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.”
[Illustration: COPPER DRIP PIPE AFTER SEVENTEEN DAYS’ EXPOSURE IN SALT
WATER TO THE ACTION OF ELECTRICITY. Half size.]
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.
[Illustration: 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.]
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 hard, dense, even-grained cast iron, containing
alloyed rather than combined carbon, have not been appreciably
corroded. At Dayton, Ohio, on the other hand, seventy-seven thousand
dollars’ worth of damage has already resulted. One peculiarity of
electrolyzed 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.
[Illustration: 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.]
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.
[Illustration: 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.]
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.
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.
[Illustration: LEAD SERVICE PIPE SHOWING THE IRREGULARITY OF
ELECTROLYTIC ACTION, OR WHAT IS TECHNICALLY KNOWN AS “PITTING.”]
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.
[Illustration: LEAD SERVICE PIPE ILLUSTRATING THE LOCAL EFFECTS OF
EIGHT MONTHS’ ELECTROLYSIS.