JP2628331B2 - Zinc-chrome electroplating method - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a method for producing a Zn-Cr electroplated steel sheet having excellent corrosion resistance used for automobiles, home appliances, building materials, etc., and particularly to industrially stable continuous production. The present invention relates to a method for maintaining and replenishing a metal ion concentration of a plating solution for performing the method.

(Prior art) Zn-Cr containing Cr in Zn or Zn-based alloy plating
As a system electroplating method, for example, JP-B-61-36078,
JP-B-58-56039, JP-A-61-270398, and the like are known, but these are very small amounts of eutectoids of Cr up to 0.005 to 5%. Therefore, although there is no particular disclosure regarding the replenishment of Cr ³⁺ ions in the plating solution, it is not impossible to cope with the known method of adding a chemical such as chromium sulfate or chromium chloride.

(Problems to be Solved by the Invention) The present inventors have set out to improve the corrosion resistance remarkably.
A method for producing a Zn-Cr-based electroplated steel sheet having a higher Cr content was studied, and the aim was obtained. However, in order to industrially and stably produce over a long period of time, there is a need for a technique capable of replenishing Zn ²⁺ ions and Cr ³⁺ ions consumed in plating and maintaining the ions in the plating bath at a predetermined concentration. .

When Cr ³⁺ ions are added as salts such as chromium sulfate and chromium chloride, the concentration of anions such as SO ₄ ^2- ions and Cl ⁻ ions in the plating bath increases, and plating is hindered. This cannot be added to the plating solution as an oxide such as Cr ₂ O ₃ or metal Cr. This is because these do not dissolve even in an acidic bath having a pH of 1 or less.

It is conceivable to add it to the plating bath as a hydroxide such as Cr (OH) ₃ or a carbonate such as Cr ₂ (CO ₃ ) _2, but only partially dissolves in the acidic bath, resulting in an insoluble residue. This is because they are unstable in air and easily converted to oxides. For example, even if the formation of oxides can be suppressed, it is not realistic because the raw material cost becomes unduly high.

Although it is possible to dissolve electrically using metal Cr as the anode, the excess Cr dissolves in the plating bath with respect to the amount of Cr consumed for plating at the cathode, so the concentration of Cr ³⁺ ions is reduced. It cannot be kept constant.

As described above, there has been no efficient method of replenishing a plating source with a Cr source that can be industrially applied in Cr plating.

The present invention has been made to advantageously solve such a problem.

(Means for Solving the Problems) That is, the present invention provides an aqueous solution containing metal Zn and hexavalent Cr.
²⁺ ions and Cr ³⁺ ions contacted reaction acidified plating bath mainly zinc characterized by plating while supplying the Zn ²⁺ ions and Cr ³⁺ ions to the acidic plating bath - chromium electric It is a plating method, and Pb is used as an insoluble anode.
Using a system electrode, supply the carbonate of Sr or Ba into the system, and replenish a part of the Cr source with chromium sulfate.Fill with granular metal Zn with a particle size of 10 to 0.1 mm. A plating solution and an aqueous solution containing hexavalent Cr are press-fitted into the dissolving tank, and dissolved while suppressing the metal Zn from flowing along with the liquid flow, and the aqueous solution containing hexavalent Cr is dissolved. Chromic acid and / or chromium chromate.

(Action) Metal Zn dissolves in the acidic plating bath while generating H ₂ gas. However, when hexavalent Cr, for example, an aqueous solution of chromic acid or the like is added to this system, hexavalent Cr promotes the dissolution of metal Zn and converts itself to Cr ³⁺ ions. When metal Zn and hexavalent Cr are sufficiently contacted, Cr ³⁺ ions are generated with almost no unreacted hexavalent Cr remaining.

Metallic Zn is dissolved by a competitive reaction between dissolution by H ⁺ ions and dissolution by hexavalent Cr, so that the Cr content is particularly high.
When performing Zn-Cr electroplating, it is important to increase the contribution of the latter reaction. The latter reaction rate is hexavalent Cr
It is governed by the diffusion of ions to the metal Zn surface. Therefore, it is preferable to use a melting tank having high contact efficiency between metal Zn and hexavalent Cr.

As a melting tank, it has a hopper for charging metal Zn,
The tank is filled with metal Zn, and a plating solution and a hexavalent Cr aqueous solution flow into the tank to measure contact with the metal Zn by the liquid flow,
A reactor having a circulation system flowing out of the tank after the reaction is preferable since continuous ion supply can be performed. A batch-type dissolving tank is also applicable, but in this case, it is preferable to add a device such as shaking, stirring, or gas blowing to increase the contact efficiency. As a continuous dissolution tank, a fluidized tank,
A filling tank, a tower mill, etc. can be applied.

In particular, the dissolving method having high dissolving ability and high contact efficiency is such that, when metal Zn is filled and a plating solution and an aqueous solution of hexavalent Cr are allowed to flow in and pass through, the metal Zn moves along with the solution flow and adheres to H ₂ bubbles. Metal Zn so that it does not surface
It is preferable that a perforated plate is provided above and below the packed bed so that the fixed plate can be fixed. By doing so, the relative flow velocity can be secured, and the contact efficiency can be increased.

The liquid flow rate is preferably 0.5 cm / sec or more in the dissolving tank. More preferably, the relative velocity is 5 cm / sec or more in a filling tank in which metal Zn does not move with the liquid flow.

As the form of metal Zn, plate-like, granular, and powdery forms can be used, but in order to secure both the relative flow rate and increase the surface area, a granular form with a diameter of 10 mm to 0.1 mm is optimal. is there. If the diameter exceeds 10 mm, the surface area becomes small, and the dissolving capacity per unit volume decreases, which is not preferable. Diameter
If it is less than 0.1 mm, it becomes difficult to secure a relative flow velocity over the entire volume of the tank.

Examples of the plating solution to which the present invention is applied include an acid bath having a Zn ²⁺ ion of 10 to 150 g / l, a Cr ³⁺ ion of 10 to 150 g / l, and a pH of 3 to 0.5. Both chloride baths are applicable, and Na ⁺ , K
⁺ , NH ₄ ⁺ , Mg ^2+, etc., and has no effect, and contains buffering agents such as H ₃ BO ₃ or special additives such as polyoxyalkylene derivatives such as polyethylene glycol and colloidal fine particles such as SiO ₂ Doing so has no essential effect. Fe, Ni, Co, Mn,
A small amount of metal ions such as Cu, Sn, Cd, Pb, and Mo are inevitably mixed into the plating solution, but the method of the present invention can be applied to a small amount.

If the hexavalent Cr ion is less than 10 g / l, it does not substantially affect the Zn-Cr plating and is acceptable. The temperature of the plating solution passed through the melting tank can be applied at room temperature to 80 ° C., and it is more preferable to operate within the same range as the plating temperature of 30 to 70 ° C.

The hexavalent Cr solution to be replenished preferably does not contain other cations or anions due to the ion balance of the plating solution, and chromate, dichromate and / or chromate can be used. Chromium chromate is obtained by reacting chromic anhydride with organic acids such as alcohols and formic acid or organic substances such as starch as a reducing agent to reduce a part of hexavalent Cr to Cr ³⁺ . In this way, a product containing no other ions and having unreacted organic substances in traces can be obtained. In some cases, salts such as sodium chromate can be applied as long as the balance of Na ⁺ ions is not changed.

Next, the relationship between the plating cell and the insoluble anode will be described.

The insoluble anode does not wear out even if it is plated for a long period of time, so the shape of the anode is maintained, and the distance between the electrodes and the steel plate to be plated can be kept constant. Can operate under certain plating conditions. In addition, since the distance between the electrodes can be reduced, the voltage loss due to the liquid resistance can be reduced as much as possible, and furthermore, the economical advantage that the production operation rate can be improved without requiring frequent anode replacement like a soluble anode can be obtained. large.

However, when an insoluble anode is used, the current is transmitted through O ₂ gas generation by electrolysis of water or electrolytic oxidation reaction of a plating solution component. In the case of a Zn ²⁺ -Cr ³⁺ plating bath, Cr ³⁺ ions are oxidized to generate hexavalent Cr.
Therefore, hexavalent Cr is accumulated in the system, and it is necessary to reduce it.

According to the present invention, by supplying metal Zn into the system, hexavalent Cr generated from the insoluble anode is also reduced, and 6% in the plating bath is reduced.
It also aims to keep the valence Cr concentration low. By doing so, an insoluble anode can be applied to the Zn-Cr plating, and the above advantages can be enjoyed.

In this case, it is more preferable to use an insoluble anode for all the plating cells, but in some cases, a soluble electrode such as a Cr electrode may be applied to some of the cells. Contribution to hexavalent Cr reduction during dissolution of metallic Zn and Cr ³⁺ at insoluble anode
The anode may be selected in consideration of the current sharing ratio with respect to oxidation so that the redox balance of Cr can be obtained.

As an insoluble anode, Sn, Ag, In, Te, Tl, Sr, As, S
b, Ca alloyed Pb alloy electrode or PbO ₂ electrode
Pb-based electrodes, Pt-based electrodes in which Pt or Pt is alloyed with Ir, Pd, Ru, Rh, etc., noble metal-based electrodes such as Rh, Ru oxide electrodes, Ta
And an amorphous alloy-based electrode made of Ru, Rh, Pd, Ir, Pt, Ni, or the like.

Of these, Pb-based electrodes are the most economically advantageous and are used exclusively in sulfate-based plating baths, but trace amounts of
Pb dissolves. In Zn-Cr plating, the Pb concentration in the bath is
If it exceeds 3 ppm, the plating adhesion tends to deteriorate, which is not preferable. Therefore, Sb and / or Ba carbonate is put into the bath, and Pb can be removed by utilizing the action of co-precipitating Pb when precipitating as sulfate. In Zn-Cr plating, the action of Sr and / or Ba carbonate can be effectively used as sulfate ion removal in addition to Pb removal. That is, when Zn ²⁺ ions and Cr ³⁺ ions are consumed by plating, sulfate ions as counter ions accumulate, which hinders long-term continuous plating. The metal ions corresponding to the excess sulfate ions are replenished with the metal Zn and hexavalent chromium, but precipitate as Sr and / or Ba sulfate and the sulfate ions are removed out of the system.
The source can be partially replenished as a sulfate such as Cr ₂ (SO ₄ ) ₃ or Cr (OH) (SO ₄ ), and 6
It is possible to reduce the load related to the valence Cr reduction.

Next, an example of a plating process according to the present invention will be described with reference to FIG.

1 is a plating cell using an insoluble anode, 2 is an insoluble anode, 3 is a steel strip to be plated, and there are a plurality of plating cells 1.

4 is a plating cell using a soluble anode, 5 is a soluble anode, and there are zero or more plating cells 4.

Transfer the plating solution from the circulation tank 6 to the plating cell 1 or 1
And 4, the plating solution is plated by passing an electric current, and the plating solution is returned to the circulation tank 6 and circulated again. On the other hand, metal
Zn is charged and charged from a hopper 8, a plating solution is supplied from a circulation tank 6, and hexavalent Cr is supplied to and mixed with the plating solution from a storage tank 9 and passed through a melting tank 7. Here, hexavalent Cr is metal Zn
To convert it into Cr ³⁺ ions, while simultaneously dissolving Zn
Become ²⁺ ions. The plating solution, which has passed through the dissolving tank 7 and has been ion-supplied, is guided to the settling tank 10. From hopper 11
The Sr and / or Ba carbonate powder is put into the settling tank 10 and becomes sulfate and precipitates. The precipitate is removed out of the system by the filter 12. The plating solution returns from the settling tank 10 to the circulation tank 6 and is used for circulation. The plating solution in the circulation tank 6 is supplied from the dissolution tank 7 in an amount corresponding to the consumption of metal ions in one plating cell system, and is maintained at a predetermined concentration.

FIG. 2 shows an embodiment of a melting tank in which a plating solution and hexavalent Cr are press-fitted and metal Zn is prevented from flowing with the solution flow.

Metal Zn particles 13 are charged from the hopper 8 and filled into the melting tank 7. Metal Zn particles 13 are sandwiched between a perforated plate 14 and a pressing plate 15,
It does not flow in the liquid flow. Plating solution supply pipe
The pressure is introduced from 16, reacts with the metal Zn particles 13 in the tank 7, is discharged from the discharge port 17, and is guided to the settling tank 10 shown in FIG. The pressurizing plate 15 is pulled up through the guide bar 19 and the lot 20 by the operation of the driving device 18 when the metal Zn particles are charged, descends after the charging, contacts the top of the metal Zn particles 13, and is rotated by the motor 21, Then pressurize. By doing so, the filling height of the metal Zn particles 13 can be made uniform and pressurized.

As described above, by applying the present invention, a plating temperature of 30 to 70 ° C., a current density of 50 A / dm ² or more, and a relative flow rate of 30 to 2
Under the condition of 00 m / min, high corrosion resistance Zn-
Cr-plated steel sheets can be manufactured stably and continuously for a long time.

Example 1 Zn ²⁺ = 107 g / l, Cr ³⁺ = 40 g / l, Na ⁺ = 14 g / l, polyethylene glycol (molecular weight: 1500) = 2 g / l, pH = 1.3 using sulfuric acid as an anion From the bath, the Pb-4% Sn electrode was used as the anode,
Continuous plating up to 10,000 clones / l was performed under the conditions of a current density of 150 A / dm ² , a flow rate of 60 m / min, and a bath temperature of 50 ° C. The composition of the plating layer on the steel sheet was Cr 15% balance Zn. 10,000 clones / l
Cr ^{6+ in} the plating bath after loading increased to 0.57 g / l.

Next, 1.8 g of metal Zn powder per plating bath and a CrO ₃ aqueous solution equivalent to 0.3 g as Cr ⁶⁺ were added, and sufficiently stirred at 50 ° C. to dissolve. The Cr ⁶⁺ after dissolution was 0.1 g / l or less, and the Zn ²⁺ and Cr ₃₊ ion concentrations recovered to almost the initial concentrations.

After adjusting the liquid volume, continuous plating was performed again to 10,000 clones / l under the same conditions as above, ion replenishment was performed under the same conditions as above, and such a cycle was repeated to reduce the load to 600,000 clones / l. I took it. During this time, slight fluctuations in the added components due to drag-out and the like were corrected with the reagent.

The composition of the plating layer up to 600,000 clones / l is almost Cr15
%, The balance was Zn, and the plating appearance was good with no change.

The Zn ²⁺ ion and Cr ₃₊ ion concentrations after the recovery of ion supplementation were almost the same as those at the initial ^stage , and the Cr ⁶⁺ ion was 0.1 g / l or less, and no tendency to increase the concentration was observed.

(Example 2) Zn ²⁺ = 84 g / l, Cr ³⁺ = 49 g / l, Na ⁺ = 14 g / l, polyethylene glycol (molecular weight: 1500) = 2 g / l, pH = 1.2 using sulfuric acid as an anion Current density 1
Under the conditions of 00A / dm ² , flow rate of 60m / min, and bath temperature of 50 ° C, 10,000 clones /
Continuous plating up to l was performed. The composition of the plating layer on the steel sheet was 15% Cr and the balance was Zn. The Cr ⁶⁺ concentration in the plating bath after loading 10,000 clones / l was 0.1 g / l or less.

Next, 1.8 g of metal Zn powder per plating bath and an aqueous solution of chromium chromate equivalent to 0.3 g as Cr content were added to the solution, and 50
The mixture was sufficiently stirred at ℃ to dissolve. The chromium chromate aqueous solution is obtained by partially reducing chromic anhydride using starch, and 30% of the total Cr is Cr ³⁺ and the balance is Cr ⁶⁺ . The Cr ⁶⁺ after the dissolution reaction was 0.1 g / l or less, and the Zn ²⁺ and Cr ³⁺ ion concentrations recovered to almost the initial concentrations.

After adjusting the liquid volume, continuous plating was performed again to 10,000 clones / l under the same conditions as above, ion replenishment was performed under the same conditions as above, and such a cycle was repeated to reduce the load to 600,000 clones / l. I took it. During this time, slight fluctuations in the added components due to drag-out and the like were corrected with the reagent.

The composition of the plating layer up to 600,000 clones / l is almost Cr15
%, The balance was Zn, and the plating appearance was good with no change.

The Zn ²⁺ ion and Cr ₃₊ ion concentrations after the recovery of ion supplementation were almost the same as those at the initial ^stage , and the Cr ⁶⁺ ion was 0.1 g / l or less, and no tendency to increase the concentration was observed.

(Example 3) Zn ²⁺ = 84 g / l, Cr ³⁺ = 49 g / l, Na ⁺ = 14 g / l, polyethylene glycol (molecular weight: 1500) = 2 g / l, pH = 1.2 using sulfuric acid as an anion Pb-1% Ag electrode from the bath
Continuous plating up to 10,000 clones / l was performed under the conditions of a current density of 150 A / dm ² , a flow rate of 60 m / min, and a bath temperature of 50 ° C. The composition of the plating layer on the steel sheet was 15% Cr and the balance was Zn. 10,000 clones
Cr ^{6+ in} the plating bath after / l loading increased to 0.76 g / l. The Pb concentration in the bath was 14 ppm.

Next, 1.8 g of metal Zn powder was added per plating bath, and the mixture was sufficiently stirred at 50 ° C. to dissolve. Furthermore, as Cr content
Add an aqueous solution of chromium sulfate equivalent to 0.3 g, then add SrCO
1.6 g of ₃ powders were charged and dissolved by stirring. Cr ⁶⁺ after bath adjustment was 0.1 g / l or less, and Pb was 1 ppm or less. Zn
^{The 2+} and Cr ³⁺ ion concentrations returned to near-initial concentrations.

Adjust the liquid volume and repeat continuous plating under the same conditions as above.
Perform ion replenishment operation under the same conditions as above, and repeat this cycle to 600,000 clones / l.
Load up to / l. During this time, slight fluctuations in the added components due to drag-out and the like were corrected with the reagent.

The composition of the plating layer up to 600,000 clones / l is almost Cr15
%, The balance was Zn, and the plating appearance was good with no change. Zn ²⁺ ions and Cr ³⁺ after recovery from ion supplementation
The ion concentration was almost the same as in the initial stage, and the content of Cr ⁶⁺ was 0.1 g / l or less, and no tendency to increase the concentration was observed.

(Example 4) Metal Zn having a particle diameter of 2 mm was placed in a packed tank having a tower diameter of 500 mm shown in FIG.
330 kg of grains were filled to a height of about 300 mm.

Chromic acid for replenishment and the plating solution were previously mixed upstream and supplied to the filling tank at a pump pressure of 7 kg / cm ² and a flow rate of 3 m ² / min. In this case, the superficial velocity corresponds to 25 cm / sec. The composition of the feed solution is Zn ²⁺ = 80 g / l, Cr ³⁺ = 40 g / l, Na ⁺ = 14 g / l, and chromic acid is C
As r ⁶⁺ 0.2g / l, polyethylene glycol (molecular weight 150
0) = 1.5 g / l and sulfuric acid as anion, pH = 1.0.

After one hour of continuous operation, the dissolved amount of metal Zn was 36 kg, and the chromic acid concentration at the outlet was 0.1 g / l or less as Cr ⁶⁺ . About 90% of the dissolved Zn contributed to the reduction of hexavalent Cr.

(Effects of the Invention) As described above, the electroplating method according to the present invention enables continuous production of a Zn-Cr electroplated steel sheet having excellent corrosion resistance and high Cr content with stable quality for a long time.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an example of a plating process of the present invention, and FIG. 2 is a side sectional view showing an example of a melting tank. 1,4 ... Plating cell, 2 ... Insoluble anode, 3 ... Steel strip, 5 ...
Soluble anode, 6… circulation tank, 7… dissolution tank, 8,11… hopper, 9… storage tank, 10… settling tank, 12… filter, 13… metal
Zn particle, 14 ... perforated plate, 15 ... press plate, 17 ... outlet, 18 ... drive device for press plate.

Claims (4)

(57) [Claims] 1. A an aqueous solution containing the metal Zn and hexavalent Cr contacted response to acidic plating bath composed mainly of Zn ²⁺ ions and Cr ³⁺ ions, the acidic and Zn ²⁺ ions and Cr ³⁺ ions A zinc-chromium electroplating method, wherein plating is performed while supplying the solution to a plating bath. 2. A method using a Pb-based electrode as an insoluble anode,
The zinc-chromium electroplating method according to claim 1, further comprising supplying a carbonate of Ba into the system and supplementing a part of the Cr source with chromium sulfate. 3. A plating solution and an aqueous solution containing hexavalent Cr are pressed into a dissolution tank filled with granular metal Zn having a particle size of 10 to 0.1 mm,
The zinc-chromium electroplating method according to claim 1, wherein the metal Zn is dissolved while being suppressed from flowing along with the liquid flow, and is recycled. 4. The zinc-chromium electroplating method according to claim 1, wherein the aqueous solution containing hexavalent Cr is chromate and / or chromium chromate.