Popular Science Monthly/Volume 56/January 1900/Destructive Effects of Vagrant Electricity
|DESTRUCTIVE EFFECTS OF VAGRANT ELECTRICITY.|
REVERTING to the dictionary for a definition, electrolysis is the process of decomposing a chemical compound by the passage of an electric current through it. Electroplating is a popular illustration of this definition, having been numbered among the industrial arts for nearly a century.
If in a bath of sulphate-of-copper solution are placed a copper plate and a plumbago-covered wax mold, the passage of an electric current through the solution, from the plate to the mold, will result in the deposition of copper upon the mold, or negative electrode, and the wasting away of the plate of copper, or positive electrode. Generalizing from this and other experiments, it may be broadly stated that the passage of an electric current through a solution of electrolyzable metallic salt, from an oxidizable metal to some other conductor, will be attended by the separation of the salt into two parts: first, the metal, appearing at the negative electrode; and, second, an unstable compound of the remaining elements. This unstable compound is supposed to unite with the hydrogen of the water, liberating oxygen, and forming an acid. Both oxygen and acid appear only at the positive electrode, which is thus made subject to a double decay—a corrosion by oxygen and a solution by acid.
There is nothing new about this. It is not even a novel statement of a fundamental electro-chemical truth. In times past, however, we were wont to consider this matter as pertaining solely to the laboratory or to the electroplating industry; now we are forced to see that the reproduction of this experiment on a grand scale is attended with results as disagreeable as they are widespread.
Hidden beneath our highways lie gas pipes, water pipes, railway tracks, Edison tubes, cement-lined iron subway ducts, and lead covered cables. These are the electrodes. In contact with these conductors is the soil, containing an electrolyzable salt—chloride, nitrate or sulphate of ammonia, potash, soda, or magnesia, generally. In the presence of moisture this soil becomes an electrolyte, or salt solution. In the absence of electricity no appreciable damage occurs; but the passage of an electric current, no matter how small, from one pipe to another is sure, sooner or later, to leave its traces upon the positive conductor in the form of a decay other than
mere oxidation. It is to this decay that has been given the name of electrolysis; so that when this heading appears in the daily press or in technical journals one may interpret the term popularly as "the electrolytic corrosion of metals buried in the soil."
To produce electrolytic disintegration of pipes, etc., on a scale grand enough to cause apprehension, a bountiful source of electricity is essential. Unfortunately, this condition is not lacking to-day in any town in which the usual overhead trolley electric railway is in operation. This system of electric propulsion is based upon the use of a "ground return"—that is to say, the electricity passes out from the power house to the bare trolley wire, thence to the pole on the roof of the car, thence through the motors to the wheels, whence it is expected to return to the power house, via the rails.
As a matter of fact, however, the released electricity by no means confines itself to the rails and the copper return feeders—legitimate paths provided for it. It avails itself, on the other hand, of what may be termed, for brevity's sake, the illegitimate return—comprising all underground electrical conductors except the rails and return feeders, and including subterranean water-courses, sewers, and metallic earth veins.
In the light of our experience of the last eight years, it is easy to identify as electrolysis the effects shown in the accompanying cuts of buried metals that have been actually subjected to a flow of electricity. It is not to be inferred that the destructive action here depicted is universal throughout our towns, but, rather, that the damage occurs in spots, its rate of progress being dependent upon the amount of current and the duration of the flow. Dry, sandy soils tend to keep down the flow of current by interposing a high resistance, so that in such localities electrolytic effects are not as pronounced as in wet, loamy soils. In the same way, the character of the pipe surface or coating, if there be any—acts as a partial barrier to check the passage of electricity.
Until recently it was generally supposed that cast iron was not attacked—at least not rapidly enough to cause alarm. In Brooklyn the water mains, of very
Wrought-Iron Service Pipe for Water after One Year's Burial beneath a Trolley Track The fibrous appearance of the surface is characteristic of wrought iron and steel.
Lead Service Pipe after Eight Months' Burial in Builders' Sand. The collapsed appearance of the pipe is due entirely to the removal of the lead by electrolysis, the bore retaining its original shape. The dark spot on the upper surface of the pipe is the point of rupture. One third size.
cast iron is that the original shape is usually retained, the iron being eaten away and leaving a punky formation of pure or nearly pure graphite. In such a case a superficial examination detects nothing wrong, and it requires a mechanical scraping to show that the strength is not there. For this reason good photographs of cast-iron electrolysis are somewhat hard to obtain.
The reason for the comparative immunity of cast iron is not as yet definitely understood. It certainly does not lie particularly in the asphaltic varnish usually applied, for this varnish affords little or no protection when used upon wrought iron or other metals. Nor can it be accounted for by the composition of cast iron itself, inasmuch as a fractured or brightly scraped surface of cast iron shows approximately the same symptoms as other metals when acted upon by a given current for a given time. Whether the iron oxide is the saving feature, or whether the "skin" due to the process of casting acts as an insulator, is not yet settled.
When the trouble first appeared in Boston, in 1891, its cause was promptly identified. The electric-railway construction of those days was so crude, however, that many well-informed electricians fell into the error of assuming that heavier rails, more and larger return feeders, and better bonding (i. e., wire connections from rail to rail, around the joints, designed to decrease the resistance) would prove a panacea for all electrolytic ills. Indeed, this view is still held by a surprisingly large number of men versed in matters electrical.
I am of the opinion that it is impossible, from a financial standpoint, to provide so satisfactory a legitimate return that considerable electricity will not seek a path through pipes, cable covers, etc.; for, in order to confine the electric current to the rails, the resistance of the earth and its contained pipes would have to be infinitely great, and this condition can be realized only by making the resistance of the rail infinitely small as compared with that of the earth. The cost of arriving at this condition is prohibitive, and the improved track return is, and always must be, a palliative merely, not a cure.
Assuming, then, that under the most favorable character of electric-railway construction some of the current may be expected to stray from the straight and narrow path, it behooves us to consider how it may best be cared for in order that it may not cause electrolysis. Since corrosion of this nature occurs only at those points where electricity leaves the metal, one might suppose that the attachment of a conducting wire to the affected part would result in the harmless carrying away of the current. In isolated cases, in small towns, such a plan might accomplish the desired result. It is open to the objection, however, that it in a measure legalizes the conveyance of electricity on conductors other than those designed for the purpose.
Lead Service Pipe showing the Effects of Eight Months' Electrolytic Action, and clearly illustrating the Fact that Damage occurs only where the Electricity leaves the Conductor. The interior surface Is unattacked.
In larger towns, with more than one power house and with car lines radiating from and circumscribing the business center, the attachment of conducting wires entails a ceaseless disturbance of the electrical equilibrium, curing the evil in spots and developing new danger points. Furthermore, these connections tend to decrease the resistance of the total illegitimate return, thereby tempting a greater flow of electricity along other paths than the rails and track feeders. It has been generally believed that this increased current would develop electrolysis at the ends of the pipes, due to the jumping of the electricity around the presumably high resistance of the joints; and, indeed, many samples of such corrosion are in existence. I have found, however, that it is possible to calk a bell-and-spigot joint in cast-iron pipe in such a manner that the resistance is practically nil; and as for wrought iron or steel, the joint resistance may be made as low as we please by fitting the surfaces so carefully that white-leading is unnecessary. Arguing from the fact that the negative electrode is not attacked, it has been suggested to employ an auxiliary dynamo and a special system of wiring, in order to maintain the pipes, etc., at all times and at all points, negative to the rails. Could this ideal condition be realized, the rails alone would suffer. We can not
hope, however, to thus easily solve the problem in towns where the distribution of buried conductors is at all complex.
It has been suggested, also, to discourage the flow of electricity along pipes and cable covers by inserting insulating sections of wood or terra cotta. This plan has never been tried on a scale large enough to afford a suitable demonstration of its utility. while it might reasonably be tried on new construction, its application to old work is almost prohibited by the attendant expense.
Attacking the problem from a directly opposite standpoint, there seems to be a chance of successfully invoking the aid of some purely chemical method of rendering lead and iron innocuous, electrolytically
speaking. If we can obtain an insulating oxide, lacquer, or varnish that will retain its high-resistance properties during the ordinary lifetime of the buried metal, it will be possible to effectually protect pipes and cable coverings by coating them prior to burial. Or, if we can stumble upon an electrolysis-proof alloy, formed by the addition of a few per cent of some foreign metal to the pipe material during manufacture, the buried conductor will need no protection whatever.
But, supposing that we discover this lacquer or this alloy and by such means guard against damage to all new construction, how are we to care for the metals already buried? We can not dig them all up and paint them, neither can we attempt to replace them by the new alloy. I do not see that the state of the art to-day presents any solution of the difficulty, other than the banishment of the single trolley system. None of the electrical remedies (so called) offers more than partial and temporary relief, and the chemical field is just beginning to be explored.
Permit me to state most emphatically that this is not intended as an argument in favor of the abolishment of single trolley systems. Our civilization owes more to them than could be rehearsed in catalogue form within the limits of one issue of this magazine. We have nothing at present that can be employed as a satisfactory substitute for the ordinary electric railway. The underground trolley is a safe substitute, but the great expense of installation renders it available for very few localities. The overhead trolley, with two wires and no ground return, is cumbersome, vexatious, and unsightly. The storage battery is more or less experimental in its nature. The electro-magnetic contact systems, with plates set in the pavement at stated intervals, make no pretense of avoiding electrolytic troubles. The compressed-air motor has yet to receive popular approval.
There seems to be a mistaken impression abroad that the railway companies are indifferent to this subject. So far as my experience and information go, this is not the case. They are only too anxious to find a remedy—not, as some electricians have stated, to save their coal-pile, for energy is wasted in forcing the electricity back to the power house, no matter what the path, but because they fear that at some future date the taxpayer, the corporation, and the municipality will band together, present overwhelming bills for damages, and sweep the trolleys off the face of the earth. The instinct of self-preservation, if nothing else, demands that the electric-railway companies should put forth every endeavor to solve the electrolysis problem.
And yet, conservative judgment requires that the railway companies should not take the initiative. It is one of boyhood's maxims not to shoot arrows at a hornet's nest unless one has mud handy to apply to the subsequently afflicted part. Thus it happens that the railway company remains apparently inactive, bearing the burden of public condemnation, while we, whose lethargy is responsible for failing pipes, read electrolysis articles in the daily press and wonder how soon the impending catastrophe is likely to occur.
This condition of affairs is deplorable; for, while we may not care how extensively or how frequently the city authorities or the private corporations are obliged to renew their underground metals, we are at least vitally concerned as to whether the stray electricity is endangering our steel office buildings, our bridges, our water supply, our immunity from conflagrations, and the safety of the hundred and one appliances that go to make up our modern civilization.
Are the Brooklyn Bridge anchor plates going to pieces, or are they not? Are the elevated railroad structures about to fall apart, or are they not? The consulting electrical engineer says "Yes," the railway man says "No." The municipal authorities say nothing. "When doctors disagree———"
I deem it doubly unfortunate that so much valuable brain energy has been inefficiently expended in the discussion of electrolysis. Each writer has viewed it from his own standpoint. Electrical literature has acquired in this way a series of views, interesting and instructive, but also bewildering. There is no composite view, such as might be obtained from the report of a commission composed of a technical representative of each of the interests affected. So far as I am able to learn, such a commission has never existed.