Method for controlling corrosion using molybdate compositions

Abstract: A method for inhibiting corrosion in aqueous systems comprising adding to the system being treated an effective amount of a composition comprising a molybdate ion source and a water-soluble component selected from polymaleic acid anhydride, amine adducts of polymaleic anhydride, maleic anhydride copolymers, carboxylic acid/sulfonic acid polymers, salts of the above-described polymers, phosphonates, phosphino carboxylic acids, polyphosphoric acid and esters of polyphosphoric acid.

Description: BACKGROUND OF THE INVENTION The instant invention relates to a method for inhibiting the corrosion of metallic surfaces in contact with aqueous systems and to compositions for use in such a method, particularly where the water of the aqueous system is oxygen-bearing. More particularly, the present invention relates to the use of compositions comprising a combination of a molybdate ion source and water soluble polymers containing a sulphonic acid moiety, a carboxylic acid moiety, a polyalkylene oxide moiety and salts thereof to inhibit the corrosion of metallic surfaces of water-carrying systems. The term "aqueous system" as used herein, is intended to describe any system which contains water in any physical state, including water which contains one or more dissolved or dispersed substances such as inorganic salts. The term "metallic" as used herein, is intended to include ferrous and ferrous-containing materials. The corrosion of a metallic surface in an aqueous system consists of the destruction of the ferrous metal by chemical or electrochemical reaction of the metal with its immediate environment. Where the corrosion is electrochemical in nature, a transfer or exchange of electrons is necessary for the corrosion reaction to proceed. When corrosion of the metal takes place, at least two electrochemical processes occur, and must occur, simultaneously. There is an anodic oxidation reaction in which metal ions go into solution, leaving behind electrons; and at least one cathodic reduction reaction in which species in solution are reduced by consuming the electrons produced by the anodic reaction. With respect to ferrous or ferrous containing materials, when the water contains oxygen and is at a neutral pH or above, these processes may be illustrated by the following equations: Anodic oxidation: Fe → Fe⁺² + 2e⁻ Cathodic reaction: 2H₂O + O₂ + 4e⁻ → 4OH⁻ The two ionic reaction products, ferrous ion and hydroxyl ion, combine to form ferrous hydroxide, Fe(OH)₂, which is then oxidized to form ferric hydroxide, Fe(OH)₃ (rust). For ferrous or ferrous-containing materials as well as other metals in aqueous systems, the principle factors influencing the corrosion process are the characteristics of the water in the system, including but not limited to the rate of water flow, the temperature of the system and contact between dissimilar metals in the system. Variable characteristics of the water which impact upon its corrosiveness are its dissolved oxygen concentration, carbon dioxide content, pH and hardness. The presence of dissolved oxygen in the water of an aqueous system is primarily the result of contact between the water and the atmosphere. The oxygen solubility in water is temperature and pressure dependent, with increases in pressure increasing solubility and increases in temperature lowering oxygen solubility. Corrosion produced by the presence of oxygen in the water of an aqueous system can take place in the form of small pits or depressions and/or in the form of general metal loss. As a corrosive process continues, pits or depressions generally increase in depth. The corrosive attack is more severe when it causes pits or depressions, since the deeper penetration of the metal causes more rapid failure at these points. U.S. Patent 4,640,793 discloses synergistic scale and corrosion inhibiting admixtures containing carboxylic acid/sulphonic acid polymers and molybdates. U.S. Patent 4,618,448 discloses the use of carboxylic/sulphonic/polyalkylene oxide polymers for use as scale and corrosion inhibitors. However, none of the prior art references described above in any way suggest the synergistic results obtained with the novel compositions of the instant invention. According to a first aspect of the present invention there is provided a method for inhibiting corrosion in an aqueous system comprising adding to said system an effective amount of a corrosion inhibiting composition comprising: (a) a molybdate ion source; and (b) a polymer or salts thereof, the polymer prepared from 50-70%, by weight, acrylic acid or methacrylic acid, 10-40%, by weight, 2-acrylamido-2-methylpropyl sulfonic acid or 2-methyacrylamido-2-methylpropyl sulfonic acid and 10-30%, by weight, of a polyalkyleneoxide compound, wherein the weight of (a):(b), on an active basis, ranges from 10:1 to 1:10. According to a second aspect of the present invention there is provided a composition comprising: (a) a molybdate ion source; and (b) a polymer or salts thereof, the polymer prepared from 50-70%, by weight, acrylic acid or methacrylic acid, 10-40%, by weight, 2-acrylamido-2-methylpropyl sulfonic acid or 2-methyacrylamido-2-methylpropyl sulfonic acid and 10-30%, by weight, of a polyalkyleneoxide compound, wherein the weight of (a):(b), on an active basis, ranges from 10:1 to 1:10. The weight ratio of the molybdate ion source to the water-soluble polymer ranges from 10:1 to 1:10. The corrosion inhibiting compositions of this invention may optionally contain other known corrosion inhibitors, such as zinc salts, triazoles or an ortho-phosphate source. The present invention also concerns the novel compositions used in the method of the present invention for inhibiting corrosion. The instant compositions are especially effective over a pH range of from 6.0 to 9.0, preferably 7.0 to 8.0, and these compositions are effective in waters of various hardness. Any source of molybdate ions can be used. The preferred sources are water soluble molybdate salts, and the most preferred molybdate salts are magnesium molybdate, ammonium molybdate and alkali metal molybdates such as lithium molybdate, sodium molybdate and potassium molybdate. The carboxylic/sulfonic polymer of the instant invention may be any water soluble polymer having an intrinsic viscosity of 0.05 to 2.5 dl/g prepared from: (a) 50 to 70%, by weight, acrylic acid methacrylic acid; (b) 10 to 40%, by weight, 2-acrylamido-2-methylpropyl sulfonic acid or 2-methyacrylamido-2-methylpropyl sulfonic acid; and (c) 10 to 30%, by weight, of a polyalkyleneoxide compound. Examples of suitable monomers of the polyalkylene oxide moiety include allyl polyethylene glycols, methallyl polyethylene glycols, polyethylene glycol acrylates, polyethylene glycol methacrylates, methoxy allyl polyethylene oxides, alloxyallyl polyethylene oxides and the polypropylene equivalents thereof. Also, mixtures of polyethers formed from polyethylene oxide with other polyalkylene oxides, such as propylene or butylene oxide, may be used. The polyether chain may be capped with an alkyl, aralkyl, sulfonate or phosphonate group metal or ion, or it may be uncapped. The preferred polyalkylene oxides are polyethylene glycol methacrylates containing up to 20 (OCH₂CH₂) groups, most preferably 3-10 (OCH₂CH₂) groups. Also, other monomers may be used. For example, non-ionic monomers such as acrylamide, methacrylamide and acrylonitrile may also be present in the polymers. The most preferred polyalkylene oxides are polyethylene glycol methacrylates. These polymers may be prepared by mixing the monomers in the presence of a free radical initiator. Theoretically, any free radical initiator may be used. Examples of preferred initiators include peroxides, azo initiators and redox systems. The polymerization may also be initiated photochemically. The preferred catalysts are sodium persulfate and sodium metabisulfite. The polymerization may be conducted by any of a variety of procedures, for example, in solution, suspension, bulk or emulsion. Polymers of this type are usually characterized by intrinsic viscosity. The intrinsic viscosity should be 0.05 to 2.5, preferable 0.05 to 0.5 dl/g, in 1.0 M sodium chloride (measured on a 75 Cannon Ubbelohde capillary viscometer). Water soluble salts may also be used. The ratio of component (a) to component (b) in the instant compositions ranges from 1:10 to 10:1, on an active weight basis, preferably from 5:1 to 1:5. An effective amount of the instant compositions should be used. As used herein, the term "effective amount" refers to that amount which inhibits or prevents the corrosion of metallic surfaces in contact with the aqueous system being treated. Preferably, the instant compositions should be added at a dosage of from 0.1 to 200 ppm, on an active weight basis, based on the total weight of the water in the aqueous system being treated. Components (a) and (b) can be added separately or in combination, which ever is most convenient. The instant method is especially effective at pH's ranging from about 6.0 to about 9.0, preferably from about 7.0 to about 8.0. Also, the instant method is effective at various levels of hardness. Other known corrosion inhibitors, such as zinc salts or azoles, may be used in conjunction with the instant compositions. EXAMPLES The following examples further illustrate this invention. However, they are not intended to limit the scope of this invention in any way. Corrosion studies were initiated by precleaning 1"x2" carbon steel coupons with xylene, Calclean, (an alkaline silicate phosphate cleaner available from Calgon Corporation), water and acetone, respectively in an ultrasonic bath, then drying them with house air. The coupons were weighed and then hung in eight liter test solutions which were adjusted to and maintained at pH 7.0 or 8.0, heated to and maintained at 50 o C, circulated and aerated. Three test solutions of varying hardness were used. Soft water was prepared by adding 1.40L of 4X Pittsburgh water to 6.60L of deionized water. 4X Pittsburgh water is a solution of 50.2 mg/L MgCl · / 2 6H₂O, 43.2 mg/L Na₂SO₄, 13.8 mg/L NaHCO₃ and 379.5 mg/L CaSO · / 42H₂O. Moderately hard water was prepared by adding 7.30L of 4X Pittsburgh water to 0.70 L of deionized water. Hard water was prepared by adding 43.26 grams of 50.0 g/L CaCL · / 22H₂O to 8.0 L of 4X Pittsburgh water. Inhibitor stock solutions were made up at an active concentration of 8.0 g/L and were added individually to the various test solutions before coupon immersion. The MoO -2 / 4 source was Na₂MoO · / 42H₂O. For tests at pH 8.0, 15 mLs of an 8.0 g/L active solution of an acrylic acid/acrylamidosulfonic acid/polyalkylene oxide inhibitor was added to each test solution, in addition to the inhibitor stock solution, to prevent Ca⁺²/MoO -2 / 4 and/or Ca⁺²/PO -3 / 4 precipitation. The inhibitor tested was: a 70/20/10 AA/AMPSA/polyethylene glycol methacrylate terpolymer prepared using 70%, by weight, acrylic acid; 20%, by weight, 2-acrylamido-2-methylpropylsulfonic acid and 10%, by weight, CH₂ = C₂H₄ - CO - (OCH₂CH₂) n OH where n = 5, having a molecular weight of approximately 10,000. After seven days, coupons were removed and cleaned with inhibited acid, water and acetone, respectively in an ultrasonic bath. Inhibited acid contains 50.0 g SnCl₂ and 20.0 g Sb₂O₃ per liter of 1:1 HCl. Coupons were then dried using house air and reweighed. From the coupon weight losses, the corrosion rates in mpy were calculated. The results are shown in Table 1. Table 2 presents the results of Table 1 in "% inhibition" format.

Claims: A method for inhibiting corrosion in an aqueous system comprising adding to said system an effective amount of a corrosion inhibiting composition comprising: (a) a molybdate ion source; and (b) a water-soluble component comprising a polymer prepared from 50-70%, by weight, acrylic acid or methacrylic acid, 10-40%, by weight, 2-acrylamido-2-methylpropyl sulfonic acid or 2-methyacrylamido-2-methylpropyl sulfonic acid and 10-30%, by weight, of a polyalkyleneoxide compound, wherein the weight of (a):(b), on an active basis, ranges from 10:1 to 1:10, or a salt of said polymer. A composition comprising: (a) a molybdate ion source; and (b) a water-soluble component comprising a polymer prepared from 50-70%, by weight, acrylic acid or methacrylic acid, 10-40%, by weight, 2-acrylamido-2-methylpropyl sulfonic acid or 2-methyacrylamido-2-methylpropyl sulfonic acid and 10-30%, by weight, of a polyalkyleneoxide compound, wherein the weight of (a):(b), on an active basis, ranges from 10:1 to 1:10, or a salt of said polymer.

Inventors: BOFFARDI BENNETT P, REY SUSAN P, BOFFARDI, BENNETT P., REY, SUSAN P.
Applicants: CALGON CORP, CALGON CORPORATION
Application year: 1989
Main classification: C23F11
IPC codes: C23F11