JP2003129264A - Method for preventing corrosion of nonpassivated metallic body and method for preventing corrosion of boiler - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a nonpassivated metallic body such as a boiler heat exchange tube from being corroded under the influence of water. SOLUTION: The method for corrosion prevention comprises the step of setting the concentration contained in water influencing a nonpassivated metallic body such as a boiler heat exchange tube to at least 150 mg/l. Corrosion controllable by this method is exemplified by local corrosion occurring in the direction of the thickness of a nonpassivated metallic body.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for suppressing corrosion, and more particularly to a method for suppressing corrosion that occurs in a non-passivated metal body such as a heat transfer tube of a boiler under the influence of moisture.

[0002]

2. Description of the Related Art Once-through boilers, which belong to the category of special circulation boilers specified in Japanese Industrial Standards (JIS), are
It is equipped with a heat transfer tube for heating the feed water to generate steam. Since such heat transfer tubes are formed using non-passivating metals such as carbon steel, the parts that come into contact with boiler water are damaged due to corrosion due to the effect of boiler water, which is fatal to the life of the once-through boiler. May have a negative impact. Therefore, in order to stably operate the once-through boiler for a long period of time, it is necessary to effectively suppress the corrosion of the heat transfer tube.

Therefore, JIS B 8223: 1999
Sets various management items regarding the water quality of the boiler water of the special circulation boiler from the viewpoint of suppressing the above-mentioned corrosion that occurs in the heat transfer tubes, and stipulates the recommended standards thereof.

By the way, the corrosion of the heat transfer tube is usually evaluated based on the following three types of indexes. (1) mdd (mg / dm ² / day): It represents the amount of mass reduction (mg) per day per unit surface area (1 dm ² ) of the contact surface with water. (2) ipy (inch / year):
This expresses the reduction amount (inch) of the thickness (wall thickness) of the heat transfer tube. (3) Number of pits / cm ² : Unit surface area of contact surface with water (1 cm
² ) It expresses the number of pits generated per hit. Incidentally, the pit means a localized corrosion from the contact surface side with the water of the heat transfer tube toward the opposite side in the thickness direction, that is, a pit formed by pitting corrosion (for example, issued by Nikkan Kogyo Shimbun Co., Ltd. See pages 31-33 of "Corrosion Prevention Technical Handbook" edited by the Association).

However, JIS B 8223: 1999
The once-through boiler is operated so as to meet the boiler water management standards recommended in the above, and the progress of corrosion of the heat transfer tubes is evaluated based on the above-mentioned indicators, and the transmission at the contact point with the boiler water is evaluated. Even if it can be determined that the condition of corrosion of the heat pipe is not such that the heat pipe is damaged, the portion of the heat transfer pipe may be unexpectedly damaged due to corrosion. According to this, the boiler water management standard recommended by JIS is not always effective for suppressing the corrosion of the heat transfer tube.

An object of the present invention is to suppress corrosion that occurs in a non-passivated metal body such as a heat transfer tube of a boiler due to the influence of moisture.

[0007]

A method for inhibiting corrosion of a non-passivated metal body according to the present invention is a method for inhibiting corrosion occurring in a non-passivated metal body under the influence of moisture. The method includes the step of setting the concentration of silica contained in the water that affects the metallized body to at least 150 mg / l. Corrosion that can be suppressed by this method is, for example, localized corrosion.

A method for inhibiting corrosion according to another aspect of the present invention is a method for inhibiting corrosion occurring in a heat transfer tube of a boiler, wherein the concentration of silica contained in the boiler water in the boiler is at least 150 mg / l. It includes the step of setting. Corrosion that can be suppressed by this method is, for example, localized corrosion from the contact surface side of the heat transfer tube with water toward the opposite side in the thickness direction.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION An outline of a steam boiler apparatus equipped with a once-through boiler to which the corrosion suppression method of the present invention can be applied will be described with reference to FIG. In the figure, the steam boiler device 1 is
The once-through boiler 2 and the water supply device 3 are mainly provided.

The once-through boiler 2 is, as shown in FIG. 2, a storage portion 4 for storing the water supply supplied from the water supply device 3.
And a plurality of heat transfer tubes 5 (an example of a non-passivating metal body) provided so as to stand upright with respect to the storage section 4, and a load device (not shown) provided at the upper end of the heat transfer tube 5 A header 6 having a supply path 6a for supplying steam and a heating device 7 for heating the supply water to generate steam are mainly provided. The storage section 4 and the header 6 are set to have an annular planar shape. Further, the storage unit 4 has a discharge port 4a provided with an opening / closing valve (not shown) for discharging the water supply (boiler water W described later) stored therein.

The heat transfer tube 5 is a member formed by using a non-passivating metal, that is, a non-passivating metal body. Here, the non-passivating metal means a metal that does not passivate naturally in a neutral aqueous solution, and is usually a metal except stainless steel, titanium, aluminum, chromium, nickel, zirconium and the like. Specifically, carbon steel, cast iron, copper and copper alloys are used. Note that carbon steel may be passivated in the presence of a high concentration of chromate ion even in a neutral aqueous solution, but this passivation is due to the effect of chromate ion. It is hard to say the natural passivation inside. Therefore, carbon steel belongs to the category of non-passivating metals here. Also, for copper and copper alloys, the electrochemical series (emf series) is in a noble position,
It is usually considered to be a metal that is unlikely to corrode due to the influence of water, but it does not passivate spontaneously in a neutral aqueous solution, so it belongs to the category of non-passivating metal here.

The water supply device 3 is for supplying water to the once-through boiler 2, and is made up of a make-up water injection channel 8, a water supply tank 9 for storing make-up water from the water injection channel 8 and a storage of the once-through boiler 2. A water supply channel 10 for supplying water to the section 4 is mainly provided (FIG. 1).

Here, the water injection passage 8 is provided with a water softening device 11 and a deoxidizing device 12 in this order. Water softening device 11
Is for replacing various hardness components contained in the makeup water with sodium ions to convert the makeup water into soft water. On the other hand, the deoxidizer 12 is for mechanically removing dissolved oxygen contained in the makeup water.

When the steam boiler device 1 described above is operated, makeup water is supplied from the water injection passage 8 to the water supply tank 9, and this makeup water is stored in the water supply tank 9. The feed water stored here is water that has been treated by the water softening device 11 and the deoxidizing device 12, that is, deoxidized soft water. Then, a pump (not shown) is operated to supply the water supply stored in the water supply tank 9 to the once-through boiler 2 through the water supply passage 10.

In the once-through boiler 2, the water supply supplied through the water supply passage 10 is stored in the storage section 4 as boiler water W. The boiler water W stored in the storage unit 4 rises in each heat transfer tube 5 while being heated by the heating device 7, and gradually becomes steam. The steam generated in each heat transfer tube 5 is collected in the header 6 and supplied to the load device through the supply path 6a.

During operation of the steam boiler device 1 as described above, the heat transfer tubes 5 used in the once-through boiler 2 are arranged in the same manner as in FIG.
The lower end portion as indicated by the alternate long and short dash line III, that is, the connection portion with the storage portion 4 is in continuous contact with the boiler water W. Therefore, the heat transfer tube 5 is easily affected by the boiler water W and corrodes in such a portion. In particular,
At the lower end portion of the heat transfer tube 5, local corrosion is likely to occur in addition to the wall-thinning corrosion of the inner peripheral surface, which may cause minute holes to be damaged.

Here, the local corrosion refers to FIG. 3 (II in FIG. 2).
As shown in (enlarged view of portion I), hole-like corrosion from the contact surface side of the heat transfer tube 5 with water toward the opposite side in the thickness direction, that is, in the thickness (wall thickness) direction of the heat transfer tube 5 occurs. Refers to the corrosion of holes. Hereinafter, such a phenomenon of occurrence of localized corrosion is referred to as "pitting corrosion", and the pitting corrosion caused by this pitting corrosion is referred to as "pitting" (indicated by reference numeral 5a in FIG. 3).

Therefore, during operation of the steam boiler device 1, in order to suppress damage to the heat transfer tube 5 due to corrosion, the boiler water W
Continuously measure the concentration of silica contained in the boiler water W
The concentration of silica contained therein (that is, the concentration of silicon dioxide (SiO ₂ )) is at least 150 mg / l (that is, 150 mg / l or more), and preferably at least 300 mg / l (that is, 300 mg / l or more). Set.

Since silica contained in the boiler water is considered to be a scale generation factor in the heat transfer tube 5, it is generally considered preferable to suppress its concentration as much as possible. On the other hand, the present invention sets silica in boiler water as a positive factor for corrosion inhibition. Therefore, the above-mentioned silica concentration of boiler water is not mentioned in JIS B 8223: 1999, and is the water quality control standard of the boiler water proposed for the first time by the present invention.

Incidentally, it is considered that silica exists as anions or negatively charged micelles in the boiler water W. The silica concentration set here is the concentration as silica (SiO ₂ ). Such silica concentration in the boiler water W is usually JIS K 010.
1: 1998 can be measured according to the molybdenum yellow absorptiometry.

When the concentration of silica contained in the boiler water W is adjusted as described above, the heat transfer tube 5 suppresses the wall-thinning corrosion at the contact portion with the boiler water W, and the formation of the pit 5a. And growth is also suppressed, corrosion (especially pit 5
Damage due to a) is less likely to occur. In other words, if the silica concentration in the boiler water W is less than 150 mg / l, J
Even if other control standards recommended by IS B 8223: 1999 (for example, pH of boiler water, chloride ion concentration, etc.) are set to required conditions, the heat transfer tube 5 is corroded, especially the pit 5a caused by pitting corrosion. Is likely to occur.

When the silica concentration of the boiler water W is set as described above, the corrosion of the heat transfer tube 5 is suppressed because the dissolved oxygen and chloride which are the corrosion promoting factors of the heat transfer tube 5 contained in the boiler water W are suppressed. It is considered that silica acts on the components eluted from the heat transfer tube 5 due to the influence of the substance ions, etc., and a corrosion resistant film (anticorrosion film) is formed on the inner surface of the heat transfer tube 5. In particular, dissolved oxygen and chloride ions may cause a local anode to appear in the heat transfer tube 5 and thereby promote pitting corrosion. However, silica contained in the boiler water W contains anions or anions as described above. Since it exists as a negatively charged micelle,
It is easy to be adsorbed on such an anode and it is easy to selectively form an anticorrosive film on the relevant part. Therefore, if the silica concentration of the boiler water W is adjusted as described above, the progress of pitting corrosion in the heat transfer tube 5 can be suppressed particularly effectively.

In the steam boiler apparatus 1, the silica concentration of the boiler water W depends on, for example, the quality of the feed water supplied to the once-through boiler 2 through the feed water passage 10 (in particular, the silica concentration), That is, it can be set as described above by appropriately adjusting the concentration ratio by heating the boiler water W). Incidentally, if the amount of discharge (so-called blow) of the boiler water W from the discharge port 4a is suppressed, the concentration ratio of the boiler water W can be increased. On the other hand, if the boiler water W is appropriately discharged from the discharge port 4a while the boiler water W is diluted by the water supplied from the water supply passage 10, the concentration ratio of the boiler water W can be reduced.

[Other Embodiments] (1) In the above embodiment, the concentration of silica contained in the boiler water W is set within the above range by appropriately adjusting the concentration ratio of the boiler water W. By injecting a chemical for adjusting the silica concentration into the feed water, the boiler water W
The present invention can be carried out in the same manner even when the silica concentration is set to the above range. In this case, as shown by the alternate long and short dash line in FIG. 1, an injection device 13 for injecting the drug into the water supply is attached to the water supply passage 10, and the drug is appropriately injected into the water supply from the injection device 13. The chemical used here is not particularly limited as long as it can increase the silica concentration in the feed water, but usually an aqueous solution of silicate is preferably used. As the silicate, it is preferable to use one that does not easily give hardness components such as calcium and magnesium to the feed water, for example, sodium silicate or potassium silicate. Incidentally, it is preferable that the injection amount of such a chemical agent is appropriately adjusted according to the concentration ratio of the boiler water W in the once-through boiler 2.

(2) In the above-described embodiment, the corrosion suppression method of the present invention has been described by taking the case of suppressing the corrosion of the heat transfer tube used in the once-through boiler as an example. However, the corrosion suppression method of the present invention is not limited to this. It is not something that will be done. For example, heat transfer tubes of boilers other than once-through boilers, water tanks used in steam boiler devices that employ various boilers such as once-through boilers, condensate pipes and water supply pipes, and other various heat equipment other than boilers (for example, water heaters, A member (non-passivating metal body) made of the non-passivating metal as described above, such as heat transfer tubes, water storage tanks, and various pipes used in absorption refrigerators, cooling towers, etc., such as water and steam. The corrosion inhibition method of the present invention should be similarly applied to those that may be corroded under the influence of the water content, especially those that may cause the localized corrosion (pitting corrosion) as described above. You can

When the corrosion inhibiting method of the present invention is applied to a non-passivating metal body other than the heat transfer tube of the boiler, the silica concentration in the water which affects the non-passivating metal body is set to the above. Like (ie at least 150 mg /
l, preferably at least 300 mg / l). For example, when suppressing pitting corrosion in the water tank of the steam boiler device, the silica concentration of the water (water supply) stored in the water tank is set as described above.

[0027]

Example: M alkalinity (acid consumption (pH 4.8)) 41 mgCaCO ₃ / l, hardness 0.1 mgCaCO ₃ /
1, a sulfate ion concentration of 30 mg / l and a dissolved oxygen concentration of 0.8 mg / l and chloride ion concentrations of 1.7 mg / l, 9.5 mg / l, 17.1 mg / l,
The boiler manufactured by the applicant of the present invention was operated while supplying the feed water set to either 31.0 mg / l or 39.9 mg / l. At this time, an aqueous sodium silicate solution was appropriately injected into the feed water to adjust the silica concentration in the feed water. Also,
Regarding the operating conditions of the boiler, the operating time was set to 48 hours, the operating pressure was set to 0.3 MPa, the feed water temperature was set to 50 ° C., and the boiler water concentration ratio was set to 10 times. According to this, the chloride ion concentration of boiler water becomes 10 times the chloride ion concentration of feed water, and JIS B 8223: 199
It becomes the range of 400 mg / l or less recommended in 9.

For the boiler operated under the above conditions, the relationship between the silica concentration in the boiler water and the maximum value of the depth (μm) of the pit formed in the heat transfer tube was investigated. The results are shown in Fig. 4. According to FIG. 4, with respect to boiler water having any chloride ion concentration, when the silica concentration exceeds 150 mg / l (particularly, when it exceeds 300 mg / l), the depth of the pit formed in the heat transfer tube tends to decrease. is there. From this, it can be seen that by setting the silica concentration of the boiler water to at least 150 mg / l, it is possible to effectively suppress the corrosion of the heat transfer tube, especially the damage of the boiler due to the progress of pitting corrosion.

[0029]

EFFECT OF THE INVENTION In the method for inhibiting corrosion of a non-passivated metal body according to the present invention, since the silica concentration in water affecting the non-passivated metal body is set to at least 150 mg / l,
Corrosion that occurs in the non-passivated metal body, especially localized corrosion, can be suppressed.

Further, in the method for suppressing corrosion of a boiler according to the present invention, the silica concentration in the boiler water in the boiler is at least 15%.
Since it is set to 0 mg / l, the corrosion that occurs in the heat transfer tube,
In particular, local corrosion can be suppressed.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a steam boiler apparatus including a once-through boiler to which a corrosion suppression method of the present invention can be applied.

FIG. 2 is a schematic partial cross-sectional view of the once-through boiler.

FIG. 3 is an enlarged view of a portion III in FIG.

FIG. 4 is a graph showing the results of examining the relationship between the silica concentration of boiler water and the maximum depth of pits formed in the heat transfer tube.

[Explanation of symbols]

2 once-through boiler 5 heat transfer tubes 5a pit W boiler water

Continued front page    (72) Inventor Keita Mizoue             7 Horie-cho, Matsuyama City, Ehime Prefecture Miura Industrial Co., Ltd.             In the company (72) Inventor Junichi Kato             7 Horie-cho, Matsuyama City, Ehime Prefecture Miura Industrial Co., Ltd.             In the company F-term (reference) 4K062 AA03 AA10 BA14 CA05 DA10                       FA06

Claims (4)

[Claims] 1. A method for suppressing corrosion that occurs in a non-passivated metal body under the influence of water, wherein the concentration of silica contained in the water that affects the non-passivated metal body is at least 150 mg. A method for inhibiting corrosion of a non-passivated metal body, which comprises the step of setting to 1 / l. 2. The method for suppressing corrosion of a non-passivated metal body according to claim 1, wherein the corrosion occurring in the non-passivated metal body is localized corrosion. 3. A method for suppressing corrosion occurring in a heat transfer tube of a boiler, comprising the step of setting the concentration of silica contained in boiler water in the boiler to at least 150 mg / l. Method. 4. The method for suppressing corrosion of a boiler according to claim 3, wherein the corrosion is localized corrosion from the contact surface side of the heat transfer tube with water toward the opposite side in the thickness direction.