Methods of Inhibiting Corrosion Using Halo-Benzotriazoles

The present invention relates to the control of corrosion in aqueous systems. More particularly, the present invention relates to the inhibition of corrosion of steel and copper alloys in aqueous systems through application of halo-benzotriazoles to the aqueous system.

The use of triazoles for inhibiting the corrosion of copper and iron alloys in a wide variety of aqueous and non-aqueous systems is well known. In industrial cooling water systems, benzotriazole and tolyltriazole are used most often. Tolyltriazole is generally preferred because of its lower cost. Triazoles are film forming materials that provide efficient coverage of metal or metal oxide surfaces in a system thereby providing protection against corrosive elements present in an aqueous system. In addition to the film forming tendency of various azoles, they also precipitate soluble, divalent copper ions. The precipitation prevents transport of copper ions to ferrous surfaces, where galvanic reactions between copper ions and iron atoms leads to pitting corrosion of the ferrous metal.

While the use of azoles for corrosion inhibition is widespread, there are drawbacks to their use, specifically with tolyltriazole. The most important drawbacks are experienced when azoles are used in combination with oxidizing halogens. Oxidizing halogens such as elemental chlorine, bromine, their hypohalous acids, or their alkaline solutions (i.e., solutions of hypochlorite or hypobromite ion) are the most common materials used to control microbiological growth in cooling water systems. When copper or iron alloys that have previously been protected with azoles are exposed to an oxidizing halogen, corrosion protection breaks down. After breakdown, it is difficult to form new protective films in tolyltriazole treated cooling systems that are being chlorinated, particularly continuously chlorinated. Very high dosages of tolyltriazole are frequently applied in an attempt to improve performance, often with limited success.

The degradation of protection of azole films in the presence of oxidizing halogens is well-documented in the literature. For example, R. Holm, et al., concluded that hypochlorite penetrates an intact triazole film, leading to higher corrosion rates, and that secondly, hypochlorite attacks the prefilmed triazole surface, disrupting or degrading the film (53rd Annual Meeting of the International Water Conference, Paper No. IWC-92-40, 1992). Lu, et al., also studied interactions of triazole films with hypochlorite on copper and copper alloy surfaces ("Effects of Halogenation on Yellow Metal Corrosion: Inhibition by Triazoles", Corrosion, 50, 422 (1994)). Lu, et al., concluded:

(a) prefilmed tolyltriazole on copper and brass surfaces undergoes decomposition during chlorination;

(b) the stability of prefilmed tolyltriazole on copper and brass to NaOCl was improved when tolyltriazole was added to the hypochlorite solution;

(c) clean (i.e., non-prefilmed) copper surfaces did not develop good protective films when placed in solutions containing mixtures of tolyltriazole and NaOCl.

Thus, the combination of tolyltriazole with NaOCl did not produce a composition capable of efficient film formation and corrosion inhibition.

The nature of the reaction products when azoles are exposed to oxidizing halogens in a cooling water system is not clear. The literature teaches that a compound is formed when chlorine and tolyltriazole are combined in cooling waters, and that it responds to analytical tests for chlorine. For example, Vanderpool, et al., state that chlorine reacts reversibly with tolyltriazole to produce N-chloro-tolyltriazole. They specifically state, "presumably this compound is not itself an inhibitor." Rather, they teach that it is readily hydrolyzed to the original tolyltriazole and hypochlorous acid so that free tolyltriazole becomes available for corrosion inhibition ("Improving the Corrosion Inhibitor Efficiency of Tolyltriazole in the Presence of Chlorine and Bromine", NACE Corrosion/87, Paper No. 157 (1987)). Hollander and May stated they were able to isolate 1-chloro-tolyltriazole from stored, more highly concentrated solutions, but they also teach that "at low concentrations (less than 10 mg/L) rapid hydrolysis made it impossible to isolate the chloro adducts." Based upon proton NMR analysis, the material Hollander and May isolated was chloro-tolyltriazole.

Another observation is that a very characteristic odor is present whenever tolyltriazole and chlorine are combined in cooling waters.

In contrast, the present authors have shown that chloro-tolyltriazole does not respond to analytical tests for chlorine, despite extended boiling. And solutions of chloro-tolyltriazole, surprisingly, do not produce the characteristic odor. Thus chloro-tolyltriazole is clearly different from the tolyltriazole-chlorine reaction product that forms in situ in cooling water systems.

There are also references in the literature to 5-chlorobenzotriazole (i.e., CAS number ›94-97-3!). In "The Water Drop", Volume I No. 2, 1985, Puckorius & Associates state that chlorinated tolyltriazole is effective as a corrosion inhibitor and cite R.P. Carr as a reference. A literature review of published work by Carr indicates that he actually teaches that reactions between tolyltriazole and chlorine do not occur under cooling water conditions ("The Performance of Tolyltriazole in the Presence of Sodium Hypochlorite Under Simulated Field Conditions", NACE Corrosion/83 Paper No. 283, 1983). In this Corrosion/83 paper, Carr does discuss the inhibiting action of a chloro-azole but it is a reference to earlier literature and specifically to the action of 5-chlorobenzotriazole and related aryl substituted azoles in sulfuric acid solutions ("Effects of Substituted Benzotriazole on the Electrochemical Behavior of Copper in H.sub.2 SO.sub.4 ", Wu et al., Corrosion, Volume 37, No. 4,223 (1981)). Since the 1985 Puckorius reference, there has been widespread use of tolyltriazole in chlorinated cooling systems with well established performance difficulties, indicating a continuing, unsolved problem in the art.

Other problems are well-known when tolyltriazole and oxidizing halogens are combined in cooling waters. These include a loss in the extent of precipitation of transition metal ions such as copper, thus leading to improved transport and galvanic corrosion, a change in the response of the standard spectrophotometric test for tolyltriazole, leading to unintentional overfeed, and the objectionable odor mentioned above. This odor can be sensed even when the cooling water originally contained 1 ppm tolyltriazole, or less. Since cooling water often passes over cooling towers, evaporation and drift release the objectionable odor to the local environment.

The present inventors believe that the odorous material is N-chloro-tolyltriazole, that it forms reversibly with tolyltriazole in dilute solution, and that it is absent in the final product when the reaction is run in concentrated solution, i.e., tolyltriazole+OCl.sup.-.fwdarw. N-chloro-tolyltriazole-(intermediate).fwdarw.chloro-tolyltriazole. The present inventors have found no evidence of reversion of chloro-tolyltriazole to either the odorous intermediate or to tolyltriazole. Nor is there any evidence of reactions between hypochlorite and chloro-tolyltriazole in dilute aqueous solutions.

The present inventors have discovered that halo-benzotriazoles such as chloro-tolyltriazole and bromo-tolyltriazole are more effective than tolyltriazole in inhibiting corrosion in aqueous systems. The halo-benzotriazoles are substantially more effective than tolyltriazole in the presence of chlorine. Furthermore, when chloro-tolyltriazole is exposed to chlorine, an objectionable odor does not form and the quantity of chlorine that is required to produce a residual in the aqueous system is reduced. A process for preparation of aqueous solutions of halo-benzotriazoles is also disclosed.

The present inventors have discovered that halo-benzotriazoles such as chloro-tolyltriazole and bromo-tolyltriazole are more effective than tolyltriazole in inhibiting corrosion in aqueous systems. The halo-benzotriazoles are substantially more effective corrosion inhibitors than tolyltriazole in the presence of chlorine. The efficacy of the present invention is surprising given the prior knowledge that chlorination of an azole treated system leads to degradation of corrosion inhibition performance. Furthermore, the halo-benzotriazoles of the present invention are not subject to the formation of objectionable odors when exposed to chlorine as is tolyltriazole, the quantity of chlorine that is required to produce a residual in the aqueous system is notably reduced in comparison to systems treated with tolyltriazole, and the treatment is effective in the presence of sulfide ions.

It was discovered that the ex situ preparation of a halo-benzotriazole provided a corrosion inhibitor which exhibited a surprising and unexpected activity when compared to a treatment comprising a mixture of a benzotriazole and a halogen. The results of the studies of the present invention clearly show that mere mixtures of a benzotriazole and a halogen in a cooling water system do not provide the corrosion inhibiting effect of the addition of a halo-benzotriazole prepared ex situ. As further evidence of the surprising activity of an ex situ prepared halo-benzotriazole, the present inventors found that the chlorine demand of a system treated in accordance with the present invention was significantly reduced. Furthermore, in systems treated in accordance with the present invention the objectionable odor common to systems treated with a triazole and halogen was absent.

The halo-benzotriazoles of the present invention can include chloro-, fluoro-, bromo- and iodo- as well as haloalkyl (trifluoromethyl) benzotriazoles. Preferred are chloro-tolyltriazole and bromo-tolyltriazole. The azole may include tolyltriazole, benzotriazole, butylbenzotriazole, mercaptobenzothiazole and the like. The preferred azole is tolyltriazole.

The preferred benzotriazole, tolyltriazole, is such that the preferred halo-benzotriazole is chloro-tolyltriazole or bromo-tolyltriazole. The preparation of the preferred chloro-tolyltriazole can be by any suitable means. Examples of preparation methods include but are not limited to reactions with hypochlorite, N-chlorosuccinimide, and other chlorinating agents. A method of forming chloro-tolyltriazole is through the reaction of tolyltriazole with hypochlorite, in which case the final reaction mixture is an alkaline solution that can be used with or without further modification. Alternatively, chloro-tolyltriazole can be formed through the reaction of tolyltriazole with hypochlorite in acetic acid solutions, (i.e., hypochlorous acid) and then isolated as a solid. For convenience of application, the solid can be redissolved in alcohols such as methanol or 2-propanol, aqueous solutions of alcohols or strong alkaline solutions such as sodium hydroxide or potassium hydroxide.

The preparation of bromo-tolyltriazole can be by any suitable means. Examples of preparation methods include but are not limited to reactions with hypobromite, bromine, and other brominating agents. A method of forming bromo-tolyltriazole is through the reaction of tolyltriazole with bromine in an aqueous solution and then isolating it as a solid. For convenience of application, the solid can be dissolved in a strong alkaline solution such as sodium hydroxide or potassium hydroxide.

In treating an aqueous system in accordance with the present invention, the chloro-tolyltriazole (hereinafter Cl-TTA) is preferably fed continuously to the water. A preferred treatment concentration ranges from about 0.5 to 10 parts per million, most preferably at about 3 parts per million. Continuous feed is not, however, a requirement. The chloro-tolyltriazole can be fed at a concentration sufficient to form a protective film and thereafter feed can be discontinued for extended periods of time.

The halo-benzotriazole treatment of the present invention can be used in combination with other corrosion and/or deposit inhibiting treatments known in the art including but not limited to phosphates, phosphonates, acrylic homo- and copolymers, chelants, and oximes.

The present inventors have discovered a process for producing a high pH aqueous solution of halo-benzotriazole, the process comprises adding a sodium hypochlorite solution to a slurry of 4(5)-methylbenzotriazole/tolyltriazole (TTA) in water at 47.degree.-50.degree. C. The resulting reaction produces a high pH solution containing Cl-TTA as its major component. This new process eliminates the use of acetic acid and the need to isolate solid Cl-TTA. The resulting aqueous solution is easily transported, handled and applied as a corrosion inhibitor in aqueous systems. While the reaction of sodium hypochlorite with aqueous solutions of the sodium salt of TTA also resulted in the formation of an aqueous solution of Cl-TTA, the yield was lower so this reaction is less desirable.

In one example of the preferred method of preparation of Cl-TTA, an aqueous slurry of TTA was reacted with sodium hypochlorite in a 1:1 molar ratio. The temperature was 47.degree.-50.degree. C. and the pH ranged from 8-9 to about 12 at the end of the reaction. The resulting mixture contained 1.2-1.4 wt. % residual TTA. During the initial stages the reaction mixture is a slurry, but as the pH increases over 10, a solution forms.

Examples 13 and 14 below relate to the new method of preparation of Cl-TTA and Br-TTA while comparative Example 15 relates to the adaptation of the prior "acetic acid" method of preparing Cl-BZT to the preparation of Cl-TTA. These examples show that the new method of this invention results in a higher yield without the need to handle acetic acid or solid product.

The present invention will now be further described with reference to a number of specific examples which are to be regarded solely as illustrative and not as restricting the scope of the present invention.

The preparation of the solid samples was as follows:

Tolyltriazole (hereinafter TTA) (30 g, 0.225 mol) was dissolved in aqueous acetic acid (60 mL, 1:1 ratio) by heating to 32.degree. C. Sodium hypochlorite (366 g, 5.25% sodium hypochlorite as a bleach solution) was added while maintaining the reaction temperature at .about.20.degree. C. Following the addition, the reaction mixture was stirred at room temperature for 24 hours. A sticky precipitate formed during this time. The solid was filtered and taken into methylene chloride. The solid that did not dissolve was filtered and identified as a mixture of Cl-TTA with minor amounts of TTA and dichloro-tolyltriazole (di-Cl-TTA). The methylene chloride was removed to obtain a yellow solid which was identified as a mixture of Cl-TTA with minor amounts of di-Cl-TTA. Unless noted, this latter solid was used in the following Examples.

A slurry of TTA (50 g, 0.376 mol) in 25 g of water was warmed to 35.degree. C. Sodium hypochlorite (27.9 g, 0.376 mol, added as 226.8 g of a 12.3% sodium hypochlorite solution) was added over a period of 2 hours. After the addition, the reaction was kept at 45.degree. C. for one hour. During the addition the pH of the reaction mixture increased to 12 and the solids dissolved. The final product was analyzed by .sup.1 H and .sup.13 C NMR and LC-UV and found to be composed of 81.9% Cl-TTA, 8.8% residual TTA, and 9.3% di-Cl-TTA based on the relative areas in the UV spectra.

On dilution to 1 to 100 ppm azole, with or without pH adjustment to about 7.2, there was no odor from the halo-benzotriazole solution of the present invention.

In the schemes below, TTA was present at 100 ppm, in contrast to Example 2 where the initial slurry contained about 200,000 ppm. "x" denotes a stoichiometric ratio. ##STR1##

Cl-TTA, prepared as a solid according to Example 1, was dissolved in methanol and charged to a simulated cooling water solution. The solution contained 319 ppm Ca (calculated as CaCO.sub.3), 7 ppm Mg (calculated as CaCO.sub.3), 190 ppm NaHCO.sub.3, 882 ppm Na.sub.2 SO.sub.4, 1184 ppm NaCl, 5 ppm Cl-TTA, and 2.4 ppm of hydroxyethylidene diphosphonic acid (HEDP). Hypochlorite was absent. The solution was maintained at 120.degree. F. by an admiralty brass heater tube and at pH=7.2 to 7.5 by a pH controller equipped to feed sulfuric acid on demand. The solution was recirculated past the heater and past both admiralty and copper/nickel alloy corrosion rate meters (CRM). After 1 hour the solution was drained and replaced by an identical solution with no Cl-TTA. This solution was fed to overflow which replenished the system with fresh solution at a rate of about 4% by volume per hour. This system was maintained under these conditions continuously until the bright admiralty tube was tarnished, at which point the experiment was terminated. Comparisons were made to identical experiments with TTA and benzotriazole.

Corrosion tests were carried out in the apparatus described in Example 4 with water containing 500 ppm Ca, 250 ppm Mg, 25 ppm Malk, 15 ppm o-PO.sub.4, 3 ppm tetrapotassium pyrophosphate, 10 ppm of a 3:1, low molecular weight, acrylic acid/allyl 2-hydroxypropyl sulfonate ether copolymer, 2.4 ppm HEDP, a 3 ppm of either Cl-TTA or TTA. The pH was maintained at 7.2 with a blended mixture of air and carbon dioxide at 120.degree. F. for 18 hours. Electrochemical corrosion rates were measured using admiralty brass (ADM) and low carbon steel (LCS) working electrodes. All tests also had both admiralty and LCS coupons in contact with the solution. The method differed from Example 4 in that the azole was fed continuously at 3 ppm during these experiments. The azole was supplied by dissolving the solid in potassium hydroxide solution and then diluting it into the feedwater for the system. Each experiment was duplicated: once with an admiralty brass heated tube, and once with a low carbon steel heated tube. Corrosion rates were measured as in Example 4 from admiralty and LCS working electrodes, and by weight changes of admiralty and LCS coupons. Rates for the coupons were measurer for the initial day of each run, and a "differential" rate was calculated for the remaining days of the run by offsetting the initial rate from the overall rate.

The method of Example 5 was followed, except a solution of sodium hypochlorite was added after 20 hours and continued for an additional 72 hours. The feed rate of the sodium hypochlorite was controlled to produce a "chlorine residual" of about 0.1 to 0.3 ppm as Cl.sub.2 using a standard DPD spectrophotometric test on the recirculating water. For the experiment with Cl-TTA, the feed rate of the sodium hypochlorite was about 30% of that required for TTA. For TTA, the characteristic odor was detected immediately after the first hypochlorite was added. With Cl-TTA, there was no odor upon initiating hypochlorite addition, and only a trace was sensed just prior to concluding the four day run.

Solutions of azole at 6 ppm were made in deionized water, and the pH was adjusted to 7.0. Cu.sup.+2 ion was added (0.1 ppm from cupric sulfate) and the pH was again adjusted to 7.0. A sample was digested with nitric acid, analyzed for copper, and a second sample was filtered (0.2 micron pore size), digested, and analyzed for copper. The ratio was expressed as "% soluble Cu":

Admiralty brass corrosion coupons and working electrodes were coated with a sulfide layer by exposing the metal to a sodium sulfide solution for 18 hours. These samples were rinsed and dried. Corrosion tests were carried out in aqueous solutions in stirred beakers containing 500 ppm Ca, 250 ppm Mg, 25 ppm Malk, 15 ppm o-PO.sub.4, 3 ppm tetrapotassium pyrophosphate, 10 ppm of a 3:1, low molecular weight, acrylic acid/allyl 2-hydroxypropyl sulfonate ether copolymer, 2.4 ppm HEDP, and the pH was maintained at 7.2 with a blended mixture of air and carbon dioxide at 120.degree. F. for 18 hours. Electrochemical corrosion rates were measured using admiralty brass or low carbon steel working electrodes. All tests also had both admiralty and LCS coupons in contact with the solution.

Each solution was tested with and without addition of sodium hypochlorite (added after 1 hour exposure). In a separate, but otherwise identical experiment, clean low carbon steel working electrodes were used in place of the sulfide-exposed admiralty brass, but the sulfide-exposed brass coupons were present as a source of copper. At the conclusion of the experiment, a sample of the supernatant solution was taken and analyzed for copper. Analyses were taken with and without filtration through a 0.2 micron membrane filter.

A synthetic sea water was formulated from deionized water plus 1010 ppm Ca as CaCO.sub.3, 5226 ppm Mg (as CaCO.sub.3), 18971 ppm Cl, 2660 ppm SO.sub.4, 117 ppm M-alkalinity (as CaCO.sub.3), 5 ppm azole (see below), and the pH was maintained at 7.8 with a blended mixture of air and carbon dioxide at 100.degree. F.

Admiralty brass electrodes were exposed to this medium for 1 hour and then they were transferred to identical water with no azole present. Electrochemical corrosion rates were measured for 18 hours.

Sodium hypochlorite (12.2%, 204.9 g, 0.336 mol) was added over 90 minutes to a stirring slurry of benzotriazole (40 g, 0.336 mol) in 30 g of water at room temperature. Following the addition, the reaction mixture was held at 45.degree.-50.degree. C. for one hour. Upon cooling, a precipitate formed. A clear yellow solution was obtained after adjusting the pH to 11. The final product was analyzed by LC/MS and .sup.13 C and .sup.1 H NMR and found to be composed of 54.6% chloro-benzotriazole (Cl-BZT), 23.9% residual benzotriazole, and 21.5% di-chloro-benzotriazole (di-Cl-BZT).

Bromine (12.5 g, 0.078 mol) was added to a stirring slurry of TTA (10 g, 0.075 mol) in 66 g of water in a reactor protected from light, while maintaining the temperature at <25.degree. C. After the addition, the reaction mixture was held at 35.degree.-40.degree. C. for one hour. Upon cooling, adjusting the pH to 11-12 did not produce a clear solution. The small amount of precipitate that formed upon standing was removed by filtration, the pH of the filtrate was adjusted to neutral, and the resulting precipitate filtered. This solid was characterized by LC/MS and .sup.13 C and .sup.1 H NMR and found to be composed of 90.5% bromo-tolyltriazole (Br-TTA), 4.9% residual TTA, and 4.2% di-bromo-TTA.

The method of Example 8 was followed, using samples from Examples 2, 11 and 12 at 1 to 4 ppm total actives. The following were the 18 hour averaged electrochemical corrosion rates:

A 75 gallon glass lined reactor was charged with 174.1 lb. of water and 75 lb. of TTA. After sealing the reactor, 50.4 lb. of a 12.5% sodium hypochlorite solution (this represents 15% of the total charge of 335.8 lb.) was transferred to the reactor. The reaction mixture was heated to 45.degree. C., and after the temperature was stable the remaining sodium hypochlorite solution was added over a period of 2 hours while maintaining the temperature at 47.degree.-50.degree. C. The two step addition of sodium hypochlorite was done due to the design characteristics of the mixer being employed. After the sodium hypochlorite feed was complete, the temperature was held at 50.degree. C. for 1 hour. The product was analyzed by LC-UV area percent and .sup.13 C NMR. The composition of the product was as follows: 8.5% residual TTA, 80.3% Cl-TTA, 10.8% di-Cl-TTA, <0.01% residual chlorine, pH 11.5.

A sodium hypobromite solution was generated by mixing sodium bromide (21.4 g, 0.208 mol) dissolved in 50 g of water with sodium hypochlorite (100.5 g, 12.2% solution, 0.165 mol) prior to use. The sodium hypobromite solution was added to a stirred slurry of TTA (20.0 g, 0.15 mol) in 76.5 g of water over a period of 2 hours. The temperature was maintained at 30.degree.-35.degree. C. The equipment was covered with aluminum foil to protect it from light. After the sodium hypobromite addition was complete, the reaction mixture was held at 35.degree.-40.degree. F. for 4 hours. The product was analyzed by LC-UV area percent and .sup.13 C NMR. The composition of the product was as follows: 6.1% residual TTA, 81.5% Br-TTA, 12.1% di-Br-TTA, pH 9.9.

TTA (30 g, 0.225 mol) was dissolved in aqueous acetic acid (60 mL, 1:1 ratio) by heating to 32.degree. C. The solution was cooled with an ice bath and held below 20.degree. C. during the addition of sodium hypochlorite solution (366 g, 5.25% sodium hypochlorite, 0.259 mol). After the addition, the reaction mixture was allowed to reach room temperature and was stirred for 24 hours. During this time a sticky material separated. The aqueous layer was decanted and the sticky yellow material was dissolved in methylene chloride (300 mL). A flock formed and was filtered off (solid 1, 13.9 g). The methylene chloride was removed to obtain a waxy yellow solid (solid 2, 27.5 g). NMR analysis showed an apparent separation of isomers between the two solids. This was confirmed by LC/MS analysis. The composition of the solids was as follows:

The above examples show that the halo-benzotriazoles of the present invention are effective corrosion inhibitors even in the presence of chlorine.

While the present invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of this invention will be obvious to those skilled in the art. The appended claims and this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the present invention.