The corrosion of underground cables

Abstract

Modern telephone systems utilize extensive networks of bare lead-sheathed cables drawn into earthenware conduit. As with other underground structures, the cable sheaths are liable to corrosion. In the absence of electrolytic stimulation the rate at which this takes place depends more on the nature of the soil waters, etc., than on the composition of the sheathing material. The danger is increased by the electrolytic action of stray currents passing between the sheath and its environment by ionic paths. Such currents may be the result of leakage from electric traction systems with uninsulated return, or may arise from the setting-up of primary-cell actions due to inhomogeneities in the environment of the cable.Laboratory examination of samples of corroded sheathing, together with chemical analysis of the corrosion products and soil waters, enable definite conclusions to be reached as to whether there has been electrolytic stimulation. In general; this produces a pitted surface, an intercrystalline type of attack and, with normal soil waters, a corrosion product rich in chloride. The actual field measurement of the intensity of the current discharge from the sheath in the dangerous anodic areas does not yet appear possible. Useful information has, however, been obtained from apparatus recording the sheath current at two points a short distance apart. Increasing use is also being made of standard “half-cell” electrodes for the accurate determination of the small potential differences existing between a sheath and its immediate surroundings.Electrical drainage has been used extensively abroad for the protection of cable sheaths in the stray-current fields of electric tramways. In Italy this method has been used with shunted insulating gaps in the cable sheaths in such a way that there is no increased danger to other buried pipe systems. The British Post Office has been successful in obtaining protection solely by the planned insertion of numerous insulating gaps in a cable system. Protective coverings are in use, although they make the pulling of cables into and outlof cable ducts more difficult. It has been found, however, that the chemical attack of corrosive soil waters may be retarded by the addition of a chemical inhibitor to the lubricant used when the cable is first drawn in.