Grain boundary transport of hydrogen in nickel

Scripta METALLURGICA

Vol. 16, pp. 575-578, 1982 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

GRAIN BOUNDARYTRANSPORT OF HYDROGENIN NICKEL

T. Tsuru and R.M. Latanision Department of Materials Science and Engineering Massachusetts I n s t i t u t e of Technology Cambridge, MA 02139

(Received February 8, 1982) (Revised February 25, 1982) Introduction The i n t e r g r a n u l a r , hydrogen-induced embrittlement of nickel has been widely discussed and the role of various phenomena such as solute ( p a r t i c u l a r l y metalloid) segregation ( I ) , hydrogen transport by l a t t i c e d i f f u s i o n as well as by mobile d i s l o c a t i o n s (2), hydrogeninduced p l a s t i c i t y (3), e t c . , has been considered. Birnbaum (4) has recently reviewed the hydrogen related fracture of metals. Grain boundary transport of hydrogen has been considered less l i k e l y and previous gas phase (5) as well as e l e c t r o l y t i c (6) permeation measurements have f a i l e d to produce evidence of grain boundary d i f f u s i o n . Recent ion microprobe (IMA) experiments suggest that hydrogen (duterium) segregation occurs at grain boundaries in Nb and Ni (4). The object of t h i s communication is to report evidence of grain boundary d i f f u s i o n of hydrogen in nickel using IMA and electrochemical methods. Experimental Strips of Ni 270 (99.97% Ni) were cold r o l l e d to thickness ranging from 0.15 to 0.06 mm. A f t e r a mechanical p o l i s h , the specimens were annealed at I050°C f o r 3 min and then water quenched. To induce the segregation of i m p u r i t i e s , some specimens were aged at 600°C for 24 hr under an argon atmosphere and then water quenched. The grain diameter of these specimens was 50 to 150pm. The method developed by Devanathan and Stachurski (7) was used f o r the measurement of hydrogen permeation rate. O.IN-H2SO4 and O.IN-NaOH were used as catholyte and anolyte, respectively. I n i t i a l l y , anodic p o l a r i z a t i o n in O.IN-NaOH at 0.I V (SCE) was performed f o r more than 24 hr. When the anodic current decayed to less than I0 nA/cm 2 and the current t r a n s i e n t became n e g l i b l e , the catholyte was f i l l e d i n t o the cathodic compartment of the c e l l and galvanostatic cathodic p o l a r i z a t i o n (6.6 or 1 mA/cm2) f o r hydrogen charging started immediately. Subsequent increase in anodic current was a t t r i b u t e d to the permeation f l u x of hydrogen. I t should be noted that Pd was not plated onto the anode surface, as is often the case, in these experiments in order to achieve low background anodic current densities (8) For IMA measurements, the anodic side of specimen was polished with O.3pm alumina paste and covered by a lacquer. A f t e r hydrogen charging for a certain period from the uncoated cathode side, the specimen was wrapped in aluminum f o i l and kept in a l i q u i d nitrogen vessel. Just before IMA (CAMECA) analysis of the anode side, the lacquer was dissolved o f f in acetone. This was intended to avoid contamination and minimize the escape of hydrogen from the anodic surface. Results and Discussion A t y p i c a l r e s u l t of the e l e c t r o l y t i c permeation measurement is shown in Fig. I . The permeation f l u x due to l a t t i c e d i f f u s i o n increased over a period of several hours depending upon the specimen thickness and then reached steady state a f t e r more than ten hours. In the early stages of permeation, a very small incremental increase in the anodic or e x i t current

575 0036-9748/82/050575-04503.00/0 Copyright (c) 1982 Pergamon Press Ltd.

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E

o

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I0

15

o

o Time,tlh

Time,t/h FIG. l Electrolytic hydrogen permeation transients showing l a t t i c e and grain boundary (insert) transport.

(o) , t l r n i n ( o )

FIG. 2 Logarithmic dependence of the permeation flux on time.

occurred after a few minutes and reached a steady statq current within lO to 20 min, as shown in Fig. I. Since this increment was a few nA/cm~ which corresponded to about one thousandth of the permeation flux through l a t t i c e diffusion, i t was presumably buried in the noise in some e a r l i e r measurements (6) and sensitive to the surface preparation. I t is presumed that this small transient corresponds to a grain boundary diffusion flux. Similar observations have been made recently in gas phase charging experiments by Fidelle (9). The diffusion constant was calculated from the breakthrough time t b and the lag time tlag as follows (7): L2 L2 [2] tb 15.3 D [l] and tlag = 6 D where L is the thickness of the specimen and D is the diffusion constant. I t has also been shown (7) that the plot of log ( P , - Pt)/P~ against time, for the rise transient, is a straight line with a slope I / t o , where t o = L2/x2D

[3]

Fig. 2 shows those plots for the l a t t i c e diffusion and the early stage of permeation which presumably corresponds to grain boundary diffusion. Both l a t t i c e and grain boundary diffusion constants were independent of the sample thickness, which affected only the steady state fluxes. Similarly, the addition of a promoter for hydrogen absorption, 3 mg/l of NaAsO2, increased the steady state flux but did not sensibly affect the diffusion constants. The mean values and standard deviations of measured diffusion constants are given in Table I. The l a t t i c e diffusion constant compares well with that determined by gas phase charging (5). The grain boundary diffusion constant was approximately 60 times (between 20 to lO0) larger than that corresponding to l a t t i c e diffusion. This is not as large differences as may be expected but may be reasonable for i n t e s t i t i a l hydrogen. Notice in

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TABLE 1 Measured Diffusion Constants Lattice DLffusion D lat(CmL/sec) Annealed (I050°C) Annealed + Aged (Aged at 600°C/24 hr)

Grain Boundarv Diffusion Dgb (cm2/sec)

(3.52 + 1.02) x I0 -I0 ~ = II

(2.05 + 1.50) x 10-8 - n = 5

(3.38 + 0.91) x I0 -I0 n=4

(2.13 + 0.93) x I0 -8 n=2

n : number of independent measurements Table 1 that both l a t t i c e and grain boundary diffusion constants were only s l i g h t l y affected by thermal treatment (aging) intended to induce grain boundary segregation of sulfur. S i m i l a r l y , Birnbaum (4) has observed (thermally charged) hydrogen accumulation at grain boundaries both in the presence of segregated sulfur and in i t s absence. I t i s , of course, possible that an alternate thermal treatment might give rise to the segregation of other species and to a quite d i f f e r e n t response. The influence of thermal treatment is s t i l l under study. Since the average grain size of the specimen wa~ around lO0~m, we can roughly estimate the length of grain boundary lines as 200 cm in 1 cm surface of the specimen. Assuming° a grain boundary width which is e f f e c t i v e for the rapid grain boundary d i f f u s i o n as I0 A ( I 0 ) , the e f f e c t i v e area of grain boundary, Sgb, is considered as 2 x 10-5 cm2 per unit surface area. So the grain boundary and l a t t l c e fluxes per unit surface area can be written as ig b = k SgbDgb

and

i l a t : k Dla t

[4]

where k is a constant. Since the r a t i o of the diffusion constants was abou~ 60 and the steady state f l u x through l a t t i c e d i f f u s i o n was measured to be about 2~A/cm , the steady state f l u x through grain boundary d i f f u s i o n should be around 2.4nA/cm L. This rough e s t i mation agrees f a i r l y well with experimental results. An IMA measurement was made on a specimen of 0.12 mm thickness which corresponds to a breakthrough time f o r the l a t t i c e and grain boundary d i f f u s i o n of about 8 hr and 8 min, respectively. The specimen was charged with hydrogen for 2 hr; in short, the charging time is long enough for hydrogen to permeate the specimen by grain boundary transport but not by l a t t i c e d i f f u s i o n . The r e s u l t of a step scan analysis of 60Ni+ and IH+ (lO~m each step and 13~m resolution) is shown in Fig. 3 in which three hydrogen peaks can be seen. This suggests that there were spots where the concentration of hydrogen was very large compared with the matrix. Fig. 4 shows the sputtered surface where the IMA measurement was performed and the l i n e of c i r c l e s which corresponds to the trace of the IMA spot measurements. These peaks correspond to the points of intersection of the IMA scan direction vector with grain boundaries in the specimen as shown in Fig. 4. In essence, the concentration of hydrogen at grain boundaries was found to be very large compared to background hydrogen. Notice as well that the nickel signal is l i t t l e affected in crossing grain boundaries - i . e . , the r a t i o IH+/6ON.i+ increases s i g n i f i c a n t l y . We consider these observations to be consistent with the suggestlon of grain boundary d i f f u s i o n that emerged from the e l e c t r o l y t i c permeation experiments described e a r l i e r .

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FIG. 3 II%A data showing the increased hydrogen concentration at grain boundaries on the e x i t surface of a nickel specimen c a t h o d i c a l l y charged for 2 hours at the opposite or entry surface (cathode).

FIG. 4 IMA scanning direction superimposed on the optical micrograph of the surface correspondingto Fig. 3. The hydrogen peaks in Fig. 3 correspond to grain boundary intersections.

Conclusion The present experiments suggest that the grain boundary diffusion constant for hydrogen in pure nickel is larger than that for l a t t i c e diffusion. Both l a t t i c e and grain boundary diffusion constants appear to be independent of the specimen thickness, the segregation of sulfur and the presence of promoters of hydrogen absorption in the electrolyte. IMA measurements are compatible with the rapid diffusion of hydrogen along grain boundaries. Acknowledgements The authors thank Dr. I. Kohatsu at MIT for help in performing the IMA measurements. The support from the Office of Naval Research under Contract number N00014-78-C-0002 i s gratefully acknowledged. References I. 2. 3. 4. 5. 6. 7. 8. 9. lO.

R.M. Latanision, M. Kurkela and F. Lee, in Hydrogen Effects in Metals, eds., I.M. Bernstein and A.W. Thompson, p. 379, (TMS-AIME, Warrendale) 1981. M. Kurkela and R.M. Latanision, Scripta Met., 13, 927 (1979). T. Masumoto, J. Eastman and H.K. Birnbaum, Scrip~ta Met., L5, I033 (1981). H.K. Birnbaum, in Proc. NATOAdvanced Research Institute on Atomistics of Fracture, eds., R.M. Latanision and J.R. Pickens, (Plenum Press, NY) in press. W.M. Robertson, Z. Metallkd., 64, 436 (1973). M. Kurkela, Sc.D. Thesis, Massachusetts Institute of Technology, 1981. M.A.V. Devanathan and Z. Stachurski, Proc. Roy. Soc., A270, 90 (1962). M. Kurkela and R.M. Latanision, Scripta Met., 15, I 1 5 7 ~ 8 1 ) . J.P. Fidelle, private communication, September'981. P.G. Shewmon, Diffusion in Solids, p. 164, (McGraw-Hill, NY) 1963.

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