Microbiologically stable yellow metal corrosion inhibitor

Abstract: The invention provides a composition and a method of preventing the corrosion of yellow metal surfaces in contact with water. The method comprising the step of adding to the water a bio-stable tolyltriazole composition including at least 45% of the 4-methylbenzotriazole isomer by weight.

Description: Background of the Invention Field of the Invention The present invention is directed to a microbiologically stable corrosion inhibitor and a method of preventing yellow metal corrosion in aqueous systems. The composition and method provide superior corrosion performance. More particularly, the invention provides a composition including 4-methylbenzotriazole (4-MBT) which is used as a yellow metal corrosion inhibitor in aqueous systems. Description of the Prior Art Tolyltriazole has two isomers, 4-methylbenzotriazole (4-MBT) and 5-methylbenzotriazole (5-MBT). Tolyltriazole, as the mixture of the two isomers, has traditionally been one of the most effective corrosion inhibitors for copper and its alloys in a wide variety of cooling water environments. A commercially available preparation of the mixed tolyltriazole isomers is COBRATEC® TT-100, available from PMC Specialties, Cincinnati, Ohio. Mixed tolyltriazole isomer preparations used as corrosion inhibitors include at least 60% by weight of the 5-MBT isomer. Generally, the tolyltriazole isomers are added to cooling water to inhibit corrosion. The tolyltriazole isomers prevent corrosion by adsorbing to metal surfaces to produce a protective surface film which inhibits corrosion. It is believed that the surface film is a monolayer film. In GB-A-1 398 988 corrosion inhibiting compositions of benzotriazole and tolyltriazole are disclosed wherein the triazole has the isomer composition of 40% 4-methylbenzotriazole and 60% 5-methylbenzotriazole. The present invention advantageously provides a tolyltriazole composition which is not biodegradable and, therefore, will provide a corrosion inhibitor which is easier to dose, more economical and not wasted in systems which have microbiological contamination. Unexpectedly, the novel composition and method of the present invention significantly and unexpectedly provide excellent corrosion inhibition while being microbiologically stable. Summary of the Invention One aspect of the invention provides a method of preventing the corrosion of the yellow metal surfaces of a cooling system in contact with water. The method comprises the step of adding to the water a tolyltriazole composition including at least 45 % by weight 4-methylbenzotriazole. Preferably, the 4-methylbenzotriazole is added to the water in a final concentration of from about 0,01 to about 100 parts per million. The 4-methylbenzotriazole is added to the water either intermittently or continuously. Other known non-tolyltriazole corrosion inhibitors may also be added to the water. According to another aspect the invention provides a microbiologically stable corrosion inhibitor preferably for use of preventing the corrosion of cooling system surfaces in contact with water, in particular water containing microorganisms, and especially for use of preventing the corrosion of cooling system yellow metal surfaces in contact with water, in particular water containing microorganisms, which corrosion inhibitor comprises a tolyltriazole composition containing at least 45 % by weight of 4-methylbenzotriazole and less than 55 % by weight of 5-methylbenzotriazole, optionally in admixture with a non-tolyltriazole corrosion inhibitor. According to preferred embodiments of the present invention the tolyltriazole composition includes at least 60 % by weight, preferably at least 80 % by weight, and most preferably at least 95 % by weight of 4-methylbenzotriazole, and less than 40 % by weight, preferably less than 20 % by weight, and most preferably less than 5 % by weight of 5-methylbenzotriazole. Brief Description of the Drawings FIG. 1 graphically represents the biodegradation of 5-MBT after tolyltriazoles spike. FIG. 2 graphically represents bacterial populations as a function of dosage of 5-MBT, 4-MBT and distilled water. FIG. 3 graphically represents the data obtained from a respirometry experiment demonstrating the aerobic biodegradation of 5-MBT. Description of the Preferred Embodiments The present invention provides a composition and a method of preventing the corrosion of cooling system yellow metal surfaces in contact with water. Although the invention is not limited to any particular source of water, preferably, cooling water systems, such as cooling water towers, once-through cooling systems, cooling lake or pond systems, and spray ponds, are treated by the method and compositions of the invention. These cooling water systems are described in detail in the Nalco Water Handbook, 2nd ed., Ch. 34 (1988). The term yellow metal is intended to include copper, bronze, and copper alloys. According to the method of the invention, an amount of a tolyltriazole composition sufficient to prevent the corrosion of the yellow metal surfaces in contact with cooling water is added to the water. According to one embodiment of the invention, the tolyltriazole composition of the invention includes at least 45% by weight of the 4-methylbenzotriazole (4-MBT) isomer of tolyltriazole. As will be described below in more detail, the present inventor has discovered that the 4-MBT isomer of tolyltriazole is biologically stable whereas the 5-methylbenzotriazole (4-MBT) isomer is not. The 4-MBT is stable in cooling water including naturally occurring or added micro-organisms. Thus, with the present invention, since 4-MBT is not biodegraded, the level of corrosion inhibition is maintained in the presence of microbiological contamination. More preferably, the tolyltriazole compositions of the invention include at least 60%, and more preferably, 80% by weight of the 4-MBT isomer. Most preferably, the tolyltriazole composition of the invention includes from about 90 to about 99% by weight of the 4-MBT isomer. According to one preferred embodiment of the invention, a tolyltriazole composition consisting essentially of the 4-MBT isomer is added to an industrial or commercial cooling system in an electric utility to prevent yellow metal corrosion. The 4-MBT isomer is preferably added in a dosage of from 0.01 to about 100 parts per million (ppm). More preferably, the 4-MBT is added to the cooling water in a final concentration of from 0.1 to about 20 ppm. The dosage of 4-MBT in the cooling water will depend on how corrosive the cooling water is, and on whether the yellow metal surfaces of the cooling water tower have been previously treated with corrosion inhibitors. In one embodiment of the invention, 4-MBT is added to the cooling water continuously at a controlled rate to maintain a concentration of from 0.01 - 100 ppm. Since 4-MBT is biostable, 4-MBT is preferably added intermittently to achieve a concentration of 4-MBT in the water from 0.05 to about 20 ppm. The cooling water may also contain non-tolyltriazole corrosion inhibitors, such as biocides, phosphates, benzotriazole, napthatriazole, molybdates, and polymer treatment programs. These other non-tolyltriazole corrosion inhibitors may be added with the 4-MBT or separately. As shown in the Examples below, surprisingly and unexpectedly, 5-MBT is biodegraded; however, 4-MBT is not effected. Thus, over time 4-MBT adsorbs to yellow metal surfaces more effectively. Thus, treatment with 4-MBT provides a better protective film over yellow metal surfaces; and therefore, provides a superior protective barrier against the corrosive cooling water. Furthermore, since 4-MBT is not biodegraded, the task of maintaining a constant protective concentration of 4-MBT in the cooling water is significantly simplified. Also, chemical is not wasted through biodegradation. Thus, the treatment of the invention is more economical to the operator. The Examples below further show that unadsorbed 4-MBT is not biodegraded by microbes in a cooling tower or other cooling system. Therefore, the 4-MBT which is not biodegraded remains in the cooling water and continues to prevent corrosion. The present invention prevents the loss of chemical seen using the mixed isomer preparations currently being used while providing superior protection against corrosion. Furthermore, the use of the present invention provides a constant concentration of corrosion inhibition in the water. Thus, cooling system operators are better able to control corrosion. The following examples are presented to describe preferred embodiments and utilities of the invention. Example 1 Three copper electrodes were polished with 600 grit sanding paper (Buehler) and rinsed with water. These electrodes were immersed in three separate Greene cells containing four cycles Chicago Tap Water (360 Ca, 200 Mg, and 440 "M" alkalinity, all as CaCO 3 ). After a half hour immersion period, the initial corrosion rate was obtained using electrochemical measurements (Linear Polarization Resistance). One of the Greene cells was then spiked with 2 ppm of 5-MBT. The second cell was spiked with 2 ppm of 4-MBT. The third cell was left as is. After an 18 hour immersion period, the corrosion rates were measured again. It was found that the corrosion rate of copper in the flask spiked with 5-MBT had decreased from an initial value of 0,009 mm/year (0.36 mpy) to 0,00008 mm/year (0.0033 mpy) (a 100 fold decrease). Similarly, for the 4-MBT spiked sample, the corrosion rate had decreased from 0,012 to 0,0002 mm/year (0.4779 to 0.0089 mpy). For the flask left as is, the corrosion rate had decreased from an initial value of 0,012 mm/year (0.46 mpy) to 0,005 mm/year (0.2 mpy) (only a 2-fold decrease). This example illustrates that both 5-MBT and 4-MBT are very effective yellow metal corrosion inhibitors. Example 2 A field sample of discharge from a utility treated with a mixed tolyltriazole preparation was analyzed for 4- and 5-MBT using HPLC and found to contain only 4-MBT. This sample was spiked with 2 ppm of a mixed isomer tolyltriazole (TT) preparation (1.16 ppm 5-MBT and 0.84 ppm 4-MBT). It was found that the 5-MBT levels had not changed in about ten hours. When measured at the end of 40 hours, 5-MBT had completely disappeared (Figure 1). 4-MBT levels, on the other hand, remained constant throughout the experiment. This type of extremely selective degradation (5-MBT vs. 4-MBT) following an initial acclimation period, is very typical of microbiological processes. Addition of sulfuric acid (up to 15%), in order to lyse any bacteria, did not result in recovery of 5-MBT ruling out processes such as adsorption by cell walls. This example illustrates that 4-MBT is resistant to microbiological degradation in a cooling water environment, whereas 5-MBT is not. Example 3 A field sample from a utility was analyzed for TT by HPLC and found to contain only 4-MBT. The sample was split into eight fractions. One fraction was left as is and spiked with 2 ppm TT. The other seven fractions were subjected to one of the following processes and then spiked with TT: Additionally, the eighth sample was spiked with 2 ppm TT and chilled in a refrigerator at 4°C. It was found that in the field sample with no treatment, 5-MBT disappeared in approximately 2 days. In samples 2 through 8, 5-MBT was stable for up to one month (analysis was not performed after this time). Since all the treatments listed in sample nos. 2 through 8 either have bactericidal effect or inhibit bacterial metabolism, preservation of 5-MBT in these samples seems to point to a microbiological mode of degradation When sample no. 8 (chilled sample) was kept at room temperature, the 5-MBT disappeared in about two days. This example provides evidence of microbiological mechanism of degradation of 5-MBT. Example 4 The field water sample from Example 2 was split into four portions. The first portion was contained in a brown glass bottle and completely covered in aluminum foil. The second portion was contained in a transparent volumetric flask. The third portion was contained in a plastic bottle, and the fourth container was contained in a plastic bottle and covered with aluminum foil. All of the samples were spiked with 2 ppm of TT from Example 2. After two days, the samples were assayed for TT using HPLC. It was found that the 5-MBT isomer had disappeared in all of them. However, 4-MBT concentrations did not change. This example illustrates that disappearance of the 5-MBT isomer is not due to a container effect (e.g. adsorption on plastic, etc.) or a photochemical phenomenon. Example 5 A Pilot Cooling Tower water sample known to degrade 5-MBT was split into three parts. To the first portion, 5-MBT was repeatedly spiked (each time waiting for the previous spike to disappear). A cumulative concentration of 1050 ppm was spiked to this portion. Whenever 5-MBT was spiked to the first portion, the same concentration of 4-MBT was spiked to the second portion and distilled water was spiked to the third portion. Samples were withdrawn at various intervals and assayed for total aerobic counts and for 4- or 5-MBT using HPLC. The results showed that 5-MBT concentrations in the first portion decreased to zero following each spike of 5-MBT. However, 4-MBT concentrations in the second portion steadily increased, consistent with amount of 4-MBT spiked to the sample. Figure 2 depicts total aerobic bacterial counts as a function of cumulative dosage of 4-MBT, 5-MBT and distilled water. It can be clearly seen that addition of 5-MBT to the first flask and its subsequent degradation results in a significant increase in total cell counts. No such increase was found for the 4-MBT isomer and the control sample. At the end of the experiment, 1.9 ml of the first portion was spiked into a Greene cell containing a copper electrode immersed in 1000 ml of standard No. 13 Chicago Tap Water. This would result in 2 ppm of 5-MBT based on a cumulative spike of 1050 ppm. Similarly, 1.9 ml of the second and third portion was spiked into separate Greene cells containing copper electrodes immersed in standard No. 13 Chicago Tap Water. Electrochemical corrosion measurements (Linear Polarization Resistance) showed that the corrosion rate of 4-MBT spiked sample had decreased roughly hundred fold to less than 0,00013 mm/year (0.005 mpy) in 20 hours. The 5-MBT spiked sample and the sample spiked with distilled water, on the other hand, showed only a two-fold decrease. This example illustrates that 5-MBT is aerobically degraded in the presence of certain bacteria. This degradation is irreversible, and the degradation product is not a yellow metal corrosion inhibitor. 4-MBT is completely stable in the presence of bacteria and is, therefore, preferred for yellow metal corrosion inhibition over 5-MBT in such situations. Example 6 Three liters of a solution containing 1 ml/ℓ of heavy metals, 1 g/ℓ of NH 4 Cℓ, 0.5 g/ℓ potassium phosphate (dibasic salt) and 0.1 g/ℓ MgSO 4 was prepared and the pH adjusted to 7.0 with H 3 PO 4 . The solution was then split into three parts. To the first part, 50 ppm of 5-MBT was spiked. To the second part, 50 ppm of 4-MBT was spiked. To the third part, distilled water was spiked. To each of the parts, 8 ml of an inoculum containing bacteria acclimated with 5-MBT (from 5-MBT spiked sample in Example 5) was added. The three solutions were then transferred to respirometry bottles and the oxygen consumption by the bacteria in the bottles was measured as a function of time. It was found that the 5-MBT spiked samples showed a significantly higher oxygen consumption (55 ppm per 50 ppm of 5-MBT) than 4-MBT and the distilled water spiked samples. The 5-MBT spiked sample was repeatedly spiked with 100, 150, 200 and 250 ppm of 5-MBT, each time waiting for the oxygen consumption from the previous spike to level off. The results are shown in Figure 3. This example illustrates that 5-MBT is aerobically degraded by certain bacteria, whereas 4-MBT is not.

Claims: A method of preventing the corrosion of cooling system yellow metal surfaces in contact with water, the method comprising the step of adding to the water a mixed isomer tolyltriazole composition including at least 45% by weight of 4-methylbenzotriazole [and less than 55% by weight 5-methylbenzotriazole]. The method of Claim 1, wherein the tolyltriazole composition is added to the water in a final concentration of from 0.01 to about 100 parts per million of 4-methylbenzotriazole. The method of Claim 1 or 2, wherein the tolyltriazole composition is added to the water intermittently. The method of Claim 1 or 2, wherein the tolyltriazole composition is added to the water continuously. The method of any of Claims 1-4, wherein the tolyltriazole composition is added to the water in a final concentration of from about 0.1 to about 20 parts per million of 4-methylbenzotriazole. The method of any of Claims 1-5, wherein the method includes a further step of adding a non-tolyltriazole corrosion inhibitor to the water. A method of preventing the corrosion of yellow metal surfaces of a cooling water tower in contact with water which contains microorganisms, the method comprising the steps of adding to the water a mixed isomer tolyltriazole composition which includes at least 60% by weight of 4-methylbenzotriazole [and less than 40% by weight 5-methylbenzo.triazole]. The method of Claim 7 wherein the mixed isomer tolyltriazole composition includes at least 80% by weight of 4-methylbenzotriazole [and less than 20% by weight 5-methylbenzotriazole]. The method of Claim 7 or 8, wherein the mixed isomer tolyltriazole composition includes at least 90% by weight of 4-methylbenzotriazole [and less than 10% by weight 5-methylbenzotriazole]. The method of any of Claims 7-9 wherein the mixed isomer tolyltriazole composition includes at least 95% by weight of 4-methylbenzotriazole [and less than 5% by weight 5-methylbenzotriazole]. The method of claim 10, wherein the method includes a further step of adding to the water a non-tolyltriazole corrosion inhibitor. The method of any of Claims 7-11, wherein the mixed isomer tolyltriazole composition is added to the water in a final concentration of from 0.01 to about 100 ppm of 4-methylbenzotriazole. A microbiologically stable corrosion inhibitor comprising a tolyltriazole composition which contains at least 45 % by weight of 4-methylbenzotriazole, optionally in admixture with a non-tolyltriazole corrosion inhibitor. The corrosion inhibitor of Claim 13, wherein the tolyltriazole composition includes at least 45 % by weight of 4-methylbenzotriazole and less than 55 % by weight of 5-methylbenzotriazole. The corrosion inhibitor of Claim 13 or 14, wherein the tolyltriazole composition includes at least 60 % by weight, preferably at least 80 % by weight of 4-methylbenzotriazole, and less than 40 % by weight, preferably less than 20 % by weight of 5-methylbenzotriazole. The corrosion inhibitor of any of Claim 13 to 15, wherein the tolyltriazole composition includes at least 90 % by weight, preferably at least 95 % by weight of 4-methylbenzotriazole, and less than 10 % by weight, preferably less than 5 % by weight of 5-methylbenzotriazole. Use of the corrosion inhibitor of any of Claims 13 to 16 for preventing the corrosion of cooling system surfaces in contact with water, in particular water containing microorganisms. Use of the corrosion inhibitor according to Claim 17 for preventing the corrosion of cooling system yellow metal surfaces in contact with water, in particular water containing microorganisms.

Inventors: JOHNSON DONALD A, LU FRANK Y, NGHIEM NHUAN P, RAO NARASHIMA M, JOHNSON, DONALD A., LU, FRANK Y., NGHIEM, NHUAN P., RAO, NARASHIMA M.
Applicants: NALCO CHEMICAL CO, NALCO CHEMICAL COMPANY
Application year: 1993
Main classification: C23F11
IPC codes: C23F11