This material is far shorter lived than even wrought iron, and is entirely unsuited to the plumbing system, which should be expected to render service almost as long as the house in which it is placed.
The only way in which either wrought-iron or steel pipe can be used with any degree of safety is by coating it with a non-corrosive substance such as galvanizing, which is demanded by all ordinances on plumbing. Even when so protected, there will be thin places in the coating, and whenever the pipe is cut, the coating at the ends of the pipe is more or less damaged, so that the steel or wrought iron is left bare. At such points corrosion gets in its work. A scale is formed by this galvanic action, over the exposed surface, which in time exposes a fresh surface to be acted upon, the scale forming again, and again falling off. Thus the action continues until a hole has been eaten entirely through the pipe. The action of gases and acids in the sewage, and in the vapors and steam that rise from the sewage, tends to increase this corrosive action in a marked degree. Cast iron, however, is much more free from such corrosion, for it simply rusts over on any exposed surface, but does not scale, the rust actually forming a sort of protection for the piping.
An important agent in the corrosion of wrought iron and steel is the condensation of vapors on the sides of the pipe in the form of drops of water, which quickly oxidize any exposed surface which they come in contact with.
Mild steel is especially objectionable, as it is so filled with impurities that it rapidly decays wherever they exist.
The vent system is open to the injurious effects of corrosion to an even greater extent than the drainage system, for the latter is often covered with a slime which acts as a protection against such action.
While the screw joint is the strong arguing point in favor of the Durham system, it is right at this point that the most serious trouble may be expected, both on the drainage and on the vent lines. Wherever a thread is cut, the material of the pipe is entirely exposed, and whenever threads project out from the joint, which often happens, there is not only abundant opportunity for corrosive action to take place, but there is a large surface to act upon, because of its being threaded, and owing to the depth of the thread there is less thickness of metal to be eaten through, before the pipe is punctured. In the case of mild steel, especially, it takes only a few years to accomplish such a result under the above conditions.
It is a very easy matter for most users to be imposed upon in deciding from the appearance of pipe, whether it is wrought iron or steel. A very large part of the pipe now turned out is of steel.
The following shows some of the differences between iron and steel. Iron pipe looks rough and has a heavy scale, while the scale on steel pipe is much lighter and in the form of small bubbles, with a smooth and rather white surface beneath.
Steel pipe, when spread out, seldom breaks, while iron pipe breaks easily. A break in the former shows a very fine grain, while that of the latter is much coarser.
Steel pipe is not hard and its threads tear rather than break. Dies that are used on steel pipe may also be used on wrought-iron pipe, but blunt dies that work satisfactorily on wrought-iron pipe will tear the softer threads of steel pipe.
A few remarks concerning the length of life of wrought- and cast-iron pipes under actual working conditions, and the conditions which act to protect or destroy them, may be of interest. A case is on record of the complete decay of an entire underground wrought-iron gas-supply system in eleven years, the cause being in this case traced entirely to external conditions and not to the gas which the pipes were carrying. In the same town experience shows that wrought-iron water-service pipes have a life generally of about seven years. Cast-iron pipes have been known to fail through softening of the metal after a period of use underground of from thirty-five to fifty years. This action, however, is very rare, and the failure of cast-iron pipes, when laid underground, may generally be traced to defects in manufacture.
A few years ago in the city of Los Angeles, the cast-iron water mains were uncovered in over three hundred places, and the pipes, which had been laid nearly thirty years previous, were found to be in almost perfect condition.
It was found that the coating of asphalt had almost entirely disappeared, that in sandy soil the bare pipe had not rusted, and that in other moist soil it had rusted somewhat but was almost uninjured. In conclusion, it would seem advisable to use cast-iron pipe for drainage purposes wherever possible, and that when impossible or impracticable, nothing but wrought-iron pipe heavily galvanized should be used. Steel pipe should never be used.