JP2003082481A - Method for suppressing corrosion of nonpassivated metallic body, and method for suppressing corrosion of boiler - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress local corrosion occuring in a nonpassivated metallic body such as a heat transfer tube of a boiler caused by the influence of moisture. SOLUTION: The method for suppressing corrosion includes a stage where the pH of moisture giving an influence on a nonpassivated metallic body such as a heat transfer tube of a boiler is set in the range of 11 to 12.5. In this method, the pH of the moisture is preferably set in the above range in accordance with the concentration of dissolved oxygen in the moisture.

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 a heat transfer tube is formed by using a non-passivating metal such as carbon steel, the portion that comes into contact with the boiler water does not corrode due to the influence of dissolved oxygen contained in the feed water or the pH of the boiler water. Therefore, it may be damaged and may have a fatal effect on the life of the once-through boiler. 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
Refers to the control standard for 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 tube, and in the special circulation boiler, the pH of the boiler water is from 11.0 to 11.8. It is recommended to set the range.

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. Therefore, the present inventors, as a result of investigating the cause of damage to the heat transfer tube in such cases, the heat transfer tube was not damaged due to the thinning corrosion of the contact portion with the boiler water, and all In the example, it was found that the heat transfer tube was damaged due to minute pitting due to the pitting caused by the progress of localized corrosion (the above-mentioned pitting corrosion) from the water contact surface side to the opposite side in the thickness direction. I found it. According to this, the boiler water management standard recommended in JIS is effective for suppressing the wall-thinning corrosion of the heat transfer tube, but is effective for suppressing the pitting corrosion generated in the heat transfer tube. Is hard to say.

An object of the present invention is to suppress local corrosion which occurs in a non-passivated metal body such as a heat transfer tube of a boiler under 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 localized corrosion that occurs in a non-passivated metal body under the influence of moisture. The pH of the water that affects the passivated metal body is 11 to 1
It includes the step of setting to the range of 2.5. In this corrosion inhibition method, the pH of water is usually set within the above range according to the concentration of dissolved oxygen in water.

Further, a corrosion inhibiting method according to another aspect of the present invention is a method for inhibiting localized corrosion occurring in a heat transfer tube of a boiler, wherein the pH of boiler water in the boiler is 11 to 1.
It includes the step of setting to the range of 2.5. In this corrosion suppression method, the pH of boiler water is usually set within the above range according to the dissolved oxygen concentration of the feed water supplied to the boiler. Alternatively, this corrosion suppression method further includes the step of setting the dissolved oxygen concentration of the feed water supplied to the boiler to 1.0 mg / l or less.

[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.

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 path 8 is provided with a water softening device 11 and a deoxidizer 12 in this order. The water softening device 11 is for replacing various hardness components and the like contained in makeup water with sodium ions to convert into soft water. On the other hand, the deoxidizer 12 is for mechanically removing dissolved oxygen contained in the makeup water. Also,
The water supply passage 10 includes a pump (not shown) for supplying the water supply stored in the water supply tank 9 to the once-through boiler 2, and an injection device 13 for injecting a drug into the water supply in the water supply passage 10. There is.

Here, the chemicals injected from the injection device 13 into the water supply are mainly the pH adjusting agent and the oxygen scavenger. pH
The adjusting agent is for adjusting the pH of the boiler water of the once-through boiler 2 within a predetermined range described later, and is usually one that can adjust the pH of the boiler water to the alkaline side, such as lithium hydroxide and sodium hydroxide. Alternatively, it is an aqueous solution containing an alkali metal hydroxide such as potassium hydroxide. On the other hand, the oxygen scavenger is for chemically removing dissolved oxygen contained in the feed water, for example, reducing sugars such as glucose and dextrin, sodium sulfite, potassium sulfite,
Examples thereof include ascorbic acid and erythorbic acid.

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. For this reason, the heat transfer tube 5 may be locally corroded due to the influence of the boiler water W in such a portion, which may cause minute holes and 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 the operation of the steam boiler apparatus 1, the pH of the boiler water W of the once-through boiler 2 is continuously maintained in order to prevent the heat transfer tube 5 from being damaged by the pit 5a caused by the above-mentioned pitting corrosion. taking measurement. Then, the pH is appropriately adjusted from the injection device 13.
The pH of the boiler water W is adjusted while injecting the adjusting agent, and the pH is set to be in the range of 11 or more and 12.5 or less. As a result, the heat transfer tube 5 is prevented from being corroded in a portion in contact with the boiler water W, and at the same time, the occurrence and growth of the pit 5a is also suppressed, and the pit 5a is less likely to be damaged. Incidentally, when the pH set here is less than 11, the heat transfer tube 5 is likely to be corroded together with the pit 5a, and the heat transfer tube 5 is likely to be damaged. On the other hand, even if the pH is set to exceed 12.5, the effect of suppressing the pit 5a is unlikely to increase, and the pH adjuster is wastefully used, which is uneconomical.

The pH of the boiler water W is adjusted from the viewpoint of more effectively suppressing the pits 5a of the heat transfer tubes 5 from the once-through boiler 2
It is preferably set according to the dissolved oxygen concentration of the feed water supplied to the. In other words, if the dissolved oxygen concentration in the feed water is high,
The pH of the boiler water W is preferably set higher in the above range.

The method for suppressing corrosion of the heat transfer tube 5 as described above is
Dissolved oxygen concentration of feed water supplied to once-through boiler 2 is 1.0 m
If it is set to g / l or less, the effect is likely to increase.
Such a dissolved oxygen concentration in the feed water can be achieved by appropriately adjusting the treatment of makeup water in the deoxidizer 12 and the type and amount of the oxygen scavenger injected into the feed water from the injector 13. However, even if the dissolved oxygen concentration of the feed water exceeds 1.0 mg / l, if the pH of the boiler water W is set to a high value within the above range (for example, a range of over 11.8 and under 12.5). Setting), the generation and growth of the pits 5a in the heat transfer tube 5 can be effectively suppressed.

In this 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, but the corrosion suppression method of the present invention is not limited to this. Not a thing. 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 applying the corrosion inhibiting method of the present invention to a non-passivated metal body other than the heat transfer tube of the boiler,
PH of water affecting the non-passivated metal body
Is set to the above range (that is, the range of 11 or more and 12.5 or less). For example, when suppressing pitting corrosion in a water tank of a steam boiler device, water stored in the water tank (water supply)
PH of the above is set in the above range. Also, the pH of this water
Is preferably set according to the concentration of dissolved oxygen contained in the water, as in the case described above. That is, when the dissolved oxygen concentration in the water is high, the pH of the water is
Is preferably set higher in the above range. By the way, the dissolved oxygen concentration in the water is 1.0 mg /
When it is set to 1 or less, the corrosion inhibiting effect of the non-passivated metal body is more likely to be enhanced.

Verification Example During the 10 years from October 1989 to October 1999, 29 cases of corrosion damage of heat transfer tubes were reported in the boiler manufactured by the applicant. However, in all of these reported cases, the index (mdd) indicating the condition of corrosive corrosion of the heat transfer tube indicates that the heat transfer tube is not corroded and damaged. Therefore, when we examined the damage patterns of the heat transfer tubes in each reported case, the damage modes in all cases were not due to the thinning corrosion of the heat transfer tubes, but the small holes due to the pits caused by the progress of pitting corrosion. It turned out to be open.

In order to investigate the above causes, the present inventors have
The company's once-through boiler was operated using several types of water with dissolved oxygen concentrations. At this time, tap water of Matsuyama City, Ehime Prefecture (M alkalinity (acid consumption (pH 4.8)) = 50)
mgCaCO ₃ / l, silica concentration = 17 mg / l, hardness <1.0 mgCaCO ₃ / l, chloride ion concentration = 13
mg / l, sulfate ion concentration = 30 mg / l) was used.
The operating conditions of the boiler were set such that the operating time was 48 hours, the heat load was 4.19 kW / m ² , the pressure was 0.29 MPa, and the feed water temperature was 50 ° C.

Regarding the boiler operated under the above-mentioned conditions, first, the relationship between the pH of the boiler water and the corrosion weight loss of the heat transfer tube was investigated. Note that the corrosion weight loss is the weight loss of the heat transfer tube (mg)
Means The results are shown in Fig. 4. According to FIG. 4, in the heat transfer tube, the higher the pH of the boiler water, the less the corrosion weight loss. From this, the setting target of boiler water pH recommended in JIS B 8223: 1999 is "11.
It can be said that "0 to 11.8" is effective for suppressing the corrosion weight loss of the heat transfer tube, that is, suppressing the wall-thinning corrosion.

On the other hand, regarding the boiler operated under the above conditions, the pH of the boiler water and the depth of the pit (μ) generated in the heat transfer tube
The relationship with the maximum value of m) was investigated. Results are shown in FIG. According to FIG. 5, when the pH of the boiler water is in the range of approximately 10 to 12, the depth of the pit formed in the heat transfer tube becomes large. In particular, when the dissolved oxygen concentration in the feed water is a high concentration exceeding 1.0 mg / l, the depth of the pit formed in the heat transfer tube becomes significantly large when the pH of the boiler water is in the range of about 11 to 11.8. .

Comparing FIG. 4 and FIG. 5, in the heat transfer tube of the boiler, when the pH of the boiler water is set in the range recommended by JIS, the corrosion weight loss is suppressed, but the pitting caused by pitting corrosion is On the contrary, it is possible to grow. From this, in order to suppress damage due to pitting corrosion of the heat transfer tube as occurred in the above-mentioned example, according to the dissolved oxygen concentration of the feed water,
It will be preferable to set the pH of the boiler water in the range of 11 to 12.5. More specifically, it is preferable that the higher the dissolved oxygen concentration of the feed water, the higher the pH of the boiler water is set within the range of 11 to 12.5.

According to the results shown in FIGS. 4 and 5, when the pH of the boiler water is set in the range of 11 to 12.5 and the dissolved oxygen concentration of the feed water is set to 1.0 mg / l or less, the heat transfer tube is set. It can be seen that the pitting corrosion can be suppressed more effectively. On the other hand, even if the concentration of dissolved oxygen in the feed water is high, exceeding 1.0 mg / l, setting the pH of the boiler water in the range above 11.8 and below 12.5 results in effective pitting of the heat transfer tube. It can be seen that it can be suppressed.

[0029]

EFFECTS OF THE INVENTION The method for inhibiting corrosion of a non-passivated metal body according to the present invention sets the pH of water affecting the non-passivated metal body to 1 or less.
Since it is set in the range of 1 to 12.5, it is possible to suppress local corrosion that occurs in the non-passivated metal body.

Further, in the boiler corrosion inhibiting method according to the present invention, since the pH of the boiler water in the boiler is set within the range of 11 to 12.5, local corrosion occurring in the heat transfer tube can be suppressed. it can.

[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 investigating the relationship between the pH of boiler water and the corrosion weight loss of heat transfer tubes in a boiler.

FIG. 5 is a graph showing the results of investigating the relationship between the pH of boiler water and the maximum depth of pits formed in the heat transfer tube of the boiler.

[Explanation of symbols]

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

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yasuo Nogami             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 BA08 BA11 BA14 BC01                       BC02 CA03 DA01 FA06 FA16

Claims (5)

[Claims] 1. A method for suppressing localized corrosion that occurs in a non-passivated metal body due to the influence of moisture, wherein the pH of the moisture that affects the non-passivated metal body is set to 11
A method for inhibiting corrosion of a non-passivated metal body, which comprises the step of setting the range to 12.5. 2. The method for inhibiting corrosion of a non-passivated metal body according to claim 1, wherein the pH of the water is set within the range according to the concentration of dissolved oxygen in the water. 3. A method for suppressing localized corrosion that occurs in a heat transfer tube of a boiler, the method comprising the step of setting the pH of boiler water in the boiler within a range of 11 to 12.5. Suppression method. 4. The method for suppressing corrosion of a boiler according to claim 3, wherein the pH of the boiler water is set in the range according to the dissolved oxygen concentration of the feed water supplied to the boiler. 5. The method for suppressing corrosion of a boiler according to claim 3, further comprising the step of setting the dissolved oxygen concentration of the feed water supplied to the boiler to 1.0 mg / l or less.