JPH07188971A - Method for galvanizing aluminum and aluminum alloy sheet - Google Patents

Abstract

PURPOSE:To form a highly adhesive electrogalvanizing film an Al sheet. CONSTITUTION:The content of impurities in a plating soln. contg. >=0.01mol/l fluorine is controlled to <=3ppm Pb and Sn and to <=0.5ppm Sb, As and Ge, and the relative speed between the surface to be plated and a plating soln. is controlled to >=0.2m/sec to conduct electroplating. Consequently, the effect of the impurites disturbing the electrodeposition of zinc is eliminated, the oxide film on the surface of an aluminum sheet is uniformly removed, zinc is uniformly and successively deposited, and a highly adhesive film is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a surface-treated aluminum thin plate material used as a coating base, and more particularly to a technique for obtaining an electroplating film of zinc or a zinc alloy having excellent adhesion.

[0002]

2. Description of the Related Art Cold-rolled steel sheets and surface-treated steel sheets thereof are widely used for automobile bodies or housings of home appliances. In recent years, cold-rolled steel sheets and surface-treated steel sheets have been used for the purpose of weight reduction and corrosion resistance improvement.
The use of aluminum and aluminum alloys (hereinafter simply referred to as aluminum) having a low specific gravity has been studied.

One of the subjects to be studied is to impart a function suitable for the purpose of use, in which surface treatment as a coating base is being studied. The surface treatment includes plating treatment in addition to anodizing treatment and chemical conversion treatment.

In the case of the plating treatment, if the surface of the aluminum thin plate which is easily oxidized is directly electroplated, sufficient adhesion between the aluminum base material and the plating film cannot be obtained, and peeling of the plating film cannot be avoided.

Therefore, after performing a preliminary treatment for activating the aluminum surface, a displacement plating treatment of zinc is once performed, and then a desired plating is performed thereon. However, this method is inferior in productivity because the treatment process is complicated and takes time, and only a thin plating film can be obtained only by the displacement plating, and there is a problem in controlling the amount of adhesion.

Recently, a method of directly electroplating an aluminum thin plate has been studied with an emphasis on productivity. Examples include a pulse energization method and a method using a plating bath containing fluoride.

In the pulse energization method, the surface oxide film is dissolved by pulse energization and then the plating energization is performed. The effect is seen in molten salt electrolysis, but in electroplating with an aqueous solution, the surface oxide film is not dissolved. Incomplete and sufficient plating adhesion is not obtained.

In the method using a plating bath containing fluoride, fluoride is contained in a predetermined concentration or more, and a plating film is formed while the oxide film is dissolved and removed by the action of fluoride ions. For example, Japanese Unexamined Patent Publication No. 5-65691 discloses an acidic zinc plating bath containing a fluoride ion concentration of 10 ppm or more.

[0009]

However, stable plating adhesion cannot be obtained even with the above plating bath,
If the operation is continued, the adhesiveness is deteriorated and the plating bath needs to be renewed. The present invention has been carried out in order to solve this problem, and an object thereof is to provide an electroplating method capable of obtaining a zinc-based plating film having good productivity and stability and excellent plating adhesion.

[0010]

[Means for Solving the Problems] To achieve this object, a plating solution containing a fluorine concentration of 0.01 mol / l is used, and the total Pb concentration and Sn concentration contained in this plating solution is 3 ppm. A zinc-based plating method for aluminum and aluminum alloy plates, in which the total of As concentration, Sb concentration, and Ge concentration is controlled to 0.5 ppm or less in total, and the above Pb is preferable.
This is a zinc-based plating method for aluminum and aluminum alloy plates, in which electroplating is performed with a relative flow rate of the plating solution whose concentration is controlled to the plating surface of 0.2 m / s or more.

[0011]

[Function] When performing zinc-based electroplating on an aluminum thin plate with a plating bath containing a fluoride, good plating adhesion can be obtained regardless of normal conditions such as fluorine concentration and plating conditions. In some cases, it could not be obtained, and the cause was investigated. As a result, it has been expected that specific impurities in the plating bath are greatly involved in the plating adhesion. These impurities include P
b, Sn, Sb, As, Ge.

The action of these impurities can be considered as follows. That is, when zinc is directly plated on aluminum in a bath containing a fluoride, first, an oxide film on aluminum is rapidly dissolved in the plating bath by the action of fluorine ions.

Since the aluminum oxide film has a high electric resistance, a plating current flows into a place where the oxide film is melted and the aluminum metal is exposed, and zinc is first deposited there. When the above impurities are not present, dissolution of this oxide film and deposition of zinc occur one after another, and a thin but uniform galvanized layer is formed at the initial stage of energization. After that, zinc is continuously deposited on the zinc metal to a desired thickness, so that a plating film having good adhesion is formed.

On the other hand, when the above impurities are present,
Impurities form various compounds in addition to ions in the plating bath. The product behaves differently from zinc ions and is adsorbed on an aluminum oxide film, an aluminum metal surface, or a zinc deposition surface, and interferes with the uniform electrodeposition of zinc.
Furthermore, the adsorbed compound serves as a cathode to form a local battery, which causes disturbance in the electric field and hinders uniform zinc electrodeposition.

For this reason, the zinc electrodeposition is already non-uniform in the initial stage of energization, and when it is not sufficient, the base is locally exposed. Since the electrodeposition coating grows on this, a perfect bond cannot be obtained between the base material and the plating coating, resulting in a decrease in plating adhesion.

Further, when the influence of these impurities is quantitatively investigated, the larger the amount of impurities, the greater the influence, but the impurities are roughly divided into two groups according to the degree of influence. Pb,
Sn is the group with the least influence, and is Sb, As, Ge.
Is the group with the greatest influence. In the group of Pb and Sn, if the content exceeds 3 ppm, there is a practical problem, and if it exceeds 0.5 ppm in Sb, As, and Ge, the adverse effect thereof appears and the plating adhesion deteriorates.

Within these groups, even if any one of the above-mentioned limit concentrations exceeds the above-mentioned limit, there is no effect even if a plurality of them are contained unless the limit concentration is exceeded. The same applies when there are multiple groups.

These impurities are unavoidably mixed in the plating solution as impurities used in the bath for constructing or replenishing the plating bath or as impurities in the anode. In addition, the bath may be contaminated by the plating solution used last time when replacing the plating solution.

Strict control of these impurities in the plating bath is necessary to secure the plating adhesion. In the case of chemicals and contamination from the anode, impurities accumulate and increase during operation, so it is necessary to monitor and control the amount. In particular, in the high-speed continuous plating treatment, the rate of increase is large, and in the unlikely event that the plating adhesion is lowered, the loss becomes enormous. The amount of impurities must be measured frequently or continuously and reduced.

For measuring these impurities, a rapid instrumental analysis method such as an atomic absorption method or a high frequency inductively coupled plasma emission spectroscopy (ICP) method can be used. In addition, as a control method, it can be removed by bringing the plating solution into contact with the metal zinc particles or powder, and depositing or adsorbing it on the metal zinc.

As described above, when impurities are controlled, if the plating bath contains 0.01 mol / l of fluorine, the aluminum oxide film is removed and zinc is uniformly deposited. When the content of fluorine is less than 0.01 mol / l, the effect of removing the oxide film is low and practically good plating adhesion may not be secured.

When impurities such as Pb, Sn, Sb, As, and Ge are accumulated in the bath and the plating adhesion is lowered, further adhesion of fluoride to the plating bath does not improve the plating adhesion. As described above, in addition to the method of controlling the impurities in the plating bath, there is a means of stirring the plating solution as a method of assisting the uniform electrodeposition of zinc.

By the stirring action, the zinc ion concentration in the vicinity of the aluminum surface is made uniform, and at the same time, the fluorine concentration is also made uniform and the oxide film on aluminum is also dissolved uniformly. The effect of stirring can be expressed by using the relative flow velocity of the plating solution with respect to the object to be plated as an index, and the effect increases as the relative flow velocity increases. When this flow velocity is 0.2 m / s or more, stable and good plating adhesion can be secured.

[0024]

[Examples] After degreasing and cleaning an aluminum plate, zinc-based electroplating was performed to examine the adhesion of the plated film. The plating bath used was a bath controlled according to the present invention and a bath in which the aforementioned impurities were not controlled and controlled for comparison.

The impurities were controlled by periodically sampling a sample, measuring the amount of impurities by ICP analysis, and guiding the plating bath to the circulation path and passing the Zn powder layer based on the results. The aluminum plate used is equivalent to A1200P or A5182P specified in JIS-H4000. Types of zinc-based plating are zinc plating, zinc-iron alloy plating (iron content 15%) and zinc-nickel alloy plating (nickel content 12%).

The compositions of these plating baths are shown below. A. Galvanized ZnSO4 ・ 7H2O 400g / l Na2SO4 30g / l NH4FHF Change pH 2.0 B. Zinc-iron alloy plating ZnSO4 / 7H2O 150g / l FeSO4 / 7H2O 350g / l Na2SO4 30g / l NH4F change pH 1.5 C.I. Zinc-nickel alloy plating ZnSO4 ・ 7H2O 150g / l NiSO4 ・ 7H2O 350g / l Na2SO4 30g / l NaF change pH 1.5 For plating conditions, bath temperature 50 ℃, current density 50A /
The relative flow velocity was changed at dm ² .

To evaluate the plating adhesion, a contact bending test with a bending angle of 180 ° specified in JIS-Z2248 was performed, and an adhesive tape was attached to the plated surface of the bent portion, which was forcibly peeled off and plated. The peeling state of the film was examined. No peeling was observed at all, an extremely excellent one was marked with ⊚, a slight peeling was observed and there was no problem in practical use, a peeling was clearly recognized and there was a problem in practical use with △, When peeling was observed on the entire surface at the bent portion and adhesion was poor, each was evaluated by x. The adjusted results are shown in Table 1 together with the amount of impurities in the bath.

[0028]

[Table 1]

As shown in the examples of the present invention, in a zinc-based plating bath containing a fluoride concentration of 0.01 mol / l or more with a fluoride, the material in the bath is made of pure aluminum by controlling and controlling the impurities in the bath. With (1200) or aluminum alloy (5182), a plating film having good adhesion can be directly obtained by electroplating. Particularly, when the relative flow velocity is 0.2 m / s or more, the adhesiveness that sufficiently withstands the severe adhesive bending is stably secured. In the comparative example in which the impurities are not controlled, the adhesiveness is poor and the adhesiveness cannot be secured even if the fluorine concentration in the plating bath is sufficient.

[0030]

According to the present invention, since the specific impurities in the plating bath are controlled and the relative flow rate of the plating solution is controlled, the aluminum oxide film is uniformly removed and the plating film is evenly distributed over the entire surface. To deposit. Therefore, good adhesion of the plating film can be stably and reliably obtained. As described above, the effect of the present invention, which makes it possible to reliably manufacture a zinc-based plated aluminum plate having excellent adhesion with a high yield, is great.

Claims (2)

[Claims] 1. A plating solution containing a fluorine concentration of 0.01 mol / l is used, and the Pb concentration and Sn contained in this plating solution are used.
The total concentration is less than 3ppm and the As concentration, Sb concentration and Ge
A zinc-based plating method for aluminum and aluminum alloy plates, characterized in that the total concentration is controlled to 0.5 ppm or less for electroplating. 2. The relative flow velocity of the plating solution with respect to the plating surface
The zinc-based plating method for aluminum and aluminum alloy sheets according to claim 1, wherein the plating rate is 0.2 m / s or more.