Scale remover in steam generating facility

Abstract

A scale remover that efficiently removes scale deposited inside a boiler without corroding a boiler includes polyacrylic acid and polymethacrylic acid and/or salts thereof.

Claims

1. A scale remover for removing scale deposited in a system of a steam generating facility, the scale remover comprising: polyacrylic acid and/or a salt of the polyacrylic acid having a weight-average molecular weight of more than 20,000 to 170,000; and polymethacrylic acid and/or a salt of the polymethacrylic acid having a weight-average molecular weight of more than 5,000 to wherein a weight ratio of the polyacrylic acid and/or the salt of the polyacrylic acid to the polymethacrylic acid and/or the salt of the polymethacrylic acid is 1:100 to 100:1.

2. The scale remover in a steam generating facility according to claim 1, wherein the polymethacrylic acid and/or a salt of the polymethacrylic acid has a concentration of 1 to 1,000 mg/L in a boiler water of the steam generating facility.

Description

BRIEF DESCRIPTION OF DRAWING

(1) FIG. 1 is a system diagram illustrating an embodiment of a steam generating facility for carrying out the present invention.

DESCRIPTION OF EMBODIMENTS

(2) Embodiments of the present invention will be described in detail below.

(3) [Method for Removing Scale in Steam Generating Facility]

(4) In the method for removing scale in a steam generating facility according to the present invention, scale deposited in a system of a boiler can, that is, a steam generating unit in a steam generating facility is removed by adding polyacrylic acid having a weight-average molecular weight of more than 20,000 and 170,000 or less and/or a salt of the polyacrylic acid (hereafter referred to as polyacrylic acid (salt)) to water in the steam generating facility or feed water of the steam generating facility. When the feed water of the steam generating facility contains iron, polymethacrylic acid having a weight-average molecular weight of more than 1,000 and 100,000 or less and/or a salt of the polymethacrylic acid (hereafter referred to as polymethacrylic acid (salt)) is preferably added in a combined manner.

(5) FIG. 1 is a system diagram illustrating an embodiment of a steam generating facility for carrying out the present invention. The steam generating facility includes a feed water tank 1, a boiler can (steam generating unit) 2, a condensate tank 3, a make-up water tank 4, and a location 5 at which steam is used. Feed water in the feed water tank 1 is fed to the boiler can 2 through a feed water line 11. Steam generated in the boiler can 2 is fed to the location 5 at which steam is used through a steam line 12. Condensed water is circulated to the feed water tank 1 through a circulation line 13, the condensate tank 3, and a condensate line 14.

(6) Make-up water in the make-up water tank 4 is supplied to the feed water tank 1 through a make-up water line 15. An alkaline agent is added to the feed water tank 1 through an alkaline agent addition line 16. A scale remover is added to the feed water tank 1 through a scale remover line 17. Blowdown water is discharged from the boiler can 2 through a blowdown line 18.

(7) In FIG. 1, a scale remover containing polyacrylic acid (salt) or polyacrylic acid (salt) and polymethacrylic acid (salt) is added to feed water in the feed water tank 1. The scale remover may be added to the make-up water tank 4, the condensate tank 3, or the transfer line of each water system. The scale remover may also be added to two or more of such places. When the polyacrylic acid (salt) and the polymethacrylic acid (salt) are used in a combined manner, they may be added to different places or the same place. When the polyacrylic acid (salt) and the polymethacrylic acid (salt) are added to the same place, they may be mixed in advance and added or may be separately added. The same applies to other optional components described later.

(8) Although FIG. 1 illustrates a circulation steam generating facility, the present invention is not limited to the circulation steam generating facility. The present invention can also be applied to a once-through steam generating facility and other steam generating facilities.

(9) Examples of the feed water of the steam generating facility include water obtained by treating raw water with a reverse osmosis membrane, water obtained by softening raw water, or water obtained by subjecting raw water to ion exchange.

(10) The operating conditions of the steam generating facility are not particularly limited. The working pressure is preferably 0.2 to 4 MPa and more preferably 0.2 to 3 MPa. When the working pressure is lower than 0.2 MPa, the scale removing effect is not sufficiently produced. When the working pressure is higher than 4 MPa, polymers such as polyacrylic acid (salt) and polymethacrylic acid (salt) undergo pyrolysis, which decreases a scale removing effect.

(11) Hereafter, description will be made on polyacrylic acid (salt) used as a scale removing component in the method for removing scale in the steam generating facility according to the present invention, polymethacrylic acid (salt) used together with the polyacrylic acid (salt), and other optional additional components that may be used together with these chemicals.

(12) <Polyacrylic Acid (Salt)>

(13) The polyacrylic acid is not particularly limited, and polyacrylic acid that satisfies the weight-average molecular weight described later can be employed. Examples of the polyacrylic acid salt include a sodium salt of the polyacrylic acid and a potassium salt of the polyacrylic acid. The polyacrylic acid salt can be produced by mixing polyacrylic acid with an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide or an alkali metal carbonate such as sodium carbonate or potassium carbonate.

(14) The weight-average molecular weight of the polyacrylic acid used in the present invention is more than 20,000 and 170,000 or less and preferably more than 50,000 and 120,000 or less.

(15) When the weight-average molecular weight of the polyacrylic acid is less than 20,000, the scale removing effect is not sufficiently produced. Similarly, when the weight-average molecular weight of the polyacrylic acid is more than 170,000, the scale removing effect decreases.

(16) In the case of the polyacrylic acid salt, it is sufficient that the weight-average molecular weight of polyacrylic acid serving as a base of the polyacrylic acid salt satisfies the above conditions.

(17) The polyacrylic acid (salt) is preferably added in such an amount that the concentration in the water of the steam generating unit in the steam generating facility, that is, the concentration in the boiler water is 1 to 1,000 mg/L, in particular, 10 to 500 mg/L.

(18) When the concentration of the polyacrylic acid (salt) in the boiler water is above the lower limit, the scale removing effect is easily produced. When the concentration is below the upper limit, the complication of a water discharge process due to an increase in COD is prevented, and good cost-effectiveness can be achieved.

(19) Therefore, the polyacrylic acid (salt) is added to water in each place in accordance with the concentration ratio of the steam generating facility so that the concentration of the polyacrylic acid (salt) in the boiler water is within the above range.

(20) The polyacrylic acid (salt) is preferably added in the form of an aqueous solution prepared using deionized water so as to have a concentration of 0.1 to 30 wt %, in particular, 0.5 to 10 wt %.

(21) <Polymethacrylic Acid (Salt)>

(22) When iron is contained in the feed water of the steam generating facility, polymethacrylic acid (salt) is preferably used together with the polyacrylic acid (salt).

(23) In general, iron insoluble in water, such as iron hydroxide or iron oxide, is present in feed water in the form of a suspended solids. Iron soluble in water is dissolved in water and dissociated and is present in the form of iron ions.

(24) When the concentration of such suspended iron and dissolved iron in feed water is more than 0.3 mg/L, in particular, when the concentration is as high as 0.4 to 5.0 mg/L, polymethacrylic acid (salt) is preferably used together with the polyacrylic acid (salt).

(25) The polymethacrylic acid is not particularly limited, and polymethacrylic acid that satisfies the following weight-average molecular weight is preferably used. Examples of the polymethacrylic acid salt include a sodium salt of the polymethacrylic acid and a potassium salt of the polymethacrylic acid. The polymethacrylic acid salt can be produced by mixing polymethacrylic acid with an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide or an alkali metal carbonate such as sodium carbonate or potassium carbonate.

(26) The weight-average molecular weight of the polymethacrylic acid used in the present invention is 1,000 or more and 100,000 or less and more preferably 5,000 or more and 60,000 or less.

(27) When the weight-average molecular weight of the polymethacrylic acid is less than 1,000, the iron scale or inhibition effect is sometimes not sufficiently produced. When the weight-average molecular weight of the polymethacrylic acid is more than 100,000, the effect decreases.

(28) In the case of the polymethacrylic acid salt, it is sufficient that the weight-average molecular weight of polymethacrylic acid serving as a base of the polymethacrylic acid salt satisfies the above conditions.

(29) The polymethacrylic acid (salt) is preferably added in such an amount that the concentration in the water of the steam generating unit in the steam generating facility, that is, the concentration in the boiler water is 1 to 1,000 mg/L, in particular, 10 to 500 mg/L.

(30) When the concentration of the polymethacrylic acid (salt) in the boiler water is above the lower limit, the iron scale or inhibition effect is produced and the scale removing effect produced by the polyacrylic acid (salt) can be easily maintained. When the concentration is below the upper limit, the complication of a water discharge process due to an increase in COD is prevented, and good cost-effectiveness can be achieved.

(31) Therefore, the polymethacrylic acid (salt) is added to water in each place in accordance with the concentration ratio of the steam generating facility so that the concentration of the polymethacrylic acid (salt) in the boiler water is within the above range.

(32) To sufficiently produce a synergistic scale removing effect by using the polyacrylic acid (salt) and the polymethacrylic acid (salt) in a combined manner, the polyacrylic acid (salt) and the polymethacrylic acid (salt) are preferably added so that the weight ratio of the polyacrylic acid (salt) to the polymethacrylic acid (salt) in the boiler water is 1:100 to 100:1, in particular, 1:50 to 10:1.

(33) The polymethacrylic acid (salt) is preferably added in the form of an aqueous solution prepared using deionized water so as to have a concentration of 0.1 to 30 wt %, in particular, 0.5 to 20 wt %.

(34) <Optional Additional Component>

(35) In the present invention, various additional components such as a pH adjuster (alkaline), an oxygen scavenger, a corrosion inhibitor, and a scale disperser may optionally be added in an effective amount at any place in the system of the steam generating facility as long as the object of the present invention is not impaired. These additional components can be used alone or in combination of two or more.

(36) The pH of the boiler water is preferably 11.0 or more and is preferably 12.0 or less from the viewpoint of preventing corrosion in the boiler can and in the system of the steam generating facility. Examples of a method for adjusting the pH of the boiler water to 11.0 or more include a method in which an alkaline agent is added and a method in which the concentration ratio is adjusted by controlling the blowdown amount and/or the feed water amount. Among them, a method in which an alkaline agent is added is suitable from the viewpoint of easiness of pH adjustment.

(37) Examples of the alkaline agent for pH adjustment include alkali metal hydroxides, alkali metal carbonates, alkali metal phosphates, and neutralizing amines.

(38) Examples of the alkali metal hydroxide include sodium hydroxide, potassium hydroxide, and lithium hydroxide. Examples of the alkali metal carbonate include sodium carbonate and potassium carbonate. Examples of the alkali metal phosphate include trisodium phosphate and sodium hydrogen phosphate.

(39) Examples of the neutralizing amine include monoethanolamine, cyclohexylamine, morpholine, diethylethanolamine, monoisopropanolamine, 3-methoxypropylamine, and 2-amino-2-methyl-1-propanol.

(40) Among the alkaline agents, the neutralizing amine moves to the steam and condensate system, and therefore the addition of a neutralizing amine with a high concentration produces a bad odor in the steam and condensed water and excessively increases the pH of the steam condensate system. As a result, if a copper-based alloy is present in the system, corrosion of the metal may occur. Therefore, the alkaline agent is preferably an alkali metal hydroxide, an alkali metal carbonate, or an alkali metal phosphate and more preferably sodium hydroxide, potassium hydroxide, or sodium carbonate from the viewpoint of cost effectiveness.

(41) These alkaline agents can be used alone or in combination of two or more.

(42) As in the case of the scale remover, the alkaline agent is also preferably added to make-up water or feed water. In a circulation steam generating facility, the alkaline agent may be added to steam condensate.

(43) To supply an appropriate amount of alkaline agent, the steam generating facility according to the present invention preferably includes pH measuring means on the upstream side of the boiler can and/or on the downstream side of the boiler can.

(44) Examples of the oxygen scavenger include hydroxylamines such as hydrazine, carbohydrazide, and diethylhydroxylamine; N-amino heterocyclic compounds such as 1-aminopyrrolidine and 1-amino-4-methylpiperazine; hydroquinone; hydrolyzable and condensed tannins (tannic acid) and the salts thereof; erythorbic acid and ascorbic acid and the salts thereof; aldonic acids such as gluconic acid and -glucoheptonic acid and the salts thereof; saccharides (monosaccharides and polysaccharides) such as glucose; and sulfurous acid-based substances such as sulfurous acid and metabisulfurous acid and the salts thereof. They may be used alone or in combination of two or more.

(45) Examples of the corrosion inhibitor include polyvalent carboxylic acids such as succinic acid, citric acid, and malic acid, oxycarboxylic acid, and the salts of the foregoing. They may be used alone or in combination of two or more.

(46) [Scale Remover for Steam Generating Facility]

(47) The scale remover for the steam generating facility according to the present invention contains polyacrylic acid (salt) having a weight-average molecular weight of more than 20,000 and 170,000 or less, preferably more than 50,000 and 120,000 or less, and polymethacrylic acid (salt) having a weight-average molecular weight of more than 1,000 and 100,000 or less, preferably more than 5,000 and 60,000 or less. Preferably, the scale remover contains the polyacrylic acid (salt) and the polymethacrylic acid (salt) at a weight ratio of polyacrylic acid (salt):polymethacrylic acid (salt)=1:100 to 100:1. More preferably, the weight ratio is 1:50 to 10:1.

(48) The scale remover of the present invention may optionally contain the above-described water treatment components such as a pH adjuster, an oxygen scavenger, a corrosion inhibitor, and a scale disperser as long as the object of the present invention is not impaired.

(49) In the scale remover of the present invention, the polyacrylic acid (salt) and the polymethacrylic acid (salt) may be a single formulation or may be separately supplied.

(50) As described above, the polyacrylic acid (salt) is normally used in the form of an aqueous solution prepared by dissolving the polyacrylic acid (salt) in deionized water so as to have a concentration of 0.1 to 30 wt %, in particular, 0.5 to 15 wt %. The polymethacrylic acid (salt) is normally used in the form of an aqueous solution prepared by dissolving the polymethacrylic acid (salt) in deionized water so as to have a concentration of 0.1 to 30 wt %, in particular, 0.5 to 20 wt %.

EXAMPLES

(51) Hereafter, the present invention will be more specifically described based on Examples and Comparative Examples, but the present invention is not limited to Examples.

(52) In Examples and Comparative Examples below, when artificially synthetic raw water used as feed water was prepared, CaCl.sub.2 was used for Ca hardness, MgCl.sub.2 was used for Mg hardness, Na.sub.2SiO.sub.3 was used for silica, and FeCl.sub.2 was used for Fe.

Test Example I: Examples I-1 to I-6 and Comparative Examples I-1 to I-3

(53) The following scale removing test was performed under the following conditions using the test equipment and feed water below to evaluate the scale removing effect of polyacrylic acid and polymaleic acid.

(54) <Test Equipment>

(55) Stainless test boiler (water quantity: 5 L)

(56) <Feed Water>

(57) Synthetic water A: synthetic water having a Ca hardness of 20 mgCaCO.sub.3/L and a Mg hardness of 10 mgCaCO.sub.3/L and containing silica with a concentration of 15 mg/L and sodium carbonate with a concentration of 30 mg/L

(58) Synthetic water B: synthetic water containing silica with a concentration of 15 mg/L, polyacrylic acid or polymaleic acid with a concentration of 10 mg/L, which has a weight-average molecular weight listed in Table 1, and sodium carbonate with a concentration of 32 mg/L

(59) <Test Conditions>

(60) Feed water temperature: 40 C.

(61) Working pressure: 2.0 MPa

(62) Water feeding amount: 10 L/h

(63) Concentration cycles: 10 times

(64) Boiler water pH: 1.4

(65) <Scale Removing Test>

(66) Three heat transfer tubes (made of carbon steel, surface area: 200 cm.sup.2) were weighed and recorded, and then inserted into the stainless steel-made test boiler.

(67) A 60 mesh strainer having a diameter of 20 mm was installed in a blowdown line.

(68) The stainless test boiler was operated for 24 hours while the synthetic water A was supplied under the conditions of pressure: 2.0 MPa, amount of evaporation: 9.0 L/h, blowdown amount: 1.0 L/h, and concentration cycles: 10 times. After the operation, the heat transfer tubes on which scale was deposited were taken out and weighed to calculate the scale deposition amount. Subsequently, the heat transfer tubes were inserted into the stainless test boiler again. The stainless steel-made test boiler was operated for three days under the same conditions using the synthetic water B to perform a scale removing process.

(69) After the scale removing operation, the heat transfer tubes were taken out and weighed in the same manner to calculate the scale deposition amount. A scale removal percentage was calculated from the scale deposition amounts before and after the scale removing process.

(70) The strainer after the completion of the test was observed to check the presence or absence of a deposit that may cause clogging of the strainer.

(71) Table 1 shows the results.

(72) TABLE-US-00001 TABLE 1 Scale remover Weight-average Scale removal molecular percentage Deposit on Type weight (%) strainer Comparative PAA 4,000 21 Absence Example I-1 Comparative PMA 21,000 35 Presence Example I-2 Comparative PMA 55,000 40 Presence Example I-3 Example I-1 PAA 21,000 42 Absence Example I-2 PAA 52,000 65 Absence Example I-3 PAA 75,000 71 Absence Example I-4 PAA 100,000 61 Absence Example I-5 PAA 120,000 54 Absence Example I-6 PAA 170,000 45 Absence PAA: polyacrylic acid PMA: polymaleic acid

(73) <Consideration>

(74) As is clear from Table 1, when the weight-average molecular weight of the polyacrylic acid is 20,000 or less, the scale removal percentage is low. When the weight-average molecular weight is more than 20,000 and 170,000 or less, the scale removal percentage is high. In particular, when the weight-average molecular weight is more than 50,000 and 120,000 or less, the scale removing effect is high.

(75) In contrast, the polymaleic acid produced a scale removing effect to a certain degree. However, a viscous deposit was observed in the blowdown line strainer, and most of the strainer was covered with the deposit and clogging was about to occur. This was assumed to be due to a gelled material produced as a result of a reaction of hardness components and the polymaleic acid.

Test Example II: Examples II-1 to II-6 and Comparative Examples II-1 to II-11

(76) The following scale removing test was performed under the following conditions using the test equipment and feed water below to evaluate the influence of the weight-average molecular weight of the polyacrylic acid and the combined use of the polymethacrylic acid on the scale removing effect.

(77) <Test Equipment>

(78) Stainless steel-made test boiler (water quantity: 5 L)

(79) <Feed Water>

(80) Synthetic water C: synthetic water having a Ca hardness of 40 mgCaCO.sub.3/L and a Mg hardness of 20 mgCaCO.sub.3/L and containing silica with a concentration of 30 mg/L, sodium carbonate with a concentration of 30 mg/L, and Fe with a concentration of 1 mg/L

(81) Synthetic water D: synthetic water containing silica with a concentration of 30 mg/L, polyacrylic acid with a concentration of 5 mg/L, which has a weight-average molecular weight listed in Table 2 and serves as a chemical 1, a substance listed in Table 2 with a concentration of 5 mg/L, which serves as a chemical 2, Fe with a concentration of 1 mg/L, and sodium carbonate with a concentration of 32 mg/L (the chemical 1 and the chemical 2 were not added in Comparative Example II-1 and the chemical 2 was not added in Comparative Examples II-2 to II-8)

(82) <Test Conditions>

(83) Feed water temperature: 40 C.

(84) Working pressure: 0.7 MPa

(85) Water feeding amount: 13 L/h

(86) Concentration cycle: 10 times

(87) Boiler water pH: 11.5

(88) <Scale Removing Test>

(89) Three heat transfer tubes (made of carbon steel, surface area: 200 cm.sup.2) were weighed and recorded, and then inserted into the stainless steel-made test boiler.

(90) The test boiler was operated for 21 hours while the synthetic water C was supplied under the conditions of pressure: 0.7 MPa, amount of evaporation: 11.7 L/h, blowdown amount: 1.3 L/h, and concentration cycle: 10 times. After the operation, the heat transfer tubes on which scale was deposited were taken out and weighed to calculate the scale deposition amount. Subsequently, the heat transfer tubes were inserted into the stainless test boiler again. The stainless test boiler was operated under the same conditions using the synthetic water D to perform a scale removing process. After the operation, the heat transfer tubes were taken out and weighed in the same manner to calculate the scale deposition amount. An amount of scale removed was calculated from the scale deposition amounts before and after the scale removing process. The amount of scale removed was evaluated by taking out and weighing the heat transfer tubes and inserting the heat transfer tubes again every three days. The scale removing process was performed for 9 days in total. The scale removal percentage after the scale removing process for 9 days (the ratio of the total amount of scale removed after 9 days to the amount of scale deposited at the beginning of the scale removing process) was calculated.

(91) Table 2 shows the results.

(92) TABLE-US-00002 TABLE 2 Scale remover Chemical 1 Chemical 2 Weight- Weight- Concentration average Concentration of average of chemical Scale removal molecular chemical (mg/L- molecular (mg/L-feed Amount of scale removed (mg) percentage Type weight feed water) Type weight water) 0-3 days 3-6 days 6-9 days (%) Comparative Blank 20 10 10 2.7 Example II-1 Comparative PAA 4,000 5 178 156 134 31.2 Example II-2 Comparative PAA 21,000 5 272 255 220 49.8 Example II-3 Comparative PAA 52,000 5 355 321 299 65.0 Example II-4 Comparative PAA 75,000 5 361 337 315 67.5 Example II-5 Comparative PAA 100,000 5 298 271 250 54.6 Example II-6 Comparative PAA 120,000 5 280 259 233 51.5 Example II-7 Comparative PAA 170,000 5 248 220 197 44.3 Example II-8 Comparative PAA 4,000 5 AA/AMPS 10,000 5 189 160 142 32.7 Example II-9 Comparative PAA 4,000 5 AA/HAPS 10,000 5 178 160 131 31.3 Example II-10 Comparative PAA 4,000 5 PMAA 10,000 5 216 231 221 44.5 Example II-11 Example II-1 PAA 21,000 5 PMAA 10,000 5 374 380 378 75.5 Example II-2 PAA 52,000 5 PMAA 10,000 5 475 479 498 96.8 Example II-3 PAA 75,000 5 PMAA 10,000 5 488 499 511 99.9 Example II-4 PAA 100,000 5 PMAA 10,000 5 412 422 424 83.9 Example II-5 PAA 120,000 5 PMAA 10,000 5 405 409 407 81.4 Example II-6 PAA 170,000 5 PMAA 10,000 5 375 378 388 76.1 PAA: polyacrylic acid PMAA: polymethacrylic acid AA: acrylic acid AMPS: 2-acrylamido-2-methylpropanesulfonic acid HAPS: 3-allyloxy-2-hydroxy-1-propanesulfonic acid AA/AMPS and AA/HAPS are each a mixture with a weight ratio of 80/20.

(93) <Consideration>

(94) The following is found from Table 2.

(95) In Comparative Examples II-2 to II-8, the amount of removed scale decreases with the passage of days. This is because iron scale is further deposited on the scale that has already been deposited and the treatment using the polyacrylic acid alone does not maintain a constant scale removing effect.

(96) Although two polymers are used in a combined manner in Comparative Example II-11, the removing effect is low. In Comparative Examples II-9 and II-10, the removing effect is also low.

(97) In contrast, scale is removed at a constant rate even when the number of days increases in Examples II-1 to II-6. This is because, when the polymethacrylic acid is used in a combined manner, the deposition of iron scale is prevented while at the same time a constant scale removing effect is maintained by the polyacrylic acid having a large weight-average molecular weight.

Test Example III: Examples III-1 to III-6 and Comparative Examples III-1 to III-4

(98) In Test Example II, a test piece (SPCC, 155010 mm, #400 polishing) made of carbon steel was placed to immerse in water in a test boiler can. Synthetic water containing chemicals listed in Table 3 in concentrations listed in Table 3 in the boiler can was supplied under the same conditions as those of Test Example II to perform a confirmatory test for corrosiveness. The pH of the boiler water was adjusted to 11.3.

(99) After the test, the test piece was taken out and derusted. The corrosion rate was determined from calculation formula (1) below.
Corrosion rate (mdd)=corrosion weight loss of test piece (mg)/(surface area of test piece (dm.sup.2)test duration (day))(1)

(100) Table 3 shows the results.

(101) TABLE-US-00003 TABLE 3 Scale remover Chemical 1 Chemical 2 Weight-average Concentration Weight-average Concentration Corrosion molecular of chemical molecular of chemical rate Type weight (mg/L) Type weight (mg/L) pH (mdd) Example III-1 PAA 21,000 250 PMAA 10,000 250 11.3 21 Example III-2 PAA 52,000 250 PMAA 10,000 250 11.3 16 Example III-3 PAA 75,000 250 PMAA 10,000 250 11.3 17 Example III-4 PAA 100,000 250 PMAA 10,000 250 11.3 18 Example III-5 PAA 120,000 250 PMAA 10,000 250 11.3 18 Example III-6 PAA 170,000 250 PMAA 10,000 250 11.3 20 Comparative EDTA 250 PMA 20,000 250 11.3 195 Example III-1 Comparative EDTA 500 11.3 325 Example III-2 Comparative NTA 500 11.3 274 Example III-3 Comparative EDTA 250 11.3 182 Example III-4 PAA: polyacrylic acid PMAA: polymethacrylic acid PMA: polymaleic acid EDTA: ethylenediaminetetraacetic acid NTA: nitrilotriacetic acid

(102) As is clear from the results of Table 3, the corrosion rate is much lower in the treatment that uses the polyacrylic acid and the polymethacrylic acid in a combined manner than in the treatment that uses a chelating agent such as EDTA or NTA. Thus, the treatment that uses the polyacrylic acid and the polymethacrylic acid in a combined manner has a corrosion inhibiting effect.

(103) The present invention has been described in detail using particular embodiments. However, it is obvious for those skilled in the art that various modifications can be made without departing from the intention and scope of the present invention.

(104) This application is based on Japanese Patent Application No. 2014-025459 filed Feb. 13, 2014, the entire contents of which are incorporated herein by reference.