Electrical resistance of hydrogen-charged iron wires

ELECTRICAL

RESISTANCE

OF HYDROGEN-CHARGED

D. J. VAN

OOIJEN

IRON

WIRES*

and J. D. FAST?

Pure iron wires were supersaturated with hydrogen by cathodic charging in sulphuric acid. It was found that the hydrogen precipitates at the crystal boundaries, building up high gas pressures. Microscopic cracks are thus formed, accompanied by cold working of the surrounding metal. An increase of the electrical resistance of the wires due to the cracks and an increase due to coldworking, could be measured as two distinctly different effects. No change in the resistance due to interstitial solution of hydrogen could be detected. RESISTANCE

ELECTRIQUE

DE

FILS

DE

PER

CHARGE

D’HYDROGENE

Des fils de fer pur Btaient surcharges d’hydrogene par Blectrolyse dans l’acide sulfurique. On a constate que l’hydrogene precipite aux joints de grains, developpant ainsi des hautes pressions. Des fissures microscopiques sont formees, ce qui est accompagne d’un Bcrouissage du metal environnant. Une augmentation de la resistance Blectrique des fils par suite de ces fissures et une augmentation par suite de l’ecrouissage, pouvaient Btre mesurees comme deux effets differents. On ne constatait aucun changement de la resistance, qui pourrait 6tre attribue a la presence d’atomes d’hydrogene en insertion dans le metal. ELEKTRISCHER

WIDERSTAND

WASSERSTOFFBELADENER

EISENDRAHTE

Reine Eisendrlihte wurden durch kathodische Beladung in Schwefelsiiure mit Wasserstoff iibersattigt. Es wurde festgestellt, dass der Wasserstoff sich an den Korngrenzen ausscheidet und dort hohe Gasdrticke aufbaut. Dadurch werden nicht nur mikroskopische Risse geformt, sondern das umgebende Eisen wird such plastisch deformiert. Eine Zunahme des elektrischen Widerstandes der Drahte infolge der Risse und eine Zunahme infolge der Kaltverformung konnten als zwei durchaus verschiedene Effekte gemessen werden. Eine Widerstandsanderung, welche Wasserstoff in interstitieller Losung zuzuschreiben ware, konnte nicht festgestellt werden.

1. INTRODUCTION

One of the characteristics

A combination

of hydrogen

in iron is

esting.

that its solubility falls off with decreasing temperature.

of hydrogen

The following

be expected

solubility

values for hydrogen

iron at a pressure of 1 atmosphere

have been worked

out from data in papers by Carmichael(l)

Hydrogen

in the

cooling.

A high

metal degree

[HI + 0 = pi, -

can be by

of supersaturation

fast

[HI + L = (LLW,

(about where

by electrolytic

charging. As to the manner extra

in which hydrogen

the iron, various suggestions (1) The hydrogen

may

this relatively

large

is accommodated

may

an

interstitially a vacancy

above

be precipitated

temperature

interstitially

With

at lattice imperfections in microcavities

(de

in

cold-worked

rising

The object at the surface

iron

(Petcht’));

of

whether

in

demonstrated.

in the As the

falls or the number temperature

lattice

The of

the equilibrium of the associated

quantities

to take electrical

ACTA 4

METALLURGICA,

VOL.

11, MARCH

that [H] + 0 Philips’ 1963

Gloei-

than 211

q H

or [H] + 1

or (IH),

then

increase

could

be

are

the

concentration

of

involved

the

and the temperature. resistivity

to be measured in the experiments.

will shift

was to ascertain

of this equilibrium

hydrogen,

imperfections

decided

of lattice imperfec-

will shift to the right.

of our investigations

the existence

concentration

steel chemisorption may occur at the boundaries of Fe,C particles (Hill and Johnson’s), Werner and Davisfg)). * Received August 3, 1962. t Philips Research Laboratories, N. V. lampenfabrieken, Eindhoven, Netherlands.

a dis-

energy is released.

to the left in consequence in entropy.

Garofalo et al(@). may take place

dissolved

and 1

with an imperfection

manner, a binding

tions increases the equilibrium

(4) Adsorption microcavities

represents

atom or ion, 0

When an [H] combines

in

have been made: be dissolved

(2) It may be segregated (Darkenc4)). (3) It

[H]

location.

(Plusquellec(3)).

Kazinczyt5),

can

hydrogen

reversible reactions:

hydrogen of

dissolved

its ideal structure.

2 * 10V5 at

in supersaturation

10F3 at H/at Fe) can also be achieved

quantity

between interstitially

3 * 10-s at H/at Fe at 25°C.

taken up at a high temperature

retained

the rate of diffusion

in iron is so high that interaction

and sites in the iron where the lattice departs from We had in mind the following

and Eichen-

auer et al.c2): 2 . 1O-4 at H/at Fe at 900°C; H/at Fe at 400°C;

in alpha

of (1) and (2) seemed to us inter-

Even at room temperature

It was

as the quantity If it be assumed

cause a higher resistivity a shift of the equilibrium

ACTA

212

METALLURGICA,

towards the right will manifest itself as a fall-off in resistivity. BhatiaoO) carried out calculations relating to a similar fall-off in t,he resistivity of iron due to the segregation of carbon at dislocations, and the effect has been found experimentally by Cottrell and Churchman.(n) At first sight the results of our experiments were quite understandable in terms of the equilibria discussed above. On closer investigation, however, a totally different mechanism, which is more in accordance with suggestion (3), proved to be responsible for the measured changes in resistance. 2. EXPERIMENTAL

The material used was pure iron that had been melted in vacua and cast as a rod about 15 mm in diameter. The rod was cold-hammered and drawn and in this way its diameter was reduced to 0.5 mm. Subsequently it was soft-annealed in wet hydrogen for 1 hr at 850°C; after a further annealing in vacua lasting 1 hr, it was slowly cooled to room temperature. Spectrochemical analysis revealed that the main impurity was Ni (0.04 per cent by weight). Concentrations of other impurities were at least ten times smaller. All experiments were carried out on wires that were initially in the soft-annealed state. The wires were not etched, because this would have involved the absorption of an unknown quantity of hydrogen. Charging of the wires took place in a 0.1 n H,SO, solution to which 50 mg AssO, per 1. had been added to facilitate the absorption of hydrogen by the iron. The current density was invariably 0.12 A. cm-s. The wire to be charged was fitted into a glass frame with platinum wire (anode) wound round it. The resistance measurements were made with a Diesselhorst potentiometer. Initially, all measurements were carried out at liquid nitrogen temperature (77°K) for the following reasons. In the first place the resistivity due to lattice vibrations is smaller at 77°K than at room temperature (about 15 times in the case of Fe) and hence the changes to be measured would be relatively greater. Secondly, the hydrogen cannot escape at 77”K, though it may easily do so at room temperature. Later on the measurements were performed both at 77°K and at room temperature. This change in procedure was adopted because the second consideration had ceased to apply: it was not the effect of any hydrogen present that was being measured, but only the permanent damage that had been occasioned by the hydrogen.

VOL.

11,

1963

The changes in resistance due to charging were analysed in the following way. Let the resistance of the wire prior to charging be

R=p;.

(1)

R = resistance p = resistivity I= length of the wire A = cross-sectional area of the wire. If l/A does not change due to charging, the relative increase in resistivity equals the relative increase in resistance : Ap AR -=(2)

P

To find Aplp, it of the wire at after charging. of the charging order terms:

R

will suffice to measure the resistance the same temperature before and If l/A does change in consequence of the wire, then, neglecting second-

AR -=R

Ap + A(Q)

(3)

1/A’

P

Resistance measurements at a given temperature before and after charging only provide a value for AR/R. Aplp and A(Z/A)/(Z/A) cannot be determined separately. However, they can be calculated separately if resistance before and after charging is measured at a second temperature, and if the values found for the wire under investigation are compared with those for a reference wire which does not undergo charging. Prior to charging the difference in the resistance values measured at temperatures Tl and T, is given by R(T,) -

R(T’,) = (pT1 -

(4)

PT,) $

for the wire under investigation, and by

R,(T,) - RST,) = (PT, - PT,)

”r

(5)

for the reference wire. On dividing we obtain

VA

R(TJ - R(T,) RAT,) - RAT,) =

(6)

I,lA,’

Similarly, after charging, R(T,‘) R,(T,‘)

-

l/A + A(W)

R(T,‘) R,(T,‘)

=

&IA,

*

(7)

Generally T,’ M T, w 300°K and T,’ M T, M 77’K. It is assumed in the above that Ap due to charging obeys Matthiessen’s rule. A(E/A)/(Z/A) can now be derived from equations (6) and (7). It will then be

VAN

OOISEN

~\NDFAST:

RESISTANCE

OF

HYDROGEK-CHARGED

IRON

313

possible, since AR/R is known, to detexmine Apjp with the help of equation (3). 3, RESULTS

AND DISCUSSION

3.1 On the basis of the considerations set out in the introduction, one would expect that a given concentration of hydrogen will cause a higher resistivity increase in soft-annealed iron than in cold-worked iron, as a eonsequen~e of the higher concentration of lattice defects in the cold-worked material. In order to investigate this, the relative increase in resistance as a function of charging time was measured for a wire in the soft-annealed state and for a wire that prior to charging was drawn ataroom temperature, its diameter being reduced from 0.5 to 0.3 mm. The final hydrogen content, which was determined by vacuum extraction, was about 10h3 at H/at Fe for both wires (Fig. 1).

Soft onneoled,diamQ5mm

Cold deformed, diom.j.3mm +--+-+

0

I 70

I w

1 I 30 40 ---w

I I 1 50 60 70

I I 80 9.l

FIQ. 2. Soft-annealed iron wire, diam. 0.5 mm, after electrolytic oharging with hydrogen. Crack formation at crystal boundaries due to high pressures built up by molecular hydrogen. Etched; 190 x .

It will be seen from Table 1 that the A(l/A)/(@l) term is considerable. Microscopic examination of the charged wires revealed cavities at the crystal boundaries which had not been present before charging (Figs. 2 and 3). It is to these cavities that we attribute the A(Z/A)/(Z/A) contribution to AR/R, for there are “dead spaces” in front of and behind the cavities (looked at in the direction of current flow), that do not, carry any current and so increase the resistance of the wire. It can be shown(12)that a wire containing spherical cavities, a proportion AVIV of its volume being occupied by these cavities. has a relative resistance increase of

Chqging time (minu f es)

Pm. I. The relative increase in resistance at 77”K, as a function of charging time at room temperature and at constant current density, for samples of soft-annealed and cold-worked iron.

In our case the cavities are not spherical but lenticular and as a consequence their orientation with

If it be assumed that equation (2) applies here, then the result of Fig. 1 is in accordance with our expectations. However, equation (2) does not apply, as is demonstxated in Table 1, where the measured values of AR/R are analysed according to equation (3); hff 7, is the relative resistivity change measured at 77°K due to cold-corking only.

Charged after soft-annealing Cold-worked Charged after cold-working

3.43 0.86

0.37

0.49

Fro. 3. Cold-deformed iron wire, diam. 0.3 mm, after electrolytic charging with hydrogen. Cracks lying mainly parallel to the wire axis. Etched; 60 x .

214

ACTA

METALLURGICA,

respect to the direction of current flow has a great bearing on the resistance increase as measured. The increase will be very much less if they lie parallel rather than perpendicular to the direction of current flow. This is more or less the case in the cold-drawn wire, where the crystals have been pulled in the direction of the wire, so that they are lying with their largest surfaces in this direction (Fig. 3). In the annealed wire the crystal surfaces, and so the cracks, are more randomly orientated. The large difference in the measured values of A(Z/A)/(Z/A) (see Table 1) can be explained in this way. An approximate value of AV/V for the crack volume can be worked out from the results of the resistance measurements, by applying equation (8). For the soft-annealed wire the value found in this manner is 4.27115 = 2.8 per cent. Direct measurements of the dimensions have shown that the diameter of the charged wire had increased by about 1.2 per cent; the increase in length was only about onetenth of this. This means that the volume had undergone an increase AVIV of at least 2.4 per cent. Thus completely different methods yield almost the same value for the volume of the cracks. The relative increase in resistivity after charging, ApIp, might be ascribed to interstitially dissolved hydrogen. However, it is more likely to be due to plastic deformation of the metal in the vicinity of the microcavities. After 24 hr at room temperature a charged, soft-annealed iron wire had lost about 95 per cent of the total quantity of hydrogen it absorbed originally, while Aplp had virtually remained constant. Heating of the wire for 2 hr at 350°C caused a decrease in ApIp,,, from the previous value of 0.9 per cent to 0.6 per cent, although it is known from extraction measurements that virtually all the absorbed hydrogen must have escaped by that time. We therefore attribute the decrease found in Ap/pT7 to partial removal of the lattice imperfections that had arisen around the microcavities. Compare this with the observation that a colddeformed wire, which had previously yielded a Ap,/p,, value of 2.7 per cent, exhibited a decrease to about 1.4 per cent after being heated at 360°C for 2 hr. If it be borne in mind that a severely deformed wire recovers its electrical resistance more quickly than one that has undergone less deformation, the decrease found in the charged wire is by all means compatible with structural recovery after colddeformation. The similar trend in coercivity values and in resistivity changes due to charging or colddeformation,

VOL.

11,

1963

+ cold

4c

.

worked

Chm-ged

0 Charged

0

1

2

3

4

-

2 6

and heat treoted ot3500Cfor2hrs

5 orfj?plOq 77

FIG. 4. Relative increase in coercivity as a function of the relative increase in resistivity due to charging or cold-deformation.

as shown in Fig. 4, provides a further argument in favor of comparing Aplp,, subsequent to charging with the resistance increase subsequent to cold deformation. 3.2 Again on the basis on what was said in the introduction, one would expect that on cold deformation of soft-annealed iron containing hydrogen the resistivity will either decrease or, if it increases, then the increase will be expected to be slower than is the case for iron not containing hydrogen. In order to test this, a soft-annealed charged wire was deformed at room temperature and the resulting change in resistance was measured, The relative change in resistivity can be calculated as a function of the elongation, Al/l, from data on the relative change in electrical resistance, AR/R, provided that it can be assumed that the product 1 x A where A is the cross-sectional area of the wire, is the same before and after deformation. The relevant expression is then R+AR 1 AP -=---(9) (1 + AZ/Z)2- ” R P The results have been plotted in Fig. 5, which seems to reveal that Ap/p falls off as the amount of deformation increases. The explanation suggested +3l-

7 (x10”) +2

77

f

+7 t O

-I r

1

2

3

+

4 5 (x702)

-2

’

-31

FIG. 5. Relative change in resistivity at 77”K, as a function of plastic elongation at room temperature, for soft-annealed iron charged with hydrogen. . ignoring crack formation + corrected for crack formation

VAN

by

OOIJEN

the hypothesis

adopted

that as the amount stitially dissolved lattice tivity.

hydrogen

that

RESISTANCE

OF

in the introduction

of deformation

imperfections, Note

FAST:

AND

increases,

is

HYDROGEN-CHARGED

If Matthiessen’s

a decrease

p is the resistivity

rule applies

in resis-

APT

of the wire

at

crystal

result of electrolytic tensile

stressing,

relation

boundaries,

1 x A = constant

permissible in

ratio,

strictly

as manifested

correlated

P77

as a

order

to

electrically,

calculate

is no

with the macroscopic

The orientation,

use the

The reason is that the

of the wire, which are those playing testing.

to

longer

dimensions

resistivity

after deformation,

we analysed

were carried out.

If the values found

assumption

The results are displayed

in Table 2 and in Fig. 5.

Al -r x 102

___ VA Assuming I x A = cofmtant x 102 x 102 2.24 3.36 4.48

1.12 1.68 2.24

Based on eleot&al measurements x 102 x 102

-1.43 -1.71 -1.89

on the contrary,

as constant.

Temperature

(“K)

189

209

228

239

257

273

286

(APT/P,,) x 10’

1.75

1.80

1.77

1.90

1.82

1.80

1.83

put forward

3.3 It was assumed in deriving

We

wish

dependent

rule, i.e. Ap, ature. which

of a soft-annealed,

(7) that

of an uncharged was measured range dividing

iron wire, serving

as a function

between

77°K

resistance

temperatures

experiment.

charged

and values

by

T,

room

the

The

same

proceeds

measured

at

PT

+

P77 +

‘PT _ AP77

various after

(10)

for the charged wire, and

R~(T)

-z-_-x

R 777

for the reference wire.

PT P77

conclusion

from Matthiessen’s

is arrived

at on

of the experimental

as follows.

of temperature

“ApT”lp7,

(11)

an

results,

is calculated

from equation

(11) and

(14) :

We now know

R(T) __

(14)

P77

that equation

(14) should

rightly

pr + APT l/A + A(llA)

=

R 77

l/A

P77

From equations second-order

(14) and (15) we obtain,

(15)

’

neglecting

terms

“APT” _ APT --X3-ip-. P77

Since it involves

PT 7

WA)

(16)

VA

pT, the second term on the right-

hand side of equation and so “Apr”l~~~

R 77

temperature-

in the introduc-

be

By

charging we obtain

R(T) = __

the

where R 7 7 is the resistance of the wire before charging, so ignoring the A(l/A) effect due to charging.

over the

at 77°K

deviation

R 77

as a reference,

temperature.

resistance

The

iron wire and

of temperature

that

as discussed

R(T) pT + “Ap,” __ =

demonstrating

equation

here

should increase with increasing temper-

as a function

in the introduction

as will be shown by the following

note

erroneous interpretation

Ap due to charging was temperature-independent (Matthiessen’s rule). This assumption is permissible, resistance

to

equilibrium,

of a charged

again

3

TABLE

from equation

when the wire is deformed;

it increases,

that the hypothesis is incorrect.

0.09 0.61 0.54

0.70 0.99 2.01

The table shows that the resistivity wire does not decrease

a,

WA) VA

ap

WA)

the values

in Table 3 and can in fact be regarded

tion, predicts a positive

2

TABLE

are constant,

(12) will have been correct;

are displayed

the meas(3).

(13)

can now be worked out for at which resistance measurements

the real change in

ured resistance changes with the help of equation

.

A value for AP~[P,~

are after all liable to alter when the wire is stretched. In order to be able to calculate

(10) and

each temperature

a part in tensile

shape and size of the cavities

from equations

X-Y __~ Y--l

AP* --zzz

charging, and which is undergoing it is not

Aplp from Al/l as measured. l/A

formed

(12)

AP77

(11) that

in the case of an iron wire which contains

microcavities

=

and in that case it follows

after charging. However,

215

inter-

moves into newly formed

so causing

IROn-

(16) is temperature-dependent

increases with increasing

ture. 3.4 The mechanism

of crack formation

working

by molecular

hydrogen

explain

or to support

temperaand cold-

can be adduced

to

the results of several experi-

ments reported in the literature on hydrogen in iron or steel. According to Rogers,(13) strain-aged iron containing

nit,rogen failed to exhibit

any yield

216

ACTA

METALLURGICA,

pointafterbeingelectrolyticallychargedwithhydrogen. During the charging process many new dislocations are formed, not yet anchored by nitrogen atoms, which start to move as soon as a tensile stress is set up in the iron. The explanation recently suggested by Besnard(l4) to the effect that the hydrogen drives the nitrogen out of the dislocations, had earlier been put forward by Rogers himself but who disproved it by experiment.(15) This mechanism also explains the results of damping measurements performed by Weiner and Gensame@) on electrolytically charged iron (Fast(17)). In the charged condition the iron shows a Snoek peak at low temperature; after cold-working of the iron a peak appears at a higher temperature, about 105°K. This new peak must be associated with an interaction of the hydrogen with the dislocations produced by the cold-working. After aging the charged iron for several days at room temperature, the 105’K peak shows up spontaneously, which in our picture is due to dislocations created around regions where molecular hydrogen under high pressure is formed. Recent X-ray investigations by Tetelman et I& have revealed that iron electrolytically charged with hydrogen exhibits the same line broadening as does cold-worked iron. These authors were unable to observe any change in the lattice parameter such as might be expected when the hydrogen was dissolved interstitially. These results tally exactly with our own. CONCLUSIONS

Pure, polycrystalline iron wires were supersaturated with hydrogen by electrolytic charging at room temperature. One may expect that the hydrogen will be present in the iron in several different ways:

VOL.

11,

1963

dissolved interstitially as hydrogen atoms or protons; bound to dislocations or other imperfections; as molecular hydrogen under high pressure at all places where enough room is available. Our investigation, which was confined to electrical resistance measurements, showed that the last effect is by far the most important one. It manifests itself as crack formation along the grain boundaries, and is accompanied by plastic deformation of the metal. Interstitial solution and trapping by dislocations, if present at all, are completely dominated by the observed effect as regards their influence on electrical resistance. REFERENCES 1. D. C. CARMICHAEL,Trans. Amer. Inst. Min. (Metall.) Engrs. 218, 826 (1960). 2. W. EICHENAUER,H. KUNZICJand A. PEBLER. 2. Metallk. 49, 220 (1958). 3. J. PLUSQUELLEC, Mim. SC. Rev. Mltall. 57,215,265 (1960). 4. L. S. DARKEN, The Physical Chemistryof MetallicSolzLtions and InterrnetallicCompounds, Proceedings of a Symposium held at the Nat. Phys. Lab., June 1958, Paper 4G (London, 1959). 5. F. DE KAZINCZY,J. IronSt. Inst. 177,85 (1954); Acta Met. 7. 706 119591. 6. $. GAROFALO,Y. T. CHOUand V. AMBE~AOKAR,Acta Met. 8, 504 (1960). 7. N. J. PETCR, Phil. Mag. 1, 331 (1956). 8. M. L. HILL and E. W. JOHNSON.Trans. Amer. Inst. Min. (Met&.) Engrs. 215, ‘717 (1959): 9. J. E. WERNERand H. M. DAVIS, Trans. Amer. Sot. Metals 53, 853 (1961). 10. A. B. BHATIA, Proc. Phys. Sot. Lond. B82, 229 (1949). 11. A. H. COTTRELLand A. T. CHURCHMAN, J. Iron St. Inst. 162, 271 (1949). 12. R. LANDAUER,J. AppZ. Phys. 23, 779 (1952). 13. H. C. ROGERS, Trans. Amer. Inst. Min. (Metall.) Engw. ” 215, 666 (1959). 14. S. BESNARD,Ann. Chim. 6, 245 (1961). 15. H. C. ROGERS,Acta Met. 4, 114 (1956). 16. L. C. WEINER and M. GENSAMER,Acta Met. 5, 692 (1957). 17. J. D. FAST, Mltauz 36, 383 and 431 (1961). 18. A. S. TETELMAN,C. N. J. WAGNERand W. D. ROBERTSON, Acta Met. 9, 205 (1961).