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Platinum Metals Rev., 1971, 15, (3), 90

Cathodic Protection of Rapid Transit System

Extensive Use of Lead-Platinum Bielectrodes

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The Bay Area Rapid Transit System (BART), connecting San Francisco, Alameda and Contra Costa counties in California, is to cover some 75 miles and boasts many innovations, among them the use of 5 ft 6 in. gauge track with long-welded rails to obtain more room in the luxuriously appointed cars and to ensure greater stability.

The main link of the BART system is a mile tube, the longest in the world, that crosses under San Francisco Bay to connect San Francisco with Oakland. The train tube is made up of 57 prefabricated concrete-lined steel shells. These sections were placed in a 56 ft wide trench dredged in the bottom of the bay to a maximum depth of 130 ft below the surface of the water. Lead-platinum anodes are used to protect the steel shells against corrosion.

Cathodic protection has been demonstrated to be the most efficient and economical method of providing underwater steel structures with protection from corrosive attack. Small pieces of platinum inserted into surfaces of lead or lead alloy anodes cause a remarkable change in their behaviour as electrodes, making them suitable for cathodic protection (1). Since such lead-platinum bielectrodes are inexpensive, robust and easily fabricated, they have been widely and successfully adopted for the cathodic protection of marine structures, for example offshore drilling rigs, cooling water culverts at power stations, jetties, and large ships (2).

A lead-platinum anode array mounted on its platform ready for installation under San Francisco Bay. (Courtesy of Lead Industries Association, Inc.)

A lead-platinum anode array mounted on its platform ready for installation under San Francisco Bay. (Courtesy of Lead Industries Association, Inc.)

The system used by BART on the tube consists of 16 lead-platinum anode arrays placed along either side of the tube at about 300 ft from the steel shell. With the exception of two platforms which are buried 20 ft below the ship channel all the anode-bearing platforms are located on the sea bottom.

Each lead-platinum anode array consists of a creosoted wood platform 30 ft long by 5 ft deep. Ballast for the platform is concrete. A lead anode 30 ft long by 3 in. diameter and weighing 1,048 lb is mounted on the platform. Each lead anode contains 75 platinum inserts spaced about 5 in. apart and these are 0.040 in. diameter by 1 in. long. The inserts extend in. into the lead anode. Rectifiers inside the tube feed 24 volt d.c. to the anode and the negative terminal is connected to the steel train tube. The lead-platinum anode supplies a current that cancels out or neutralises the local corrosion currents occurring on the steel train tube in its seawater environment. In effect the tube becomes a cathode and acts as the pole of a large battery which then corrodes only very slowly.

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References

  1. 1
    L. L. Shreir, Platinum Metals Rev., 1959, 3, ( 2 ), 44 – 46
  2. 2
    Ibid., 1968, 12, ( 2 ), 42 – 45

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