechanisms are operative.
214 4. D i s c u s s i o n
4.1. Effect of g r a i n b o u n d a r y structure a n d c h e m i s t r y on the corrosion o f Zr-2.5Nb at high t e m p e r a t u r e s The corrosion resistance of Zr-2.5Nb is known to be sensitive to the metallurgical history and the microstructure that is developed [11 ]. In the commercial pressure tubing a satisfactory corrosion resistance is obtained in tubing which is made by extrusion at 850 °C (in a-Zr+fl-Zr region), followed by air cooling and cold-working. The resulting structure consists of elongated a-Zr grains and a grain boundary network of metastable fl-Zr that contains approximately 20 wt.% Nb [12]. Upon heating at temperatures below 610 °C (the monotectoid temperature) metastable fl-Zr will slowly decompose through a number of intermediate phases, e.g. fl-enriched and w-phase, to an equilibrium structure consisting of a-Zr and fl-Nb (approximately 85 wt.%Nb) [13]. The grain boundaries in cold-worked Zr-2.5Nb pressure tubing can thus contain fl-Zr and/or its decomposition products together with impurities and other precipitates. There are thus many kinds of boundaries formed between these phases. At the beginning of the corrosion process at 300 °C, oxidation has been shown to proceed rapidly along the grain boundaries [7 ]. Therefore, oxidation at grain boundaries is faster than at the bulk a-Zr grains. Microstructural changes can, however, occur during exposure to high temperature air and steam and these changes can cause changes in corrosion behaviour. For example, after corrosion for longer times at 490 °C, the metastable flZr will have mostly decomposed to an equilibrium structure consisting of a-Zr and fl-Nb. During 300 °C corrosion, even for long times, the structures and chemical composition (phase make-up) of the grain boundaries change only slightly. The changes occurring at 490 °C, i.e. the breakup of the filaments and the change from fl-Zr to fl-Nb, should lead to a reduction in corrosion rates. However, the rate of metal oxidation increases rapidly with increase in temperature [14]. Thus, in the case of long-term corrosion testing at 490 °C, the kinetics of corrosion are more complex than at low temperatures because the competing effects of change in metal structure and change in oxidation kinetics, and mechanisms, which increase in temperature. Given the break-up of the fl-filaments, occuring at 490 °C, and the fact that the fl-filaments are the main sites for oxidation at the lower temperature, it is not surprising that the oxidation did not appear to proceed along the grain boundaries for the 490 °C exposures. 4.2. Effect o f corrosion e n v i r o n m e n t s on the o x i d e structure at or n e a r the o x i d e - m e t a l interface The oxidation rate in the 300 °C oxygen environment was much lower than that in the 300 °C pressurized lithiated water. Looking at the mechanisms of corrosion, we can distinguish two types of processes, namely chemical corrosion and electrochemical corrosion [14]. Oxidation in a 300 °C oxygen environment is an example of chemical corrosion, whereas oxidation in a
215 300 °C pressurized lithiated water environment is an example of electrochemical corrosion. Complex grain boundary structures affect electrochemical corrosion much m or e than chemical corrosion by promoting grain boundary attack. This was seen for the specimen e x posed to 300 °C oxygen where the grain b o u n d ar y oxidation was not as obvious as in the specimen exposed to 300 °C pressurized lithiated water. Since LiOH is added to the primary coolant of most pressurized water reactors (PWRs) to maintain an alkaline p H so as to control the transport of corrosion p r o d u c t s from the other (than zirconium alloy) structural materials of the r e a c t o r coolant system, the effect of LiOH on the corrosion resistance of the zirconium alloys used for fuel cladding, pressure tubing, etc. is of considerable technological interest. Several studies have been made of the effect of the LiOH concentration in aqueous solutions on the corrosion resistance of zirconium alloys, in particular the Zircaloys [ 15-21 ]. The studies of Zr - 2 .5 Nb pressure tubing clearly show that the oxidation rates in pressurized lithiated water are highly d e p e n d e n t on the LiOH concentration. Manolescu e t al. [21 [ have studied the effect of LiOH on the acceleration in corrosion of Zircaloy-2 and Z r - 2. 5N b for 0.0005 M to 1 M LiOH solutions. They found that the acceleration in corrosion owing to LiOH was greater for the Zr-2.SNb alloy. The p r es e nt authors have shown [7 and this study[ that the oxidation of cold-worked Zr-2.5Nb pressure tubing in pressurized lithiated water (4.8 g LiOH/1 deionized water) pr oc e e ds first along grain boundaries at which there are fl-Zr and its decomposition products, and then continues on a-Zr grains. The oxides f or m e d were mainly ext ended along grain boundaries, and were characterized by long filaments, which are c o m p o s e d of many fine ZrO2 grains. Short-circuit diffusion at grain boundaries is mainly related to the oxidation of grain bounda r y phases, flaws arising from fi-Zr filaments, and microcracking arising from phase transformation in ZrO2. The presence of LiOH may e n c o u r a g e the corrosion at the grain boundaries due to incorporation of lithium in the growing oxide, which generates additional anion vacancies and causes a more rapid transition from the tetragonal oxide phase to the monoclinic phase, and hence p roduces an increase in corrosion rates [22, 23]. Differences in the grain b oundary structures in Zircaloy-2 and Zr-2.5Nb, in particular the pr e s ence of /3-Zr and its decomposition p ro d u cts in Zr-2.5Nb m a y explain the difference found by Manolescu et al. [21 ] for Zircaloy-2 and Zr-2.5Nb in terms of the acceleration of corrosion due to LiOH. The oxidation rate was highest for the 490 °C air environment. Nitrogen, p res en t in air, is known to increase the corrosion rates of zirconium alloys [24]. When N ~- displaces 0 2 - in ZrO2, additional vacancies are p r o d u c e d in ZrOe lattice, which results in increased corrosion rates. The oxide at or ne a r the o x i d e - m e t a l interface in specimens e x p o s e d to air or steam at 490 °C contained interconnected porosity. Similar porosity has been seen by W a r r e t a l . [25] in their studies of steam (400 °C) prefilmed s p ecimen s o f Z r - 2 . 5 N b that had subsequently been e x p o s e d to either a v a c u u m at 500 °C or an in-reactor at 300 °C. Their explanation for the
216 p o r o s i t y w a s t h a t it r e s u l t e d f r o m o x i d e dissolution into the b a s e m e t a l u n d e r c o n d i t i o n s o f h i g h t e m p e r a t u r e ( 5 0 0 °C) a n d a w e a k l y oxidizing e n v i r o n m e n t ( i n - r e a c t o r a t 3 0 0 °C). S u c h an e x p l a n a t i o n m a y also b e a p p r o p r i a t e for the p r e s e n t tests. T h e m i c r o s t r u c t u r e at the o x i d e - m e t a l i n t e r f a c e in o u r c o u p o n s 2 2 E a n d 1H14, w h i c h h a d b e e n e x p o s e d to 4 9 0 °C s t e a m , w a s quite similar t o t h a t o b s e r v e d b y W a r t e t a l . [25] in the s t e a m p r e - f i l m e d s p e c i m e n s e x p o s e d to v a c u u m .
5. Conclusions D e t a i l e d SEM s t u d i e s s h o w t h a t the o x i d a t i o n of c o l d - w o r k e d Z r - 2 . 5 N b p r e s s u r e t u b i n g in b o t h o x y g e n a n d p r e s s u r i z e d lithiated w a t e r at 3 0 0 °C p r o c e e d s m a i n l y a l o n g t h e g r a i n b o u n d a r i e s . H o w e v e r , in t h e 3 0 0 °C o x y g e n e n v i r o n m e n t , t h e o x i d a t i o n a l o n g t h e g r a i n b o u n d a r i e s is n o t as p r o n o u n c e d as in t h e p r e s s u r i z e d lithiated water. P r e f e r e n t i a l o x i d a t i o n at t h e grain b o u n d a r i e s w a s n o t o b s e r v e d w h e n t h e Z r - 2 . 5 N b w a s e x p o s e d to e i t h e r an air or s t e a m e n v i r o n m e n t at 4 9 0 °C. T h e s e r e s u l t s are r a t i o n a l i z e d in t e r m s o f e i t h e r a c h e m i c a l or e l e c t r o c h e m i c a l c o r r o s i o n m e c h a n i s m b e i n g o p e r a t i v e . In t h e 4 9 0 °C air a n d s t e a m e n v i r o n m e n t s , t h e o x i d e s t r u c t u r e at the o x i d e - m e t a l i n t e r f a c e w e r e c h a r a c t e r i z e d b y the p r e s e n c e of i n t e r c o n n e c t e d p o r o s i t y . T h e h i g h o x i d a t i o n r a t e f o r s p e c i m e n s e x p o s e d to 4 9 0 °C air is c o n s i d e r e d to a r i s e f r o m t h e high t e m p e r a t u r e a n d t h e p r e s e n c e o f nitrogen.
Acknowledgments This work was supported by the Natural Sciences and Engineering R e s e a r c h Council o f C a n a d a ( O p e r a t i n g G r a n t A 4 3 9 1 ) a n d the C a n a d a D e u t e r i u m U r a n i u m (CANDU) O w n e r s G r o u p (COG) t h r o u g h W o r k i n g P a r t y No. 35. Mr. J o h n W. R o b i n s o n a s s i s t e d with t h e c o r r o s i o n t e s t s a n d with t h e SEM e x a m i n a t i o n s . S e l e c t e d s p e c i m e n s w e r e p r o v i d e d b y t h e Chalk River N u c l e a r L a b o r a t o r i e s o f A t o m i c E n e r g y o f C a n a d a Limited.
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