CONTROL OF CORROSION IN AQUEOUS SYSTEMS

Abstract: Corrosion of metals in contact with system water in an aqueous system is inhibited by adding to the system water an effective amount of an oxygen scavenger selected from the group of nitrones having the formula: wherein R₁ and R₂ may be the same or different and are each a member selected from the group consisting of hydrogen and hydrocarbon radicals having between 1 and 10 carbon atoms, and R₃ is a hydrocarbon radical having between 1 and 10 carbon atoms; or water soluble salts thereof.

Description: This invention relates to the addition of oxygen scavengers to aqueous systems to reduce the corrosion of metals that are in contact with water in the systems. In processes using aqueous solutions, corrosion of metal surfaces may occur at various locations including feed lines, heaters, steam lines, process tanks and return lines. Dissolved oxygen in the water can be a principal factor influencing this corrosion, particularly where iron and steel are materials of construction. The corrosion of the iron and steel pipes, boilers, and economizers of conventional boiler systems is a well known problem; and controlling the presence of oxygen in boiler systems, particularly in the feed water section, has received considerable attention. Oxygen removal may be partially accomplished by either vacuum or thermal deaeration, or both. Complete removal of oxygen cannot be effected by these means, however, and further removal by use of a chemical scavenging agent, such as sodium sulfite, has been customary. In recent times, the use of low pressure boilers (operating below about 150 psig) has been increasingly supplemented by use of boilers operating at moderate pressure (operating between about 150 psig and about 600 psig) and high pressure (operating above about 600 psig). As boiler operating temperatures and pressures have increased there has been particular interest in the performance of oxygen scavengers at these operating conditions. For example, use of sulphites at elevated temperatures and pressures may cause an increase in solids, and formation of sulfur dioxide and hydrogen sulfide, both of which can be a source of corrosion. Scavengers such as hydrazine, hydroquinone, and certain hydroxylamines have been found to perform satisfactorily in some circumstances. In other circumstances, the efficiency with which the scavenging proceeds has not been optimal. There is thus a continuing need for alternative oxygen scavengers which can be effectively used at elevated temperatures and pressures. Of particular value are single scavenging compounds which can be used in place of hydrazine. Despite the reported toxicity of hydrazine, much recent research has concerned development of corrosion inhibitors using hydrazine together with various organic products such as various quinone compounds and their derivatives as well as various aryl amine compounds and also certain organometallic complexes as catalysts, and preferably quinone compounds, including hydroquinone, and their derivatives to render the compositions compatible with phosphonate scale control agents. Other work has focused on other nitrogen-containing compounds such as various amines, hydroxylamines and oximes. We have found according to the present invention, that the addition of nitrones or their water soluble salts to the system water of aqueous systems, including boiler and steam condensate systems, inhibits the corrosion of metal in contact with the system water by removing oxygen in the system. Nitrones surprisingly promote the rapid scavenging of oxygen in aqueous systems, such as boiler water systems, containing dissolved air and/or oxygen. The invention is especially suited for aqueous systems such as boiler and steam condensate systems wherein the system liquid or steam consists essentially of water and oxygen. Indeed, the efficiency of oxygen scavenging achieved by using the additives, and the effectiveness of the additives at conditions found within boiler systems, makes boiler system use particularly advantageous. Accordingly, the present invention provides a method for the control of corrosion of a metal in contact with water in an aqueous system which comprises incorporating in the system a nitrone of the formula: wherein R₁ and R₂ are each independently hydrogen or a hydrocarbon group having 1 to 10 carbon atoms, and R₃ is a hydrocarbon group having 1 to 10 carbon atoms, or a water soluble salt thereof, as oxygen scavenger. R₁, R₂, and R₃ are typically alkyl groups, cycloalkyl groups, aryl groups, or aralkyl groups. Examples of said alkyl groups include methyl, ethyl, n-propyl, isopropyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl and n-decyl. Unsaturated hydrocarbon groups include alkenyl groups such as n-hexenyl, n-heptenyl, n-octenyl, n-nonenyl and n-decenyl groups. Examples of said cycloalkyl, aryl, and aralkyl groups include cyclohexyl, phenyl, and tolyl groups. The preferred hydrocarbon radicals are groups which have 1 to 7 carbons. Examples of preferred compounds are thus formaldehyde isopropylnitrone; formaldehyde ethylnitrone, formaldehyde methylnitrone, acetaldehyde isopropylnitrone, acetaldehyde propylnitrone, acetaldehyde ethylnitrone, acetaldehyde methylnitrone, acetone isopropylnitrone, acetone propylnitrone, acetone ethylnitrone, acetone methylnitrone, acetone n-butylnitrone, acetone benzylnitrone, formaldehyde n-hexylnitrone, methyl ethyl ketone ethylnitrone, formaldehyde cyclohexylnitrone, isobutyraldehyde isopropylnitrone, isobutyraldehyde ethylnitrone, n-butyraldehyde isoproylnitrone, n-butyraldehyde ethylnitrone, and n-butyraldehyde propylnitrone. These preferred compounds include formaldehyde alkyl nitrones having the general structural formula: acetaldehyde alkyl nitrones having the general structural formula: and acetone alkyl nitrones having the general structural formula: wherein R₃ is a saturated or unsaturated aliphatic group having 1 to 10, preferably 1 to 7 carbon atoms. Low molecular weight formaldehyde alkyl nitrones, acetaldehyde alkyl nitrones, and acetone alkyl nitrones such as those where R₃ is methyl, ethyl, propyl, or isopropyl are particularly useful where higher volatility is desired. Water soluble salts of the nitrones used in accordance with this invention may also be used. It is well known that a nitrone may be prepared by the reaction of a hydroxylamine, particularly a mono N-substituted hydroxylamine, with an aldehyde or a ketone. It is also known that alkylation of oximes using agents such as alkyl halides or sulfates yields nitrones as well as oxime ethers, the relative yield of each depending on the reaction conditions and the nature of the reagents (including the configuration of the oxime). U.S. Patent No. 4,596,874 to Murahashi et al. describes nitrone synthesis by reacting certain secondary amines with a peroxide in the presence of a catalyst. The use of nitrones as oxygen scavengers may be practiced in various water systems, such as recirculating heating or cooling systems. We have found that they are particularly suited for use as oxygen scavenging additives to boiler related systems, such as boiler feedwater and boiled water where the water is preferably maintained at a pH of 7 to 12, most preferably from 8 to 11. One suggested use is for treatment of water in a dormant boiler wherein the nitrone may be added along with sufficient amounts of alkaline agents such as sodium hydroxide to raise the pH to within the desired range. The higher alkalinity also aids in corrosion inhibition. In boiler water systems, corrosion may occur in feed lines, heaters, economizers, boilers, steam lines, and return lines; the invention is intended for broad use in any boiler water system (i.e. at pressures in the range of 0 to 1000 psig [70 kg/cm²] or higher). The volatility of nitrones included within this invention, especially those having relatively low molecular weights, also makes them useful in steam and steam condensate systems where addition may be made to either the steam or to its condensate. Oxygen scavenging is particularly effective in systems where the water reaches elevated temperatures, for example at least 190° Fahrenheit (88°C). Indeed, inasmuch as nitrones perform effectively at higher pressures and temperatures, they are considered particularly suited for boiler water systems which operate at temperatures in the range of 298°F to 637°F (148° to 336°C) and at pressures in the range of 50 psig to 2000 psig (3.5 to 141 kg/cm²). They are also considered effective scavengers for boiler systems which operate at even higher temperatures and pressures. The metal surfaces exposed to these conditions are generally iron and steel. The following Examples further illustrate the present invention. EXAMPLE I The oxygen scavenging activity of various chemicals was evaluated in an experimental boiler. This experimental boiler has been described previously in the proceedings of the Fifteenth Annual Water Conference, Engineers Society of Western Pennsylvania, pages 87-102 (1954). Essentially, it consists of a vertical steel tube fitted with three external heating loops extending from near the bottom and discharging near the center of the central tube. Boiler water circulates through each heating loop from bottom to top and steam is withdrawn from the top of the vertical tube. The heating source in each loop consists of an electrical fire-rod unit transferring heat indirectly through the walls of a heating tube. The feedwater was saturated with oxygen by continuous aeration maintaining about 8 to 9 parts per million (ppm) of dissolved oxygen. This feedwater contained a total hardness of approximately 10 ppm (as CaCO₃). Typical boiler water treatment chemicals such as sodium hydroxide (caustic soda) and disodium phosphate for alkalinity and calcium hardness control were added to the water in the boiler. The boiler water pH ranged from 10.8 to 11.2. Feedwater passed into the boiler where it was heated to a temperature of about 448°F (231°C) under a pressure of about 400 psig (28kg/cm²). After steam formation, the steam was condensed through a heat exchanger. The condensate, which had a temperature of approximately 55°F (13°C), was then passed through a chamber containing a standard oxygen probe. The dissolved oxygen in the condensate was measured in the chamber to determine the effectiveness of oxygen removal within the system. Boiler water was fed through the system without addition of any oxygen scavenger until a constant dissolved oxygen reading was observed in the condensate. Once the base dissolved oxygen level in the condensate was established, formaldehyde isopropylnitrone was added as the sole oxygen scavenger to the boiler feedwater at a dosage of 80 parts per million (ppm). The dissolved oxygen level in the condensate was then monitored to determine a final level by which the effectiveness of the nitrone could be determined. A second run was made using formaldehyde ethylnitrone in place of formaldehyde isopropylnitrone. After a base dissolved oxygen level in the condensate was established, formaldehyde ethylnitrone was added to the feedwater at a dosage of 80 ppm. The final level of the oxygen in the condensate was then measured to determine the effectiveness of the nitrone in the system. A third run was made using hydrazine. After a base dissolved oxygen level in the condensate was established, hydrazine was added to the feedwater at a dosage of 80 ppm. The final level of oxygen in the condensate was then measured to determine the effectiveness of the hydrazine in the system. A fourth run was made using sodium sulfite. After a base dissolved oxygen level in the condensate was established, sodium sulfite was added to the feedwater at a dosage of 80 ppm. The final level of oxygen in the condensate was then measured to determine the effectiveness of the sulfite in the system. The results of these runs are summarized in Table I. The results shown in Table I indicate that nitrones were effective oxygen scavengers in the boiler system. It is evident that nitrones may be utilized as oxygen scavengers in moderate and high pressure boiler systems. The iron and steel components typically found in these systems can thus be effectively protected from oxygen-related corrosion. EXAMPLE II The oxygen scavenging activity of nitrones was also evaluated and compared to that of hydrazine under simulated boiler feedwater conditions. The apparatus used for this evaluation is depicted schematically in Fig. 1. The apparatus (10) comprises a peristaltic pump (11) which is used for circulating water throughout the system, pump inlet tubing (12) and pump outlet tubing (13). A polyethylene fill-bottle (14) is connected with inlet tubing (12) and is used to feed system water into the apparatus. A valve (15), which may be selectively opened and closed, is provided between fill-bottle (14) and the inlet tubing (12) and is used to control flow therebetween. A temperature measurement chamber (16) is positioned within the pump inlet tubing (12) and contains a thermometer (17) used for monitoring the water temperature. The system water is drawn through the inlet tubing (12), into the pump (11), and then is pumped via the pump outlet tubing (13) into a glass sampling chamber (18). The sampling chamber (18) contains a pH probe (19) and a dissolved oxygen probe (20). The pH probe (19) is operably connected to a pH meter (21) used for displaying and monitoring the system water pH; and the dissolved oxygen probe (20) is operably connected to a dissolved oxygen meter (22) used for displaying and monitoring the system water dissolved oxygen level, as well as to a recorder (23) used for continuously recording the dissolved oxygen level in the system water. Tubing (24) is provided for directing system water from the glass sampling chamber (18) to the one liter stainless steel preheat reactor (25), known in the laboratory as a "Parr vessel." A glass chemical inlet chamber (26) is positioned within the tubing (24) and may be fitted with a rubber septum (not shown) through which a chemical solution of oxygen scavenger may be injected. Preheat reactor (25) contains a heating element (not shown) which may be activated to preheat the system water. Water forced out of the preheat reactor (25) by operation of the pump (11) is directed through tubing (27) to a two liter stainless steel heating reactor (28) where the system water is heated to its highest temperature. A heating element and a thermostat (not shown) are provided for maintaining the water temperature in the heating reactor (28) relatively constant. The water from the heating reactor (28) passes through tubing (29) into a water cooled heat exchanger (30) which cools the system water to a range of about 55°F to 60°F (13 to 16°C). System water from the heat exchanger (30) is then recirculated into the pump inlet tubing (12). Thus, while the system water may be heated to elevated temperatures in the preheat reactor (25) and heating reactor (28), the recirculated water passing through the pump (11) and into the sampling chamber (18) may be kept at a relatively constant temperature conducive to accurate probe operation. The full capacity of the apparatus (10) is about 4.5 liters while the pumping rate of pump (11) is about 0.30 to 0.35 liters per minute. The oxygen scavenging activities of nitrones and hydrazine were each investigated at system water conditions of 190°F (88°F) and pH 8.5. In each run distilled water saturated with oxygen (about 8 to 9 ppm) and adjusted to the requisite pH with sodium hydroxide, was fed into the system through the polyethylene fill-bottle (14). The system was filled to its capacity with water to the substantial exclusion of air, and the water was heated using the preheat reactor (25) and heating reactor (28) until the steady state was achieved at which the temperature of system water leaving the heating reactor (28), as controlled by the thermostat therein, was about 190°F, and the temperature of the system water leaving the heat exchanger (30) as measured by thermometer (17) was from about 55°F to 60°F. During this period, gas was allowed to exit from the system through the fill-bottle (14) and the chemical inlet chamber (26); and the dissolved oxygen level in the circulating water was allowed to equilibrate. Recirculation continued in each run until the oxygen level in the system water remained constant for at least 30 minutes. Once the initial oxygen reading was established, the system was effectively closed to the atmosphere and the oxygen scavenger was injected through a rubber septum fitted in the chemical inlet chamber (26). Recycling continued for 30 minutes, during which time the oxygen level in the sampling chamber (18) was monitored. The results given in Table II below were obtained on separate runs using hydrazine (Run 12) and various dosages of the nitrones formaldehyde isopropylnitrone (Runs 5 and 6), formaldehyde ethylnitrone (Run 7), acetaldehyde isopropylnitrone (Run 8), acetaldehyde ethylnitrone (Run 9), acetone isopropylnitrone (Run 10), and acetone ethylnitrone (Run 11). The results shown in Table II indicate that nitrones were effective oxygen scavengers in the system water. It is evident that nitrones may be advantageously utilized as an oxygen scavenger in feedwater systems of moderate and high temperature boilers. The iron and steel components typically found in these systems can thus be effectively protected from oxygen-related corrosion. The process of this invention for inhibiting corrosion of metal surfaces in contact with the system water of an aqueous system by scavenging oxygen from the system water comprises adding to the system water an effective amount of at least one nitrone. The preferred dosage range of nitrone is generally from 0.01 ppm to 10,000 ppm. Concentrations toward the lower end of this range, say up to about 200 ppm, are most practical in systems where additional means of oxygen removal are also provided such as boiler water systems having deaerators in which mechanical deaeration is practiced. Concentrations toward the upper end of this range, say about 20 ppm and above, are most practical where slug dosages are made to a system such as a recirculating hot water system, to provide residual nitrone over a long time period. The most preferred dosage range is generally from 0.05 ppm to 1,000 ppm. Use of nitrones is not confined to low, moderate or high pressure boiler systems, but may be utilized in a wide variety of other systems, where aqueous solutions containing dissolved air and/or oxygen are in contact with metal surfaces. The precise dosage used in each system will be somewhat dependent on the specific nitrone and the particular system and the water characteristics therein. The preferred system liquid (which may contain small amounts of certain additives normally used in systems such as boiler water systems for control of corrosion, scaling, sedimentation, pH, hardness) consists essentially of water containing dissolved oxygen. Likewise, where the nitrones are used to protect steam systems, the preferred steam composition (which may contain other gases normally found in air, as well as other system volatiles) consists essentially of water and oxygen. In systems where addition is made to system water which will not be further aerated before it contacts the metal surfaces to be protected, a mole ratio of nitrone to oxygen in the system is preferably from 0.1:1 to 1,000:1, most preferably from 0.5:1 to 50:1. Preferably the temperature of the system water reaches the range of 40°F to 700°F (4 to 371°C) as scavenging proceeds, most preferably from 60°F to 600°F (16 to 316°C). It is generally economical, especially where the systems are not operated at elevated temperatures, that the system water have a pH greater than about 5 as oxygen scavenging proceeds, preferably from 7 to 12, most preferably from 8 to 11. The preferred pH may be maintained by conventional means such as adding alkaline agents. Preferred conventional agents include sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, and phosphate salts of high alkalinity, such as disodium phosphate, and trisodium phosphate (especially trisodium phosphate). In boiler water systems, sodium hydroxide and potassium hydroxide are generally preferred over sodium carbonate and potassium carbonate because of the known potential for carbonates to contribute to carbon dioxide induced corrosion. The oxygen scavengers are preferably added directly to the system water as nitrones. However, compositions which, upon being added to an aqueous system, yield an oxygen scavenger consisting essentially of nitrones would of course provide similar corrosion protection and therefore fall within the scope of the present invention. The nitrone compositions employed in the process of this invention can typically be added to the system water by conventional bypass feeders using briquettes containing the additives, by feeding the compounds as dry powder mixtures to the water, or by feeding the treatment compounds as an aqueous feed solution. Feed solutions, when used, preferably contain from 0.5 to 95 percent by weight of nitrone oxygen scavenger, most preferably from 5 to 60 weight percent. Where, as in many boiler systems, make-up water is added to the aqueous solution, calibrated injecters can be employed to deliver predetermined amounts of the nitrones, periodically or continuously, to the aqueous solution via the make-up water line. While the scavengers may be added at locations throughout the system, the addition is preferably made at a location where there is little opportunity for additional aeration of the solution before it contacts the metal components to be protected. For example, in boiler systems, addition is preferably made to the feedwater at a location where the feedwater will not be exposed to substantial aeration before it enters the boiler. Examples of such locations might include the feedwater line itself, or depending upon the boiler system design, the return condensate lines, make-up water lines, or the storage section of deaerators.

Claims: A method for the control of corrosion of a metal in contact with water in an aqueous system which comprises incorporating in the system a nitrone of the formula: wherein R₁ and R₂ are each independently hydrogen or a hydrocarbon group having 1 to 10 carbon atoms, and R₃ is a hydrocarbon group having 1 to 10 carbon atoms, or a water soluble salt thereof, as oxygen scavenger. A method according to claim 1 wherein R₁, R₂, and R₃ are each independently a saturated or unsaturated aliphatic acyclic or cyclic group, an aryl group, or an aralkyl group. A method according to claim 1 or 2 wherein R₁ and R₂ are each independently a saturated and unsaturated aliphatic group having 1 to 10 carbon atoms or hydrogen, and R₃ is a saturated or unsaturated aliphatic acyclic group having 1 to 10 carbon atoms. A method according to claim 3 wherein R₁ and R₂ are each independently a saturated or unsaturated aliphatic hydrocarbon group having 1 to 7 carbon atoms or hydrogen. A method according to any one of claims 1 to 4 wherein R₃ is an aliphatic acyclic group having 1 to 7 carbon atoms. A method according to claim 5 wherein R₃ is a methyl, ethyl, propyl or isopropyl group. A method according to any one of the preceding claims wherein the nitrone is a formaldehyde alkyl nitrone, an acetaldehyde alkyl nitrone, an acetone alkyl nitrone, or a water soluble salt thereof A method according to claim 7 wherein the nitrone is formaldehyde isopropylnitrone, formaldehyde ethylnitrone, acetaldehyde isopropylnitrone, acetaldehyde ethylnitrone, acetone isopropylnitrone, acetone ethylnitrone, or a water soluble salt thereof. A method according to any one of the preceding claims wherein the nitrone is added at a dosage of 0.01 ppm to 10,000 ppm. A method according to any one of the preceding claims wherein the boiler system has a deaerator and deaeration is practiced. A method according to claim 10 wherein the concentration of nitrone in the system is from 0.01 ppm to 200 ppm. A method according to any one of claims 1 to 9 wherein the scavenger is added at a dosage of 0.05 ppm to 1,000 ppm. A method according to any one of the preceding claims wherein addition is made to system water which will not be further aerated before it contacts the metal surfaces, and the mole ratio of nitrone to oxygen dissolved in the system water is from 0.01:1 to 1,000:1. A method according to claim 13 wherein the scavenger is added to boiler water at a location wherein there is no substantial further aeration of the water before it contacts the metal surfaces to be protected. A method according to claim 14 wherein the scavenger is added in a mole ratio to oxygen in the boiler feedwater of 0.5:1 to 50:1. A method according to any one of the preceding claims wherein the system water is maintained at a pH from 7 to 12. A method according to any one of the preceding claims wherein the aqueous system is a boiler water system or steam condensate system and the system liquid or steam consists essentially of water and oxygen, and nitrone is added directly to the system. A method according to any one of the preceding claims wherein the nitrone is added to boiler feedwater. A method according to claim 18 wherein the boiler system operates at a pressure of at least 50 psig (3.5 kg/cm²) and a temperature of at least 298°F (148°C). A method according to any one of the preceding claims wherein the temperature of the system water reaches 40°F to 700°F (4 to 371°C) as scavenging proceeds. A method according to claim 19 wherein the boiler system reaches an operating pressure of 50 to 2000 psig (3.5 to 141 kg/cm²) and a temperature from 298°F to 637°F (148 to 336°C) as scavenging proceeds. A method according to any one of claims 1 to 17 and 19 to 21 wherein the system contains steam and addition is made to the steam.

Inventors: CUISIA, DIONISIO GUERRERO, HWA, CHIH MING, SALUTSKY, MURREL LEON
Applicants: W.R. GRACE & CO.-CONN. (A CONNECTICUT CORP.)
Application year: 1988
Main classification: C23F11
IPC codes: C23F11, C02F1