APPARATUS PERMEABILITY

FOR

INVESTIGATING OF

HYDROGEN

MATERIALS

R. I. Kripyakevich, R. I. Van'kovich, B. F. Kachmar, V . M. S i d o r e n k o , and I. V. Semchishin

UDC 620.193.29

Hydrogen p e r m e a b i l i t y of m a t e r i a l s is d e t e r m i n e d by measuring the quantity of hydrogen p a s s i n g in a unit t i m e through a unit s u r f a c e a r e a of a s p e c i m e n under specified e x p e r i m e n t a l conditions, i.e., by m e a suring g a s flows at v e r y slow r a t e s and low p r e s s u r e s . Methods used in this application can be divided into two groups: 1) collecting the gas in a v e s s e l of a known volume and m e a s u r i n g its p r e s s u r e at periodic i n t e r v a l s ; 2) continuous m e a s u r e m e n t s of the gas flow with the aid of h y d r o d y n a m i c affects (deflection of a mechanical s c r e e n , p r e s s u r e drop a c r o s s a throttle) or other p h y s i c a l f a c t o r s (velocity of p r o p a g a t i o n of u l t r a s o n i c s , t i m e taken by ionized p a r t i c l e s to p a s s through etc.). The method of m e a s u r i n g hydrogen flow should be absolute and sufficiently a c c u r a t e ; it should be suitable for c a r r y i n g out m e a s u r e m e n t s (continuously or at sufficiently frequent i n t e r v a l s ) in a wide r a n g e of g a s flow r a t e s with simple and reliable equipment. T h e s e r e q u i r e m e n t s a r e b e t t e r s a t i s fied by the method which involves the collection of hydrogen. The apparatus c u r r e n t l y used for d e t e r m i n i n g hydrogen p e r m e a b i l i t y by this method [lv5] has a n u m b e r of d i s a d v a n t a g e s . The a c c u r a c y of d e t e r m i n a t i o n is r e l a t i v e l y low and depends to a l a r g e extent on the o p e r a t i v e ' s skill b e c a u s e of the manual control and visual o b s e r v a t i o n of the m e r c u r y level in the c o m p r e s sion m a n o m e t e r . Substantial e r r o r s a r e introduced by the v a r i a t i o n in the level of liquid nitrogen in m o i s t u r e t r a p s of the v a c u u m s y s t e m , as a r e s u l t of which the t e m p e r a t u r e of the walls of the t r a p s p e r i o d i c a l l y r i s e s and the condensate v a p o r s find t h e i r way to the analytical s y s t e m . The m e a s u r e m e n t s cannot be c a r r i e d out at sufficiently frequent i n t e r v a l s b e c a u s e of the manual operation of the c o m p r e s s i o n m a n o m e t e r and c o m p l i c a t e d p r o c e d u r e s for s t a r t i n g and stopping the apparatus. Another c a u s e of the low p r o d u c tivity of the equipment is its sensitivity to f a i l u r e s of the v a c u u m s y s t e m . If, f o r i n s t a n c e , a i r finds its way into the s y s t e m when the s p e c i m e n or g l a s s p a r t s b r e a k , it t a k e s a long t i m e to r e s e t the a p p a r a t u s ; overflow of m e r c u r y , contamination with oil f r o m the pump or f a i l u r e s in the supply of w a t e r or e l e c t r i c p o w e r have the s a m e c o n s e q u e n c e s . Finally, the range of m e a s u r e m e n t s is narrow and the d e t e r m i n a t i o n of s m a l l gas flows difficult b e c a u s e of difficulties in operating the equipment at high d e g r e e s of c o m p r e s sion in the m a n o m e t e r . The a p p a r a t u s we have developed is f r e e f r o m t h e s e disadvantages. It can be used to investigate h y drogen p e r m e a b i l i t y of different m a t e r i a l s under constant e x p e r i m e n t a l conditions ( t e m p e r a t u r e , p r e s s u r e and composition of the gas on the inlet side), to study the v a r i a t i o n in p e r m e a b i l i t y with t i m e , to d e t e r m i n e the p e r m e a b i l i t y of s p e c i m e n s u n d e r the influence of e l e c t r i c and magnetic fields or m e c h a n i c a l loads, to study the p e r m e a b i l i t y of s p e c i m e n s with oxide or multiple s u r f a c e f i l m s , and to d e t e r m i n e the gas content in a s p e c i m e n by a method of h i g h - t e m p e r a t u r e e x t r a c t i o n . Five y e a r s ' operational e x p e r i e n c e p r o v e d the reliability of the a p p a r a t u s which has been used for investigating hydrogen p e r m e a b i l i t y of s t e e l s of v a r i o u s composition, e l e c t r o t r a n s p o r t of hydrogea in m e t a l s , effect of s t r e e s on h~trogen p e r m e a b i l i t y of m e t a l s , etc. [6-11]. The a p p a r a t u s (Fig. 1) c o n s i s t s of t h r e e main s u b a s s e m b l i e s : working c h a m b e r 1, hydrogen supply line 2 and automatic equipmect 3 for m e a s u r i n g the flow of ~ d m g e n and r e c o r d i n g the r e s u l t s . The w o r k Institute of P h y s i c s and M e c h a n i c s , A c a d e m y of Sciences of the Ukrainian SSR, L ' v o v . T r a n s l a t e d f r o m F i z i k o - K h i m i c h e s k a y a Mekhanika M a t e r i a l o v , Vol. 6, No. 4, pp. 72-76, July-August, 197(}. Original a r t i c l e s u b m i t t e d D e c e m b e r 26, 1969. 9 1973 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permission of the publisher. A copy of this article is available from the publisher for $15.00.

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ing Chamber is designed depending on the aims of a given i n v e s tigation. The hydrogen supply line includes equipment for e v a c uating the working c h a m b e r to p r e s s u r e s below 10 -4 mm Hg, purifying hydrogen (to " s p e c t r o g r a p h i c a l l y p u r e " grade) [12], maintaining a constant hydrogen p r e s s u r e in the working chamber and r e g e n e r a t i n g the hydrogen filter. The s y s t e m consists of roughing 4 and diffusion 5 pumps, nickel filter 6, cylinders 7 and 8 for technical grade and purified hydrogen, U - s h a p e ordial~type m a n o m e t e r 9, hydrogen cylinder 10 and various connections.

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Fig.2. E l e c t r i c circuit of the automatically controlled c o m p r e s s i o n manometer.

478

The nickel filter 6 consists of a number of thin-walled tubes (sealed at one e n d ) p l a c e d i n a stainless steel chamber heated by an e l e c t r i c furnace. The open tube ends are connected to a pure hydrogen manifo}d. T h e filter can p e r f o r m its function satisfactorily for long periods at hydrogen p r e s s u r e s (on the inlet side) below 5 arm and t e m p e r a t u r e s not exceeding 500~ In most c a s e s a 101 cylinder of purified hydrogen will be sufficient to maintain a constant p r e s s u r e (accurate to 0.3% ) during an 8 h experiment at a hydrogen flow of less than 10 -3 l / u sec. Hydrogen diffusing through a specimen p a s s e s to the analytical s y s t e m (previously evacuated to 10 -4 mm Hg) where the variation in p r e s s u r e with time is automatically r e c o r d e d by c o m p r e s s i o n m a n o m e t e r 11. The compression m a n o m e t e r was modified and automated to i n c r e a s e its a c c u r a c y (mainly by reducing the subjective e r r o r )

and efficiency. To make the analysis of automatically r e c o r d e d m e a s u r e m e n t s m o r e p r e c i s e and convenient, the m a n o m e t e r - unlike the conventional models* - w a s designed on the principle of linear scales with a provision for a c h a n g e - o v e r to different p r e s s u r e s r a n g e s depending on the magnitude of gas flows measured. The m a n o m e t e r 11 (Fig. 1) consists of a tubular f r a m e 25, a vessel 26 with m e r c u r y and a c o m p r e s sion volume 27. A calibrated platinum wire (0.2 mm diameter) filament 29 tensioned by a tungsten spring 30 is sealed in the m e a s u r i n g tube 28 of the f r a m e . The traditional r e f e r e n c e c a p i l l a r y tube has been replaced by a tube 18 mm d i a m e t e r ; to allow for the c a p i l l a r y depression, the scale has been moved. The c o m p r e s s i o n volume c o n s i s t s of s e v e r a l c h a m bers connected with c a p i l l a r i e s of the s a m e d i a m e t e r . Platinum c o n t r a c t s are sealed in the connecting capillaries for automatically stopping the m e r c u r y at various p r e d e t e r m i n e d d e g r e e s of c o m p r e s s i o n . The c o m p r e s s i o n volume is connected to the f r a m e 25 through a glass joint which facilitates its r e p l a c e m e n t . The r i s e and fall of m e r c u r y is controlled with the aid of platinum contacts A, B, C, and D sealed in the m a n o m e t e r body. The following is the sequence of operations of the automatic control of the c o m p r e s s i o n m a n o m e t e r (Fig. 2). 1. When the automatic control is switched on, the electromagnet of the pneumatic valve of the vacuum line (EMi) is activated through the normally closed contacts RVP2. 2. When mercury closes the contact C, relay R4 activates (through its normally open contacts) relay R3. 3. Relay R 3 activates (through its normally open contacts) the electromagnetically operated pneumatic valve of the air line (EM2). Mercury rises. Relay R3 becomes locked up through the normally closed contact of relay R2. 4, Contact C gets clear of mercury. Relay R3 remains locked-up. 5. Further rise of mercury closes the contact B and switches on the program time relay RVP. 6. When one of the contacts I, If, HI, IV, and V is closed by mercury, the relief relay Rk activates (through its normally open contact) relay R2. 7. Relay R2 switches off (through its normally closed contact) the electromagnet EM2. The rise of m e r c u r y is stopped. 8. After the elapse of the f i r s t period of time the n o r m a l l y open contact RVP 2 activates (with a c l o s ing delay) the m o t o r RD-09 of the automatic bridge feedback. 9. After the elapse of the second period of time the t h r o w - o v e r contact RVP 2 cuts off the e l e c t r o magnet EM 1. M e r c u r y subsides. 10. The n o r m a l l y open contact RVP 2 blocks the disconnection of RVP 1 when m e r c u r y subsides below the contact B. The n o r m a l l y closed contact RVP 3 operating together with RVP 2 cuts off the m o t o r RD-09 of the bridge. 11. When m e r c u r y c l o s e s contact C, r e l a y R 4 cuts off r e l a y RVP and activates the pneumatic valve EM2. The cycle is r e p e a t e d automatically, Since both the scale of the c o m p r e s s i o n m a n o m e t e r and the c h a r a c t e r i s t i c s of the automatic bridge are linear, the displacement of the c a r r i a g e of the s e l f - r e c o r d i n g instrument is proportional to changes in the p r e s s u r e in the s y s t e m . The s c a t t e r in the p r e s s u r e range investigated (1.0-10 -4 m m Hg) 2cr < 0.01, i.e., the m a x i m u m e r r o r of a single m e a s u r e m e n t does not exceed 1%. The quantity of hydrogen can be m e a s u r e d by two methods. The f i r s t method is used for determining low gas flow r a t e s (of the o r d e r of 10 -Y l p / s e c ) and is applied in the following way. After evacuating the outlet size of a specimen to a p r e s s u r e of 1 0 - 4 m m Hg valves 12 and 14 (Fig. 1) a r e shut and valve 13 is set in position "I" so that the chamber is connected to the c o m p r e s s i o n m a n o m e t e r . The volume of the analytical part in this method is equal to the volume of tubes 15 and 1 6 . *The existing automatic c o m p r e s s i o n m a n o m e t e r s of the MacLeod type [13, 14] have a number of disadvantages (quadratic scale, i n c o n s i s t e n c y of time intervals between the moment at which m e r c u r y c o v e r s the lower end of the capillary tube and the moment of r e c o r d i n g the reading, insufficient a c c u r a c y , etc.). 479

This method is not suitable for m e a s u r i n g high gas flow r a t e s (up to 10 -8 l / ~ e c ) because it does not ensure a constant and sufficiently low p r e s s u r e on the outlet side of the specimen (a m a r k e d i n c r e a s e in the back p r e s s u r e of hydrogen r e t a r d s the permeation p r o c e s s ) . In this case the m e a s u r e m e n t s a r e c a r r i e d out by the second method. Hydrogen diffusing through the specimen is evacuated by a m e r c u r y diffusion pump 17 through liquid nitrogen traps 18 and c o m p r e s s e d in cylinder 19 (with valve 20 shut). Valve 13 is set in position ~II". The use of a diffusion pump DPN-10 makes it possible to produce a p r e s s u r e of up to 2 mm Hg in cylinder 19. To eliminate the r i s k of the a c c u r a c y of the m e a s u r e m e n t s being affected by variations in the level of nitrogen in liquid nitrogen traps, a device has been provided for automatically feeding nitrogen to the t r a p s and maintaining a constant nitrogen level. The device works on the following principle. When the nitrogen level in trap 18 drops, the n o r m a l l y open contact 21is closed by the float pusher rod. A r e l a y 22 is activated and through its n o r m a l l y open contacts switches on an e l e c t r o m a g n e t i c valve 23 and a heater 24 mounted below the liquid nitrogen level in a Dewar supply flask. The p r e s s u r e in the flask r i s e s and n i t r o gen flows into the trap. When the n o r m a l l y closed upper contact is p r e s s e d by the float pusher rod, the r e lay is cut off. This t e r m i n a t e s the heating of r e s i s t a n c e 24, valve 23 is opened and the p r e s s u r e in the s y s tem falls to atmospheric, as a r e s u l t of which the flow of nitrogen in the trap c e a s e s . The vacuum s y s t e m is protected against breakthrough of hydrogen (due to accidental leakage through the specimen) by an e l e c t r o m a g n e t i c vacuum valve 32 which is activated by a vacuum protection gage 33. Oil f r o m the pump is prevented f r o m getting to the vacuum p a r t of the apparatus by an e l e c t r o m a g netic hose clamp 31 on the inlet side of the roughing vacuum pumps. This clamp controls also the o p e r a tion of vacuum conduits s e r v i n g the s y s t e m which r a i s e s and lowers m e r c u r y in the c o m p r e s s i o n m a n o m eter 11. The technical c h a r a c t e r i s t i c s of the apparatus are as follows. The range of gas flow r a t e s that can be m e a s u r e d = 1 x 10 -Y- 1 • 10 -3 l # / s e c ; maximum test t e m p e r a t u r e = 1000~ The p r e s s u r e of purified hydrogen on the inlet side of the specimen can be maintained constant in the r a n g e 10 m m Hg-3 atm a c c u r a t e to 0.3 %. The automatic timing of the intervals betwen consecutive m e a s u r e m e n t s can be set for any period in the range 5-60 min. Systematic e r r o r s associated mainly with inaccurate calibration do not exceed 10%of the value m e a s u r e d . The mean quadratic e r r o r of a m e a s u r e m e n t is l e s s than 4%. LITERATURE 1. 2. 3. 4. 5. 6. 7. 8.

9. 10. 11. 12. 13. 14.

480

CITED

N . I . Zakhodskaya et al., FKhMM [Soviet Materials Science], No. 3, 1967. G . V . Karpenko and R. I. Kripyakevich, Effect of Hydrogen on P r o p e r t i e s of Steel [in Russian], Metallurgizdat, 1962. McBain, Sorption of Gases by Solids, Routledge, 1932. W. Ham, J. Chem. Phys., 1, 476, 1933. H. Shenck and H. Taxhet, Arch. Eisenh~lttenwesen, 30, 661 1959. Collection: Hydrogen Charging of Metals and Combating Hydrogen E m b r i t t l e m e n t [in Russian], MDNTP, pg. 23 and 62, 1968. V . M . Sidorenko and R. I. Kripyakevich, FKhMM [Soviet Materials Science], no. 3, 1968. V . M . Sidorenko, R. I. Kripyakevich, and B. F . Kachmar, FKhMM [Soviet Materials Science], no. 2, 1969. V.M. Sidorenko and R. I. Kripyakevich, FKhMM [Soviet Materials Science], no. 2, 1969. B . F . Kachmar, V. I. Tkachev, R. I. Kripyakevich, and B. N. Romaniv, F KhMM [Soviet Materials Science], no. 5, 1969. R . I . Kripyakevich, B. F. Kachmar, V. M. Sid0renko, and V, G. Bravinskii, FKhMM [Soviet Materials Science], no. 3, 1970. A.V. Balitskii, The Technology of Manufacturing Vacuum Apparatus [in Russian], Moscow-Leningrad, 1966. P.A. Faeth, RST, 36, no. 1, 106-107,1965. R . L . Legan, RST, 37, no. 1, 116-117,1966.