US3120425A - Zinc silicofluoride composition and method of preventing corrosion - Google Patents

Description

G. B. HATCH ZINC SILICOFLUORIDE COMPOSITION AND Feb. 4, 1964 METHOD OF PREVENTING CORROSION 4 Sheets-Sheet 1 Filed March 28, 1962 BY MMXXW.

,47' 7' @QA/E y G. B. HATCH Feb. 4, 1964 3,120,425 ZINC SILICOFLUORIDE COMPOSITION AND -METHOD OF PREVENTING CORROSION Filed March 28, 1962 N ONM 4 Sheets-Sheet- 2 OIUO/ I NVENTOR.

GERGE B. HATCH BY wwow.

TTP/VEV Feb. 4, 1964 G. B. HATCH ZINC SILIGOFL UORIDE COMPOSITION AND METHOD OF PREVENTING CORROSION 4 Sheets-Sheet l5 Filed March 28, 1962 AVU/Z/w/a/v- $507 1M Ava/fra/Q/V- $907 1M INVENTOR 650,965 a. mrc# .477- p/vfy Feb. 4, 1964 Filed March 28, 1962 G. B` HATCH ZINC SILICOFLUORIDE COMPOSITION AND METHOD OF PREVENTING CORROSION 4 Sheets-Sheet 4 PIG. 7

INVENTOR. G50/Q65 HATCH Zawo United States Patent O 3,120,425 ZINC SILICFLUORHDE CMPSTIGN AND METHGD F PREVENTING CORRGSIN George B. Hatch, Aliison Parli, Pa., assigner to Calgon Corporation, a corporation of Pennsylvania Filed Mar. 23, 1962, Ser. No. 183,291 9 Claims. (Cl. 21-2.7)

This invention relates to inhibiting the corrosion of metals in contact wi-th water. In particular, it relates to the use of zinc silicoluoride to inhibit the corrosion of metals in contact with aqueous media such as industrial waters.

It is known to those skilled in the ant of corrosion inhibiting that most commercial corrosi-on inhibitors eX- hibit one or more Iundesirable characteristics wholly apart from their abilities to inhibit corrosion. One of these weaknesses is a susceptibility to destruction by oxidizing or reducing agents present in the system. Another is the tendency of some corrosion inhibitors to encourage the growth of biological organisms because of their nutrient properties. Other corrosion inhibitors have the undesirable trait off depositing ia relatively permanent stain on susceptible surfaces. I have invented methods and compositions which are excellent for inhibiting corrosion of metals and at the same time are not undesirable because of any of the above traits.

I have discovered that Zinc silicoi'luoride (ZnSiFG) provides surprisingly good corrosion inhibition in aqueous syste-ms such as in cooling towers and other industrial waters, by itself or in combination with other corrosion inhibitors.

The accompanying seven iigures illustrate the effectiveness of my invention. FIGURE 1 is a plot of the corroson inhibition curve of var-ions concentrations of zinc ion and silicofluoride ion, showing the synergistic action at the ratio of zinc silicoiluoride. FIGURE 2 is a graph of the intliuence of pH on the corrosion inhibiting action of zinc silicofluoride. FIGURE 3 is a different View of the same points plotted in lFIGURE 2 to, clearly illustrate the effect of pH. FIGURE 4 shows the corrosion inhibiting effect of zinc silicoiluoride on copper. FIGURE 5 illustrates the effect of my invention in the presence of Calgon brand sodium hexametaphosphate. FIGURE 6 depicts the etlect ot zinc silicoiluoride in the presence of still another known corrosion inhibitor, sodium dichromate. FIGURE 7 plots the eiect on galvanic attack of increasing concentrations of silicor'luoride ion.

The weight loss data in the drawings were obtained from agitated batch tests in l liter of a water closely approximating an average analysis of Pittsburgh tap water. rPhe water used in all demonstrations recited herein contained 19 ppm. calcium, 6.0 ppm. magnesium (hardness .thus calculated as 77 ppm. CaCOs), 18 ppm. chlorine, and 12 ppm. HCO3. The tests were run for live days at 35 C. $0.1". The concentrations of zinc silicoiluoride in all ligures except FIGURE l `are calculated as the hexaliydrate and represent concentrations of Zinc silicoiluoride heXahydr-ate.

FIGURE l illustrates that the action of zinc silicoiluoride is not merely an additive eieot of its ionic components. The SW6- ion alone 4is practically ineffective. Although the zinc ion alone has some inhibiting effect, it cannot compare with the results obtained when the zinc ions are in the presence of silicoiiuoride ion. If the effect were merely additive, one would expect a mixture of 50 ppm. zinc and 50` ppm. silicolluoride ion to result in a loss of about 80 mg./sq. decimeter/day. As can be seen in FIG. l, the loss is rather only about 4 mg./sq. decimeter/day.

FIG. 2 records the weight `loss in milligrams per square ecimeter per day of steel immersed in water containing various concentrations of zinc silicofluoride. It will be seen that even in concentrations over 50 ppm., zinc silicoiluor-ide `alone does not provide startlingly effective inhibition at a pH less than e although it does exhibit an inhibiting effect. It is strikingly effective at pHs above 6 as can be seen from the graph of FIG. 2.

The same points of FIG. 2 are presented differently in FIG. 3 to emphasize the eifect of pH. Although some inhibition is provided at pH 5, it is preferred to use my invention at a pI-I of fat least 6. Preferably it will be used at a pH no higher than 8 because of the danger of precipitating zinc compounds iat higher levels. As little as 5 ppm. of zinc silicoliuoride will provide a noticeable inhibiting etleot; however, it is preferred to use at least about l0 ppm. off Zinc silicofluoride hexahydrate or the equivalent less hydrous compound.

That my invention also has an excellent inhibitive eiect on the corrosion of copper is shown in FIG. 4. Again, a pI-I of 8 is considered the maximum practical limit because of precipitation problems.

A striking eiiect is produced by the use of zinc silicofluoride in the presence of sodium hexametaphosphate to inhibit the corrosion of steel. FIG, 5 compares the inhibiting etect of zinc silicouoride in various concentrations in the presence of Calgon brand sodium hexametaphosphate at a strength of ten parts per million. rThe pH at each test was 6.5, but may vary between the limits of about 5 and 8 as suggested elsewhere herein.

FIG. 6 is a similar graph illustrating the striking eiiect of a combination of Zinc silicofluoride and sodium dichromate. Again, the pI-I of the tests was 6.5, but it may vary between about 5 and about 8. An outstanding advantage of the use of zinc silicolluoride together with sodium dichromiate over sodium dichromate yalone is that the inhibiting effect will be substantially preserved during periods when the dichromate may be subject to severe reduction.

FIG. 7 demonstrates the ellect of my invention on galvanic corrosion. In this demonstration, a test panel of copper and one of steel 11/2 square were immersed in parallel planes at a distance of 1/2 inch in water containing various concentrations of SiFe ion and zinc ion as noted. Electrode leads were connected to the panels; the current generated as measured in milliamperes is plotted against time on the graph. The five solutions shown on the graph as 0%, 25%, 50%, 75%, and 100% contain varying proportions as noted on SiF6-. They were prepared from two solutions, one representing 100% SiF (a 371/2 ppm. solution of a composition consisting of 20% sodium-dichromate dihydrate and 80% zinc silicotiuoride hexahydrate) and the other representing 0% SiFG (25 ppm. oi a mixture of 70% ZnSO4.H2O and 30% Na2Cr2O7-2H2O). The solution marked 50% on the graph had 1/2 the amount of SiFS in the 100% solution. It was prepared by mixing 12 parts by weight of the 0% composition and 18% parts by weight of the 100% cornposition. The 25% and 75% solutions were made in a similar manner from the 0% and 100% compositions. Each solution thus contained the same amount of zinc and chromate ions, and in the same ratio of 1:1. As the amount of zinc silicoliuoride was reduced in each successive solution, the amount of zinc introduced through zinc sulfate was increased. Thus the 25 solution contained 3.4 ppm. of SiFS, the 50% solution contained 6.8 ppm. of SiFG, the solution contained 10.2 ppm. of SiFG, and the solution contained 13.5 ppm. of SF, while the zinc and chromate remained substantially constant.

The compositions used in the demonstrations recorded in FIG. 7 are not limited in eifect to the inhibition of galvanic attack but are also effective in aqueous systems containing only one metal.

In addition to the corrosion inhibiting effects illustrated in the graphs, the combination of zinc silicouoride with sodium hexame-taphosphate or sodium dichromate (or boh) provides the system with stabilization of aluminum. Aluminum may be present in a water system as a carryover from clarification processes, and is undesirable where corrosion is likely to occur.

The maximum amount of zinc silicofiuoride which may be of use is governed only by economics and solubility factors. 1n many industrial waters, up to 500 ppm. of zinc silicoiluoride may be carried without precipitation. When other inhibitors are present, such as sodium hexametaphosphate or sodium dichrornate, a safe practical limit is about 290 ppm. of zinc silicoiiuoride. The maximum amount of other inhibitors is generally unaffected by the presence of zinc silicoiluoride. For example, the teachings of Rice and Hatch, U.S. Patent 2,337,856, may be followed with regard to effective amounts of sodium hexarnetaphosphate. Chromates and dichromates may be used in large concentrations with zinc silicofiuoride. However, it is preferred to use chromates and dichromates in the presence of additional zinc ions according to the teachings of my US. Patent 3,022,133. Zinc silicofluoride may be combined, within the scope of the present invention, with both a chromate (with or without a separate source of zinc as in U.S. Patent 3,022,133) and a phosphate glass.

A preferred composition employing zinc silicofluoride and sodium dichromate, as suggested in the demonstration of FIG. 7, consists essentially of 35% by weight of Zinc sulfate monohydrate, 40% zinc silicoiiuoride hexahydrate, and 25% sodium dichromate. Of course, zinc silicoluoride may be used with combinations of the Zinc and CrzO, ion in other ratios. It may be incorporated in any composition providing dichromate ions with or without additional zinc ions. The use of such combinations may be expressed as a method of treating aqueous media to inhibit corrosion of metals in contact therewith comprising adding to such media (a) at least about five parts or" Zinc silicoiiuoride (calculated as the hexahydrate) per million parts of water, (b) at least one water soluble zinc salt, such as zinc sulfate, zinc chloride, zinc nitrate, and similar zinc salts capable of producing up to about ppm. zinc ions under the conditions present, and (c) sufficient Water-soluble chromate and/ or dichrornate cornpounds to produce up to about ppm. chromate and/0r dichromate ions.

As little as 10 ppm. or less of a composition of 8 parts by weight zinc silicofluoride hexahydrate, 7 parts by weight zinc sulfate monohydrate, and 5 parts by Weight sodium dichromate produces good corrosion inhibiting results in most industrial Waters, although the optimum range is considered to be about 25-50 p.p.m., preferably in a solution having a pH in the optimum range of 6.5-7.2.

From the foregoing figures and data, it will be seen that Zinc silicol'iuoride is an effective corrosion inhibitor by itself in aqueous systems of pH about 5 to about 8 in concentrations of at least about 5 ppm. As is indicated in the figures, even less than 5 p.p.m. of zinc silicoiiuoride in the presence of known corrosion inhibitors will enhance the effect thereof. As can be seen from FG. 5, as little as 1 ppm. of Zinc silicoiiuoride reduces weight loss to less than half that resulting from the use of l() ppm. sodium hcxametaphosphate alone.

Zinc silicofluoride (ZnSiF) is very diflcult to oxidize or reduce. Consequently, it is stable to oxidizing agents including those normally found in industrial Water, such as chlorine. lut the same time it is highly resistant to reduction by reducing agents occasionally found in water, such as organic matter, sulfur dioxide, and so forth. It wiil not serve as a nutrient for algae or slime bacteria. Since it is colorless, it causes no staining problems.

Having thus described and illustrated centain presently preferred embodiments of my invention and methods of practicing the same, it is to be distinctly understood that it is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

i claim:

l. Method of inhibiting the corrosion of metals in contact with water comprising adding to said Water at least about 5 ppm. of zinc silicoiiuoride calculated as the hexaliydrate and maintaining the pH between about 5 and about 8.

2. Method of inhibiting the corrosion of metals in contact with water comprising adding to said Water an effective amount of Zinc ions and SAF(- ions and maintaining the pH between about 5 and about S.

3. Method of inhibiting the corrosion of steel in contact with water comprising adding to the Water a small amount of zinc silicofiuoride and a small amount of sodium hexametaphosphate.

4. Method of inhibiting the corrosion of steel in contact with water comprising adding to the water at least about 5 ppm. of zinc silicoiluoride and a small amount of sodium dichromate.

5. A composition useful in inhibiting the corrosion of metals in aqueous systems comprising sodium hexametaphosphate and Zinc silicoiiuoride.

6. A composition useful in inhibiting the corrosion of metals in aqueous systems comprising sodium dichromate and zinc silicoiluoride.

7. A composition useful in inhibiting the corrosion of metals in aqueous systems comprising sodium dichromate, sodium hexarnetapliosphate, and zinc silicoiluoride.

8. A composition useful in inhibiting corrosion in aqueous systems comprising (a) zinc silicoiuoride,

(b) a Water-soluble Zinc salt,

(c) a water-soluble compound capable of producing in solution an anion of the group consisting of chron ate ion and dichromate ion.

9. Method of inhibiting the corrosion of metals in contact with aqueous media comprising adding to said aqueous media (a) at least about 5 parts per million of zinc silicouoride, (b) about 2 to about '1G' parts per million of additional Zinc ion, and

(c) up to about 25 parts per million of an ion from the group consisting of chromate ion and dichromate ion.

References Cited in the ille of this patent UNlTED STATES PATENTS

Claims (1)

1. METHOD OF INHIBITING THE CORROSION OF METALS IN CONTACT WITH WTER COMPRISING ADDING TO SAID WATER AT LEAST ABOUT 5 P.P.M. OF ZINC SILICOFLUORIDE CALCULATED AS THE HEXAHYDRATE AND MAINTAINING THE PH BETWEEN ABOUT 5 AND ABOUT 8.

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