Note on the Action of Water on Galvanized Iron.*
“Notes’’ bearing this caption are usually concerned with the danger to the water consumer front zinc dissolved from the coating. The case here presented is considered primarily from a different standpoint—namely, the action of a partially softened water upon the zinc coating of galvanised iron pipes as affecting their durability. A new meter with a galvanised iron cup, subjected to unusual service during twenty-four days registered 128,266 cu. ft., or 961,995 gal. Upon removing the meter cup at the end of this period, it was found to contain a soft, adherent, yellowish-white coating. This was loosened by gentle rubbing with a rubber-coated glass rod and rinsing, without abrading the harder underlying layer. The weight of the materials removed dried was 2.5 grains. Its analysis gave these results:
The cleaned surface showed a hard, brittle, black layer about 1/100 of an inch in thickness, rich in sulphides and in carbon, overlying the cast iron body of the cup, with only traces here and there of the original zinc coating, which had been almost entirely removed or converted in place into zinc oxide and basic carbonate. No pitting or tuberculation of the iron had taken place, al though one short rust-streak occurred where the iron seemed freshly exposed—-perhaps, by a scratch made by the attendant who removed the cup. The water which effected this change was fairly uniform during the period, as shown by frequent examinations, yielding the following average results;
•Paper read at the twenty-eighth annual convention of the American Water Works association; Washington, D. C., May, 1908.
The absence of force carbon dioxide and the presence of considerable quantities of bicarbonate and neutral carbonates may be considered as the important factors in affecting the zinc, in connection with the abundant supply of dissolved oxygen which the water carried at temperature ranging from 42° to 52° Fahr. The questions raised by the facts presented are: First: The danger of contamination by zinc in solution; second, The value of the zinc coating as a protection for iron pipes. As to the first: It may be said that, despite the rapid flow through the meter of almost 1,000,000 gal. of water, so large a portion of the zinc compounds remained in place that there is small chance of serious pollution from the slight quantity of basic carbonate oxide which could dissolve. Furthermore, repeated tests of this water, after exposure to zinc, and its oxide and basic carbonate for a period of an hour, show only infinitesimal traces of zinc. As to the second point: The continuous action of an excess of this water through twenty-four days was sufficient to remove almost the entire body of zinc or to convert it into the compounds above mentioned. While the coating left had sufficed thus far to prevent turberculatton of the iron, it could not be relied upon to continue this protective action indefinitely. It is true the amount of water passing the meter was equivalent to about twelve to fifteen years’ supply for the ordinary consumer: but the short time (twenty-four days) in which the action occurred points to a much shorter life of the zinc surface than fifteen years, It may he that the underlying thin layer of sulphides and carbon would retard the further action of oxidation of iron for a time. But in these very impurities in contact with the iron lies an element of danger, in that there* may develop differences of potential which would result in local couples. Unquestionably the pasty adherent zinc compounds may be considered more effective than the thin subjacent layer as a protective coat ing to the iron beneath. It must be remembered, however, that the total zinc in the 2.5 grams of compounds (=1.5 grams zinc) remaining in the cup represents but a very small portion of the original body on the surface; since it is equivalent to a film of 0.003 centimetres in thickness, or o.oot t8-inch over the surface of the cup. The larger part of the original zinc coating had been removed during its temporary solution in the formation of the compounds or forming in place, these products had been eroded by the ratlin stream of water. That the residual coatings afford no permanent protection to the iron is evidenced by the tuberculation which occur? in galvanised pipe after long use in rapid flow. A section of a 1 1/2-in. galvanised iron pipe which had been in use for an indefinite period is here shown. The tuberculation is quite well developed, and the pitting of the surface between tubercles shows how completely the protection has failed. The distribution of these tubercles suggests the action at some time of zinc-iron or some other couples. A strip of commercial zinc exposed to a trickling stream of water in a glass container, develops within twenty-four hours a series of spots of white products. If these are removed, and the zinc is cleaned, the formation recurs on further exposure to the water in the same spots distributed irregularly over the surface, but persistent each in repeated experiments. With galvanised iron the action is similar; the initial distribution of affected spots on the coating is persistent, and seems to indicate the location of a series of couples which accelerate the destructive action. Some other cases of such action occurring in St. Louis within the last few years may be cited. It is difficult in many instances to ascertain the time during which the pipe has been in service. A section of galvanised iron wipe used four months during the construction of a building was coated thinly over the entire inner surface, with spots of thicker growth irregularly distributed. The thicker portions were rich in zinc, indicating the locally developed action which was carried further in other cases mentioned below. A piece of ¾-in. pipe 10)4 in. long used eight years in the cold water supply to a kitchen boiler was cut longitudinally and the coating removed. There was collected 1.74 grams (dried to 105° Cent.) of loosely foliated light substance rich in zinc and poor in calcium and magnesium, and 2.59 grams of a denser matter containing more calcium and magnesium, with slightly less zinc and a large percentage of clay and siliceous matter. This pipe was in service four years prior to the installation of the clarification process. The larger amounts of siliceous matter in all these samples is due to the accumulated mud from the unclarificd water in the mains and service. In this case the flow of water through the pipe had been too weak to erode the zinc compounds formed; the deposit had therefore, accumulated to a thickness of from 1-16 to 1/4-in. After the removal of the deposit, the zinc coating of the pipe was found unbroken, save in one spot, where the exposed iron wasL’slightly corroded. In a section of hot water line the same condition was found. The bore of the tube was seriously constricted. The line leading from a small heater three years in service was almost closed by a similar deposit. It appears from the cases cited, which are but a few of those brought to my attention within the past two years in St. Louis, that the zinc compounds formed by the action of the water remain in place, save when the force of the flowing water is very great The action upon the zinc coating is retarded, but not stepped by the accumulation. In a wellcoated pipe there is enough zinc to cause serious constriction by its conversion into the bulky com pounds resulting. In many cases manufacturing establishments have used black iron pipe in their installations. In one case, a 2-in. line four years in use showed but slight tuberculation. In another a ¼-m pipe was completely closed by accumulated ferrous carbonate. In a third the growth of crenothrix in a large iron tank, together’with the roughening of small iron lines leading from it, had led to stoppage of the flow through small orifices supplied from this source.
The waterworks system has been satisfactorily completed at Slocomb, Ala.