JPH0688297A - Method for descaling stainless steel by neutral salt electrolysis - Google Patents

Abstract

PURPOSE:To increase the dissolving speed of scales at the time of descaling stainless steel which an anodic electrolysis and cathodic electrolysis in an sodium sulfate soln. by specifying the pH of soln. and current density at the time of a cathodic electrolysis. CONSTITUTION:A stainless steel with an oxide scale formed on its surface is passed through an aq. sodium sulfate soln., anodic electrolysis for eluting Cr2O3 and cathodic electrolysis for eluting Fe2O3 are repeated at least once, and the stainless steel is descaled by neutral salt electrolysis. The aq. sodium sulfate soln. is controlled to <=pH 3.5 by adding sulfuric acid, and the current density is kept at >=150mA/cm<2> in the cathodic electrolysis. Consequently, the dissolving rate of the oxide scale is almost doubled compared with a conventional method.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a neutral salt electrolytic descaling method for stainless steel, and more particularly to a method for efficient descaling by performing both anodic electrolysis and cathodic electrolysis at least once.

[0002]

2. Description of the Related Art Stainless steel, for example, stainless steel strip, has the opportunity to be heated for various purposes such as heating before rolling and annealing during the manufacturing and processing stage thereof, except for special cases such as bright annealing. The formation of oxide scale on the surface is unavoidable. In order to ship the product as a product, in the manufacturing process of stainless steel as described above, for example, a step of removing oxide scale generated by annealing is required, and the oxide scale is usually removed by pickling.

As the pickling method, the most conventional method is to immerse it in a mixed acid of nitric acid and hydrofluoric acid, and the acid dissolves the surface of stainless steel to remove oxide scale. However, since the oxide scale itself is essentially insoluble in acids such as a mixed acid of nitric acid and hydrofluoric acid (hydrofluoric acid nitric acid), even if stainless steel with an oxide scale is immersed in hydrofluoric acid. , Pickling does not proceed quickly. This is because the oxide scale prevents the acid from penetrating into the stainless steel matrix.

Therefore, if the oxide scale is modified by some method so that the acid can easily permeate, the pickling rate can be increased.

For this purpose, a method of electrolyzing in a neutral salt aqueous solution such as sodium sulfate is known.
The mechanism of descaling by this method is described in (1) below.
By the reaction of Eq. (2), chromium oxide (Cr ₂ O ₃ ), which is the main component of the oxide scale, is eluted as chromic acid and dichromate ions, and the oxide scale becomes porous. It is thought to be easier.

[0006] _{Cr 2 O 3 + 5H 2 O} → 2CrO 4 - + 10H + + 6e ··· (1) Cr 2 O 3 + 4H 2 O → Cr 2 O 7 - + 8H + + 6e ··· (2) above As can be seen from the reaction equation of, the dissolution of Cr ₂ O ₃ proceeds during anodic electrolysis (when positive electricity is applied).

[0007]

However, even with such a conventional method, the dissolution rate of oxide scale is not sufficient, and particularly when high-speed processing is performed aiming at reduction of the processing cost as in recent years, the dissolution rate is high. Further improvement is required. It is an object of the present invention to provide a neutral salt electrolytic descaling method for stainless steel in which the dissolution rate of oxide scale by neutral salt electrolysis is made as rapid as possible.

[0008]

Therefore, the present inventors have
As a result of various studies on the conditions for increasing the dissolution rate of the oxide scale of stainless steel by the neutral salt electrolysis method, the inventors have found that the following is effective and completed the present invention.

Both the anodic electrolysis and the cathodic electrolysis are performed at least once, and the current density of the cathodic electrolysis is 150 mA / cm ² or more. Lower the pH of the neutral salt electrolyte (eg 3.5 or less).

Therefore, the gist of the present invention is that both anodic electrolysis and cathodic electrolysis are performed at least once in an aqueous sodium sulfate solution, and the current density during cathodic electrolysis is 150 mA / cm ² or more. This is a neutral salt electrolytic descaling method for stainless steel.

From another aspect, the present invention is to perform both anodic electrolysis and cathodic electrolysis at least once in an aqueous sodium sulfate solution whose pH is adjusted to 3.5 or less by adding sulfuric acid. It is characterized by the neutral salt electrolytic descaling method of stainless steel.

[0012] From still another aspect, the present invention is to carry out both anodic electrolysis and cathodic electrolysis at least once in an aqueous solution of sodium sulfate whose pH is adjusted to 3.5 or lower by adding sulfuric acid, and Current density during electrolysis is 150 mA / c
It is a neutral salt electrolytic descaling method of stainless steel, which is characterized in that it is at least m ² .

[0013]

Next, the reason why the descaling conditions are limited as described above in the present invention and the mechanism showing the effect of accelerating the dissolution of the oxidized scale of the stainless steel are considered as follows.

That is, the main component of the oxide scale of stainless steel is chromium oxide (Cr ₂ O ₃ etc.), but usually contains a large amount of iron oxide. In particular, trivalent iron oxide (Fe ₂ O ₃ ) is difficult to dissolve in acid, so even if Cr ₂ O ₃ is eluted by anodic electrolysis, if Fe ₂ O ₃ remains as it is, the pickling rate with nitric hydrofluoric acid will increase. Become slow.

However, by carrying out cathodic electrolysis at a current density of 150 mA / cm ² or more, trivalent iron is reduced to divalent iron and easily dissolved, and at the same time coexisting hydrogen ions (H
It is presumed that the action of ( ⁺ ) causes the reaction of the following formula (3) to occur and the iron oxide is dissolved.

Fe ₂ O ₃ + 6H ⁺ + 2e → 2Fe ⁺⁺ + 3H ₂ O (3) On the other hand, the dissolution reaction of the trivalent iron oxide is more likely to proceed as the H ⁺ ion concentration is higher, so It is speculated that the dissolution of the oxide scale was accelerated by lowering the pH of the electrolytic solution instead of increasing the density. Of course, by using both of these means together, the effect is further enhanced.

Therefore, according to the present invention, by alternately performing anodic electrolysis and cathodic electrolysis under the above-mentioned conditions,
Elute Cr ₂ O ₃ during anodic electrolysis, and Fe ₂ O 3
The elution of ₃ is aimed at, and the specific conditions at that time may be appropriately determined depending on the amount of oxide scale and the like.

A concrete preferred operation mode will be described by taking a stainless steel strip and a stainless steel wire as an example. First, a plurality of anode electrolysis cells and cathode electrolysis cells are alternately arranged, and one of them is maintained under respective processing conditions. Continuously feed stainless steel strip or wire from one side to the other. In addition,
In this case, preferably, the efficiency of oxide scale removal is further improved by performing the first and last electrolysis with the treated material surface serving as an anode. Next, the working effects of the present invention will be described in more detail with reference to examples.

[0019]

【Example】

(Example 1) Cold rolled steel plate of SUS304 stainless steel (thickness 0.5 m
m) was annealed in a combustion atmosphere of propane gas at 1120 ° C for 60 seconds as a test material, and this was subjected to anode and cathodic electrolytic treatment in a 20% Na ₂ SO ₄ aqueous solution at 80 ° C. Fe
Was analyzed by ICP (inductively coupled plasma) emission spectroscopy to examine the elution amount. The pH of the electrolytic solution was adjusted by adding sulfuric acid. The results are shown in Table 1 together with the electrolysis conditions.

The examples shown in Table 1 are ones in which anodic electrolysis and cathodic electrolysis were repeated alternately 3 or 6 times for electrolysis, and in both cases, the ratio of anodic electrolysis time to cathodic electrolysis time was 2: 1. In this example, the total electrolysis time is 9 seconds.

[0021]

[Table 1]

As can be seen from the results shown in Table 1, the electrolyte solution
Even if the pH is 5.0 as before, the current density is 150 mA / cm ²
By the above, the elution of Cr and Fe is increased (see No. 1). The cathode current density is 80 mA / cm ² , but the electrolyte with a lower pH (pH 3.0 to 1.7) (No. 9 to 11) also contains Cr.
The elution amount is 2.5 to 3.4 mg / dm ² , and the Fe elution amount is 0.8 to 1.4 mg / dm ² .

On the other hand, when the current density of the cathodic electrolysis is 180 mA / cm ² or more (No. ^{2 to} 8), the Cr elution amount per sample area of 1 dm ² is 3.2 mg or more and Fe elution is all. The amount was more than 1.5 mg. In all three cases, the comparison method (No.
It was found that the dissolution rate of oxide scale was fast, which was clearly higher than that of Cr and Fe (12, 13) and almost doubled.

Experiments were also conducted in which the time and ratio of anodic electrolysis and cathodic electrolysis, and the number of repetitions were variously changed. In any case, it was found that the cathode current density was 150 mA / cm ² or more. It was confirmed that / or lowering the pH to 3.5 or lower is the most effective in accelerating the dissolution of the scale.

(Example 2) Cold rolled steel plate of SUS430 (thickness 0.5 m
m) was annealed at 830 ° C for 60 seconds in a combustion atmosphere of propane gas as a test material, and this was electrolyzed in a 20% Na ₂ SO ₄ aqueous solution at 80 ° C, and then Cr and Fe in the solution were removed. The amount of elution was examined by analysis by ICP (inductively coupled plasma) emission spectroscopy. The pH of the electrolytic solution was adjusted by adding sulfuric acid. The results are shown in Table 2.

[0026]

[Table 2]

As can be seen from Table 2, when the current density of the cathodic electrolysis is 180 mA / cm ² or more (No. 1 to 8), the elution amount of Fe is 2.5 to 3.0 mg / dm ² , and the elution of Cr is The amount is 0.7 to 1.0 mg / dm ^2, which is clearly higher than the elution amount of the comparative examples (No. 12, 13). In addition, the ratio of the elution amount of Fe and Cr is SUS304 of Example 1.
The reason for the reverse of the case is that the Fe content in the oxide scale of SUS430 is several times that of Cr. It was also confirmed that those with a cathode current density of -60 mA / cm ² and a low electrolyte pH (3.0 to 1.7) (No. 9 to 11) also had a higher elution amount (especially Fe) than the comparative example. did.

[0028]

As described above, according to the present invention,
The dissolution rate of oxide scale is almost doubled compared with the conventional method, and the processing speed can be increased accordingly.
It can meet today's technical requirements.

Claims (3)

[Claims] 1. A stainless steel characterized in that both anodic electrolysis and cathodic electrolysis are performed at least once in an aqueous sodium sulfate solution, and the current density during cathodic electrolysis is 150 mA / cm ² or more. Neutral salt electrolytic descaling method. 2. Neutral salt electrolysis of stainless steel, characterized in that both anodic electrolysis and cathodic electrolysis are performed at least once in an aqueous sodium sulfate solution whose pH is adjusted to 3.5 or lower by adding sulfuric acid. Liberation scale method. 3. Both anodic electrolysis and cathodic electrolysis are performed at least once in an aqueous solution of sodium sulfate whose pH is adjusted to 3.5 or less by adding sulfuric acid, and the current density during cathodic electrolysis is 150 mA / The neutral salt electrolytic descaling method of stainless steel, which is characterized by making it cm ² or more.