JPS58189391A - Manufacture of electrolytically chromated steel plate - Google Patents

Abstract

PURPOSE:To stably deposit a prescribed amount of Cr oxide hydrate on a metallic Cr layer formed on the surface of a steel plate by cathodic electrolysis, by carrying out anodic electrolysis in a plating bath contg. Cr<6+> to convert part of the metallic Cr layer into Cr oxide hydrate of Cr<3+>. CONSTITUTION:A metallic Cr layer is formed on the surface of a steel plate by cathodic electrolysis using an electrolytic soln. contg. Cr2O3 and NaSCN. The plate having the Cr layer is electrolyzed as an anode using an electrolytic soln. contg. >=2g/l Cr<6+> at >=3A/dm<2> (ASD) current density and ordinary temp. - 80 deg.C. >=1 Kind of salt amlong the alkali salts and NH4 salts of chromic anhydride and dichromic acid is used as a compound contg. Cr<6+>. Part of the metallic Cr layer is converted into Cr oxide hydrate of Cr<3+> by the electrolysis to obtain the desired electrolytically chromated steel plate having a metallic Cr layer as an under layer and a layer of Cr oxide hydrate of Cr<3+> as an upper layer on the surface.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing an electrolytically chromate-treated steel sheet, which allows a predetermined amount of hydrated chromium oxide to be stably deposited.

An electrolytic chromate-treated steel sheet is a plated steel sheet that has two layers on the surface of the steel sheet, the upper layer being hydrated chromium oxide and the lower layer being metallic chromium. Therefore, the amount or quality of hydrated chromium oxide in the upper layer has a large effect on various quality characteristics such as corrosion resistance and paint adhesion, and controlling the amount of chromium oxide deposited on industrial production lines is necessary to obtain stable quality. most important. However, in industrial production lines, intermittent electrolysis is unavoidable due to restrictions on electrode arrangement, and when no current is applied, in the conventional production method, due to the influence of sulfate groups coordinated in hydrated chromium oxide, Hydrated chromium oxide dissolves into the liquid. In addition, the amount dissolved varies depending on the operating conditions, so
It is not easy to electrically control the amount of hydrated chromium oxide that is ultimately deposited.

Conventional electrolytic chromate treated steel sheets are mainly manufactured by the following method.

(1) A method in which a lower layer of metallic chromium and an upper layer of hydrated chromium oxide are simultaneously formed on the surface of a steel sheet by cathodic electrolysis in a chromate treatment solution (-liquid method).

(2) In the chromium plating process, a metal chromium layer is deposited on the surface of a steel sheet, and then hydrated chromium oxide is formed on the metal chromium layer by cathodic electrolysis in another chromate treatment solution (double liquid method).

First, in method (1), in order to simultaneously form metallic chromium and hydrated chromium oxide on the surface of the steel sheet, the presence of sulfate radicals is essential in the electrolyte. Sulfate groups are coordinated in the hydrated chromium oxide.

Due to this sulfate radical, the hydrated chromium oxide formed on the surface of the steel sheet dissolves in the electrolyte when no current is applied. Also (1)
In the case of the above method, it is necessary to secure a certain amount of metallic chromium on the surface of the steel sheet, so there are restrictions on the concentration of sulfate radicals in the electrolyte, and it is necessary to reduce the sulfate radicals in hydrated chromium oxide on the surface of the steel sheet. It is not possible to extremely reduce the concentration of sulfate groups in the electrolyte for this purpose. Therefore, in method (1), the dissolution of hydrated chromium oxide on the steel plate surface is unavoidable when no current is applied, and the amount of metallic chromium precipitated and the amount of hydrated chromium oxide produced on the steel plate surface are reduced. The electrolyte composition and electrolytic conditions must be selected so that the dissolved amount is well balanced.

In addition, in method (2), after forming metallic chromium on the surface of the steel sheet in the chromium plating process, hydrated chromium oxide is formed separately, which reduces sulfate radicals in the electrolyte in the subsequent process. It is possible to reduce the sulfate radicals in hydrated chromium oxide on the steel plate surface. but,
Even if no sulfate radicals are added, industrial chemicals inevitably contain a small amount of sulfate radicals, which coordinate in the hydrated chromium oxide on the surface of the steel sheet. In addition, in the chromium plating process, sulfate radicals are added to the electrolytic solution in order to form metallic chromium on the surface of the steel sheet, and when metallic chromium is formed, hydrated chromium oxide containing sulfate radicals also forms on the surface of the steel sheet. generated. The hydrated chromium oxide on the surface of the steel sheet is dissolved and removed to some extent by non-current immersion, but a trace amount of hydrated chromium oxide remains without being removed. The sulfate groups in the remaining hydrated chromium oxide participate in the electrolytic reaction in the subsequent step, and the sulfate groups are also coordinated in the hydrated chromium oxide generated on the surface of the steel sheet during the second step.

Therefore, in the method (2), dissolution of the hydrated chromium oxide obtained on the surface of the steel sheet when no current is applied into the electrolytic solution is inevitable.

From the above, in methods (1) and (2), it is inevitable that the hydrated chromium oxide obtained on the surface of the steel sheet will dissolve into the electrolyte when no current is applied. This fact indicates that when a steel plate passes through many treatment tanks in an industrial manufacturing line, due to restrictions on the electrode arrangement method, hydrated chromium oxide generated on the surface of the steel plate is absorbed into the chromate treatment solution in the electrolytic tank. It also dissolves in the chromate treatment solution that adheres to the steel plate taken out during driving.

Further, this dissolution rate varies depending on various operating conditions such as the electrolyte temperature, current density, or degree of fatigue of the electrolyte over time, which adversely affects the control of hydrated chromium oxide adhesion. In addition, even if the amount of electricity is constant, if the line speed changes, the amount of hydrated chromium oxide deposited on the steel plate surface will change due to the change in the dissolution time of hydrated chromium oxide, so simple electrical control is not possible. It is not easy to control the amount of hydrated chromium oxide adhered to the surface of the steel plate, and it is necessary to stably adhere a predetermined amount of hydrated chromium oxide to the surface of the steel plate.

Therefore, this invention was made to solve the above problems, and involves cathodic electrolysis treatment of a steel plate to form a metallic chromium layer on its surface (first step), and then Cr"2
．． In a plating bath containing 9/l or more, the current density is 3
Above A/di (ASD), the steel plate is
By electrolytic treatment, a part of the metallic chromium layer formed on the surface of the steel sheet is changed into hydrated chromium oxide of Cr3+, and the metallic chromium layer is formed as a lower layer on the surface of the steel sheet. The present invention is characterized in that it is a method for manufacturing an electrolytically chromate-treated steel sheet, in which a hydrated chromium oxide layer of Cr3+ is formed as an upper layer (second step).

The first feature of the present invention is that by generating hydrated chromium oxide containing no sulfate radicals on the surface of the steel sheet in the second step, the hydrated chromium oxide is not dissolved in the liquid during non-current immersion. There are many things that can be mentioned.

In the second process, a steel plate is used as an anode, so according to past knowledge,
It was known that metallic chromium is oxidized to hexavalent chromium and dissolved in liquid. However, as a result of various studies by the inventors and others by changing various electrolytic conditions, if an appropriate current density (more than 3 ASD) is selected, hydrated chromium oxide, which is mainly composed of trivalent chromium, is produced in steel sheets by anodizing. discovered that it forms on the surface.

Further, the hydrated chromium oxide on the surface of the steel sheet becomes a film that does not contain sulfate groups even if sulfate groups are present in the electrolyte in the second step. As described above, although the mechanism of formation of hydrated chromium oxide that does not contain sulfate groups is not clear, it can be estimated as follows.

That is, in the case of a cathode electrolytic reaction in an electrolytic solution containing hexavalent chromium, hydrated chromium oxide mainly composed of trivalent chromium is generated on the steel sheet surface by the reduction reaction of hexavalent chromium ions in the electrolytic solution. During the reaction, sulfate groups in the electrolyte coordinate into the hydrated chromium oxide.

On the other hand, in the case of an anodic electrolysis reaction, an oxidation reaction occurs from solid metallic chromium already deposited on the surface of the steel sheet.

The second step of the present invention is anodization, so that the metal chromium on the surface of the steel sheet is first oxidized and converted into hydrated chromium oxide mainly consisting of solid trivalent chromium, and then solid water It is estimated that a portion of the chromium oxide is oxidized to hexavalent chromium and dissolved in the electrolyte. Therefore, even if 8042- is present in the electrolytic solution in the second step, there is no room for it to participate in the electrolytic reaction.

In addition, the second feature is that S Q 2- is present in the hydrated chromium oxide that is generated simultaneously with the metallic chromium in the first step on the surface of the steel sheet, and its film inevitably forms on the surface of the steel sheet. However, this sulfate group is completely removed by the second step of anodization. The reason for this is that the hydrated chromium oxide remaining on the surface of the steel sheet in the first step is oxidized by the anodizing reaction in the second step and is dissolved in the electrolyte in the form of hexavalent chromium. At the same time, it is presumed that SO1'- in this hydrated chromium oxide is also dissolved and removed.

This second feature allows for the residual amount of hydrated chromium oxide on the surface of the steel sheet in the first step, and also reduces the amount of 804' in the hydrated chromium oxide remaining on the surface of the steel sheet. Since this is removed regardless of the electrolyte, there are no restrictions on the electrolytic conditions or electrolyte in the first step.

From the above, the hydrated chromium oxide produced on the surface of the steel sheet after the second step of anodic electrolysis does not contain SO4'', so it is carried out in the chromate treatment solution in the electrolytic tank and during running in the industrial production line. Hydrated chromium oxide does not dissolve in the chromate treatment solution that adheres to steel sheets.As mentioned earlier, this is because the amount of hydrated chromium oxide adhered takes into account changes in the operating conditions of industrial production lines. Since it is not necessary, it is easy to control the amount of attached chromium oxide.Also, since the amount of attached hydrated chromium oxide is directly proportional to the current density in the second step,
The amount of hydrated chromium oxide deposited can be controlled by simple current value control, and the amount of deposited is stabilized.

The electrolytic solution used in the first step is a conventional electrolytic chromate electrolytic solution or a chromium plating electrolytic solution. Examples include an electrolytic solution containing chromic acid and rhodan soda, an electrolytic solution containing chromic acid, rhodan soda, and a fluorine compound, and an electrolytic solution containing chromic acid and sulfate. In this electrolyte, a steel plate is used as a cathode for electrolytic treatment,
Generates metallic chromium on its surface.

In the subsequent second step, the electrolyte is converted into hexavalent chromium (Cr”
) as a solution containing 29/1 or more, or a diluted solution of the first step electrolyte containing 2 g/1 or more of hexavalent chromium. For example, examples of compounds containing hexavalent chromium include aqueous solutions containing one or two of chromic anhydride, alkali salts of dichromic acid, and ammonium salts. The reason why the concentration of hexavalent chromium is set to 29/1 or more is because if it is less than 21/1, the bath resistance will increase, the electrical loss will be large, and the production efficiency of hydrated chromium oxide on the steel plate surface will also decrease. In the above electrolytic solution, the electrolytic chromate-treated steel sheet in the first step is used as an anode, and electrolytically treated at a current density of 3 ASD or more and a solution temperature of room temperature or higher and 80° C. or lower. The reason why the N flow density is set to 3 ASD or more is because if it is less than 3 ASD, the production efficiency of hydrated chromium oxide on the surface of the steel sheet decreases. Further, even if the liquid temperature is 80° C. or higher, the production efficiency of hydrated chromium oxide does not change and heat loss increases.

Next, the anodizing effect in the second step of the present invention,
This will be explained using a test example.

[Test Example 1] As the electrolytic chromate treatment liquid in the first step, CrO3:
2509/1. H2SOl: 4. Oi/l, using a cold rolled steel plate as a cathode, liquid temperature 45°C1 current density 3
After treatment with 0 ASD and electrolysis time of 4 sec, 3 sec
A film consisting of 150 m97 m'' of metallic chromium and 5~/m' of hydrated chromium oxide is produced by continuous electric current dipping for 300 m2.Then, as a second step, the electrolyte is
3: 40 g/11 The liquid temperature was 35°C, the chromate-treated steel plate was used as an anode, and the current density was 0. 1
．． 3. 5. 10. 15 and 20 ASD,
The electrolysis time is 005 seconds. The amount of chromium deposited on the resulting sample was measured using fluorescent X-rays.

The first step is to measure the total amount of Cr deposited, and the second step is to immerse it in hot water (100°C) for 10 minutes and measure the amount of soluble chromium (because it is water-soluble, it is (Estimated to be chromium) Then, in the third step, hot alkali (1ON, NaOH, 120°C) was used to remove the hydrated chromium oxide.
) for 10 minutes and measure the amount of adhesion.

By the above procedure, the amount of hydrated chromium oxide on the sample surface and the hexavalent chromium and trivalent chromium therein can be separated and fixed. These results are shown in FIG.

From FIG. 1, it can be seen that hydrated chromium oxide mainly composed of trivalent chromium was generated on the sample surface by the second step of anodization. In addition, from the amount of decrease in the amount of metallic chromium deposited on the sample surface due to anodization, it was found that metallic chromium on the sample surface was changed from trivalent chromium to trivalent chromium, or from metallic chromium to 6%.
Figure 2 shows the results of calculating the respective distribution ratios in the oxidation reaction to valent chromium. From Figure 2, the current density is 3
It was found that when the current density is less than 3 ASD, the distribution ratio from metallic chromium to hexavalent chromium is large on the sample surface, and the generation efficiency of hydrated chromium oxide, which is mainly composed of trivalent chromium, is reduced, and the current density is 3 ASD or more. (More preferably, 5
ASD or higher).

Furthermore, the results of measuring the sulfate groups in the hydrated chromium oxide on the sample surface using fluorescent X-rays are shown in FIG. From Figure 3, the sulfate radicals in the hydrated chromium oxide are reduced by the anodizing treatment in the second step, and the sulfate radicals generated in the first step are removed, and a sample of hydrated chromium oxide containing no sulfate radicals is obtained. It can be seen that it can be generated on the surface (
3 ASD or more, preferably 5 ASD or more). Further, FIG. 4 shows the change in the amount of hydrated chromium oxide on the sample surface after electrolysis in the second step and non-current immersion in the electrolytic solution. As shown in FIG. 4, it can be seen that the hydrated chromium oxide produced on the sample surface in the second step does not dissolve in the electrolyte. Further, even by non-current immersion in the electrolyte solution in the first step, this hydrated chromium oxide does not dissolve in the solution. From the above, the present invention has the above-mentioned two characteristics: "generation of hydrated chromium oxide containing no sulfate groups" and "removal of 8042- from the hydrated chromium oxide remaining in the first step". Since the hydrated chromium oxide formed on the surface does not dissolve by non-current immersion, it has been demonstrated that this is a manufacturing method that allows for easy fine control of adhesion and, therefore, a stable amount of adhesion.

Next, the present invention will be explained in detail.

A comparison was made between the steel sheet treated according to the present invention and a comparative electrolytic chromate treated steel sheet other than the present invention as shown below.

[Example 1] A cold rolled steel plate was used as a cathode, Cry3: 100 g/l,
Na5CN: 0.5 i/l, l'Ja2S
Hydration was achieved by performing electrolytic chromate treatment in a luminophore treatment solution containing iFa: 711/11 at a temperature of 50°C, a current density of 20 ASD, and an electrolytic time of 2 seconds, followed by non-current immersion for 2 seconds. 5 m9/m' of chromium oxide and 150 m9/m' of metallic chromium were deposited on the surface of a cold-rolled steel sheet. Next, this steel plate was used as an anode in an electrolytic chromate treatment solution containing 50 g/l of Crys at a temperature of 40° C. and a current density of 10 A.
The treatment was carried out at SD1 electrolysis time of 0.5 SeC.

[Example 2] The electrolyte for the second step of anodization was CrO3: 50 g/
A cold rolled steel sheet was treated under the same conditions as in Example 1, except that ILH2So4: 0.39/l was contained.

[Comparative Example 1] Using a cold rolled steel plate as a cathode, Cry: 100 g/6
．． Na5CN: 0.59/L Na2SiF6: 7
The hydrated chromium oxide 5 m9/m',
Metallic chromium 1001n9/m' was attached to the surface of a cold-rolled steel sheet. Next, using this steel plate as a cathode, 50 g of CrO3 was added.
/l in an electrolytic chromate treatment solution containing 50%
'C1 Chromate treatment was performed at a current density of 30 ASD and a treatment time of 1 sec.

[Comparative example 2] −4−・　　゛゛　Second process The electrolyte in the cathodic treatment was CrO3: 501/l.

A cold-rolled steel sheet was treated under the same conditions as in Comparative Example 1, except that it contained 0.3 g/l of H2SO4, a current density of 30 ASD, and an electrolytic time of 0.5 sec, except that the techromate treatment was not performed.

[Comparative example 3] Using a cold rolled steel plate as a cathode, CrO3: 1009/l.

Na5CN: 0.5 E/ / i Na25iF
a: 75'/l! as the electrolyte, the liquid temperature is 50℃
, current density 30 ASD, electrolysis time], 5 Sec.

Regarding the above-treated steel sheets, the amount of Cr deposited and the amount of SO42 during oxidation of hydrated comb were measured using fluorescent X-rays, and the results are shown in Table 1.

Table 1 From Table 1, it can be seen that even if the electrolyte in the second step of the steel sheet of the present invention contains sulfate groups as in Example 2, sulfate groups are present in the water-use chromium oxide generated on the steel sheet surface. It can be seen that there is no coordination. Furthermore, as shown in Comparative Examples 1 to 3, the smaller the amount of sulfate radicals in the electrolyte, the lower the amount of sulfate radicals in the hydrated chromium oxide. Even if it is not added, SO42 is present in the hydrated chromium oxide on the steel sheet surface.
- coordinates (it seems that the sulfate group in the hydrated chromium oxide remaining in the first step coordinates). In addition, Examples 1 to 2
For Comparative Examples 1 to 3, the results of measuring the hydrated chromium oxide on the steel plate surface using fluorescent X-rays after 5 sec to 20 sec of non-current immersion in each electrolytic solution after treatment are shown below. It is shown in Figure 5. From FIG. 5, it was found that in all comparative examples, the hydrated chromium oxide on the steel plate surface was reduced by non-current immersion, and that the hydrated chromium oxide was dissolved in the electrolytic solution. On the other hand, in all of the examples of the steel sheets of the present invention, it was found that the hydrated chromium oxide on the surface did not decrease by non-current immersion, and the hydrated chromium oxide did not dissolve in the electrolytic solution. This is probably due to the fact that the steel sheet of the present invention does not contain so4''- in the hydrated chromium oxide, as described above.

As explained above, according to the present invention, since the hydrated chromium oxide on the surface of the steel sheet does not dissolve in the electrolytic solution during non-current immersion, the hydrated chromium oxide on the surface of the steel sheet can be hydrated by simple electrical control on the industrial production line. It is possible to stably control the amount of chromium oxide.

[Brief explanation of drawings]

Figure 1 shows the relationship between hydrated chromium oxide dispersion on the steel plate surface and the second process current density, and Figure 2 shows the distribution of trivalent and hexavalent chromium produced by oxidation from metallic chromium on the steel plate surface. Figure 3 is a diagram showing the relationship between the So content in the hydrated chromium oxide on the steel plate surface and the second process current density, Figure 4 is a diagram showing the relationship between the So content and the second process current density. A diagram showing the relationship between the amount of hydrated chromium oxide on the steel plate surface after non-current immersion and the immersion time in the second step electrolyte solution, Figure 5 shows the relationship between the non-current immersion time and the amount of hydrated chromium oxide on the steel plate surface Applicant Nippon Steel Tube Co., Ltd. Agent Keita Tsutsumi and one other person Figure 1 Figure 2 Obani Kogyoden JL Atsushi 71 (ASD) Figure 3

Claims (1)

[Claims] A steel plate is subjected to cathodic electrolytic treatment to form a metallic chromium layer on its surface, and then the above-mentioned plating is carried out at a current density of 3 A/dm or more in a plating bath containing Cr2jj/l! or more. By subjecting the steel plate to anodic electrolytic treatment, a part of the metallic chromium layer formed on the surface of the steel plate is changed into a hydrated chromium oxide of Crs, and the metal chromium is added to the surface of the steel plate as a lower layer. A method for manufacturing an electrolytically chromate-treated steel sheet, characterized in that a hydrated chromium oxide layer of Cr3+ is formed as an upper layer of the metal chromium layer.