Chemical apparatus and process disinfecting – deodorizing – preser – Process disinfecting – preserving – deodorizing – or sterilizing – Maintaining environment nondestructive to metal
Reexamination Certificate
1999-07-26
2001-03-13
Thornton, Krisanne (Department: 1744)
Chemical apparatus and process disinfecting, deodorizing, preser
Process disinfecting, preserving, deodorizing, or sterilizing
Maintaining environment nondestructive to metal
C210S696000, C210S697000, C210S701000, C210S754000, C210S764000, C252S389200, C252S389210, C252S389220, C252S389230, C252S389240, C422S007000, C422S015000, C422S018000
Reexamination Certificate
active
06200529
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods for inhibiting corrosion of corrodible metals present in contact with water in distributive water systems, particularly those associated with distribution of drinking water.
BACKGROUND OF THE INVENTION
In 1992, the United States Environmental Protection Agency adopted a standard for lead and copper in municipal drinking water. The standard sets action levels for lead and copper in standing samples collected from faucets with the highest risk for elevated lead and copper levels. The action level for lead is 15 &mgr;g/liter of water; the action level for copper is 1.3 mg/liter of water. The source of these and other metals in drinking water is primarily corrosion of plumbing system components, which include copper and lead-based solder as well as carbon steel and brass. Current anticorrosion additives, which include phosphate salts and/or zinc salts, have been in use for many years, but do not always provide adequate protection. There remains a need for improved corrosion inhibitors which are not themselves likely to present a health hazard when added to water.
Co-owned U.S. Pat. Nos. 5,202,058 and 5,510,057 describe the use of stannous salts, such as stannous octanoate or stannous chloride, typically in combination with other additives, in an alcoholic solvent medium, for reducing metal corrosion in, for example, cooling water tower systems. However, these additional components, including the alcoholic solvent, would be undesirable or even prohibited for use in drinking water supplies. Alcoholic solvents can provide a food source for the growth of bacteria and thus impact the sterility of drinking water supplies. Reduction in microbial growth would be beneficial in industrial water supplies as well. Accordingly, there is a need for effective corrosion inhibitors which minimize organic content.
SUMMARY OF THE INVENTION
The present invention includes, in one aspect, a method for inhibiting corrosion of corrodible metal in contact with water in a distributive water system. According to the method, stannous halide alone, a composition consisting essentially of stannous halide, or a composition consisting essentially of an aqueous stannous halide solution, is added to the water, and a concentration of the stannous halide corresponding to a tin level of about 0.01 to about 75 ppm is maintained in the water system.
The halide is preferably selected from stannous fluoride, stannous chloride, and stannous bromide, and is most preferably stannous chloride. The stannous halide is preferably added in the form of an aqueous solution. The weight percent of stannous chloride in an aqueous stannous chloride solution is preferably about 5 to 90 percent, and more preferably about 40 to 80 percent. The concentration of stannous chloride corresponding to a tin level of about 0.01 to about 75 ppm is about 0.016 ppm to about 120 ppm SnCl
2
; a preferred concentration range for stannous chloride in the water system is about 0.05 to 25 ppm SnCl
2
.
The corrodible metal is typically a ferrous metal, a brass metal, a copper-containing metal, or a lead-containing metal. In a preferred embodiment of the method, the distributive water system is a municipal drinking water system. Such a system may initially contain an alkali metal phosphate, such as a pyrophosphate, an orthophosphate, a hexametaphosphate, a hypophosphate, a polyphosphate, or a combination thereof, typically in an amount effective to provide a concentration of about 0.01 ppm to about 5 ppm of the alkali metal phosphate in the water system. The water system may also be treated to contain at least one component selected from a dispersing agent, a chelating agent, and a biocide
In another aspect, the invention provides a related method for inhibiting corrosion of corrodible metal in contact with water in a distributive water system. In this method, stannous halide alone, a composition consisting essentially of stannous halide, or a composition consisting essentially of an aqueous stannous halide solution, is added to the water system, in combination with an alkali metal phosphate selected from a pyrophosphate, an orthophosphate, a hexametaphosphate, a hypophosphate, and a polyphosphate. A concentration of the stannous halide corresponding to a tin level of about 0.01 to about 75 ppm is maintained in the water system. Again, stannous chloride is the preferred halide, and is preferably maintained at a level of about 0.05 ppm to about 25 ppm in the water system. The alkali metal phosphate, e.g. sodium hexametaphosphate or sodium orthophosphate, is typically present in an amount effective to provide a concentration of about 0.01 ppm to about 5 ppm in the water system.
In a further aspect, the invention provides a metal corrosion inhibiting composition, consisting essentially of (i) stannous halide or a an aqueous stannous halide solution and (ii) an alkali metal phosphate selected from a pyrophosphate, an orthophosphate, a hexametaphosphate, a hypophosphate, and a polyphosphate. In preferred embodiments, the stannous halide is stannous chloride, and the stannous chloride and alkali metal phosphate are present in relative amounts effective to produce concentrations of about 0.05 ppm to about 25 ppm and about 0.01 ppm to about 5 ppm, respectively, in a distributive water system. The composition may also include one or more substances typically added to a distributive water stream for purposes other than inhibition of corrosion, such as a dispersing agent, chelating agent, or biocide. In one embodiment, the composition includes an acrylate copolymer, e.g. an acrylate/sulfonate copolymer.
REFERENCES:
patent: 4029577 (1977-06-01), Godlewski et al.
patent: 5202058 (1993-04-01), Riggs
patent: 5376331 (1994-12-01), Buchner et al.
patent: 066 347 (1982-12-01), None
patent: 265 723 (1988-05-01), None
A. S. Incorporated
Gorthey LeeAnn
Thornton Krisanne
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