JP2015183285A - Metal corrosion control method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of suppressing metal corrosion which can keep a good anticorrosive effect without environmental pollution problems or excessive rise of pH under low-concentration conditions, e.g. at the time of start-up of an open circulation cooling water system.SOLUTION: A method of suppressing corrosion of a metal in a water system comprises adding 30-150 mg/L of one or more polymers selected from those of maleic acid and/or its water soluble salts to water of a water system having a 'ranjeria' index of smaller than 1.5 or under a condition of [SiO]×[CaH]<2000, where [SiO] is the concentration (mg/L) of SiOin water and [CaH] is the calcium hardness (mg/L) as CaCOin water.

Description

  The present invention relates to a method for inhibiting corrosion of a metal, and in particular, corrodes a metal in contact with water, particularly a metal member of an open circulating cooling water system, without causing environmental pollution problems such as toxicity and eutrophication in a closed water area. The present invention relates to a method for effectively suppressing.

  Metal members provided in the cooling water system, such as heat exchangers, reaction kettles and piping made of carbon steel, copper, or copper alloys, are generally corroded by contact with cooling water. Processing has been applied.

  For example, phosphorus compounds such as orthophosphate, hexametaphosphate, hydroxyethylidene diphosphonate, and phosphonobutanetricarboxylate have been conventionally used to suppress corrosion of carbon steel heat exchangers, reaction kettles and piping. It is added to the cooling water. Further, a heavy metal salt such as zinc salt or dichromate may be added alone or in combination. However, large amounts of these phosphorus compounds and heavy metal salts, especially heavy metal salts, can contaminate the water quality and have a serious impact on the environment. I need it.

As a method for effectively suppressing metal corrosion without causing such environmental problems, Patent Document 1 discloses that the Langeria index is 1.5 or more and [SiO ₂ ] × [CaH] ≧ 2000 (however, , [SiO ₂ ] is SiO ₂ concentration in water (mg / L), and [CaH] is aqueous water adjusted to have calcium hardness (mg / L) as CaCO ₃ in water. And / or one or more polymers selected from water-soluble salts thereof, one or more polymers selected from maleic acid, maleic anhydride and water-soluble salts thereof, and one or more of nonionic monoethylenically unsaturated monomers And a method for inhibiting corrosion of metals by adding a copolymer thereof. However, a good anticorrosive effect can be obtained by this method only when the concentration ratio of cooling water is relatively high, and anticorrosion under low concentration conditions at the start of operation cannot be expected.

  Therefore, even in a cooling water system in which non-phosphorus and non-zinc treatment is performed during normal operation, a method of adding a phosphorus compound and / or heavy metal salt as an initial treatment is generally employed during low concentration.

In Comparative Example 3 of Patent Document 1, a maleic acid polymer and a maleic acid-isobutylene copolymer were added to an open circulation cooling water system having a Langeria index of 1.8 and [SiO ₂ ] × [CaH] = 1200. It is described that there is much local corrosion even when 20 mg / L (1 mg of maleic acid polymer is added) of a 1: 1 mixture of Further, in Comparative Example 4 of Patent Document 1, the Langerian index is 0.7, and the maleic polymer and maleic acid-isobutylene are combined in an open circulating cooling water system of [SiO ₂ ] × [CaH] = 6000. It is described that even when a 1: 1 mixture with a polymer is added at 20 mg / L (10 mg / L as a maleic acid polymer), a general corrosion tendency occurs.

Patent Document 2 discloses an open circulation in which the calcium hardness in the circulating water is 10 to 300 mg-CaCO ₃ / L as a method for effectively suppressing corrosion of the metal under the condition that the concentration factor is low without causing environmental problems. 20 to 300 mg-CaCO ₃ / L of alkali metal hydroxide is added to the cooling water system, and the Ritzner index at 40 ° C. of the circulating water is maintained at 4.0 to 6.0. A method of adding a scale inhibitor such as finopolycarboxylic acid, phosphonocarboxylic acid, maleic acid polymer, acrylic acid polymer is described. However, since the pH is increased by adding an alkali metal hydroxide to the low hardness water quality, amphoteric metals such as zinc and aluminum increase the risk of corrosion. Moreover, there is also concern about scale adhesion due to the increase in pH caused by the addition of alkali metal hydroxide.

JP2007-111983 JP2005-200721

  The present invention suppresses corrosion of metals that can maintain a good anticorrosion effect without excessively increasing the pH under low concentration conditions such as at the start of operation of an open circulating cooling water system without causing environmental pollution problems. It aims to provide a method.

The present inventors have found that an excellent anticorrosive effect can be obtained by adding a maleic acid polymer even in an aqueous system having a low Langerian index and [SiO ₂ ] × [CaH] value. Completed the invention.
That is, the method for inhibiting corrosion of a metal according to the present invention is a method for inhibiting corrosion of an aqueous metal, wherein the Langerian index is less than 1.5 or [SiO ₂ ] × [CaH] <2000 (where [SiO ₂ ] is SiO ₂ concentration in water (mg / L), [CaH] is calcium hardness (mg / L) as CaCO _{3 in} water) 1 is selected from maleic acid and / or water-soluble salts thereof in the aqueous water 30 to 150 mg / L of a polymer of at least seeds (hereinafter referred to as “maleic acid polymer A”) is added.

  In this invention, it is preferable that the weight average molecular weights of a maleic acid type polymer are 500-2500.

According to the method for inhibiting corrosion of a metal of the present invention, even if it is an aqueous system having a Langeria index <1.5 or [SiO ₂ ] × [CaH] <2000, a phosphorus compound or heavy metal salt is not used. Thus, corrosion of water-based metals can be effectively prevented or suppressed without causing environmental pollution problems, which can contribute to stable operation of cooling water systems and the like.

  The effect of the present invention is considered to be achieved by the adsorption of one or more polymers selected from maleic acid and / or water-soluble salts thereof onto the metal surface to form an anticorrosion film. One or more polymers selected from maleic acid and / or water-soluble salts thereof are known to easily bind to calcium ions in water and form an insolubilized product. Although it is thought that it cannot be obtained, it is considered that a high anticorrosive effect can be obtained because it is difficult to form an insolubilized material under the condition of low calcium hardness.

Generally, in an open circulating cooling water system, when the concentration of water increases with the passage of operating time, the concentration of calcium ions and bicarbonate ions contained in the water increases, and corrosion is suppressed by the formation of a CaCO ₃ film. That is well known. As a scale indicating the water-based CaCO ₃ precipitation tendency, the Langerian saturation index has been proposed. This shows the CaCO ₃ precipitation tendency of the water as one standard from the calcium concentration, M alkalinity, total dissolved solid, water temperature, and pH. When the Langeria index is 1.5 or more, the tendency of the CaCO ₃ film to adhere increases, and the corrosion resistance of water also increases. Silica contained in the water system also plays an extremely important role in preventing corrosion. In Patent Document 1, a maleic acid polymer is added to an aqueous system having a Langeria index of 1.5 or more and [SiO ₂ ] × [CaH] ≧ 2000 to obtain an anticorrosive effect. On the other hand, the present invention has a sufficient anticorrosive effect even in low-concentrated water quality at the start of operation of an open circulating cooling water system in which the Langeria index is less than 1.5 or [SiO ₂ ] × [CaH] is less than 2000. Is what you get.

  Embodiments of the metal corrosion inhibiting method of the present invention will be described in detail below.

[Maleic acid polymer]
The maleic acid polymer is a polymer of maleic acid and / or a water-soluble salt of maleic acid. Examples of the water-soluble salt of maleic acid include sodium maleate, potassium maleate, and ammonium maleate. The weight average molecular weight of the maleic acid polymer is preferably about 500 to 2500, and particularly preferably about 800 to 1500.

  The amount of maleic polymer added (retention concentration in water) is 30 to 150 mg / L, preferably 30 to 100 mg / L, as the solid content. In addition, there exists a possibility of gelatinizing that the addition amount of a maleic acid type polymer is over 150 mg / L. The maleic acid polymer is preferably added continuously or intermittently.

[Other additives]
In the present invention, only the maleic acid polymer may be added to the aqueous system, or the acrylic acid polymer may be added together with the maleic acid polymer. By using an acrylic acid polymer in combination, the magnesium silicate scale can be suppressed, and the effect of dispersing iron rust can be obtained. Examples of the acrylic acid polymer include a copolymer of acrylic acid and 2-acryloside-2-methylpropanesulfonic acid. Its molecular weight is preferably about 5000 to 50000, particularly preferably about 10000 to 30000. When an acrylic acid polymer is used in combination, the addition amount is preferably about 5 to 50 mg / L.

  When a maleic acid polymer and an acrylic acid polymer are used in combination, they may be added individually or in advance. When added individually, these may be added at the same location or at different locations.

  In the present invention, other corrosion inhibitors, scale inhibitors, dispersants, slime control agents, release agents, antifoaming agents, etc. may be used in combination, and in combination with various water treatment equipment such as filters. Is possible. For example, when a copper material is included in the aqueous system, the anticorrosion performance for the copper material can be improved by using an azole derivative such as benzotriazole or tolyltriazole in combination.

[Treatment target water system]
In the present invention, the water system to be treated has a Langeria index of less than 1.5, or [SiO ₂ ] × [CaH] of less than 2000, particularly less than 1000. The maleic acid polymer is added in an amount of 30 mg / L or more even in an aqueous system in which the Langerian index and / or [SiO ₂ ] × [CaH] of the water to be treated is small and the CaCO ₃ and / or silica concentration in the water is low. As a result, CaCO ₃ and silica are entrained to form a good anticorrosion film on the metal surface, and an excellent anticorrosion inhibiting effect can be obtained.

The aqueous silica concentration is preferably about 1 to 150 mg / L, particularly about 10 to 100 mg / L. If it exceeds 150 mg / L, silica scale may be generated. [CaH] is preferably about 10 to 500 mg / L, particularly about 20 to 200 mg / L. The M alkalinity is preferably 10 to 500 mg / LasCaCO _{3 and the} pH is preferably 7 to 9.

  As such an aqueous system, a cooling water system is mentioned, For example, an open circulation type cooling water system etc. are mentioned. More specifically, it may be the case when the cooling water cannot be maintained at a high concentration, for example, at the start of operation of the open circulating cooling water system (during initial processing) or when the thermal load is reduced during steady operation. In an open circulating cooling water system, it is preferable to add a maleic acid polymer to the pit of the cooling tower.

  EXAMPLES The present invention will be described more specifically with reference to the following examples and comparative examples. However, the present invention is not limited to the following examples unless it exceeds the gist.

Examples 1-5, Comparative Examples 1-4
<Anticorrosive (polymer)>
In the following Examples and Comparative Examples, maleic acid polymers, maleic acid / isobutylene copolymers (monomer ratio 50:50), acrylic acid polymers, acrylic acid / 2-acrylamido-2-methylpropanesulfone are used as anticorrosive agents. An acid (AMPS) copolymer (monomer ratio 80:20) and an acrylic acid / 2-hydroxyethyl methacrylate (HEMA) copolymer (monomer ratio 85:15) were used. Table 2 shows the weight average molecular weight of each polymer or copolymer.
<Test method>
After adding sodium hydrogen carbonate aqueous solution, sodium silicate aqueous solution, polymer aqueous solution, magnesium sulfate aqueous solution, sodium chloride aqueous solution, calcium chloride aqueous solution to pure water, adjust the pH with a small amount of sodium hydroxide aqueous solution and sulfuric acid aqueous solution, and test water AD It was. Table 1 shows the quality of each test water.

Take 1 L of the above test water in which the concentration of each anticorrosive agent (polymer) is as shown in Table 2 in a beaker, keep it at 30 ° C., stir the test water at 150 rpm using a stir bar, and test pieces (carbon steel, longitudinal 50 mm, width 1.5 mm, thickness 1 mm). After 72 hours, the test piece was taken out, the corrosion rate was measured, and evaluated according to the following evaluation criteria. The results are shown in Table 2.
○: Less than 20 mdd x: 20 mdd or more After 72 hours, the test piece was pulled up, acid-washed with hydrochloric acid containing an inhibitor, air-dried, and the weight was measured. The corrosion rate (mg / dm ² / day) was calculated from the difference in weight before and after the test.

<Discussion>
1) When Examples 1 to 3 added with 100 mg / L of polymer were compared with Comparative Examples 1 to 5, no corrosion was observed in the visual observation when the maleic acid polymer was used (Example 1, In contrast to 2), corrosion was observed when other polymers were used (Comparative Examples 1 to 5).
2) Even when a maleic acid polymer and a polymer having no corrosion-inhibiting effect were used in combination, the occurrence of corrosion could be suppressed (Example 3).
3) In test water B and C, when the concentration of the maleic acid polymer was less than 30 mg / L, sufficient anticorrosive effect was not obtained (Comparative Examples 6 to 9).
4) In the test waters B and C, when the concentration of the maleic acid polymer was 30 mg / L or more, a sufficient anticorrosive effect was obtained (Examples 4 to 6).
5) In test waters B and C, if the concentration of one or more polymers selected from maleic acid and / or water-soluble salts thereof is 30 mg / L or more, the anticorrosion function can be obtained even if other polymers are added. There was no inhibition (Example 6).
6) Even in the polymers of Comparative Examples 8 and 9, corrosion is suppressed in the case of test water having a high hardness component like test water D, but corrosion is not suppressed in other test waters A to C.

  From the above Examples and Comparative Examples, it can be seen that according to the present invention, a good metal anticorrosion effect can be maintained without excessively increasing the pH under low concentration conditions such as at the start of operation of an open circulating cooling water system. It was.

Claims (5)

In a method for suppressing corrosion of water-based metals,
Langerian index is less than 1.5 or [SiO ₂ ] × [CaH] <2000 (where [SiO ₂ ] is SiO ₂ concentration (mg / L) in water, [CaH] is calcium hardness as CaCO _{3 in} water (Mg / L))
Addition of 30 to 150 mg / L of one or more polymers selected from maleic acid and / or water-soluble salts thereof (hereinafter referred to as “maleic acid polymer”) .   The method for inhibiting corrosion of a metal according to claim 1, wherein the maleic acid polymer has a weight average molecular weight of 500 to 2500. 3. The method for inhibiting corrosion of metal according to claim 1, wherein [SiO ₂ ] × [CaH] is 1000 or less.   The method for inhibiting corrosion of a metal according to any one of claims 1 to 3, further comprising adding an acrylic acid polymer to the aqueous system.   5. The method for inhibiting corrosion of a metal according to claim 1, wherein the water system is an open circulation cooling water system.