The influence of halide ion concentration on the effects of surface active inhibitors in strong electrolytes

Corrosion Science, 1973, Vol. 13, pp. 697 to 706. Pergamon Press. Printed in Great Britain

T H E I N F L U E N C E OF H A L I D E I O N C O N C E N T R A T I O N O N THE E F F E C T S OF S U R F A C E ACTIVE INHIBITORS I N S T R O N G ELECTROLYTES* P. KIRKOV~ Centre for Radioisotope application in Science and Industry of SRM-Skopje, Department of Physical and Electrochemistry, Faculty of Technology and Metallurgy, University of Skopje, Yugoslavia Abstract--The effect of CI-, Br- and I - on the efficiency of diphenyl amines, thiomea and hydroxylamines in the corrosion of the low carbo~ steel alloys was investigated in aqueous solutions of H~SO4 and Na2SO4, at a total concentration between 10-3 and 5M, in the pH range 0-6'5. The inhibitors efficiency was determined by the equation: AG - - AG~. I00 AG where AG and AG~ represent the loss of metal weight per surface unit during 24 h, in the presence or absence of inhibitors or halide ions respectively. It has been found that in the presence of halides the inhibitor efficiency increases with increasing halide concentration to an optimal value (maximum efficiency). Above this value, the increase of halide concentration decreases the inhibitor's efficiency. The optimal concentration of the halide which increases inhibitor's efficiency, decreases with the increase of the halide ion radius as follows CI- > B r - > I - i.e. in the same order as that of the peaks on the halide concentration-inhibitor efficiency curve. The effect of the ratio between the halide and inhibitor concentrations on the efficiency of corrosion protection has been determined. Rfsumf---On a examin~ l'effet de CI-, Br- et I- sur l'effieacit6 inhibitrice des diph6nylamine, thiour&e et hydroxylamine /t l'6gard de la corrosion d'aciers /t bas carbone dans des m61anges aqueux de H2SO4 et Na2SO4, ~ des concentrations comprises entre 10- s e t 5M, pH 0 ~t 6,5. L'efficacit6 inhibitrice est dgtermin6e par la relationp -----100 AG-AGdAG, oh AG et AGi sont les pertes de poids joumali~res par unit6 de surface m6tallique, respectivement en pr6sence et en absence d'inhibiteurs et d'halog6nures. En pr&sence d'halog6nures, l'efficacit6 des inhibiteurs augmente d'abord avec la teneur en halog6nures, atteint un maximum et diminue ensuite. La teneur optimale en haloggnures diminue avec l'augmentation du rayon ionique de l'halog6nure, soit darts l'ordre Cl- > Br- > I-, done d a m le m6me ordre que celui des pies des courbes de teneurs en halog6nures/efficacit6 des inhibiteurs. L'effet du rapport entre les concentrations d'halog6nures et d'inhibiteurs et l'efficacit6 anticorrosion a 6galement 6t~ examin6. Zusammeafassung--Der Einfluss yon CI-, B r - und I - aaf die Wirkung yon Diphenylaminen, Thioharnstoff und Hydroxylaminen bei der Korrosion yon Stahllegierungen niedrigen Kohlenstoffgehalts wurde in w/issrigen Lfsungen yon H2SO4 und Na2SO, einer Gesamtkonzentration zwischen I0 -a und 5 m i m pH-Bereich 0 bis 6,5 untersucht. Der Wirkungsgrad der Inhibitoren wurde nach der Gleichung

~G - AG AG

100

bestimmt, in der AG und AG~ den Verlust an MetaUgewicht je Oberffiicheneinheit wiihrend des *Manuscript received 17 November 1972; in revised form 10 March 1973. fPresented at the Yugoslav-Belgian conference on Corrosion and Protection of Materials, Dubrovnik, April 1972 (Corrosion week of Cebelcor 1972--65th Event, European Federation of Corrosion). 697

698

P. KmKov

Zeitraums yon 24 Stunden in An- oder Abwesenheit yon Inhibitorert bze. Halogenidionen bedeuten. Es stellte sich heraus, dass in Anwesenheit der Halogensalze die lnhibitorwirkung mit steigeader Salzkonzentration. bis zu einem optimalen Wert (maximale Wi~kung) gesteigert wird. Oberhalb dieses Wertes setzt eine Zunahme der Halogensalzkonzentration die Inhibitorwirkung herab. Die optimale Konzentration yon Halogensalz, das die Wirkung von Inhibitoren steigert, nimmt mit der Zunahme des Halogensalz-Ionenradius wie folgt ab: CI- > B I - > I-, d.h. in der gleichen Keihenfolge wie der der Maxima der Halogensalzkonzentration-Inhibitorwirkung-Kurve. Del Einfluss des Verh~ltnisses zwischen tier Halogensalz- und Inhibitorkonzentration auf die Wirksamkeit des Korrosionsschutzes wurde bestimmt.

INTR.ODUCTION

INrtmn'i~o effects of Br- and I- ions, in electrolyte solutions have been reported by many authors, t-62 This effect was interpreted by a specific halide adsorbtion, which could be proved with great precision in the case of I-. However, many contrary opinions have been expressed about the effect of other halides on the corrosion activity; some authors have shown that the presence of halides, especially of CI-, stimulates the corrosion. However, analysis of the experimental data show that these effects were observed under different experimental conditions, which doubtless increased the possibility for contradictory conclusions.~8-sa For this reason, we wished to determine the dependence of inhibitor efficiency on concentration, and nature of ions, in the first place CI-, Br- and I-. Therefore, we decided to investigate the corrosion of low carbon steel in strong electrolyte solutions. Taking into account that the concentration of H + essentially effects the dissolution mechanism of iron, we have investigated the effect of pH on the inhibitor efficiency at different electrolyte concentrations, with and without adding CI-, Br-, I- and inhibitors. EXPEKIMENTAL

Corrosion rates were investigated in aqueous solutions, in the concentration range of 10-a-5M H2SO4 and in Na~.SO4. By mixing equal concentrations of dectrolytes, the pH of the solution was adjusted between 0 and 6.5; additions of NaC1, NaBr and NaI in the concentration of 10-5-1M, were also made as appropriate. In order to be able to follow successfully the halide ion effect on inhibitor efficiency, three types of inhibitors were chosen, namely (a) hydroxylamine, (b) pentyldiamine, and (c) diphemylamine, which show specific differences in the size and structure of their molecules. The investigations were performed on steel sheet samples JUS CE 55 type, 150 × 150 × 2.5 mm in dimension. The samples were cleaned with a metal brush, washed in carbon tetrachloride, rinsed in ethyl alcohol-ether mixture (in the ratio 1 : l) and dried in a clean nitrogen atmosphere, previously scrubbed in a system of five washing columns with a total length of 75 cm of the solution. The samples were cleaned in order to remove colloidal particles and other impurities which exist in the laboratory atmosphere. It has been shown by using washed air in a series of 30 samples, that the reproducibility of the measurement was within the error allowed for the weight loss, whereas with samples prepared in unwashed air, the deviations exceeded by more than 5 times the average variation found by the application of washed air.

The influence of halide ion concentration

699

The samples were exposed to corrosion under the conditions mentioned above at the laboratory temperature of 21 ° -4- 2°C, were washed every 24 h under water at a pressure of 5 arm, rinsed with redistilled water, washed with a 1 : 1 ethyl alcoholether mixture, and dried with dehydrated Nz in a chamber at room temperature. The weight difference between every two successive measurements A G, and AGsoi.e. the difference between each measurement and the results obtained for pure, starting electrolyte solutions, were determined for these samples. AGni and AGsl were obtained in a similar manner adding inhibitors or halides, and halides with inhibitors. All the results were recalculated and expressed as the weight loss in mg Fe/cm 2 per 24 h. On the basis of AGn, AGso, AGnland AGsi values the protection efficiency was determined using the expressions: p, _ AGsn -- AG,I AGo

100

(1)

p_

100.

(2)

A G o - AGni AGo

With increasing corrosion rate in the presence of additions P, and P are decreased, while with decreasing corrosion rate they are increased. This means that the efficiency of additions is increased, too. P, = P, except in the cases when the addition is changed during the corrosion, by which the corrosion conditions, i.e. the corrosion and inhibitor mechanism, are also changed. RESULTS AND DISCUSSION The inhibitor efficiency, with and without the presence of halides in pure sulphuric acid in concentrations from I0 -a to 5M, is optimal when the inhibitor concentration varies from 0.02 to 2.5 g/100 ml of the solution, and with the concentrations of CI-, Br- and I - up to I0 -~ M. The region of maximum inhibitor efficiency is not essentially I00 I'-- ~

ve l 2CC o2B

6O ^

P

40

Ir~

+ ~A

30 I~P

× 2A

20 tlO o I

0 3A I

DPA

I

,

I

0'5

f

~ I

I

I'0

I

1"5

I

2'0

I

2'5

g / I 0 0 nil

FIG. 1. Inhibitor efficiency,(A) hydroxil amine, (/3) thiourea, ((2) dyphenil amine in dependence of (1). 5, (2). 10-1 and (3). 10-3M concentrations of H2SO4.

700

P. IG-RKOV

changed with increasing H2SO4 concentration which is not the case with halide ions. This can be seen in Figs. 1-4. Figure 1 shows the dependence of inhibitor efficiency on its concentration in 10-a, 10-1 and 5M H2SO4 without halide additions. In Fig. 2 the same dependence is given for 1M H=SO~, at the concentration up to 10-8, 10-3 and 1M of NaCI, NaBr and NaI. As can be seen in Fig. 2 the concentration of maximum inhibitor efficiency also appears in the presence of halides but its value does not depend on halide concentration which is not the case with the maximum efficiency value. This dependence is best shown in Fig. 3, where the inhibitor efficiency at its optimum concentration is given as a function of NaCI, NaBr and NaI concentrations. The effect of halides in pure Na2SO4 electrolytes is similarly independent of inhibitor concentration and electrolyte solution strength as can be seen in Figs. 4 and 5. I 0 0 [ ~ ~ 8

8~

0

¢,

oo

~

~

~L......=..~......_

60 +

40

Q

fl

'"

x

o

%,

,+l

0"5

,

,

,

I

I'0

P,

,

I

1"5

,

,

,t,

2"0

,

, t-r-,

2"5

,

g /100 rnt FIG. 2. The change of inhibitor et~ciency A, in 1M H=SO4 with additions of (1) 10-~MNaC1; (2) 10-SMNaCI; (3) 1M NaCl; (4) 10-SMNaBr; (5)10-SMNaBr; (6) I M NaBr; (7) 10-SM NaI; (8) 10-SM NaI; (9) 1M NaI. ,oo

60

" ~ ~ 2 B

40 //""~

\x~ ' x ~ ~ c -,o,. ~ ' I B

2C ,5

~IA

I 4

I 3 C=

FIG.3.

l 2 g mol/L

I I

0 -logC

The efficiency of optimum inhibitor concentrations A, B and C as a function of concentrations (I) NaCI; (2) NaBr; (3) NaI in 5M H2SO4.

The influence of halide ion concentration I00

701

• IC o2C

o3C

o 8o

~

~

C /

~

o

~_

v2B

~3B

~

+IA

40

o 2B

xsc

2O

r

I

0"5

I

15

I

2-0

2'5

g / I00 ml.

DPA FIG. 4.

[

I-0

Inhibitor efficiency A, B and C as a function of concentration in (1) 5M; (2) 10-XM and (3) 10-3M Na~SO,.

I00

v

v

9

v

i=

80

-- ~

60

-

+

~

~

4O

20

~

~

• .×

I 05

I I'0

I 3

-r------'-'×

I

I

!

1"5

2"0

2"5

g / 100m.L FIG. 5. The change of inhibitor efficiency A, in 1M Na=SO, with additions of (1) 10-SM NaC1; (2) 10-3M NaCI; (3) 10-ZM NaC1; (4) 10-~M NaBr; (5) 10-SM NaBr; (6) 10-1M NaBr; (7) 10-SM NaI; (8) 10-3M NaI and (9) 10-XM NaI.

Figure 4 shows the effect of inhibitor concentration on its efficiency in pure solutions, whilst in Fig. 5 the same dependence is shown in solutions with optimum inhibitor concentration as a function of halide concentration. By changing the ratio between H2SO4 and Na2SO4, the p H of the solution was altered from 0 to 6.5. Thus, we obtained a dependence which shows that inhibitor efficiency, i.e. its optimum concentration in pure solutions, regularly shifts to higher values with decreasing the pH. That slope decreases with the increase of electrolyte concentration, as it has been shown in Fig. 6. In Fig. 6 the dependence between p H of the solution and inhibitor efficiency, with

702

P. Kim~ov

L 80 ~

i---' ~

j

~

2

4°t

t

I

I

I

I

I

2

3

4

5

6

DPA

pH

Fxo. 6. The pH inhibitor efficiencyratio (with optimum concentration) in the solutions of (a) 10-1; (b) IM and (c) 5M, of the total concentration of H=SO4and Na2SO4 with (1) 10-1M NaCl; (2) 10-1M NaBr and (3) 10-1M NaI. a n d without halide additions is shown from electrolyte concentrations of l0 -2, 1M

and 5M. As it can be seen from the diagrams (1-6) there is the dependence between the inhibitor efficiency and halide concentration which is most probably due to the covering of the metal surface with inhibitor and adsorbed halide ions as a monolayer. The halide ions exercise an inhibiting influence, as can be seen from the change of corrosion rate in the HzSO4 and Na2SO4 solutions without organic inhibitors, where the halide additions cause the appearance of one maximum in the p - c diagram, so that by increasing halide concentration (except for I-), p value (efficiency) becomes negative indicating the increase of the corrosion, as shown in Fig. 7. As shown in Fig. 7, the addition of low concentrations of halides (10 -5 gmol/l.) increases the corrosion (negative p value). The increase concentration of halides

100

- -

80 6O 40 P

ZO 0 -20 ' -40 5 DPA

4

3

2

i -log C

C=gmol/L.

FIO. 7. The effect of hafide concentration on the protection efficiency in 1M H=S04 Na, SO,, pH ---- 3 for (I) NaCI; (2) NaBr and (3) NaI.

-I-

The influenceof halide ion concentration

703

decrease corrosion until an optimal concentration. The optimal concentration of halides, as well as the peaks sharpness on the p - c diagram, depends upon the radius of halides ions. The increase ionic radius in order C1- < Br- < I - the" sharpness decrease and the maximal p value dependence which slope depends from the structure of inhibitors. The above-mentioned relation is shown on Fig. 8.

ioo

-

°

°°-

p

/

80

70

60

5o 1"8 OPA

I

I

I

1'9

20

21

_

I

I

22

2-5

r -A

FIG. 8. The optimal inhibitor efficiency as a function of halide ion radius in 0'IM HzSO, + NaSO~ for inhibitors A, B and C at (1) pH = 1; (2) pH = 3-2 and (3) pH = 5'I. This phenomenon could be explained by means of the boundary layer structure. At low halide concentrations the surface layer is not compact, which speeds up the activity of the local charges and the increase of corrosion. Increasing concentration of halides stimulates the monolayer formation so that at the optimal concentration a homogeneously monolayer is formed (maximum p - c diagram). The increase concentration of halide ions causes a multilayer ion adsorbtion, which decreases the stability of the protection boundary layer and stimulates the corrosion processes. Simultaneous formation of the surface layer with the molecules of surface-active organic inhibitors and adsorbed halide ions, as a function of their structures at determined concentrations, increases the stability of the boundary layer by preventing the reorientation of inhibitor molecules. In this way, the corrosion rate is decreased; this effect has been described earlier by determination of inhibitor efficiency as a function of its structure. 5,~x It is very important to emphasize that the effect of p H on inhibitor efficiency in the presence of halides causes the FeX~ and FeX 8 (X = halide ions) compounds to show regular decrease of dissolution, with increasing halide ion radius in ratio CI- > Br- > I - and therefore also the transition of H + on the metal surface, which

704

P. KmKOV Inhibifors A OH

I

OH I ,

0t

/ \"

/

c k l ,. c , / \

H

I-I

Fe

C

B S II c

\

/ \N

H

H

H

\ / ", H 5~0:

ct

/ \" I-I

S II c

H

c\ly I-I

Fe

N/

H

H Ct--Fe -CI

s

o. OH

N --H

[ H

Cl

I Ct--Fe--Ct

H--N /

I H

I

s

II

N

N/ON CL

H--

Cl

N

I

I H

:°.5 %

,

/C N N--H

O6H \ 5 / C6H5

C6H5

/ ~N

N [

Cl

N I

I

H

I

J

I

H

H

CE--Fe--CI

H

Ct - - Fe --Of

DPA ~ o . 9.

Schematic presentation of the inhibitor efficiency stabilization in the presence of halides. I. Optimal concentration of the inhibitors. II. Optimal concentration of halides (C1-).

is shown by the change of the pH efficiency dependence of the inhibitor. The increase of FeX~. and FeX 3 molecules cross-section decrease reorientation rate with increase p value. This conclusion is proved by the experiments (see Fig. 8). The increase of the inhibitor efficiency with the increase of halide ion radius can be seen from the slope of relation on Fig. 8. The slope ofp-r (efficiency-radius) relation increases if inhibitor efficiency without halide is lower. However, this effect is evident only at the optimum concentration of halides and inhibitors. It means that the inhibitor efficiency increases in the presence of halide ions, only within the range of optimum stability of the monolayer. The formation of the protective surface layer schematically can be present as in Fig. 9. On the basis of the presented results, it can be concluded that in solution of surface active of electrolytes and inhibitors, the optimal concentration of both corrosion protection agents can be determined. In this way, the presence of halides influences the etficiency of corrosion protection in cases when they supported stability of protective layer. That is a reason which influences on the different opinion and evident observations about effect of halide ions on corrosion in presence and absence of inhibitors. This paper shows the influence of the concentration of halides on the protection and activation of the corrosion. REFERENCES 1. J. I. GP,~OM.~u'q,Corrosion lnhibitors. Macmillan, New York (1963). 2. J. HORVATH, L. HACVJ~and A. RAUSCr~R, C.r. du 2~me Syrup. Europ~en sur les Inhibiteurs de Corrosion, Annali Univ. Ferarra, N.S. Sez. V, Suppl. 4, 477 (1966). 3. H. Pmm~, C.r. du 2~me Symp. Europ6en sur les Inhibiteurs de Corrosion, Annali Univ. Ferarra, N.S. Sez. V, Suppl. 3, 1 (1961). 4. H. F/SCr~R and W. SF.ILB~, C,r. du 2~me Syrup. Europ~en sur les Inhibiteurs de Corrosion, Annali Univ. Ferarra, N.S. Sez. V, Suppl. 4, 19 (1966).

The influence of halide ion concentration

705

5. P. Kuucov, 3rd European Symp. on Corrosion Inhibitors, Annali Univ. Ferarra, N.S. Sez. V, 303 (1970). 6. K. J. VE'rrER, Elektrochemisct, e Kinetik, pp. 588-641. Springer, Bet l in (1961). 7. H. H. UrmLIG, Corrosion Control, p. 259. Wiley, New York (1965). 8. J. HORVATH and H. H. UHLIG, J. electrochem. Soc. 115, 791 (1968). 9. H. P. LECKllEand H. H. UHUG, J, electrochem. Soc. 113, 1262 (1966). 10. 1. L. R.OSENFELD and P. MAXIMTSCHLrK,Z. phys. Chem. 225, 25 (1960). 11. M. PRAZAK, V. SPANIL'¢ and J. TOUSEK, Corrosion Week, p. 766. Academic Edn., Budapest (1970). 12. B. E. CONWAY, Transactions o f the Sympo~hon on Electrode Processes. Wiley, New York (1961. 13. T. MURAKAWZand N. HACKERMAN, Corros. Sci. 4, 387 (1964). 14. G. M. SCHMID and N. HACKERMAN, J. electrochem. Soc. 109, 243 (1962). 15. A. N. FRUMrdN and N. DAMAS:ON,J. electroanaL Chem. 3, 36 (1962). 16. Ya. M. KOLOTYRKIN, J. electrochem. Soc. 108, 209 (1961). t7. K. SCHWABE, Corros. Sci. 4, 156 (1964). 18. Z. A. JOFA, V. V. BATRAKOVand B. A. CHO-NGOG, Electrochhn. Acta 9, 1645 (1964). 19. J. W. LORENZ and H. FISCHER, C.r. du 26me Symp. Europ6en sur les lnhibiteurs de Corrosion, Annali Univ. Ferrara, N.S. Sez. V, Suppl. 4, 81 (1966). 20. Z. A. Fogotrus, C.r. du 26me Syrup. Europ6en sur les Inhibiteurs de Corrosion, Annali Univ. Ferrara, N.S. Sez. V, Suppl. 4, 285 (1966). 21. Z. A. JOFA, C.r. du 26me Symp. Europ6en sur les Inhibiteurs de Corrosion, Annali Univ. Ferrara, N.S. Sez. V, Suppl. 4, 93 (1966). 22. A. N. FRUMKIN, Z. Phys. Chem. A164, 121 (1933). 23. A. N. FRtrMKIN, Advances in Electrochemistry and Electrochemical Engineering, Vol. I (Ed. P. DELAHAY). Interscience, New York (1961). 24. L. I. ANTROPOV, 1st International Congress on Metallic Corrosion, p. 147. Butterworths, London (1962). 25. J. O. M. BOCKRIS, IV[. A. DEVANATHAN and K. MOLER, Proc. R. Soc. A274, 54 (1963). 26. N. HACr,.ERMAN, E. S. SNAVELYand J. S. PAVNE,3". electrochem. Soc. 113, 677 (1966). 27. V. CA~tASSITI, G. TRABANELLIand F. ZtJCCHI, C.r. du 2/~me Symp. Europ6en sur les Inhibitems de Corrosion, Annali Univ. Ferrara, N . S Sez. V, Suppl. 4 (1966). 28. L. FELLONI and A. CozzT, C.r. du 2~me Symp. Europ6en sur les Inhibiteurs de Corrosion, Annali Univ. Ferrara, N.S. Sez. V, Suppl. 4, 253 (1966). 29. N. HACg2ERA~ANand A. C. M_AKTRIDES,J. phys. Chem. 59, 707 (1955). 30. L. CErtVENY, C.r. du 26me Symp. Europ6en sur les Irthibiteurs de Corrosion, AnnaH Univ. Ferrara, N.S. Sez. V, Suppl. 4, 701 (1966). 31. S. LEVIN, P. VINOGRADOV, V. KUCHINSKY, O. TroKAI and S. KDSCINKSAYA, Corrosion Week, p. 712. Academic Edn., Budapest (1970). 32. J. HORVaTH and L. HACI
706 52. 53. 54. 55. 56. 57.

P. Kz~k"ov

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