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Hydrogen trapping by TiC particles in iron
Acro mcmll. Vol. 32, No. 1. pp. 131-136. 1984 Prime@ in Great Britain. All rights reserved
Copyright 8
OOLII-6l60/84.$3.00+0.00 1984 Peremon Press Ltd
HYDROGEN TRAPPING BY TiC PARTICLES IN IRON H. G. LEE and JAI-YOUNG Department
of Materials
Science and Engineering,
LEE
Korea Advanced Institute
of Science and Technology,
P.O. Box 150 Chongryang, Seoul, Korea (Received 26 February 1983; in revised form 26 May 1983)
At&a&-The interaction of hydrogen with the interface of Tic inclusion in iron was studied by thermal analysis technique using gas chromatro~aph as hydrogen detector. The amount of hydrogen evolved from trap site was measured and its relations with activation energy and trap binding energy were studied. The evolution rate peak appeared at 996 K when heated with 3 K/min heating rate. 11is due to the hydrogen release from the interface of Tic inclusions. The trap activation inergy needed to escape from the interface of Tic inclusion is obtained from the relation between peak temperature and heating rate as 86.9 kJ/mol and the trap binding energy obtained from the relation between peak area and the hydrogen charging temperature is 28.1 Wfmol. The energy level of the hydrogen around the trap site is estimated from the above values. R&sum&Nous avons ttudit l’interaction de l’hydrog&ne avec l’interface d’une inclusion de Tic dans du fer par la technique de l’analyse thermique, en utilisant un chromatographe B gaz comme ditccteur de l’hydroghne. Nous avons mesun la quantite d’hydrog&ne issue des sites de pi&cage et nous avons 6tudiC ses relations avec l%nergie d’activation et l%nergie de liaison du pi&e. L.e pit de la vitesse de dbgagement apparaissait B 996 K pour une vitesse de chase de 3 K/min. n provient du dbgagement d’hydro&ne z4par& de l’interfae des inclusions de TIC. Nous avons obtmu l%nergie d’activation du pi&e n&cessaire pour tiapper de l’interface de l’inclusion de Tic, g partir de la relation entre la temp&rature du pit et la vitesse de chauffagc, soit 86.9 kJ/mol, ainsi que 1’Cnergie de liaison du pi&e, obtenue i partir de la relation entre la surface du pit et la temp&ature de la charge en hydrogtne, soit 28,l kJ/mol. Nous avons estimit le niveau d&e&e de I’hydro&ne autour des sites de pi&eage, g partir des vaicun ci-dessus. medic W~h~i~r~ng von Wasserstoff mit der Grenzflsiche zwischen ~C-Ein~hi~~ und der Eisenmatrix wurde mittels thermischer Analytik und einem Gaschromatographen als Wasserstoffdetektor untersucht. Die aus den Fallen entweichcnde Wasserstoffmcngc wurde gemessen und mit Aktivivngsund Bindungsenergien verglichen. Das Maximum der Freigabe trat -bei einer Aufheiz~hwindigkeit von 3 K/min- bei 996 K auf und tirt von dem AblSsen des Wasserstoffs aus der Grenzfliiche her. Die Aktivienmgsenergic f”r das L&en dcs Wasserstoffs aus der Fe/Tic-Grenzfliiche ergibt sich aus dem Zusammenhang zwischen Temperatur des Maximums und der Au~ei~h~ndi~eit. Sic bctriigt 86,9 kJ/Mol. Die Energie fur Bmdung an die Falle wird erhaltcn aus dem Zusammenhang zwischen der F&he unter dem Maximum und der W~~to~~~~~~mt~ und betriigt 28,l kJ/Mol. Das Energieniveau des Wasserstoffs in der N%e der Falle wird aus diesen Werten abgeschatzt.
1. ~ODU~ON Hydrogen embrittlemcnt phenomena in steels have been the subject of much research, but the mechanism has not been understood fully yet, It is important to know the mechanism of hydrogen transport in iron in order to study the hydrogen ~b~t~~~t phe nomena. The diffusivities of hydrogen measured at low temperature are considerably smaller than the values extrapolated from high temperature region and deviate markedly compared with the high temperature values 11J. Many researchers think that these abnormal hydrogen diffusion phenomena appear by the trapping of hydrogen in the trap site at low temperatures. The hydrogen trapping phenomenon is an attractive interaction between hydrogen atom and the structural imperfection and there are many nsearches which intend to analyze the hydrogen embrittfement phenomenon by the hydrogen trapping in recent years (24.
It is known that almost all kinds of defects in metal can act as trapping sites, i.e. the dislocation [5,6], microvoid [7,8], grain boundary [9], the ferrite-cementite interface [lo] and the interface of non-metallic inclusions [l l-15]. McNabb and Foster [16] proposed a mathematical model which describes the effect of trap on hydro~n diffusion by considering trapping rate and detrapping rate independently. Oriani [17] derived the apparent diffusivity of hydrogen in metals with the assumption of local equilibrium between trapping site and normal interstitial site. But Kowia [18] derived the apparent diffusi~ty equation by considering the substitutional impurity atom as a trapping center. At any rate the apparent diffusivity of hydrogen in metals is the function of trap activation energy, trap binding energy and trap density when trap sites are present in the specimen. Therefore it is important to measure these values of various trap sites to studying hydrogen diffusion in metals. I31
LEE and LEE: HYDROGEN TRAPPING IN IRON
132
The hydrogen trapping phenomenon by the interface of Tic inclusion was studied by some researchers. Asaoka (141 injected the tritium into F&.5 wt% Ti alloy and observed it with the autoradiographic method and insisted that the hydrogen trap binding energy of interface of TiC inclusion is larger than 61 kJ/mol. Pressouyre [IS] has analyzed the difference of 1st and 2nd permeation experiment and calculated the trap activation energy as 95kJ/mol. But the above two experimental results have difficulty in analyzing quantitatively the sole trapping effect of the interface of TX inclusion [19] in iron. In this research, the trap activation energy and trap binding energy of the interface of Tic inclusion which is isolated from the effect of other trapping sites were obtained by thermal analysis method. 2. THEORETICAL BACKGROUND The hydrogen evolution reaction from trap site can be written as equation (1) and the energy level of hydrogen around the trap site can be expressed as in Fig. 1.
qIm,*
Otr.,, + K,.
(1)
The hydrogen evolution rate from trapping site is written as equation (2), because equation (1) is the thermally activated process [20].
h = A (1 -
dt
xr)exp
- $G (
site. When the hydrogen charged specimen is heated with a uniform heating rate, the hydrogen evolution rate peak is formed at a certain temperature related to the trap activation energy of each trap site. It is thermal analysis that observe the peak of hydrogen evolution rate formed by each trap site, and it is possible to separate the effect of each trap site by thermal analysis [21]. 2.1. Trap iakntt$cation of trap sire and the calculating of rrap acrivation energy [21]. Because there ex&s several types of trap sites in one specimen several peaks appear in the thermal analysis for one specimen. The height of each peak is determined by the amount of hydrogen at that trap site. In this research the specimens had different amounts of Tic inclusions while the amount of other trap sites were kept constant and minimized and were prepared artificially, in order to observe the trapping effti of interface of Tic inclusion. The temperature at which the evolution rate peak of the interface of Tic appears, can be identified by carrying out the thermal analysis with these specimens at a heating rate ($), because only the height of the peak related to Tic inclusion will increase with the increase of the amount of Tic inclusion in the specimen. At the maximum of hydrogen evolution rate the first derivative of equation (2) is zero, so equation (3) is derived
(2)
(3)
>
where
where
xr = (Gl - CXr)/CXo c ro = the amount of hydrogen in trapping site at t-o CxT= the amount of hydrogen in trapping site at r#O A = reaction constant R = gas constant T = absolute temperature. In equation (2). the term (1 - xj) expresses the amount of hydrogen remained at trapping site and exp( - EJRT) means the probability of hydrogen to overcome the energy barrier from trap site to lattice
4 = heating rate T, = peak temperature. Equation (4) is derived by taking logarithm of both sides of equation (3) and differentiating with respect to l/T, (4) The trap activation energy (E,,T) needed to escape from the trap site to the normal lattice site is calculated from the slope of ln($/Tt) vs (l/T,) plot by measuring the change of peak temperatures with the heating rates. 2.2. Calculation of the trap binding energy [22-241
Fig. 1. Energy level of hydrogen around a trapping site. E,,, = activation energy of hydrogen diffusion in normal lattice; E, = saddle point energy; E, = trap binding energy; EoT= trap activation energy; S, = trapping site; S, = normal interstitial site.
The trap binding energy (E#) between the trap site and hydrogen is obtained from the relation between the hydrogen charging temperature and the amount of hydrogen trapped at the trap site. The variation of hydrogen occupied fraction of the trap site (dn/dr) with time is expressed as equation (5) from McNabb and Foster’s model [ 161 dn x
=kC,(l
-n)-pn
133
LEE snd LEE: HYDROGEN TRAPPING IN IRON Table 1. Chemical composition of ekctro&tic iron (em)
where
= C, exp( - E,/RT) hydrogen occupied fraction in trap site the trapping rate of hydrogen to trap site, v, exp( - EJRT) the detrapping rate of hydrogen from trap site, = v, exp(-(E, $ EJRT) heat of solution saddle point energy vibration frequencies of the hydrogen at normal and trapping sites respectively.
C, = hydrogen concentration
n= k = = P= E. = 4, = v,,, v, =
If equilibrium state is established when hydrogen is charged to the specimen at a certain temperature, the trap occupancy fraction of trap site in the specimen is constant (dnjdt = O), and generally it is very small (I- n) N 1. Equation (6) is derived
(--Ei).
n=fCoexp
It can be assumed that the vibration frequencies (v,,, v,) are the same, then equation (6) can be expressed as equation (7)
C
-_
$0
N .._~_.
S
P
Ni
Cr
Si
-
50
40
-
-
-
Mn 50
Table 2. Chemical Fomposition of the spccimens used in this research (wtp/,) Element Specimen Ti __ .~ _.. _. -.A 0.10 B 1.00 C 1.50 D 2.00
C 0.02 0.24 0.37 0.u)
FC --99.98 98.16 98.13 97.50
The weight ratio of the amount of Ti to carbon were controlled to be 4:l in order to minimize the amount of free carbon and titanium, and the chemical compositions of electrolytic iron and specimens used are shown in Tables 1 and 2, respectively. These specimens were heat treated under vacuum for 24 h at 973 K to minimize or remove the amount of microvoid and dislocation in the specimen. The ingots were cut to the rectangular shape and their surfaces were ground to emery paper No. 1200. 3.2. The hydrogen charging and thermd anafysis
(7)
where C, = the concentration N, = the trap density. Combining derived
equations
of trapped hydrogen, (7) and (8), equation
C,=N,C,exp
q (
H >
(9) is
The specimen was charged with hydrogen under 0.1 MPa hydrogen pressure in the apparatus as shown in Fig. 2. In order to obtain the trap activation energy, hydrogen is charged at 673 K for 3 h. But in case of the experiment to obtain trap binding energy hydrogen was precharged at 773 K for 2 h and temperature is lowered to the charging temperature and maintained for 5 h in order to rapidly establish the equilibrium state. Hydrogen charged specimens were
(9) To H To vacuum
where T,., = the hydrogen char&g
temperature.
By taking logarithm of both sides of equation (9) lnC,=lnEI,C~+~. II
(10)
The amount of hydrogen trapped at a certain trap site can be obtained from the area of peak related to that trap site and .& and C,, values are obtained from published lattice hydrogen solubility equation. Therefore the trap binding energy is calculated from the slope of the In (C,) vs (l/T,) plot and the trap density is obtained from the intercept [22-241.
3.1. Specimen preparation
Specimens were prepared by melting electrolytic iron with Ti and C in an arc furnace under argon atmosphere.
Fig. 2. Schematic diagram of hydrogen ehargiag apparatus. (a) sample holder, (b) vycor tube; (c) false bottom; (d) furnace; (e) aluminium finer; Q magna (g) steel rod; (h) gathering bottle (if cooling chamber.
LEE and LEE: HYDROGEN TRAPPING IN IRON
134 Argon
carrier
surface with palladium to prevent contamination of
gos
the iron surface was the same as for that of nonel~tr~e~sited specimen. The evolution rate peak of escaped hydrogen from one type of trap site appears at a constant temperature, and its’ height is consistent for the specimens with the same history [12,19,21-241. From these, it is believed that there is no effect of the surface impedance layer for hydrogen entry and/or hydrogen evolution in this experiment. 4. JIXPERIMENTALRESULTS
Fig. 3. schematic diagram of thermal analysis app~at~. specimen;(B) furnace; (C) temperatureprogrammer ID) measuring thermocouolc; (El and (F) solenoidvalve;
(A,
(Gj ice bath, @) swagel&k &&etiori; (I) metal to glass seal;(1) gas chromatograph. quenched into ice water to prevent the evolution of hydrogen from trap site. After that the specimens were brought to the apparatus shown in Fig. 3 and held under the argon atmosphere at room temperature for 4 h to remove the mobile hydrogen from the specimen so that only the trapped hydrogen remained. Then the specimen was heated with a uniform heating rate and the amount of hydrogen evolved from specimen was measured by gas chromatograph to give the hydrogen evolution rate. At first, the quartz reaction tube was isolated by closing valve E and positioning three way valve F at position 1 in Fig. 3, then the evolved hydrogen was gathered in the reaction tube. The hydrogen gathered during uniform time was delivered to the gas chromatograph by opening valve E and turning three way valve F to the position 2, and its amount was measured by the thermal conductivity detector of gaschromatograph. The amount of evolved hydrogen was calculated from the detected value by calibrating thermal conductivity detector by injecting the known amount of hydrogen into the gas ~hromato~aph using a gas tight syringe. The ice trap was used to correct the effect of gas temperature difference on the detected value. There was no problem of the surface impedance layer which prevented the entry of hydrogen into the specimen and/or evolution of hydrogen from the specimen, as the surface impedance layer can be fulty removed by the treatment of specimen for 1 h at
under atmospheric hydrogen pressure flO,21]. Hong and Lee [IO] have found that the hydrogen
673 K
diffusivity of the specimen electrodeposited
on to
Figure 4 shows the results of thermal analysis with the heating rate of 3 KImin for the specimen with different amount of Tic inclusion. There appeared three peaks at 473, 773 and 996 K. The peak height at 996 K was increased as the amount of TiC inclusion increased while other peaks remained constant. From this result it is bclicved that the peak at 996 K appeared by the release of hydrogen trapped at the interface of Tic inclusions. It was verified by previous research that the peaks at 473 and 773 K were caused by the trapped hydrogen at dislocation and oxide inclusion, respectively [12,24,28]. By comparing the height of peaks in Fig. 4, the amount of hydrogen trapped at Tic is very much larger than that of dislocation. As heating rate is increased in the thermal analysis for specimen D in Table 2 at which the effect of other trap site is minimum, the peak temperature of the interface of Tic inclusion is increased as shown in Table 3. Figure 5 is the plot of In 4/T: vs l/T, from the data of Table 3. From the slope of Fig. 5, the trap activation energy (E,,) of hydrogen evolution from the interface of Tic inclusion is calculated as 86.9 kJ/mol. The relationship between the hydrogen charging temperature and the amount of hydrogen trapped - o 2.00Ti -O.SOC l 1.50 Tl -0.3X a I.OOTi -0.24C A 0.10 Ti -0.02C
0
0
8-O
0
0 0
0
0
0
0
0
0
I
‘D
0 = 2
6
0 0 0
.*
l***.
l
0 l
.
0
e4
5 ._ “1
D 5
2
673
773
673
973
Temperature
( K1
1073
Fig. 4. Dependence of the heights of pks on the amount of Tic-ferrite interface at constant heating rdte of 3 K/min.
LEE
and LEE:
HYDROGEN
TRAPPING
IN
IRON
Table 3. Change in the peak temperature of TLC interface with various heating rates Heating rate (K/min)
Trap activation Peak energy temperature (kJ/mol) (K)
99i
25 3 4 5 6
996 1023 1045 1060
86.9
was investigate using specimen D. As the hydrogen charging temperature increased the amount of hydrogen trapped at the interface of Tic inclusion was decreased as shown in Fig. 6. In order to calculate the trap binding energy (E,) from the slope of Fig. 6, it is necessary to known the lattice solubility of hydrogen in iron. In this research equation (I 1) derived by Gonzalez 1251is used C,= 3.7 P’/*exp[-27.3
& 1.4(kJ/mol-‘)/RT] x
cm3 H&M cm3 Fe
(11)
5. DISCUSSI0N The trap activation energy of hydrogen evolution from the. interface of Tic inclusion, 86.9 kJ/mol, is nearly the same as that by Pressouyre et af. [15]. However they cannot obtain the trap binding energy of it because of the difficulty of experiment. Therefore until now, it has not been found that the energy level of hydrogen at the interface of Tic inclusion. In this experiment, the trap binding energy of hydrogenthe interface of Tic inclusion is measured, 58.8 kJ/mol. In order to understand the energy level of hydrogen around the interface of Tic inclusion, it is also necessary to know the lattice activation energy of hydrogen diffusion in iron. Published data for the lattice diffusivity of hydrogen in a-iron applied are D, = 2.24 x 10-3exp[18.9 ~~rnol)/~~
IQ*
IO'/7-l K) Fig. 5. Dependence
of the peak temperature on the heating rate.
of
3
2
lO’/r(Kt
Fig. 6. Dependence of the amount of hydrogen trapped at Tic interface on the hydrogen charging temperature.
by Baukoloh and Wenzel [26]
DL= 1.4 x 10-3exp[13.44(kJ/mol)/RTf by Eichenauer et al. [27]. Therefore the lattice diffusivity of hydrogen used in this paper is
The obtained binding energy between hydrogen and the interface of TiC inclusion is 28.1 kJ/mol and the trap density in specimen D is 10” trap sites/cm3 Fe obtained applying equation (9).
IO.0
.f
Tic interface
L?, = 2.24 x 10-j expIl6.8 ~J/mol)/~~ where R is gas constant, T is absolute temperature. Figure 7 shows the energy level around the interface of TiC inclusion estimated from present data. Table 4 shows the comparison of the present results with those obtained by other researchers for another trap site. Trap activation energy of the interface of TiC inclusion is very much larger than the values of other trap site while the value of trap binding energy is not. It is verifmd that the saddle point energy is greatly larger than the activation energy needed for lattice diffusion. The reason of large saddle point energy is not explained by the present results but it is noticed that the values of the interface of inclusion such as Tic, MnS 112,131 and iron oxide 1241are generally larger than the activation energy of lattice diffusion. The reversibility of hydrogen between trapping site and normal interstitial site will decrease as the value of (E, - E,,,) increases. Therefore it is thought that the retarding effect of hydrogen diffusion around the Tic inclusion by the trapping phenomenon is not so large at low ~m~rature regions. In other words, steady state flux of hydrogen during the permeation
I 1
\I (unit: KJ/moll Fig. 7. Energy level of hydrogen face.
around
Tic-ferrite
inter-
LEE and LEE: HYDROGEN TRAPPING IN IRON
136
Table 4. Comparison between the trapping ctTectsof Tic interface and other trapping sites
E.r Trapping site
(2)
E#
(Umol)
0wmol)
a.8
4 Nmol)
Grain boundary [21,29] Fe-Fe, C interface [I 0,23.28] Dislocation [21.29] Microvoid [21,2!2] Fe oxide interface [24] MaS interface [12.13]
378(3.1)’
17.2
393 (3.0) 495 (2.6) 573 (2.9) 695 (2.9) 768 (3.4)
la.5 26.9 35.3 47.0 72.2
2::: 29.0 14.3 -
014 34::
Tic interface
996 (3.0)
86.9
28.1
58.8
ti
-
&Trnt$
-a.4 -8.0 - 16.4 -10.5 15.9 42.0
‘Values in parenthesisesdisplay heating rate K/min.
experiment at a low temperature region, 300 K, decreases as the area of interface of Tic inclusion in specimen increases, because the trapping rate (k) and detrapping rate (p) from and to the interface of Tic inclusion are very slow, considering the thermal energy and trapping energies, i.e. saddle point energy and trap activation energy. Therefore, at low tcmperature region, the amount of hydrogen trapped at interface of Tic inclusion is very little. Oriani (17 assumed a local equilibrium between normal lattice site and trapping site to treat the abnormality of the hydrogen diffusion in iron mathematically. According to this assumption, the interaction energy between trapping site and hydrogen only decides the mass of trapped hydrogen, and hydrogen can easily evolve from the trapping site, independent of the trap activation energy (E,&, to maintain the local equilibrium. However, our result indicates that a distinct energy is needed to evolve hydrogen from a type of trapping site, in the form of thermal energy. Because of this, the local equilibrium theory is thought to be quite a bold assumption, but the mathematical models of Koiwa [18] and McNabb [16], which consider the trapping and evolution rates from and to trapping site, seem a more proper way to treat the anomalous behavior of hydrogen in iron.
6. CONCLUSIONS 1. The hydrogen evolution rate peak of Tic inclusion is observed at 996 K when heating rate is 3 K/min. 2. The trap activation energy of hydrogen needed to escape from the interface of Tic inclusion is calculated as 86.9 k.I/mol. 3. The trap binding energy between hydrogen and the interface of TIC inclusion is calculated as 28.1 kJ/mol. 4. The hydrogen energy level around the interface of TIC inclusion is suggested using above values.
REFERENCES A. S. Nowick, Dif/urion in Soliak Academic Press, New York (1975). G. M. Pressouyre and I. M. Bernstein, Actu metall. 28, 895 (1980). 0. M. Pressouyre and I. M. Bernstein, Melall. Trans. 12A, 835 (1981). J. O’M. Bockris and P. K. Subramanyan. J. Efectrothem. Sot. 118, 1114 (1971). 5. A. M. Adair and R. E. Hook, Acta meraIl. 10, 741 (1962). 6. A. J. Kumnick and H. H. Johnson, Metall. Trans. SA, 1199 (1974). I. G. M: Ev& and E. C. Rollaaon, Japan Iron Steel Inst., p. 1484 (1%9). 8. D. M. Allen-Booth and J. Hewitt, Ada metd. 22, 171 (1974). 9. H. Hargi. Y. Hayashi and L. L. Shreir, Corros. Sci. 11, 25 (1971). 10. G. W. Hong and J. Y. Lee, J. Mater. Sci. 1%271 (1983). 11. T. Asaoka, Proc. JIMIS-2, Hydrogen in Metals, pp. 161-164 (1980). 12. J. L. Lee and J. Y. Lee, Metals Sci. In press. 13. J. L. Lee, KAIST MS. Thesis, Seoul, Korea (1982). 14. T. Asaoka, G. Lapasset. M. Auwuturier and P. Lacombe. Corrosion 34, 39 (1978). 15. G. M. Pressouyre and I. M. Bernstein, Meroll. Trans. 9A, 1571 (1978). 16. A. McNabb and P. K. Foster, Trm. Am. hr. Min. Engrs 227, 618 (1963).
17. R-A. Oriani, Ado &all. 18, 147 (1970). 18. M. Kowia. Acra me&II. 22. 1259 (1974). 19. H. G. Leei KAIST M.S. T&is. !&I&~ Korea (1983). 20. H. E. Kissinger, Analyl. Chem. 29, 1702 (1957). 21. W. Y. Choo and J. Y. Lee, Metoll. Trans. 134, 135 (1982). 22. G. W. Hong. KAIST Ph.D. Thesis. Seoul, Korea (1983). -23. G. W. Hong and J. Y. Lee, ScrQNamekdl. 17,823 (1983). 24. G. W. Hong and J. Y. Lee, J. Mater. Sci. Engng. In g?l Gonzalez Trons TMS-AIME 245 607 (1969) ii: W. Baukoloh add W. &nzel. Archs hsenfhzitt. Il. 27; (1937). 27. W. Eichenauer, H. Kunzig and A. Pebler. Z. Metal/c. ,* 49, 220 (1958). _.,. W. Y. Choo and J. Y. Lee, J. Mater. Sci. 19, 1930 (1982). 29.
W. Y. Choo and J. Y. Lee, Metall. Trans. 14A, 1299 (1983).
Copyright 8
OOLII-6l60/84.$3.00+0.00 1984 Peremon Press Ltd
HYDROGEN TRAPPING BY TiC PARTICLES IN IRON H. G. LEE and JAI-YOUNG Department
of Materials
Science and Engineering,
LEE
Korea Advanced Institute
of Science and Technology,
P.O. Box 150 Chongryang, Seoul, Korea (Received 26 February 1983; in revised form 26 May 1983)
At&a&-The interaction of hydrogen with the interface of Tic inclusion in iron was studied by thermal analysis technique using gas chromatro~aph as hydrogen detector. The amount of hydrogen evolved from trap site was measured and its relations with activation energy and trap binding energy were studied. The evolution rate peak appeared at 996 K when heated with 3 K/min heating rate. 11is due to the hydrogen release from the interface of Tic inclusions. The trap activation inergy needed to escape from the interface of Tic inclusion is obtained from the relation between peak temperature and heating rate as 86.9 kJ/mol and the trap binding energy obtained from the relation between peak area and the hydrogen charging temperature is 28.1 Wfmol. The energy level of the hydrogen around the trap site is estimated from the above values. R&sum&Nous avons ttudit l’interaction de l’hydrog&ne avec l’interface d’une inclusion de Tic dans du fer par la technique de l’analyse thermique, en utilisant un chromatographe B gaz comme ditccteur de l’hydroghne. Nous avons mesun la quantite d’hydrog&ne issue des sites de pi&cage et nous avons 6tudiC ses relations avec l%nergie d’activation et l%nergie de liaison du pi&e. L.e pit de la vitesse de dbgagement apparaissait B 996 K pour une vitesse de chase de 3 K/min. n provient du dbgagement d’hydro&ne z4par& de l’interfae des inclusions de TIC. Nous avons obtmu l%nergie d’activation du pi&e n&cessaire pour tiapper de l’interface de l’inclusion de Tic, g partir de la relation entre la temp&rature du pit et la vitesse de chauffagc, soit 86.9 kJ/mol, ainsi que 1’Cnergie de liaison du pi&e, obtenue i partir de la relation entre la surface du pit et la temp&ature de la charge en hydrogtne, soit 28,l kJ/mol. Nous avons estimit le niveau d&e&e de I’hydro&ne autour des sites de pi&eage, g partir des vaicun ci-dessus. medic W~h~i~r~ng von Wasserstoff mit der Grenzflsiche zwischen ~C-Ein~hi~~ und der Eisenmatrix wurde mittels thermischer Analytik und einem Gaschromatographen als Wasserstoffdetektor untersucht. Die aus den Fallen entweichcnde Wasserstoffmcngc wurde gemessen und mit Aktivivngsund Bindungsenergien verglichen. Das Maximum der Freigabe trat -bei einer Aufheiz~hwindigkeit von 3 K/min- bei 996 K auf und tirt von dem AblSsen des Wasserstoffs aus der Grenzfliiche her. Die Aktivienmgsenergic f”r das L&en dcs Wasserstoffs aus der Fe/Tic-Grenzfliiche ergibt sich aus dem Zusammenhang zwischen Temperatur des Maximums und der Au~ei~h~ndi~eit. Sic bctriigt 86,9 kJ/Mol. Die Energie fur Bmdung an die Falle wird erhaltcn aus dem Zusammenhang zwischen der F&he unter dem Maximum und der W~~to~~~~~~mt~ und betriigt 28,l kJ/Mol. Das Energieniveau des Wasserstoffs in der N%e der Falle wird aus diesen Werten abgeschatzt.
1. ~ODU~ON Hydrogen embrittlemcnt phenomena in steels have been the subject of much research, but the mechanism has not been understood fully yet, It is important to know the mechanism of hydrogen transport in iron in order to study the hydrogen ~b~t~~~t phe nomena. The diffusivities of hydrogen measured at low temperature are considerably smaller than the values extrapolated from high temperature region and deviate markedly compared with the high temperature values 11J. Many researchers think that these abnormal hydrogen diffusion phenomena appear by the trapping of hydrogen in the trap site at low temperatures. The hydrogen trapping phenomenon is an attractive interaction between hydrogen atom and the structural imperfection and there are many nsearches which intend to analyze the hydrogen embrittfement phenomenon by the hydrogen trapping in recent years (24.
It is known that almost all kinds of defects in metal can act as trapping sites, i.e. the dislocation [5,6], microvoid [7,8], grain boundary [9], the ferrite-cementite interface [lo] and the interface of non-metallic inclusions [l l-15]. McNabb and Foster [16] proposed a mathematical model which describes the effect of trap on hydro~n diffusion by considering trapping rate and detrapping rate independently. Oriani [17] derived the apparent diffusivity of hydrogen in metals with the assumption of local equilibrium between trapping site and normal interstitial site. But Kowia [18] derived the apparent diffusi~ty equation by considering the substitutional impurity atom as a trapping center. At any rate the apparent diffusivity of hydrogen in metals is the function of trap activation energy, trap binding energy and trap density when trap sites are present in the specimen. Therefore it is important to measure these values of various trap sites to studying hydrogen diffusion in metals. I31
LEE and LEE: HYDROGEN TRAPPING IN IRON
132
The hydrogen trapping phenomenon by the interface of Tic inclusion was studied by some researchers. Asaoka (141 injected the tritium into F&.5 wt% Ti alloy and observed it with the autoradiographic method and insisted that the hydrogen trap binding energy of interface of TiC inclusion is larger than 61 kJ/mol. Pressouyre [IS] has analyzed the difference of 1st and 2nd permeation experiment and calculated the trap activation energy as 95kJ/mol. But the above two experimental results have difficulty in analyzing quantitatively the sole trapping effect of the interface of TX inclusion [19] in iron. In this research, the trap activation energy and trap binding energy of the interface of Tic inclusion which is isolated from the effect of other trapping sites were obtained by thermal analysis method. 2. THEORETICAL BACKGROUND The hydrogen evolution reaction from trap site can be written as equation (1) and the energy level of hydrogen around the trap site can be expressed as in Fig. 1.
qIm,*
Otr.,, + K,.
(1)
The hydrogen evolution rate from trapping site is written as equation (2), because equation (1) is the thermally activated process [20].
h = A (1 -
dt
xr)exp
- $G (
site. When the hydrogen charged specimen is heated with a uniform heating rate, the hydrogen evolution rate peak is formed at a certain temperature related to the trap activation energy of each trap site. It is thermal analysis that observe the peak of hydrogen evolution rate formed by each trap site, and it is possible to separate the effect of each trap site by thermal analysis [21]. 2.1. Trap iakntt$cation of trap sire and the calculating of rrap acrivation energy [21]. Because there ex&s several types of trap sites in one specimen several peaks appear in the thermal analysis for one specimen. The height of each peak is determined by the amount of hydrogen at that trap site. In this research the specimens had different amounts of Tic inclusions while the amount of other trap sites were kept constant and minimized and were prepared artificially, in order to observe the trapping effti of interface of Tic inclusion. The temperature at which the evolution rate peak of the interface of Tic appears, can be identified by carrying out the thermal analysis with these specimens at a heating rate ($), because only the height of the peak related to Tic inclusion will increase with the increase of the amount of Tic inclusion in the specimen. At the maximum of hydrogen evolution rate the first derivative of equation (2) is zero, so equation (3) is derived
(2)
(3)
>
where
where
xr = (Gl - CXr)/CXo c ro = the amount of hydrogen in trapping site at t-o CxT= the amount of hydrogen in trapping site at r#O A = reaction constant R = gas constant T = absolute temperature. In equation (2). the term (1 - xj) expresses the amount of hydrogen remained at trapping site and exp( - EJRT) means the probability of hydrogen to overcome the energy barrier from trap site to lattice
4 = heating rate T, = peak temperature. Equation (4) is derived by taking logarithm of both sides of equation (3) and differentiating with respect to l/T, (4) The trap activation energy (E,,T) needed to escape from the trap site to the normal lattice site is calculated from the slope of ln($/Tt) vs (l/T,) plot by measuring the change of peak temperatures with the heating rates. 2.2. Calculation of the trap binding energy [22-241
Fig. 1. Energy level of hydrogen around a trapping site. E,,, = activation energy of hydrogen diffusion in normal lattice; E, = saddle point energy; E, = trap binding energy; EoT= trap activation energy; S, = trapping site; S, = normal interstitial site.
The trap binding energy (E#) between the trap site and hydrogen is obtained from the relation between the hydrogen charging temperature and the amount of hydrogen trapped at the trap site. The variation of hydrogen occupied fraction of the trap site (dn/dr) with time is expressed as equation (5) from McNabb and Foster’s model [ 161 dn x
=kC,(l
-n)-pn
133
LEE snd LEE: HYDROGEN TRAPPING IN IRON Table 1. Chemical composition of ekctro&tic iron (em)
where
= C, exp( - E,/RT) hydrogen occupied fraction in trap site the trapping rate of hydrogen to trap site, v, exp( - EJRT) the detrapping rate of hydrogen from trap site, = v, exp(-(E, $ EJRT) heat of solution saddle point energy vibration frequencies of the hydrogen at normal and trapping sites respectively.
C, = hydrogen concentration
n= k = = P= E. = 4, = v,,, v, =
If equilibrium state is established when hydrogen is charged to the specimen at a certain temperature, the trap occupancy fraction of trap site in the specimen is constant (dnjdt = O), and generally it is very small (I- n) N 1. Equation (6) is derived
(--Ei).
n=fCoexp
It can be assumed that the vibration frequencies (v,,, v,) are the same, then equation (6) can be expressed as equation (7)
C
-_
$0
N .._~_.
S
P
Ni
Cr
Si
-
50
40
-
-
-
Mn 50
Table 2. Chemical Fomposition of the spccimens used in this research (wtp/,) Element Specimen Ti __ .~ _.. _. -.A 0.10 B 1.00 C 1.50 D 2.00
C 0.02 0.24 0.37 0.u)
FC --99.98 98.16 98.13 97.50
The weight ratio of the amount of Ti to carbon were controlled to be 4:l in order to minimize the amount of free carbon and titanium, and the chemical compositions of electrolytic iron and specimens used are shown in Tables 1 and 2, respectively. These specimens were heat treated under vacuum for 24 h at 973 K to minimize or remove the amount of microvoid and dislocation in the specimen. The ingots were cut to the rectangular shape and their surfaces were ground to emery paper No. 1200. 3.2. The hydrogen charging and thermd anafysis
(7)
where C, = the concentration N, = the trap density. Combining derived
equations
of trapped hydrogen, (7) and (8), equation
C,=N,C,exp
q (
H >
(9) is
The specimen was charged with hydrogen under 0.1 MPa hydrogen pressure in the apparatus as shown in Fig. 2. In order to obtain the trap activation energy, hydrogen is charged at 673 K for 3 h. But in case of the experiment to obtain trap binding energy hydrogen was precharged at 773 K for 2 h and temperature is lowered to the charging temperature and maintained for 5 h in order to rapidly establish the equilibrium state. Hydrogen charged specimens were
(9) To H To vacuum
where T,., = the hydrogen char&g
temperature.
By taking logarithm of both sides of equation (9) lnC,=lnEI,C~+~. II
(10)
The amount of hydrogen trapped at a certain trap site can be obtained from the area of peak related to that trap site and .& and C,, values are obtained from published lattice hydrogen solubility equation. Therefore the trap binding energy is calculated from the slope of the In (C,) vs (l/T,) plot and the trap density is obtained from the intercept [22-241.
3.1. Specimen preparation
Specimens were prepared by melting electrolytic iron with Ti and C in an arc furnace under argon atmosphere.
Fig. 2. Schematic diagram of hydrogen ehargiag apparatus. (a) sample holder, (b) vycor tube; (c) false bottom; (d) furnace; (e) aluminium finer; Q magna (g) steel rod; (h) gathering bottle (if cooling chamber.
LEE and LEE: HYDROGEN TRAPPING IN IRON
134 Argon
carrier
surface with palladium to prevent contamination of
gos
the iron surface was the same as for that of nonel~tr~e~sited specimen. The evolution rate peak of escaped hydrogen from one type of trap site appears at a constant temperature, and its’ height is consistent for the specimens with the same history [12,19,21-241. From these, it is believed that there is no effect of the surface impedance layer for hydrogen entry and/or hydrogen evolution in this experiment. 4. JIXPERIMENTALRESULTS
Fig. 3. schematic diagram of thermal analysis app~at~. specimen;(B) furnace; (C) temperatureprogrammer ID) measuring thermocouolc; (El and (F) solenoidvalve;
(A,
(Gj ice bath, @) swagel&k &&etiori; (I) metal to glass seal;(1) gas chromatograph. quenched into ice water to prevent the evolution of hydrogen from trap site. After that the specimens were brought to the apparatus shown in Fig. 3 and held under the argon atmosphere at room temperature for 4 h to remove the mobile hydrogen from the specimen so that only the trapped hydrogen remained. Then the specimen was heated with a uniform heating rate and the amount of hydrogen evolved from specimen was measured by gas chromatograph to give the hydrogen evolution rate. At first, the quartz reaction tube was isolated by closing valve E and positioning three way valve F at position 1 in Fig. 3, then the evolved hydrogen was gathered in the reaction tube. The hydrogen gathered during uniform time was delivered to the gas chromatograph by opening valve E and turning three way valve F to the position 2, and its amount was measured by the thermal conductivity detector of gaschromatograph. The amount of evolved hydrogen was calculated from the detected value by calibrating thermal conductivity detector by injecting the known amount of hydrogen into the gas ~hromato~aph using a gas tight syringe. The ice trap was used to correct the effect of gas temperature difference on the detected value. There was no problem of the surface impedance layer which prevented the entry of hydrogen into the specimen and/or evolution of hydrogen from the specimen, as the surface impedance layer can be fulty removed by the treatment of specimen for 1 h at
under atmospheric hydrogen pressure flO,21]. Hong and Lee [IO] have found that the hydrogen
673 K
diffusivity of the specimen electrodeposited
on to
Figure 4 shows the results of thermal analysis with the heating rate of 3 KImin for the specimen with different amount of Tic inclusion. There appeared three peaks at 473, 773 and 996 K. The peak height at 996 K was increased as the amount of TiC inclusion increased while other peaks remained constant. From this result it is bclicved that the peak at 996 K appeared by the release of hydrogen trapped at the interface of Tic inclusions. It was verified by previous research that the peaks at 473 and 773 K were caused by the trapped hydrogen at dislocation and oxide inclusion, respectively [12,24,28]. By comparing the height of peaks in Fig. 4, the amount of hydrogen trapped at Tic is very much larger than that of dislocation. As heating rate is increased in the thermal analysis for specimen D in Table 2 at which the effect of other trap site is minimum, the peak temperature of the interface of Tic inclusion is increased as shown in Table 3. Figure 5 is the plot of In 4/T: vs l/T, from the data of Table 3. From the slope of Fig. 5, the trap activation energy (E,,) of hydrogen evolution from the interface of Tic inclusion is calculated as 86.9 kJ/mol. The relationship between the hydrogen charging temperature and the amount of hydrogen trapped - o 2.00Ti -O.SOC l 1.50 Tl -0.3X a I.OOTi -0.24C A 0.10 Ti -0.02C
0
0
8-O
0
0 0
0
0
0
0
0
0
I
‘D
0 = 2
6
0 0 0
.*
l***.
l
0 l
.
0
e4
5 ._ “1
D 5
2
673
773
673
973
Temperature
( K1
1073
Fig. 4. Dependence of the heights of pks on the amount of Tic-ferrite interface at constant heating rdte of 3 K/min.
LEE
and LEE:
HYDROGEN
TRAPPING
IN
IRON
Table 3. Change in the peak temperature of TLC interface with various heating rates Heating rate (K/min)
Trap activation Peak energy temperature (kJ/mol) (K)
99i
25 3 4 5 6
996 1023 1045 1060
86.9
was investigate using specimen D. As the hydrogen charging temperature increased the amount of hydrogen trapped at the interface of Tic inclusion was decreased as shown in Fig. 6. In order to calculate the trap binding energy (E,) from the slope of Fig. 6, it is necessary to known the lattice solubility of hydrogen in iron. In this research equation (I 1) derived by Gonzalez 1251is used C,= 3.7 P’/*exp[-27.3
& 1.4(kJ/mol-‘)/RT] x
cm3 H&M cm3 Fe
(11)
5. DISCUSSI0N The trap activation energy of hydrogen evolution from the. interface of Tic inclusion, 86.9 kJ/mol, is nearly the same as that by Pressouyre et af. [15]. However they cannot obtain the trap binding energy of it because of the difficulty of experiment. Therefore until now, it has not been found that the energy level of hydrogen at the interface of Tic inclusion. In this experiment, the trap binding energy of hydrogenthe interface of Tic inclusion is measured, 58.8 kJ/mol. In order to understand the energy level of hydrogen around the interface of Tic inclusion, it is also necessary to know the lattice activation energy of hydrogen diffusion in iron. Published data for the lattice diffusivity of hydrogen in a-iron applied are D, = 2.24 x 10-3exp[18.9 ~~rnol)/~~
IQ*
IO'/7-l K) Fig. 5. Dependence
of the peak temperature on the heating rate.
of
3
2
lO’/r(Kt
Fig. 6. Dependence of the amount of hydrogen trapped at Tic interface on the hydrogen charging temperature.
by Baukoloh and Wenzel [26]
DL= 1.4 x 10-3exp[13.44(kJ/mol)/RTf by Eichenauer et al. [27]. Therefore the lattice diffusivity of hydrogen used in this paper is
The obtained binding energy between hydrogen and the interface of TiC inclusion is 28.1 kJ/mol and the trap density in specimen D is 10” trap sites/cm3 Fe obtained applying equation (9).
IO.0
.f
Tic interface
L?, = 2.24 x 10-j expIl6.8 ~J/mol)/~~ where R is gas constant, T is absolute temperature. Figure 7 shows the energy level around the interface of TiC inclusion estimated from present data. Table 4 shows the comparison of the present results with those obtained by other researchers for another trap site. Trap activation energy of the interface of TiC inclusion is very much larger than the values of other trap site while the value of trap binding energy is not. It is verifmd that the saddle point energy is greatly larger than the activation energy needed for lattice diffusion. The reason of large saddle point energy is not explained by the present results but it is noticed that the values of the interface of inclusion such as Tic, MnS 112,131 and iron oxide 1241are generally larger than the activation energy of lattice diffusion. The reversibility of hydrogen between trapping site and normal interstitial site will decrease as the value of (E, - E,,,) increases. Therefore it is thought that the retarding effect of hydrogen diffusion around the Tic inclusion by the trapping phenomenon is not so large at low ~m~rature regions. In other words, steady state flux of hydrogen during the permeation
I 1
\I (unit: KJ/moll Fig. 7. Energy level of hydrogen face.
around
Tic-ferrite
inter-
LEE and LEE: HYDROGEN TRAPPING IN IRON
136
Table 4. Comparison between the trapping ctTectsof Tic interface and other trapping sites
E.r Trapping site
(2)
E#
(Umol)
0wmol)
a.8
4 Nmol)
Grain boundary [21,29] Fe-Fe, C interface [I 0,23.28] Dislocation [21.29] Microvoid [21,2!2] Fe oxide interface [24] MaS interface [12.13]
378(3.1)’
17.2
393 (3.0) 495 (2.6) 573 (2.9) 695 (2.9) 768 (3.4)
la.5 26.9 35.3 47.0 72.2
2::: 29.0 14.3 -
014 34::
Tic interface
996 (3.0)
86.9
28.1
58.8
ti
-
&Trnt$
-a.4 -8.0 - 16.4 -10.5 15.9 42.0
‘Values in parenthesisesdisplay heating rate K/min.
experiment at a low temperature region, 300 K, decreases as the area of interface of Tic inclusion in specimen increases, because the trapping rate (k) and detrapping rate (p) from and to the interface of Tic inclusion are very slow, considering the thermal energy and trapping energies, i.e. saddle point energy and trap activation energy. Therefore, at low tcmperature region, the amount of hydrogen trapped at interface of Tic inclusion is very little. Oriani (17 assumed a local equilibrium between normal lattice site and trapping site to treat the abnormality of the hydrogen diffusion in iron mathematically. According to this assumption, the interaction energy between trapping site and hydrogen only decides the mass of trapped hydrogen, and hydrogen can easily evolve from the trapping site, independent of the trap activation energy (E,&, to maintain the local equilibrium. However, our result indicates that a distinct energy is needed to evolve hydrogen from a type of trapping site, in the form of thermal energy. Because of this, the local equilibrium theory is thought to be quite a bold assumption, but the mathematical models of Koiwa [18] and McNabb [16], which consider the trapping and evolution rates from and to trapping site, seem a more proper way to treat the anomalous behavior of hydrogen in iron.
6. CONCLUSIONS 1. The hydrogen evolution rate peak of Tic inclusion is observed at 996 K when heating rate is 3 K/min. 2. The trap activation energy of hydrogen needed to escape from the interface of Tic inclusion is calculated as 86.9 k.I/mol. 3. The trap binding energy between hydrogen and the interface of TIC inclusion is calculated as 28.1 kJ/mol. 4. The hydrogen energy level around the interface of TIC inclusion is suggested using above values.
REFERENCES A. S. Nowick, Dif/urion in Soliak Academic Press, New York (1975). G. M. Pressouyre and I. M. Bernstein, Actu metall. 28, 895 (1980). 0. M. Pressouyre and I. M. Bernstein, Melall. Trans. 12A, 835 (1981). J. O’M. Bockris and P. K. Subramanyan. J. Efectrothem. Sot. 118, 1114 (1971). 5. A. M. Adair and R. E. Hook, Acta meraIl. 10, 741 (1962). 6. A. J. Kumnick and H. H. Johnson, Metall. Trans. SA, 1199 (1974). I. G. M: Ev& and E. C. Rollaaon, Japan Iron Steel Inst., p. 1484 (1%9). 8. D. M. Allen-Booth and J. Hewitt, Ada metd. 22, 171 (1974). 9. H. Hargi. Y. Hayashi and L. L. Shreir, Corros. Sci. 11, 25 (1971). 10. G. W. Hong and J. Y. Lee, J. Mater. Sci. 1%271 (1983). 11. T. Asaoka, Proc. JIMIS-2, Hydrogen in Metals, pp. 161-164 (1980). 12. J. L. Lee and J. Y. Lee, Metals Sci. In press. 13. J. L. Lee, KAIST MS. Thesis, Seoul, Korea (1982). 14. T. Asaoka, G. Lapasset. M. Auwuturier and P. Lacombe. Corrosion 34, 39 (1978). 15. G. M. Pressouyre and I. M. Bernstein, Meroll. Trans. 9A, 1571 (1978). 16. A. McNabb and P. K. Foster, Trm. Am. hr. Min. Engrs 227, 618 (1963).
17. R-A. Oriani, Ado &all. 18, 147 (1970). 18. M. Kowia. Acra me&II. 22. 1259 (1974). 19. H. G. Leei KAIST M.S. T&is. !&I&~ Korea (1983). 20. H. E. Kissinger, Analyl. Chem. 29, 1702 (1957). 21. W. Y. Choo and J. Y. Lee, Metoll. Trans. 134, 135 (1982). 22. G. W. Hong. KAIST Ph.D. Thesis. Seoul, Korea (1983). -23. G. W. Hong and J. Y. Lee, ScrQNamekdl. 17,823 (1983). 24. G. W. Hong and J. Y. Lee, J. Mater. Sci. Engng. In g?l Gonzalez Trons TMS-AIME 245 607 (1969) ii: W. Baukoloh add W. &nzel. Archs hsenfhzitt. Il. 27; (1937). 27. W. Eichenauer, H. Kunzig and A. Pebler. Z. Metal/c. ,* 49, 220 (1958). _.,. W. Y. Choo and J. Y. Lee, J. Mater. Sci. 19, 1930 (1982). 29.
W. Y. Choo and J. Y. Lee, Metall. Trans. 14A, 1299 (1983).