JPH0762572A - Pickling method for stainless steel - Google Patents

Abstract

PURPOSE:To maintain good surface finishing and to prolong the life of a pickling soln. relating to electrolytic conditions of the pickling method to be applied to an electrolytic pickling device for stainless steels. CONSTITUTION:A nitric acid soln. of 25 to 200g/l nitric acid (NHO3) concn. is stored as the pickling soln. into the electrolytic pickling device 9 and both anode and cathode electrodes 2, 2 are arranged to face each other in this soln. 3. The pickling device is so constituted that a stainless steel sheet 1 can be passed between these electrodes. A salt bridge 6 is disposed near the transported stainless steel sheet 1 so that the potential of the steel sheet is measured by a voltmeter 8 via a reference electrode 7. Constant potential electrolysis is executed by controlling the impressed voltage and maintaining the set voltage constant. Since the surface potential of the stainless steel is set in a secondary passive region, pitting, etc., are hardly generated on the surface of the steel sheet and the good surface finishing is made possible.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to electrolytic conditions of a pickling method applied to an electrolytic pickling apparatus for stainless steel and other electrolytic pickling apparatus for metals exhibiting passivation behavior.

[0002]

2. Description of the Related Art A conventional stainless steel electrolytic pickling apparatus is shown in FIG.

In the figure, the annealed stainless steel plate 1 is introduced into an electrolytic pickling apparatus 9 and immersed in a pickling solution 3.

While passing between the two electrodes 2 arranged in the pickling device 9, an oxidative dissolution reaction of the base material is caused by an anode current by an applied current and a reduction dissolution reaction of an oxide scale is caused by a cathode current. Oxide scale on the steel plate surface is removed.

The applied current at this time is a constant current (density) during the pickling time.

Further, the applied current value is selected from an empirical value and a finishing condition after pickling, and is not quantitatively selected and determined in view of required quality.

[0007]

By the way, in the conventional pickling of stainless steel, the pickling of the stainless steel is completed with a constant current from the start to the end of pickling, and the pickling is finished. Since the judgment depends on the operator's visual inspection, the steel surface condition at the end is a product that is non-uniform in quality.

In other words, if the set time becomes long, it causes overpickling, resulting in a roughened surface, and from the viewpoint of productivity, excessively dissolving the base steel causes a problem of promoting deterioration of the pickling solution.

On the other hand, if the set time is short, there is a problem that the chromium deficient layer is left excessively or in some cases, the oxide scale is left and the quality is deteriorated.

An object of the present invention is to solve the above problems and to provide a new method for pickling stainless steel, which maintains a good surface finish and extends the life of the pickling solution.

[0011]

In order to achieve the above object, a method of pickling stainless steel according to the present invention is a method of immersing a stainless steel plate in a pickling solution in an electrolytic pickling apparatus, and facing the pickling solution. In a pickling method for stainless steel, which removes oxide scale by an applied current while passing between the arranged positive and negative electrodes, a nitric acid solution having a nitric acid concentration of 25 to 200 g / l is used as the pickling solution, and the pickling is performed. The surface potential of the stainless steel in the liquid is set in the secondary passivity region and a salt bridge is installed in the vicinity of the stainless steel plate for constant potential electrolysis, and the potential of the steel plate is measured with a voltmeter through a reference electrode, The feature is that the applied voltage of the power supply is controlled.

[0012]

OPERATION FIG. 2 shows the time-dependent change in the polarization characteristics (potential / current characteristics) of the steel surface during electrolytic pickling of stainless steel with oxide scale attached.

When the surface potential of the stainless steel plate in the pickling solution is set to the secondary passivation potential region and constant potential electrolysis is performed as in the present invention, electrolytic pickling is performed in the secondary passivation potential region as shown in FIG. It can be seen that the melting current drastically decreases with the progress of. Therefore, the finished state of the surface of the stainless steel plate can maintain a good surface finish without being affected by the setting of the pickling time.

On the other hand, in the superpassive potential region and the like, the dissolution current hardly changes regardless of the progress of pickling.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A method according to an embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic view of an electrolytic pickling apparatus for carrying out a pickling method for stainless steel according to an embodiment of the present invention, in which 1 is an annealed stainless steel plate, 2 is an electrode, and the electrode is an electrolytic pickling apparatus 9 It is immersed in the pickling solution 3 stored therein.

The stainless steel plate 1 is sent into the electrolytic pickling device 9 and, while passing between the two positive and negative electrodes 2 and 2 facing each other, the oxide scale adhered to the surface of the steel plate is removed by the current applied to the electrodes by the power source 4. To be done.

A salt bridge 6 is installed near the stainless steel plate 1 to be transferred, and the potential of the steel plate 1 can be measured by a voltmeter 8 via a reference electrode 7.

The voltmeter 8 is connected to the power source 4, and an ammeter 5 is installed in the circuit connecting the power source 4 and the electrode 2.

Next, the pickling method of the present invention using the electrolytic pickling apparatus having the above-mentioned structure will be explained with reference to Example 1. The pickling solution is nitric acid (HNO ₃ ) containing 10 g / l of iron at a liquid temperature of 50 ° C. Using a solution, in the solution, set potential 14
The potentiostatic electrolysis of the annealed SUS430 stainless steel plate was performed with 00 mVAg / Agcl.

FIG. 3 shows the change with time of the current at this time. As can be seen from the figure, the current value decreases to about 1/10 of the initial value in the electrolysis time of 6 to 12 seconds.

However, when the nitric acid concentration is 250 g / l, the current value increases immediately after the decrease. Therefore nitric acid concentration 200g
It is necessary to be less than / l.

However, when the nitric acid concentration becomes 25 g / l or less, the secondary passivation behavior becomes unclear.

Therefore, as in the present invention, the nitric acid concentration is 25 to 200.
In the range of g / l, the permissible (error) range of the pickling time (electrolysis time) is widely applicable under the industrial pickling working conditions.

Further, in order to compare the surface finishing states of the constant potential method (this method) and the constant current method (conventional method), annealing was performed in a pickling solution having a nitric acid concentration of 100 g / l and an iron concentration of 10 g / l at a liquid temperature of 50 ° C. S
Electrolytic pickling of a US430 stainless steel plate was performed for 10 seconds.

In the constant potential method, the potential is 1400 mVag / A.
The current was set to gcl and the current was set to 10 mA / cm ² by the constant current method.

Then, in order to observe the state of scale adhesion on the surface after pickling, surface composition analysis by ESCA was performed as shown in FIG.

From the analysis results, there is no oxide scale remaining, and there is no significant difference in the surface condition of the pickling material between the two methods when the electrolysis (pickling) time is 10 seconds.

However, the microscopic observation shows that the constant current has about twice as many pitting corrosion as the constant potential method of the present invention.

In both of the pickling materials prepared by the above two methods, the oxide scale does not remain, but the accumulated electric quantity of the constant current method is about 10 times that of the constant potential method. It can be seen that in proportion to this, a large amount of elution is shown.

Although the method of the first embodiment of the present invention has been briefly described above, the present invention is not limited to the above-mentioned embodiment and can be variously modified within the scope of the technical idea of the present invention. It belongs to the technical scope of the present invention.

[0031]

The method of pickling stainless steel according to the present invention has the following effects. 1. Since the electric current decreases as the pickling process progresses, the finish state of the surface is not affected by the setting of the pickling time. 2. Since overpickling can be prevented and the steel base material is not excessively dissolved, the life of the pickling solution can be extended. 3. Since the electrolytic potential is maintained in the secondary passivation region, pitting corrosion is less likely to occur and a good surface finish can always be maintained.

[Brief description of drawings]

FIG. 1 is a schematic diagram of an electrolytic pickling apparatus for carrying out a pickling method for a stainless steel plate according to an embodiment of the present invention.

FIG. 2 is a polarization characteristic diagram of oxidized scale-adhered stainless steel.

FIG. 3 is a diagram showing a change with time of current in a constant potential electrolytic pickling.

FIG. 4 is a scale analysis diagram by ESCA.

FIG. 5 is a diagram of a conventional electrolytic pickling apparatus.

[Explanation of symbols]

 1 stainless steel plate 2 electrode 3 pickling solution 4 power supply 5 ammeter 6 salt bridge 7 reference electrode 8 voltmeter 9 electrolytic pickling device

Front page continuation (72) Inventor Yukio Kanda 4-6-22 Kannon Shinmachi, Nishi-ku, Hiroshima City Mitsubishi Heavy Industries Ltd.Hiroshima Works (72) Inventor Issei Sakurai 4976 Nomura Minamimachi, Shinnanyo, Yamaguchi Prefecture Nisshin Steel Co., Ltd. Company Shunan Steel Works

Claims (1)

[Claims] 1. A stainless steel in which an oxide scale is removed by an applied current while a stainless steel plate is immersed in a pickling solution in an electrolytic pickling apparatus, and is passed between negative and positive electrodes facing each other in the pickling solution. In the pickling method, the nitric acid solution having a nitric acid concentration in the range of 25 to 200 g / l is used as the pickling solution, and the surface potential of the stainless steel in the pickling solution is set to the secondary passivation region for constant potential electrolysis. Therefore, a salt bridge is installed in the vicinity of the stainless steel plate, the potential of the steel plate is measured with a voltmeter through a reference electrode, and the applied voltage of the power source is controlled, so that the pickling method for stainless steel is characterized.