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Thermal outgassing properties of mechanically polished and of sand- and bead-blasted Inconel 600 surfaces up to 500°C
Vacuum/volume32/number 5/pages 227 to 232/1982
0042-207X/82/050227-06503.00/0 Pergamon Press Ltd
Printed in Great Britain
Thermal outgassing properties of mechanically polished and of sand- and bead-blasted Inconel 600 surfaces up to 500°C * F R e i t e r and J C a m p o s i l v a n , Commission of the European Communities, Joint Research Centre, Ispra Establishment, 21020 Ispra (Va), Italy
received 19 February 1981;in revisedform 3 August 1981
In the present work, two different types of Inconel 600 samples have been investigated. The first one was mechanically polished and chemically cleaned in several steps and had a total hemispherical emissivity at 75~C of e1 =0.146 and a surface roughness of CLAI =0.08 pm, while the second sample was sand- and bead-blasted with E==0.382 and CLA== 1.2 pm. The experimental studies were composed of the determination of desorption spectra from 200C up to 550°C at different heating rates, the investigation of the decrease of thermal outgassing rates during a longer heating interval at about 500°C and the measurement of equilibrium outgassing rates at higher temperatures of samples which have been heated several days at about 500"C. The total outgassing rates and the partial outgassing rates of the main components H=, 14=0, CO and CO= have been determined in these studies. Activation energies of desorption, rate constants and equilibrium surface coverages have been calculated from the experimental data using the theory of Redhead.
1. Introduction An important problem in fusion technology is the desorption of molecules, which are adsorbed on the surface or are absorbed in the bulk of the wall surrounding the plasma by physical or chemical forces. During the first start-up of any fusion reactor or fusion experiment, large numbers of molecules will be desorbed by thermal energy, by the impact ofcharged and neutral particles and by radiation. Hereby the plasma will be cooled and the ignition of the fusion reaction can be hindered. Desorption from metallic surfaces depends strongly on the mechanical, chemical and thermal pre-treatment. In this study, the surfaces of two differently prepared samples are investigated. These surfaces form the first wall and the rear surface of protection shields of JET.
thermal emissivity and it will form the rear surface of the protection shields of the bellows which connect the sections of the reaction chamber of JET. These shields will have a much higher temperature during the discharge than the surrounding walls and can deliver heat to the surroundings mainly by thermal radiation. Therefore, the rear surface of the shields must be enlarged in order to increase their cooling. A metallurgical examination of these samples has been carried out after their mechanical, chemical and thermal preparation both by optical and scanning electron microscopy of the surface and of a transverse section and by measurements of hardness, roughness and thermal emissivity. The results of measurements of roughness and total hemispherical emissivity at 75°C are: Sample A: CLA 1=0.08 #m Sample B: CLA 2 = 1.2/zm
e 1=0.146 e2 =0.382
3. Experimental system and processing 2. Preparation and characterization of the samples Two differently prepaxed surfaces of Inconel 600 have been investigated. The first surface (sample A) is a polished and clean surface which has been pre-treated mechanically and chemically, as foreseen for the vacuum wall of JET. The second (sample B) is a sand- and bead-blasted surface with higher surface roughness and
* This report is an extended version with additional results of a work presented at the Eighth International Vacuum Congress, Cannes, 22-26 September 1980.
The schematic arrangement of the thermal outgassing apparatus is presented in Figure 1. A detailed description was given in a previous report 1. Thermal outgassing measurements have been carried out from ohmic heated specimens of 200 mm length, 2 mm width and 0.15 mm thickness. The gas released from a specimen during a certain interval of time in volume V1 is expanded at the end of this interval in the large volume V1 + V2. During this expansion, the pressure increase in V1 + V2 is measured by a Bayard-Alpert gauge BA and the gas composition is determined by a high frequency resonance mass filter MF. Errors from gas pumping of BA and M F and
227
F Reiter and J Camposilvan:
Thermal outgassing properties of Inconel 600 surfaces Table 1. Temperature ( K ) o f maxima ofthermal outgassing rates
pump
T goS inlet
Mass
Run 1(0.136°C s -~)
Run 2 (0.0125°C s -~)
2 18 28 44
656 551 626 574
582 519 561 538
system
•
Vl
Adsorption volume
V2
expansion volume
BA
B o ¥ o r d - A l p e r t gauge
MF
mass f i l t e r
MV
magnetic valve
LV
leak valve
BH
20°C - b o t h
Activation energies of desorption, which approximately equal heats of adsorption, can be calculated from these temperatures by a theory given by Redhead:. The activation energy of desorption Ei is given by the transcendental equation
E~ R T~
vi fl exp
_
_
(1)
(T~=temperature of maximum of reduced thermal outgassing rate, fl = heating rate, v~= rate constant, the index i indicates the gas species i), when (a) the characteristic pumping time
Figure I. Thermal outgassing measurements. Experimental set-up.
V chemical reactions at the hot filaments of BA and MF are avoided by the following procedure. Total gas pressures and gas compositions after a gas release interval are extrapolated to the moment, when the gas is expanded from Vt to V~ + V2.
( V= V~ + V2, S = pumping speed, t, = duration of the temperature sweep) and when (b) the desorption of the various gas species from an Inconel 600 surface in the temperature range up to 500°C are first order reactions. A value of the rate constant must be assumed, e.g.
4. Results
-o
Total gas release
+ Mass 45O
-v
Ei
- -
RT~
/
Z8
/
~
d log fl
-
-
(4)
d log T i
r~
/'% \
IN ~
%
5OO
I
~
~'E 400--× Moss 44 o/ / ~-Temperoture (rightscale)/ I//~/~ - " ~50 -~/ ~ / e / E 25o __-~
+ 2 -
and equation (1) itself. Condition (a) is satisfied in a step by step heating cycle. On the contrary, condition (b) is valid only for the low temperature
/
MOSS 18
o Mass
Alternatively, rate constants and activation energies ofdesorption can be determined from two desorption spectra at different heating rates using the differentiated equation (1)
/
2
(3)
t'i= 1013 S-1
4.1. T h e r m a l outgassing spectra. Figures 2 and 3 show thermal outgassing spectra at two different mean heating rates of 0.136 and 0.0125°C s- ~. In Run 1, the specimen (sample A-1 ) has been heated in 13 steps from 20°C up to about 550°C; each desorption interval was 5 min. In Run 2, another Inconel 600 sample (sample A-2) has been heated in 48 steps from 20°C up to about 550°C and the desorption interval was 15 rain. The temperatures of the maxima of thermal outgassing rates of the main components H 2, H20, CO and CO_, are given in Table 1. N> O2 and hydrocarbons could not be detected, their contribution was smaller than 0.1%.
500
(2)
l
l/ ~ !L~,__
400 Run l SampleA.
3OO p-
200
2OO
15O
IOO 50
I00
15
30
45
t Is] Figure 2. Thermal outgassing spectrum of an Inconel 600 strip of 200 mmx 2 mm x 0.15 mm. Mean heating rate: 0.136C s- ~. 228
F Reiter and J Camposilvan: Thermal outgassing properties of Inconel 600 surfaces
40 - " +
36-v 32 o - x
u 28E
--
Total gas release Moss 2 Moss 18 Moss Ze
/ /
/~ ~ \
J
~'
~.
Moss 4 4
[
r ~ \
Temperature (right scale)
/ ~
-500
Tj ~
-- 400
~ r-r'
\
~
Sample .&2
16 -
,2
-- 200
-
4 -
J
300
~"+.+.+.-,.S+,, • ,.x
o '~='~=~=~a~,~
"~'~==~'~'~"-"~4~ t I-s]
Figure 3. Thermal outgassing spectrum of an Inconel 600 strip of 200 mm x 2 mmx 0.15 mm. Mean heating rate: 0.0125°Cs-1.
desorption (up to 500°C) of H20, CO and CO 2 and not for H 2, as the desorption of hydrogen above room temperature is a second order reaction. Following the theory of Redhead, the activation energy of a second order reaction can be estimated from the change of the maximum desorption temperature with surface coverage. Nevertheless. we have calculated the H2-values in Table 2 as for a first order reaction and set the results in parenthesis, because the surface coverage of a second order reaction cannot be determined from these measurements.
Table 2. Activation energies of desorption
Mass
E, (kcal/mole) calculated by equation ( I ) and values of Run 2 E~ (kcal/mole) assuming calculated by ~, = 101a s- i equation (4)
t', (s - 1) calculated by equation (1) and E, of column 3
2 18 28
(42.8) 38.1 41.3
(22.5) 40.3 23.3
(1.2. 105) 9. I013 6. 105
44
39.5
38.5
4. 1012
Table 2 gives in column 2 the activation energies ofdesorption calculated by equation (1) assuming ri = 10 ~3 s- i. In column 3 the activation energies calculated from equation (4) and in column 4 the rate constants calculated by equation (1) and the activation energies of column 3 are presented. The calculated rate constants of H20 and CO 2 are in the expected range, while those of H_, and CO differ from probable values (l'i,~lO 13 S - 1 ) by many orders of magnitude. The determination of rate constants from the shifting of outgassing maxima in thermal outgassing spectra at different heating rates requires the determination of these maxima with a precision better than I~C, as a variation of 3°C of one of the maxima changes r by more than one order of magnitude. However, a determination of the outgassing maxima within 1°C is probably not possible in step by step heating cycles with heating steps of about 12°C and 40°C.
A more precise determination of the rate constants and the activation energies of desorption would be possible, if the heating rate /3 is varied by two or more orders of magnitude, as recommended by Redhead 2. However, the heating rate could be varied only by one order of magnitude in these measurements. Moreover, the experimental determination of rate constants and activation energies of desorption of first order reactions is usually done from surfaces which have been carefully degassed before and have then been exposed to one gas species only in order to have the same conditions for the outgassing spectra measurements at different heating rates. Further measurements at different gas exposures have to be done for second order reactions. On the contrary, the specimens used in Run 1 and 2 have probably not exactly the same pre-history and their outgassing rate curves depend not only on the heating rates but also on their pre-history. The anomalous rate constant of H_, can be explained by the non-validity of the second assumption and by a superposition of the desorption of molecules originally adsorbed on the surface or absorbed in the bulk of the specimen. The desorption of hydrogen in this temperature range starts from atomic states and is therefore a second order reaction. Furthermore, the outgassing rate curve of H2 is not only determined by the desorption of atoms originally adsorbed on the surface but also by the desorption of atoms which come from the bulk and pass through the series of processes: diffusion in the bulk, diffusion in the near surface region, entering in the surface, recombination and desorption.
4.2. Decrease of outgassing rates during a baking interval at higher temperatures. The decrease of outgassing rates of Inconel 600 samples has been studied during a baking interval of 7 days. Sample A-2 and B-2 were baked at 500°C. The results of these measurements are presented in Figures 4 and 5. In Run 3, the measurements have been started about 6 h after the beginning of heating, while they have been started immediately in Run 7. However, the initial thermal outgassing rates in Run 7 could not be measured, because these values were outside the range of the measuring instruments and at least 100 times larger than the first recorded values, 10 minutes after beginning of heating. That means, the initial total thermal outgassing rates of Run 7 were larger than 10 -6 mbar. 1.s-~.cm -2. 229
F Reiter and J Camposilvan." Thermal o u t o a s s i n g properties of Inconel 6 0 0 surfaces Run 3 LOs
• T01'o( QOS release + MOSS 2
"
]1 ]l~
v Mass 18 o Moss 28 x MOSS 4 4
|~..
~lt
E
--~-,~, _ - - - - - . . - ~ a .
_
"~. ~v
~.
v"
"--,,
vx
v ~5-. . . . . "~"---~ . . . . . . . Qr"C~. . . . . . . . . . . . . . . . . . . . . .
"~" ~
x I~ ~z
l
-
25
I
50
~
E)
o'
x
x
i
75
x
I
L
I00
125
l
_
150
J
t [h3 Figure 4, Decrease of outgassing rate at 500°C. Sample A-2.
+
l
Run 7 • ÷ v o
IO-B~|
x
A
\
'
~~.,X fo~,
Total gas rele0se Mass 2 Moss 18 Mass 2 8 M o s s 44
"e--~6-"
............
--8
~ \,,xv
io-~z
0
I
40
I
80
1
l
120
~60
I
200
I
240
J
1' [ h i
Figure 5. Decrease of outgassing rate at 500:C. Sample B-2.
The total thermal outgassing rates decrease by one order of magnitude in less than 10 rain, by two orders of magnitude in about 1 h and by three orders of magnitude in about 15 h. The further decrease of outgassing has approximately an exponential slope. The total thermal outgassing rate after a 7 days baking interval amounts to about 10-xa m b a r . l . s - t . c m - 2 and will decrease slowly during a longer heating interval. During baking at higher temperatures, the outgassing process starts with the desorption of molecules chemisorbed on the surface and will be continued with the desorption of material originally absorbed in the bulk of the material. The kinetics of this diffusion-desorption process has been studied by Calder and Lewin 3. Following this theory, the decrease of outgassing rate of H 2 from Inconel 600 should take place much more rapidly, if one assumes the known diffusion rates of the system stainless steel-hydrogen * for the unknown data of the system Inconel 230
600 hydrogen. This discrepancy between experiment and theory can be explained either by smaller diffusion rates for the system Inconel-hydrogen, or by the assumption that bulk diffusion is not the rate-determinant step of the desorption process or, more probably, by the fact that the ends of the specimens are at lower temperatures during the baking interval and that consequently diffusion of hydrogen to the surface was much slower. In Run 3 (Sample A-2, Figure 4), the desorbed gas is composed of about 90°/, of hydrogen and of several percents of H 20, CO and CO2; in Run 7 (Sample B-2, Figure 5), the desorbed gas is composed of about 60 % of hydrogen, of about 30 % of CO and of several percents of HzO and CO2. In both runs, 0 2, N 2 and hydrocarbons could not be detected, their contribution is in the range of 0.1% or smaller. The total thermal outgassing rate and the partial thermal outgassing rate of H 2 after a 7 days heating at 500°C ofsample B-2 (Figure 5, Run 7) are smaller than the corresponding ,~alues of
F Reiter andJ Camposilvan:
Thermal outgassing properties of Inconel 6 0 0 surfaces
Run 4 (sample A-2), and Run 8 (sample B-2). The results are presented in Figures 6 and 7. The equilibrium outgassing rate at a given temperature has been determined from an extrapolation of outgassing rate measurements during 48 h. The gas composition at a given temperature has been averaged from several measurements. The scatter of a single gas composition measurement was considerable because these measurements have been carried out at the limit of the sensitivity of the partial pressure analyser. The total outgassing rate measurements are also linked with a considerable uncertainty, as the outgassing rate of the sample at a given temperature in Run 4 and 8 and that of volume V 1 at 20°C are in the same order of magnitude. Equilibrium outgassing rates have been determined in Run 4 and 8 from 200°C up to 500°C. Total outgassing rates are in the range from 10 -12 up to 10 -1° mbar. l . s - ~ . c m -2. The released
sample A-2 (Figure 4, Run 3), while the partial thermal outgassing rates of CO and CO2 of sample A-2 are smaller than those of sample B-2. The sand- and bead-blasted process cracks the oxideand carbon-rich layers which form a barrier for H-diffusion, increases the initial H2-outgassing rate by increased H-diffusion from the bulk and leads therefore to a smaller H2-outgassing rate after baking at 500°C for several days. On the contrary, the thermal outgassing of CO and CO2 is mainly determined by particles originally adsorbed on the surface; this leads to higher CO- and CO2-outgassing rates of the sand- and bead-blasted sample B-2, because its real surface area is larger than that of the polished sample A-2. 4.3. Equilibrium thermal outgassing rates and equilibrium surface coverages. Equilibrium outgassing rates of Inconel 600 samples after a 7 days baking at higher temperatures have been studied in
Run 4 5C0°C
io-IO
300"C
400*C
200"C
r
+ o x
+~ .......
_-
Mass 2
~
Mass 28 Mass 44
--
+ ~
E u
'++c,,.",.~ . o
I0-11
+
* ~ " ~ .
~
+
~.,,q.
~
'm
*
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x
~
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o
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iO-12 Z
.
x
~
O
~
~.
~..,,,,.
0 ~.~.
× x iO-13 12
I
I
I
I
I
,3
,4
,s
,6
,7
I
I L8
19
I
I
20
I
21
22
I03/T [K"] Figure 6. Equilibrium outgassing rate of sample A-2 after a 7 days baking at 500°C. Run 8
500% [
10-IO
400% I
300% I
200"C I + o x
o
÷
-
~,
-
E
_
÷
~""--.,;_ -x--.---r
13
I
14
I
15
~
~
.....
~ - - " •: : ~ - ~
I
16
I
17
.....+~
÷
~.._~ .... .---
i6,2
2
Mass 4 4
~
lO.J 1
,..,
2 28
Mass Mass
' ~'~-
"" - -
0
' "~3""".~....0
I
18
~ ". . . . .
F
19
X
0
1
20
[
21
I
22
I03/T [I<']
Figure 7. Equilibrium outgassing rate of sample B-2 after a 7 days baking at 500°C. 231
Thermal outgassing properties of Inconel 600 surfaces
F Reiter and J Camposilvan:
gas consists mainly of hydrogen and of smaller quantities of CO and CO2. A comparison of the thermal outgassing rates after'baking at 500°C for several days and the corresponding equilibrium outgassing rates shows quite good agreement for sample A-2 {Figures 4 and 6) for C O and CO2; for sample B-2 {Figures 5 and 7) it shows again quite good agreement for C O and poor agreement for CO2. The corresponding values for H2 differ by a factor of 2 for both samples indicating further decrease of H : outgassing rates during the measurements of Run 4 and 8. In Tables 3 and 4, equilibrium surface coverages cr~are given of the mechanically polished sample A-2 baked for 7 days at 500°C and of the sand- and bead-blasted sample B-2 baked for 7 days at 500°C. These data have been calculated fromZ:
(w~=equilibrium outgassing rate of gas species i), using rate constants and activation energies ofdesorption of column 3 and 4 of Table 2 and equilibrium outgassing rates of Figures 6 and 7. These surface coverages are only rough estimations, as they are calculated from experimental values linked with considerable uncertainties. Extrapolations to room temperature give anomalously high values of equilibrium surface coverage. Probably this is conditioned by the temperature decrease at the ends of the strips during the baking interval and during the measurements. By this the absolute values and temperature dependences of the surface coverages will be changed.
wi=vitriexp(---~-~)
Thermal outgassing properties of mechanically polished and of sand- and bead-blasted lnconel 600 surfaces have been investigated in the temperature range from 2 f f C up to 500'C. A comparison of the thermal outgassing rates of samples pretreated by these two mechanical methods and baked at 500~C for several days showed smaller values for H 2 from sand- and beadblasted surfaces and for C O and CO_, from polished surfaces. The smaller H_,-equilibrium outgassing rates attained after sand- and bead-blasting of the surface could be explained by cracking of the oxide- and carbon-rich surface layers which form a H-diffusion barrier and consequently by a faster decrease of the H-outgassing rate with time. On the contrary, the outgassing of C O and CO_, starts from the surface or very near surface regions, as diffusion is very small in this temperature range, and depends on the real surface area which is considerably smaller for the polished surface. H.,, C O and CO., are contaminants in a thermonuclear fusion experiment or reactor. The allowable concentration limits of contaminants decrease with increasing atomic number. However, the equilibrium outgassing rates of H_, were considerably higher than those of C O and CO_, for both surface treatments. Therefore, the two surface treatments seem to be approximately equal for the pre-treatment oflnconel 600 surfaces used as first wall material in a fusion device.
(5)
Table 3. Equilibrium surface coverage (molecules/cm 2) of sample A-2 after a 7 days baking at 500°C
T(°C) 200 250 300 350 400 450 500
o'2
al~
o'25
3. 1013
--
8.1012
--
2.1012 8.1011 3.1011 1.5. 1011 8. 101°
-__ __ ---
1.2 2 7 2 9 4 2
a~.~ 1012
1011 10 I° 101° 10 9
109 109
1. 4 3 3 4 8 2
1012 101° 10 9
10~ l0 T 106 106
Table 4. Equilibrium surface coverage (molecule;/cm2) of sample B-2 after a 7 days baking at 500°C
T(°C) 200 250 300 350 400 450 500
232
o"2 4. 1013 5. 1012
als
tr28
---
2 3 6 1.7 5 2 1.0
1 . 0 . 1012
--
3.1011 8. 101° 3.101° 1.2. 101°
-----
a.~a 1012
IOI1 101o 101o 109 IO 9 10 9
5. Conclusions
1.0.1013
2 7 5 5 5 3
10 t t 109 l0 s 107 106 105
References 1 F Reiter and J Camposilvan, Thermal Desorption of Inconel, EUR 5792 {1977). 2 p A Redhead, Vacuum12, 203 119621. 3 R Calder and G Lewin, Br J Appl Phys, 18, 1459 (1967). "~H L Erschbach, F Gross and S Schultien, I:twuum,13, 543 11963).
0042-207X/82/050227-06503.00/0 Pergamon Press Ltd
Printed in Great Britain
Thermal outgassing properties of mechanically polished and of sand- and bead-blasted Inconel 600 surfaces up to 500°C * F R e i t e r and J C a m p o s i l v a n , Commission of the European Communities, Joint Research Centre, Ispra Establishment, 21020 Ispra (Va), Italy
received 19 February 1981;in revisedform 3 August 1981
In the present work, two different types of Inconel 600 samples have been investigated. The first one was mechanically polished and chemically cleaned in several steps and had a total hemispherical emissivity at 75~C of e1 =0.146 and a surface roughness of CLAI =0.08 pm, while the second sample was sand- and bead-blasted with E==0.382 and CLA== 1.2 pm. The experimental studies were composed of the determination of desorption spectra from 200C up to 550°C at different heating rates, the investigation of the decrease of thermal outgassing rates during a longer heating interval at about 500°C and the measurement of equilibrium outgassing rates at higher temperatures of samples which have been heated several days at about 500"C. The total outgassing rates and the partial outgassing rates of the main components H=, 14=0, CO and CO= have been determined in these studies. Activation energies of desorption, rate constants and equilibrium surface coverages have been calculated from the experimental data using the theory of Redhead.
1. Introduction An important problem in fusion technology is the desorption of molecules, which are adsorbed on the surface or are absorbed in the bulk of the wall surrounding the plasma by physical or chemical forces. During the first start-up of any fusion reactor or fusion experiment, large numbers of molecules will be desorbed by thermal energy, by the impact ofcharged and neutral particles and by radiation. Hereby the plasma will be cooled and the ignition of the fusion reaction can be hindered. Desorption from metallic surfaces depends strongly on the mechanical, chemical and thermal pre-treatment. In this study, the surfaces of two differently prepared samples are investigated. These surfaces form the first wall and the rear surface of protection shields of JET.
thermal emissivity and it will form the rear surface of the protection shields of the bellows which connect the sections of the reaction chamber of JET. These shields will have a much higher temperature during the discharge than the surrounding walls and can deliver heat to the surroundings mainly by thermal radiation. Therefore, the rear surface of the shields must be enlarged in order to increase their cooling. A metallurgical examination of these samples has been carried out after their mechanical, chemical and thermal preparation both by optical and scanning electron microscopy of the surface and of a transverse section and by measurements of hardness, roughness and thermal emissivity. The results of measurements of roughness and total hemispherical emissivity at 75°C are: Sample A: CLA 1=0.08 #m Sample B: CLA 2 = 1.2/zm
e 1=0.146 e2 =0.382
3. Experimental system and processing 2. Preparation and characterization of the samples Two differently prepaxed surfaces of Inconel 600 have been investigated. The first surface (sample A) is a polished and clean surface which has been pre-treated mechanically and chemically, as foreseen for the vacuum wall of JET. The second (sample B) is a sand- and bead-blasted surface with higher surface roughness and
* This report is an extended version with additional results of a work presented at the Eighth International Vacuum Congress, Cannes, 22-26 September 1980.
The schematic arrangement of the thermal outgassing apparatus is presented in Figure 1. A detailed description was given in a previous report 1. Thermal outgassing measurements have been carried out from ohmic heated specimens of 200 mm length, 2 mm width and 0.15 mm thickness. The gas released from a specimen during a certain interval of time in volume V1 is expanded at the end of this interval in the large volume V1 + V2. During this expansion, the pressure increase in V1 + V2 is measured by a Bayard-Alpert gauge BA and the gas composition is determined by a high frequency resonance mass filter MF. Errors from gas pumping of BA and M F and
227
F Reiter and J Camposilvan:
Thermal outgassing properties of Inconel 600 surfaces Table 1. Temperature ( K ) o f maxima ofthermal outgassing rates
pump
T goS inlet
Mass
Run 1(0.136°C s -~)
Run 2 (0.0125°C s -~)
2 18 28 44
656 551 626 574
582 519 561 538
system
•
Vl
Adsorption volume
V2
expansion volume
BA
B o ¥ o r d - A l p e r t gauge
MF
mass f i l t e r
MV
magnetic valve
LV
leak valve
BH
20°C - b o t h
Activation energies of desorption, which approximately equal heats of adsorption, can be calculated from these temperatures by a theory given by Redhead:. The activation energy of desorption Ei is given by the transcendental equation
E~ R T~
vi fl exp
_
_
(1)
(T~=temperature of maximum of reduced thermal outgassing rate, fl = heating rate, v~= rate constant, the index i indicates the gas species i), when (a) the characteristic pumping time
Figure I. Thermal outgassing measurements. Experimental set-up.
V chemical reactions at the hot filaments of BA and MF are avoided by the following procedure. Total gas pressures and gas compositions after a gas release interval are extrapolated to the moment, when the gas is expanded from Vt to V~ + V2.
( V= V~ + V2, S = pumping speed, t, = duration of the temperature sweep) and when (b) the desorption of the various gas species from an Inconel 600 surface in the temperature range up to 500°C are first order reactions. A value of the rate constant must be assumed, e.g.
4. Results
-o
Total gas release
+ Mass 45O
-v
Ei
- -
RT~
/
Z8
/
~
d log fl
-
-
(4)
d log T i
r~
/'% \
IN ~
%
5OO
I
~
~'E 400--× Moss 44 o/ / ~-Temperoture (rightscale)/ I//~/~ - " ~50 -~/ ~ / e / E 25o __-~
+ 2 -
and equation (1) itself. Condition (a) is satisfied in a step by step heating cycle. On the contrary, condition (b) is valid only for the low temperature
/
MOSS 18
o Mass
Alternatively, rate constants and activation energies ofdesorption can be determined from two desorption spectra at different heating rates using the differentiated equation (1)
/
2
(3)
t'i= 1013 S-1
4.1. T h e r m a l outgassing spectra. Figures 2 and 3 show thermal outgassing spectra at two different mean heating rates of 0.136 and 0.0125°C s- ~. In Run 1, the specimen (sample A-1 ) has been heated in 13 steps from 20°C up to about 550°C; each desorption interval was 5 min. In Run 2, another Inconel 600 sample (sample A-2) has been heated in 48 steps from 20°C up to about 550°C and the desorption interval was 15 rain. The temperatures of the maxima of thermal outgassing rates of the main components H 2, H20, CO and CO_, are given in Table 1. N> O2 and hydrocarbons could not be detected, their contribution was smaller than 0.1%.
500
(2)
l
l/ ~ !L~,__
400 Run l SampleA.
3OO p-
200
2OO
15O
IOO 50
I00
15
30
45
t Is] Figure 2. Thermal outgassing spectrum of an Inconel 600 strip of 200 mmx 2 mm x 0.15 mm. Mean heating rate: 0.136C s- ~. 228
F Reiter and J Camposilvan: Thermal outgassing properties of Inconel 600 surfaces
40 - " +
36-v 32 o - x
u 28E
--
Total gas release Moss 2 Moss 18 Moss Ze
/ /
/~ ~ \
J
~'
~.
Moss 4 4
[
r ~ \
Temperature (right scale)
/ ~
-500
Tj ~
-- 400
~ r-r'
\
~
Sample .&2
16 -
,2
-- 200
-
4 -
J
300
~"+.+.+.-,.S+,, • ,.x
o '~='~=~=~a~,~
"~'~==~'~'~"-"~4~ t I-s]
Figure 3. Thermal outgassing spectrum of an Inconel 600 strip of 200 mm x 2 mmx 0.15 mm. Mean heating rate: 0.0125°Cs-1.
desorption (up to 500°C) of H20, CO and CO 2 and not for H 2, as the desorption of hydrogen above room temperature is a second order reaction. Following the theory of Redhead, the activation energy of a second order reaction can be estimated from the change of the maximum desorption temperature with surface coverage. Nevertheless. we have calculated the H2-values in Table 2 as for a first order reaction and set the results in parenthesis, because the surface coverage of a second order reaction cannot be determined from these measurements.
Table 2. Activation energies of desorption
Mass
E, (kcal/mole) calculated by equation ( I ) and values of Run 2 E~ (kcal/mole) assuming calculated by ~, = 101a s- i equation (4)
t', (s - 1) calculated by equation (1) and E, of column 3
2 18 28
(42.8) 38.1 41.3
(22.5) 40.3 23.3
(1.2. 105) 9. I013 6. 105
44
39.5
38.5
4. 1012
Table 2 gives in column 2 the activation energies ofdesorption calculated by equation (1) assuming ri = 10 ~3 s- i. In column 3 the activation energies calculated from equation (4) and in column 4 the rate constants calculated by equation (1) and the activation energies of column 3 are presented. The calculated rate constants of H20 and CO 2 are in the expected range, while those of H_, and CO differ from probable values (l'i,~lO 13 S - 1 ) by many orders of magnitude. The determination of rate constants from the shifting of outgassing maxima in thermal outgassing spectra at different heating rates requires the determination of these maxima with a precision better than I~C, as a variation of 3°C of one of the maxima changes r by more than one order of magnitude. However, a determination of the outgassing maxima within 1°C is probably not possible in step by step heating cycles with heating steps of about 12°C and 40°C.
A more precise determination of the rate constants and the activation energies of desorption would be possible, if the heating rate /3 is varied by two or more orders of magnitude, as recommended by Redhead 2. However, the heating rate could be varied only by one order of magnitude in these measurements. Moreover, the experimental determination of rate constants and activation energies of desorption of first order reactions is usually done from surfaces which have been carefully degassed before and have then been exposed to one gas species only in order to have the same conditions for the outgassing spectra measurements at different heating rates. Further measurements at different gas exposures have to be done for second order reactions. On the contrary, the specimens used in Run 1 and 2 have probably not exactly the same pre-history and their outgassing rate curves depend not only on the heating rates but also on their pre-history. The anomalous rate constant of H_, can be explained by the non-validity of the second assumption and by a superposition of the desorption of molecules originally adsorbed on the surface or absorbed in the bulk of the specimen. The desorption of hydrogen in this temperature range starts from atomic states and is therefore a second order reaction. Furthermore, the outgassing rate curve of H2 is not only determined by the desorption of atoms originally adsorbed on the surface but also by the desorption of atoms which come from the bulk and pass through the series of processes: diffusion in the bulk, diffusion in the near surface region, entering in the surface, recombination and desorption.
4.2. Decrease of outgassing rates during a baking interval at higher temperatures. The decrease of outgassing rates of Inconel 600 samples has been studied during a baking interval of 7 days. Sample A-2 and B-2 were baked at 500°C. The results of these measurements are presented in Figures 4 and 5. In Run 3, the measurements have been started about 6 h after the beginning of heating, while they have been started immediately in Run 7. However, the initial thermal outgassing rates in Run 7 could not be measured, because these values were outside the range of the measuring instruments and at least 100 times larger than the first recorded values, 10 minutes after beginning of heating. That means, the initial total thermal outgassing rates of Run 7 were larger than 10 -6 mbar. 1.s-~.cm -2. 229
F Reiter and J Camposilvan." Thermal o u t o a s s i n g properties of Inconel 6 0 0 surfaces Run 3 LOs
• T01'o( QOS release + MOSS 2
"
]1 ]l~
v Mass 18 o Moss 28 x MOSS 4 4
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t [h3 Figure 4, Decrease of outgassing rate at 500°C. Sample A-2.
+
l
Run 7 • ÷ v o
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Total gas rele0se Mass 2 Moss 18 Mass 2 8 M o s s 44
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40
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80
1
l
120
~60
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200
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240
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Figure 5. Decrease of outgassing rate at 500:C. Sample B-2.
The total thermal outgassing rates decrease by one order of magnitude in less than 10 rain, by two orders of magnitude in about 1 h and by three orders of magnitude in about 15 h. The further decrease of outgassing has approximately an exponential slope. The total thermal outgassing rate after a 7 days baking interval amounts to about 10-xa m b a r . l . s - t . c m - 2 and will decrease slowly during a longer heating interval. During baking at higher temperatures, the outgassing process starts with the desorption of molecules chemisorbed on the surface and will be continued with the desorption of material originally absorbed in the bulk of the material. The kinetics of this diffusion-desorption process has been studied by Calder and Lewin 3. Following this theory, the decrease of outgassing rate of H 2 from Inconel 600 should take place much more rapidly, if one assumes the known diffusion rates of the system stainless steel-hydrogen * for the unknown data of the system Inconel 230
600 hydrogen. This discrepancy between experiment and theory can be explained either by smaller diffusion rates for the system Inconel-hydrogen, or by the assumption that bulk diffusion is not the rate-determinant step of the desorption process or, more probably, by the fact that the ends of the specimens are at lower temperatures during the baking interval and that consequently diffusion of hydrogen to the surface was much slower. In Run 3 (Sample A-2, Figure 4), the desorbed gas is composed of about 90°/, of hydrogen and of several percents of H 20, CO and CO2; in Run 7 (Sample B-2, Figure 5), the desorbed gas is composed of about 60 % of hydrogen, of about 30 % of CO and of several percents of HzO and CO2. In both runs, 0 2, N 2 and hydrocarbons could not be detected, their contribution is in the range of 0.1% or smaller. The total thermal outgassing rate and the partial thermal outgassing rate of H 2 after a 7 days heating at 500°C ofsample B-2 (Figure 5, Run 7) are smaller than the corresponding ,~alues of
F Reiter andJ Camposilvan:
Thermal outgassing properties of Inconel 6 0 0 surfaces
Run 4 (sample A-2), and Run 8 (sample B-2). The results are presented in Figures 6 and 7. The equilibrium outgassing rate at a given temperature has been determined from an extrapolation of outgassing rate measurements during 48 h. The gas composition at a given temperature has been averaged from several measurements. The scatter of a single gas composition measurement was considerable because these measurements have been carried out at the limit of the sensitivity of the partial pressure analyser. The total outgassing rate measurements are also linked with a considerable uncertainty, as the outgassing rate of the sample at a given temperature in Run 4 and 8 and that of volume V 1 at 20°C are in the same order of magnitude. Equilibrium outgassing rates have been determined in Run 4 and 8 from 200°C up to 500°C. Total outgassing rates are in the range from 10 -12 up to 10 -1° mbar. l . s - ~ . c m -2. The released
sample A-2 (Figure 4, Run 3), while the partial thermal outgassing rates of CO and CO2 of sample A-2 are smaller than those of sample B-2. The sand- and bead-blasted process cracks the oxideand carbon-rich layers which form a barrier for H-diffusion, increases the initial H2-outgassing rate by increased H-diffusion from the bulk and leads therefore to a smaller H2-outgassing rate after baking at 500°C for several days. On the contrary, the thermal outgassing of CO and CO2 is mainly determined by particles originally adsorbed on the surface; this leads to higher CO- and CO2-outgassing rates of the sand- and bead-blasted sample B-2, because its real surface area is larger than that of the polished sample A-2. 4.3. Equilibrium thermal outgassing rates and equilibrium surface coverages. Equilibrium outgassing rates of Inconel 600 samples after a 7 days baking at higher temperatures have been studied in
Run 4 5C0°C
io-IO
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21
22
I03/T [K"] Figure 6. Equilibrium outgassing rate of sample A-2 after a 7 days baking at 500°C. Run 8
500% [
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400% I
300% I
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o
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Figure 7. Equilibrium outgassing rate of sample B-2 after a 7 days baking at 500°C. 231
Thermal outgassing properties of Inconel 600 surfaces
F Reiter and J Camposilvan:
gas consists mainly of hydrogen and of smaller quantities of CO and CO2. A comparison of the thermal outgassing rates after'baking at 500°C for several days and the corresponding equilibrium outgassing rates shows quite good agreement for sample A-2 {Figures 4 and 6) for C O and CO2; for sample B-2 {Figures 5 and 7) it shows again quite good agreement for C O and poor agreement for CO2. The corresponding values for H2 differ by a factor of 2 for both samples indicating further decrease of H : outgassing rates during the measurements of Run 4 and 8. In Tables 3 and 4, equilibrium surface coverages cr~are given of the mechanically polished sample A-2 baked for 7 days at 500°C and of the sand- and bead-blasted sample B-2 baked for 7 days at 500°C. These data have been calculated fromZ:
(w~=equilibrium outgassing rate of gas species i), using rate constants and activation energies ofdesorption of column 3 and 4 of Table 2 and equilibrium outgassing rates of Figures 6 and 7. These surface coverages are only rough estimations, as they are calculated from experimental values linked with considerable uncertainties. Extrapolations to room temperature give anomalously high values of equilibrium surface coverage. Probably this is conditioned by the temperature decrease at the ends of the strips during the baking interval and during the measurements. By this the absolute values and temperature dependences of the surface coverages will be changed.
wi=vitriexp(---~-~)
Thermal outgassing properties of mechanically polished and of sand- and bead-blasted lnconel 600 surfaces have been investigated in the temperature range from 2 f f C up to 500'C. A comparison of the thermal outgassing rates of samples pretreated by these two mechanical methods and baked at 500~C for several days showed smaller values for H 2 from sand- and beadblasted surfaces and for C O and CO_, from polished surfaces. The smaller H_,-equilibrium outgassing rates attained after sand- and bead-blasting of the surface could be explained by cracking of the oxide- and carbon-rich surface layers which form a H-diffusion barrier and consequently by a faster decrease of the H-outgassing rate with time. On the contrary, the outgassing of C O and CO_, starts from the surface or very near surface regions, as diffusion is very small in this temperature range, and depends on the real surface area which is considerably smaller for the polished surface. H.,, C O and CO., are contaminants in a thermonuclear fusion experiment or reactor. The allowable concentration limits of contaminants decrease with increasing atomic number. However, the equilibrium outgassing rates of H_, were considerably higher than those of C O and CO_, for both surface treatments. Therefore, the two surface treatments seem to be approximately equal for the pre-treatment oflnconel 600 surfaces used as first wall material in a fusion device.
(5)
Table 3. Equilibrium surface coverage (molecules/cm 2) of sample A-2 after a 7 days baking at 500°C
T(°C) 200 250 300 350 400 450 500
o'2
al~
o'25
3. 1013
--
8.1012
--
2.1012 8.1011 3.1011 1.5. 1011 8. 101°
-__ __ ---
1.2 2 7 2 9 4 2
a~.~ 1012
1011 10 I° 101° 10 9
109 109
1. 4 3 3 4 8 2
1012 101° 10 9
10~ l0 T 106 106
Table 4. Equilibrium surface coverage (molecule;/cm2) of sample B-2 after a 7 days baking at 500°C
T(°C) 200 250 300 350 400 450 500
232
o"2 4. 1013 5. 1012
als
tr28
---
2 3 6 1.7 5 2 1.0
1 . 0 . 1012
--
3.1011 8. 101° 3.101° 1.2. 101°
-----
a.~a 1012
IOI1 101o 101o 109 IO 9 10 9
5. Conclusions
1.0.1013
2 7 5 5 5 3
10 t t 109 l0 s 107 106 105
References 1 F Reiter and J Camposilvan, Thermal Desorption of Inconel, EUR 5792 {1977). 2 p A Redhead, Vacuum12, 203 119621. 3 R Calder and G Lewin, Br J Appl Phys, 18, 1459 (1967). "~H L Erschbach, F Gross and S Schultien, I:twuum,13, 543 11963).