Metallic cable sheaths can utilize anti-corrosion coatings to achieve passivation protection. In the past, lead was primarily used for cable sheaths. Nowadays, cables employing steel, copper, or aluminum as sheath materials or for armor are increasingly common. Under the same anodic current density, the corrosion rate of lead is 2.5 times that of iron. It is important to note that the wall thickness of telephone cables is much thinner than that of pipelines, making lead-sheathed cables more susceptible to damage than pipelines. Consequently, uncoated lead-sheathed cables are no longer used.
When directly buried, lead sheaths are protected from corrosion by being wrapped alternately with impermeable impregnated paper and viscous asphalt layers. Small-diameter cables are mechanically protected by closely wound round steel wires, while large-diameter cables use flat steel strips to enhance mechanical strength. An impregnated jute layer is wrapped over the reinforcing material, offering some anti-corrosive effect, but it does not insulate the cable sheath from the soil. Additionally, thick polyethylene sheaths are used, which are seamless and free of pinholes. Therefore, cathodic protection is mainly applied to jute-wrapped lead-sheathed cables and plastic-sheathed cables with an outer metallic reinforcement. Cables with external metallic sheaths can be included in cathodic protection systems but require special arrangements.
When using corrugated steel-sheathed cables, non-overlapping carbon steel strips are welded lengthwise along the cable conduit to form a sealed closure. The core wire bundle sheathed using this method is corrugated, making it more flexible. The corrugated bends are filled with a viscous material that bonds the metal to the plastic, and the outside is wrapped with a plastic tape. A polyethylene outer jacket is then extruded over this layer. This plastic sheath is effectively free of pinholes, thus providing good anti-corrosion protection. Only the sleeves and mechanically damaged areas are exposed to the soil.
