JPH06146091A - Zinc-chromium alloy electroplating method - Google Patents

Abstract

PURPOSE:To improve corrosion resistance by executing plating while supplying the Cr<3+> and Zn<2+> obtd. by electrolytically reducing Cr<6+> with Pt electrodes or by bringing metal Zn and plating liquid into reaction with a plating bath contg. the Cr<3+>, the Zn<2+> and the Cr<6+>. CONSTITUTION:The acidic plating bath cotg. the Cr<3+> ions and the Zn<2+> ions and the hexavalent chromium (chromic acid and/or chromium chromate) is housed in a tank 6. An electrolytic reduction cell 7 is connected to this tank 6 where the hexavalent chromium is electrolytically reduced to the Cr<3+>. This Cr<3+> is returned to the tank 6. The Pt electrodes are used and DC current is intermittently supplied thereto or while the polarities of the electrodes 8, 9 are inverted, these electrodes are energized. The Cr<3+> ions and Zn<2+> obtd. by reaction of the hexavalent chromium in the metal Zn and the plating liquid in an electrolytic cell 20 are returned into the tank 6. The plating bath in the tank 6 is supplied to a cell 1 and the steel plate 3 is subjected to Zn-Cr alloy plating by impressing current between the insoluble anode 2 and the steel plate 3.

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. While supplying Cr ³⁺ ions obtained by electrolytic reduction of hexavalent chromium and Cr ³⁺ ions and Zn ²⁺ ions obtained by contacting a plating solution with metal Zn, the plating bath is plated. At the time of the electrolytic reduction, at least one of the anode and the cathode of the electrode for electrolytic reduction is used as a Pt-based electrode to apply a direct current.

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, by connecting a dissolution bath to the plating bath.
Bond and send part of plating solution containing hexavalent chromium
Cr obtained by applying electrolytic reduction treatment³⁺Io
Achieved by returning the resin to the plating bath again
To be done.

Further, the present invention is characterized in that at the time of the above-mentioned electrolytic reduction, at least one of the anode and the cathode of the electrode for electrolytic reduction is used as a Pt-based electrode to apply a direct current. That is, in the case of a Pt-based electrode, unlike the Pb-based electrode, oxidation near the electrode is suppressed, so that Cr ³⁺ ions are
Preventing the phenomenon of oxidation to Cr ⁶⁺ ions
Prevents the increase of ⁶⁺ ions as ^much as possible. As such a Pt-based electrode, Pt or Pt alloyed with Ir, Pd, Ru, Rh, or the like can be used. Further, it is preferable that a direct current be intermittently applied during the electrolytic reduction because an appropriate oxidation balance near the electrodes is maintained and the increase of Cr ⁶⁺ ions is suppressed. Furthermore, when both the anode and cathode of the electrode for electrolytic reduction are Pt-based electrodes and the current is applied while reversing the polarities of the electrodes, they are deposited at the cathode
This is preferable because Zn is dissolved and removed when it becomes an anode due to the inversion of the electrode, and the electrolytic reduction efficiency is improved.

Further, in the present invention, in addition to the above-mentioned supply of Cr ³⁺ ions, plating is performed while also supplying Cr ³⁺ ions and Zn ²⁺ ions obtained by the catalytic reaction of metal Zn with a plating solution. This is characterized by promoting the generation of Cr ³⁺ ions and performing zinc-chromium electroplating with a high Cr content. That is, since metal Zn dissolves in an acidic plating bath by a competitive reaction between dissolution by H ⁺ ions and dissolution by hexavalent chromium, in order to perform zinc-chromium electroplating with a particularly high Cr content, the latter reaction It is necessary to increase the contribution rate. Since this reaction rate is the rate-determining diffusion of hexavalent chromium to the surface of metallic Zn, it is preferable to improve the contact efficiency between metallic Zn and hexavalent chromium.

As a dissolution tank having such a high contact efficiency, a plating solution and a hexavalent chromium aqueous solution filled with metal Zn in the tank and reacted with the metal Zn by a liquid flow are circulated through a circulation path. It is preferable to use a continuous type having a fluidized tank, a filling tank, a tower mill or the like. In particular, in order to increase the dissolution ability and the contact ability, it is preferable to be able to fix the metal Zn in the tank so that the metal Zn does not move along with the liquid flow or float by the H ₂ bubbles, whereby the relative flow velocity can be increased. It can be ensured and contact efficiency can be improved. As the liquid flow rate, when metal Zn is fixed in the tank,
The relative velocity is preferably 5 cm / sec or more.

The form of metal Zn is plate-like, granular,
A powdery material can be used, but a granular material having a diameter of 0.1 to 10 mm is preferable in order to secure a predetermined relative flow velocity and contact area. If the particle size is less than 0.1 mm, it becomes difficult to secure the relative flow velocity throughout the melting tank, while the particle size is 10 mm.
If it exceeds, the surface area becomes small and the dissolution capacity per unit volume decreases, which is not preferable.

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 liquid to be replenished, it is preferable to contain no other cation or anion from the viewpoint of ion balance of the plating liquid, 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 to achieve efficient production of plated steel sheets.

The insoluble electrode may be 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.

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. 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. At this time, since a direct current is applied by using at least one of the anode and the cathode of the electrode for electrolytic reduction as a Pt-based electrode, it is possible to suppress the oxidation of Cr ³⁺ ions to Cr ⁶⁺ ions in the vicinity of the Pt-based electrode. ^Therefore , the increase of Cr ⁶⁺ ions is prevented as much as possible in the entire system, and the electrolytic reduction is efficiently performed. Zn
As for the supply of the source, a part of the plating solution is introduced from the circulation tank 6 into the dissolution tank 20 filled with the metal Zn particles charged from the storage tank 19 to dissolve the plating solution, and the solution obtained by the contact reaction is settled in the calming tank 15.
It is carried out by returning to the circulation tank 6 again via. At the same time, hexavalent chromium is also reduced to Cr ³⁺ ions,
Cr ³⁺ ions will be replenished. If necessary, the zinc carbonate storage tank 11 is charged into the mixing tank 17, and a part of the plating solution is introduced into the mixing tank 17 from the circulation tank 6 to be melted and returned to the circulation tank 6, or the calming tank 15 is further added.
It can also be returned to the circulation tank 6 via. 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 a part of the plating solution is supplied from the circulation tank 6 to the mixing tank 17. It may be introduced and dissolved and returned to the circulation tank 6, or may be further returned 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 is precipitated as a co-precipitate of Ba sulfate or Sr sulfate and Pb by the carbonate supplied from the storage tank 14 and a part of the plating solution introduced from the circulation tank 6 in the settling tank 15. Then, it is removed from the system by the filter 16.

FIG. 2 shows an example of a melting tank in which the flow of metal Zn is suppressed. Metal Zn particles 31 are charged from the hopper 19 and filled in the melting tank 20, but a porous plate 32 is provided. It is sandwiched between the pressure plates 33 and does not flow. When the plating solution is introduced under pressure from the supply pipe 34, it reacts with the metal Zn particles 31 in the melting tank 20, and the discharge port 3
It is configured to be guided to the sedation tank 15 of FIG. The pressure plate 33 is a driving device 3 when metal Zn is charged.
6 is pulled up through the guide bar 37 and the lot 38 by the operation of 6, and is lowered after charging and comes into contact with the tops of the metal Zn particles 31, the motors 39 are rotated to smooth the tops and then pressurize. This makes it possible to make the filling height of the metal Zn uniform and apply pressure. As described above, by applying the present invention, the plating temperature is 30 to 70 ° C., the plating current density is 50 to 300 A / dm ² , and the relative flow rate is 30 to 20.
Under the condition of 0 m / min, a highly corrosion resistant zinc-chromium alloy-plated steel sheet having a high Cr content of 5 to 40% can be stably manufactured for a long period of time.

[0018]

[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 listed in the table and catalytic reaction with metallic zinc was performed. This cycle is repeated up to 200,000 coulombs per liter, and the actual reduction time during the entire reduction operation when the composition in the plating bath is maintained in the initial state (the rest time for dissolution of Zn deposited on the electrode and The ratio of the time (excluding the time for switching off due to electrode wear) was evaluated as the operating efficiency. As a measure of the degree of freedom of operation, the limiting current density that does not generate Cr ⁶⁺ ions was measured and evaluated. 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 evaluations under the same plating bath conditions, the same plating conditions, etc. but without catalytic reduction, and the conditions for electrolytic reduction of hexavalent chromium were changed. However, as can be seen from the comparison with this comparative example, the present invention has extremely high operating efficiency and is stable and high for a long period of time while surely suppressing the increase of Cr ⁶⁺ ion concentration in the plating bath. It was confirmed that quality plating treatment can be performed continuously. In addition, in the present invention, there is no particular limit to the current density, so that it was confirmed that the degree of freedom in operation is large and the operation is easy.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

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.

FIG. 2 is a cutaway side view showing an example of a melting tank according to the present invention.

[Explanation of symbols]

 1 Plating Cell 2 Insoluble Electrode 3 Steel 4 Plating Cell 5 Soluble Electrode 6 Circulation Tank 7 Electrolytic Reduction Tank 20 Melting Tank

Claims (7)

[Claims] 1. A plating solution for Cr ³⁺ ions obtained by electrolytic reduction of hexavalent chromium and a metal Zn in an acidic plating bath mainly containing Cr ³⁺ ions and Zn ²⁺ ions and containing hexavalent chromium. While plating while supplying Cr ³⁺ ions and Zn ²⁺ ions obtained by the catalytic reaction, at the time of the electrolytic reduction at least one of the anode and cathode of the electrode for electrolytic reduction is a Pt-based electrode and a direct current is applied. A zinc-chromium alloy electroplating method comprising: 2. The zinc-chromium alloy electroplating method according to claim 1, wherein a direct current is intermittently applied during the electrolytic reduction. 3. The zinc-chromium alloy electroplating according to claim 1 or 2, wherein both the anode and the cathode of the electrode for electrolytic reduction are Pt-based electrodes and current is applied while reversing the polarities of the electrodes. Method. 4. The zinc-chromium alloy electroplating according to claim 1 or 2 or 3, wherein the hexavalent chromium liquid to be supplied to the plating bath is chromic acid and / or chromium chromate. Method. 5. A Pb-based electrode is used as an insoluble anode of a plating cell, and a carbonate of Sr and / or Ba is supplied into the system, claim 1 or claim 2 or claim 3. Alternatively, the zinc-chromium alloy electroplating method according to claim 4. 6. The zinc according to claim 1, wherein both the Pb-based electrode and the Pt-based electrode are used for the anode of the plating cell. -Chromium alloy electroplating method. 7. A plating bath and an aqueous solution containing hexavalent chromium are pressed into a dissolution tank filled with granular metal Zn having a particle size of 0.1 to 10 mm to suppress the flow of metal Zn along with the liquid flow. 2. The solution is dissolved while being circulated and reused.
Alternatively, the zinc-chromium alloy electroplating method according to claim 2 or claim 3 or claim 4 or claim 5 or claim 6.