Leadlead dioxide bielectrodes

Leadlead dioxide bielectrodes

StecnoMimxn Act, 1977. Vet _2, pp 197-2113 . Pergamon Pros . Printed in Great Britain LEAD-LEAD DIOXIDE BIELECTRODES K . C . NAaASttm xtu and H . V ...

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StecnoMimxn Act, 1977. Vet _2, pp 197-2113 . Pergamon Pros . Printed in Great Britain

LEAD-LEAD DIOXIDE BIELECTRODES K . C . NAaASttm xtu and H . V . K. UDUPA Central Electrochemical Research Institute . Karaikudi, 623006, India (Received 4 February 1975 ; and in revised form 7 August 1975) Abstract-Anodic polarization of lead and lead alloys at high cds in chloride solution results in the formation of a thick, voluminous, porous deposit of lead chloride . When a platinum microelectrode is introduced into lead . lead dioxide is formed on the surface of lead in chloride medium during anodic polarization at high cds . As a replacement for costly platinum, lead dioxide pieces which fell during the electrodeposition of lead dioxide were shaped into small cylindrical microelectrodes, inserted into the lead or lead alloys and anodically polarized in chloride solution . In this case lead dioxide is also formed on the surface of the lead or lead alloy. It has been found that the formation of lead dioxide on the lead-lead dioxide bielectrode is good in 3% NaCl solution at an anode cd of 3 A/dm~ and at room temperature . [he effect of withdrawal and insertion of lead dioxide microelectmde, and of the relative area of lead dioxide microelectrode to lead on the formation of lead dioxide is also studied . Studies on the weight change during anodic polarization of lead and lead alloys embedded with lead dioxide microelectrode, in chloride solution and in synthetic sea water show that lead-silver (1%) alloy is by far the best anode from the point of view of the formation of a compact and crackfrcc deposit of lead dioxide on the surface .

1 . INTRODUCTION

eat case also does not introduce any impurities . Lead dioxide pieces fallen from the graphite anode during electrodeposition of lead dioxide from lead nitratecopper nitrate bath [11, 12] have been shaped into small cylindrical rods and inserted in the lead sheet. The present paper describes the results of potentialtime studies as well as weight change during anodic polarization of lead and lead alloys embedded with lead dioxide microelectrodes, in chloride solutions .

The rise of lead and lead alloys as anodes in electrolytic processes is only possible ff the electrolytic conditions are such that a lead dioxide coating is formed and maintained on the lead surface. Lead dioxide, which is characterized by a high electronic conductivity, can be formed by the oxidation of plumbous ions in solution at an inert electrode such as platinum, graphite, etc or by the oxidation of lead itself [1-3] . The formation of lead dioxide on lead and its alloys during anodic polarization in sulphuric acid solution is well known and Think and Wynne-Jones [4] and Ruetschi and Cahan [5, 6] studied in detail the anodic polarization of lead and lead alloys in sulphuric acid . The behaviour of lead in chloride solution is quite different and both lead chloride and lead dioxide are formed simultaneously, but at low cds a film of lead dioxide gradually consolidates and the lead can then act as a relatively inert electrode [3] . The presence of SOa - in chloride solutions, as in sea water, facilitates the formation of lead dioxide and anodes of 1-2% Ag-Pb and 1 Ag--6'% Sb-Pb are used for cathodic protection of marine structures . Anodic polarization of lead and lead alloys at high cots in chloride solution results in the formation of thick, voluminous, porous deposits of lead chloride . Shreir and Weinraub [7-9] demonstrated for the first time the formation of lead dioxide on the surface of lead and lead alloys in chloride medium during anodic polarization at high cads also, by the introduction of platinum microelectrode into lead . Lead-platinum bielectrodes have been widely and successfully adopted for the cathodic protection of marine structures [3,10] . Platinum being costly, it has been considered to replace the same with a cheaper material ail lead dioxide since the lead dioxide is reported to have almost the same oxygen overvoltage and in the pres-

2 . EXPERIMENTAL

2 .1 . Lead and lead alloy anodes Panels (1 x 1 cm) with 99 .99% pure lead were used giving electrical connections on the top portion of the panel. However, for the experiments on weight change during anodic polarization, a cylindrical rod of 2.5 cm dia x 2 .5 cm long (total area exposed 19 .5 em 2) with rounded end was used . Connection was given through a copper wire (0 .3 cat dia) screwed on to the top portion of the lead rod . The copper wire was covered with an unplasticized PVC holder and rubber washer was used between lead and PVC holder to prevent ingress of the electrolyte (see Fig . 1). Lead alloys like lead-si'ver (1%), lead-antimony (6%) were made using the pure metals and in the same shape described above. 2 .2. Preparation of lead dioxide microelectrode Lead dioxide plates (0 .3-0 .5 cm thick) removed from the graphite substrate after electrodeposition of lead dioxide from lead nitrate upper nitrate electrolyte employing optimum conditions established earlier [11-14], were shaped into cylindrical rods (02 x 0 .3 cm) with the help of the grinding wheel . These microelectrodes were fitted at the centre of the lead or leadd alloy panels . Two lead dioxide microelectrodes (035-0 .45 crn dia x I cm long) were introduced into the cylindrical lead or lead alloy rods by 197

198

K . C . NAxasnt HAm AND H . V . K . UDUPA

3. RESULTS 3.1 . Effect of lead dioxide and platinum oricroelectrode on lead

I. 2. 3. 4. 5.

Lead or lead alloy anode Lead dioxide microelectrode Rubber washer PVC rod Copper wire threaded into lead anode

Fig. 1 . Lead

anode embedded with microelectrode .

lead

dioxide

drilling holes and inserting the microelectrodes one at the bottom and another in the side as shown in Fig . 1 .

2 .3 . Cell assembly The cell was a Pyrex trough of t I capacity fitted with a PVC cover . Two mild steel cathodes (2 .5 cm wide x 9 cm long) were fixed to the PVC cover on either side of the anode with an inter electrode distance of 6 cm . Temperature of the cell was maintained by placing the cell in another vessel containing water . A Pyrex trough of 2.5 1 capacity was used for experiments on weight change . The cathode (5 cm wide x 15 cm long) was mild steel sheet fitted to the cell cover. The inter electrode distance was 15 cat. 2 .4. Electrolyte 3% (w/v) sodium chloride (analar) solution was used as electrolyte. 650 ml of fresh electrolyte was used for each experiment . For experiments using synthetic sea water, it was prepared with the composition given in BS . 1391 [15] . Two litres of the solution was used for weight change experiments and the electrolyte was replaced every day to ensure that hypochlorite formed during electrolysis was kept to a minimum .

Figure 2 shows the anode potential-time relationship for pure lead, lead with lead dioxide microclectrode and lead with platinum microelectrode. The behaviour of lead when anodically polarized in 3% sodium chloride electrolyte was not reproducible . The anode potential rose to nearly 2 V and later was fluctuating with further passage of current at 3 A/dm 2 (tilde Curve I). But when the polarization was carried out at 3 A/dtn2 , the curves with lead-lead dioxide microelectrode (Curve II) and lead-platinum microelectrode (Curve III) were characlerircd by an initial increase in potential and after reaching a peak potential of 1 .76 V vs see for lead-lead dioxide bielectrode and 1 .44 V vs see for lead-platinum bielectrode, the potential had gradually fallen to a value of about 1 .3 V (vs see) . The evolution of gas on these two electrodes appeared to coincide with the peak potential . On removal from the electrolyte, the anodes were found to be coated with a deposit of lead dioxide although even after 4h, small areas of lead chloride were visible . Under certain conditions, it was found to be possible to obtain lead dioxide formed around the microelectrode and then extended over the lead surface . The effect, however, was not reproducible and lead dioxide appeared to form on all parts of lead surface- Figure 3 is the photograph of lead and lead--lead dioxide bielectrode after polarization in 3% sodium chloride solution. 3 .2 . Effect of anode current density on the formation of lead dioxide In Fig . 4, at an anode current density of I A/dm 2 , the anode potential remained almost steady at 1 .45 V vs see. The anode potential became very high at 10 A/dm 2 (2 .34 V vs see) and it was found that formation of lead dioxide was not good at 6 Aidm 2 and above. Hence other experiments were carried out at an anode cd of 3 A,idm 2. Figure 5 shows the photographs (a, b and c) of lead dioxide formation at different cds (ie 3, 1 and 6 A/dm 2) .

2 .4

I

e .o

2 .5. Electrolysis The potential of the lead-electrolyte interphase was determined by a Loggia capillary and calomel half cull (saturated KC1, E = 0 .2422 V) in conjunction with a vacuum tube voltmeter (Philips GM 6020). To avoid interfering with film formation, the capillary was so arranged that it was very close to but not actually touching the lead surface . The small it drop through the electrolyte was considered negligible . Duplicate experiments were conducted in all the cases.

I Pure Lead It Lead with Pb0 r microelectrode m Load with Pt microelectrode

Fig. 2. Variation of anode potential with time for lead with different microclectrodes .



1 99

Izad-lead dioxide bielectrodes

3(a)

3(b)

Fig . 3 . Photograph of the surface of lead in 3% sodium chloride solution : (a) Without microelectrode ; (b)' With lead dioxide microelectrode . 3 .3 Effect of concentration of sodium chloride on the formation of lead dioxide Different concentrations of sodium chloride solution :i2 0.5, 1 . 3 and 6; (w/v) were prepared and potential-time studies were carried out using the lead anodes fitted with lead dioxide microelectrode (Fig. 6) . At lower concentrations (0.5 and 1%), patches of lead chloride were also seen along with the lead dioxide formation. The anode potential slowly increased, reached peak potential after 120 min and then gradually decreased . At higher concentration of sodium ;/v) the potential increased rapidly even chloride (6% w within 60 min and after 80 min became more than 5 V . No lead dioxide formation took place on the lead surface . The behaviour in 3" sodium chloride solution, is already shown .

Fig. 5 . Photograph of the surface of Pb-PbO 2 bielectrode in 3'/'. sodium chloride solution : (a) 3 A/dmr ; (b) 1 A/dm' ; (c) 6 A/dm . potential of 2.04 V (vs see) decreased to 1 .5-1 .55 V (vs see) . The lead dioxide deposit on the surface of lead was loosely adherent . 3 .5. Effect of withdrawal and insertion of lead dioxide microelectrode on the formation of lead dioxide Curve I in Fig. 8 was obtained by Burst arindiually polarizing lead without lead dioxide microelectrode at 3 A/dm 2 for 2 h, the potential fluctuating between 2 and 3 V . The anode, which was covered with lead chloride, was then removed from the electrolyte, a lead dioxide microelectrode (0 .2 x 0 .3 cm) inserted in the middle and the bielectrode put back in the electrolyte and electrolysis continued. The anode potential decreased to 1 .2 V and slowly raised from 1 .2 V to 1 .5 V (vs see) in about 30 min and then decreased to 1 .2 V (vs see) after 3 h. Lead surface was covered with lead dioxide .

3 .4. Effect of temperature on the formation of lead dioxide Figure 7 represents the variation of anode potential with time at different temperatures. At 15°C, the potential increased and after reaching a peak potential of 1 .82V (vs see) started decreasing up to 1 .59 V (vs see). Patches of white lead chloride were visible in certain portions ou the lead along with lead dioxide deposit . The deposit was adherent upto 40`C . At 50°C, the anode potential, after reaching a peak

40

60

120

160 200

Time,

Fig. 4. Variation of anode potential with time for Pb-PbO, bielectrode at different cds .

240

min

Fig . 6 . Variation of anode potential with time for Pb-PbO, bielectrode at different concentrations of NaCl.



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K . C . NARASIMUAM AND H . V . K . UoUPA

was formed uniformly on lead . Removal of the lead dioxide microelectrode resulted in a rapid increase of potential (more than 10 V in about 80 min) and the simultaneous formation of lead chloride on the surface of the lead.

so

20 Time,

160 mir

240

Fre. 7 . Variation of anode potential with time for Pb--Pb0 2 bielectrode at different temperatures. Curve II in the Fig . 8 shows the effect of removing the lead dioxide microelectrode (0 .2 x 0.3 cm) from the lead-lead dioxide bielectrode, which had been anodically polarized at 3 A/dm 2 for 3h . The steady state potential was 1 .05 V (vs see) and lead dioxide

I Introduction of PbCp microelectrode II Removal of Pbo 2 microelectrode

i

I More

than IC V

3.6 . Effect of relative area of lead dioxide microelectrode to lead As the size and number of lead dioxide microelectrodes are of practical importance, experiments were conducted by taking different ratios of lead/lead dioxide microelectrode and Table I shows the results of the same . The potential-time curves for the area relationship are shown in Fig. 9 . All curves were characterized by a rapid increase in potential followed by a gradual decrease to a steady state potential of 1 .2-1.4 V (vs see) . In general, it was noticed that with increase in the ratio of lead to lead dioxide microelectrode, there was an increase in the bath voltage. However, even -the smallest microelectrode appears to be capable of stabilizing a coating of lead dioxide on the metal surface . Behaviour of lead, lead-antimmiy (6%) and leadsilver (1%) alloys with lead dioxide microelectrode in 3% (w/v) sodium chloride and synthetic sea water Studies on potential-tune and changes in weight during anodic polarization were carried out with Ph, Pb -Sb (6%) and Pb-Ag (1%) in sodium chloride solution (3% w/v) and synthetic sea water at a temperature of 30-32`C . Anode cds of 3 and 2.3 A/dma were 3 . 7.

0 .4

0 1 ._ I I I 100

ISO 220 260 300 Time, min

Time . min

Fig . 8 . Effect of insertion and removal of microelectrode Fig . 9 . Effect of ratio of area of microelectrode and mac roelectrode on the potential-time behaviour of lead . on the potential-time behaviour of lead. Table 1 . Variation of ratio of area of lead and lead dioxide microelectrode Size of S1. No .

Lead (cm)

1

1 .1 x 1 .0

2 3 4

2.1 x1 .0 1 .1 x 1 .0 5 .0 x 3 .0

Area of Lead dioxide microelectrode (cm)

Lead (em')

0.2 x 0.3 03 x 0.3 0.15 x 0.15 0.2x0.2

2 .2 4 .2 2 .2 15 .0

Lead dioxide microelectrode (cm 2)

Maximum voltage (V)

0 .12 0 .18

3 .65 3 .90

0 .045

4.00 5 .25

0 .04

(Area of lead)/ (Area of lead dioxide)

18 .3 23 .3 48 .9 375 .0



20 1

Lead-lead dioxide bielectrodes

I Lead a Lead-Antimony (6Y.) m Lead-5ilver(1%)

60

5 .0

r Lead II Lead-Antimony (6%) m Lend-Silver (1%)

t

20

Q 10 I I I I r 1 40 80 120 16o 200 240

Time, min

0

Fig. 10. Variation of anode potential with time for Pi embedded lead and lead alloys in 3% NaCl . used for potential-time studies and determination of changes in weight respectively . 3 .7 .1 . Potential-rune curves . Potential-time curves obtained during anodic polarization for Pb, Pi (6%) and Pi (1%) in sodium chloride solution (3°0) and synthetic sea water are shown in Figs. 10 and 14 respectively . In sodium chloride solution it can be seen that the potential of lead-antimony alloy rises to 1 .82 V (os see) and remains almost constant for 2h and then decreases to a value of 1 .45 V (vs see). The lead-silver alloy, on the other hand, not only registers a lower potential (1 .?0 vs see) but also exhibits periodic oscillations in potential with time . The behaviour of lead is already described . In synthetic sea water the nature of potential-time curves is quite similar to that obtained above for lead and its alloys except that the oscillations are less pronounced for leadsilver alloy, and for lead-antimony lsco 1200

T and T (0)Pure Lead If and II f a) Lead-Antimony (6°e) In and III( a) Lead- Silver {1 °e)

m

ego

v

400 0

III (a)

a

Ifa)

3

-400 x'

I I I I 0,8 1,2 1 .8 0 .4 log Quantity et electricity

2.

Fig . 12 . Weight loss with logarithmic quantity of electricity for MID, embedded lead and lead alloys in 3% NaCl . the potential continues to decrease with time after reaching a maximum value-. 3 .72. Weight change of the anode and appearance of the deposit . Figure 11 shows the changes in the weight of lead and lead alloys in sodium chloride solution (3%) and synthetic sea water . The loss of weight is maximum with lead in sodium chloride solution, and for lead-antimony the loss tremondously decreases within a short period after the initial loss being equal to pure lead . This indicates that after an initial rapid dissolution, film formation sets in . The lead silver alloy shows an increase in loss of weight in the initial stages, but the loss becomes less and less with time, which shows that the film formation is accompanied by a greater dissolution . From Fig . 12 it will appear that the coverage of the surface is nearly complete after about 40A . h . The surface characteristics of the anodes after polarization for 20 days in sodium chloride solution show that the coating on pure lead (sec Fig . 13a) is powdery and patchy . Whereas the film formation on lead-antimony (Fig . lab) is less powdery but still patchy, the same on lead-silver alloy is more uniform and less powdery (Fig . 13c). However in synthetic sea water, lead-antimony and lead-silver alloys gain in weight and becomes wnstant with lime, whereas the lead anode does not show any net change in weight with time, which may he due to periodic dissolution and film formation. The film formed in all these cases after polarization

I soI I I 1 1 120 ISO 200 240

40

Quantity of electricity, A-h e Fig. 11 . Chan g of weight with duration for PbO, embedded lead and lead alloys in 3% NuCI and synthetic said water. ew. 22,2-i

Fig. 13 . Photograph of Pb-PbO 2 bielectrode after anodic polarization in 3% sodium chloride solution : (a) Lead ; (b) Lead-antimony (6%) ; (c) Lead-silver(1%).



2 02

K . C . NAassIMHAat AND H . V . K. UDUPA

24-

r Leod II Lead-Antimany(6%) M Lead-Sllver (1%) a

0 .4-

I I __J- I I I 0 40 a0 120 160 200 240

Time, min Fig. 14 . Variation of anode potential with time for PbO2 embedded lead and lead alloys in synthetic sea water . for 20 days is more adherent and compact than obtained in sodium chloride solution as can be seen from Fig. 15 (a, b and c). However, the films formed on lead and leadantimony exhibit minor cracks . 4.

DISCUSSION

It is clear from the above study that a microelectrode of lead dioxide fitted to a lead surface can also result in the formation of a stable coating of lead dioxide on lead during anodic polarization in chloride solutions like the insertion of noble metal on a lead surface reported by Shreir [8, 16] for the first time . Lead, when anodically polarized in chloride solutions, gives sparingly soluble anodic reaction products [17] ; if the chloride concentration and current density are sufficiently high, non-conducting lead chloride forms on the lead surface and at constant current, the potential increases due to passivation [18, 19] . The relevant reactions have already been given by Shreir [16, 20] . The mode of crystallisation of lead chloride on lead anode will depend on the activity of chloride and lead ions at the metal/electrolyte interphase, which will depend in turn on the concentration of chloride in the electrolyte. cd, temperature, agitation, etc. It is apparent, however, that the behaviour of lead in chloride electrolytes differs from its behaviour in other electrolytes which form insoluble lead salt (eg sulphates, chromates) . Thirsk and Wyime-Jones [21] have shown that in sulphuric acid, lead sulphate forms initially at the reversible PbpPbSO 4 H 2 S0 4

Fig. 15 . Photograph of Pb-PbO 2 bielectrode after anodic polarization in synthetic sea water : (a) Lead ; (b) Lead-Antimony (6%) ; (c) Lead-silver (1%).

potential ; the potential then increases rapidly to the PbIPbO z H 2 SO4 potential and plumbous ions are then oxidised to lead dioxide . Although a similar oxidation can occur in chloride electrolytes, the lead dioxide is not maintained on the lead surface as it is rapidly penetrated by chloride ions with formation of lead chloride on the metal surface and consequent detachment of the lead dioxide . The potential-time curves obtained with the leadlead dioxide bielectrode (Figs 2 and 14) are similar to those obtained with lead-platinum bielectrode by Shreir [7] in that it is characterised by an initial rise in potential and a subsequent fall to a steady value . However, it should be noted that in the case of leadsilver (1%) and lead-antimony (6%) alloys in 3% sodium chloride solution the peaks are not so pronounced (Fig. 10) . The rise in potential to a maximum is associated with the initial evolution of chlorine from the microelectrodes and then a gradual fall to a steady state potential for which Shreir [9] has reported an explanation . At high cds (ey 6 A/dm 2 ), the evolution of oxygen may be more predominant than the chlorine evolution and hence not a suitable dioxide is formed At concentrations greater than 30 (w/v) sodium chloride, the loud dioxide formation has not been observed . Shreir [9,16] reported that with lead-platinum bielectrode the lead dioxide formation did not occur beyond 1 .3 M sodium chloride solution and this was explained due to the formation of soluble chluroeomplexes. However, with lead dioxide microelectrode, the limiting concentration value of sodium chloride is lower than that reported for platinum microelectrodc . Not much change is expected with variation of temperature of electrolyte except lowering of potential . But at temperature of 50°C, the peak potential is higher and loosely adherent lead dioxide deposit is formed. This may perhaps be due to the decomposition of bypochlorite at higher temperature and the formation of chlorate which can act as more powerful penetrating anion . The insertion or removal of lead dioxide microelecIrote clearly indicates that the microelectrode is necessary to obtain the film of lead dioxide on lead surface in chloride medium by acting as a 'chlorine valve' similar to observations given by Shrew [9] with platinum microelectrode . The considerable weight loss observed in the case of lead and its alloys in 3% sodium chloride solution (Fig . 11) indicates that a large quantity of lead has to dissolve before the formation of lead chloride film and subsequent oxidation to lead dioxide . It is of interest to note that lead-antimony (6%) alloy exhibits much lower weight loss than lead or lead-silver (1%) . However, one observes only gain in weight in synthetic sea water for lead-silver (1%) and lead-antimony (6%) alloys . So it is to be borne in mind that the changes in the weight of the anodes during anodic polarization not only depend on the nature of alloy but also on the nature of the medium . It is perhaps possible that the sulphate present in sea water helps in the immediate rise in potential, facilitating the formation of lead dioxide . The difference in the behaviour of the 3 systems (lead, lead-antimony and leadsilver) cannot be easily explained, though one may



Lead-lead dioxide bielectrodes conjecture that the alloying components facilitate the formation of the lead dioxide . It is worth pointing out here that Shreir [9] also reported the gain in weight to be much more for lead-antimony (6%) than the lead-silver (1%) alloy in synthetic sea water. It may also be pointed out that there is no simple relationship between potential-time curves (Figs . 10 and 14) and changes in weight (Fig. 11) excepting in case of lead where the higher dissolution can be explained due to lower anodic potential at which simultaneous dissolution of lead and formation of lead dioxide take place. The higher potentials and the lower dissolution rate (or gain in weight) may be associated with more complete formation of lead dioxide in the case of lead-antimony and lead-silver alloys . The comparison of the changes in weight as well as anode potential for lead-silver (1%) and lead-antimony (6p) alloys in 3% sodium chloride solution can be explained in similar terms, hut the same explanation is not very convincing in the case of synthetic sea water . Though it would appear from the weight changes and potential-time curves, the lead-antimony (6%) alloy is the most suitable anode in 3% sodium chloride and synthetic sea water, the nature of deposits obtained under these conditions indicate that leadsilver (1%) alloy is the most suitable one from the point of view of formation of compact and crack-free deposit . s. CONCLUSION The use of lead dioxide microelectrode in combination with lead helps in the formation of lead dioxide on the surface of lead in chloride .solutions, similar to the use of platinum microelectrodes reported by Shrier (7-9, 16] . Thus in this process, the costly platinum is replaced with a cheap lead dioxide microelec trode . Though the lead-antimony (6%) alloy appears to be better from weight changes, the lead-silver (1%) alloy is by far the best anode from the point of view of formation of compact and crack-free deposit in 3%

203

sodium chloride solution as well as in synthetic sea water. Acknowledgement-The authors thank Miss S Vasundara for her occasional help during the experiments conducted on weight change measurements .

REFERENCES

1 . P . Delahay, M . Puurbaix and P . Van Rysseleberghe, J. electrochem Soc. 98- 57 (1951). 2. W. M . Latimer, Oxidation Potentials. Prentice Hall, New York (1956). 3 . L. L . Shreir, Platin, Metals Rev 12, 42 (1968) . 4 . H . R . Thirsk and W. F. K . Wynne-Jones, Tiara . Insf . Metal Finish 29, 264 (1953) . 5 . P . Ruetsehi and B . D . Cahan, J . efectrochem. Soc. 104, 406 (1957) . 6 . P . Ruetschi and B . D . Cahan, J . etectrochem. Soc. 105, 369 (1958) . 7. L . L . Shreir and .A, Weinraub, Chem . Ind. (London) 1326 (1958). 8. L . L. Shreir, Platin. Metals Rev . 3, 44 (1959) . 9 . L . L. Shreir, Corrosion 17, 90 (1961) . 10. Anon, Plain . Metals Rev. 15. 90 (1971), 11 . K . C . Narasimham and H . V . K, Udupa, Proc . Symp . Electrolytic Cells, Central Electrochemical Research Institute, Karaikudi p. 22 . (1961). 12. H . V . K. Udupa and K . C . Narasimham, Indian Pat . 66, 195 (1958) . 13. K . C . Narasimham, S. Sundarairajan and H . V . K . Udupa, J. elecrrochem . Soc. 108, 798 (1961). 14. F . D. Gibson Jr., U .S. Pat. 2, 945, 791 (1960). 15 . Synthetic seawater, B .S. 1391 (1952) . 16. E . L. Littauer and I .. L. Shreir, Proc. lst Int . Cony . Metal Corrosion, p. 374. Butterworth, London (1961) . 17 . W . J . Muller, Trans. Faraday Soc . 27, 737 (1931) . 18 . G . W . D. Briggs and W. F . K . Wynne-Jones, J . chern_ Soc . 3, 2966 (1956) . 19. L . J. Kurtz, C .r. Akad. Sri ., USSR 8, 305 (1935). 20. H . Helber and E . L . Littauer, C'nrros . Sri . 10, 411 (1970). 21 . H. R . Think and W . F. K . Wynne-Jones, Extrait du J . chem . Phyc . 49, 131 (1952).