JPH0242919B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of alkaline galvanizing by supplying zinc ions to a plating bath.

The conventional method for galvanizing the inner surface of steel pipes, etc. is to place a zinc rod as an anode in the center of the inner surface of the steel pipe, etc., and then energize it. Alternatively, by lowering the amount of current, a gray-black film called a passivation film falls off from the surface of the zinc rod and adheres to the fastening surface, causing so-called roughness on the cathode. Furthermore, as the zinc rod melts, the surface area of the rod decreases, so changes in current density increase, making it impossible to maintain a constant zinc dissolution rate. Furthermore, the zinc rod, which has a reduced surface area, must be replaced frequently, resulting in a significant decrease in work efficiency. Due to these drawbacks, the method of galvanizing the inner surface of steel pipes, etc., as described above, is hardly ever practiced at present.
Instead of the above method, various methods have been developed for supplying zinc ions to the plating solution using the following insoluble anode.

(i) Similar to the normal method of alkaline galvanizing the inside of a steel pipe, this is a method in which an iron anode is placed inside the steel pipe and a zinc anode is placed outside the steel pipe, and each wire is connected in parallel to conduct electrical plating. This method is not only inconvenient as it is necessary to apply a seal etc. to the unplated surface of the object to prevent plating from adhering, but also because of the distance between the electrodes, the solubility of the zinc anode is extremely high. This has the disadvantage that the required uniform electrodeposition on the inner surface becomes significantly worse.

(ii) In an alkaline galvanizing bath, a zinc metal and a substance with a low hydrogen overvoltage, such as high carbon steel, high temperature oxidized steel, cast iron, etc. are brought into contact to form a battery of zinc and hydrogen, and according to the reaction formula below: Zn＋2H ⁺ +2(OH ^- )→Zn(OH) ₂ +H ₂ ↑ Zn(OH) ₂ +2NaOH→Na ₂ Zn(OH) ₄ Na ₂ Zn(OH) ₄ is prepared and this liquid is supplied to the plating bath. This is a method of replenishing zinc ions. In this method, the generation rate of Na ₂ Zn (OH) ₄ is slow, and the electromotive force of the hydrogen-zinc battery fluctuates as it is reduced by the hydrogen generated from the surface of the oxide in contact with it. ₂ The disadvantage is that the production rate of Zn(OH) ₄ cannot be achieved.

(iii) This is a method of supplying zinc ions by dissolving zinc oxide or zinc hydride in caustic alkali in an alkaline galvanizing bath. In this method, the amount of caustic alkali in the plating bath gradually increases, making it difficult to stabilize the composition of the plating bath.

The inventors of the present invention solved the deficiencies of these methods and conducted extensive research in search of a new method for supplying zinc ions. As a result, they found that an electrolytic solution with the same composition as the alkaline galvanizing bath had a low hydrogen overvoltage, and hydrogen A cathode made of a ferrite oxide that generates but does not precipitate zinc and a zinc anode are arranged and electrolyzed to elute zinc, and this zinc solution is supplied to the plating solution to replenish zinc ions and circulate the plating solution. It has been found that galvanizing can be carried out for a long period of time without any problems by letting the steel coat.

The present invention is a method based on this knowledge, which uses a cathode made of a ferrite oxide that has a low hydrogen overvoltage and does not deposit zinc in an electrolytic solution having the same composition as the plating solution used when performing alkaline electrogalvanizing. and a zinc anode are arranged to electrolyze and elute zinc, and the resulting zinc solution is supplied to the plating treatment bath to replenish and circulate zinc ions. This method supplies zinc ions.

In the present invention, the ferrite oxide used for the cathode of the electrolytic cell is an oxide represented by the general formula MO.Fe ₂ O ₃ , where M is Mn, Mg, Ni, Fe,
Indicates divalent metals such as Co, Cu, and Zn. The most typical of these is magnetite (Fe ₃ O ₄ or FeO.
Fe ₂ O ₃ ). In addition, quenched steel, which is made by treating the surface of iron-based materials at high temperatures to produce ferrite-based substances;
Cast iron is also used.

In the present invention, when a ferrite-based oxide is used as a cathode, no zinc is deposited at all current densities, and zinc ions can be supplied for a long time.

However, since ferrite materials have poor electrical conductivity, it is preferable to avoid high current density (30 A/dm ² or higher) work, as this generates a lot of heat and requires cooling.

The relationship between the Zn concentration in the bath and the amount of current applied when a ferrite cathode is used when M is nickel metal is shown in FIG. The electrolysis conditions are as follows.

Liquid amount 4 (bath preparation liquid composition; NaOH 120g/,
Metallic Zn0g/) Electrolysis conditions Zn cathode: 1.5dm ² (Surface area) Ferrite cathode (manufactured by TDK) 0.3dm ² ) Current 3A Cathode current density 2A/dm ^2Cathode current density 10A/dm ^2In this experiment, zinc is 96~ It was dissolved with a current efficiency of 100%, and as shown by the rate of increase in zinc concentration, no zinc precipitation was observed on the ferrite cathode, and the current efficiency of zinc was close to 0%.

Fig. 2 is a drawing showing the principle of the present invention, and Fig. 3 is a drawing showing the principle of the present invention.
FIG. 4 and FIG. 4 are explanatory diagrams showing embodiments of carrying out the present invention, of which FIG. 3 shows a case in which zinc ions are supplied when zinc ions are insufficient while galvanizing is carried out in one tank. FIG. 4 shows a case where the electrolyzed zinc ion solution produced in one tank is put into the next galvanizing tank, and the solution discharged from the galvanizing tank is circulated to the zinc ion production tank.

2 to 4, 1 is a plating object, 2 is an insoluble cathode, 3 is a zinc non-depositing cathode, 4 is a zinc anode for supplying zinc ions, 5 is a power supply for plating, 6 is a power supply for supplying zinc ions, 7 is a plating 8 is a plating tank, 9 is an electrolytic tank for supplying zinc ions, 10 is a partition wall for liquid flow, and 11 is a circulation pump.

When plating the inner surface of a steel pipe using the present invention, first determine the total surface area of the cathode to be plated and the cathode current density, and roughly estimate the zinc deposition rate.
Next, determine the current density and surface area of the anode zinc that correspond to the zinc deposition rate. Place the insoluble anode so that it is equidistant from the plated cathode surface. The ferrite cathode is arranged evenly with respect to the zinc anode, and the current density is determined by considering heat generation. By designing the plating apparatus in this way, it is possible to keep the zinc concentration in the plating bath constant for a long time by finely adjusting the zinc dissolving current.

Although the above description has mainly been given to the case of galvanizing the inner surface of a steel pipe, the present invention is also applicable to the case of galvanizing objects of other shapes.

Example As shown in Fig. 3, a bath for continuous plating of zinc on the inner surface of a steel pipe was used, and the composition of the plating solution used was Zn 10g/NaOH 120g/brightener 6c.c. (However, the brightener was manufactured by Dipsol Co., Ltd.) (I used NZ-65) plating solution 20, length 150mm x depth 400mm
I put it in a pot for plating.

Cathode current density on plating bath side: 3A/dm ² Insoluble anode current density: 57A/dm ² Bath temperature: 14.2ml/dm to maintain a constant appearance of brightener at 25±2°C
When plating was carried out every 30 minutes while replenishing at a rate of 100 AH, the zinc deposition rate was 2.34 g/AH. On the other hand, on the electrolytic cell side, ferrite cathode current density was 21 A/dm to supply zinc ions. ² Zinc anode current density When electrolysis was carried out at 2A/ ^dm2 , the dissolution rate of zinc ions was
It was 2.40g/AH.

The relationship between the zinc concentration in the bath and the amount of current applied during the plating period is as shown in Figure 5.
(1000AH), the zinc ion concentration in the bath could be maintained at approximately 10g/. Furthermore, the plating speed and appearance of the plating film remained unchanged from when the bath was first constructed.

[Brief explanation of the drawing]

FIG. 1 shows the relationship between the zinc concentration in the bath and the amount of current applied when a zinc anode and a ferrite cathode are placed in the bath solution and electricity is applied. Figure 2 is an explanatory diagram of the principle of the present invention, and Figures 3 and 4 are simplified diagrams showing embodiments of the present invention, and Figure 3 shows a plating tank and an electrolytic tank in one tank. Figure 4 shows the case where the plating solution in the first tank and the electrolytic zinc solution in the second tank are circulated. Next, Figure 5 shows the third
It is a drawing which shows the relationship between the zinc ion concentration in a bath and the amount of electricity applied when plating is performed using the bath shown in the figure. In Figures 2 to 4, 1... covered object, 2...
Insoluble anode, 3... Zinc non-depositing cathode, 4... Zinc anode for supplying zinc ions, 5... Power source for plating,
6... Power source for supplying zinc ions, 7... Stirring blade, 8
...Plating tank, 9...Electrolytic cell for supplying zinc ions,
10... Liquid flow diaphragm, 11... Circulation pump.

Claims (1)

[Scope of Claims] 1. When performing alkaline electrogalvanizing, a cathode made of a ferrite oxide that has a low hydrogen overvoltage and does not deposit zinc, and a zinc anode are placed in an electrolytic solution having the same composition as the plating solution used. supplying zinc ions to an alkaline zinc plating bath, characterized in that the zinc ions are supplied and circulated by supplying the generated zinc solution to the plating bath and supplying zinc ions to the plating bath. how to. 2 As a cathode made of a ferrite-based oxide, an oxide represented by the general formula MO.Fe ₂ O ₃ (where M is Mn,
At least one divalent metal consisting of Mg, Ni, Fe, Co, Cu and Zn) is used to add zinc ions to the alkaline galvanizing bath according to claim 1. How to supply. 3. A method for supplying zinc ions to an alkaline galvanizing bath according to claim 2, characterized in that magnetite (Fe ₃ O ₄ ) is used as the cathode made of a ferrite oxide.