DECEMBER, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
iron, and it also tends to prevent the precipitation of iron which was initially present in the water. The present investigation has been concerned chiefly with the corrosion of steel by waters which simulate, in composition and temperature, those encountered normally in cold-water distribution systems. The quantitative data have all been obtained with Pittsburgh tap water, with and without further chemical treatment. However, qualitative tests of the effect of threshold treatment upon the corrosion of steel have been conducted on a number of municipal water supplies with a portable apparatus essentially similar to that used in the present laboratory investigation (Figure 1); the effect of the treatment was evaluated by comparison of the appearance of steel wool which had been exposed to the water supply with that which had been exposed to the threshold-treated supply. A rather wide range of types of municipal supplies has been tested in this manner-soft waters typical of the Atlantic Coast, hard bicarbonate waters of the Midwest, and the effluents of both zeolite and lime-soda softeners-and in all cases the treatment with small amounts of hexametaphosphate has been found to result in a marked decrease in the rate of corrosion. Numerous tests in the field, which specifically show the effect of threshold treatment upon corrosion, are yielding promising results (8,17,22).It might be mentioned that much industrial heat-exchange equipment is being maintained in a scale-free condition by the use of threshold treatment with hexametaphosphate without a resultant difficulty with corrosion. However, data are not available concerning the corrosive attack on bare metal surfaces in these systems by the untreated water, owing to the heavy scale upon these surfaces prior to the treatment. Investigation of the effect of threshold treatment upon the inhibition of corrosion under a variety of special conditions, as well as upon the stabilization of dissolved iron, is being continued in this laboratory. The ease with which continuous-flow tests of the type described in this paper may be conducted has led other investigators to make direct tests upon various industrial and municipal water systems: rather than to rely solely upon chemical analysis as a guide to the probable behavior of their waters. As the results of these field tests multiply, the range of conditions under which corrosion may be controlled by threshold treatment should become more clearly defined. The application of threshold treatment to a system is extremely simple; the only precaution which must be observed is that the treatment should not be applied ahead of a coagulation or a cold lime-soda softening process, since
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both of these are inhibited to some extent by the treatment. The hexametaphosphate is fed from solution, and any proportioning device of reasonable accuracy may be employed. The treatment is not sensitive to intermittent variations in the feed-probably due to the hysteresis effect previously discussed-and fluctuations up to several hundred per cent can be tolerated as long as the average remains fairly constant. Sodium hexametaphosphate, even in concentrations far above those employed in threshold treatment, has no harmful physiological action (13)nor does it impart any taste or odor which might render the water unfit for human consumption. This, combined with low cost as a result of the low concentrations employed, makes corrosion control with sodium hexametaphosphate particularly promising.
Acknowledgment The writers wish to acknowledge the assistance of C . S. B. Freese with much of the analytical work described in this paper.
Literature Cited Am. Pub. Health Assoc., Standard Methods of Water Analysis
8th ed., 1936. Baylis, J. R., IND.ENG.CHEM.,19, 777 (1927).
Baylis, J. R., J. Am. Water W o r k s Assoc., 9, 408 (1922). Beaver, J. J., J . Optical SOC.Am., 18, 41 (1929). Daugherty, T. H., and Kaufman, C. E., Div. Water, Sewage, and Sanitation Chem., A. C. S. Meeting, Cincinnati, 1940. Enslow, L. H., Water Works & Sewerage, 86,238 (1939). Fortune, W. B., and Mellon, M. G., IND.ENG.CRIM., And. Ed., 10, 60 (1938). Gidley, H. T., and Weston, R. S., J. Am. Water Works Assoc., 32, 1484 (1940). Hatch, G. B., Ohio Conf. Water Purification Ann. Rept., 19, 104 (1940). Hatch, G. B., and Rice, Owen, IND. ENG.CHEM.,31, 51 (1939). Hoover, C. P., J. Am. Water Works Assoc., 30, 1802 (1938). Hoover, C. P., and Rice, Owen, Water Works & Sewerage, 86, 10 (1939). Jones, K. K., J. Am. Water W o r k s Assoc., 32, 1471 (1940). Langelier, W. F., Ibid., 28, 1500 (1936). Rice, Owen, and Hatch, G. B., Ibid., 31, 1171 (1939). Rice, Owen, and Partridge, E. P., IND.ENG.CHEW,31, 68 (1939). Rogers, A. H., J. Am. Water Works Assoc., 32, 1498 (1940). Rosenstein, L., U. 9. Patent Reissue 20,754 (1938). Saywell, L. G., and Cunningham, B. B., IND.ENQ. CHIM., Anal. Ed., 9, 67 (1937). Speller, F. N., and Kendall, V. V., IND.ENG.CHEM.,15, 134 (1923). Speller, F.N., and Texter, C. R., Ibid., 16,393 (1924). Trax, E. C., J. Am. Water Works Assoc., 32, 1495 (1940). PRESENTED before the Division of Water, Sewage, and Sanitation Chemistry a t the 99th Meeting of the American Chemical Society, Cincinnati, Ohio.
Correspondence-Cyanamide, Dicyandiamide, and Melamine SIR: With reference to the paper on the above subject, AND published in the September, 1940, issue of INDUSTRIAL ENGINEERING CHEMISTRY(pages 1187-8); our attention has been called to an article by K. Heydrich [ Z . Kryst., 48, 243-305 (1911)] giving optical and crystallographic properties for melamine and dicyandiamide. Published under the title “The Relation between Density and the Index of Refraction in Solid, Crystalline, Isomeric, Organic Compounds”, the article covers a broad field and compares the relation between density and index of refraction of inorganic compounds with that of organic ones. Being so general in scope it was not indexed under melamine and dicyandiamide in Chemical Abstracts and thus was missed when the authors made their search of the literature.
Heydrich carefully described for both melamine and dicyandiamide the refractive indices a,p, and y (to four decimal places), 2V, optical sign, axial ratios, the most common crystallographic forms, many face angles, etc. There is some duplication of this data in our article; and there is agreement within experimental error for p index, 2V, and the optical sign for dicyandiamide, and for the optical sign for melamine. Other values for refractive index are not comparable directly because we gave values for extinction directions in the planes of the common crystal faces, rather than in imaginary crystallographic directions, the purpose being to include data for determinative purposes rather than classical crystallographic description. T. G. ROCHOW
