In recent years great damage has been done to all kinds of underground piping by the action of electric currents, chiefly from electric railway systems. This damaging action afreets water mains and service pipes, gas mains and service pipes, the lead sheathing of underground telephone and telegraph lines, and in fact any line of underground piping, regardless of the nature of the metal of which it is made. In the action of the ordinary galvanic battery, such as is used for house bells, two metallic plates are used, one of these generally being zinc, and the other some metal which will not oxidize so readily as zinc.
When two such plates are immersed in a saline solution, and a circuit completed by connecting a wire from one plate to the other, it is a well-known fact that the more easily oxidized plate will be acted upon chemically and decomposed. It is for the reason that this chemical action in time destroys the zinc plate that battery zincs must be replaced in batteries at longer or shorter intervals. This destruction of a metal by means of the passage of an electric current, is known as electrolysis, and is an action which is constantly going on underground, in the vicinity of trolley tracks.
It is the practice in the operation of most electric-railway systems to carry the electric current to the end of the line through large wires, and to carry it back to the dynamos through the rails. As the rails are not separated or insulated from the surrounding earth in any way, there is nothing to prevent a part of the current from escaping from the rail and passing into and through another near-by conductor. An electric current will always take the path that is easiest for it; that is, the path that has the least resistance. Whenever an electric current passes a point where it may take either of two or more paths, it will always divide, a part of it passing through each path that is open to it, and the path that presents the least amount of resistance to its passage will receive the largest part of the current. If the rails of the trolley system were welded together and therefore one continuous conductor, the action of electrolysis would be much less prevalent. As it is, however, the rails must be bonded, and at these joints the greatest resistance is to be met. Even though two rails might have their ends pressed together as closely as possible, there would still be at this joint a resistance to the passage of the current many times greater than the resistance it would meet at any intermediate points in the rail. Even when the rails are connected together by means of copper wire attached to the rails in the most approved manner, the resistance at the points of connection will be very great. It is at such points of resistance as these that the electric current will jump from the rail to some other conductor which offers less resistance, and this easier path for the current is often supplied by a near-by line of underground piping. If the current would only continue in the pipe, and not leave it, the pipe would not be damaged, any more than the rail is damaged by having the current pass through it.
It is at the points where the electric current jumps from the pipe to the rail again, or to some other conductor, that the damage comes, and also at fittings. The current in passing from the pipe, through the joint and into a fitting, does specially harmful work. It is not at the point where the current enters the pipe, or at intermediate points along the pipe that the pipe is destroyed, but at those points where the current leaves it. This point is not generally understood.
While all kinds of piping are subject to the action of electrolysis, and valves as well, cast iron is probably less harmfully acted upon than the other metals, although there are many instances where cast-iron water mains have been very seriously damaged.
There are, however, several instances recorded, where serious damage was done to wrought-iron and lead pipes, while the cast-iron mains, which were apparently subject to the same conditions, were practically unharmed. An explanation of this result is not clear, although it has been suggested that in the casting of the iron pipes in sand moulds, a sort of silicious coating forms over the pipe, which acts as a protection to it. The plumber is naturally much interested in the methods that may be employed to prevent the action of electrolysis. It may truthfully be said that there is really no practicable remedy which may be applied at an expense which is not prohibitory. The owners of electric-railway systems may often considerably reduce the cause of damage, but that is not the part of the question in which the plumber is interested. If the pipe that is affected can he surrounded by some suitable non-conductor, the trouble may be remedied, but it is a most difficult matter to provide a suitable nonconductor. Many materials that above ground might be used as non-conductors, cannot be used underground for the same purpose, as they absorb moisture and become conductors. The use of as-phaltum, resin, wax, and other substances has been tried, but they are not generally practicable, as a coating of such material is liable to crack and fall off, and in addition is too expensive to apply. In some cases, about the only thing that can be done is to provide for taking out sections of pipe, that are being constantly destroyed, in as easy a manner as possible. Sometimes it is well to encase the pipe in another pipe, in which case the current will often act on the outer pipe only.
The action of electrolysis has caused the plumber an endless amount of annoyance in a great many instances, as one pipe after another has often been destroyed, and the cause many times being unknown, the plumber has been blamed for results that are practically beyond his power to remedy.
In addition, the gas and water and telephone and telegraph companies have suffered enormous losses. In the case of the gas and water companies, especially the former, the loss has not been entirely on the piping, but loss of great extent has occurred in the waste of gas or water carried in the pipes.
The action of electrolysis is not confined alone in its destructive action to underground piping. The steel frames of large city buildings, the steel framework of elevated railways, and much other construction work of a similar nature has also been very seriously impaired from the same cause.
The great losses due to the action of electrolysis, and the danger attending the results of such action, have become of such importance that a very large amount of money has been offered by a leading scientific institution for a practicable remedy that will overcome its effects.