"OF CAST-IRON, IN SIMPLE CONTACT WITH ZINC, IMMERSED AT AN INDEFINITELY SMALL DEPTH IN FRESH WATER. "Cast-iron, free from initial rust, so exposed in contact with an equal surface of zinc, is oxidized from the first moment of exposure. The zinc is oxidized from the first, also. "A plate of iron, whose entire surface was covered with zinc in metallic contact [zinced or galvanized iron], was immersed for twenty-five months in fresh water. On examination, much flocculent zine had been formed, and lay at the bottom of the glass vessel, which, in some places, was stained with red oxide of iron. The zinc surface was found, in irregularly scattered patches, wholly removed down to the iron, which was covered with peroxide. Hence, about two years appears to be the limit of the preservative power of zinc to iron in fresh water, applied in fusion over its whole surface by the ordinary method. It is to be observed that the zinc surface was removed by solution, unequally or in patches, indicating local action ab initio; and it has been shown before that as soon as oxidation takes place at any point upon the iron surface, the protective power of the zinc is diminished at once or rendered null. [The corrosion of both zinc and iron then ensues more rapidly.] "The conditions the most favorable possible for rapid oxidation of iron consist in its exposure to wet and dry or to air covered with an indefinitely thin film of water, constantly renewed; thus circumstanced, zinc has no protective power over iron in fresh water, and, on the whole, it may be affirmed that, under all circumstances, zinc has not yet been so applied to iron to rank as an electro-chemical protector towards it, in the strict sense." In a report,* made by Prof. Max Pettenkofer, in reply to the inquiry, how thick a covering of zinc is required to insure permanent protection against the oxidation of iron, the author gives the results of a series of experiments, undertaken by himself, with zinc plate taken from the roof of a building in Munich, where it had been exposed to the atmospheric influences during twenty-seven years. The outer surface was found to be covered with a thick, whitish, oxidized layer, of varying depth, showing that the oxidation had followed the crystalline structure of the metal. By calculation, he determined approximatively, that upon a piece of the zinc, one and a half feet square, there were present 4.264 grammes of zinc rust. By experiment, also, he estimated the amount which had been removed during these twentyseven years, in the rain-water, in solution and by mechanical displacement, as about 4.117 grammes, making a total of 8.381 grammes. The preceding observations, which have been made at * Abhandlungen des naturwissenschaftlich-technischen Commission in München, Vol. I., 1857. different periods, and were derived from a variety of sources, will be sufficient to illustrate the fact that the zinc of galvanized iron is acted upon by water; that, when allowed to stand in reservoirs or to flow through pipes of this material, water will contain a greater or less amount of zinc, for a longer or shorter period; finally, that, sooner or later, the whole of the zinc will be removed. With reference to the second part of our first inquiry, -namely, What are the actual products of this action of water upon the zinc?-the conclusions at which we shall be able to arrive will not be so definite. We know that various circumstances, conditions and processes combine to render water, in its ordinary state, a very complex fluid. Receiving its constituents from the air and ground, in various combinations, the laws of which are imperfectly understood, it contains mineral, vegetable and animal matter in suspension, and gaseous, organic and mineral matter in solution. The mode of combination of these various substances in solution cannot be determined, at least with the means at our command at the present day. It is a popular custom, however, for chemists to ascertain, by analysis, the amount of each constituent and then to calculate the probable chemical combinations which have taken place. Carrying out this latter idea, chemists have reported that they have detected the presence of various soluble salts of zinc in water which has been in contact with this metal; the sulphate and the chloride have been reported, principally, and, in some cases, the statement has been made that water has been found "strongly impregnated” with these salts. The real basis of these conclusions is founded upon isolated chemical experiments, made in the laboratory, like the following:-* "Zinc is rapidly dissolved in a very dilute solution of common salt [chloride of sodium] in water, and may be found in the solution, or water, as the muriate [chloride] of zinc. This would be the action of the common salt in rain-water, and it is the source of the corrosion of zine roofs." "Galvanized iron, introduced into a solution of copperas [protosulphate of iron] in water, very dilute, acts thus: I soon found iron-rust rapidly falling on the galvanized pipe. In a short time all the iron was precipitated from the water, and fell in a coat of rust, while its place in the water was * Extract from a report to the City Council of Lowell, Mass. 1842. From Appendix to "Lead Discases," by L. Tanquerel des Planches, Lowell. 1848. supplied by zinc. In other words, copperas, or green vitrol, was exchanged for white vitrol." "Galvanized iron, in a mingled solution of salt and of copperas, such as is found in several wells in Lowell, is rapidly destroyed; the water becomes charged with salts of zinc." Without entering upon the palpable sources of error in the above experiments, judging them from the published account which is given here, it may be stated that the direct inference implied, that similar re-actions always take place between zinced iron pipes and water passing through them, and containing the above-mentioned constituents,-is unwarranted. At the request of the writer, Professor Wm. Ripley Nichols presents the following communication with reference to the action of water upon zinced pipes, and the products of this action: MASS. INSTITUTE OF TECHNOLOGY, CHEMICAL LABORATORY, MY DEAR SIR:-With regard to the action of water on zinc, it is well known that, when-zinc is exposed to moist air, it quickly becomes covered with a film of oxide, which soon changes, under the influence of the carbonic acid of the air, into a basic carbonate. The oxide at first formed has been regarded as a sub-oxide (Zn, O), but is now generally held to be the ordinary oxide of zinc (ZnO). That this coating subsequently changes to a basic carbonate, and that the white compound of zinc, which is often found in suspension in water which has been in contact with "galvanized" iron pipes, is a (hydrated) basic carbonate, seems to be sufficiently well established. That the compound is not perfectly definite in its composition, but contains sometimes more, sometimes less, carbonic acid, in proportion to the oxide of zinc, is also an accepted fact. What, however, is the state in which the zinc exists dissolved in water, we do not know, and probably cannot know. Although it has been stated in some cases that a given water contained in solution so much chloride of zinc, or so much sulphate, such statements rest upon purely gratuitous assumptions. We have good reason to believe that absolutely pure water would have no action on absolutely pure zinc; but ordinary water contains a quantity, more or less considerable, of different salts, such as chlorides, sulphates, carbonates, and in what form the small amount of zinc in solution exists, it would be impossible to say. We know that zinc is attacked by a solution of chloride of sodium (common salt), and that a portion goes into solution, hydrogen being at the same time evolved. In the case of a strong solution of chloride of sodium, the amount of zinc that is taken up is so considerable, that it is not unnatural to suppose that a portion of the zinc exists as the double chloride of zinc and sodium; but as undissolved oxide of zinc is also a product of the re-action, and as the solution is found to be alkaline, it is probable that, at the same time, some compound of oxide of zinc and oxide of sodium (zincate of sodium?) is also formed. In the case of a drinking-water, which is a dilute solution of a variety of salts, the case would be very different, and although we know of this action of chlorides on zinc, we also know that nitrates and sulphates and other salts likewise attack the metal and are capable also of dissolving its oxide; we know further, that the oxide and all the carbonates of zine dissolve in water containing carbonic acid, so that we are unable to say whether the trace of zinc found in solution existed as chloride, nitrate or sulphate, or as a salt of some organic acid, as (acid?) carbonate, (or carbonate held in solution by carbonic acid), or whether a portion existed in each and all these different states. I may, perhaps, make my meaning more clear by using an illustration. If we mix together very dilute solutions of chloride of calcium and of sulphate of magnesium, we obtain a mixture which is not distinguishable in appearance from the solutions from which it was produced. If we submit it to chemical examination, we find that it contains a sulphate (or sulphates) and a chloride (or chlorides); also, that it contains magnesium and calcium, Analysis does not, and cannot, show whether the solution contains chloride of calcium and sulphate of magnesium or chloride of magnesium and sulphate of calcium, or whether it contains some chloride and some sulphate of calcium and some chloride and some sulphate of magnesium. The latter view, in fact, has the greater probability; the proportions in which the distribution occurs taking place according to some law at present not understood. But according to the fashion, formerly universal, which even now prevails to a certain extent, the solution, if analyzed, would be said to contain so much sulphate of calcium and so much chloride of magnesium, and for this reason: If the solution be concentrated by evaporation, sulphate of calcium will crystallize out, and may be obtained nearly free from chloride of magnesium. This, however, does not prove the previous existence of all the calcium as sulphate, for the condition of things in the liquid is changed by concentration. It is a general law, that when solutions of two chemical substances are mixed, if such a re-arrangement of the acid and basie radicals is possible, as to form a compound, insoluble in the liquid employed, or a gaseous compound, such compound will be formed; but where no insoluble or gaseous compound is formed, we cannot judge of the change which takes place. Therefore, I do not hesitate to say, that we do not, and cannot, know what compound of zinc is present in solution in the case of water which has passed through "galvanized" iron pipes. Yours respectfully, WM. RIPLEY NICHOLS. DR. W. E. BOARDMAN. Vauquelin and Deyeux,* Devaux and Dejaer,† Mallet,* Schaufele,* Gaultier de Claubry, Tardieu,§ Pettenkofer,* Brande and Taylor,|| Bouchardat and Fonssagrives, ¶ W. R. * Loc. cit. + Procès-verbal de la Séance, publ. de la Soc., établie à Liege. 1813. § Dict. d'Hygiène Publique, t. 3, p. 706. 1854. Chemistry, Am. Ed. 1863. Journal de Chimie Médicale, t. 10, p. 594. 1864. Nichols, all state-indeed, it is a well-known fact-that zinc, when exposed to the action of common, potable water, acquires a coating of oxide, which is practically insoluble in water. This coating, subsequently, is acted upon by the carbonic acid, which comes into contact with it, and it results from this, that the layer is finally composed of oxide, carbonate and a combination of these two, regarded as oxyhydrocarbonate of zinc, by Pettenkofer.† By mechanical and galvanic action and solution, the removal of this layer is effected gradually, and the water then contains more or less of these compounds in suspension, while the remainder enters into solution. This much, then, is all that can be stated positively, at present, with regard to the nature of the products in ques tion. In regard to the amount of zinc, in all forms, metallic or other, which may be present in the water, many influences come into consideration. The water may contain ingredients, abnormal in kind or quantity, which will act with unusual energy upon the zinc, or it may be of such purity as to have but a slight action upon the metal. Again, as shown by Mallet, imperfect construction of the material-if the iron be not properly freed from initial rust or if the zinc be incompletely applied, will favor the corrosion of the zinc, for as soon as the iron is exposed, the destruction of the zinc goes on more rapidly. The texture of the zinc, too, whether fine or coarse, affects the results. If impure zinc be employed, it will be more readily destroyed. The length of time during which pipes have been in use, also, is to be taken into consideration. The action of potable waters of the purity of the Cochituate is comparatively feeble. We have seen that this water, drawn through pipes which have been in use for eight or nine years, contained only 0.062 grain of metallic zinc to the gallon, while some chemists have reported the presence of from two to six grains § in the gallon of other waters, and this latter * See communication in this Report. + Loc. cit. Pettenkofer, loc. cit. § Boston Med. and Surg. Jour., Jan., 1871, p. 13. |