JP2012097336A - Scale removing method of chromium-containing steel material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a scale removing method of a chromium-containing steel material for sufficiently removing, with a simple facility, scale of a bilayer structure having a lower layer containing high chromium oxide content formed on the surface of the chromium-containing steel material and an upper layer mainly made of iron oxide.SOLUTION: The scale removing method for a surface of the chromium-containing steel material, where the scale has the bilayer structure having the upper layer and lower layer and the chromium oxide content of the lower layer is higher than that of the upper layer, includes: a first step of removing the upper layer with acid; and a second step of applying oxidation treatment to the lower layer to solubilize the chromium oxide of the lower layer and then performing acid treatment. Preferably, the second step is repeated until the whole lower layer is removed. As the acid treatment, treatment by permanganate is suitable.

Description

  The present invention relates to a method for removing scale of chromium-containing steel material, and more particularly to a method suitable for removing steam oxidation scale from a chromium-containing tube used in a steam system.

  A chromium-containing steel material having excellent high-temperature heat resistance, such as Cr-Mo steel or SUS steel, is used for pipelines of boiler superheaters and reheaters. When the inner wall of such a pipe is oxidized by superheated steam flowing through the inside, a steam oxidation scale made of chromium oxide, iron oxide or the like is formed.

  If the oxide scale adhering to the pipe wall is peeled off in this way, it accumulates on the U-bent portion of the pipe and causes overheating of the superheater pipe, or the peel scale scatters and causes erosion of the turbine. Therefore, it is necessary to periodically remove such steam oxidation scale.

  Such oxide scale is generally generated not only on the inner wall of the pipeline but also on the wall surface exposed to steam. Therefore, it may occur on the outer wall of the pipeline, and may occur on the same wall surface as well as the pipeline.

As described in paragraph 0015 of JP-A-2004-211996 (Patent Document 1), the steam oxidation scale on the inner surface of the chromium-containing steel pipe has a two-layer structure composed of inner and outer layers (upper and lower layers). The outer layer (upper layer) scale is mainly composed of iron oxide (particularly magnetite (γ-Fe ₃ O ₄ )), and the inner layer (lower layer) scale is an oxide containing Fe, Cr, Mo and the like.

  The scale of a normal steel pipe inner surface is mainly composed of iron oxide and can be removed relatively easily by acid cleaning. However, the scale of the two-layer structure generated by steam oxidation in the chromium-containing steel material is as follows. The lower oxide layer contains a large amount of hardly soluble chromium oxide, and is difficult to remove by ordinary acid cleaning.

  In Japanese Patent Laid-Open No. 9-87877 (Patent Document 2), as a method for removing oxidized scale of a boiler main steam pipe, a scale is formed by inserting a nozzle into the main steam pipe and injecting pickling liquid from the nozzle at a high pressure. It is described to be removed.

  However, in this case, a high-pressure injection device for pickling solution is required, which increases the cost of cleaning equipment and has the disadvantages that it is impossible or difficult to apply to curved pipe portions and small-diameter pipes.

  Japanese Patent Application Laid-Open No. 10-259489 (Patent Document 3) describes a method of film scale on the surface of hot rolled stainless steel, first dipping in a permanganate solution and then dipping in an acid to dissolve and remove the scale. ing.

  However, when this method is applied to the removal of the two-layer scale of the chromium-containing steel material, the permanganate hardly reaches the two-layer scale chromium oxide-rich lower layer when immersed in the permanganate solution, and then the acid treatment is performed. However, the lower layer cannot be removed sufficiently.

  In Japanese Patent Laid-Open No. 2003-193276 (Patent Document 4), as a method of removing a chromium oxide coating layer of a gas turbine engine, a contaminant layer composed of a hydrocarbon residue on the surface is first removed by treatment with alkali, and then It is described that the substrate is immersed in an alkaline permanganate treatment solution and then acid-washed.

  However, even when this method is applied to the removal of a two-layer scale in a chromium-containing steel material, as in the case of Patent Document 3, the permanganate is formed into a lower layer rich in chromium oxide when immersed in the permanganate solution. Since it hardly reaches, the lower layer cannot be sufficiently removed even by acid treatment.

JP 2004-211996 A Japanese Patent Laid-Open No. 9-87877 Japanese Patent Laid-Open No. 10-259489 JP 2003-193276 A

  The present invention can sufficiently remove the scale of a two-layer structure including a lower layer having a high chromium oxide content formed on the surface of the chromium-containing steel material and an upper layer mainly composed of iron oxide with simple equipment. An object of the present invention is to provide a method for removing the scale of a chromium-containing steel material.

  The present invention (Claim 1) is a method for removing scale on the surface of a chromium-containing steel material, the scale comprising a two-layer structure comprising an upper layer and a lower layer, and the lower layer of chromium oxide content being higher than that of the upper layer. In the descaling method of the chromium containing steel material which has, it has the 1st process of removing an upper layer with an acid, and the 2nd process of oxidizing the lower layer, solubilizing the lower layer, and then acid-treating It is characterized by this.

  A second aspect of the present invention is characterized in that the second step is performed a plurality of times in the first aspect.

  A third aspect is characterized in that, in the first or second aspect, the oxidation treatment is a treatment in which a permanganate solution is brought into contact with a scale.

  A fourth aspect of the present invention provides the method according to the third aspect, wherein the permanganate solution is circulated and brought into contact with the scale, the chromium ion concentration in the circulating solution is measured, and oxidation is performed until the rate of increase of the chromium ion concentration becomes a predetermined value or less. Processing is performed.

  A fifth aspect of the present invention is the method according to any one of the first to fourth aspects, wherein the acid treatment in the first and second steps is a treatment in which an acid solution is circulated and brought into contact with the scale, and the iron ions in the circulating solution The concentration is measured, and the acid treatment is performed until the rate of increase of the iron ion concentration becomes a predetermined value or less.

  A sixth aspect of the present invention is characterized in that, in any one of the first to fifth aspects, the chromium content in the chromium-containing steel material is 12 wt% or less.

  In the method for removing the scale of the chromium-containing steel material of the present invention, when removing the scale of the two-layer structure having the lower layer rich in chromium oxide and the upper layer mainly composed of iron oxide on the surface of the chromium-containing steel material, the scale is first converted into an acid. The upper layer is dissolved and removed by the treatment (first step), and then the remaining lower layer is oxidized to make the lower layer acid soluble, and then the acid treatment is performed to remove the lower layer (second step). Thus, since the lower layer is oxidized after removing the upper layer, the lower layer becomes sufficiently acid-soluble, and the lower layer is sufficiently removed by the acid treatment.

  In addition, as the oxidation treatment for solubilizing the lower layer, it is simple and effective to contact a permanganate solution, which is preferable.

  By repeating the second step of acid treatment after solubilizing the lower layer by oxidation treatment, the lower layer can be sufficiently removed.

It is a flowchart explaining the method of this invention. It is explanatory drawing of the liquid passing method of an Example. It is a graph which shows an experimental result. It is a graph which shows an experimental result. It is a graph which shows an experimental result. It is a graph which shows an experimental result.

  Hereinafter, the present invention will be described in more detail.

  The method of the present invention is a method for removing scale on the surface of a chromium-containing steel material, the scale comprising a top layer and a bottom layer, and having a layer structure in which the chromium oxide content in the bottom layer is higher than that of the top layer The method for removing the scale of the contained steel material includes a first step of removing the upper layer with an acid, and a second step of oxidizing the lower layer to solubilize the lower layer of chromium oxide and then acid-treating the lower layer.

  The present invention is particularly suitable for application to steel pipes, in particular, alloy steel pipes for boiler heat exchangers, stainless steel pipes for boiler heat exchangers, carbon steel pipes for boiler heat exchangers, and the like, for example, Cr-Mo alloy steel pipes such as STBA20-24. Examples include various stainless steel pipes and high Cr carbon steel pipes such as STB 25-29. Specific boiler tubes include superheater tubes, reheater tubes, main steam tubes, and the like.

  In the present invention, the scale of a two-layer structure having a chromium oxide-rich lower layer and an upper layer mainly composed of iron oxide formed on the surface of such a chromium-containing steel material by steam oxidation or the like is removed. The lower layer of the scale generated by steam oxidation on the surface of the superheater tube or reheater tube made of the above-mentioned chromium-containing steel material generally contains chromium oxide in an amount of 3 wt% or more, for example, about 5 to 15 wt%. In many cases, the upper layer contains iron oxide at 90 wt% or more, for example, about 92 to 98 wt%, and chromium oxide at 0.5 wt% or less, for example, about 0.1 to 0.2 wt%. When the total thickness of the upper and lower layers of the scale is about 0.2 to 0.6 mm, the proportion of the lower layer thickness is usually 30 to 70%, particularly about 40 to 60%.

  In the present invention, when removing the scale from the chromium-containing steel material having such a two-layered scale, first, the acid solution is brought into contact with the scale to dissolve and remove the upper layer (first step).

  The acid used for the acid treatment may be any of an inorganic acid, an organic acid, and a chelating agent. Examples of the inorganic acid include hydrochloric acid, sulfuric acid, and hydrofluoric acid. Examples of the organic acid include citric acid, ammonium citrate, formic acid, malonic acid, oxalic acid, acetic acid, glycolic acid, malic acid and the like. These acids may use only 1 type and may use 2 or more types together. Examples of the chelating agent include EDTA and EDTA-ammonium salt.

  The concentration of the acid solution is preferably about 3 to 10 wt%, particularly about 5 to 8 wt% in the case of an inorganic acid, and about 3 to 20 wt%, particularly about 3 to 15 wt% in the case of an organic acid or a chelating agent.

  The acid solution preferably contains a corrosion inhibitor, a reducing agent or the like in order to prevent corrosion of the base steel material. Examples of the corrosion inhibitor include primary to tertiary amines, polymer amine compounds, imidazoline compounds and the like. Examples of the reducing agent include hydrazine, erythorbic acid, L-arcosbic acid and salts thereof, and erythorbic acid or L-arcosbic acid is particularly preferable.

  To bring the acid solution into contact with the scale, the chromium-containing steel material may be immersed in an acid solution. However, if the chromium-containing steel material is a tube and the scale is formed on the inner surface of the tube, the acid solution Is preferably passed through the tube.

  The liquid flow rate at the time of liquid passage is preferably 0.01 to 5.0 m / sec, particularly about 0.1 to 3.0 m / sec, but is not limited thereto. The liquid temperature at the time of passing the liquid may be normal temperature or may be heated, but in order to increase the dissolution rate, it is 40 ° C. or higher. In the case of inorganic acid, 50 ° C. to 60 ° C., especially organic acid or chelate In the case of an agent, it is preferable to heat to about 80-95 degreeC.

  The acid treatment time for removing the upper layer is preferably set so that preferably 80% or more, particularly 95% or more of the upper layer is removed.

  When the upper layer is dissolved and removed with acid, the treatment time may be a predetermined time set in advance, but the acid solution is circulated through the pipe and the Fe ion concentration in the circulating liquid is measured. You may make it continue until a typical raise speed becomes below a predetermined value. In this case, it is preferable to carry out this acid treatment until it is confirmed that the increase in the Fe ion concentration per 0.5 hour is 5% or less of the eluted Fe ion concentration.

  After performing the acid treatment (first step) for removing the upper layer, the process proceeds to the second step. First, the lower layer remaining on the surface of the chromium-containing steel material is oxidized and solubilized. As this oxidation treatment, treatment with permanganic acid, ozone treatment, hydrogen peroxide treatment or the like can be adopted, but treatment with permanganate which is easy to handle is suitable. As the permanganate, either potassium permanganate or sodium permanganate may be used, but potassium permanganate is preferred because it is inexpensive. The concentration of the permanganate solution is preferably about 0.1 to 10 wt%, particularly about 0.5 to 5.0 wt%.

By the oxidation treatment with permanganate or the like, the chromium oxide in the lower layer is changed from Cr ₂ O ₃ to Cr ₂ O ₇ ²⁻ and becomes soluble.

  The pH of the permanganate solution is preferably adjusted according to the steel type. In the case of carbon steel, it is preferable to use an alkaline permanganate solution having a pH of 8 or more, for example, about 10 to 14, or a neutral permanganate solution.

  In order to make the permanganate solution alkaline, it is preferable to use potassium hydroxide or sodium hydroxide as the alkali.

  The permanganate solution may further contain a surfactant or the like.

  In order to bring the permanganate solution into contact with the lower layer, the chromium-containing steel material may be immersed in the solution, and when the scale is formed on the inner surface of the tube, the solution is passed through the tube. Is preferred.

  The liquid flow rate at the time of liquid passage is preferably 0.01 to 5.0 m / sec, particularly about 0.1 to 3.0 m / sec, but is not limited thereto. The liquid temperature at the time of passing the liquid may be normal temperature or may be heated, but in order to increase the oxidation rate, it is preferable to heat to 40 ° C. or higher, particularly about 70 ° C. to 95 ° C.

  When the solubilization treatment by oxidation of the lower layer is performed with the permanganate solution, the treatment time may be a predetermined time set in advance, but the permanganate solution is circulated through the tube and Cr ions in the circulatory fluid are circulated. The concentration may be measured and continued until the rate of increase in Cr ion concentration with time falls below a predetermined value. In this case, it is preferable to carry out this solubilization treatment until it is confirmed that the increase in Cr ion concentration per 0.5 hour is 5% or less of the eluted chromium ion concentration.

  After the lower layer is oxidized, the lower layer is dissolved and removed with an acid.

  The acid solution in this case is preferably the same as the acid solution in the first step. The concentration of the acid solution in this case may be equivalent to the acid solution in the first step, and may be higher or lower than that. As will be described later, when the lower layer oxidation treatment and the acid treatment are repeated for a plurality of cycles, the concentration of the permanganate solution and the acid may be lower in the later cycle than in the previous cycle.

  A preferred embodiment of the acid treatment method may be dipping treatment as in the acid treatment during the upper layer dissolution and removal treatment, or may be passed through the tube. Suitable conditions for passing through the tube are the same as in the upper layer dissolution and removal treatment.

  When the lower layer is dissolved and removed with an acid, the treatment time may be set to a predetermined time, but the acid solution is circulated through the pipe and the Fe ion concentration in the circulating liquid is measured. You may make it continue until a typical raise speed becomes below a predetermined value. In this case, it is preferable to perform this acid dissolution treatment until it is confirmed that the increase in Fe ion concentration per 0.5 hour is 5% or less of the eluted Fe ion concentration.

  In the present invention, the second step comprising the lower layer oxidation treatment and the acid dissolution removal treatment may be repeated.

  That is, as shown in FIG. 1 (a), after removal of the upper layer (first step), the lower layer oxidation treatment and lower layer dissolution removal (second step) can be performed only once to remove all the scale. In some cases, the second step need only be performed once. If the lower layer on the surface of the steel material is not sufficiently removed by the second step only once, the second step consisting of the lower layer oxidation treatment and the acid dissolution removal treatment is performed twice as shown in FIG. 1 (b). It is preferable to carry out the above.

  In addition, after performing the 2nd process, the surface of cleaning objects, such as a boiler pipe, is observed, and if the scale is removed enough, the 2nd process will be ended and the scale removal is insufficient. May perform the second step once more. Instead of observing the surface of the object to be cleaned, the Cr ion concentration in the acid treatment step effluent of the second step is measured, and if this Cr ion concentration is below a predetermined value, the second step is terminated, When the value is exceeded, the second step may be performed once more.

  Normally, if the second step is performed about 2 to 3 times, the steam oxidation scale on the inner surface of the boiler tube made of the chromium-containing steel material can be sufficiently removed. After the scale is sufficiently removed by performing the second step one or more times, the object to be cleaned such as a boiler tube is washed with water.

  The present invention can also be applied to scale removal of a two-layer structure on the surface of a chromium-containing steel material other than a boiler tube. In addition, the scale may have another layer.

  Hereinafter, examples, comparative examples, and reference examples will be described.

[Example 1]
A tertiary superheater tube (inside diameter 37.6 mm, outside diameter 50.8 mm) made of STBA27 (Cr 9 wt%) of a coal-fired boiler for power generation was cut out from the boiler to a length of 60 mm to obtain a sample tube.

A scale having an average thickness of 0.23 mm (150 mg / cm ² ) was adhered to the inner surface of the sample tube (third superheater tube). When analyzed by EPMA, this scale has an average thickness of 0.11 mm containing about 9 to 15 wt% chromium and 50 to 60 wt% iron, and an average thickness containing 60 to 70 wt% iron and no chromium. It had a two-layer structure consisting of an upper layer of 0.13 mm.

  The three sample tubes were connected in series so that circulation was possible as shown in FIG. The liquid in the tank 1 is passed through the pump 2, the circulation outward pipe 3, the samples 4 and 4, the circulation return pipe 5, and the tank 1 in this order. The tank 1 is provided with a heater 6 so that the liquid can be heated.

  In Example 1, the second step was repeated three times. That is, the first acid treatment → the first oxidation treatment → the second acid treatment → the second oxidation treatment → the third acid treatment → the third oxidation treatment → the fourth acid treatment.

  As an acid solution for the first to third acid treatments, an organic acid solution having the following composition was used, and the liquid temperature was 90 ± 5 ° C.

Glycolic acid: 4.5 wt%
Malonic acid: 4.5wt%
L-ascorbic acid: 1.0 wt%
Corrosion inhibitor: 1.0 wt%
Further, as the acid solution for the fourth acid treatment, an organic acid solution having the following composition was used, and the liquid temperature was set to 90 ± 5 ° C.

Glycolic acid: 1.5 wt%
Malonic acid: 1.5wt%
L-ascorbic acid: 1.0 wt%
Corrosion inhibitor: 1.0 wt%
As the corrosion inhibitor, IBIT 30ES manufactured by Asahi Chemical Industry Co., Ltd. (“IBIT” is a registered trademark of Sumitomo Chemical Co., Ltd.) was used.

  As an oxidizing agent for the oxidation treatment, a mixed solution of 1.5 wt% potassium permanganate and 3.5 wt% potassium hydroxide was used, and the liquid temperature was 80 ± 5 ° C. For the third oxidation treatment, a mixed solution of 1 wt% potassium permanganate and 2 wt% potassium hydroxide was used.

  At the time of acid cleaning, 1.3 L of the above acid solution (unused) is stored in the tank 1 and circulated through at 8 L / min, and the total Fe (T-Fe) concentration of the liquid in the tank 1 and The total Cr (T-Cr) concentration was measured over time. And until the time-dependent increase in the Fe ion concentration in the tank 1 stops, specifically, until the increase in the Fe ion concentration per 0.5 hour confirms two points of 5% or less of the eluted Fe ion concentration, The acid wash was continued. After the completion of the acid cleaning, the acid solution was taken out, washed with pure water, and then subjected to the following oxidation treatment.

  At the time of oxidation treatment, 1.3 L of the above-mentioned permanganate solution (unused) is placed in the tank 1 and circulated through at 8 L / min, and the total Fe (T-Fe) in the liquid in the tank 1 Concentration and total Cr (T-Cr) concentration were measured over time. And until the time-dependent increase in Cr ion concentration in the tank 1 stops, specifically, until the increase in Cr ion concentration per 0.5 hour confirms two points of 5% or less of the eluted Cr ion concentration, The oxidation treatment was continued. The results are shown in Table 1, FIG. 3 and FIG.

  As shown in Table 1 and FIG. 4, in the first acid cleaning step for removing the upper layer, the increase in Fe ion concentration almost stopped at 11,100 mg / L after 25 hours. It shifted to the lower layer oxidation treatment process with the second potassium permanganate.

  In the first lower layer oxidation treatment step, the increase in Cr ion concentration almost stopped at 216 mg / L after 24 hours had elapsed, so the process shifted to the next second acid treatment.

  In the second acid treatment step, the increase in Fe ion concentration almost stopped at 1,000 mg / L after 12 hours had passed, so the process shifted to the second lower layer oxidation treatment step with potassium permanganate. In the second lower layer oxidation treatment step, the increase in Cr ion concentration almost stopped at 260 mg / L after 11 hours had elapsed, and the process shifted to the next third acid treatment. In this third acid treatment step, the increase in Fe ion concentration almost stopped at 500 mg / L after 21 hours had elapsed, so the process shifted to the next third lower layer oxidation treatment step with potassium permanganate.

  In this third lower layer oxidation treatment step, the increase in Cr ion concentration almost stopped at 114 mg / L after 8 hours had passed, so the process shifted to the next fourth acid treatment. In the fourth acid treatment step, after 8 hours, the increase in Fe ion concentration almost stopped at 250 mg / L and the increase in Cr ion concentration almost stopped at 13 mg / L. With this, the descaling process was terminated, and the acid solution remaining in the pipe was discharged and washed with pure water. When the inner surface of the sample tube was observed, the scale was almost completely removed.

[Comparative Example 1]
Four sample tubes identical to those used in Example 1 were similarly connected in series and set as shown in FIG. The tank 1 contains 1.3 L of the same unused acid solution as in Example 1 and the first to third acid treatments, and is passed for 30 hours at a liquid temperature of 90 ° C. and in the liquid in the tank 1. T-Fe concentration and T-Cr concentration were measured.

  Next, all the remaining liquid in the tank 1 and the sample 4 is discharged, and 1.3 L of the same unused acid solution is again stored in the tank 1, and the liquid in the tank 1 is passed through at a liquid temperature of 90 ° C. for 30 hours. The T-Fe concentration and T-Cr concentration in the medium were measured. The results are shown in Table 2, FIG. 5 and FIG. As shown in FIGS. 5 and 6, elution of Fe and Cr becomes extremely slow after 10 hours, especially 20 hours.

  Thereafter, the inner surface of the sample was observed, and a brownish scale remained at an average thickness of 0.15 mm. Although the total acid cleaning time was almost equal to 65 hours in Example 1 and 60 hours in Comparative Example 1, it was confirmed that the scale lower layer was hardly removed in Comparative Example 1.

  As in Example 1, even in the case of a chromium-containing steel material, it is possible to dissolve the lower layer by removing the upper layer of the scale and then solubilizing the lower layer with permanganate and performing acid treatment. And the lower layer can be sufficiently removed by repeating the acid treatment a plurality of times.

1 tank 4 sample tube 6 heater

Claims (6)

A method for removing scale on the surface of a chromium-containing steel material,
The scale is composed of an upper layer and a lower layer, and the chromium-containing steel material has a two-layer structure in which the chromium oxide content of the lower layer is higher than that of the upper layer.
A first step of removing the upper layer with an acid;
And a second step of solubilizing the lower layer by oxidizing the lower layer and then acid-treating the lower layer, and a method for removing the scale of the chromium-containing steel material.   The scale removal method for a chromium-containing steel material according to claim 1, wherein the second step is performed a plurality of times.   3. The method for removing the scale of a chromium-containing steel material according to claim 1, wherein the oxidation treatment is a treatment in which a permanganate solution is brought into contact with the scale.   In claim 3, the permanganate solution is circulated and brought into contact with the scale, the chromium ion concentration in the circulating solution is measured, and the oxidation treatment is performed until the rate of increase of the chromium ion concentration becomes a predetermined value or less. A method for removing scale of a chromium-containing steel material. 5. The acid treatment in any one of claims 1 to 4, wherein the acid treatment in the first and second steps is a treatment in which an acid solution is circulated and brought into contact with a scale.
A method for removing the scale of a chromium-containing steel material, comprising measuring an iron ion concentration in a circulating solution and performing an acid treatment until an increasing rate of the iron ion concentration becomes a predetermined value or less.   6. The method for removing a scale of a chromium-containing steel material according to any one of claims 1 to 5, wherein a chromium content in the chromium-containing steel material is 12 wt% or less.

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