Lead-Platinum Bielectrodes for Cathodic Protection
Lead-Platinum Bielectrodes for Cathodic Protection
Advantages in Marine Applications
The insertion of small pieces of platinum into the surface of lead or lead alloy anodes causes a remarkablechange in their behaviour as electrodes. Such lead-platinum bielectrodes are inexpensive, robust and easily fabricated and can be used successfully for the cathodic protection of marine structures. In this article the author describes the principles involved and reviews ten years of experience in a variety of applications.
It has been established for some years that lead and lead alloys can be used as inert anodes in electrolytic processes provided that a film of lead peroxide is formed and is maintained on the surface.
Lead peroxide, which is a thermodynamically stable form of lead at elevated electrode potentials, can be formed by the oxidation of Pb2+aq in solution at an inert electrode, such as platinum, or by the oxidation of lead itself. The oxide is chemically stable and is characterised by a high electronic conductivity (about 50 per cent that of lead) so that lead with a film of PbO2 will act as an inert anode, provided the oxide remains in electrical contact with the metal and is reformed should a discontinuity be produced in the film. Thus anodic polarisation of lead in sulphuric acid results in the formation of a thin film of PbO2 at the Pb/PbO2−H2SO4 reversible potential; with further passage of charge the predominant reaction is oxidation of water to oxygen and there is practically no thickening of the film.
The position in chloride solutions is quite different, and both PbCl2 and PbO2 are formed simultaneously, but at low current densities a film of the latter gradually consolidates and the lead can then act as a relatively inert electrode. The presence of SO42− in chloride solutions, as in sea-water, facilities the formation of PbO2, and anodes of 1 to 2 per cent silver-lead and 1 per cent silver – 6 per cent antimony-lead are used for the cathodic protection of marine structures.
It has been shown however (1), that the insertion of a platinum microelectrode into lead has a remarkable effect on the anodic behaviour of the latter, and that lead-platinum bielectrodes can be anodically polarised in chloride solutions at high current densities (2). Chlorine evolution on platinum takes place at low overpotentials (3), so that this reaction occurs in preference to oxygen evolution, although the latter is the thermo-dynamically preferred process; platinum in contact with lead will therefore tend to act as a potentiostat and to limit the potential of the lead/solution interface to about 2.5 volts. It has been shown (4) that the PbO2 film thickens when anodically polarised at high current densities in sea-water, and since the volume ratio of PbO2/Pb is greater than unity, the film is under considerable expansive stress. This results in the formation of blisters which eventually rupture, with subsequent formation of PbCl2 and corrosion of the exposed lead and, at constant current density, the potential of the electrode will increase since the PbCl2 is nonconductive.
This can be demonstrated by forming PbO2 on a lead-platinum bielectrode and then removing the platinum microelectrode, when the potential immediately increases. When platinum is in contact with the lead this increase in potential cannot occur, and PbO2 will re-form at the lead exposed at a ruptured blister without excessive corrosion.
Design and Construction of Bielectrodes
The lead-platinum bielectrode consists of an extruded bar of lead or lead alloy, 1 to 1.5 inch diameter and up to 12 feet in length, into which are inserted platinum micro-electrodes at 6 to 12 inch intervals. The micro-electrodes consist of small wires of platinum (with a small percentage of an alloying metal to increase hardness) 0.5 inch long by 0.030 inch diameter, inserted in the lead by drilling a 0.025 inch diameter hole, tapping in the wire until it is flush with the lead surface and then peening the surrounding lead to ensure good electrical contact.
The nature of the lead used in the bielectrode is important and recently an extensive series of tests on cast lead and lead alloys and on dispersion-hardened lead have been carried out in collaboration with D. P. Peplow of the Central Electricity Generating Board. The illustration overpage shows cast lead alloys (1 inch long by 1 inch diameter) and extruded dispersion-hardened lead (1.5 inch long by 0.5 inch diameter) on test in a water box at Marchwood Power Station. These anodes were polarised at 50 A/sq. ft for one year in order to assess the effect of the composition on the growth and spalling of the lead peroxide, and the results can be summarised as follows:
The use of lead-platinum anodes in cathodic protection installations is largely confined to Metal and Pipeline Endurance Limited (MAPEL) in England, and to Lockheed Aircraft Service Corporation of Ontario, California, both of whom, on the basis of over ten years’ experience, are using them for a variety of marine structures.
One of the earliest applications by MAPEL was for the cathodic protection of the cooling water culverts in a power station in Malacca, Malaya. These anodes have now been operating satisfactorily for ten years at 25 A/sq. ft. Other examples of large structures protected by MAPEL are the jetties in Europort and North Sea gas platforms such as that shown in the illustration on page 43. Here the bielectrode is mounted on a steel frame which is attached to the legs of the platform so that both are protected.
The Lockheed Aircraft Company have used these anodes extensively for the protection of bulk carriers, tankers, liners, offshore drilling rigs and oil-wells and this company has recently been awarded the contract (5) for the protection of the San Francisco Bay Area Rapid Transit Tube, a steel tube four miles long and some 130 feet in circumference which will carry rail and road traffic under the Bay; this is probably the largest cathodic protection installation ever undertaken.
Advantages of Lead-Platinum Anodes
Lead-platinum bielectrodes are cheap, robust and easily fabricated, and can be used economically in sea-water over a range of current densities, from 10 to 70 A/sq. ft; a large anode operating at a low current density is sometimes an advantage when a uniform current distribution is required on a large structure. The presence of an a.c. ripple on the d.c. produced by transformer-rectifiers causes the slow corrosion of platinum, which can be serious when the platinum is in the form of a very thin coating on a valve metal. The effect of a.c. corrosion on the relatively massive platinum microelectrode is insignificant, and this has the advantage that single-phase equipment produces no problems when lead-platinum anodes are used in a cathodic protection system.
The experience gained over the past ten years has shown that the lead-platinum bielectrode provides a cheap and reliable anode for the cathodic protection of marine structures, and it is envisaged that its importance will increase in the future.
L. L. Shreir and A. Weinraub, Chem. and Ind., 1958, 1326 ; L. L. Shreir, Platinum Metals Rev., 1959, 3, 44
E. L. Littauer and L. L. Shreir, Proc. First Int. Congr. Metall. Corros., p. 374, Butter-worths, London, ( 1961 ); L. L. Shreir, Corrosion, 1961, 17, 90 ; E. L. Littauer and L. L. Shreir, Electrochim. Acta, 1966, 11, 465
E. L. Littauer and L. L. Shreir, Electrochim. Acta, 1966, 11, 527
D. B. Peplow and L. L. Shreir, Corrosion Technology, 1964, 11, ( 4 ), 16
E. L. Littauer, Private communication