College of Earth andMineral Sciences

PENNSTATEDEPARTMENT OF MATERIALS SCIENCE

IIETALLURGY PROGRAM

0C - ITECHNICAL REPORT

October 1988

OFFICE OF NAVAL RESEARCH

Contract No. N00014-84-k-0201

A MECHANISTIC ANALYSIS OF HYDROGEN ENTRY INTO METALS

DURING CATHODIC HYDROGEN CHARGING D T ICE LECT E

Howard W. Pickering OCT 1 81988

l)eoartment of Material; Science and Engineering HThe Pennsylvania State University

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4. TITI.E (and Subtie) S. S OP REPORT 6 PERIOD COVERCD

A Mechanistic Analysis of Hydrogen Entry IntoMetals During Cathodic Hydrogen Charging Technical Report

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Howard W. Pickering NOQ 1 4-84-k-020 1i. PERIORMING ORGANIZATION NAME AND0 ADORES8S 10. GRMP"EEL. RJC.TS

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20. ABSTRACT (Coninue an reverse side it necessary and Identity' by block numeber)

The present paper seeks to analyze the h.e.r. mechanism and to predict therelationship between the permeation flux and the charging and evolution(recombination) fluxes. A thorough development of the model and actualcomputations of rate constants and hydrogen coverages will appear elsewhere.

DD I AN7 1473 EOITION OF I NOV 65 IS OBSOLETE

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Scripta METALLURGICA Vol. 22, pp. 911-916, 1988 Pergamon Press plcPrinted in the U.S.A. All rights reserved

A MECHANISTIC ANALYSIS OF HYDROGEN ENTRY INTO METALSDURING CATHODIC HYDROGEN CHARGING

Rajan N. Iyer and Howard W. PickeringDepartment of Materials Science and Engineeng

The Pennsylvania State UniversityUniversity Park, PA 16802

andMehrooz Zamanzadeh

Professional Services Industries - PTL Division850 Poplar Street

Pittsburgh, PA 15220

(Received March 9, 1988)(Revised April 5, 1988)

Entry of hydrogen into metals is of serious concern to metallurgists and engineers, since itseverely degrades the mechanical properties of metals (1). These problems arise in the cathodicprotection of metals, power plants, and in environments where H2S is present as in petroleum refining(2) where the hydrogen evolution reaction (h.er.) and hydrogen permeation reaction take place in thecorroding or cathodically polarized metal. By performing hydrogen charging experiments of thinsamples using the Devanathan-Stachurski cell, the permeation characteristics have been extensivelystudied (3,4). The present paper seeks to analyze the h.e.r. mechanism and to predict the relationshipbetween the permeation flux and the charging and evolution (recombination) fluxes. A thoroughdevelopment of the model and actual computations of rate constants and hydrogen coverages willappear elsewhere (5).

Anaz

Essentially, three steps are involved during cathodic hydrogen charging of metals. They are: (1)the hydrogen discharge reaction (proton tunneling), (2) hydrogen recombination reaction either bychemical recombination or electrochemical desorption, and (3) hydrogen permeation (mainly by bulkdiffusion). Of these three, the bulk diffusion step is usually the slowest. A detailed schematic of thereactions is given in Fig. 1 below.

The charging current (ic) is given byic = io' (I - 8s) e'a"q (I

The hydrogen evolution current (it) (assuming chemical recombination of H atoms) is given byir = F k3 s2 (2)

9110036-9748/88 $3.00 + .00

Copyright (c) 1988 Pergamon Press plc

912 HYDROGEN ENTRY INTO METALS Vol. 22, No. 6

H2

I ~ :FIG.!1. SchematicSshowing hydrogen

discharp,C 7' c.()I recombination,"( k (2b) k_2 (4) k4 -.H + per ton and

H30+ . (2)k 2 , ...... H--'+ -I-+ - selvedge reactions.2

[Aa c noe C I

Surfaoe "bswface exit side(xO) N- ) N-0

[H]

0 .,.--, x

The steady state hydrogen permeation cuent (i..) is given by

i. a F- 21. C,(3)

In equatois 1, 2 and 3: io' Fk = io/(l-ee); io the exchange curnt density; e the eCquiibrium

swfacecoverage of hydrogen ; kl= the discharge rate coefficient- k, CH+C -aEeq (acid)

l aotEe (akaine); ki a the rate constant for the forward reaction; cH+ = H + ion concentraton; a =

F/RT - 38.94 (volts) 1 at T - 300K; a - the transfer coefficient; E q = the equilibrium potential for the

h.er, Os - the surface coverage of hydrogen; 'n = the hydrogen overvoltag¢ - E applied -Eeq; k3 - therecombination rate coefficient; Di = the hydrogen diffusion coefficient in the metal L = the membranethickness.

The model considers a selvedge reaction (as a result of proton tunneling) that is quite fast andconstitutes a transition layer of a thickness that could range upward from - 1 nm (that has yet to bedetermined by special experiments). This establishes a metal subsurface hydrogen concentration, ci, atthe boundary of the selvedge. Thus, hydrogen diffuses out to either surface, though mostly to thecharging side to recombine to form H2 molecules. An equilibrium will be established between thesurface covered (adsorbed) hydrogen atoms and hydrogen just below the surface (in the adsorbed state,

with concentration cs). This equilibrium has been analyzed before (6,7) giving Os = cs/k', where k' =

Vol. 22, No. 6 HYDROGEN ENTRY INTO METALS 913

the equilibrium absorption - adsorption constant and cs = ci - Cg. (However, it is to be emphasized heretha the selvedge reaction is not critical to the development and applicatoi of this model Considerationof selvedge, on the other hand, helps to generalize the model; in the absence of a selvedge, cg -0 ands - ci). Using these relations along with equations (1), (2) and (3) one can awive at the following set

of equatio, which for the first time have taken into account the effect of i.e on the h.e. kinetics:

i. k' + (4)and

ieM (bio' (" +io 5

" ) Lb i(5)

where b - LI(FDI) - a constant for a metal; also,

MV 'dni 1 d11 di11 v'i- ~ i1 < i <- <2 _24

c d log 1 logi- 1 dlogi decade (6)

for the model.

The transfer coefficient is given bya-- " -/a (6a)dI j

Details of the derivations are shown elsewhere (5).

The potential range (1 to Nl) in which the recombination reaction will be coupled with the

discharge reaction (commonly observed on many metals) is given by (5)

lIl ( 31klk] / (aa) (7a) OT

,, [In (k,/(10k3))] / (ad) (7b)

where the superscripts I and u refer to the lower and upper limits of the overpotential range.

Results and Discussion

In hydrogen permeation experiments, ic is set and when the permeation current becomesindependent of time, i., is measured. Then ir = ic - i,.

If the plots of ie vs ir (equation (4)) and ic e aOczi vs [i. - (equation (5)) are linear,t!I eqaioth)er "er then an oof the coefficients k', k3, cx and io' can be calculated. Such calculations have been done onexperimental data from theliterature for iron and nickel membranes and found to verify the model (5)since these two plots are linear. An example of such an analysis is given for the polarization and

ByDistribution/

I Availability CodesAvail and/or

Dist Special

a.

914 HYDROGEN ENTRY INTO METALS Vol. 22, No. 6

peadon data of Bockis al (3) obtained on Armco iron in .N H2SO4 solution. Fig. 2 shows

the plot Of i vs ir and ig. 3 shows ic 0 KM vs Di..e]-. plot for thedaza ofBockris et al (3). Itis

easily seen that these plots ae linear and hence the model can be applied to detennine the rate constantsand exchange current density (Table 1). Then, foai Eqn. (2), Os can be calculated using the k' and io'values obtained from the slope and intercept, respectively, of Fig. 3, the k3 value obtained from the

slope of Fig. 2 and the ir value from ic - i... The surface coverage (0s) vs the hydrogen overvoltage

(TI) plot (for the Bockris et al data) is shown in Fig. 4. It can be seen that the coverage is quite low inthe potential range of perimentation.

Ey

rC

4 I-

,0 F_ 00,U.

0.0 0.2 0.4 0.6 0.6 1.0 1.2 4 a

4"1r (mA/sq.cm.) (i,, - c. /b) (CJA/sq.Cm.)

FIG. 2. Analysis of Data of Bockris et al (3). FIG. 3. Analysis of Data of Bockris et al (3).

TABLE ICalculated Values by Applying the

Model to the Data of Bockris et al (3)

* io - 0.5 ILA/sq.cm.>okl - 5.5 x 10- 12 mol / cm2 .s

k3 - 2.3 x 10- 5 o /(cm2.sle 2.6 x 10-5 mol /cm 3

=600mV; Tu -8 10mV

4 40 " '_ __'_ _ _ __,_ _

0.006 0.010 0.015 0.020 0.025

eos

FI0. 4. H Coverage for the Data of Bockris et -.1(3).

V

Vol. 22, No. 6 HYDROGEN ENTRY INTO METALS 915

Thus, this model effectively accounts for the contribution of i.e to the overall kinetics. The mostimportant prediction of this model is that i. is proportional to 4ir and not to '4ic as assumed in earliermodels (3). This type of relationship (i.e a o4ir) has been previously observed (8).

The above relafionships assume that il >> RT/F so that the backward reactions can be neglected.Also, the Langmuir isotherm of hydrogen coverage was utilized in order to simplify the derivation.Howeve, in many cases, the reactions (dischare and recombination) are activated in which caseFrumkin-Temkin corrections (9) have to be applied for 8s in the equations for ic and ir. Equation (1)then becomez

ic - i, (I -e,) Ce'Se'' (8)and equation (2) becomes

i k 3iO.ceCe: '' (9)

where f - sRT, y being the gradient of the apparent standard free energy of adsorption with coverage.The value of f = 4 to 5 for H coverages (9). In the problem of enhanced hydrogen entry in thepresence of H2S, such considerations have been shown to be necessary (10). The modifiedrelationships between i., ir and ic are given by

and In (f(i. i.)) -c a (-11) + In (i,') (11)where

S(af bi.)

f(ic, .) -(1- .- )

k'

Thus, equation (10) tells us that i. will not be linearly related to '/ir when fo, meaning the dischargeand recombination reactions are activated, probably due to a side reaction of H2S with a hydratedelectron, e'aq (10), as follows:

H2S + Cq -+ H2 S (12)and

H2 S-+H++M -+ M-H+H 2S (13)

'dirIf In (.) vs i.e and In (f(iciae)) vs Ti are linear, then this side reaction and overall mechanism can bei0e

said to be operating. Then, the coefficients k', k3, a and io' can be computed from the slopes andintercepts of these plots, in conjunction with the iterative solution of equations (10) and (11).Eventually, 0s vs T1 can be plotted; and 8e, io and kI can be computed.

Once again, the potential ranges, where the recombination and discharge reactions are coupled,can be estimated from equations (7a) and (7b). With increasing H2S concentration, these potentialranges have been found from analysis of available data according to the above model to become lessnegative, k3 progressively decreases suggesting a decreasing surface diffusivity of Had atoms, and ki

916 HYDROGEN ENTRY INTO METALS Vol. 22, No. 6

piogressively increases suggesting that the discharge reaction is enhanced by the side reaction, equation(12), followed by equation (13). This will also explain why the overvoltage actually decreas ratherthan increases in the presence of H2S.

(

A summary of a recently completed analysis of hydrogen electrode reactions during aqueouscathodic charging of metals is presenited. The analysis for the first time takes into account the effect ofhydrogeng-em nd. into the metal an the h.e.r. Prom the model all of the kinetic parameters arecomptae without use of any adjustable parameter. For the first time, surface coverages and rateconstants am determinable from the measured charming and permeation currents. The enhancement ofhydrogen entry in the presence of poisons, such asi±S- can be analyzed with the model, taking intoaccount the Fnmldn-Temkin isotherms in the discharge and recombination reaction kinetics. fr-' 1 , d. '

Financial support by the Office of Naval Research under Contract No. N0OO14-84k-0201 isgratefully acknowledged.

References

1. M. Smialowski, Hydrogen in Steel, Pergamon Press, Oxford (1962).2. C. M. Hudgins Materials Protection, Vol. 8, p. 41 (1969).3. J. OM. Bockris, J. McBreen and L Nanis, J. Electrochem. Soc., Vol. 112, p. 1025 (1965).4. M. Zamanzadeh, A. Allam, H. W. Pickering and G. K. Hubler, 3. Electrochem. Soc., Vol. 127,

No. 8, p. 1688 (1980).5. R. N. lyer, M. Zamanzadeh and H. W. Pickering "Analysis of Hydrogen Evolution and Entry

into Metals for the Coupled Discharge-Recombination Mechanism," (submitted to the J.Electiochemical Society).

6. C. D. Kim and B. E. Wilde, J. Electrochem. Soc., Vol. 118, p. 202 (1971).7. B. E. Wilde and C. D. Kim, Corrosion, Vol. 42, No. 4, p. 243 (1986).8. E. G. Dafft, L Bohnenkamp and H. J. Engell, Corrosion Science, Vol. 19,f. 591 (1979).9. E. Gileadi and B. F. Conway, Modern Aspects of Electrochemistry, No. 3, . OX. Bockris and

B. E. Conway eds., Butterworths, Washington (1964), pp. 347-442.10. R. N. Iyer, I. Takeuchi, M. Zamanzadeh and H. W. Pickeing, "Hydrogen Sulfide Effect on

Hydrogen Entry in Iron - A Mechanistic Study" (to be submitted to Corrosion).

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