Inhibition of Corrosion in Aqueous Systems

The present invention relates to the treatment of water to inhibit and control corrosion of metals in contact with aqueous systems. More particularly, the present invention relates to the use of a modified polyepoxysuccinic acid to inhibit or prevent corrosion of ferrous-based metals in contact with aqueous systems.

In industrial cooling systems, water such as from rivers, lakes, ponds, etc., is employed as the cooling media for heat exchangers. The cooling water from heat exchangers is typically passed through a cooling tower, spray pond or evaporative system prior to discharge or reuse. In these systems, the cooling effect is achieved by evaporating a portion of the water passing through the system. Because of the evaporation which takes place during cooling, dissolved materials in the water become concentrated, making the water more corrosive.

In cooling systems, corrosion causes two basic problems. The first and most obvious is the failure of equipment, resulting in replacement costs and plant downtime. Also, decreased plant efficiency occurs due to the loss of heat transfer. The accumulation of corrosion products causes heat exchanger fouling, resulting in the loss of heat transfer.

Ferrous-based metals, e.g., iron metal and metal alloys containing iron (mild steel), are routinely used in the construction of cooling systems due to their low cost and availability. As the system water passes over or through these ferrous-based metal containing devices, they are subjected to corrosion processes. Corrosion inhibitors are generally added as part of a water treatment program in cooling systems to prevent and inhibit the corrosion of ferrous-based metal containing devices.

Chromates, molybdates, zinc, phosphates or polyphosphates, and phosphonates have been used to inhibit the corrosion of ferrous-based metals in contact with the system water of cooling systems. Each treatment, however, presents certain drawbacks. Chromate is highly toxic and presents handling and disposal problems. Phosphates, polyphosphates, and phosphonates contribute to the eutrophication of the receiving water upon discharge, leading to restriction of their discharge by regulatory bodies. The discharge of cooling tower blowdown containing zinc, a heavy metal, is also regulated due to its aquatic toxicity. Molybdate and tungstate are not effective at low concentrations and generally are combined with other conventional inhibitors, such as phosphonates, to be cost effective.

There exists a need, therefore, for a more environmentally acceptable corrosion inhibitor of ferrous-based metals in contact with aqueous systems. In particular, there is a need for a non-phosphorus containing organic corrosion inhibitor.

Preventing the corrosion and scaling of industrial heat transfer equipment is essential to the efficient and economical operation of a cooling water system. Excessive corrosion of metallic surfaces can cause the premature failure of process equipment, necessitating downtime for the replacement or repair of the equipment. Additionally, the buildup of corrosion products on the heat transfer surface reduces efficiency, thereby limiting production or requiring downtime for cleaning.

The present invention provides an effective method and novel compounds for inhibiting and controlling corrosion of metals, particularly ferrous-based metals in contact with aqueous systems.

The method of the present invention comprises treating industrial waters with a modified poly›epoxysuccinic acid! of the general formula: ##STR2## wherein R, when present, is H, a substituted or non-substituted alkyl or aryl moiety having a carbon chain up to the length where solubility in an aqueous solution is lost, or a repeat unit obtained after polymerization of an ethylenically unsaturated compound; R' and R" each independently are hydrogen, C.sub.1-4 alkyl or C.sub.1-4 substituted alkyl; Z is O, S, NH, or NR, where R is as described above; n is a positive integer greater than 1; f is a positive integer; and M is hydrogen, a water soluble cation (e.g., NH.sub.4.sup.+, alkali metal), or a non-substituted lower alkyl group having from 1 to 3 carbon atoms. (When R is not present, Z may be MO.sub.3 S, where M is as described above).

In a preferred embodiment of the invention, R is a C.sub.1 -C.sub.20 alkyl or substituted alkyl moiety, or a C.sub.4 -C.sub.9 aryl or substituted aryl moiety, R' and R" are hydrogen, Z is NH, n is greater than 1, f is 1-2, and M is Na.sup.+.

In one particularly preferred embodiment of the invention, R is --CH.sub.2 C.sub.6 H.sub.4 CH.sub.2 -- moiety, R' and R" are hydrogen, Z is NH, n is greater than 1, f is 2, and M is Na.sup.+. In another particularly preferred embodiment of the invention, R is HOCH.sub.2 (CHOH).sub.3 C(CO.sub.2 H)--, R' and R" are hydrogen, Z is --O--, n is greater than 1, f is 1, and M is Na.sup.+.

The compositions of the present invention should be added to the aqueous system for which corrosion inhibition activity of the ferrous-based metal parts in contact with an aqueous system is desired, in an amount effective for the purpose. This amount will vary depending upon the particular system for which treatment is desired and will be influenced by factors such as the area subject to corrosion, pH, temperature, water quantity and respective concentrations in the water of corrosive species. For the most part, the present invention will be effective when used at levels of from about 0.025-500 parts per million (ppm) of water, and preferably from about 0.05-100 ppm of water contained in the aqueous system to be treated. The present invention may be added directly to the desired water system in a fixed quantity and in a state of an aqueous solution, continuously or intermittently.

The present invention pertains to a novel method of inhibiting and controlling corrosion of ferrous-based metals in contact with aqueous systems, e.g., cooling water, steam generating, gas scrubbing and pulp and papermaking systems. Specifically, the method of the present invention comprises adding to an aqueous system a modified poly›epoxysuccinic acid! of the general formula: ##STR3## wherein R is H, a substituted or non-substituted alkyl or aryl moiety having a carbon chain up to the length where solubility in an aqueous solution is lost, or a repeat unit obtained after polymerization of an ethylenically unsaturated compound; R' and R" are hydrogen, C.sub.1-4 alkyl or C.sub.1-4 substituted alkyl; Z is O, S, NH, or NR, where R is as described above; n is a positive integer greater than 1; f is a positive integer; and M is hydrogen, a water soluble cation (e.g., NH.sub.4.sup.+, alkali metal), or a non-substituted lower alkyl group having from 1 to 3 carbon atoms.

The compounds of the present invention can be prepared by incorporating reagents containing an .alpha.-hydroxycarboxylic acid (.alpha.HCA) functionality into a poly›epoxysuccinic acid! (PESA) polymer matrix. The .alpha.HCA compounds can be obtained by the ring opening reaction of a suitable reagent (R--›--Z--H!.sub.f) with a salt or ester of epoxysuccinic acid (ESA). The .alpha.HCA compound can be synthesized prior to the incorporation reaction (Scheme A) or be generated in situ by conducting the polymerization of ESA in the presence of a suitable ring-opening reagent (Scheme B). ##STR4## (R, R',R", n, Z, f and M are as described above)

For a general review of ring-opening reactions of epoxides to prepare .alpha.HCA compounds, see March, "Advanced Organic Chemistry-Reactions, Mechanisms, and Structures", 2nd Edition, Chapter 10, McGraw-Hill, New York, 1977.

Methods for conducting the polymerization of ESA, Scheme A-Step 2 and Scheme B, are described by Pearson et al., U.S. Pat. No. 3,776,850 and Bush et al., U.S. Pat. No. 4,654,159, both incorporated by reference.

The reaction can be performed neat, or in aqueous or non-aqueous solvents. If the resulting product is non-aqueous it should be modified by traditional techniques known to those skilled in the art to yield a water soluble product (e.g., hydrolysis of ester derivatives).

In a preferred embodiment of the invention, aqueous solutions of the compounds of the present invention are prepared by reacting an amine with an aqueous solution of disodium epoxy-succinate (ESA.Na.sub.2) in the presence of calcium hydroxide. The reaction is typically conducted under atmospheric conditions at about 30.degree. C.-100.degree. C., preferably from 80.degree. C. to 100.degree. C. The molar ratio of the ring opening reagent R--›--Z--H!.sub.f to ESA.Na.sub.2, relative to functionality (f) may fall within the range of about 1:2 to 1:1000, with a range of 1:5 to 1:100 being preferred. The molar ratio of calcium hydroxide to ESA.Na.sub.2 or ESA.Na.sub.2 +.alpha.HCA may fall within the range of 1:20 to 1:3, with a ratio of 1:10 being preferred.

It will be appreciated that certain by-products (e.g., disodium tartrate, PESA, and .alpha.HCA compounds) may be produced along with the compounds of the present invention in the course of the above reaction schemes. The desired reaction products can be readily recovered from the reaction product by known methods; however, it is feasible and economical to employ the compounds of the present invention as produced without separation or purification.

The treatment levels of compound added to an aqueous system can range from about 0.025 to 500 parts per million of water, and preferably from about 0.05 to 100 parts per million of water contained in the aqueous system to be treated. The concentration of compound necessary to provide effective corrosion inhibition will, of course, vary from system to system. The treatment level will vary, in part, with changes in temperatures and pH, water quantity and respective concentrations in the water of corrosive species.

The compounds may be added directly into the desired water system in a fixed quantity and in a state of an aqueous solution, continuously or intermittently. The compounds of the present invention are also expected to exhibit scale inhibition, e.g., calcium carbonate, barium sulfate, calcium oxalate, calcium sulfate and silica/silicate activity. In addition, the compounds of the present invention may also be used with topping agent components in order to enhance the scale controlling and corrosion inhibition properties thereof. Such topping components are readily known to those skilled in the art. Details of such compounds are disclosed by Chen, U.S. Pat. No. 4,659,481, incorporated herein by reference. It is expected that the compounds of the present invention may be used in conjunction with the polymers and topping components of Chen '481 to provide treatment programs which effectively inhibit corrosion and scale deposition in water systems.

Suitable topping agents include polyacrylates, phosphoric acid and water soluble salts thereof, phosphonic acids and water soluble salts thereof, polyvalent metal salts, azole compounds, molybdate and tungstate compounds and mixtures thereof.

A suitable polyacrylate is represented by the following formula: ##STR5## wherein R.sup.1 is H or lower alkyl (C.sub.1 -C.sub.3); R.sup.2 is OH, OM or NH.sub.2 ; M is a water soluble cation; R.sup.3 is a hydroxy substituted alkyl or alkylene radical having from 1 to 6 carbon atoms or a non-substituted alkyl or alkylene radical having from 1 to 6 carbon atoms; X, when present, is an anionic radical selected from SO.sub.3, PO.sub.3, PO.sub.4 and CO.sub.2 ; Z, when present, is H or any water soluble cation or cations which together counterbalance the valence of the anionic radical; a is 0 or 1, the molar ratio of x:y of the polymer being between 30:1 and 1:20.

The phosphoric acid may be orthophosphoric acid or pyrophosphoric acid or a water soluble salt thereof. The phosphonic acid may be 1-hydroxyethane-1,1-diphosphonic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid or hydroxyphosphonoacetic acid. The polyvalent metals may be Zn.sup.2+, Mn.sup.2+, or Sn.sup.2+. The azole compound may be 1,2,3-tolyltriazole, benzotriazole or butylbenzotriazole. The molybdate compound may be sodium molybdate or potassium molybdate. The tungstate compound may be sodium or potassium tungstate.

The topping agents may be added to the system in an amount of about 0.01 to 500 ppm of said system.

The 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.

Preparation of aspartic acid, 3-hydroxy, N-›2-ethane sulfonic acid!- trisodium salt via Reaction Scheme A, Step 1.

A suitable reaction flask was equipped with a magnetic stirrer, reflux condenser, nitrogen sparge, thermometer, and addition ports. Taurine (99%, 12.64 g, 0.1 mole) and 67 ml of deionized water were charged to the flask and sparged with nitrogen. Aqueous sodium hydroxide (50%, 8 g, 0.1 mole) was then charged to the flask to yield a clear, colorless liquid followed by ESA.Na.sub.2 (90%, 20.54 g, 0.105 mole) being charged to the flask. The resulting slurry was heated at 90.+-.2.degree. C. for 16.5 hours under a nitrogen atmosphere. The resulting clear solution was isolated and diluted to 130 g with deionized water.

The structure of the resulting aspartic acid, 3-hydroxy, N-›2-ethanesulfonic acid!-trisodium salt, example reference .alpha.HCA(8), was confirmed by .sup.13 C NMR spectroscopy. The product yield was estimated to be 93.3 mole % via integration of the methine carbons of the .sup.13 C NMR spectrum.

Preparation of poly›oxy(1,2-dicarboxylic acid-1, 2-ethanediyl)!.alpha.-hydro .omega.-›(ethanesulfonic acid)2-amino!-sodium salt via Reaction Scheme A, Step 2.

To a reactor setup similar to that described in Example I was charged aqueous aspartic acid, 3-hydroxy, N-›2-ethane sulfonic acid!-trisodium salt (29.2%, 11.07 g, 0.01 mole), 48 ml deionized water, and ESA.Na.sub.2 (90%, 37.16 g, 0.19 mole). The solution was sparged with nitrogen and adjusted to a pH of 10.8 with aqueous sodium hydroxide (50%). Calcium hydroxide (98%, 1.51 g, 0.02 mole) slurried in 20 ml of deionized water was then charged to the flask and the mixture was heated to 80.+-.2.degree. C. for 15.5 hours. The resulting solution was then filtered, diluted to 130 g with deionized water, and collected.

The structure of the product, example reference 10, was verified by .sup.13 C NMR spectroscopy. Residual 3-hydroxy, N-›2-ethanesulfonic acid!-trisodium salt was also detected. Approximately 23.4 mole % of the ESA.Na.sub.2 hydrolyzed to disodium tartrate under these reaction conditions.

Preparation of poly›oxy(1,2-dicarboxylic acid-1,2-ethanediyl)!.alpha.-hydro-.cndot.-›(ethanesulfonic acid)2-amino!-sodium salt via Reaction Scheme B.

To a reactor setup similar to that described in Example I was charged ESA.Na.sub.2 (90%, 19.56 g. 0.1 mole), 27 ml deionized water, and taurine (99%, 0.63 g, 0.005 mole). The solution was sparged with nitrogen and adjusted to a pH of 10.1 with aqueous sodium hydroxide (50%). Calcium hydroxide (98%, 0.76 g, 0.01 mole) slurried in 10 ml of deionized water was then charged to the flask and the mixture was heated at 80.+-.20.degree. C. for 17 hours. The resulting solution was then filtered, diluted to 65 g with deionized water, and collected.

The .sup.13 C NMR of the product, example reference 9, was similar to that of Example II. No residual taurine was detected. Approximately 22.2 mole % of the ESA.Na.sub.2 hydrolyzed to the disodium tartrate by-product under these reaction conditions.

Using the above-described preparative techniques, several other modified PESA analogs were prepared. The final products were typically a mixture of the modified PESA analog, residual .alpha.HCA, and unmodified PESA (collectively considered the "actives" portion in testing), and sodium tartrate by-product. The results of these preparations are set forth in Table 1. Several .alpha.HCA analogs (Formula I, n=1) were also prepared for evaluation. These compounds are also listed in Table I for reference.

The corrosion inhibition activity of the present invention was evaluated with a Corrosion Test Apparatus, composed of a 2 liter beaker equipped with a LCS coupon, a LCS electrochemical probe, and a stirrer. The test solution volume is 1.9 liters.

Electrochemical corrosion rate data (EC) are obtained during the test. Additional corrosion data is obtained from the coupon and the electrochemical probe by standard weight loss measurement techniques. All tests were conducted under the following conditions unless otherwise noted:

Under these conditions, the test water is supersaturated with respect to calcium carbonate. It is known in the art that a precipitated film of calcium carbonate will inhibit corrosion, resulting in much lower corrosion rates than that resulting from the inhibitors themselves. Therefore, all tests were conducted with a base treatment to inhibit the precipitation of calcium carbonate under the test conditions. The blank runs reported for all tests include the addition of the base treatment.

It was observed during the course of the testing that, on average, the test coupons were more sensitive to the lack of an effective inhibitor than was the test probe. Consequently, tests of inhibitors with slight to moderate effectiveness exhibited higher corrosion rates for the coupon weight loss data than for rates obtained for the electrochemical data or test probe weight loss data. Such compounds are not judged to be ineffective, but rather are less effective than compounds which produced consistent and lower corrosion rates for coupon, probe and electrochemical test data.

In all tests, the blank corrosion rates are expressed in mils per year (mpy). The corrosion rates for the inhibitor compounds are expressed as a reduction in the corrosion rate relative to the blank calculated according to the following equation: ##EQU1##

For the purpose of this invention, a particularly effective corrosion inhibitor will reduce the coupon weight loss corrosion rate by at least 80% compared to the blank. All tests were conducted as parts per million of the inhibitor molecule as the sodium salt. Compound .alpha.HCA(11) was evaluated as ppm of the free acid.

Table II illustrates the percent corrosion inhibition results for the compounds of the present invention, and some of their corresponding .alpha.HCA analogs. The base treatment is made up of a combination of 1 ppm of 1-hydroxyethane-1,1-diphosphonic acid (HEDP), 10 ppm acrylic copolymer (see Chen, U.S. Pat. No. 4,659,481) and 15 ppm molybdate as MoO.sub.4.sup.2-. The molybdate content of the base treatment is not sufficient to provide adequate corrosion protection, as demonstrated by the high corrosion rate for the blank.

These results demonstrate that, in general, the monofunctional derivatives of PESA (Formula I, f=1; n>1, Examples 1 to 11) are significantly more effective than the corresponding .alpha.HCA analogs. The difunctional derivatives of PESA (Formula I, f=2; n>1, Examples 12 to 16) were comparable to the corresponding .alpha.HCA analogs.

While this 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 in 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.