WO2007014362A1

WO2007014362A1 - Corrosion inhibition method for use in boiler water systems

Info

Publication number: WO2007014362A1
Application number: PCT/US2006/029623
Authority: WO
Inventors: William E. Stapp
Original assignee: A.S. Incorporated
Priority date: 2005-07-27
Filing date: 2006-07-27
Publication date: 2007-02-01

Abstract

Methods are described using stannous salt for inhibiting corrosion of corrodible metals present in contact with water in boiler water systems, and in particular methods effective in high pressure/high temperature boilers. The compositions employed are particularly effective at high temperatures and/or high hardness levels, and form little or no deposits on the metal surface.

Description

CORROSION INHIBITION METHOD FOR USE IN BOILER WATER SYSTEMS

FIELD OF THE INVENTION

The present invention relates to methods for inhibiting corrosion of corrodible metals present in contact with water in boiler water systems, and in particular to methods effective in high pressure/high temperature boilers.

BACKGROUND OF THE INVENTION

Metal surfaces in boiler water systems, including low pressure/high temperature and high pressure/high temperature boilers, are subject to high levels of corrosion. Trace levels of oxygen react rapidly with the internal metal surfaces under typical operating conditions. Moreover, the process of generating steam can result in local concentration conditions that differ from those found in bulk boiler water. Boilers are usually treated with caustic to bring pH up to 10 to improve pH control. Nevertheless, either high or low pH can cause deterioration of the protective iron oxide layer that forms on steel surfaces, thus exposing the underlying metal to further corrosion.

Phosphates are conventionally employed in such systems as corrosion inhibitors.

The phosphate salts (typically sodium salts) react with dissolved calcium to form calcium phosphate, which forms a barrier deposit on the metal surface. However, excessive deposition is a common problem which impairs heat transfer and often requires disassembly and cleaning of the equipment.

Accordingly, a corrosion inhibitor which is effective to inhibit corrosion under these conditions, but which does not form excessive deposits on the metal surface, would be desirable.

SUMMARY OF THE INVENTION

In one aspect, the invention provides a method for inhibiting corrosion of corrodible metal in contact with water in a boiler water system, particularly a high pressure boiler water system, e.g., one that operates at pressures greater than about 15 psi, up to about 1500 psi or greater, and at temperatures greater than 250°F, up to about 70O⁰F₅ about

800°F or greater. Typically, the pH of the boiler water in such systems is 8.5 or higher.

The corrodible metal is typically a ferrous metal, copper, brass, or lead, and is most commonly a ferrous metal, such as carbon steel. The method comprises: (a) adding to the boiler feed water, a composition consisting essentially of a stannous salt of a non-carbon acid, or an aqueous solution thereof, and (b) maintaining in the boiler water a concentration of said stannous salt which corresponds to a concentration of tin (Sn⁺²) between about 0.1 and about 25 ppm, preferably between about 0.5 and about 10 ppm, and more preferably between about 1 and about 5 ppm.

In one embodiment, the stannous salt is preferably selected from the group consisting of stannous bromide, chloride, sulfate, nitrate, and oxide, and is more preferably stannous chloride. Preferably, the stannous salt is added in the form of an aqueous solution.

These and other objects and features of the invention are made more fully apparent in the following detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic of an exemplary low pressure boiler system.

Figure 2 is a schematic of an exemplary high pressure boiler system.

DETAILED DESCRIPTION I. Definitions "Corrosion" of a metal in contact with water, as used herein, refers to degradation of the metal due to chemical reaction with its environment, in this case, water and substances present in the water, including oxygen. Such corrosion ultimately leads to dissolution or dispersion of the metal or metal compound in the water, observed as a loss of mass of the metal. "Boiler feed water" refers to water that is fed into a boiler, and may include makeup water that is fed during operation, as typically required for high pressure boilers.

A "non-carbon" acid is an acid containing no carbon atoms; examples include hydrochloric, sulfuric, phosphoric, and nitric acid.

As used herein, an "aqueous solution" of a stannous salt of a non-carbon acid refers to a composition of the salt in water, preferably in a high concentration of about 5 to 50 percent by weight. The concentration of the stannous salt may be 5, 10, 20, 30, 40, or 50 percent by weight or greater, if the salt is of sufficient solubility in water to achieve this concentration. In a preferred embodiment, the concentration of the stannous salt is 5 percent by weight. In another preferred embodiment, the concentration of the stannous salt is 50 percent by weight. The composition may also include an acid, e.g., HCl, H₂SO₄, or HNO₃ to promote dissolution of the stannous salt.

A "composition consisting essentially of a stannous salt of a non-carbon acid (or aqueous solution) refers to the stannous salt, or aqueous solution, in combination with one or more optional components which do not materially affect the metal corrosion inhibiting characteristic(s) of the composition. Such components would typically be substances added to a distributive water stream for purposes other than inhibition of corrosion, as defined above. For example, antiscaling agents, including dispersants, chelating agents, and/or soluble anionic polymers (e.g. polyacrylates or acrylate copolymers), may be employed to reduce scale formation on solid surfaces.

A "concentration of stannous salt which corresponds to a concentration of tin" in a given concentration range is determined from the relative molecular weights of the components. For example, 1.0 ppm tin corresponds to approximately 1.8 ppm SnSO₄, 1.6 ppm SnCl₂, or 2.3 ppm SnBr₂, "Caustic" refers to a basic solution, i.e., one that has a pH greater than 7.0. Basic solutions may be referred to as "caustic" or "alkaline."

"Hydrogen ion concentration" refers to the number of moles of hydrogen ion per cubic meter.

"Ion concentration" refers to the strength of a solution and is expressed as the number of molecules of a substance in a given volume (e.g., moles/cubic meter).

"Percent by weight" or "weight percent" or "percent by mass" refers to the mass of a solute divided by the total mass of the solution and multiplied by 100%.

The term "passivates", as used herein, refers to treating or coating a metal in order to reduce the chemical reactivity of its surface. "Stannous" refers to a chemical compound containing tin in the valence state.

A "stannous salt of an inorganic acid" refers to a compound that results from replacement of part or all of the acid hydrogen of an acid by a stannous ion.

"Stannous chloride" dissociates from SnCl₂ to Sn⁺² and 2Cl^".

Abbreviations

"Parts per million" or "ppm" and "parts per billion" or "ppb" refers to parts by weight in water. Thus, 5 ppb stannous ion refers to 5 parts by weight of stannous per 1 billion parts by weight of water.

"Pounds per square inch" or "psi" or "PSI" or "lb/in²" or "lb/sq in" is a unit of pressure measuring force per unit area. Normal atmospheric pressure is 14.7 psi, meaning that a column of air one square inch in area rising from the Earth's atmosphere to space weighs 14.7 pounds.

"Atmosphere" or "arm" refers to atmospheric pressure wherel aim = 14.6956 psi = 760 torr.

II. Corrosion Inhibition in Boilers

The corrosion inhibition methods disclosed herein are applicable to both low pressure and high pressure boilers. Low pressure boilers, i.e., those operating below about 15 psi to about 100 psi, are typically used for heating water or steam (approx. 250-300°F) to be circulated in a building for heating purposes. High pressure boilers, i.e., those operating above about 15 psi, and up to about 1500 psi, at temperatures up to about 700- 800°F, are found in industry, and are used to generate steam for a variety of uses, e.g., steam engine locomotion, wood pulp processing, and manufacture of various items such as rubber products or food products. Typically, the pH of the boiler water in such systems is about 8.5 or higher. These boilers typically require constant "make up" water during operation as the steam is used. Embodiments of methods disclosed herein are particularly advantageous for high pressure boilers, where corrosion is a great concern. Such systems include those with operating pressures above about 100 psi, above about 500 psi, above about 1000 psi, and up to about 1500 psi or more, and operating temperatures above about 300°F, above about 500°F, and up to about 700-800°F or more.

Conventionally, boiler feed water is softened or demineralized to reduce scale, which can form during normal operation as a result of the precipitation of normally water- soluble solids that become insoluble as temperature and/or pressure are increased. Typical components of boiler scale include calcium carbonate, calcium sulfate, and calcium silicate, although it will be appreciated that other components may form scale. Chelating agents, suspending agents, and calcium-solubilizing polymers are typically used to address problems of scaling.

Boiler feed water is also conventionally deaerated. Chemical oxygen scavengers, as exemplified by sodium sulfite (Na₂SO₃), typically for low pressure boilers, or volatile agents such as hydrazine, hydroquinone, diethylhydroxyethanol, methylethylketoxime, etc., typically for high pressure boilers, may be used to remove traces of dissolved oxygen. However, even when deaerated water and such oxygen scavengers are employed, corrosion is still a critical concern in high pressure boilers.

As discussed above, phosphates (such as, for example, monosodium phosphate or trisodium phosphate) are conventionally used as corrosion inhibitors in such systems, and react with dissolved calcium to form calcium phosphates, which form a barrier deposit on the metal surface. This phenomenon occurs in part due to the fact that calcium reacts faster with phosphate than with carbonate. However, excessive deposition is a common problem which impairs heat transfer, and typically, the affected equipment must be cleaned periodically through a blow down at least once per shift. The equipment must also be disassembled for inspection to ensure safe operation.

III. Use of Aqueous Stannous Salt Solutions in Corrosion Inhibition The addition of small amounts of an aqueous stannous salt, preferably a salt of a non-carbon acid, such as stannous chloride, to boiler feed water is effective to inhibit corrosion of internal boiler surfaces. Although the invention is not limited by mechanism of action, this treatment is believed to precipitate a thin layer of tin on the metal surfaces contacted by the water. Because the stannous salt is also a reducing agent, it may also be effective as a trace oxygen scavenger. Optimal treatment levels fall within the range of about 0.1 to about 25 ppm, preferably about 0.5 to about 10 ppm, and more preferably about 1 to about 5 ppm.

In a preferred embodiment, a composition consisting essentially of a stannous salt of a non-carbon acid, preferably stannous chloride, or an aqueous solution thereof, is added to boiler feed water, and a concentration of the stannous salt which corresponds to a concentration of tin (Sn⁺²) between about 0.1-25 ppm, preferably about 0.5-10 ppm, more preferably about 1-5 ppm, is maintained in the water system. The concentration of Sn⁺² in the water system may be monitored by techniques known in the art and adjusted as necessary to maintain the desired concentration.

The stannous salt compositions are effective, at relative low concentrations, to inhibit corrosion of metals under conditions of high temperature, as noted above, and do not form excessive deposits on the metal surface, as is common with conventional phosphate based inhibitors. In one embodiment, the compositions are effective to inhibit corrosion of ferrous metals, such as carbon steel, as well as other metals such as copper, lead, and brass. The stannous salt compositions may also operate to remove existing corrosion product from the boiler surfaces. For example, for steel surfaces, stannous ion reduces ferric ion (Fe⁺³) to ferrous ion (Fe⁺²). Moreover, the reducing activity of stannous ion may be effective to scavenge trace oxygen in the boiler water. To form the corrosion inhibiting compositions, any stannous salt of a non-carbon acid having sufficient aqueous solubility to be maintained in a boiler water system at a concentration corresponding to a selected tin level in the range of about 0.1 to 25 ppm, preferably about 0.5 to about 10 ppm, may be used. Preferred salts include stannous bromide, chloride, sulfate, nitrate, oxide, pyrophosphate, perchlorate, tetrafluoroborate, monofluorophosphate, ammonium fluoride, sodium fluoride, and fluorosilicate.

Particularly preferred salts include stannous bromide, chloride, sulfate, nitrate, and oxide. In one embodiment, the salt is stannous chloride.

Stable aqueous solutions of these compounds are often more readily prepared by adding water to the salt, rather than the reverse order of addition. For example, highly concentrated aqueous stannous chloride solutions, i.e., about 50 percent by weight up to about 90 percent by weight SnCl₂, can be conveniently prepared by adding water to stannous chloride in the desired weight ratio and stirring for a brief period of time. Dissolution is enhanced by using slightly acidified {e.g., dilute HCl) water.

Again, although the invention is not limited by mechanism of action, the formulation is believed to reduce corrosion by depositing a thin layer of tin very close to the surface of the metal, thereby forming a barrier on cast iron, copper, steel, lead, and other metals. This layer (1) becomes a layer of sacrificial material that is replaced by a continuous treatment of less than 0.35 ppm of product; (2) approximates metal once it has been established; (3) passivates the active metal surfaces, leaving them all with the same charge, and also protecting them with a barrier of ceramic tin; and (4) reduces the electrical activity of lead, copper, cast iron, and steel.

IV. Method

In one embodiment, the stannous salt may be added to the boiler as part of a multi- component formulation. In an embodiment, this formulation is comprised of stannous chloride (an oxygen scavenger, exemplified by sodium sulfite), and a dispersant polymer or softener such as orthophosphate. In one embodiment, the dispersant polymer contemplated is ACCUMER 3100® (Rohm and Haas). In another embodiment, orthophosphate, a generic chemical sold most frequently as phosphoric acid, having the chemical formula H₃PO₄, is contemplated.

As stated above, stannous chloride operates to prevent boiler internals from corrosion which may be due to oxygen. Sodium sulfite (Na₂SO₃) reacts with oxygen to form sulfate (Na₂SO₄). The dispersant polymer or orthophosphate will react with trace quantities of calcium or magnesium to minimize carbonate scale formation and deposition. This multi-component product may be especially useful for lower pressure boilers, i.e., those operating within a pressure range of about 15-150 psi.

In another embodiment, a formulation comprising a stannous chloride solution is added to the boiler system. In a preferred embodiment, the formulation comprises at least about a 50% active stannous chloride solution with about 5-10% hydrochloric acid, with the balance of the formulation being a solvent such as water. The activity level may be decreased to better accommodate the needs of the system.

It will be appreciated that an ideal dosage level for either formulation will fluctuate depending on water chemistry, and operating history; however, it is contemplated that an ideal feed rate will most generally fall within the range of about 2-15 ppm Sn⁺².

An exemplary low pressure boiler system is described in Figure 1. The present method is described with regard to this exemplary system; however, it will be appreciated that the present method will further be applicable to other boiler systems. The system 10 generally comprises a boiler feed water tank 16 connected to a boiler 20 via at least one conduit 19. In this exemplary system, the water 12 is added to the water tank 16.

Softeners, as known in the art, and including those that remove calcium and magnesium ions, may be added to the water as long as they do not react adversely with the stannous salt. These include, but are not limited to, phosphates and polyphosphate-dispersants. The water is moved from the tank to the boiler via at least one pump 18. The water in the boiler is then heated by way of a heat source 22. It will be appreciated that any source of heat for the boiler may be used. Exemplary heat sources include the combustion of fuels such as wood, coal, oil or natural gas. Further, electric boilers use resistance or immersion type heating elements. Nuclear fission may also be used as a heat source for generating steam. The water is heated, creating steam 24, which moves through a heat exchange 26 to create energy. Ideally, the formulation will be added to the system upstream of the boiler feed water tank pump as exemplified by the arrow "Stannous salt" in Figure 1. However, it will be appreciated that the formulation may further be added at any appropriate place in the system. Figure 2 shows an exemplary high pressure boiler system. For convenience, similar element numbering is retained in Figures 1-2 to identify like structural features. For example, the boiler feed water tank is numbered 16 in Figure 1 and 160 in Figure 2, etc. Again, the system 100 generally comprises a boiler feed water tank 160 connected to a boiler 210 via at least one conduit 190. The water 120 is added to the water tank 160. Demineralizers 140 (as known in the art, including those that remove silica, anions and cations, etc) may be added to the water as long as they do not react adversely with the stannous salt. Ideally, the formulation will be added to the system upstream of the boiler feed water tank pump as exemplified by the arrow "treatment." It will be appreciated, however, that the formulation may further be added at any appropriate place in the system. The water is moved from the tank to the boiler 210 via at least one pump 180. The water is passed through a deaerator 200. The water in the boiler is then heated by way of a heat source 220. Again, the water is heated, creating steam 240, which moves through the heat exchange 260. The condensate 280 is returned to the deaerator 200 for repeat passage. While the invention has been described with reference to specific methods and embodiments, it will be appreciated that various modifications may be made without departing from the spirit invention.

The foregoing description provides specific details for an understanding of, and enabling description for, embodiments of the apparatus. One skilled in the art will understand, however, that the invention may be practiced without these details. In other instances, well-known structures and functions have not been shown or described in detail to avoid unnecessarily obscuring the description of the embodiments of the invention.

It will be appreciated that embodiments described with respect to one aspect may be applicable to each aspect of the method described. It will further be appreciated that embodiments may be used in combination or separately. It will also be realized that sub- combinations of the embodiments may be used with the different aspects. Thus, although the embodiments have been described with many optional features, these features are not required unless specifically stated.

Claims

IT IS CLAIMED:

1. A method for inhibiting corrosion of corrodible metal in contact with water in a boiler water system, comprising (a) adding to said water system, a composition comprising a stannous salt of a non- carbon acid, or an aqueous solution thereof, and (b) maintaining in said water system a concentration of said stannous salt which corresponds to a concentration of tin (Sn⁺²) between about 0.5 and about 10 ppm, wherein said boiler water is operated at a temperature of between 25O⁰F and 800°F, and a pressure greater than 15 psi, during standard operating conditions.

2. The method of claim 1, wherein the stannous salt is selected from the group consisting of stannous bromide, chloride, sulfate, nitrate, and oxide.

3. The method of claim 2, wherein the stannous salt is stannous chloride.

4. The method of any one of claims 1-3, wherein the concentration of stannous salt is maintained in the water system at a concentration which corresponds to a concentration of Sn⁺² between about 1 and about 5 ppm.

5. The method of any previous claims, wherein in step (i) the stannous salt is added in the form of an aqueous solution.

6. The method of any previous claims, wherein said boiler water attains a temperature of at least 250°F during standard operating conditions.

7. The method of any previous claims, wherein the pH of said boiler water is 8.5 or higher.

8. The method of claim 1, wherein the corrodible metal is selected from a ferrous metal, copper, brass, and lead.

9. The method of claim 8, wherein the corrodible metal is a ferrous metal.

10. The method of claim 8 or 9, wherein the corrodible metal is carbon steel.

11. The method of any previous claims, further comprising monitoring the concentration of Sn⁺² in the water system.