JP5879699B2 - Corrosion prevention method for boiler water supply system - Google Patents

Description

  The present invention relates to a water supply system including an economizer in a boiler water system and a corrosion prevention method in a boiler can, and relates to a method for effectively preventing corrosion of a water supply pipe and an economizer that have already been corroded.

Generally, in boiler water systems, water from which hardness components of calcium and magnesium such as soft water and pure water are removed is used as make-up water to prevent scale, and as a method for preventing corrosion on the water side in the can, Injecting a chemical | medical agent, controlling pH to an appropriate range, removing dissolved oxygen, or forming a corrosion prevention film on the steel material surface, etc. are performed.
At this time, in order to inject the drug in consideration of the concentration in the boiler can, in the non-concentrated part such as a water supply pipe or an economizer, the pH is low and the chemical concentration is low. It will be enough. Further, in the water supply pipe and economizer, the residence time is short, and since the temperature is lower than that in the boiler can, the deoxygenation by the oxygen scavenger is slow and the deoxygenation is insufficient. For this reason, in spite of an increase in water temperature, particularly in an economizer, corrosion and pitting corrosion occur at locations where the dissolved oxygen concentration is high, resulting in failure of boiler operation.

Conventionally, phosphoric acid has been used as a chemical for scale prevention, and it is set to 20 to ₄₀ mg PO ₄ ³⁻ / L in the boiler water quality standard of the JIS B 8223 special circulation boiler. It is a low concentration of about 1/20.
In addition, water supply pipes and economizers are often not properly treated during the period from production to operation, and rusting occurs on the steel surface during this period, so corrosion has occurred since the new boiler was in operation. Is the actual situation. The rusting surface on the steel surface is more susceptible to corrosion because the chemicals cannot act uniformly than a smooth and healthy surface.

In Patent Document 1, tartaric acid or a salt thereof 10 to 50 mg / L, citric acid or a salt thereof 10 to 50 mg / L, and an alkaline agent in an amount capable of adjusting the pH of the boiler feed water to 8 to 12 with respect to boiler feed water A method for preventing corrosion and pitting corrosion of a boiler to which is added is disclosed.
Patent Document 2 discloses a boiler corrosion prevention method in which bisphosphinocarboxylic acid or a salt thereof is added.
Patent Document 3 discloses a corrosion prevention method in which a hydroxyphosphonocarboxylic acid such as 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) or hydroxyphosphonoacetic acid or a salt thereof is added.

Japanese Patent No. 4277072 Japanese Patent No. 3010893 Japanese Patent No. 2575026

The test condition of Patent Document 1 is an operation on a test piece on a polished surface assuming a newly installed pipe, and there is no suggestion of an anticorrosion effect on a steel surface that has already been corroded, and the anticorrosion effect is low. There is.
Patent Document 2 also does not suggest any anticorrosion effect on the steel surface on which corrosion has already occurred, and there is a problem that the anticorrosion effect is low.
Patent Document 3 is intended for an aqueous system containing calcium, and does not suggest any effect on a boiler water supply system using soft water or pure water that does not contain a hardness component, and an anticorrosive effect on a steel material surface on which corrosion has already occurred. There is no suggestion.
The present invention has been made under such circumstances, and in a boiler water supply system that does not substantially contain hardness components of calcium and magnesium, a water supply pipe and an economizer are applied to a rusted surface that has already been corroded. It aims at providing the anticorrosion method which exhibits the anticorrosion effect in non-concentrating parts, such as.

As a result of intensive studies to solve the above problems, the present inventors have added a compound having a phosphono group to a boiler feedwater so as to have a predetermined concentration in terms of PO ₄ ^3- We found that the problem could be solved. The present invention has been completed based on such findings.

That is, the present invention provides the following [1] to [3].
[1] A method for preventing corrosion of a boiler water supply system, wherein a compound having a phosphono group is added to boiler feed water so as to have a concentration of 15 to 100 mg / L in terms of PO ₄ ³⁻ .
[2] The boiler feed water according to [1], wherein the compound having a phosphono group is at least one compound selected from the compounds represented by the following general formulas (1) and (2), and orthophosphoric acid and derivatives thereof: System anticorrosion method.

(In the formula, R ¹ represents a hydrogen atom or an optionally substituted alkyl group, alkenyl group, aryl group or aralkyl group, and R ^{2 to} R ⁷ each independently represents a hydrogen atom or an alkyl group. And A represents an alkylene group or an alkylidene group.)
[3] The compound having a phosphono group is at least one selected from 2-phosphonobutane-1,2,4-tricarboxylic acid, 1-hydroxyethylidene-1,1-diphosphonic acid, hydroxyphosphonoacetic acid, and orthophosphoric acid. The boiler anti-corrosion method according to [2] above.

  According to the present invention, in a boiler water supply system that does not substantially contain calcium or magnesium hardness components, the anticorrosion effect is exerted on non-concentrated parts such as water supply pipes and economizers on rusted surfaces that have already been corroded. An anticorrosion method can be provided.

It is a schematic diagram of a semi-sealed container with a lid of 1.2 L that contains 1 L of a test solution used in a test example of the present invention.

The boiler water supply system anticorrosion method of the present invention (hereinafter also simply referred to as “the anticorrosion method of the present invention”) has a concentration of 15 to 100 mg / L of a compound having a phosphono group in terms of PO ₄ ³⁻ in boiler supply water. It is characterized by adding as follows.
The target water system in the anticorrosion method of the present invention is a boiler water system, and any of pure water supply, reverse osmosis (RO) water supply, and soft water supply water can be applied as the water supply type.
The boiler type is not particularly limited, and can be widely used in boilers such as special circulation boilers, water tube boilers, round boilers, exhaust heat recovery boilers and the like.
Although there is no restriction | limiting in particular in the pressure of the boiler to apply, Since it becomes easy to thermally decompose the compound which has a phosphono group when a pressure becomes high too much, Preferably it is used under the pressure of 3.0 MPa or less, More preferably, 2.0 MPa or less It is preferable to do.

[Compound having phosphono group]
In the anticorrosion method of the present invention, a compound having a phosphono group is added to boiler feed water. The phosphono group is a group represented by the following general formula (3).

(In the formula, R ² and R ³ each independently represent a hydrogen atom or an alkyl group.)
Examples of the alkyl group represented by R ² and R ³ in the general formula (3) include linear or branched alkyl groups having 1 to 6 carbon atoms. Specific examples include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, various pentyl groups, and various hexyl groups.

  As the compound having a phosphono group, at least one compound selected from the compounds represented by the following general formulas (1) and (2) and orthophosphoric acid and derivatives thereof can be used.

(In the formula, R ¹ represents a hydrogen atom or an optionally substituted alkyl group, alkenyl group, aryl group or aralkyl group, and R ^{2 to} R ⁷ each independently represents a hydrogen atom or an alkyl group. And A represents an alkylene group or an alkylidene group.)
Examples of the alkyl group which may have a substituent, which is R ¹ in the general formula (1), include a linear or branched alkyl group having 1 to 12 carbon atoms, and has a substituent. As an alkenyl group which may be sufficient, a C2-C12 linear or branched alkenyl group is mentioned.
In addition, examples of the aryl group which may have a substituent include aryl groups having 6 to 12 carbon atoms such as a phenyl group and a naphthyl group, and examples of the aralkyl group which may have a substituent include carbon. Examples of the aralkyl group of 7 to 12 include a benzyl group, a phenethyl group, and a naphthylmethyl group. As said substituent, a carboxyl group is suitable, for example, and there is no restriction | limiting in particular about the number of substituents.

The alkyl group is R ² to R ⁷ in the general formula (1) and (2) may be the same as that shown by R ² and R ³ in the general formula (3).
A in the general formula (2) is an alkylene group or an alkylidene group, and examples of the alkylene group include linear or branched alkylene groups having 1 to 4 carbon atoms. Specific examples include a methylene group, an ethylene group, a trimethylene group, a 1-methylethylene group, a tetramethylene group, a 1-methyltrimethylene group, and a 2-methyltrimethylene group.
On the other hand, the alkylidene group includes an alkylidene group having 2 to 4 carbon atoms, and specifically includes an ethylidene group, 1,1-propylidene group, 2,2-propylidene group, 1,1-butylidene group, 2,2 -Butylidene group etc. are mentioned.

  Examples of the compound having a phosphono group include phosphonic acid, methylphosphonic acid, ethylphosphonic acid, phenylphosphonic acid, benzylphosphonic acid, and mono- or dialkyl esters of these phosphonic acids (the alkyl group preferably has 1 to 6 carbon atoms). 2-phosphonobutane-1,2,4 tricarboxylic acid (PBTC) represented by the following chemical formula (4), 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) represented by the following chemical formula (5), Examples thereof include hydroxyphosphonoacetic acid represented by chemical formula (6) and orthophosphoric acid. Among these, 2-phosphonobutane-1,2,4-tricarboxylic acid, 1-hydroxyethylidene-1,1-diphosphonic acid, hydroxyphosphonoacetic acid, and orthophosphoric acid are preferable.

In this invention, the compound which has the said phosphono group can be used individually by 1 type or in combination of 2 or more types.
The amount of the compound having a phosphono group added to the boiler feed water is such that the concentration is 15 to 100 mg / L in terms of PO ₄ ³⁻ . If the concentration of the compound having a phosphono group in the boiler feed water is less than 15 mg / L, the anticorrosion effect is insufficient, and even if it exceeds 100 mg / L, the effect corresponding to the added amount cannot be obtained, which is uneconomical. . When the concentration is within the above range, it is possible to effectively prevent corrosion of water supply pipes, economizers, and the like. The concentration is more preferably in the range of 30 to 100 mg / L.
The compound having a phosphono group may be added as a pre-prepared aqueous solution or separately in the same aqueous system. However, from the upstream side of the economizer, a boiler is used so as to obtain a uniform concentration in the system. It is preferable to add in proportion to the flow rate with respect to the water supply. Accordingly, it is possible to effectively prevent corrosion in combination with the water supply pipe, the economizer, and the soft water supply system in the boiler can which do not substantially contain calcium or magnesium hardness components.

[Other additive components]
In the anticorrosion method of the present invention, existing alkaline agents, scale inhibitors, scale removers, oxygen scavengers, anticorrosive agents, neutralizing amines and the like can be used in combination as other additive components. These other additive components may be arbitrarily mixed and added to the boiler water system within a range not impairing the object of the present invention, or may be added separately and mixed in the boiler water system.

(Alkaline agent)
The alkaline agent maintains the pH of the boiler water within a certain range (for example, pH 11.0 to 11.8, which is a management standard for boiler water of a special circulation boiler shown in JIS B 8223), thereby suppressing metal corrosion. Can be used for
Examples of the alkali agent include alkali metal hydroxides such as sodium hydroxide, potassium hydroxide and lithium hydroxide, alkali metal carbonates such as sodium carbonate and potassium carbonate, alkali metals such as trisodium phosphate and sodium hydrogen phosphate. A phosphate etc. are mentioned. Among these, alkali metal hydroxides are preferable from the viewpoint of pH adjustment effect and carbon dioxide is not generated by thermal decomposition, and sodium hydroxide, potassium hydroxide and the like are more preferable from the viewpoint of economy.
The said alkali agent can be used individually by 1 type or in combination of 2 or more types.
Further, the pH of the aqueous system is preferably 8.5 or more, and the higher the pH within the preferable range, the higher the anticorrosion effect. However, in the boiler aqueous system, the concentration occurs in the latter stage. From the viewpoint of influence, pH is preferably 8.5 to 10.5, more preferably pH 8.5 to 10.0, and still more preferably pH 8.5 to 9.5.

(Scale inhibitor, scale remover)
Examples of the scale inhibitor and the scale remover include various phosphates, water-soluble polymer compounds such as polyacrylic acid, polymaleic acid, and sodium salts thereof, phosphonates, and chelating agents.

(Oxygen absorber)
Examples of the oxygen scavenger include hydrazine, carbohydrazide, 1-aminopyrrolidine, 1-amino-4-methylpiperazine, N, N-diethylhydroxylamine, tannin (acid) and its salt, ersorbic acid and its salt, ascorbic acid And salts thereof.
The oxygen scavenger can be used alone or in combination of two or more.
Moreover, you may use together with oxygen removal apparatuses, such as a nitrogen substitution type, a film | membrane type, and a vacuum type.

(Neutralizing amine)
Examples of neutralizing amines include monoethanolamine (MEA), cyclohexylamine (CHA), morpholine (MOR), diethylethanolamine (DEEA), monoisopropanolamine (MIPA), 3-methoxypropylamine (MOPA), 2-Amino-2-methyl-1-propanol (AMP) or the like can be used.
The said neutralizing amine can be used individually by 1 type or in combination of 2 or more types.

  By adding the oxygen scavenger to the water supply system, the neutralizing amine raises the pH of the water feed to a predetermined value, and the oxygen scavenger is eluted from the water feed line by removing dissolved oxygen in the water feed. Reduce the concentration of iron brought into the boiler. The amine compound that has been transferred to the boiler together with the water supply raises the pH of the boiler water to a predetermined value and suppresses corrosion in the boiler can. In addition, some of the oxygen scavengers composed of amine compounds and neutralizing amines used in combination move to steam and prevent the condensate system by raising the pH of the condensate, and supply water from the condensate to the boiler. Reduce iron and copper brought in.

[Adjustment of water quality]
When the M-alkalinity of boiler feed water and silica are insufficient, the anticorrosion effect is further improved by adding an alkali agent and a silicate (Na ₂ SiO ₂ or the like) together with a compound having a phosphono group. The
The added concentration of the alkali agent and silicate is such that the M-alkalinity of the feed water is 5 mg CaCO ₃ / L or more and the silica is 5 mg SiO ₂ / L or more, preferably the M-alkalinity is 10 mg CaCO ₃ / L or more and the silica is 10 mg SiO ₂ / L or more, more preferably, the dosage is adjusted so that the M-alkalinity is 15 mg CaCO ₃ / L or more and the silica is 15 mg SiO ₂ / L or more.

  EXAMPLES Next, although an Example demonstrates this invention further in detail, this invention is not limited at all by these examples.

Examples 1-10 and Comparative Examples 1-6
(1) Test example 1
A test piece [“SPCC-SB”, which is fixed to both ends of the support bar 3 after filling the semi-sealed container 1 with a capacity of about 1.2 L shown in FIG. 1 with 1 L of synthetic water and adjusting to pH 9. Then, emery paper # 400 polishing] was immersed, and then the container was placed in a hot water bath heated to 60 ° C., and the test was conducted for 24 hours while rotating 100 times per minute to produce a rust test piece. Next, prepare 1 L of test solution 2 adjusted to pH 9 in advance, immerse the removed rust test piece in test solution 2, and perform the test for 46 hours under the same conditions of 60 ° C. and 100 revolutions per minute. And raised the test piece. Thereafter, the test piece was post-treated according to JIS K 100, and the corrosion rate was calculated. The corrosion rate was calculated as the result of 46 hours by subtracting the corrosion weight loss average value (N = 20) of the rusting test piece in the first 24 hours from the corrosion weight loss result after the test. The results are shown in Table 1.
<Test solution>
Liquid in which chemicals are added to the following synthetic water so as to have the concentration shown in Table 1 <Composition of synthetic water>
Base: Ultrapure water NaHCO ₃ : 40 mg / L as CaCO ₃
NaCl: Cl ^- as 40mg / L
Na ₂ SO ₄ : 40 mg / L as SO ₄ ^2-
Na ₂ SiO ₃ : 30 mg / L as SiO ₂

Details of * 1 to * 4 in Table 1 are as follows.
* 1: Bisphosphinocarboxylic acid: Concentration conversion when m + n = 4 in the following formula (7)

* 2: PBTC: 2-phosphonobutane-1,2,4-tricarboxylic acid (chemical formula [4])
* 3: HEDP: 1-hydroxyethylidene-1,1-diphosphonic acid (chemical formula [5])
* 4: Hydroxyphosphonoacetic acid (chemical formula [6])

From the results of Table 1, Examples 1 to 10 using compounds having a phosphono group are Comparative Examples 2 and 3 in which citric acid and tartaric acid in a total amount of 60 mg / L are used in combination, and comparison using bisphosphinocarboxylic acid. It turns out that the anticorrosion effect is high compared with Examples 4-6 and the comparative example 7 with the low density | concentration of the compound which has a phosphono group.
In Examples 1 to 10 using a compound having a phosphono group, the higher the PO ₄ ^3- concentration in terms of corrosion protection effect tended to be higher.

  The anticorrosion method of the present invention is a pure water system including an economizer in a boiler water system and an anticorrosion method in a boiler can, and can effectively prevent corrosion of a water supply pipe or an economizer that has already been corroded.

1 Semi-closed container with lid 2 Test solution 3 Support rod 4 Test piece

Claims (2)

In a boiler water supply system including a economizer, pure water supply, reverse osmosis water supply or soft water supply water supply, and a corrosion prevention method for a boiler water supply system having a corroded surface. ₄ A corrosion prevention method for a boiler water supply system, which is added so as to have a concentration of 15 to 100 mg / L in terms of ^3- .   The boiler water supply system anticorrosion method according to claim 1, wherein the compound having a phosphono group is hydroxyphosphonoacetic acid.

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