JPH06146088A - Zinc-chromium alloy electroplating method - Google Patents

Abstract

(57) [Summary] [Object] To provide a zinc-chromium alloy electroplating method capable of stably and continuously producing a zinc-chromium alloy electroplated steel sheet having a high chromium content and excellent corrosion resistance for a long period of time. [Constitution] Plating was carried out while supplying Cr ³⁺ ions obtained by electrolytic reduction of hexavalent chromium to an acidic plating bath containing mainly Cr ³⁺ ions and Zn ²⁺ ions and containing hexavalent chromium. .

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a zinc-chromium alloy electroplating method capable of stably and continuously producing a zinc-chromium alloy electroplated steel sheet having a high chromium content and excellent corrosion resistance. is there.

[0002]

2. Description of the Related Art Zinc-chromium alloy electroplated steel sheets have been widely used for automobiles, home appliances, building materials, etc. because of their excellent corrosion resistance. The zinc-chromium alloy electroplating method for the steel sheet is, for example, Japanese Patent Publication No. 58-5.
Various methods are known, as disclosed in JP-B No. 6039 and JP-B No. 61-36078, but all of them have a low eutectoid ratio of chromium of 5% or less.

Therefore, the applicant of the present invention catalytically reacts metallic zinc to supply Zn ²⁺ ions and Cr ³⁺ ions into the plating bath,
By developing a plating method capable of obtaining a zinc-chromium alloy electroplated steel sheet having a high chromium content by maintaining the concentration in the plating bath at a predetermined value, Japanese Patent Laid-Open Publication No. 1-215997 and Japanese Laid-Open Patent Publication No. 1-215998
It was proposed as an issue gazette. However, in these plating methods, Cr ³⁺ ions are oxidized near the Pb-based electrode to increase Cr ⁶⁺ ions, which significantly affects the plating efficiency and reduces the production efficiency, and also allows stable continuous operation for a long period of time. There was a problem that it could not be manufactured.

[0004]

DISCLOSURE OF THE INVENTION The present invention solves the conventional problems as described above, and continuously produces a zinc-chromium alloy electroplated steel sheet having a high chromium content and excellent corrosion resistance for a long period of time. The present invention has been completed for the purpose of providing a zinc-chromium alloy electroplating method that can be manufactured.

[0005]

The zinc-chromium alloy electroplating method of the present invention, which has been made to solve the above-mentioned problems, comprises an acid containing hexavalent chromium mainly composed of Cr ³⁺ ions and Zn ²⁺ ions. It is characterized in that plating is performed while supplying Cr ³⁺ ions obtained by electrolytically reducing hexavalent chromium to a plating bath.

In the present invention, electrolytic reduction is used to
Hexavalent chromium of Cr³⁺Obtained after reducing to ions
Cr³⁺Cr in the plating bath³⁺While supplying ions
It is characterized by the fact that
Cr near the electrode³⁺Cr generated by oxidation of ions⁶⁺Ionic
In addition to suppressing the increase, Cr consumed in the plating process ³⁺
Predetermine the concentration of each ion in the plating solution to compensate for the lack of ions
Maintain the value and perform efficient plating. Incidentally, this
Una Cr³⁺Ions can be supplied, for example, in a plating bath in an electrolytic reduction tank.
And send a part of the plating solution containing hexavalent chromium
And Cr obtained by applying electrolytic reduction treatment³⁺I
Reached by having to turn the ON back on to the plating bath
Is made.

The plating solution to which the present invention is applied is, for example, 10 to 150 g / liter of Zn ²⁺ ions and 10 of Cr ³⁺ ions.
~ 150g / l, ph3-0.5 sulfuric acid bath or /
And acid bath of hydrochloric acid bath is used, but Na ⁺ , K
⁺ , NH ^{4 +} etc. unrelated cations, H ₃ BO ₃ etc. buffers, polyethylene glycol etc. polyoxyalkylene derivatives, SiO ₂ etc. There is no. In addition, Fe, Ni, Co,
Metal ions such as Mn, Cu, Sn, Cd, Pb, and Mo are inevitably mixed in the trace amount plating solution, but a small amount does not affect the method of the present invention.

As the hexavalent chromium solution to be replenished, it is preferable to contain no other cation or anion from the viewpoint of ion balance of the plating solution, and chromic acid and / or chromium chromate can be applied. This chromium chromate is obtained by reacting chromic anhydride with a reducing agent such as an alcohol, an organic acid such as formic acid, or an organic substance such as starch to reduce a part of hexavalent chromium into Cr ³⁺ ions. Depending on the case, salts such as sodium chromate can also be applied in a range that does not change the balance of Na ⁺ ions.

Next, the relationship with the insoluble electrode of the plating cell will be described. Since the insoluble electrode does not wear even if it is continuously plated for a long period of time, the shape of the anode is maintained and the distance from the steel strip that is the object to be plated is kept constant, so that it remains continuous for a long time under stable conditions. It has a number of advantages, such as effective plating treatment and the need for frequent electrode replacement as in the case of soluble anodes, which can improve the production operation rate. However, when an insoluble electrode is used, Cr ³⁺ ions are oxidized near the electrode to generate hexavalent chromium and accumulate in the system, which adversely affects the plating process. Therefore, in zinc-chromium plating, it is necessary to maintain the amount of hexavalent chromium in the system at least below the allowable range of 10 g / liter, and in the present invention, the hexavalent chromium generated by using this insoluble electrode is required. Chromium is electrolytically reduced to maintain the hexavalent chromium concentration in the plating bath at a low level. Also at the same time, got
Cr ³⁺ ions are supplied to the plating bath to replenish Cr ³⁺ ions that have been spent in the plating process and are insufficient, thereby achieving efficient production of plated steel sheets.

The insoluble electrode is a PbO ₂ electrode or a Pb-based electrode such as a Pb alloy electrode obtained by alloying Pb with Sn, Ag, In, Te, Sr, As or the like, Pt or Pt with Ir, Pd, Pt-based electrodes alloyed with Ru, Rh, etc., precious metal-based electrodes such as Ru, Rh, etc. oxide electrodes, amorphous alloy-based electrodes made of Ta and Ru, Rh, Ir, Pd, Ni, Pt, etc. It is applicable, and the Pb-based electrode is the most economically advantageous. However, when a Pb-based electrode is used, a trace amount of Pb is dissolved in the plating bath and, for example, when the Pb concentration in the bath exceeds 3 ppm, the plating adhesion tends to deteriorate. It is preferable to supply the above-mentioned carbonate into the system and remove Pb by utilizing the effect of co-precipitating Pb at the time of precipitation of sulfate to maintain the Pb concentration in the bath below a predetermined value.

Further, when a Pt-based electrode is used as the insoluble electrode, Cr ³⁺ ions are not oxidized in the vicinity of the electrode to generate hexavalent chromium, so that the facility scale of the electrolytic reduction apparatus is Cr ⁶⁺ as Cr source consumed by plating
It only needs to reduce the ions and can be small, but the Pt-based electrode is extremely expensive, and it is a coating electrode and it is necessary to provide a sufficient space between it and the steel strip to avoid damage due to contact with the steel strip. However, there is a drawback that the power consumption for the plating process increases. Therefore, Pb
Economical plating can be achieved by using both system electrodes and Pt system electrodes and considering the oxidation balance in the vicinity of the electrodes while optimizing the facility cost and power consumption cost of the electrolytic reduction apparatus. When both Pt-based electrodes and Pb-based electrodes are used, the number of Pt-based electrodes is 5-80% of the total.
It is preferable to arrange so that If it is less than 5%, the facility cost of the electrolytic reduction apparatus increases, while if it is more than 80%, the power cost rises undesirably.

Next, one embodiment of the plating process of the present invention will be described with reference to the drawings. In the figure, 1 is a plating cell using an insoluble electrode 2, 3 is a steel body to be plated, and one or more plating cells 1 are provided.
4 is a plating cell using the soluble electrode 5, and an arbitrary number is provided as necessary. 6 is the plating cell 1,
4 is a circulation tank that supplies and feeds back the plating solution to and from 4. An electrolytic reduction tank 7 is connected to the circulation tank 6, and in the present invention, a part of the plating solution is sent to the electrolytic reduction tank 7 to form anodes 8a and 8b and cathodes 9a and 9b which are insoluble or soluble electrodes. The hexavalent chromium in the plating solution is reduced to Cr ³⁺ ions by electrolytic reduction by the power supply 10, and the obtained Cr ³⁺ ions are returned to the circulation tank 6,
Plating is performed while supplying Cr ³⁺ ions to the plating bath. The Zn source is basically supplied from the zinc carbonate storage tank 11 directly to the circulation tank 6. However, the Zn source is supplied from the zinc carbonate storage tank 11 to the mixing tank 17, and then the plating solution is supplied from the circulation tank 6 to the mixing tank 17. It is also possible to introduce a part of the above to dissolve and return it to the circulation tank 6, or to return it to the circulation tank 6 via the calming tank 15. Similarly, the Cr source is basically supplied from the storage tank 12 directly into the circulation tank 6. However, the Cr source is supplied from the storage tank 12 to the mixing tank 17, and then the mixing tank 17 is supplied with the plating solution from the circulation tank 6. It is also possible to introduce a part and dissolve it and return it to the circulation tank 6, or return it to the circulation tank 6 via the calming tank 15. In addition, additives, sulfuric acid, etc. are added from the storage tank 13 as needed in the same manner as the replenishment of the Zn source. On the other hand, Pb eluted from the Pb-based electrode
In some cases, the carbonate supplied from the storage tank 14 and a part of the plating solution introduced from the circulation tank 6 are mixed with Ba sulfate or Sr sulfate.
It precipitates as a Pb coprecipitate and is removed from the system by the filter 16. As described above, by applying the present invention, the plating temperature is 30 to 70 ° C. and the plating current density is 50.
~300A / dm ^2, a relative flow rate of 30 to 200 m / min condition,
A highly corrosion resistant zinc-chromium alloy-plated steel sheet having a high Cr content of 5 to 40% is stable and can be continuously manufactured for a long period of time.

[0013]

[Example] Under the plating bath conditions and plating conditions shown in Table 1, a cold-rolled steel plate was used as a cathode to continuously perform plating up to 10,000 coulombs / liter to adjust the ions consumed by the replenishment reagent. In addition, hexavalent chromium was electrolytically reduced under the conditions shown in the table. This cycle is repeated up to 200,000 coulombs / liter, the plating composition after 200,000 coulombs / liter is compared with the initial one, and the Cr ⁶⁺ ion concentration (g / liter) in the plating bath is measured to perform plating. It was evaluated whether the bath could maintain the initial state. The evaporation of water in the plating solution and slight fluctuations in the components of the plating bath due to dragout and the like were adjusted by reagents. Table 2 shows, as a comparative example, the results of evaluation in the case where the plating bath conditions, the plating conditions and the like are the same as those described above, but hexavalent chromium is not electrolytically reduced.
As can be seen from the comparison with this comparative example, in the present invention, stable and high-quality plating treatment can be continuously performed for a long period of time while reliably suppressing the increase of Cr ⁶⁺ ion concentration in the plating bath. It could be confirmed.

[0014]

[Table 1]

[0015]

[Table 2]

[0016]

As is clear from the above description, the present invention is capable of stably and continuously producing a zinc-chromium alloy electroplated steel sheet having a high chromium content and excellent corrosion resistance. is there. Therefore, the present invention is a zinc-chromium alloy electroplating method that eliminates the conventional problems, and has a great contribution to the industrial development.

[Brief description of drawings]

FIG. 1 is a block diagram showing an example of a plating process of the present invention.

[Explanation of symbols]

 1 Plating cell 2 Insoluble electrode 3 Steel body 4 Plating cell 5 Soluble electrode 6 Circulation tank 7 Electrolytic reduction tank

Claims (4)

[Claims] 1. Plating while supplying Cr ³⁺ ions obtained by electrolytic reduction of hexavalent chromium to an acidic plating bath mainly containing Cr ³⁺ ions and Zn ²⁺ ions and containing hexavalent chromium. And a zinc-chromium alloy electroplating method. 2. The zinc-chromium alloy electroplating method according to claim 1, wherein the hexavalent chromium liquid supplied to the plating bath is chromic acid and / or chromium chromate. 3. A Pb-based electrode is used as an insoluble anode,
The zinc-chromium alloy electroplating method according to claim 1 or 2, wherein a carbonate of Sr and / or Ba is supplied into the system. 4. The zinc-chromium alloy electroplating method according to claim 1, 2 or 3, wherein both the Pb-based electrode and the Pt-based electrode are used as the anode.