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INDUSTRIAL AND ENGINEERING CHEMISTRY
male had been prevented, a large proportion had not formed. It was to be expected that piping cleaned with inhibited acid would be somewhat more likely t o corrode than new piping; yet with the silicate treatment the piping is in good condition after a longer period than was required to clog parts of i t when no treatment was used. There is every indication that the piping will be useful for some years to come. When the cost of replacing the piping in a building of this type is considered (estimated at several hundred thousand dollars in 1927), the minor cost of the silicate treatment is hardly worth considering. Hale’s tests indicated that yellow brass was not attacked, or so little attacked that it could be used safely for New York water. Actually there have been many failures of such pipe in the city. These frequently take the form of dezincification in localized areas, such as Figure 4 shows. A typical service pipe with this type of dezincification appears in Figure 5 , as well as a similar pipe protected with silicate. The dark spot at the top of the latter is where a portion of the silicate film chipped off. Sodium silicate prevented the failure of any yellow brass pipes in this area over a period of years. One factor which should be watched is whether sufficient silicate is fed so that, under stagnant or semistagnant conditions, enough residual soluble silica is present to maintain or restore the protective film. If the surface of the metal exposed is large compared with the volume of water, it is easy to see how the silica might be withdrawn, particularly locally, to well below the 4 p.p.m. residual required. There is a n evident need of more data from field cases. If these were available for correlation with laboratory tests, i t might be possible to eliminate the contradictions or at least determine the causes. PRESENT STATUS
This article has dealt with the use of sodium silicates for the control of corrosion in office buildings, laundries, apartment houses, and dwellings. The treatment is not limited to these alone but has been used in industrial processes which would have to be considered separately, although many of the basic principles are the same. The following summarizes present knowledge about the use of sodium silicates a8 inhibitors: 1. The minimum dosage of a sodium silicate required to give a good degree of inhibition of corrosion is equivalent to 8 p.p.m. silica in usual waters. 2. Larger amounts of silicate speed up the formation of the protective film but increase the chances of removing accumulated products of corrosion from old piping.
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3. The preferred silicate for waters in which the p H is above 6.0 is NmO: 3.3SiOt; for those in which the pH is 6.0 or below, Nas0:2.1Si02. 4. The former silicate can be used in hot water systems in tEe form of glass. 5. Larger amounts of silicates are required when the water contains considerable amounts of chlorides, but a dosa e of not over five times that given above is satisfactory even on %rines. 6. After the protective a m is formed, it can be maintained by feeding less silicate, but at least 4 p.p.m. residual silica must be present in usual waters. 7. The silica in sodium silicate solutions, whether present as ions or as micelles, carries a negative charge. 8. Because of this charge the silica will migrate t o anodic areas where at least part of the charge will be neutralized. 9. An extremely labile film will be produced by this process. 10. To obtain a more stable film there must be a reaction with the metallic ions being formed at the anode. Such films can be demonstrated at high current densities. 11. These films are electrical insulators. 12. They protect both gross anodic areas and the anodes of local concentration cells which cause pits. 13. I n stagnant water the sup ly of silica is soon exhausted, either in the region within the e8ective range of the electrical forces around the anode or in small containers throughout the entire solution. 14. If only part of the area is protected, the remainder takes all the attack of the corrosive medium. Therefore it is important to use enough inhibitor. 15. The efficacy of the silicate treatment varies with the kind of metal. 16. The treatment has checked corrosion in systems where two metals are in contact. LITERATURE CITED
(1) Hale, F. E., J. Am. Water Works Assoc., 26, 1 3 1 5 4 7 (1934). (2) Huntington, L. I., Plumbers TradeJ.,Jan. 15,1925:Aprill5, 1926. (3) N. Y. Dept. of Health, Sanitary Code Sections 170 and 191 (Dec. 12, 1938). (4) Russell, R. P., Am. Dyestuf Reptr., 15, 61-5 (1926). (5) Shuldener, H . L.,U. S.Patent 1,796,407 (March 17, 1931). (6) Shuldener, H . L., Yale Sci. Mag., summer issue, 1940. (7) Speller, F. N., “Corrosion, Causes end Prevention”, 1st ed.. 1). 353, New York, McGraw-Hill Book Co., 1926. (8) Speller, F. N . , J. F r a n k l i n I m t . , 193, 519-20 (1922). (9) Speller, F. N., personal communication. (IO) Speller, F. N., ChappelI, E. L., and Russell, R . P., Trans. A m . Inst. Chem. Engrs., 19, 165-7 (1927) ; Chem. & Met. Eng.. 34, 4 2 3 4 (1927). (11) Stericker, Wm., Am. Petroleum Inst., Div. of Produrtioii Southwest District, Feb., 1941. (12) Stericker, Wm., IND. ENO.CHEM., 30, 348-51 (1938). (13) Texter, C. R., J. Am. Water Works Assoc., 10, 764-72 (1933). (14) Texter, C. R.. Sanitary Heating Eng., Nov. 14, 1924. (15) Thresh, J. C., A n d u s t , 47, 459-68, 500-5 (1922). (16) Weston, R. S.,J. New Enol. Water Works Assoc., 30,850-l(1938). (17) Whitman, W. G . , Russell. R. P., and Davis, G. H. B . , J. Am Chem. SOC.,47, 70-9 (1925).
Discussion o f Paper 6 y Stericker HENRY L. SHULDENER W a t e r Service Laboratories, New York,
I
T HAS always been extremely difficult to get sufficient correlated field data to serve as a basis for definite conclusions. This is particularly true in corrosion work. However, as indicated by Dr. Stericker, sodium silicate has a field history of approximately twenty-five years and in that time certain fundamentals have emerged. Unfortunately, no substitute for field experience has been found and no substitute for time. This lack of correlation between laboratory and field results was dramatically illustrated by the Ashokan experiments mentioned by Dr. Stericker. Frank E. Hale was good enough to dig out the old samples of yellow brass pipe which had carried hot raw water for four years during these experiments. Careful metallurgical
N. Y.
examination confirmed his conclusions that no dezincification had occurred. However, as the result of an investigation st‘arted in 1938, during which over five hundred samples of pipe have been removed from hot water systems in New York City buildings, it is now known that yellow brass pipe is subject to the corrosive attack known as dezincification. The successful retardation of ferrous corrosion with silicate at the Bankers Trust Building and the U. S. Steel Building at 71 Broadway has been repeated a hundred fold in other buildings. Corrosion of yellow brass pipe has also been successfully anticipated over a period of more than ten years. Such length of treatment of brass hot water piping was incidental to the treatment of
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
August, 1945
the complete water supply in order to check corrosion of the iron cold water piping. (Many buildings have been built with brass hot water piping and iron cold water piping because of the reduced resistance t o corrosion of iron in hot water.) It is interesting t o note the cost of one dollar per person per year for silicate treatment arrived a t by Dr. Stericker. He expresses this as the cost of the chemical alone. A complete service, including weekly application of the silicate, has been applied for years for less than one dollar per person per year in buildings containing over sixty families. It may be of further interest to state that the complete cost of servicing apartment houses, hotels, or office buildings with a sodium silicate treatment has been found to be less than the interest on the investment required to replace the piping system. Such corrosion prevention is indeed good pipe insurance. As Dr. Stericker says, the protective siliceous films do not build up. They are extremely thin and do not appear to be any different after seventeen years of continuous treatment than after two yeam. With respect t o the amount of silicate t o be added, it appears important t o maintain the silida increase between 4 and 8 p.p.m. at all times. The residual would therefore be the silica content of the raw water plus the silica added. The old idea of making the initial charge high and then reducing it has not been conducive to good results.
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With respect to the limited “length of carry”, mentioned in the first edition of F. N. Speller’s book, I think this early conclusion was based on the observation of precipitated matter near the point of application. It was undoubtedly due t o the crude method of feeding in the early years which led t o considerable overtreatment a t times. An important contribution to the effectiveness of sodium silicate in corrosion prevention has been the uniform and continuous method of feeding. The intermittent method was based on the erroneous theory that the protective film was stable for a long time and did not need residual silica t o maintain its effectiveness by stabilizing the film. The paper quotes C. R. Texter’s statement made in 1923 that the treatment was “applicable to old rusty hot water heaters and almost worn out pipe lines as well as t o new installations”. Experience in many hundreds of buildings in six cities for more than fifteen years amply confirms this statement. Many of these buildings have been under continuous treatment for ten to seventeen years and many of them were more than fifteen years old when treatment was started. I n concluding these comments, I should like to point out that paragraph two of the summary may create the impression that sodium silicate is capable of removing rust. This has not been my experience.
EFFECT OF CHROMATE AND LIME ADDITIONS TO WATER H.
R. COPSON
The International Nickel Company, Inc,, Bayonne,
CONDITIONS
GALVANIC CORROSION tests were made on steel coupled With a 3 to 1 area ratio of nickel to steel, galvanic corrosion of the steel was appreciable but not excessive. Treating the water with 300 parts per million of sodium chromate practically inhibited corrosion, provided the steel was rubbed occasionally. Undisturbed steel was liable to pit. Treating the water with lime to p H 11 cut down the total corrosion but localized it, with the result that the maximum rate of penetration was increased.
to nickel in tap water.
ICKEL-CLAD steel had been specified as the material for constructing a canal lock, a t a time when nickel wm becoming scarce owing to wartime restrictions. It was considered expedient to complete the lock with both plain carbon steel and nickel-clad steel. The gate waa t o be made of painted carbon steel, and the slots in which it would move were to be lined with nickel-clad steel. Included in the assembly which would operate in the slots would be unprotected steel tracks and rollers. Conditions would be such that the area of unprotected steel would be about one third the area of nickel in its immediate vicinity. It was feared that appreciable galvanic corrosion of the steel might occur. Laboratory testa were undertaken to determine the probable extent of the galvanic effects and t o investigate means of minimizing them. It was known that the canal water in the slots would be stagnant and would be replaced a t infrequent intervals, such as once a month. Therefore the use of a chromate inhibitor or of lime to raise the pH of the water seemed practical. Bayonne t a p water is similar enough to the canal water t o be suitable for the testa. Table I gives an analysis of the tap water.
N
N. 1.
The following couples were studied a t 30” C.: 1 and 2: steel-Bayonne tap water (pH 8)-nickel 3 and 4: steel-tap water 300 p.p.m. NaaCrOl (pH 8)nickel lime (pH 11)-nickel 8 and 6 : steel-tap water
+ +
The procedure described by Wesley (4) was followed closely. Separate Pliter glass jars were used for each couple, and normally corroding uncoupled specimens were included in each jar. The electrodes were shorted through external one-ohm resistances. The steel specimens were 1 X 5 X 0.060 inch with long tabs inch wide. The nickel specimens were 3 X 5 X 0.031 inch with similar tabs. The tabs served for electrical connections. The immersed part of the tab was protected with wax. This made the area ratios 3 to 1. For the first 36 days, for the sake of close control of conditions, the solutions were saturated with air and the specimens moved in a vertical circular path at a rate of 16 feet per minute (3). The Table 1.
Analysis of Bayonne Tap Water (Parts per Million) 2.4 Soap hardness aa CaCOs 2 5 . 0 0.02 Iron 0.08 Nz 88 albuminoid ammonia 0.05 Calcium 7.8 NI a8 nitritee 0.00 Manganese 0.05 Nn as nitrates 0.03 Magnesium 1.46 Chlorides 8.4 Silioa 2.9 Sulfatea 12.4 Total dissolved solids 47.8 Alkalinity 50.7 On consumed Nn a8 f ree ammonia
