JPH0321636B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention relates to a method for suppressing the elution of Pb ions that elute from the anode and reduce the corrosion resistance of the plating layer when electroplating Zn-based alloys using a Pb-based electrode as an anode. . (Prior Art) Electroplating of Zn-based alloys is usually carried out in a sulfuric acid acid plating bath, but sometimes an insoluble electrode is used as an anode. Platinum-based electrodes and Pb-based electrodes are used as this insoluble electrode, and Pb-based electrodes are particularly popular because they are inexpensive. However, even though Pb-based electrodes are insoluble electrodes, they elute in extremely small amounts during electrolysis, reducing the corrosion resistance of Zn-based alloy plating. Particularly, among Pb-based electrodes, pure Pb electrodes have a large amount of elution, so they cannot be used due to the corrosion resistance of the plating layer. For this reason, conventional Pb-based electrodes use a few percent of Ag or Sn as a depolarizing agent.
Pb-Ag alloys and Pb-Sn alloys were commonly used. However, even if a Pb-based electrode containing such a depolarizing agent is used, it is extremely difficult to maintain the Pb ²⁺ concentration in the plating bath below 0.2 ppm without reducing the corrosion resistance of the plating layer. Furthermore, small amounts of SrCO ₃ or BaCO ₃ powder were continuously added directly to the plating bath to remove the eluted Pb ²⁺ by adsorption. (Problem to be solved by the invention) Since Pb ²⁺ is eluted from the Pb-based electrode, the addition of this SrCO ₃ or BaCO ₃ powder is necessary for the Pb-based electrode and the overlying material that opposes this electrode. If it is placed in or near the electrodeposition area between Zn and Zn, the eluted Pb ²⁺ can be immediately adsorbed and removed, reducing the amount of Pb that is deposited on the plating layer.
As mentioned above, plating of the alloys is carried out in a sulfuric acid plating bath, so the SrCO ₃ and BaCO ₃ that have been added are converted into poorly soluble SrSO ₄ and BaSO ₄ , and the plating method is continuous plating. In this case, it adhered to the strip and was pressed by a roll or the like, resulting in scratches on the strip. Furthermore _SrCO3
The method of injecting BaCO ₃ into the electrodeposition part or its vicinity is as follows:
Since it is difficult to uniformly disperse SrCO ₃ or BaCO ₃ in a plating bath quickly and continuously, Pb ²⁺ cannot be effectively removed. For this reason, conventionally, when adding SrCO ₃ or BaCO ₃ , the electrolytic cell was connected to a large external storage tank (such as a plating solution storage tank or circulation tank) so that the plating solution could circulate. This was done in the storage tank, where sufficient time was spent to remove the Pb ²⁺ , and then the plating solution was filtered and sent back to the electrolytic cell. This allows for
Pb ²⁺ concentration: 0.1 to 0.1, which poses no problem in terms of corrosion resistance of the plating layer
A substance that can be maintained at 0.2ppm and that does not contain SrCO ₃ or
Although it was designed to prevent BaCO ₃ from adhering to the strip, it could not prevent some of the Pb ²⁺ eluted from the Pb-based electrode from being deposited immediately, so the Zn-based alloy plated with the Pb-based electrode Its corrosion resistance was inferior to that plated with platinum-based electrodes. (Means for solving the problem) In this way, with the conventional method, it is difficult to immediately remove ^{the Pb 2+ eluted} from the Pb-based electrode. Since it was difficult to prevent Pb 2+ from depositing, the present inventors aimed to fundamentally suppress the elution of Pb ²⁺ into the deposited area.
As a result of researching various methods, we found that Cr, Mn, Co,
A small amount of one or more of Ti and Mo metal ions, specifically 0.001~ for Cr ions.
0.03 mol/, 0.001 to 0.2 mol/ for Mn ions
, 0.01 to 0.1 mol/for Co ions, 0.002 to 0.2 mol/ for Ti ions, and 0.002 to 0.2 mol/ for Mo ions.
When added 0.001 to 0.02 mol/, the
They discovered that Pb ²⁺ elution can be significantly suppressed. The attached drawing shows that each metal ion mentioned above is added to a Zn-Ni alloy plating bath and a Pb-Ag alloy electrode is used.
This figure shows the increase and decrease due to oxidation of the elution of the electrode depending on the electrolysis time when electroplating at 20A/ ^dm2 , and the increase and decrease of the electrode according to the present invention is compared to the case where each of the above metal ions is not added. extremely few,
Pb electrode elution is extremely low. In the present invention, the amount of each metal ion added is set as described above because if each metal ion is below the lower limit, the effect of suppressing Pb ²⁺ elution from the Pb-based electrode is not observed.
Moreover, if the upper limit is exceeded, the effect of inhibiting Pb ²⁺ from bathing out becomes saturated, and further addition will have no effect. In the case of the present invention, if the amount of each metal ion added is within the above range, the Pb content contained in the plating layer can be reduced to 1/4 or less of the conventional value. As described above, in the present invention, when a small amount of each of the metal ions mentioned above is added, the elution of Pb ²⁺ from the Pb-based electrode is suppressed, but the reason for this is not necessarily clear. However, when a small amount of each of the above metal ions is added, the metal ions of the present invention not only promote the growth of the Pb oxide film that is generated on the surface of the Pb-based electrode during electrolysis, but also make the oxide film extremely dense. It is presumed that this is because the metal ions added according to the present invention form a poorly soluble film of the ions on the surface of the system electrode, thereby making it difficult to elute Pb ²⁺ by some method. Regardless of whether the elution of Pb ²⁺ is suppressed by any of these methods, a film related to the elements of the added metal ions according to the present invention cannot be identified even if the surface of the Pb-based electrode is analyzed by X-ray diffraction. The film formed on the surface of the system electrode is thought to be amorphous or extremely thin. In addition, in the present invention, the amount of Pb ²⁺ deposited can be further suppressed by using a conventional Pb ²⁺ removal method using chemicals such as SrCO ₃ and BaCO ₃ in parallel with the addition of each of the metal ions mentioned above. It is possible. (Example) A Pb-based electrode was used in a Zn-based alloy plating bath containing the electrode elution-suppressing elements of the present invention shown in Table 1 and in a conventional Zn-based plating bath without the above-mentioned elements. After performing electrolysis for 1 hour at a density of 20 A/dm ² , Zn-based alloy plating was applied to a steel plate (plating coverage: 20
g/m ² , one side). Thereafter, the plating layer composition and corrosion resistance (based on a salt spray test in accordance with JIS Z2371) of the obtained dampened steel sheet were investigated. The results are shown in Table 1. In addition, when performing electrolysis and plating in a plating bath to which an element for suppressing electrode elution is added or a plating bath to which the above element is not added, in both cases, be sure to connect the electrolytic bath of the plating bath to an external storage tank. Connected so that the soaking liquid circulates, and energized SrCO ₃ in the storage tank at an amount of 0.2 per 7000 coulombs.
A Pb ²⁺ removal method was carried out in parallel, in which the plating solution was filtered on the outlet side while continuously charging the plating solution.

[Table] As is clear from Table 1, the Pb content in the plating layer of the plated steel sheet plated by suppressing Pb ²⁺ from the electrode by the method of the present invention is less than 1/4 of that plated by the conventional method. Therefore, it has excellent corrosion resistance. (Effects) As described above, according to the present invention, the electrodeposition of Pb ²⁺ eluted from the Pb-based electrode is suppressed, so the Pb content in the plating layer can be reduced, and the corrosion resistance of the plating layer can be improved. can be improved.

[Brief explanation of the drawing]

The attached drawings show a conventional Zn-Ni alloy plating bath and a Zn-Ni alloy plating bath to which the Pb ²⁺ elution suppressing element of the present invention is added, using a Pb-Ag electrode at 20 A/dm ^{2 .}
This figure shows the increase and decrease of the electrode depending on the electrolysis time when electroplating is performed.

Claims (1)

[Claims] 1. When electroplating a Zn-based alloy using a Pb-based electrode as an anode, Cr, Mn, Co,
Pb of a Pb-based electrode characterized by adding one or more of Ti and Mo metal ions
Ion elution suppression method. 2. The method according to claim 1, characterized in that 0.001 to 0.03 mol/of Cr ions are added.
Method for suppressing Pb ion elution from Pb-based electrodes. . 3. The method according to claim 1, characterized in that 0.001 to 0.2 mol/Mn ion is added.
Method for suppressing Pb ion elution from Pb-based electrodes. 4 Pb according to claim 1, characterized in that 0.01 to 0.1 mol/Co ion is added.
Pb ion 5 of the system electrode Pb according to claim 1, characterized in that 0.001 to 0.2 mol/Ti ion is added.
Method for suppressing Pb ion elution from system electrodes. . Claim 1, characterized in that 0.0012 to 0.02 mol/6 Mo ions are added.
Method for suppressing Pb ion elution from Pb-based electrodes.