JPH02310398A - Method for electrolytically descaling cold-rolled stainless steel - Google Patents

Abstract

PURPOSE:To control the consumption of an anode and to electrolytically descale a cold-rolled stainless steel in an aq. neutral salt soln. over a long period by using the Ti coated with the oxide of a platinum-group element for the anode. CONSTITUTION:The oxide scales on a cold-rolled stainless steel sheet 3 are electrolytically removed. In this case, the sheet 3 is passed through an inlet-side chamber 5A contg. a 20% soln. 4 of the neutral salt of Na2SO4 and in which plural sets of anodes 1 are arranged above and below the sheet in the traveling direction and passed through an outlet-side chamber 5B contg. the same soln. 4 and in which plural sets of cathodes 2 are arranged above and below the sheet 3 in its traveling direction, and a current is applied to the cathode and anode to electrolytically remove the oxide scales on the surface of the sheet. In this case, an electrode of the Ti coated with the oxide of a platinum-family element such as IrO2 is used as the cathode, high-silicon cast iron is used for the cathode, the anode current density is controlled to 2-12A/dm<2>, the anode potential is adjusted to 2.1V based on a silver-silver chloride electrode, and electrolytic descaling is carried out.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an electrolytic descaling method for cold-rolled stainless steel, and in particular to a method for continuously removing oxidized scale on the surface of a cold-rolled stainless steel strip over a long period of time. This paper relates to an electrolytic descaling method for cold-rolled stainless steel materials.

[Conventional technology]

When cold-rolled stainless steel strip is subjected to heat treatment such as annealing or quenching in an oxidizing atmosphere, oxide scale is formed on the surface of the steel strip, so descaling treatment is generally performed to remove this oxide scale.

Pickling using sulfuric acid, hydrochloric acid, nitric-fluoric acid (a mixed acid of nitric acid and hydrofluoric acid), etc. is generally used for descaling, but the oxide scale that forms on cold-rolled stainless steel strips is dense. Because it is so strong, it is difficult to completely descale it. Therefore, as pretreatment to facilitate pickling, methods such as immersion treatment in molten alkali salt (salt treatment) or electrolytic descaling treatment in a neutral salt aqueous solution as shown in Japanese Patent Publication No. 3B-12162 have been developed. It has been put into practical use.

The electrolytic descaling treatment in the neutral salt aqueous solution employs an indirect electrolysis method as shown in FIG. That is, in the +1 JL solution 4 filled in the electrolytic tank 5,
In this method, a positive electrode 1 and a negative electrode 2 are arranged in the direction in which the copper strip moves, sandwiching the stainless steel strip 3 from above and below, and a DC voltage is applied between the electrodes 1 and 2.

As the electrode material, a lead electrode, a high silicon (Si) cast iron electrode, or the like is used.

Electrolytic descaling treatment in a neutral salt aqueous solution has the advantage that it is easier to obtain beautiful surface textures than Tsuruto treatment, and because the solution is neutral, it provides an excellent working environment.

[Problem to be solved by the invention]

In the conventional electrolytic descaling treatment method in a neutral salt aqueous solution, both electrodes 1.
Of these, at the negative electrode 2, only hydrogen is mainly generated due to the electrolysis of water, and the electrode itself suffers little wear.

However, in the anode electrode 1, in addition to the generation of oxygen accompanying the electrolysis of water, a dissolution reaction of the anode material occurs, so the electrode itself wears out considerably, and the electrode material needs to be replaced every 1 to 3 months, requiring a long period of time. The problem was that it was difficult to maintain stable operations over a long period of time.

As a countermeasure, it is possible to suppress the consumption of the anode (it is possible to divide the electrolytic tank into multiple parts and replace only the electrolyte in which the anode is immersed with a less corrosive solution, but this would require complicated equipment. It is not practical due to many other difficulties, such as the need to carry in liquids and liquids.

Therefore, the present invention has been made by focusing on the above-mentioned conventional problems, and its purpose is to cold-roll stainless steel in a neutral salt aqueous solution, which can be operated continuously for a long period of time. The object of the present invention is to provide an electrolytic descaling method for steel materials.

[Means to solve the problem]

The present invention achieves the above objects by electrolytically treating cold rolled stainless steel in a neutral salt aqueous solution using a positive electrode made of a titanium material coated with an oxide of a platinum group element. A name characterized by

The current density of the positive electrode is 2 A/d m2 to 12 A/
dm2 range.

Also, the electrode potential of the positive electrode is set to 2 with respect to the silver/silver chloride electrode.
．． It can be lowered to 1 V or less.

[Function] When an anode electrode made of a titanium material coated with an oxide of a platinum group element is used, the current density of the anode electrode is 2 A/dm2 to 12 A.
/dm” range or by maintaining the electrode potential of the anode electrode at 2.1 V or less based on the silver/chloride 8m electrode, the phenomenon of precipitation of manganese (Mn) oxide from the neutral salt aqueous solution is suppressed. Therefore, deterioration and consumption of the positive electrode material due to manganese oxide can be prevented, and the electrode material remains stable even during long-term operation.

The present invention will be explained in more detail below.

The inventors of the present invention have developed a positive electrode material suitable for neutral salt electrolysis and its appropriateness in order to reduce the consumption of the positive electrode material in the electrolytic descaling treatment of cold rolled stainless steel materials in a neutral salt aqueous solution. We focused on the need to find suitable usage conditions.

First, we conducted an electrolytic descaling experiment in a neutral salt aqueous solution using various positive electrode materials, and measured the amount of consumption of the positive electrode materials. Table 1 shows the results.

From the above results, titanium (T
i) The amount of wear of the coated substrate is extremely low;
It was found that the durability as an electrode was good and was 7°.

Therefore, we next prepared a platinum foil welded positive electrode formed by spot welding platinum foil onto a Ti substrate, and a platinum coated positive electrode formed by coating a Ti substrate with platinum. We performed electrolytic descaling treatment of cold-rolled stainless steel strip in a neutral salt aqueous solution. As a result, as expected, the platinum foil welded on the platinum foil welded positive electrode peeled off from the contact area and became unusable after three months. This peeling and peeling of the platinum foil occurred because current was concentrated at the spot weld.

On the other hand, it was expected that the platinum-coated positive electrode would be able to operate continuously for 6 months, but in reality, the surface of the platinum coating material turned black and the electrolytic voltage increased by 30% after 3 months. It ended up not being able to withstand long-term use. In addition, it was observed that the platinum was peeled off from a part of the platinum-coated surface where the electrolytic voltage increased, and the Ti of the substrate was exposed.

In order to find out the cause of this, an attempt was made to analyze the components of the blackened part of the platinum-coated positive electrode, and large amounts of manganese (Mn) and oxygen (0) were detected. From this, in the process of descaling stainless steel, Mn ions gradually eluted into the electrolyte from the stainless steel scale or base metal are oxidized by oxygen generated at the anode due to long-term electrolysis.
It is presumed that it precipitated as manganese oxide.

Furthermore, regarding the increase in electrolytic voltage, the following was found. That is, due to the deposition of precipitated manganese oxide, excessive force is applied to the platinum coating layer, and part of the platinum is peeled off, exposing Ti on the substrate. The surface of this exposed Ti is covered with an electrically insulating oxide formed as an anode by electrolysis in a neutral salt aqueous solution, making it impossible to conduct electricity (see Table 1). As a result, the effective current density at the positive electrode increases and the electrolytic voltage increases.

After all, the causes of the blackening of the platinum-coated anode and the increase in electrolytic voltage are both caused by the anodic oxidation reaction of Mn ions in the neutral salt aqueous solution. By the way, is neutral salt water used as an electrolyte? (For l, potassium (K) or sodium (Na) sulfate.

Nitrates and the like are mainly used, and Mn ions are not included at the time of bath preparation. However, during the process of electrolytic descaling, Mn ions are gradually precipitated and concentrated in the electrolytic solution from the scale or base material of the cold rolled stainless steel material.

Thus, as long as long-term electrolytic descaling is performed on cold-rolled stainless steel materials, the elution of Mn ions into the electrolyte is unavoidable, and it is difficult to remove Mn and ions from the electrolyte.

Therefore, among the various electrode materials shown in Table 1 above, the present inventors discovered a Ti-Pt coating material (platinum-coated electrode material) and a Ti-1rO□ coating material (iridium oxide-coated electrode material), which have significantly less consumption. The anodic oxidation reaction of Mn ions during the electrolytic descaling treatment of cold-rolled stainless steel in a neutral salt aqueous solution was studied in more detail, and the results shown in Figure 1 were obtained.

That is, it has been found that there is a specific range of current density for each electrode material in order for Mn oxide to precipitate from the neutral salt electrolyte onto the surface of the positive electrode plate. Regarding the electrode potential, the precipitation of Mn oxide from the neutral salt electrolyte to the positive electrode plate is due to the following:
It ranges from 2.1 to 2.9 square meters using a silver chloride electrode as a reference electrode. Outside this range, Mn oxide will not precipitate. In addition, there is a considerable difference in electrode potential between platinum-coated electrode materials and iridium oxide-coated electrode materials when used as positive electrodes, and in the case of iridium oxide-coated electrode materials, it is difficult to use electrolytic descaling treatment for 6 months. It was found that Mn oxides were not produced even in the neutral salt electrolyte after the reaction, if the current density was 12 A/dm2 or less.

On the other hand, in the case of platinum-coated electrode materials, if the current density is not suppressed to 6 A/d m2 or less in a neutral salt electrolyte after being used for electrolytic descaling treatment for 6 months, Mn It was found that the generation of oxides could not be prevented. However, since current concentration is generally likely to occur at the ends of electrode plates, it can be concluded that stable operation over a long period of time is difficult with platinum-coated electrode materials.

Regarding the specific configuration of the present invention based on the above research results,
This will be explained below.

Regarding various conditions such as the type of neutral salt used in the neutral salt aqueous solution of the present invention, the concentration of the neutral salt aqueous solution, and the temperature and current density of the electrolytic treatment, conventional conditions can also be applied to the present invention.

In other words, types of neutral salts include N such as sulfuric acid and nitric acid.
A salt and a salt can be used alone or in combination. From the point of view of economy and surface finish, 1. Preference is given to using sodium sulfate.

The appropriate concentration of the neutral salt aqueous solution is 100 to 300 g/i, and the appropriate liquid temperature is 70 to 90°C.

Regarding the current density, the appropriate range for both the anode reaction current density and the cathode reaction current density when processing cold rolled stainless steel strip is 2 to 12 A/dm. 2A/d.
If the current density is less than m'', the required electrode length becomes long, which is disadvantageous in terms of equipment.

On the other hand, if it exceeds 12 A/dm'', manganese oxide is gradually deposited, resulting in peeling of the electrode coating layer.

Expressed electrochemically, by using the electrode potential of the anode at 2.1 V or less using a silver/silver chloride electrode as a reference electrode,
Precipitation of manganese oxide can be prevented, and consumption of the positive electrode material can be prevented over a long period of time (see Figure 1).

As the electrode material, for the positive electrode, Ti is used as the base material, and ruthenium oxide (RuO□), which is an oxide of a platinum group element, is used as the positive electrode.
Rhodium oxide (Rhzo:+), palladium oxide (P
An electrode material coated with osmium oxide (Os04), iridium oxide (IrO2), etc. is used. When Ti is used as the base material, there is an advantage that high adhesion with the oxide of a platinum group element can be obtained when the oxide of the platinum group element is fired onto the surface.

Furthermore, if a part of the oxide of the platinum group element were to peel off, it would be re-coated with the oxide of Ti due to the anodic oxidation, thereby preventing dissolution.

High-silicon cast iron or various types of stainless steel are used as the negative electrode.

There are no particular restrictions on the electrode arrangement.

In the case of stainless steel, which has relatively good descaling properties, it is possible to descale it simply by electrolysis in a neutral salt aqueous solution, but if this is insufficient, it can be completely removed by subsequent pickling treatment. It becomes possible to remove scale.

For pickling after electrolytic treatment in a neutral salt aqueous solution, the same treatment as conventional methods is applied, that is, nitric acid immersion or nitric acid electrolysis is mainly applied to ferritic and martensitic stainless steels. Nitrofluoric acid immersion is mainly applied to austenitic stainless steel.

[Example] Neutral salt electrolytic descaling treatment was carried out in stainless steel cold rolled steel strip annealing and pickling equipment, and changes in electrolytic voltage (electrolytic sliding voltage) over a long period of time were measured between Ti-PL coating material (platinum-coated electrode material) and A comparative study was conducted with Ti-1rOz coating material (iridium oxide coated electrode material).

The electrolytic cell shown in FIG. 2 was used. That is, the inside of the electrolytic cell 5 was divided into two parts in the direction of movement of the cold-rolled stainless steel strip 3 as the object to be treated. On the entry side 5A, a plurality of opposing ¥5 electrodes 1 were arranged in the steel strip traveling direction so as to sandwich the stainless steel cold rolled steel strip 3 therebetween. On the outlet side 5B, a plurality of opposing negative electrodes 2 were arranged in the same manner with cold-rolled stainless steel strips 3 interposed therebetween. Due to the reduction action of hydrogen generated at the negative electrode 2 and the scale exfoliation action caused by the electrolyte stirring caused by released hydrogen, the cold rolled stainless steel strip 3
is descaled.

The electrolyte solution 4 in both tanks 5A and 5B was a 20% neutral salt aqueous solution of sodium sulfate, and the solution temperature was maintained at 80°C. Electrolyte 4 was mixed in a circulation manner.

The current density varies depending on the size and steel type of the cold-rolled stainless steel strip 3, but for the positive electrode 1 it is 6 to 12 A/dm.
and 3 to 6 A/dm'' for the negative electrode 2.

The test was first conducted for three months using all the positive electrodes 1 as Ti-PC coating materials (platinum-coated electrode materials). Thereafter, the positive electrode 1 was entirely replaced with a Ti-1rO□ coating material (iridium oxide-coated electrode material).

Figure 3 shows the change in electrolytic voltage over time during the test.

In each case, the voltage was measured and collected while processing the same size and steel type.

As is clear from Figure 3, in the case of platinum-coated electrode material,
After 40 days, the electrolytic voltage increased significantly. After 90 days, the voltage limit of the power supply was reached and it became unusable.

Thereafter, when the positive electrode 1 was pulled up from the electrolytic cell 5 and observed, the entire surface turned black and significant wear was observed at the electrode end.

On the other hand, in the case of the iridium oxide coated electrode material, there was no significant blackening even after 180 days, and no damage was observed at the electrode end.

〔Effect of the invention〕

As explained above, according to the present invention, in electrolytic descaling treatment of cold-rolled stainless steel strip, a positive electrode made of a titanium material coated with an oxide of a platinum group element is used to increase the current density to 2 A/dm2. Electrolytic treatment was carried out in a neutral salt aqueous solution while maintaining the electrode potential of the positive electrode at 2.1 V or less with respect to the silver/silver chloride electrode.

This causes manganese (Mn) damage to the positive electrode.
The generation of oxides is suppressed, and as a result, it is possible to provide an electrolytic descaling method for cold-rolled stainless steel in a neutral salt aqueous solution that can be operated continuously over a period of V.

[Brief explanation of the drawing]

Fig. 1 is a graph showing the generation conditions of manganese oxide in an example of the descaling treatment of the present invention in comparison with conventional ones, Fig. 2 is a schematic sectional view of an example of the electrolytic cell of the present invention, and Fig. 3 The figure is a graph showing an example of the change in electrolytic voltage in a long-term descaling treatment test of the present invention in comparison with a conventional electrolytic cell, and FIG. 4 is a schematic cross-sectional view of a conventional electrolytic cell. ■ is positive electrode, 2 is negative electrode, 3 is stainless steel cold rolled steel strip, 4
is a neutral salt electrolyte, and 5 is an electrolytic cell.

Claims (3)

[Claims] (1) During the electrolytic descaling treatment of cold-rolled stainless steel, a positive electrode made of a titanium material coated with an oxide of a platinum group element is used to conduct the electrolytic treatment in a neutral salt aqueous solution. Electrolytic descaling method for rolled steel. (2) The current density of the positive electrode is 2A/dm^2~12A/d
The electrolytic descaling method for cold-rolled stainless steel material according to claim 1, wherein the descaling method is performed in a range of m^2. (3) The electrolytic descaling method for cold-rolled stainless steel material according to claim (1) or (2), characterized in that the electrode potential of the positive electrode is set to 2.1 V or less with respect to the silver/silver chloride electrode.