Materials Science, VoL 30, No. i, 1994

KINETICS OF HYDROGEN RELEASE ON NEWLY FORMED AND HYDROGENATED METAL SURFACES

I. I. Dykyi, I. I. Vasylenko, and I .M. Protsiv UDC 620.194

We study the regularities of cathodic processes accompanied by hydrogen depolarization as a function of the energy state of the metal surface, including its purity and degree of hydrogenation. It was established that hydrogen release on a newly formed metal surface (NFS) is activation-free and diffusion-controlled and its rate is determined by the exchange current. Hydrogenation of the surface increases the work fun- ction of escape of an electron, decreases the probability of escape of electrons from a metal into solution by changing the structure of the electric double layer, and, thus, slows down hydrogen release.

Along with strains that stimulate heterogeneous electrode reactions in metals [1, 2], surface purity appears to be an important factor for phase processes. It determines the degree of saturation of the free valences of surface atoms and controls not only the rate of escape of metal atoms into solution but also the kinetics of the reduction process. As far as the reaction of hydrogen release is concerned, the transition of protons to adatoms is limited by the height of the energy barrier, which depends, in turn, on the energies of protons in solution and in hydrogen atoms [3, 4]. In the case of an adsorption-type interaction between hydrogen atoms and the metal, the energy of protons in atoms

is less by the value of the heat of adsorption (Ead). Various sources give different values of Ead - - from

66.7 kcal/mole [5] to 47 kcal/mole [6], but, for any value of Ead from this interval the energy of protons in atoms

adsorbed on iron (Epr = - 2 6 2 kcal/mole; adsorption interaction is absent) is less than the energy of hydration of protons ( - 263 kcal/mol). This relationship between the energies of protons in the initial and final states indicates the possibility of activation-free reaction on a newly formed surface (NFS), which is a good adsorbent of hydrogen atoms. Note that there are no oxide films on an NFS and the formation of the latter is accompanied by significant plastic strains. Since these are, in fact, additional conditions that promote a decrease in the work function of escape

of electrons into solution (I42OqbM) [7, 8], one may assume that the kinetic dependences describing the atomization

of protons on an NFS differ from similar dependences typical of a metal under stationary conditions. At the same time, the energy state of a newly formed metal surface at a crack tip changes with time both due to the formation of adsorption and oxide films and as a result of hydrogenation of the metal. This is true because any increase in the

concentrations of adsorbed and absorbed hydrogen leads to changes in the initial values of Epr and HzO~M [9] and,

therefore, affects the rate of reaction. The goal of the present paper is to establish kinetic dependences of hydrogen release from the metal surface

near the crack tip. For this purpose, we study the overvoltage of the reaction, determine the capacity of the electric

double layer, and find the activation energy (Ea) of the reaction both on the NFS and for various concentrations of

hydrogen dissolved in the metal (CHabs). The tests were carried out in a 3 % NaC1 solution with pH7 and in a solution acidified to pH 2. To eliminate the influence of oxygen on the kinetics of the reaction, the solution was blown through with hydrogen prior to and during the tests. NFS were formed by fracturing hardened U8 steel specimens having V-shaped stress concentrators. The outer surface of the specimens was insulated with a water- proof varnish. The varnish having dried, the specimens were notched over the tip of the concentrator. At the time of formation of a new surface, the (preset) metal potential was shifted 200 mV in the negative direction as com- pared to the equilibrium potential of the hydrogen electrode. In some cases, the surface was cleaned by grinding the metal in a hydrogen-bubbled solution at a potential equal to that for the NFS. Our tests demonstrate that the

procedure for obtaining clean surfaces has absolutely no effect on the shape of the q0 - lg i curves; the spread in numerical data is explained by an error in the estimation of the area of the surface. The roughness (texture) factor of the fracture surface was assumed to be equal to three.

Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Mater- ialov, Vol. 30, No. 1, pp. 53-60, January-February, 1994. Original article submitted September 25, 1992.

50 1068-820X/94/3001-0050 $12.50 © 1995 Plenum Publishing Corporation

KINETICS OF HYDROGEN RELEASE ON NEWLY FORMED AND HYDROGENATED METAL SURFACES 51

The activation energy (Ea) of the reaction was estimated by the temperature-kinetic method in the tempera-

ture range 292 - 318 K. The value of Ea on the NFS and its dependence on the degree of hydrogenation of the NFS

were estimated on the same fracture of the specimen. The specimens were hydrogen-charged at 1] = 350 mV in the

solutions under investigation. The concentration of absorbed hydrogen was assumed to be proportional to the

amount of hydrogen released on the metal surface. In the present work, the values of the electrode potentials are

always referred to the potential of a silver chloride electrode.

• o .<_ 0 7

,_~ -0.1 z oa 0.6

-0 .2 0.5

0.4

- 0.3 0.3 6

- 0.4 0.2

-0 .5 0.1

0

-0 .6 -0.1

07 I 32 33 34 32 33 34

1/T" 10-4, K -t 1/T" 10-4, K -1

F i g . 1 F i g . 2

F i g . 1. Temperature-kinetic dependences of hydrogen release rates in a 3 % NaC1 solution with pH 7: on newly formed surfaces (curves 1-4) and after hydrogenation for 10 min (curves 5-8) and for 150 min (curves 9-12) at various values of the potential q0: 1, 5, 9-0.80 V; 2,6, 10-0.85V; 3,7, l l -0.90V; 4,8, 12-0.95V.

F i g . 2. Temperature-kinetic dependences of hydrogen release rates in a 3 % NaCI solution with pH 2: on NFS (curves 1-6) and after hydrogenation for 150 min (curves 7-11) at various values of the potential q0: 1, 7 - 0.675 V; 2, 8 - 0.650 V; 3, 9 - 0.625 V; 4, 10 - 0.600 V; 5, 11 - 0.575 V; 6 - 0.550 V.

The activation energy of the reaction was found from the slopes of temperature-kinetic curves of hydrogen re-

lease rates (tan o~ =Ea /4 .57 ) at various values of the electrode potential of the metal in NaC1 solutions with pH 7

and p H 2 (Figs. 1 and 2). For both solutions, the value of Ea on the NFS is 1 . 8 - 2 k c a l / m o l e and remains almost

constant as the potential shifts toward the cathode side. The absence of an electrochemical component in the param-

eter E a ( E a = A g + (oF, where Ag is the difference between the chemical potentials of protons in atoms and in a

solution, q0 is the jump of the potential in the Helmholtz layer, and F is the Faraday constant) indicates that the

rate of the reaction on the NFS is controlled solely by diffusion processes. Earlier, similar dependences were estab-

lished in [2] for the reaction of proton discharge on highly strained iron specimens (5 _> 20 %). The hydrogenation

of an NFS increases the value of Ea, which is a function of the concentration of hydrogen absorbed by the metal.

For an NaC1 solution with pH 2, E a takes a constant value of 5 kcal/mole e provided that the metal is saturated with

hydrogen; for a solution with pH 7, the curves E a - CHabs take a max imum value ( E a = 6.4kcal /mole) and then

decrease to the value characterizing the reaction in acid solution (Ea = 5.0 kcal/mole). To obtain direct estimates of reaction rates for various degrees hydrogenation, the experimental data (Figs. 1

and 2) were replotted in the coordinates q0 - log i (Figs. 3 and 4). By comparing the results, we conclude that the

dependence of the reaction rate on the concentration of hydrogen dissolved in the metal correlates with the

52 I . I . DYKYI, I. I. VASYLENKO, AND I. M . PROTSIV

analogous dependence E a - CHab s- The highest rate is observed on the NFS in a solution with pH 2; a s CHabs

increases, it decreases linearly. For example, for the maximum value of CHaw the current i was 3.9 and 2.9 times

less than that recorded on the NFS at the potentials - 600 and - 650 mV, respectively. In an NaC1 solution with

pH 7, the hydrogen release rate is much lower than in a solution with pH 2 and the same overvoltage although the

activation energies E a in these media are equal. Hydrogenation of the metal also slows down the reaction but, as in

the case of the dependence E a - CHaw the function log i - CHabs has a maximum, i.e., the reaction rate starts to

increase again beginning with a certain concentration of hydrogen dissolved in the metal.

-0.75 >.

-O.2O

-a#5

-0.60

-0.55 I I

0 8,2

/

/

/

/

I O.4

y/

/, ¢

I

0,¢ lgi [A/m 2]

Fig. 3. Polarization curves of hydrogen release on an iron surface in a 3 % NaCI solution (pH 2): 1 - on the NFS; on the specimen sur- face hydrogenated for 2 - 10 min; 3 - 20 min; 4 - 90 rain; 5 - 200 min.

Our results indicate that each system is characterized by distinctive behavior of the reaction kinetics as a

function of the degree of hydrogenation of a metal. Data on hydrogen release rates on oxidized surfaces of Armco

iron specimens in 3% NaC1 (pH2) and HC1 (pH2) solutions serve as additional evidence of this observation. In

the first solution, the dependence of i on CH,bs is similar to that on an NFS but the quantitative changes are much

smaller (Fig. 5). In an HC1 solution, the influence of hydrogenation on the reaction kinetics may be different. At

low potentials, hydrogenation speeds up the reaction while, at high potentials, the reaction is slowed down. The

higher the value of CHabs, the lower the potentials at which the reaction is suppressed (Fig. 6).

Fig. 4.

- 0.95 >-

- 0.85

- 0.80

- 0.75

I

I ! [ ! I I

- 1 . 0 - 0 . 9 - 0 . 8 - 0 , 7 - 0 . 6 - 0 . 5 lg i [ A / m 2 ]

Polarization curves of hydrogen release on an iron surface in a 3 % NaCI solution (pH 7): 1 - on the NFS; on the specimen sur- face hydrogenated for x : 2 - 10 min; 3 - 90 rain; 4 - 150 min.

KINETICS OF HYDROGEN RELEASE ON NEWLY FORMED AND HYDROGENATED ]~IETAL SURFACES 53

>

~ t 7 u . 1

0.6

0.5

k ^ 4 ¢ / ; ~ 0 .7

./j2y , 0.6

L 1~: i n s ~2w x ~ ~ L / J x x l

i . . . . . . .

- L 0 -0.5 0 0.5 i.0

I / 7

; R I

- i .0 -0.5 0

Ig i [A/m 2] M i

0.5 1.0

Fig. 5 Fig. 6

Fig. 5.

Fig. 6.

Polarization curves of hydrogen release on an iron surface in a 3 % NaCl + HCI solution with pH 2: ! - on the original surface;

on the surface hydrogenated for (ihyd = 0.3 mA) "~: 2 - 1(~nin; 3 - 30min; 4 - 45 min; 5 - 60 min; 6 - 90 min.

Polarization curves of hydrogen release on an iron surface in a 3 % HC1 solution with pH 2 : 1 - on the original surface; on the

surface hydrogenated for x: 2 - 10 min; 3 - 30 rnin; 4 - 60 min; 5 - 90 rain.

The q~ - C curves were taken from the RC scheme by the method of overvoltage decay after switching-off the cathode current (44 kHz). As can be seen from these curves, not only the capacity but also its behavior depends on the state of the surface apd on the nature of the solution (Fig. 7). In the case where an NaC1 solution with pH 7

interacts with the NFS, the capacity and, hence, the space factor of the electrochemical reagent ( Q ) do not change within the potential range - 700 - (- 900) mV.

For a hydrogenated surface, the function q0 - C indicates that the transition from one value of the potential to

another is accompanied by an increase in the electrode capacity and the absolute values of C are higher than for a pure surface for the entire potential range under investigation.

6O

e¢5

~5

8o

7o

go I I I

-ag -0.8 ~ V -02

® I [ I

-g.~ -o.6o ~, V -o.55

F i g . 7 . Dependence of the differential capacity of an iron electrode on the potential in a 3 % NaC1 solution; (a) with pH 7 : 1 - NFS,

2 - hydrogenation for 90 min; (b) with pH 2 : 1 - NFS, 2 - hydrogenation for 90 min, 3 - hydrogenation for t50 min.

54 I . I . DYKYI, I. I. VASYLENKO, AND I. M. PROTSIV

An additional indication of different types of dependences Q - q) on an NFS and on a hydrogenated metal sur- face is given by data on the time of stabilization of the metal potential (overvoltage) when the current density

changes from one value to another. If Q is constant, the time of stabilization of the potential is mainly determined

by the rate of change of the double-layer charge; at same time, changes in Q increase the period needed for the potential to take its new value. The first dependence is typical of an NFS, the second of a hydrogenated metal. In a 3 % NaC1 solution with pH 2, the capacity of the electric double layer is high on an NFS; hydrogenation decreases its value.

In the potential range studied, the value of C and, hence, the parameter Q are controlled by the potential, and

the correlation between C and q0 is higher for a hydrogenated surface than for an NFS. An increase in the concen-

tration of hydrogen dissolved in the metal at q~ = const leads to a drop in the electric double layer capacity. The discharge of hydrogen ions is a heterogeneous reaction; its rate can be described, for convenience, by the

kinetic equation v k = K [H ÷ ] [e-] of a bimolecular reaction according to which the magnitude of the Faraday cur- rent is proportional to the concentrations of discharging particles and electrons that participate in the reaction. This approach reflects the mixed nature of the control over the kinetics of the reaction as a function of the properties of the medium and the metal surface; this means that the reaction rate can be controlled both by mass transport of dis-

charging particles toward the electrode surface and by electron transport. The constant K appearing in this equation

depends on the structure of the electric double layer. Different values of b on the NFS in solutions with pH 2 and pH 7 indicate that the contribution of charge transport to the proton atomization rate is not the same for these systems. If we suppose electron transport to be responsible for the reaction rate in a solution with pH 2 at

b = 240 mV (the hydrogenated surface), then the role of mass transport would be much higher for the NFS. In other words, the rate of the reaction on the NFS is largely restricted by insufficient concentration of particles discharging on the metal surface. The lack of electron acceptors causes a situation where, under conditions of cathodic polarization, some of the of additional high-energy electrons are responsible for electrode charging and do

not participate in the electrochemical process. For this reason, the q0 - log i curve is steeper on the NFS than on the hydrogenated surface.

All that has been said about the reaction kinetics on the NFS in a solution with pH 2 remains true for a solution

with pH 7. Judging from the magnitude of the Tafel constant b = 1600 mV, the NFS behaves in this case like an ideally polarizing electrode, i.e., the rate of electrochemical discharge is not affected by the potential but is governed by the exchange current [4]; this means that the reaction is controlled only by diffusion. An increase in the concen- tration of hydrogen dissolved in the metal to a certain level increases the portion of the delivered electricity that

participates in reduction (b = 600 mV), i.e., the control becomes mixed with a pronounced kinetic component. The changes in the type of control and in the kinetics of the reaction observed in the investigated solutions on

passing from the NFS to the hydrogenated metal surface can be explained by the influence of dissolved hydrogen on the work function of escape of electrons in view of the quantum-mechanical effect of radiationless transition of an electron to a discharging particle [9, 10] and the data on the capacity of the electric double layer. Thus, the hydro-

genation of a metal increases the work function of escape of electrons into a vacuum (qbm) [9], and the latter is con- nected with the work function of escape of electrons into solution by the relation [6]

H2OdPm = qb m - e m V H 2 0 - H2oUv,

where Vn2 o is the Volta potential and U v is the energy of electron-solvent interaction.

As follows from the formula, hydrogenation suppresses the emission of high-energy electrons into solution and this increases the overvoltage of the reaction and changes the contribution of the diffusion and kinetic components to hydrogen release rate. At the same time, according to the Franck-Condon principle, the probability of an elemen- tary discharge is not equal to one in the general case but depends on the overlap of the wave functions of the initial and final states of the electron. The greater the distance of a reacting particle from the electrode surface, the lower the probability of the transition of an electron; therefore, the reaction rate is maximum only for direct adsorption of ions on the metal. An increase in the surface concentration of hydrogen adatoms in the process of hydrogen charg- ing of a metal leads to adsorption of particles discharging on a substrate; this increases their distance from the metal

KINETICS OF HYDROGEN RELEASE ON NEWLY FORMED AND HYDROGENATED METAL SURFACES 55

surface and, therefore, decreases the probability of electron transitions. The measurements of the differential capacity of the electric double layer in an NaC1 solution with pH 2 de-

monstrate that its structure changes in the hydrogenated metal and these changes depend on CHabs (Fig.7). If the

electric double layer is treated as a plane capacitor, then the decrease in its capacity C = ( ~ - ~0) / d in the

hydrogenated metal may be caused by changes in its electric permittivity (~) or in the distance between its plates

(d). However, for the same medium and the same type of discharging particles, the value of C is mainly deter- mined by the parameter d. Hence, the decrease in the capacity of the electric double layer (e.g., for an NFS

potential of 600 mV) from 62 mF/cm 2 to 48 mF/cm 2 caused by hydrogenation is explained by an increase in the distance between a proton and the metal surface. Thus, the increase in the work function of escape of an electron into solution together with the decrease in the probability of an electron transition decelerates the cathodic process of hydrogen release from a solution with pH 2 and, at the same time, gives rise to the kinetic component of reaction control on the hydrogenated surface, in view of certain distinctive features of hydrogen release from neutral media, this explanation is also applicable to a 3 % NaC1 solution with pH 7. For example, the effect of dissolved hydrogen on the work function of escape of electrons must be independent of the properties of the medium; therefore, the asymmetry in the reaction rates for different degrees of hydrogenation results from changes in the structure of the electric double layer, which is demonstrated by experimental data (Fig. 7). Unlike an NaC1 solution with pH2,

where the hydrogenation of the NFS reduces the capacity C, in a 3 % NaC1 solution with pH 7, hydrogenation has

the opposite effect. At low CHabs, one observes an insignificant increase in the capacity; at a certain concentration

of hydrogen dissolved in the metal, C increases sharply.

Different types of behavior exhibited by the capacity and the reaction kinetics in the investigated solutions for various degrees of hydrogenation can probably be explained by a difference in the structure of water molecules on the NFS and the hydrogenated surfaces. An effect of the molecular structure of water on the reaction rate is observ- ed even for the same state of the metal provided that cathodic polarization is sufficiently strong ( q~ > - 950 mV) that changes in the electrode charge result in structural reconstruction of water molecules. Experimental data on electron reflectance spectra [11] corroborate the decisive role played by the structure of the electric double layer in the process of molecular cischarge and in changes in its structure as the surface charge increases. It is worth noting that a transition from suppression of the reaction to its acceleration accompanied by an abrupt increase in the double layer capacity occurs at b = 600 mV when reaction control is of mixed type with a significant contribution of the kinetic component.

If saturation of the metal surface with hydrogen caused only an increase in the work function of escape of an electron and there were no other effects associated with the reaction kinetics, the polarization curves would be parallel; on the contrary, according to the experimental data, they have different slopes. Different values of the

Tafel constant b indicate that the asymmetry of the reaction energy barrier is violated. The magnitude of the

relevant changes depends on the metal potential. Since, at any potential, the reaction rate (It) is the resultant one

and is equal to the difference between the rates of the direct (ic) and back (ia) reactions (in our situation, the lat-

ter is the ionization of hydrogen atoms), the variation of the constant b is a consequence of unequal contributions of the direct and back reactions at different values of the potential. The contribution of the back reaction to the overall process is determined by the energy barrier: The lower the barrier for the direct reaction, the higher the barrier for the back one.

This assertion can be illustrated by the Br6nsted-Polanyi formula Ea = Ea Hg - 1/2 ( E M e - H - EHg-H), which in- cludes the binding energy of an adsorbed hydrogen atom on the metal surface and the activation energy of the reac-

tion of hydrogen overvoltage. Thus, the calculation of Ea according to this formula with EHg-H = 29.6 kcal/mole,

EFe-H = 47 kcal/mole, and Ea I4g (1"1 = 0) = 23.2 kcal/mole [12] shows that the activation energy of the reaction on iron coated with an adsorptive oxygen film is 14.5 kcal/mole in a good agreement with the experimental value (see [2]). This formula also implies that the lower the energy of adsorption of atoms on the metal, the higher the energy barrier for the direct reaction and the lower the barrier for the back reaction.

For the NFS, the high energy barrier of the back reaction suppresses the H --~ H ÷ transition and the role of ic in the resultant rate Ir is maximum. An increase in the concentration of surface of atoms makes the back reaction more important (the heat of absorption decreases); as a result, Ir drops, i.e., the overvoltage increases and the con-

56 I. I. DYKYI, I. I. VASYLENKO, AND ][. M. PROTSIV

stant b changes its value. For the maximum concentration of adatoms on the surface, the contribution of the back

reaction becomes stable due to the properties of the system and the value of % It should be taken into account that

the greater contribution of i a to the kinetics of the overall process is explained not only by the lower value of the

parameter Ea of the back reaction but also by the growth of the surface concentration of the reduced form because

the rate of the direct reaction ia is proportional to the content of hydrogen absorbed by the metal Cnabs.

For fixed values of the potential and CHabs , the ratio ic / ia is a distinctive characteristic that reflects the bal-

ance between the direct and back reactions. This balance can be shifted either by changing the potential q~ for

CHab~ = const or by changing the concentration CI-I,b~ for q0 = const. Note that a shift in q) toward negative

values changes the balance toward suppression of ionization; moreover, the greater the shift, the more significant

these changes.

Thus, the small work function of escape of an electron into solution and the maximum heat of adsorption of hy-

drogen atoms on the NFS are the main factors that govern the diffusion control over the atomization of protons on

high-energy surfaces. Surface hydrogenation increases the work function of escape and decreases the probability of

transition of an electron from the metal into solution in view of the changes in the structure of electric double layer;

this is why it increases the overvoltage of the direct reaction of proton discharge and decreases the overvoltage of

the back reaction of ionization of hydrogen atoms. Different contributions of the rates of the direct and back re-

actions (which depend on the concentration of absorbed hydrogen and the electrode potential of the metal) lead to

different slopes of the polarization curves for the NFS and the hydrogenated surfaces.

R E F E R E N C E S

1. I. 1. Dykyi and I. M. Protsiv, "Kinetics of hydrogen release on strained iron surfaces," Zashch. Met., No. 1, 42-44 (1994). 2. I.I. Dykyi and I. M. Protsiv, "Changes in the activation energy and the rate of hydrogen release from neutral media due to deforma-

tion," Fiz.-Khim. Mekh. Mater., 29, No. 6, 16-22 (1993). 3. L.I. Antropov, Theoretical Electrochemistry [in Russian], Vysshaya Shkola, Moscow (1969). 4. V.V. Skorchelletti, Theoretical Electrochemistry [in Russian], Khimiya, Leningrad (1974). 5. N.E. Khomutov, "Hydrogen overvoltage and the multiplet theory of catalysis due to A. A. Balandin. Electrochemical studies,"

Trudy Moskov. Khim. Tekhn. Inst., XXXII, 120-124 (1961). 6. G.A. Martynov and R. R. Salem, "On the mechanism of the reaction of hydrogen depolarization," Zashch. Met., No. 2, 221-228

(1985). 7. B.M. Tsarev, Contact Voltage [in Russian], Vysshaya Shkola, Moscow; Leningrad (1949). 8. K. Vetter, Electrochemical Kinetics [Russian translation], Khimiya, Moscow (1967). 9. N.A. Galaktionova, Hydrogen in Metals [in Russian], Metallurgiya, Moscow (1967).

10. A.M. Brodskii, Yu. Ya. Gurevich, Yu. V. Pleskov, and Z. A. Rotenberg, Modern Photoelectrochemistry. Photoemission Phenom-

ena [in Russian], Nauka, Moscow (1974). 11. R.I. Lazorenko-Manevich, L. A. Sokolova, and Ya. M. Kolotyrkin, "Modulation-electroscopic study of adsorption on electrodes.

Acidity of water adsorbed on iron," Elektrokhimiya, 14, No. 12, 1779-1786 (1978). 12. L.I. Krishtalik, Electrode Reactions. Mechanisms of Primary Processes [in Russian], Nauka, Moscow (1979).