JPS63297599A - Method for removing scale of stainless steel - Google Patents

Abstract

PURPOSE:To remove the scales of a stainless steel in an extremely short time, by electrifying between a stainless steel and a counter electrode separately immersed in an acid aqueous solution containing specific amounts of nitrate ions and fluorine ions. CONSTITUTION:A stainless steel and a counter electrode are separately immersed in an acid aqueous solution which is an aqueous solution composed principally of nitrate ions and fluorine ions and containing the nitrate and fluorine ions by 1.5-40wt.%, in total, expressed in terms of nitric-acid and hydrofluoric-acid concentrations, respectively. It is desirable that any of <=about 20% inorganic acid, <=about 20% organic acid, <=about 3% surfactant, etc., is further added to the above-mentioned aqueous solution. Subsequently, electrification is applied to the position between the above-mentioned stainless steel and counter electrode. By this method, the scales of the stainless steel can be efficiently removed.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for efficiently removing oxide scale from steel materials, and particularly to a method for removing oxide scale from stainless steel materials, which is difficult to remove.

<Prior art and its problems> Stainless steel materials, especially thick plates, oil country tubular goods, and steel pipes for line pipes, etc., are usually manufactured without going through a descaling process until they become final products, unlike thin stainless steel sheets, so they are prone to oxidation scale. It is also thicker and more difficult to remove than thin steel plates.

For this reason, the conventional method was to perform shot blasting on not only the external surface but also the internal surface, and then immerse it in an acidic solution (nitrofluoric acid) or molten alkali salt for a long time (several hours) to remove the oxidized scale. It was normal.

However, with the conventional method, production efficiency is poor, descaling of the contact area between the steel pipe and the support is insufficient, and in terms of material quality, hydrogen generated from the pickling reaction during long pickling times is There were problems such as intrusion into the rope and deterioration of the toughness of the rope.

That is, the problems with the conventional method for pickling stainless steel materials are as follows.

(1) Pickling by immersion in a pickling solution or molten alkali salt usually takes 2 to 3 hours for martensitic or austenitic stainless steel pipes, and 8 hours for duplex stainless steel pipes, even if shot blasting is performed in advance. Requires ~10 hours of descaling time. Moreover, without shot blasting, descaling time is required to be 5 to 6 times longer, which significantly reduces productivity.

(2) During long-term pickling, a hydrogen generation reaction occurs on the surface of the steel material at the same time as the scale dissolution reaction, and some of the generated hydrogen enters the steel material, which should normally be 2 to 3 ppm or less. There is a risk that the hydrogen concentration will increase to 10 ppm or more, which will adversely affect the mechanical properties of the steel, especially the toughness.

Conventionally, stainless thin steel sheets have been subjected to an electrolytic pickling method using an indirect energization method (for example, Japanese Patent Application Laid-Open No. 59-59900). However, the scale on stainless thin steel plates is very thin, such as temper color, whereas the scale on thick plates and steel pipes is very thick and strong, formed by approximately 1150 tons of solution treatment during the manufacturing process. ,
The above methods cannot efficiently remove scale, and at present, scale is not generally removed by electrolytic pickling from steel pipes and thick plates.

<Object of the Invention> The object of the present invention is to provide a method for removing scale from stainless steel materials, which can remove scale from stainless steel materials such as thick plates and steel pipes in a very short time without shot blasting, which has been difficult to remove in the past. try to.

<Structure of the Invention> The method for removing scale from stainless steel materials of the present invention is to use an aqueous solution mainly containing nitrate ions and fluoride ions, convert nitrate ions to nitric acid concentration, fluorine ions to hydrofluoric acid concentration, and combine both. The stainless steel material and the counter electrode are immersed separately in an acidic aqueous solution containing 1.5 to 40*t%,
Electricity is applied between the stainless steel material and the counter electrode.

The details of the scale removal method of the present invention will be described below. The total concentration of nitrate ions and fluoride ions in the aqueous solution converted to nitric acid concentration and hydrofluoric acid concentration is 1.5 to 40 wt%.
The reason for limiting this is that these ions have the ability to greatly improve the descaling power, but if it is less than 1.5 wt%, the effect is not so great, and if it exceeds 40 wt%, the derusting power decreases. The content was set at 1.5 to 4.0 wt%.

The preferable concentrations of nitrate ions and fluorine ions in terms of acid are 5 to 30 wt% for nitrate ions and 1 to 10 wt% for fluorine ions in terms of hydrofluoric acid.

In other words, if the nitrate ion content is less than 5 wt% (calculated as nitric acid), the corrosion reaction due to hydrofluoric acid becomes dominant, which tends to impair the smoothness of the stainless steel surface after descaling.
If the amount is larger, passivation by nitric acid becomes dominant, making it difficult to remove scale. In addition, fluorine ion is equivalent to 1
If it is less than 10wt%, it is difficult to remove scale because the passive film of the stainless steel material cannot be sufficiently destroyed.
If it exceeds %, the corrosion reaction due to hydrofluoric acid becomes dominant, which tends to impair the smoothness of the surface of the stainless steel material after descaling, and also increases the difficulty of handling from the standpoint of environmental hygiene.

In the electrolytic descaling method of the present invention, an acidic aqueous solution containing mainly nitrate ions and fluorine ions as described above is used. It is desirable to use one of these liquids.

(1) Aqueous solution containing nitric acid and fluorine ions at specific concentrations.

(2) A specific inorganic acid or its salt other than sulfuric acid or sulfurous acid is added to an aqueous solution containing a specific concentration of nitric acid or fluorine ions.

(3) A specific organic acid or its salt added to an aqueous solution containing a specific concentration of nitric acid or fluorine ions.

(4) Specific inorganic acids and organic acids other than sulfuric acid and sulfite, or their salts, are added to an aqueous solution containing nitric acid and fluorine ions at specific concentrations.

(5) An aqueous solution containing a specific concentration of sulfuric acid or sulfite ions in an aqueous solution containing a specific concentration of nitric acid or fluorine ions.

(6) A solution containing a specific concentration of sulfuric acid/sulfite ions and a specific inorganic acid or its salt to an aqueous solution containing a specific concentration of nitric acid/fluorine ions.

(7) A solution containing a specific concentration of sulfuric acid/sulfite ions and a specific organic acid or its salt to an aqueous solution containing specific concentrations of nitric acid/fluorine ions.

(8) A solution containing a specific concentration of sulfuric acid or sulfite ions and a specific inorganic acid or organic acid or their salts to an aqueous solution containing a specific concentration of nitric acid or fluorine ions.

-(9) A surfactant added to the above (1) to (8).

Regarding (2) to (8) above, to state the names of specific substances and their concentrations (converted to acids), for example, the inorganic acids or their salts used include hydrochloric acid, phosphoric acid, chromic acid,
One or two inorganic acids such as hydrobromic acid, or salts containing these ions f! It is preferable to add more than i and less than 20+vt%.

These inorganic acids and salts have the function of uniformly promoting scale removal.

However, if it is added in excess of 20 wt%, the rust removal ability will be reduced, so it is set at 20 wt% or less.

The organic acids or salts thereof to be used include organic acids such as carboxylic acids and oxycarboxylic acids, or salts containing them.
It is preferable to add less than wt%.

These organic acids and salts also have the function of promoting scale removal. However, when added in excess of 20wt%,
Since it lowers the rust removal ability, the content was set to 20 wt% or less.

It is preferable that one or both of sulfate ions and sulfite ions be contained in a total amount of 20 wt % or less.

The reason is that sulfate ions and sulfite ions have the function of promoting scale removal and making the surface beautiful after descaling, but if added in excess of 20 wt%, the descaling power will decrease, so it should not exceed 20 wt%. did.

It is preferable to add 3 wt% or less of the surfactant. If there is oil or other stains on the surface of the steel material, the rust removal ability can be improved by adding a surfactant.

Any surfactant may be used as long as it is stable in an aqueous solution containing nitric acid and fluorine ions. For example, nonionic surfactants are representative. Surfactants activate the interface of steel and increase rust removal ability, but 3w
Addition of more than t% is not very effective and increases cost, so it is set to 3% or less.

In the method of the present invention, electrolysis is performed using a stainless steel material such as a stainless steel vibrator from which scale is removed in an aqueous solution mainly containing nitrate ions and fluorine ions as described above, as an anode. Stainless steel vibes are subjected to solution treatment at about 1150°C during the manufacturing process, so they have very thick and strong scales with a thickness of 10 to 60 mm.

Note that alternating current can also be used as a power source, but the rust removal efficiency is slightly inferior to direct current.

In addition, the higher the current value, the more effective the rust removal effect, but if the current is increased too much, the temperature of the vibrator and bath will rise sharply, the environment will deteriorate due to the generated acid vapor, and a large amount of evaporation from the bath will occur. In such cases, it is preferred to use means for cooling the bath.

Any material may be used as the counter electrode to the stainless steel material, but a stainless steel vibrator or a stainless steel plate such as 5US316 is preferable.

An example of an electrolytic apparatus for carrying out the method of the present invention is shown in FIG. 1, but the present invention is not limited thereto.

In FIG. 1, lobes 1 are wrapped around both ends of a stainless steel vibrator 4 to be cleaned, and the vibrator 4 is immersed in an electrolytic bath 4ffS containing the above-mentioned specific acidic aqueous solution.

A cylindrical inner electrode 3 and an outer electrode 2 are provided on the inside and outside of this stainless steel vibrator 4 coaxially with the stainless steel vibrator. The inner electrode 3 and the outer electrode 2 are supported by lobes or the like (not shown), similarly to the stainless steel vibrator 4.

A DC power supply 6 is installed at an appropriate position outside the bathtub 5, the stainless steel vibrator 4 and the anode of the DC power supply 6 are electrically connected, and the outer electrode 2 and the inner electrode 3 are electrically connected to the cathode, respectively.

<Example> The present invention will be specifically described below with reference to Examples.

, (Example and Comparative Example) 2 with a length of 300 mm, an outer diameter of 178 mm, and an inner diameter of 162 mm.
Phase stainless steel seamless pipes were immersed in various pickling solutions and electrolytically pickled using the apparatus shown in FIG.

Note that the surfactant used was a nonionic surfactant.

Further, the electrolytic pickling conditions and the rust removal status are shown in Table 1. The rust removal status was evaluated by observing the sample after pickling with the naked eye and with a 10x magnifying glass, and evaluating the surface cleanliness.

The evaluation criteria were as follows.

(1) Surface cleanliness ×; Insufficient descaling △: Some scale remains O; Slight pickling residue remains, but no practical problem ◎; Very clean finish (no pickling marks) (2) Overall evaluation ×: Practically unusable △: Some practical problems O: Fully practical ◎: Very good When the method of the present invention was used, all rusting (descaling) was achieved within 60 minutes.

However, sample No. 13.14.41.42 shown in the comparative method
．． When F- or NO3- is not used as in No. 43, the descaling ability is poor. Also, sample No. 15.16.
In the case of simple immersion without electric current, as in No. 44, 45, and 46, the descaling power was very poor, and even after 30 hours of immersion, the scale could not be removed sufficiently.

<Effects of the Invention> By using the method of the present invention, scales from stainless steel materials can be removed in a much shorter time than conventional methods.

[Brief explanation of drawings]

FIG. 1 is a diagram showing one example of an apparatus for carrying out the method of the invention. Explanation of symbols 1...lobe, 2...outer electrode, 3...
...Inner electrode, 4...Pipe, 5...Bathtub, 6...DC power supply Patent applicant Kawasaki Steel Corporation San-Ai Oil Co., Ltd.' [Representative Patent Attorney Minoru Watanabe Nozomi FIG, 1

Claims (1)

[Claims] (1) An aqueous solution mainly containing nitrate ions and fluoride ions, where the nitrate ions are converted to nitric acid concentration and the fluorine ions are converted to hydrofluoric acid concentration, and the total content of both is 1.5 to 40 wt%. A method for removing scale from a stainless steel material, comprising immersing the stainless steel material and a counter electrode in a spaced manner, and applying electricity between the stainless steel material and the counter electrode.