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Effects of iodine concentration on iodine-induced stress corrosion cracking of zircaloy-4 tube
194
Journal
of Nuclear
Materials 152 (1988) 194-201 North-Holland, Amsterdam
EFFECTS OF IODINE CONCENTRATION ON IODINE-INDUCED STRESS CORROSION CRACKING OF ZIRCALOY4 TUBE Woo Seog RYU
‘, Young
Hwan
KANG
’ and Jai-Young
LEE 2
’ Nuclear Fuel Performance Division, Korea Advanced Energy Research Institute, ’ Department of Materials Science and Engineering, Korea Advanced Institute Cheongtyang, Seoul, Korea
Received
26 August
1987; accepted
4 December
P. 0. Box 7, Daedukdanji, Chungnam, Korea of Science and Technology, P.O. Box 131,
1987
Iodine-induced stress corrosion cracking (I-XC) experiments on Zircaloy-4 tube were undertaken in the iodine concentration range of O-4 mg/cm*, the temperature range of 330-400 ’ C and a nominal hoop stress of 475 MPa, using the internal pressurization method. The time-to-failure, failure strain and strain rate were measured as a function of the iodine concentration. An apparent activation energy for I-SCC was calculated from the temperature dependence of I-SCC. A fractographic interpretation was also made through scanning electron microscopic examination. The results suggest that the iodine concentration has an influence on the crack propagation step to increase the propagation rate of I-SCC, promoting the I-SCC susceptibility. It is found that the critical iodine concentration means the phenomenological value to show the iodine concentration dependence of I-SCC susceptibility, not the critical value to determine the occurrence of I-SCC. The I-SCC behavior of Zircaloy is discussed from the viewpoint of the crack initiation and propagation process.
1. Introduction The iodine-induced stress corrosion cracking (I-SCC) of Zircaloy has been extensively studied in order to understand the mechanism of pellet-cladding interaction failures. The major interests have been concentrated on the elucidation of the I-SCC mechanism as well as the effects of metallurgical factors of Zircaloy and of environmental factors on I-SCC. There has been some research [l-6] on chemical aspects of I-SCC of Zircaloy. Iodine acts as a corrosive species in I-SCC, and there is the critical iodine concentration to induce I-SCC [7-111. The critical concentration, generally measured from the iodine concentration dependence of the time-to-failure or failure strain in I-SCC experiments, is reported to have various values, depending on the experimental method or condition. Creep mechanisms 112,131 are representative of mechanically-induced damage in the I-SCC process, because I-SCC experiments of Zircaloy are performed under a gaseous iodine atmosphere. When corrosive species are present, certain creep-related parameters are adjusted accordingly to represent the embrittlement of Zircaloy and stress corrosion cracks occur. The concentration of corrosive species influences these parame-
0022-3115/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
ters and I-SCC behavior under the conditions of stress, time and temperature. In this work, creep curves were measured as a function of the iodine concentration in the tests using the internal pressurization method. The deformation rate, crack propagation rate, time-to-failure, and failure strain were calculated from these curves. The physical meanings of the critical iodine concentration as well as I-SCC mechanism were also discussed.
2. Experimental The specimens used in this study were reactor-grade, stress-relieved Zircaloy-4 tubing. The nominal outside diameter and wall thickness of the tubing were 13.08 and 0.42 mm, respectively. The specimens about 100 mm in length were cut from the tubing and were tested in the as-received condition. Test apparatus is shown in figs. 1 and 2. Fig. 1 shows the tube pressurization system. An air-driven gas booster was used to pressurize the bottled high-purity argon gas and pump it into the specimen. The internal pressure of the specimen was measured using a pressure transducer and a digital voltmeter. Fig. 2 shows the
B.V.
W.S. Ryu et al. / Effects of iodine concentration
on I-SCC
195
The nominal hoop stress of the specimen was calculated from the equation
fJ f
);L y$;‘s”e”
R;-Rf
vent
4:
AI
gas boostet
-IT
Zircaloy tube
-C Y
gas
Fig. 1. Schematic diagram of the internal pressurization system.
temperature control and time-to-failure measurement system. An atmosphere retort for vacuum or inert gas was located in a uniform-temperature zone ( f lo C) 15 cm in length of an electric resistant three-zone furnace. The retort internal pressure was monitored as a function of holding time, using a pressure transducer and chart recorder. The test specimen, which had been connected to the pressurization system via specially modified stainless steel tube fittings and valves, was pressurized after being evacuated at room temperature. A limited amount of iodine was loaded into the specimen using a glass ampule. When the required internal pressure was reached, the specimen was closed by using an high-temperature/pressure valve and was isolated from the pressurization system. The isolated specimen was held and monitored in the retort, which had been heated to the test temperature. The change in outer diameter of the specimen was measured by a micrometer every two or three hours after cooling the specimen to room temperature during tests with different iodine concentrations.
temperature
’
where P is the internal pressure of the specimen at test and temperature, R, and Ri are the outer diameter inner diameters of the specimen, respectively. The value of P was calculated from the Beatie-Bridgeman equation of state of a real gas [14]. The iodine concentration is defined as the amount of iodine loaded divided by the total surface area of the specimen. The time-to-failure was determined from the pressure ramp in the retort due to the release of the high internal pressure of the specimen at failure. Primary and secondary strain rates were determined from the slope of the change in specimen outer diameter versus holding time plot. Average strain was determined using the relation, i, = failure strain/ time-to-failure. Failure strain was measured from the outer diameter of the specimen at failure. To examine the SEM fractographs, the failure section was broken open in liquid nitrogen by bending the half ring, which was cut from the failed specimen.
3. Results Fig. 3 shows curves of the holding time versus increase in the diameter of tube specimen with varying iodine concentrations up to 4 mg/cm’ at 330° C and under hoop stress of 475 MPa, from which the time-strain curves were calculated. The iodine-free curve represents a general creep curve of the tube. The second stage of creep had a short period, while the third stage of creep started relatively early and continued to fail by the axial-split mode after 110 h. At the third stage, local deformation was observed. The curves with iodine showed similar behavior
controller
Fig. 2. Schematic diagram of the temperature and time-to-failure measurement apparatus.
W.S. Ryu et al. / Effects of Iodine concentrutmn on I-SCC I
I
1
I
I
I
split(0.2)
4
I
I
I
I
20 holding time
40
I
split(O)
strain as a function of iodine concentration. The timeto-failure at all the test temperatures decreased slowly at first but drastically decreased above a certain concentration and decreased slowly again with increasing iodine concentration, in agreement with Peehs et al. [I] and Wood [ll]. The critical iodine concentration, at which the time-to-failure started to decrease drastically, was observed to increase from 0.6 to 2.0 mg/cm* as the temperature increased from 330 to 400 o C. The failure strains in fig. 6 also showed the shape of sigmoid curves which were similar to the case of timeto-failure. The critical iodine concentrations obtained from the failure strain were consistent with those in time-to-failure curves. The failure strain, however, at a given iodine concentration above the critical concentration tended to increase with temperature, which indicates that the SCC susceptibility of Zircaloy decreases with increasing temperature, while the failure strain decreased with temperature below the critical concentration, because the elongation minimum phenomenon occurs at about 400 o C in tensile behavior under a non-corrosive atmosphere [15,16].
I
60
t hr)
Fig 3. Holding time - increase in diameter curves with iodine concentration at 330° C, 475 MPA. Numeral: iodine concentration, mg/cm2; P: pinhole-type failure; split: axial split-
type failure.
to the iodine-free curve with three stages of creep. Curves with iodine, however, had higher strain rate, shorter time-to-failure, and smaller failure strain than the curve without iodine. The types of failure appearing in the experiments were the axial split and the pinhole. As the temperature increased to 360 or 400 o C under a stress of 475 MPa, the time-strain curves were similar to that for 330°C, as shown in figs. 4 and 5, but the strain rate, time-to-failure and failure strain were different from those for 330” C. From those time-strain curves, the variations of time-to-failure, failure strain and strain rate with iodine concentration and test temperature were investigated and are summarized below. 3. I. Effects on the time-to-failure and failure strain holding
The SCC susceptibilities can be represented indirectly by the time-to-failure and failure strain at a given condition. Fig. 6 shows both time-to-failure and failure
tima
Fig. 4. Holding time - increase in diameter curves with iodine concentration at 360 o C, 475 MPa. Numeral: iodine concentration, mg/cm2.
197
W’.S. Ryu et al. / Effects of iodineco~cenfratjonon I-SCC
O--J-,
-.
h
\
\
\
h,\ \‘x_
I
tl,:33o’c) _ ,. --e tq:40&
. a 0
2
4
6
8
IO
12
14
holding timethr)
Fig. 5. Holding time - increase in diameter curves with iodine concentration at 400 a C, 475 MPa. Numeral: iodine concentration, mg/cm’.
4iZ-li
iodinr conckation tmp/cfift ’ Fig. 6. Iodine concentration dependence of time-to-failure and failure strain at constant hoop stress. t: time-to-failure; I,: failure strain; split-type
Although the SCC susceptibility was observed to be almost independent of iodine concentration below the critical concentration, the time-to-failure or failure strain in this work was different from that for iodine-free atmosphere. At a relatively low iodine ~n~ntration of 0.2 mg,/cm’, the time-to-failure and failure strain at 330°C were 62 h and 168, respectively, which were much lower values compared with those of 110 h and 29% for iodine-free conditions. This indicates that iodine influences the deformation behavior of Zircaloy even below the critical concentration. 3.2. Effects on the strain rate Table 1 shows the primary, secondary and average strain rate with iodine concentration and temperature, which were measured from the time-strain curves. All of the strain rates lie in the general strain rate range of SCC [17], higher than the creep rate but still lower than the strain rate at mechanical ruptures. All of the strain rates at 33O’C increased with iodine concentration in
numeral: test temperature: open symbol: axial failure; closed symbol: pi~ole-TV failure.
the range from 5 x 10F7 to 2 x 10e6 s. Even below the critical concentration, the secondary strain rate was higher than those for the iodine-free condition, although the primary strain rate had a similar value. At elevated temperatures of 360 and 4OO*C, the primary strain rates increased with iodine concentration in the range from 7 x 10e6 to 9 x 10M6/s and from 2 x 10m5 to 4 x lO-5/s, respectively. The secondary strain rate was not able to be measured because the secondary region was difficult to distinguish clearly due to a relatively fast strain rate and short time-to-failure. The comparison among the arithmetic average strain rates at each temperature did not have a significant meaning because of excessive local deformation for the axial-split failure. In general, all the strain rates increased with iodine concentration, but an abrupt change was not observed in strain rates near the critical concentration obtained from the time-to-failure and failure strain.
198
W.S. Ryu et al. / E/feca
Table 1 Values of strain rate of I-WC T:
< C, 0.2 1.0 2.0 4.0
with iodine concentration
of iodineconcentration
and temperature
330°c
on I-SCC
at constant
hoop stress
360°C
4o0°c
c,
CS
if
6,
E’r
(1
if
1.4x 1o-6 1.6~10-~ 1.9x 1o-6 1.9x 1o-6
5.9x 10-7 7.0x 10-7 9.1 x 10-7 1.0x10-6
7.5 x 10-7 1.2X10~6 1.2x10-6 1.4x 10-6
7.6x 1O-6 7.9x1o-6 8.7~10~~ 8.7~10-~
1.7x10-5 9.9x10m6 6.5~10~” 7.4x1o-6
2.4 x lo-’ 2.7 x lo-’ 3.2x10-’ 3.4x 10-5
2.7~10-~ 3.1 x10-5 3.1 x10-s 3.4x 10-S
C,: iodine concentration
i,: primary strain rate;
(mg/cm*);
i,: secondary strain rate; i,: average strain rate.
As the I-SCC experiments of Zircaloy are performed under a constant load in a gaseous iodine atmosphere, the I-XC behavior has an intimate relationship with creep behavior. There are, however, only a few differences between them due to the initiation and propagation of cracks induced by the corrodant. The strain rate of I-SCC, thus, should be the sum of the creep rate and the strain rate resulting from the crack initiation and propagation. In this work, although the variation of strain rates was not quantitatively related to the crack initiation and propagation, the primary strain rate as well as secondary and average strain rates were observed to increase with iodine concentration, in agreement with the results of Peehs et al. [l] in which the strain rate was increased when iodine was supplied in C-ring tests.
crack propagation rate, however, was slower at 1.0 mg/cm2 than at 0.6 mg/cm2. This suggests that the failure mode is determined by the crack propagation rate rather than iodine concentration.
3.3. Effects on the crack propagation rate Fig. 7 shows the mean crack propagation rates, which were obtained by dividing the depth of brittle fracture in fractographs by time-to-failure, as a function of iodine concentration. The fact that the propagation rate increases as the cracks become larger was neglected. The crack propagation rates increased in the range from 10-7 to 10-4 mm/s with iodine concentration even below the critical concentration at all test temperatures, although the time-to-failure and failure strain with iodine concentration had almost constant values, as shown in fig. 6. Above the critical concentration, the crack propagation rates became saturated with iodine concentration. There were two types of failure, pinhole and axiasplit. In general, pinhole-type failures occurred at high iodine concentrations and axial-split at low concentrations. But at 330 o C, 1.0 mg/cm2 of iodine concentration, an axial-split type failure occurred, in spite of a pinhole at lower concentration of 0.6 mg/cm2. The
0 330% A 360-C n 400%
ICY”1 0.1
I
I
1111111
I
I
lllll
1.0
iodine concentration tmg/crh Fig. 7. Iodine concentration dependence of crack propagation rate at constant hoop stress. Open symbol: axial split-type failure; closed symbol: pinhole-type failure.
W.S. Ryu et al. / Effecrs
ofiodineconcentration
3.4. Calculation of apparent activation energy Since I-SCC of Zircaloy is a thermally activated process, time-to-failure, failure strain and average crack propagation rate were dependent on temperature, as shown in the previous sections. Fig. 8 represents the apparent activation energies calculated at a iodine concentration of 2.0 mg/cm’ and under a stress of 475 MPa, at which I-SCC occurred with a pinhole failure. It was found that the activation energy changed around 360°C with repeated experiments at various iodine concentrations, although the data were not sufficient in the medium temperature range. The activation energy calculated from the time-to-failure, average strain rate and average crack propagation rate had values between 33 and 43 kcal/mol in the low-temperature region, which was higher than the values of between 19 and 33 kcal/mol in the high-temperature region. I-SCC of Zircaloy occurs by the combined effects of mechanical and chemical factors. The activation energy of creep, which is most important among the mechanical factors, is known to have a value between 50 and 70
I
I
1
I
I
IO-
19KcoVmo
5.0 -
\
on I-SCC
199
kcal/mol [13,18]. On the other hand, the Zrl, formation reaction, diffusion of iodine or iodine compounds, and adsorption of these are important as chemical factors. The activation energy of Zrl, formation reaction is about 27 kcal/mol [12] and those of adsorption and diffusion of iodine or iodide are supposed to be lower than that of the Zrl, formation reaction. Therefore, the change in the activation energy between the low and high temperature ranges may be attributed to the difference in the relative contribution of those two factors. The mechanical factors are dominant in the low temperature range, while the chemical factors are dominant in the high temperature range. The activation energy calculated from the strain-rate data was lower than that of creep, suggesting that the strain behavior of I-SCC of Zircaloy is affected not only by creep but also by the initiation and propagation of cracks due to stress corrosion, which was consistent with the previous results in which the strain rate with iodine was faster than the creep rate without iodine. The activation energy calculated from the crack propagation rate was lower than that from the strain rate, but higher than that of surface diffusion of 7 kcal/mol estimated by Shann and Olander [19], and lower than that of the Zrl, formation reaction in the high temperature range. This indicates that crack propagation was not controlled by creep deformation or the Zrl, formation reaction but by a chemical factor with a lower activation energy. The difference between the activation energy calculated from the crack propagation rate and that from the time-to-failure can be understood considering the results that the time-to-failure is determined both by the crack propagation rate and by the depth of brittle fracture surface which decreased with temperature.
LO-
3.5. Observation of fracture surfaces AI
-
20.3 -
5
-
.Z 0 *;
-
0
_
._ ! 1
0.1 -
OSJ 1.45
I 1.60
I 1.55
I 1.60
I 1.65
I 1 1.70
Fig. 8. Apparent activation energy of I-SCC of Zircaloy-4 at 2.0 mg/cm2, 475 MPa. t: time-to-failure; 2,: average strain rate; U: average crack propagation,
Fig. 9 shows the inside surfaces of specimens failed at each condition. Fig. 9(a) shows the specimen with axial split failure characterized as a long time-to-failure and a large failure strain tested below a critical concentration Pitting was not observed on the surface but there were a number of microcracks parallel to the specimen axis. Most of them had a small depth and a short length, whereas a few were deep and long. The long cracks seemed to be developed by the linking of many nearly aligned short cracks. The short cracks were similar to those observed in the specimen under a burst test without iodine. Fig. 9(b) shows the specimen tested near the critical concentration and split axially with an intermediate time-to-failure and failure strain. A shal-
200
W.S. Ryu et 01. / Ejects
of Iodineconcentration
on I-SCC
and short crack, as in fig. 9(a). was scarcely observed. Most of the cracks were long with a length of a few tens or hundreds of microns. Pitting was observed on the whole surface, but did not have a definite relationship with the cracks. Fig. 9(c) shows the inside surface of a specimen around a pinhole with a small time-to-failure and failure strain, tested at a high concentration above the critical level. Although a number of branches were observed near both edges of the pinhole, other cracks were not observed on the inside surface. The dark region in the image was representative of a thin layer of iodine compound on the surface, judging from the fact that these regions dissolved in acetone with a brown color. Pitting was observed on the whole surface at higher magnification. From the above observation, the shallow and short cracks were more abundant at a low or zero iodine concentration with the large failure strain, while they were rarely observed in a specimen with a small strain at a high iodine concentration, in agreement with Peehs et al. [l]. Thus, such shallow and short cracks are thought to have an intimate relationship with the failure strain rather than the iodine concentration. But, the deep and long cracks, which were formed by linking a few microcracks together, were well developed at a high iodine concentration. It is concluded from these results that cracks should continue to initiate everywhere on the inside surface irrespective of pitting with increasing strain and propagate as long as iodine is supplied up to the crack tip. Fig. 10 represents a typical fracture surface near the inside of a tube with an axial-split failure. Ductile low
Fig. 9. Morphologies of inside surface of Zircaloy-4 tube after I-XC experiments. (a) 0.2 m&cm*, 330” C; (b) 1.0 mg/cm*, 330 ’ C; (c) 2.0 mg/cm2, 360 ’ C; bar; magnification, pm).
Fig. 10. Fractograph near inside of Zircaloy-4 tube after I-SCC experiment at 0.2 mg/cm*, 330 ’ C. Bar: magnification, pm.
W.S. Ryu et al. / Effects of iodine concentration on I-SCC
Table 2 Values of I-SCC depth with iodine concentration and temperature at constant hoop stress Cl
b-s/cm2 )
0.2 1.0 2.0 4.0
I-SCC depth (pm) 330°c
36O’C
4oo”c
45 70 390 400
12 350 400 400
23 53 323 113
fracture with microvoids took place after brittle fracture, indicating that mechanical failure occurs when cracks initiate at the inside of the tube and propagate up to a certain depth at which the material cannot endure the load. The portion of brittle fracture in the fracture surface tended to increase as the iodine concentration increased. The trackings were not clear but appeared to be transgranular, considering the results of Cox and Wood [20] and Cubicciotti and Jones [8] who reported that predominantly transgranular cracking is the most common mode of cracking of stress-relieved Zircaloy in I-SCC tests. For the pinhole failures with high iodine concentrations, most of the fracture surface was predominantly transgranular and showed no differnce between the features at the beginning of the crack compared with those at more advanced stages of propagation. Table 2 summarizes the depth of a brittle fracture surface as a function of iodine concentration. Even in the case of the same fracture mode of axial-split, the depth of brittle fracture increased with iodine concentration.
4. Discussion 4.1. Critical iodine concentration Iodine reacts chemically with zirconium, forming zirconium iodides of condensed and gaseous phases [3,5,6]. The active substance to induce I-SCC of Zircaloy is the gaseous phase rather than the condensed one [5]. For the experiment in which a limited amount of iodine crystal is loaded, an effective concentration of active iodine corresponds to the concentration of gaseous iodine. Thus, the generally defined iodine concentration, which is calculated arithmetically under the assumption that all the iodine crystal supplied acts as an active substance, was higher than the effective concentration of active iodine. The critical iodine con-
201
centration obtained in this work with a limited amount of iodine, therefore, was higher than the effective critical value. The critical iodine concentrations are reported to have various values depending on the method and conditions employed in I-SCC tests [5]. The experimental time is an important factor to determine the critical iodine concentration. The results of Wood Ill] showed that times-to-failure fall into three bands depending on iodine concentration: less than 3 h above the iodine concentration of 3 mg/cm’, in the range of 200-1000 h at intermediate concentrations, and no failure occurs within 1000 h below 0.02 mg/cm*. Peehs et al. [l] performed tests up to 1000 h and reported that iodine stress corrosion can be observed when the iodine concentration exceeds the order of magnitude of 10e3 mg/cm*, in accord with Cubcicciotti and Jones [8] and Busby et al. [9]. In this work within 100 h, the critical iodine concentrations, obtained from the time-to-failure or failure strain versus iodine concentration curves, are similar with the results of Wood, but higher than those of Peehs et al. As it was difficult to dose an exact amount of iodine of less than the order of a milligram, the tests were not performed under the conditions below the iodine concentration of 0.2 mg/cm* in this work. It is not certain that other drastic changes in the time-to-failure or in the failure strain will occur in longer time below 0.2 mg/cm’. I-SCC is generally known not to occur below the critical concentration. I-SCC, however, was observed even below the critical concentrations in this work. Being almost independent of the concentration below the critical concentration, the time-to-failure or failure strain had small values compared with the iodine-free condition. There were numerous microcracks of a few tens of microns in depth in the inner surface. Moreover, the crack propagation rates were very sensitive to the variation of iodine concentration, although it had a very low value corresponding to the low boundary of the general propagation rate in SCC [17]. These results suggest that even below the critical concentration, crack initiation occurred but propagation rates were so low that no I-SCC appeared to occur. In fact, however, I-SCC did occur. I-SCC behavior of Zircaloy, therefore, is dependent on the iodine concentration over all the concentration range in this work and the critical iodine concentration obtained from the time-to-failure or failure strain versus iodine concentration curves is not an absolute value to determine the occurrence of I-SCC, but a phenomenological value to show the concentration dependence of time-to-failure or failure strain of Zircaloy I-SCC.
202
W.S. Ryu et al / Effects of lodrneconcentrationon I-SCC
4.2. Relation to I-SCC mechanism The iodine concentration dependence of Zircaloy I-SCC will be discussed from the viewpoint of the crack initiation and propagation process. Cracks have been known to initiate at the site of microstructural discontinuities or chemical inhomogeneities [3,8]. In this work, however, cracks were randomly formed on the whole surface regardless of pitting. The shape of microcracks on the inside surface tested with iodine was also similar to that without iodine, in agreement with others [21-231, who reported that the crack initiation has an intimate relationship with slip. In addition, the number of cracks on the inside surface was small at high iodine concentrations with low strain, whereas it was large at low iodine concentrations with high strain, as shown in the results of Peehs et al. [I]. These results suggest that initiation of I-SCC should have a direct relationship with strain rather than iodine concentration. On the other hand, There have been several reports [1,2,24,25] on the existence of an incubation period before cracks initiate. In the results of this work, the stress corrosion cracks initiate and propagate immediately by transgranular cracking. The initial strain rate should be the same in the tests both with and without iodine if there is an incubation period. However, the initial strain rate with iodine was faster than that without iodine, indicating that the iodine-induced cracks initiate without an incubation period, in agreement with Haddad and Cox (241 and Cubicciotti and Jones (81. Cracks will continue to propagate if iodine is supplied to the crack tip and weaken Zircaloy. The crack propagation process can be classified into three steps: a supply of iodine (or iodides) to the crack tip by surface or volume diffusion; an adsorption of iodine (or iodides) at the crack tip; and a chemical reaction between iodine (or iodides) and Zircaloy. If diffusion or Zr-I reaction step controlled the overall propagation process, the crack propagation rate or time-to-failure should have a linear relationship with iodine concentration. In this work, however, the crack propagation rate or timeto-failure was saturated above a certain iodine concentration, The apparent activation energy was lower than that of creep of Zircaloy or that of a chemical reaction of Zrl, and was higher than that of surface diffusion in the high temperature range. Therefore, the propagation process of cracks should be controlled by an adsorption of iodine or iodides at the crack tip, consistent with some other studies [3,5,24]. The failure mode can be explained by the interrelation between the initiation rate and the propagation rate of cracks. If the crack propagation rate is slow, new
cracks will continue to initiate with increasing strain, resulting in an increase of the surface density of cracks. The nearly aligned short cracks are linked together ultimately and develop to shallow and long cracks parallel to the axis of the tube specimen, reducing the constraint effect of the matrix around a crack [S]. Reducing the effective thickness of the specimen by the depth of crack increases the true stress at the Iong crack tip up to the ultimate tensile stress of the material to rupture with the axial-split mode. This is in agreement with the result in which, on the fracture surfaces with the axial-split mode, the brittle failure due to SCC was observed on the inner side along the tube axis whereas a ductile failure with microvoid appeared on the outer side. On the other hand, for a fast propagation rate of the crack, cracks propagate rapidly to grow into the pinhole before the cracks are linked together to form a long crack on the surface.
5. Conc~~ions (1) Iodine has a significant influence on the susceptibility of Zircaloy to iodine-induced stress corrosion cracking in the iodine concentration range of O-4 mg/cm’. The time-to-failure, failure strain and failure mode of Zircaloy I-SCC are determined by the crack propagation rate, which is a function of the iodine concentration. (2) The critical iodine concentration, obtained from the dependence of time-to-failure or failure strain on iodine concentration, has a simple phenomenological significance to show the iodine concentration dependence of the I-SCC susceptibility rather than an absolute meaning to determine the occurrence of I-see. (3) I-SCC behavior of Zircaloy can be described as a relationship between the rates of crack initiation and propagation. Cracks initiate without an incubation period on the specimen surface, regardless of chemical inhomogeneities, where iodine and strain exist. The propagation of cracks is controlled by the adsorption of iodine or iodides on the metal surface at the crack tip. The iodine concentration has a more pronounced effect on the propagation of cracks than on the initiation.
Acknowledgements We gratefully acknowledge the assistance of Mr. S.J. Park in performing the experiments and Mr. D.H. Rim for the SEM observations.
W.S. Ryu et al. / Effects of iodine concentraiion on I-SCC
References [l] M. Peehs, H. Stehle and E. Steinberg, in: Zirconium in the Nuclear Industry, ASTM STP 681 (ASTM, 1979) pp. 244-260. [2] D. Cubicciotti, B.C. Syrett and R.L. Jones, Electric Power Research Inst. Rep. EPRI-NP-1329 (1979). (31 D. Cubicciotti, R.L. Jones and B.C. Syrett, in: Zirconium in the Nuclear Industry, ASTM STP 754 (ASTM, 1983) pp. 146157. [4] D. Gotzmann, J. Nucl. Mater. 107 (1982) 185. [S] P. Hofmann and J. Spino, J. Nucl. Mater. 114 (1983) 50. (61 H. Feuerstein, Oak Ridge National Laboratory Rep. ORNL-4543 (1970). [7] K. Une, J. Nucl. Sci. Technol. 16 (1979) 577. [8] D. Cubicciotti and R.L. Jones, EPRI Rep. NP-717 (1978). 191 C.C. Busby, R.P. Tucker and J.E. McCauley, J. Nucl. Mater. 55 (1975) 64. [lo] D.R. Olander, J. Nucl. Mater. 110 (1982) 343. [ll] J.C. Wood, J. Nucl. Mater. 45 (1972/73) 105. [12] R.E. Williford, Nucl. Engrg. Des. 78 (1984) 23. [13] D.L. Douglass, in: The Metallurgy of Zirconium, Atomic Energy Review Suppl. 1971 (IAEA, 1971) pp. 265-274. [14] J.S. Hsieh, Principles of Thermodynamics (McGraw-Hill, Kogakusha, Japan, 1975).
203
[15] S.I. Hong, W.S. Ryu and C.S. Rim, J. Nucl. Mater. 116 (1983) 314. [16] S.I. Hong, W.S. Ryu and C.S. Rim, J. Nucl. Mater. 120 (1984) 1. [17] R.N. Parkins, in: Stress Corrosion Cracking - The Slow Strain Rate Technique, ASTM STP 665 (ASTM, 1979) pp. 5-25. [18] G. Senski and A. Kunick, Trans. 5th Int. Conf. on Structural Mechanics in Reactor Technology, Berlin, Germany, 1979, Paper C3/3. [19] S.H. Shann and D.R. Olander, J. Nucl. Mater. 113 (1983) 234. [20] B. Cox and J.C. Wood, in: Corrosion Problems in Energy Conversion and Generation, Ed. C.S. Tedman, Jr. (Electrochemical Sot., New York, 1974) p. 275. [21] A. Garlick, J. Nucl. Mater. 49 (1973/74) 209. [22] S. Shimada and M. Nagai, J. Nucl. Mater. 114 (1983) 222. [23] T. Kubo, Y. Wakashima, H. Imahashi and M. Nagai, J. Nucl. Mater. 132 (1985) 126. [24] R. Haddad and B. Cox, J. Nucl. Mater. 138 (1986) 81. [25] K. Videm and L. Lunde, in: Zirconium in the Nuclear Industry, ASTM STP 681 (ASTM, 1979) pp. 229-243.
Journal
of Nuclear
Materials 152 (1988) 194-201 North-Holland, Amsterdam
EFFECTS OF IODINE CONCENTRATION ON IODINE-INDUCED STRESS CORROSION CRACKING OF ZIRCALOY4 TUBE Woo Seog RYU
‘, Young
Hwan
KANG
’ and Jai-Young
LEE 2
’ Nuclear Fuel Performance Division, Korea Advanced Energy Research Institute, ’ Department of Materials Science and Engineering, Korea Advanced Institute Cheongtyang, Seoul, Korea
Received
26 August
1987; accepted
4 December
P. 0. Box 7, Daedukdanji, Chungnam, Korea of Science and Technology, P.O. Box 131,
1987
Iodine-induced stress corrosion cracking (I-XC) experiments on Zircaloy-4 tube were undertaken in the iodine concentration range of O-4 mg/cm*, the temperature range of 330-400 ’ C and a nominal hoop stress of 475 MPa, using the internal pressurization method. The time-to-failure, failure strain and strain rate were measured as a function of the iodine concentration. An apparent activation energy for I-SCC was calculated from the temperature dependence of I-SCC. A fractographic interpretation was also made through scanning electron microscopic examination. The results suggest that the iodine concentration has an influence on the crack propagation step to increase the propagation rate of I-SCC, promoting the I-SCC susceptibility. It is found that the critical iodine concentration means the phenomenological value to show the iodine concentration dependence of I-SCC susceptibility, not the critical value to determine the occurrence of I-SCC. The I-SCC behavior of Zircaloy is discussed from the viewpoint of the crack initiation and propagation process.
1. Introduction The iodine-induced stress corrosion cracking (I-SCC) of Zircaloy has been extensively studied in order to understand the mechanism of pellet-cladding interaction failures. The major interests have been concentrated on the elucidation of the I-SCC mechanism as well as the effects of metallurgical factors of Zircaloy and of environmental factors on I-SCC. There has been some research [l-6] on chemical aspects of I-SCC of Zircaloy. Iodine acts as a corrosive species in I-SCC, and there is the critical iodine concentration to induce I-SCC [7-111. The critical concentration, generally measured from the iodine concentration dependence of the time-to-failure or failure strain in I-SCC experiments, is reported to have various values, depending on the experimental method or condition. Creep mechanisms 112,131 are representative of mechanically-induced damage in the I-SCC process, because I-SCC experiments of Zircaloy are performed under a gaseous iodine atmosphere. When corrosive species are present, certain creep-related parameters are adjusted accordingly to represent the embrittlement of Zircaloy and stress corrosion cracks occur. The concentration of corrosive species influences these parame-
0022-3115/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
ters and I-SCC behavior under the conditions of stress, time and temperature. In this work, creep curves were measured as a function of the iodine concentration in the tests using the internal pressurization method. The deformation rate, crack propagation rate, time-to-failure, and failure strain were calculated from these curves. The physical meanings of the critical iodine concentration as well as I-SCC mechanism were also discussed.
2. Experimental The specimens used in this study were reactor-grade, stress-relieved Zircaloy-4 tubing. The nominal outside diameter and wall thickness of the tubing were 13.08 and 0.42 mm, respectively. The specimens about 100 mm in length were cut from the tubing and were tested in the as-received condition. Test apparatus is shown in figs. 1 and 2. Fig. 1 shows the tube pressurization system. An air-driven gas booster was used to pressurize the bottled high-purity argon gas and pump it into the specimen. The internal pressure of the specimen was measured using a pressure transducer and a digital voltmeter. Fig. 2 shows the
B.V.
W.S. Ryu et al. / Effects of iodine concentration
on I-SCC
195
The nominal hoop stress of the specimen was calculated from the equation
fJ f
);L y$;‘s”e”
R;-Rf
vent
4:
AI
gas boostet
-IT
Zircaloy tube
-C Y
gas
Fig. 1. Schematic diagram of the internal pressurization system.
temperature control and time-to-failure measurement system. An atmosphere retort for vacuum or inert gas was located in a uniform-temperature zone ( f lo C) 15 cm in length of an electric resistant three-zone furnace. The retort internal pressure was monitored as a function of holding time, using a pressure transducer and chart recorder. The test specimen, which had been connected to the pressurization system via specially modified stainless steel tube fittings and valves, was pressurized after being evacuated at room temperature. A limited amount of iodine was loaded into the specimen using a glass ampule. When the required internal pressure was reached, the specimen was closed by using an high-temperature/pressure valve and was isolated from the pressurization system. The isolated specimen was held and monitored in the retort, which had been heated to the test temperature. The change in outer diameter of the specimen was measured by a micrometer every two or three hours after cooling the specimen to room temperature during tests with different iodine concentrations.
temperature
’
where P is the internal pressure of the specimen at test and temperature, R, and Ri are the outer diameter inner diameters of the specimen, respectively. The value of P was calculated from the Beatie-Bridgeman equation of state of a real gas [14]. The iodine concentration is defined as the amount of iodine loaded divided by the total surface area of the specimen. The time-to-failure was determined from the pressure ramp in the retort due to the release of the high internal pressure of the specimen at failure. Primary and secondary strain rates were determined from the slope of the change in specimen outer diameter versus holding time plot. Average strain was determined using the relation, i, = failure strain/ time-to-failure. Failure strain was measured from the outer diameter of the specimen at failure. To examine the SEM fractographs, the failure section was broken open in liquid nitrogen by bending the half ring, which was cut from the failed specimen.
3. Results Fig. 3 shows curves of the holding time versus increase in the diameter of tube specimen with varying iodine concentrations up to 4 mg/cm’ at 330° C and under hoop stress of 475 MPa, from which the time-strain curves were calculated. The iodine-free curve represents a general creep curve of the tube. The second stage of creep had a short period, while the third stage of creep started relatively early and continued to fail by the axial-split mode after 110 h. At the third stage, local deformation was observed. The curves with iodine showed similar behavior
controller
Fig. 2. Schematic diagram of the temperature and time-to-failure measurement apparatus.
W.S. Ryu et al. / Effects of Iodine concentrutmn on I-SCC I
I
1
I
I
I
split(0.2)
4
I
I
I
I
20 holding time
40
I
split(O)
strain as a function of iodine concentration. The timeto-failure at all the test temperatures decreased slowly at first but drastically decreased above a certain concentration and decreased slowly again with increasing iodine concentration, in agreement with Peehs et al. [I] and Wood [ll]. The critical iodine concentration, at which the time-to-failure started to decrease drastically, was observed to increase from 0.6 to 2.0 mg/cm* as the temperature increased from 330 to 400 o C. The failure strains in fig. 6 also showed the shape of sigmoid curves which were similar to the case of timeto-failure. The critical iodine concentrations obtained from the failure strain were consistent with those in time-to-failure curves. The failure strain, however, at a given iodine concentration above the critical concentration tended to increase with temperature, which indicates that the SCC susceptibility of Zircaloy decreases with increasing temperature, while the failure strain decreased with temperature below the critical concentration, because the elongation minimum phenomenon occurs at about 400 o C in tensile behavior under a non-corrosive atmosphere [15,16].
I
60
t hr)
Fig 3. Holding time - increase in diameter curves with iodine concentration at 330° C, 475 MPA. Numeral: iodine concentration, mg/cm2; P: pinhole-type failure; split: axial split-
type failure.
to the iodine-free curve with three stages of creep. Curves with iodine, however, had higher strain rate, shorter time-to-failure, and smaller failure strain than the curve without iodine. The types of failure appearing in the experiments were the axial split and the pinhole. As the temperature increased to 360 or 400 o C under a stress of 475 MPa, the time-strain curves were similar to that for 330°C, as shown in figs. 4 and 5, but the strain rate, time-to-failure and failure strain were different from those for 330” C. From those time-strain curves, the variations of time-to-failure, failure strain and strain rate with iodine concentration and test temperature were investigated and are summarized below. 3. I. Effects on the time-to-failure and failure strain holding
The SCC susceptibilities can be represented indirectly by the time-to-failure and failure strain at a given condition. Fig. 6 shows both time-to-failure and failure
tima
Fig. 4. Holding time - increase in diameter curves with iodine concentration at 360 o C, 475 MPa. Numeral: iodine concentration, mg/cm2.
197
W’.S. Ryu et al. / Effects of iodineco~cenfratjonon I-SCC
O--J-,
-.
h
\
\
\
h,\ \‘x_
I
tl,:33o’c) _ ,. --e tq:40&
. a 0
2
4
6
8
IO
12
14
holding timethr)
Fig. 5. Holding time - increase in diameter curves with iodine concentration at 400 a C, 475 MPa. Numeral: iodine concentration, mg/cm’.
4iZ-li
iodinr conckation tmp/cfift ’ Fig. 6. Iodine concentration dependence of time-to-failure and failure strain at constant hoop stress. t: time-to-failure; I,: failure strain; split-type
Although the SCC susceptibility was observed to be almost independent of iodine concentration below the critical concentration, the time-to-failure or failure strain in this work was different from that for iodine-free atmosphere. At a relatively low iodine ~n~ntration of 0.2 mg,/cm’, the time-to-failure and failure strain at 330°C were 62 h and 168, respectively, which were much lower values compared with those of 110 h and 29% for iodine-free conditions. This indicates that iodine influences the deformation behavior of Zircaloy even below the critical concentration. 3.2. Effects on the strain rate Table 1 shows the primary, secondary and average strain rate with iodine concentration and temperature, which were measured from the time-strain curves. All of the strain rates lie in the general strain rate range of SCC [17], higher than the creep rate but still lower than the strain rate at mechanical ruptures. All of the strain rates at 33O’C increased with iodine concentration in
numeral: test temperature: open symbol: axial failure; closed symbol: pi~ole-TV failure.
the range from 5 x 10F7 to 2 x 10e6 s. Even below the critical concentration, the secondary strain rate was higher than those for the iodine-free condition, although the primary strain rate had a similar value. At elevated temperatures of 360 and 4OO*C, the primary strain rates increased with iodine concentration in the range from 7 x 10e6 to 9 x 10M6/s and from 2 x 10m5 to 4 x lO-5/s, respectively. The secondary strain rate was not able to be measured because the secondary region was difficult to distinguish clearly due to a relatively fast strain rate and short time-to-failure. The comparison among the arithmetic average strain rates at each temperature did not have a significant meaning because of excessive local deformation for the axial-split failure. In general, all the strain rates increased with iodine concentration, but an abrupt change was not observed in strain rates near the critical concentration obtained from the time-to-failure and failure strain.
198
W.S. Ryu et al. / E/feca
Table 1 Values of strain rate of I-WC T:
< C, 0.2 1.0 2.0 4.0
with iodine concentration
of iodineconcentration
and temperature
330°c
on I-SCC
at constant
hoop stress
360°C
4o0°c
c,
CS
if
6,
E’r
(1
if
1.4x 1o-6 1.6~10-~ 1.9x 1o-6 1.9x 1o-6
5.9x 10-7 7.0x 10-7 9.1 x 10-7 1.0x10-6
7.5 x 10-7 1.2X10~6 1.2x10-6 1.4x 10-6
7.6x 1O-6 7.9x1o-6 8.7~10~~ 8.7~10-~
1.7x10-5 9.9x10m6 6.5~10~” 7.4x1o-6
2.4 x lo-’ 2.7 x lo-’ 3.2x10-’ 3.4x 10-5
2.7~10-~ 3.1 x10-5 3.1 x10-s 3.4x 10-S
C,: iodine concentration
i,: primary strain rate;
(mg/cm*);
i,: secondary strain rate; i,: average strain rate.
As the I-SCC experiments of Zircaloy are performed under a constant load in a gaseous iodine atmosphere, the I-XC behavior has an intimate relationship with creep behavior. There are, however, only a few differences between them due to the initiation and propagation of cracks induced by the corrodant. The strain rate of I-SCC, thus, should be the sum of the creep rate and the strain rate resulting from the crack initiation and propagation. In this work, although the variation of strain rates was not quantitatively related to the crack initiation and propagation, the primary strain rate as well as secondary and average strain rates were observed to increase with iodine concentration, in agreement with the results of Peehs et al. [l] in which the strain rate was increased when iodine was supplied in C-ring tests.
crack propagation rate, however, was slower at 1.0 mg/cm2 than at 0.6 mg/cm2. This suggests that the failure mode is determined by the crack propagation rate rather than iodine concentration.
3.3. Effects on the crack propagation rate Fig. 7 shows the mean crack propagation rates, which were obtained by dividing the depth of brittle fracture in fractographs by time-to-failure, as a function of iodine concentration. The fact that the propagation rate increases as the cracks become larger was neglected. The crack propagation rates increased in the range from 10-7 to 10-4 mm/s with iodine concentration even below the critical concentration at all test temperatures, although the time-to-failure and failure strain with iodine concentration had almost constant values, as shown in fig. 6. Above the critical concentration, the crack propagation rates became saturated with iodine concentration. There were two types of failure, pinhole and axiasplit. In general, pinhole-type failures occurred at high iodine concentrations and axial-split at low concentrations. But at 330 o C, 1.0 mg/cm2 of iodine concentration, an axial-split type failure occurred, in spite of a pinhole at lower concentration of 0.6 mg/cm2. The
0 330% A 360-C n 400%
ICY”1 0.1
I
I
1111111
I
I
lllll
1.0
iodine concentration tmg/crh Fig. 7. Iodine concentration dependence of crack propagation rate at constant hoop stress. Open symbol: axial split-type failure; closed symbol: pinhole-type failure.
W.S. Ryu et al. / Effecrs
ofiodineconcentration
3.4. Calculation of apparent activation energy Since I-SCC of Zircaloy is a thermally activated process, time-to-failure, failure strain and average crack propagation rate were dependent on temperature, as shown in the previous sections. Fig. 8 represents the apparent activation energies calculated at a iodine concentration of 2.0 mg/cm’ and under a stress of 475 MPa, at which I-SCC occurred with a pinhole failure. It was found that the activation energy changed around 360°C with repeated experiments at various iodine concentrations, although the data were not sufficient in the medium temperature range. The activation energy calculated from the time-to-failure, average strain rate and average crack propagation rate had values between 33 and 43 kcal/mol in the low-temperature region, which was higher than the values of between 19 and 33 kcal/mol in the high-temperature region. I-SCC of Zircaloy occurs by the combined effects of mechanical and chemical factors. The activation energy of creep, which is most important among the mechanical factors, is known to have a value between 50 and 70
I
I
1
I
I
IO-
19KcoVmo
5.0 -
\
on I-SCC
199
kcal/mol [13,18]. On the other hand, the Zrl, formation reaction, diffusion of iodine or iodine compounds, and adsorption of these are important as chemical factors. The activation energy of Zrl, formation reaction is about 27 kcal/mol [12] and those of adsorption and diffusion of iodine or iodide are supposed to be lower than that of the Zrl, formation reaction. Therefore, the change in the activation energy between the low and high temperature ranges may be attributed to the difference in the relative contribution of those two factors. The mechanical factors are dominant in the low temperature range, while the chemical factors are dominant in the high temperature range. The activation energy calculated from the strain-rate data was lower than that of creep, suggesting that the strain behavior of I-SCC of Zircaloy is affected not only by creep but also by the initiation and propagation of cracks due to stress corrosion, which was consistent with the previous results in which the strain rate with iodine was faster than the creep rate without iodine. The activation energy calculated from the crack propagation rate was lower than that from the strain rate, but higher than that of surface diffusion of 7 kcal/mol estimated by Shann and Olander [19], and lower than that of the Zrl, formation reaction in the high temperature range. This indicates that crack propagation was not controlled by creep deformation or the Zrl, formation reaction but by a chemical factor with a lower activation energy. The difference between the activation energy calculated from the crack propagation rate and that from the time-to-failure can be understood considering the results that the time-to-failure is determined both by the crack propagation rate and by the depth of brittle fracture surface which decreased with temperature.
LO-
3.5. Observation of fracture surfaces AI
-
20.3 -
5
-
.Z 0 *;
-
0
_
._ ! 1
0.1 -
OSJ 1.45
I 1.60
I 1.55
I 1.60
I 1.65
I 1 1.70
Fig. 8. Apparent activation energy of I-SCC of Zircaloy-4 at 2.0 mg/cm2, 475 MPa. t: time-to-failure; 2,: average strain rate; U: average crack propagation,
Fig. 9 shows the inside surfaces of specimens failed at each condition. Fig. 9(a) shows the specimen with axial split failure characterized as a long time-to-failure and a large failure strain tested below a critical concentration Pitting was not observed on the surface but there were a number of microcracks parallel to the specimen axis. Most of them had a small depth and a short length, whereas a few were deep and long. The long cracks seemed to be developed by the linking of many nearly aligned short cracks. The short cracks were similar to those observed in the specimen under a burst test without iodine. Fig. 9(b) shows the specimen tested near the critical concentration and split axially with an intermediate time-to-failure and failure strain. A shal-
200
W.S. Ryu et 01. / Ejects
of Iodineconcentration
on I-SCC
and short crack, as in fig. 9(a). was scarcely observed. Most of the cracks were long with a length of a few tens or hundreds of microns. Pitting was observed on the whole surface, but did not have a definite relationship with the cracks. Fig. 9(c) shows the inside surface of a specimen around a pinhole with a small time-to-failure and failure strain, tested at a high concentration above the critical level. Although a number of branches were observed near both edges of the pinhole, other cracks were not observed on the inside surface. The dark region in the image was representative of a thin layer of iodine compound on the surface, judging from the fact that these regions dissolved in acetone with a brown color. Pitting was observed on the whole surface at higher magnification. From the above observation, the shallow and short cracks were more abundant at a low or zero iodine concentration with the large failure strain, while they were rarely observed in a specimen with a small strain at a high iodine concentration, in agreement with Peehs et al. [l]. Thus, such shallow and short cracks are thought to have an intimate relationship with the failure strain rather than the iodine concentration. But, the deep and long cracks, which were formed by linking a few microcracks together, were well developed at a high iodine concentration. It is concluded from these results that cracks should continue to initiate everywhere on the inside surface irrespective of pitting with increasing strain and propagate as long as iodine is supplied up to the crack tip. Fig. 10 represents a typical fracture surface near the inside of a tube with an axial-split failure. Ductile low
Fig. 9. Morphologies of inside surface of Zircaloy-4 tube after I-XC experiments. (a) 0.2 m&cm*, 330” C; (b) 1.0 mg/cm*, 330 ’ C; (c) 2.0 mg/cm2, 360 ’ C; bar; magnification, pm).
Fig. 10. Fractograph near inside of Zircaloy-4 tube after I-SCC experiment at 0.2 mg/cm*, 330 ’ C. Bar: magnification, pm.
W.S. Ryu et al. / Effects of iodine concentration on I-SCC
Table 2 Values of I-SCC depth with iodine concentration and temperature at constant hoop stress Cl
b-s/cm2 )
0.2 1.0 2.0 4.0
I-SCC depth (pm) 330°c
36O’C
4oo”c
45 70 390 400
12 350 400 400
23 53 323 113
fracture with microvoids took place after brittle fracture, indicating that mechanical failure occurs when cracks initiate at the inside of the tube and propagate up to a certain depth at which the material cannot endure the load. The portion of brittle fracture in the fracture surface tended to increase as the iodine concentration increased. The trackings were not clear but appeared to be transgranular, considering the results of Cox and Wood [20] and Cubicciotti and Jones [8] who reported that predominantly transgranular cracking is the most common mode of cracking of stress-relieved Zircaloy in I-SCC tests. For the pinhole failures with high iodine concentrations, most of the fracture surface was predominantly transgranular and showed no differnce between the features at the beginning of the crack compared with those at more advanced stages of propagation. Table 2 summarizes the depth of a brittle fracture surface as a function of iodine concentration. Even in the case of the same fracture mode of axial-split, the depth of brittle fracture increased with iodine concentration.
4. Discussion 4.1. Critical iodine concentration Iodine reacts chemically with zirconium, forming zirconium iodides of condensed and gaseous phases [3,5,6]. The active substance to induce I-SCC of Zircaloy is the gaseous phase rather than the condensed one [5]. For the experiment in which a limited amount of iodine crystal is loaded, an effective concentration of active iodine corresponds to the concentration of gaseous iodine. Thus, the generally defined iodine concentration, which is calculated arithmetically under the assumption that all the iodine crystal supplied acts as an active substance, was higher than the effective concentration of active iodine. The critical iodine con-
201
centration obtained in this work with a limited amount of iodine, therefore, was higher than the effective critical value. The critical iodine concentrations are reported to have various values depending on the method and conditions employed in I-SCC tests [5]. The experimental time is an important factor to determine the critical iodine concentration. The results of Wood Ill] showed that times-to-failure fall into three bands depending on iodine concentration: less than 3 h above the iodine concentration of 3 mg/cm’, in the range of 200-1000 h at intermediate concentrations, and no failure occurs within 1000 h below 0.02 mg/cm*. Peehs et al. [l] performed tests up to 1000 h and reported that iodine stress corrosion can be observed when the iodine concentration exceeds the order of magnitude of 10e3 mg/cm*, in accord with Cubcicciotti and Jones [8] and Busby et al. [9]. In this work within 100 h, the critical iodine concentrations, obtained from the time-to-failure or failure strain versus iodine concentration curves, are similar with the results of Wood, but higher than those of Peehs et al. As it was difficult to dose an exact amount of iodine of less than the order of a milligram, the tests were not performed under the conditions below the iodine concentration of 0.2 mg/cm* in this work. It is not certain that other drastic changes in the time-to-failure or in the failure strain will occur in longer time below 0.2 mg/cm’. I-SCC is generally known not to occur below the critical concentration. I-SCC, however, was observed even below the critical concentrations in this work. Being almost independent of the concentration below the critical concentration, the time-to-failure or failure strain had small values compared with the iodine-free condition. There were numerous microcracks of a few tens of microns in depth in the inner surface. Moreover, the crack propagation rates were very sensitive to the variation of iodine concentration, although it had a very low value corresponding to the low boundary of the general propagation rate in SCC [17]. These results suggest that even below the critical concentration, crack initiation occurred but propagation rates were so low that no I-SCC appeared to occur. In fact, however, I-SCC did occur. I-SCC behavior of Zircaloy, therefore, is dependent on the iodine concentration over all the concentration range in this work and the critical iodine concentration obtained from the time-to-failure or failure strain versus iodine concentration curves is not an absolute value to determine the occurrence of I-SCC, but a phenomenological value to show the concentration dependence of time-to-failure or failure strain of Zircaloy I-SCC.
202
W.S. Ryu et al / Effects of lodrneconcentrationon I-SCC
4.2. Relation to I-SCC mechanism The iodine concentration dependence of Zircaloy I-SCC will be discussed from the viewpoint of the crack initiation and propagation process. Cracks have been known to initiate at the site of microstructural discontinuities or chemical inhomogeneities [3,8]. In this work, however, cracks were randomly formed on the whole surface regardless of pitting. The shape of microcracks on the inside surface tested with iodine was also similar to that without iodine, in agreement with others [21-231, who reported that the crack initiation has an intimate relationship with slip. In addition, the number of cracks on the inside surface was small at high iodine concentrations with low strain, whereas it was large at low iodine concentrations with high strain, as shown in the results of Peehs et al. [I]. These results suggest that initiation of I-SCC should have a direct relationship with strain rather than iodine concentration. On the other hand, There have been several reports [1,2,24,25] on the existence of an incubation period before cracks initiate. In the results of this work, the stress corrosion cracks initiate and propagate immediately by transgranular cracking. The initial strain rate should be the same in the tests both with and without iodine if there is an incubation period. However, the initial strain rate with iodine was faster than that without iodine, indicating that the iodine-induced cracks initiate without an incubation period, in agreement with Haddad and Cox (241 and Cubicciotti and Jones (81. Cracks will continue to propagate if iodine is supplied to the crack tip and weaken Zircaloy. The crack propagation process can be classified into three steps: a supply of iodine (or iodides) to the crack tip by surface or volume diffusion; an adsorption of iodine (or iodides) at the crack tip; and a chemical reaction between iodine (or iodides) and Zircaloy. If diffusion or Zr-I reaction step controlled the overall propagation process, the crack propagation rate or time-to-failure should have a linear relationship with iodine concentration. In this work, however, the crack propagation rate or timeto-failure was saturated above a certain iodine concentration, The apparent activation energy was lower than that of creep of Zircaloy or that of a chemical reaction of Zrl, and was higher than that of surface diffusion in the high temperature range. Therefore, the propagation process of cracks should be controlled by an adsorption of iodine or iodides at the crack tip, consistent with some other studies [3,5,24]. The failure mode can be explained by the interrelation between the initiation rate and the propagation rate of cracks. If the crack propagation rate is slow, new
cracks will continue to initiate with increasing strain, resulting in an increase of the surface density of cracks. The nearly aligned short cracks are linked together ultimately and develop to shallow and long cracks parallel to the axis of the tube specimen, reducing the constraint effect of the matrix around a crack [S]. Reducing the effective thickness of the specimen by the depth of crack increases the true stress at the Iong crack tip up to the ultimate tensile stress of the material to rupture with the axial-split mode. This is in agreement with the result in which, on the fracture surfaces with the axial-split mode, the brittle failure due to SCC was observed on the inner side along the tube axis whereas a ductile failure with microvoid appeared on the outer side. On the other hand, for a fast propagation rate of the crack, cracks propagate rapidly to grow into the pinhole before the cracks are linked together to form a long crack on the surface.
5. Conc~~ions (1) Iodine has a significant influence on the susceptibility of Zircaloy to iodine-induced stress corrosion cracking in the iodine concentration range of O-4 mg/cm’. The time-to-failure, failure strain and failure mode of Zircaloy I-SCC are determined by the crack propagation rate, which is a function of the iodine concentration. (2) The critical iodine concentration, obtained from the dependence of time-to-failure or failure strain on iodine concentration, has a simple phenomenological significance to show the iodine concentration dependence of the I-SCC susceptibility rather than an absolute meaning to determine the occurrence of I-see. (3) I-SCC behavior of Zircaloy can be described as a relationship between the rates of crack initiation and propagation. Cracks initiate without an incubation period on the specimen surface, regardless of chemical inhomogeneities, where iodine and strain exist. The propagation of cracks is controlled by the adsorption of iodine or iodides on the metal surface at the crack tip. The iodine concentration has a more pronounced effect on the propagation of cracks than on the initiation.
Acknowledgements We gratefully acknowledge the assistance of Mr. S.J. Park in performing the experiments and Mr. D.H. Rim for the SEM observations.
W.S. Ryu et al. / Effects of iodine concentraiion on I-SCC
References [l] M. Peehs, H. Stehle and E. Steinberg, in: Zirconium in the Nuclear Industry, ASTM STP 681 (ASTM, 1979) pp. 244-260. [2] D. Cubicciotti, B.C. Syrett and R.L. Jones, Electric Power Research Inst. Rep. EPRI-NP-1329 (1979). (31 D. Cubicciotti, R.L. Jones and B.C. Syrett, in: Zirconium in the Nuclear Industry, ASTM STP 754 (ASTM, 1983) pp. 146157. [4] D. Gotzmann, J. Nucl. Mater. 107 (1982) 185. [S] P. Hofmann and J. Spino, J. Nucl. Mater. 114 (1983) 50. (61 H. Feuerstein, Oak Ridge National Laboratory Rep. ORNL-4543 (1970). [7] K. Une, J. Nucl. Sci. Technol. 16 (1979) 577. [8] D. Cubicciotti and R.L. Jones, EPRI Rep. NP-717 (1978). 191 C.C. Busby, R.P. Tucker and J.E. McCauley, J. Nucl. Mater. 55 (1975) 64. [lo] D.R. Olander, J. Nucl. Mater. 110 (1982) 343. [ll] J.C. Wood, J. Nucl. Mater. 45 (1972/73) 105. [12] R.E. Williford, Nucl. Engrg. Des. 78 (1984) 23. [13] D.L. Douglass, in: The Metallurgy of Zirconium, Atomic Energy Review Suppl. 1971 (IAEA, 1971) pp. 265-274. [14] J.S. Hsieh, Principles of Thermodynamics (McGraw-Hill, Kogakusha, Japan, 1975).
203
[15] S.I. Hong, W.S. Ryu and C.S. Rim, J. Nucl. Mater. 116 (1983) 314. [16] S.I. Hong, W.S. Ryu and C.S. Rim, J. Nucl. Mater. 120 (1984) 1. [17] R.N. Parkins, in: Stress Corrosion Cracking - The Slow Strain Rate Technique, ASTM STP 665 (ASTM, 1979) pp. 5-25. [18] G. Senski and A. Kunick, Trans. 5th Int. Conf. on Structural Mechanics in Reactor Technology, Berlin, Germany, 1979, Paper C3/3. [19] S.H. Shann and D.R. Olander, J. Nucl. Mater. 113 (1983) 234. [20] B. Cox and J.C. Wood, in: Corrosion Problems in Energy Conversion and Generation, Ed. C.S. Tedman, Jr. (Electrochemical Sot., New York, 1974) p. 275. [21] A. Garlick, J. Nucl. Mater. 49 (1973/74) 209. [22] S. Shimada and M. Nagai, J. Nucl. Mater. 114 (1983) 222. [23] T. Kubo, Y. Wakashima, H. Imahashi and M. Nagai, J. Nucl. Mater. 132 (1985) 126. [24] R. Haddad and B. Cox, J. Nucl. Mater. 138 (1986) 81. [25] K. Videm and L. Lunde, in: Zirconium in the Nuclear Industry, ASTM STP 681 (ASTM, 1979) pp. 229-243.