H2SO4 electrode system

H2SO4 electrode system

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001344sf.lsss3.00+0.00 Rue Ltd.

1986. Pa(rmoa

MECHANISM OF THE ACTION OF Ag AND As ON THE ANODIC CORROSION OF LEAD AND OXYGEN EVOLUTION AT THE Pb/PbO(z_,,/H20/02/HzS04 ELECTRODE SYSTEM D. PAVLOV

and T. ROGACHEV

Central Laboratory of Electrochemical Power Sources, Bulgarian Academy of Sciences, Sofia 1040, Bulgaria

(Received 3 June 1985; in revised form 13 August 1985) Abstract-When a Pb electrode, immersed in HzSO, solution, is polarized anodically in the PbO2 potential range the Pb/PbO~Z_,,lHz0/02/HIS01 electrode system is established. Oxygen is evolved at the oxidesolution interface The oxygen atoms formed as intermediates diffuse into the anodic layer and oxidize the metal. Through a solid-state reaction, the metal is oxidized first to ret-PbO and then to Pb02. By studying the changes in the rate-potential relations of the above reactions, as well as the phase and chemical composition of the anodic layer, it was possible to elucidate the effect of Ag and As on these processes. The additives were introduced into the electrode system either by alloying with lead or by dissolving them in the HISOI solution. When added to the solution, both Ag and As lower the overvoltage of the oxygen evolution reaction. They have practically no effect on the corrosion reaction under galvanostatic polarization conditions. If alloyed in the metal, Ag reduces the oxidation rate of Pb significantly, while As enhances it. Both additives lower the stoichiometric number of the anodic oxide layer, k they retard the oxidation of PbO to PbOl. The results of these investigations were used to develop further the model of the mechanism of the reactions proceeding during the anodic oxidation of lead in H,SO, solutions.

INTRODUCTION During the anodic polarization of lead and lead alloy electrodes immersed in H2S04 solutions and polarized in the PbOl potential region (potentials 0.95 V more positive us the Hg/Hg,SO* reference) an oxide anodic layer is formed. The electrochemical properties of this electrode are related to the proeesses at the two and anodic metal-anodic interfaces: layer layer-H$O, solution. The anodic oxidation of the metal occurs at the first interface and oxygen is evolved at the second one. As a result, the following electrode Pb/non-stoichiometric system is established: Pb0JH20/02/H2S04[1]. The effect of the nature of the metal on these two reactions has not only a theoretical but also a practical aspect. The aim of this paper is to disclose the effect of silver and arsenic on the electrochemical and corrosion properties of the lead electrode polarized in the PbOl potential region. Arsenic is used as an alloying element in Pb-Sb alloys for lead acid battery grids and has been extensively studied with respect to this application[243]. Silver is introduced as an additive to enhance the corrosion stability of anodes in the electrowinning of Zn and Cd. In this connection, as well as with respect to its use as an alloying metal in some long-lived traction lead acid batteries, silver is often investigated since it also has a very pronounced inhibitory effect[f 9-183.

EXPERIMENTAL The effect of Ag and As on the electrochemical behaviour of the Pb/non-stoichiometric Pb02/H20/ 241

02/HIS0, electrode system was revealed by following the influence of these additives on the partial steady-state polarization curves of the two reactions. The polarization curves were obtained at plane electrodes (6.5 cm’) under galvanostatic conditions in 7 N H2S04 solutions. The electrode potential was measured us a Hg/Hg,S04 reference. The partial steady-state current for the metal oxidation was determined by a weight-loss method, developed in our earlier papersCl5, 163. The rate of the oxygen evolution reaction was calculated from the difference between the total polarization current and the anodic corrosion current. In view of the high conductivity of the anodic layer, it was assumed that there is no ohmic drop in this layer. Consequently, the electrode potential recorded refers to both electrode reactions. The phase composition of the anodic layer was assessed by X-ray diffraction. After the polarization the electrodes were washed, dried and X-rayed. This Xray pattern reflects the phase composition of the anodic layer at the oxide-solution interface. Then the main part of the oxide layer was brushed away, leaving only a thin oxide film firmly attached to the metal. The electrode was then X-rayed again. This diffractogram yields the phase composition of the anodic layer at the metal-oxide interface. In this manner, the potential dependence of the phase composition at both interfaces was followed. Lead alloys with varying content of Ag or As were oxidized. The alloys were prepared from pure metals: Pb99.999 %, As and Ag-99.99 %. Master alloys with high Ag and As content were obtained first. The latter were used in the preparation of alloys with different concentrations of Ag and As. The exact concentration of the alloying elements was determined by chemical analysis.

242

D. PAVLOVAND T. RESULTS

Efict of silver on the electrochemical behaviour of the lead electrode The effect of silver on the oxygen evolution and on the anodic corrosion of the metal was assessed by two methods. In the first, silver was alloyed in the lead electrode and the electrochemistry of the latter in HrS04 solution was studied. In the second, silver was added to the H2S04 solution (as Ag,S04) and pure lead electrodes were investigated. In the latter case, during anodic polarization of the test electrode, silver is deposited on the counter-electrode. In order to maintain the concentration of Ag+ in the solution within definite limits (0.03-0.06 g I- ’ ), silver was added twice a day. The Ag+ concentration in the solution was

monitored daily by chemical analysis. Figure 1 presents the current-voltage characteristics of the oxygen evolution and anodic corrosion reactions for pure lead electrodes immersed in an Agfcontaining HZS04 solution as well as for a Pb-O.58 % Ag electrode in pure H2S04 solution. To understand the effect of Ag on the abovementioned electrode systems, their V-A relationships should be compared with those of a pure Pb electrode in HzS04 solution. These relationships are indicated by dashed lines in Fig. 1 and are taken from previous work[19] where the measurements were carried out using the same method and under the same conditions as in the present study.

ROGACHEV

The oxygen overvoltage is reduced upon introduction of silver in the lead electrode. The polarization curve of the Pb-O.58% Ag electrode is shifted by 30-4OmV in the negative direction. The addition of Ag+ ions in the solution has an even stronger effect on this reaction, since the oxygen overvoltage is reduced by SO-70 mV. This difference in the polarization curves for the two methods of Ag introduction may be assigned to the different amounts of silver present in the electrode system. What is the effect of silver on the anodic corrosion of lead? It is revealed by the rate of oxidation of the metal expressed in current-density units (Fig. 1). Let us consider the case when the polarization is carried out at a constant potential (as an example, 1500 mV). When Ag+ ions are introduced into the solution, the oxygen overvoltage decreases. To keep the potential constant at 1500 mV, the rate of oxygen evolution ( z polarization current) has to be augmented by an order of magnitude. This increases the corrosion rate of the Pb eleetrode. When silver is alloyed in the metal, the rate of oxygen evolution increases less than in the preceding case, whereas the corrosion rate of the lead-silver alloy is considerably reduced. Therefore, under constant potential polarization, the introduction of Ag by both methods accelerates the oxygen evolution reaction, but it affects differently the anodic corrosion of the metal. Comparison of the rates of both reactions (corrosion and oxygen evolution) can reveal the effect of Ag in a more clear-cut manner, eliminating the influence of the oxygen overvoltage decrease. A similar presentation is given in Fig. 2. The effect of Ag+ ions in the solution on the corrosion rate is comparable to the experimental error. A similar result is presumable if we accept that the effect of Ag+ ions is limited only to the oxide-solution

-3.0

--

/

1

-LO

/

/’

-

? E

u %

d

-I

.;”

P

-20

We,, Fig. 1. Polarization curves of the oxygen evolution reaction and corrosion reaction in the eases when Ag is introduced into the solution or into the metal. The polarization relationships of both reactions for a Pb electrode in HISOI solution were taken from[19] and are indicated by dashed lines.

L

1

-30 ,A 1

-1,

crd)

Fig. 2. Relations between the corrosion

and oxygen currents in the cases when Ag is introduced into the metal or into the solution. The dashed-line curve represents the ratio between the rates of both reactions for a pure Pb electrode in HzSO, solution (taken from[19]).

Mechanism of the action of Ag and As interface. As the oxide-metal interface is isolated from the Ag+ ions by the corrosion layer, the effect of these ions on the processes on the metal surface is negligible. When Ag is alloyed in the metal, however, the corrosion rate is reduced (in the present case) by one order of magnitude. This shows that Ag in the alloy directly affects the corrosion process occurring on the metal surface. Does the concentration of silver in the lead alloy have an effect on the rates of these reactions? Figure 3 shows the rates of both electrode reactions for alloy electrodes containing increasing amounts of Ag, compared with those for a pure Pb electrode. One can immediately recognize the cathodic shift of the oxygen polarization curves with the increase of Ag concentration in the alloy, ie the oxygen overpotential drops. The Tafel polarization plots are linear with two slopes, as in the case of pure lead. The potential (EL) where the Tafel plots change their slope decreases from 1480 to 1350mV as the content of silver in the alloy grows. The relation between the rate of the oxygen evolution reaction (io,) and that of the anodic corrosion

c

243

reaction (i,) is plotted in log-log coordinates in Fig. 4 for Pb and Pb-Ag electrodes. At a constant rate of the oxygen evolution reaction the addition of Ag reduces the rate of the anodic corrosion of lead by more than an order of magnitude. The increase in the Ag concentration in the alloy enhances its inhibiting effect on the corrosion reaction. It was interesting to reveal the relation between the shape of the polarization curves and the phase composition of the anodic layer. For this purpose, by means of X-ray diffraction, we determined the phase composition of the anodic layer at both interfaces: alloy-oxide and oxide-solution. The content of a given phase (a) in the anodic layer was assessed by the relative intensity of the characteristic diffraction peak I, x lm/EIn, where Zln is the sum of the intensity of the characteristic diffraction peaks of all phases present in the anodic layer, and I,, is the intensity of the diagnostic diffraction peak of phase a. Figure S(a) illustrates the distribution of the /?-PbOz phase in the zones adjacent to both interfaces, Fig. 5(b) that of ret-PbO and fl-PbOz, and Fig. 5(c) that of tetPbO and cr-Pb02. Figure 5 reveals that up to a potential, Ek, the anodic layer contains prevailing amounts of the /3-PbOt phase. Above this potential it consists mainly of c+Pb02. The juxtaposition of Fig. S with Fig. 3 demonstrates that the change in the Tafel slopes is related to the alteration of the phase composition of the anodic layer at the oxidesolution interface. At E < Et, oxygen evolution takes place mainly on p-Pb02, whereas at potentials higher than Ek it occurs on a-Pb02. The b-PbOl phase is formed at both interfaces. The formation of &PbOz in the oxide layer next to the metal in the case of the Pb-Ag alloys is most probably induced by the presence of silver. This conclusion is supported by the finding that both the amount of p-Pb02 at the metal surface and the

- 3.0 -L.O i -5.0

1 L

1402

1YX

182

Figure 3. Current-voltage relations of the oxygen evolution reaction and the corrosion reaction for lead electrodes alloyed with different amounts of silver. The polarization relationships of both reactions for a Pb electrode in H2S01 solution were taken from[l9] and are represented by dashed fines.

-LO

-2D

-3.0

lgti ,+ .~A.cm-~)

Fig. 4. Relations between the corrosion and oxygen currents for lead electrodes alloyed with different amounts of silver. The dashed-line curve represents the ratio between the rates of both reactions for pure Pb in HpSO* solution (taken from[19]).

244

D.

PAVLOV

AND T.

R~GACHEV

potential range of its formation grow with increasing silver concentration, and also by the fact that in the case of pure Pb electrodes no &PbOz is formed at the metal-oxide interface. The diffraction peak ford = 2.78 nm is common for at the tet-PbO and /3-PbOz. PbO is unstable oxide-solution interface in H1S04 solution, and con-

,_

PtQ/solutIon

-

Fb-AglPbO,

CC-Pb02+tet-PbO

-

Pb-AgtPb02 PbOZ/solutm

LO

Ip, mV (vs. Hg/Hg2s04

20

)

1 011

c 1300 ‘p, mV,(vs

I

14XJ HglHg2S04)

1500

Fig. 5. Relationship of the relative intensity of the X-ray characteristic diffraction lines at both interfaces of Pb-Ag electrodes us oxidation potential. (a) fi-PbOz diffraction line with d = 3.50 nm; (b) t&PbO and B-PbOZ diffraction line, d = 2.79 nm; (c) tet-PbO and a-Pb02 diffraction line, d = 3.12nm.

sequently the intensity of the diffraction peak at this interface will be mainly determined by the content of/?PbOZ. Figure 5(a) shows that the content of j?-Pb02 at the metal-oxide interface is comparatively small. Hence, the intensity of the 2.78nm peak will be determined mainly by tet-PbO. It is, however, not possible to determine exactly the concentrations of tet-PbO and PbOl in the anodic layer on the basis of the X-ray diffraction analysis alone. Therefore we assessed by chemical analysis the stoichiometric number n of the oxide layer PbO,, the number of which expresses the ratio of the concentrations of tet-PbO and PbOz[19]. Table 1 presents the values of the stoichiometric number n of the oxide Table 1. Dependence of the stoichiometric number n in PbO. on the content of Ag in PbAg alloy electrodes 1300

1400 Ip, mV (~5

HglHg2%&)

1500

% Ag n

0.05 1.50

0.10 1.55

0.28 1.55

0.50 1.55

0.65 1.57

1.0 1.60

3.0 1.60

245

of the action of Ag and As

Mechanism

layer obtained by anodic oxidation of Pb-Ag alloys at 6 mA cm- 2 for 200 h as a function of the content of Ag. The stoichiometric numbers presented are overall values since the chemical analysis was performed for the total amount of the oxide layer. Figure 5 reveals that the content of PbO and Pb02 varies along the depth of the oxide layer. The variation has not been accounted for in the data of Table 1. The stoichiometric number increases with the content of silver. At the same cd, the stoichiometric number of the oxide obtained on a pure lead electrode is 1.7ql9], ie larger than that of the alloys. This implies that the introduction of silver inhibits the oxidation of PbO to Pb02. Effect of arsenic on the electrochemical behaviottr of the lead electrode The effect of As on the electrode reactions of Pb was studied by methods similar to those used in the case of Ag. Pb electrodes were polarized anodically in HzS04 solutions containing 0.01 N As2(SO& The decrease of As concentration in the solution was compensated by the daily addition of As5+ ions in amounts according to the data of the chemical analysis. We also studied electrodes of PbAs alloys in pure H2S04 solutions. Figure 6 shows the potential-reaction rate relations for both types of electrode. The relation for the pure Pb electrode in HxS04 is from our previous paper[19]. It is seen that As reduces the oxygen overvoltage as the polarization plots in both cases shift to more negative potentials. It is difficult to decide to what extent this shift is affected by the method of introduc-

-1.0

/’ -20

ing As in the electrode system or by the amount of As in the solution or in the alloy. The introduction of As also affects the rate of the anodic corrosion reaction. If we consider the three electrodes at one and the same potential, eg at 1500 mV, then it becomes apparent that the Ass+ ions added to the solution increase the corrosion rate only very slightly (and not at all at higher potentials), while when introduced into the metal (x 0.5 O/QAs enhances the corrosion rate by one order of magnitude. Figure 7 juxtaposes the rates of the oxygen evolution and the anodic corrosion reactions for the above types of electrode. The addition of As’+ ions to the H2S04 solution has practically no effect on the corrosion rate of the lead electrode. It is only at very high rates of the oxygen evolution reaction that these ions reduce the corrosion rate very slightly, as is seen from the change in the shape of the curves (Fig. 7). When introduced into the metal, As enhances the anodic corrosion of the lead electrode. Figure 8 depicts the potential-reaction rate relations of the two processes for electrodes prepared from Pb-As alloys with various As content, while Fig 9 juxtaposes the rates of these processes. The relations in these figures reveal that the increase of As content in the alloy enhances the rates of the oxygen evolution and the anodic corrosion of the alloys. Figure 10 presents the X-ray diffraction patterns revealing the phase composition of the anodic layer in and the zones adjacent to the metal-oxide oxide-solution interfaces as follows: (a) the content of /?-Pb02, (b) that of tet-PbO and &PbOl, and (c) that of tet-PbO and u-Pb02. As in the case of the Pb-Ag alloys, in the lower potential range /LPb02 is found in the anodic layer at both interfaces, its content being larger at the oxidesolution interface. In the higher potential range, the content of /I-Pb02 diminishes at the expense of the increase of a-Pb02, which above certain anodic potentials is the prevailing phase in the anodic layer.

“:

E

1

4 1

g-3.0

P’ pb - O.U% As

-4.0

- 5.0

:

l

/ -2.0

-3.0

I&,

Fig. 6. Polarization ctu-ves of the oxygen evolution reaction

and the corrosion reaction in the cases when As is introduced into the solution or into the metal. The polarization curves of both reactions for a Pb electrode in H$O, solution were taken from[19]

and are represented by dashed lines.

-1.

1 0

.IA. cm-2)

Fig. 7. Relations between the corrosion and oxygen currents for the electrodes when As is introduced into the metal or into the solution. The dashed-line curve represents the ratio between the rates of both reactions for a pure Pb electrode in H$30., solution (taken from[19]).

D. PAV~V

AND

T.

ROGACHEV

This transition is related to the change in the Tafel slope (Figs 6 and 8). The peak corresponding to d = 3.12 nm is common for both tet-PbO and a-PbO,. At very high anodic polarizations, the increase in its intensity in the bulk of the oxide layer is probably caused by the oxidation of tet-PbO to a-PbOz. In the range of lower anodic potentials, the decay of the 3.12 nm peak is probably due to the oxidation of tetPbO to p-PbOa. The overall stoichiometric number n of the oxide PbO, was determined by chemical analysis of the total Lo-

d=3.WA.

DPW*

63 -

PbAs/Pt02

20

B t t

40-

5

zo-

s 2

0

[O.lO% As ] \p, mV (K.

Hg/Hg2SOL

1

Fig. 8. Current-voltage relations of the oxygen evolution reaction and the corrosion reaction for electrodes alloyed with different amounts of As. The curves of both reactions for a Pb electrode in HaSO, solution were taken from[19] and are represented by dashed lines.

--

Pb

._

Fb - 010 % As

I

Pb-03o%as

-30 _ -

-10

Pb-056%&

/’ -LO

-20

-30 Wio,

, IA cm-3

Fig. 9. Relation between the oxygen and corrosion currents for electrodes alloyed with different amounts of As. The dashed-line curve represents the ratio between the rates of both reactions for a pure Pb electrode in H2S04 solution (taken from [19]).

1400

15co ‘p. mV (vs +lHgZSOLJ

Fig. 10. (a) and (b).

1600

7 (

Mechanism

of the action of Ag and As

247

DISCUSSION

-

Pq,/solutim

-

PbAs/p

Kabanov and co-workers[20] advanced the idea that the anodic corrosion of lead is caused by oxygen, which penetrates across the anodic layer. In two Papers we developed further this idea[l9,21], which can be illustrated by the model in Fig. 11. The oxygen evolution reaction occurs at the anodic layer-solution interface, whereby 0 atoms are obtained as intermediate products. 2H20+20H+2H++2e-

(1)

20H+O+Hz0

(2)

20 + 02.

(3)

Some of the 0 atoms diffuse into the anodic layer and on reaching the metal they oxidize it to retPbO[Z I]: Pb + 0 + tet-PbO .

1

m tet-PbO + p 0 -+ ret-PbO..

ret-PbO,, + a-PbO(, _ =).

oxide

layer

H2504 I

I

e-

ZOHO I

)_-----7-------

Pb

I

I

1 < ”
(6)

a-PbOf2_., is a non-stoichiometric oxide with the crystal lattice of a-PbOzCl.4 < (2 -x) < 21. Up to a certain potential, /3-PbOz is formed at the oxide-solution interface[l9]. This formation probably occurs when the solution takes part in the oxidation of tet-PbO. The question which arises here is: where does the action of Ag and As come in? The experimental data in Figs 1, 3, 6 and 8 demonstrate that both Ag and As reduce the overpoten-

oxide in the anodic layer. The X-ray patterns in Fig. 10 exhibit a certain distribution of stoichiometry along the thickness of the anodic layer. The overall stoichiometric number varies between 1.40 and 1.65, implying that As, like Ag, inhibits the oxidation of PbO to PbOs. Its effect on this reaction is, however, weaker than that of Ag.

Anodic

(5)

The stoichiometric number n increases up to about 1.4, whereby the concentration of defects in the crystal lattice becomes so large that the crystal lattice is transformed to that of a-Pb02[22]. The similarity between the crystal structures of ret-PbO and a-PbOZ is considerable so that the energy of phase transformation is very low.

Fig. 10. Relationship of the relative intensity of the X-ray characteristic diffraction lines at both interfaces of PbAs electrodes vs oxidation potential. (a) @-Pb02 diffraction line with d = 3.50 nm; (b) tet-PbO and gPbO,, diffraction line d = 2.79 nm; (c) ret-PbO and a-PbO,. diffraction line d = 3.12nm.

Pb

(4)

The energy for the phase transformation of Pb to ret-PbO is comparatively low, so that naturally this crystal modification is obtained. There are empty layers in the crystal lattice of tetPbO where 0 can be easily inserted[21,22]. The latter oxidizes ret-PbO to non-stoichiometric ret-PnO. (1 < n -z 1.4) without changing the crystal lattice parameter of tet-PbO[22].

I

-1.4<

k<

2

I

Fig. 11. Model of the processes in the Pb/non-stoichiometric Pb02/02/H2O/HaS04 during oxidation in the PbOl potential region.

I

electrode system

D. PAVLOVANDT. ROGACHEV

248

tial of the oxygen evolution reaction. When these additives are introduced into the solution, they most likely interact with the oxide surface, thus changing its nature facilitating reactions (1) and (2), thereby lowering the overvoltage of the oxygen evolution reaction. Hence, the potential range of oxygen evolution shifts to more negative values. Figures 2 and 7 show that at one and the same rate of oxygen evolution the corrosion rate of pure lead electrodes is practically unaffected by the introduction of Ag+ and As’+ into the solution. When the polarization proceeds at a constant potential, eg at 1500 mV, the presence of Ag+ and As5+ ions in the solution accelerates the rate of oxygen evolution (Figs 1 and 6), whereby the concentration of 0 atoms at the oxide-solution interface grows. The flux of this species across the oxide layer towards the metal surface is intensified (Fig. 11). Thus, the first parameter, through which the additive in: fluences the metal corrosion rate, is the shift of the polarization curve of the oxygen evolution reaction. The experimental results, however, also reveal that the action of the Ag and As additives on the corrosion reaction is not limited only to their effect on the oxygen evolution reaction. The corrosion process (4) is a chemical reaction preceded by an electrochemical one (the evolution of oxygen). The rate of the corrosion reaction can be expressed by i, = V, = K=C,M

(7)

where K, is the rate constant for the oxidation of the metal and Cg is the concentration of the 0 species at the oxide-metal interface. This concentration is determined by its concentration at the oxide-solution interface and the diffusion rate of these species across the oxide layer. The former, in turn, depends on the electrode potential. The effect of the potential can be eliminated if we consider the corrosion process at a constant current density of the oxygen evolution reaction (Figs 2, 4, 7 and 9). If we assume one and the same thickness of the anodic layer for all the electrodes, it might be expected that the fhtx of 0 will be determined by its diffusion rates. It is worth mentioning that some of the 0 species participate in reactions (5) and (6), whereby the tIux across the anodic layer is diminished. Figures 2 and 4 show that when Ag is alloyed in the metal it reduces the corrosion rate by more than one order of magnitude. It seems quite impossible that the influence of Ag on the diffusion coefficient of 0 will be such that it brings about this large reduction in the corrosion rate. Consequently, one may argue that Ag alloyed in the metal lowers the rate constant K, of the corrosion reaction. The data in Figs 6, 7, 8 and 9 reveal the opposite effect of As, which, when introduced into the metal, accelerates the metal corrosion. Figures 7 and 9 also demonstrate that the effect of As is not as strong as that of Ag. The promoting effect of As can be assigned either to the increase in the rate constant of the corrosion reaction (4), or to changes in the diffusion coefficient of the 0 species, or to both actions. When alloyed in the metal, the additive is oxidized together with the metal. The ions of the additive are incorporated in the oxide as a solid solution, or they can form a new oxide phase. In this way the additive affects the mobility of 0 atoms in the oxide (changing

the diffusion coefficient), the rate constants of reactions (5) and (6), and the phase composition of the anodic layer. The experimental values of the stoichiometric number of the oxide layer reveal that Ag and As reduce the oxidation rate of ret-PbO to PbOl. It is hardly possible to conclude, on the basis of the results of the present overall chemical analysis of the oxide layer, upon which of reactions (5) and (6) the additives have a more significant effect. In fact, the distribution of the individual phases in the anodic layer will depend on the ratio between the rates of reactions (4), (5) and (6). If the rate of reaction (4) is higher than those of reactions (5) and (6), then during the anodic oxidation two oxide sublayers will be formed: one of tet-PbO and a heterogeneous one composed of tef-PbO,, a-PbO(, _=) and B-Pb02. Recently Bullock and ButIerr231 established such an ele&rode in the potential range 1.26-1.32V (us Hg/Ha,SO,). If the rates of reactions (5) and (6) are higher Fhan that of reaction (4), a single heterogeneous anodic layer will be formed which will contain tet-PbO, ter-PbO,, a-PbOtt -I) and /I-PbO, in a ratio determined by their rates of formation. Figures 5 and 10 demonstrate that the anodic layer contains /LPbOz over all its thickness. With pure Pb electrodes, b-Pb02 is built up only within a definite potential range at the interface with the solution, but not with that of the metal[l9]. Hence, As and Ag enhance the formation of the (Y-modification of PbOl in the anodic layer not only at the oxide-solution interface but also in the zone of the layer close to the metal. Consequently, Figs 5 and 10 show that during the oxidation of ret-PbO,, at the same time as the basic reaction of a-Pb03 formation, Ag and As create conditions for such an arrangement of the crystal lattice that p-PbO, is formed. Similar to the case with pure lead, this occurs up to a certain limiting potential Ec. This potential shifts upon increasing the amount of the additive. These findings reveal that, depending on the amount and nature of the additive, the latter acts either as a promoter or as an inhibitor of the anodic corrosion reaction of the metal, and at the same time it controls the phase composition and the properties of the anodic oxide layer.

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347 ( 1948).

3. G. W. Mao and J. G. Larson, Metallurgia 76, 236 (1968). 4. J. D. Williams, Merallurgia 74, 105 (1966). 5. A. P. Batin, A. I. Russin, M. A. Dassoyan and G. M. Ganenko, Sbornik Rab. Him. 1st. Toka, L. Energiya 7, 16 (1972). 6. V. S. Smolkova, M. A. Dassoyan, 1. A. Silitskii, Ya. E. Neitlin and V. S. Yanchenko. Sbornik Rab. Him. 1st. Toka. L. Energiya 2, 36 (1967). 7. G. V. Krivchenko, G. N. Gordyanova and I. A. Aguf, Sbornik Rab. Him. 1st. Toka, L. Energiya 8, 17 (1973). 8. D. Pavlov and T. Rogachev, Eleclrochemical Power

Mechanism

9. 10. 11. 12 13. 14. 15.

of the action of Ag and As

Sources, p. 53. Dum Techniky CVTS, Praha, i%SR (1975). 0. Heckler and H. H%meman, 2. Metallkunde 30, 410 (1938). A. E. K&i& E. MacEvans and E. Larsen, Trans. electrochem. sot. 79, 33 (1941). C. G. Fink and A. I. Dornblatt, Trans. etectrochem. Sot. 79, 269 (1941). G. Z. Kirjakov and V. V. Stender; Zn. Prikl. Khim. 24, 1263 (1951); 25, 30 (1952). I. A. Agufand M. A. Dassojan. Yestn. Elektroprom. 62(10) (1959). M. A. Dassojan and E. I. Votubueva, Yestn. Electroprom. 62(5) (1959). D. Pavlov, M. Boton and M. Stojanova, Bull. Inst. Chim. Phys. 5, 55 (1965).

249

16. D. Pavlov and T. Rogatchev, Werkstoffe Korros. 19, 677 (1968). 17. T. W. Karoleva and D. Pavlov, Rogatchev, Metolloberpdche 11, 421 (1970). 18. L. Dawson, The Electrochemistry of Lead (Edited by A. T. Kuhn), p. 327. Academic Press, London (1979). 19. D. Pavlov and T. Rogatchev, Efectrochim Acto 23,1237 (1978). 20. I. I. Astachov, E. S. Wetsberg and B. N. Katxnov, Dokl. Acad. Nauk USSR 154. 1414 (19641. 21. D. Pavlov, Electroch&r Acta &, 845 (1978). 22 D. Pavlov and Z. Dinev, J. electrochem. Sot. 127, 855 (1980). 23. K. R. Buttock and M. A. Butter, Extended Abstracts. Las Vegas Meeting of Etectrochem. Sot., U.S.A., p. 61 (1985).