JPS62297492A - Method for plating aluminum by electrolytic activation - Google Patents

Abstract

PURPOSE:To directly form smooth plating having superior adhesion on Al without forming an intermediate layer by immersing the Al and an isoluble counter electrode in an electrolytic soln. and alternately applying positive and negative voltages between then to activate the surface of the Al. CONSTITUTION:Al or an Al alloy and an insoluble counter electrode of stainless steel, carbon or the like are immersed in an alkaline or acidic electrolytic soln. and then positive and negative voltages are alternately applied between them. Wile the Al acts as the anode, the surface of the Al is dissolved with positive electric current. While the Al acts as the cathode, hydrogen is generated with negative electric current to destroy an oxide film on the Al and to activate the surface of the al in the reducing atmosphere. activation is carried out according to the material and the kind of plating by changing the ratio of negative to positive voltage. Thus, the adhesion of the Al to plating is improved and the Al can be directly plated without forming an intermediate layer.

Description

[Detailed Description of the Invention] 3. Detailed Description of the Invention (Field of Industrial Application) The present invention is based on electrolytic activation that enables direct plating on aluminum and its alloys without using an intermediate film layer. For details on the method of plating on aluminum, the metal to be plated is placed in an alkaline or acidic electrolyte using an insoluble counter electrode such as stainless steel or carbon plate, and the polarity is periodically changed to dissolve the metal at the time of anode. In this method, the oxide film is destroyed by hydrogen generation at the cathode, and activated in a reducing atmosphere, which is repeated to clean and activate the surface of the aluminum, allowing direct plating to be performed in the same way as steel, copper alloys, etc. .

(Prior art and its problems) Aluminum is lightweight and has good workability, so it is used in a wide range of fields such as various IGL device parts, building materials, and daily miscellaneous goods. Most of these surface treatments are anodized coatings and coatings, and although aluminum plating has a long history, reliable plating methods have not been established, so its applications are limited to some areas. It is being

However, recently the requirements for aluminum surface treatment have diversified, such as solderability, magnetic properties, reduction of electrical resistance, etc.
There are demands for improved hardness, abrasion resistance, and the provision of metallic luster, which cannot be met by conventional surface treatments, and surface treatment by plating is becoming necessary.

Since aluminum is a metal that is easily oxidized, a natural oxide film is formed and plating with good adhesion cannot be obtained. In addition, since it is an amphoteric metal, it is easily corroded by acidic and alkaline solutions, and it is corroded in the plating solution, resulting in insufficient adhesion, making it impossible to obtain a plating that can be used for practical purposes only with normal pretreatment, such as with steel and copper alloys. There is a big drawback. Many techniques have been studied and proposed to compensate for these shortcomings, but the main methods currently in practical use are zinc substitution, tin substitution, and anodic oxidation. This method generates a film or porous oxide film and uses it as a foothold for plating.The plating process is extremely complicated, resulting in high costs and low reliability due to the large number of steps. It is difficult to obtain a plating with a certain

For example, the generally widely used plating process using the zinc substitution method requires 11 steps as shown by (1) to (2) before plating, as shown in FIG.

(Means for Solving the Problems) What is the purpose of the present invention for plating on aluminum? l Eliminates the complicated intermediate treatment of sub-VA substitution coating, tin substitution coating, and VA polar acid oxide coating chemical formation, and performs plating with good adhesion directly onto aluminum using a degreasing and activation machine, improving quality and simplifying the plating process. The present invention provides a method for plating on aluminum using electrolytic activation in order to reduce production equipment costs, improve productivity, and reduce costs.

That is, in order to achieve the above object, the present invention uses an insoluble counter electrode such as stainless steel or carbon plate to plate aluminum and its alloy in an alkaline or acidic electrolyte solution, and creates positive and negative polarities between the counter electrode and the aluminum plate. Voltage is applied alternately, and the surface of the metal to be treated is melted by a positive current at the anode.
The structure is such that hydrogen is generated by a negative current at the anode to destroy the oxide film, and activation is repeated in a reducing atmosphere to obtain excellent shine such as adhesion and gloss. Further, the inversion ratio at which a negative voltage is applied to the metal to be processed to which a positive voltage has been applied is changed to perform activation appropriate to the material and type of plating. In other words, as time T2 becomes longer in Fig. 2, the reversal ratio increases and the rate at which negative current flows increases, and more hydrogen gas is generated (reducing atmosphere also becomes stronger. In the opposite case, dissolution increases, If it does not increase, oxygen gas is generated and promotes oxidation of the metal surface.It is important to control both of these reactions, and the appropriate conditions differ depending on the material and type of plating, so change the inversion ratio to obtain excellent properties. The plating process according to the present invention is shown in Figure 1.
As shown in ~■, the steps are solvent degreasing - electrolytic activation -> water washing -> plating, which can be shortened to 173 degrees or less and simplified compared to the zinc substitution method. The major features of the present invention are (1) anodic dissolution by polarity conversion, negative [! The adhesion of plating can be significantly improved by electrolytic activation, which is easy to perform repeated reduction operations, and direct plating can be performed without using an intermediate layer. (
2) When an alkaline degreasing solution is used, degreasing and activation can be performed at the same time, and the gloss of the plating is improved because the metal to be treated is polished by anodic dissolution. (3) The plating process is significantly shortened, reducing equipment costs, improving productivity, and reducing costs. (4) Since high-concentration treatment liquids such as those used for zinc substitution are not used, the burden of wastewater treatment is reduced, and this is a groundbreaking invention that is significantly superior to conventional methods.

Next, the present invention will be explained in more detail with reference to Examples.

(Example 1) Degreasing liquid composition: sodium carbonate 100a/t, sodium hydroxide! Using 50t't of alkaline solution, 1
080 pure aluminum and ADC12 die-casting alloy are immersed and degreased with strike nickel and bright nickel plating, then immersed in nitric acid after immersion degreasing, and sodium hydroxide 500! ]/l, zinc oxide 100Q,'/, potassium sodium tartrate 10Q/l, ferric chloride 1CJt'
Plated by zinc substitution in a bath containing l,
The plating characteristics of stainless steel as a counter electrode in the alkaline degreasing solution of the present invention were activated for 1 minute under the conditions of a frequency of 13.3H2, a reversal ratio of 95%, and a current density of 10 A/'d 112. Shown in Table 1.

As is clear from the results in Table 1, when only immersion degreasing, which is used for general plating, .S. S-curling occurred and the adhesion was significantly poor.

When plating is performed by forming a zinc-substituted film as an intermediate layer to prevent the formation of an oxide film, adhesion improves, but 4
Poor adhesion occurs under severe conditions such as heating to 00°C or heating to 400°C followed by water cooling. On the other hand, water droplets that are electrolytically activated by periodically changing their polarity have good adhesion even under severe conditions of heating at 400° C. and cooling with water. Another major feature of Mizutoshi is that the gloss of the plating finish is significantly better than that of conventional methods, and is not affected by the type of material. The reason for this is that water droplets are activated and degreased efficiently at the same time, resulting in a clean surface.In other methods, the surface becomes rough due to etching, but with water droplets, activation and electrolytic fi4 occur through dissolution at the anode. This is because the friction progresses at the same time.

(Example 2) Real 7il! Table 2 shows the results when AlIC12 was electrolytically activated for 5 minutes by changing the polarity conversion frequency under the si conditions.

At frequency 0, that is, cathode direct current electrolytic activity, both adhesion and gloss are poor, but with polarity conversion electrolytic activity, excellent richness and gloss were obtained from a low frequency of 0.2 to a high frequency range of 100IIZ or more. . Degreasing, activation, and polishing are possible by simply changing the polarity in a commonly used degreasing solution and passing positive and negative currents alternately. Water droplets are extremely effective for directly plating onto aluminum. be.

Table 2 (Example 3) As the degreasing liquid composition, an alkaline solution containing 100Q// of trisodium phosphate and 5σl'l of sodium hydroxide was used.
Table 3 shows the results of electrolytic activation for 1 minute under the conditions of 50° C., 13.311z, and 10Az'dm2 at 50°C, 13.311z, and 10Az'dm2, and plated with strike nickel and bright nickel.

As is clear from these results, the anodic DC electrolysis with a reversal ratio of 0% did not activate and the adhesion was poor, and even with the anodic DC electrolysis with a reversal ratio of 100%, peeling occurred on the third surface when heated to 400°C and water-cooled, resulting in activation. has little effect. On the other hand, by periodically changing the polarity and changing the inversion ratio, the positive ri
When the current and the rate at which the negative current was applied were changed, in the entire reversal ratio range - 5 - <, no peeling occurred, and a firmly adhered clamp was obtained. However, the gloss level after plating differs depending on the reversal ratio. Since this tendency differs depending on the type of material and plating, it is possible to perform activation processing suitable for the material and plating by changing the inversion ratio.

(Example 4) ADC12 was electrolytically degreased for 1 minute in an alkaline degreasing solution containing 10 QQ/Z of sodium carbonate and 5 t'l of sodium hydroxide, and then degreased with a carbon plate as a counter electrode in a 10% salt III solution at room temperature. .3) Change the inversion ratio under the conditions of 1z and IOA/di', perform B electrolytic activation for 30 seconds,
Table 4 shows the results of chromium plating for 15 minutes at a flow density of OA t'dl'' of 11.

It is very difficult to obtain plating with good adhesion on aluminum because the stress is extremely large. Even if the same pretreatment as in Example 2 is performed, chromium does not precipitate, or it precipitates into a dust-like form that easily falls off or peels off. Adhesion can also be improved by adding salts containing chlorine ions, such as sodium chloride, to the alkaline degreasing solution, but electrolytic activation by polarity conversion in a hydrochloric acid solution is more effective. Even with electrolytic activation, the reversal ratio is 0%, that is, in anodic direct current electrolysis, almost no chromium plating is deposited,
In cathodic direct current electrolysis with a reversal ratio of 100%, chromium is deposited, but it peels off in the form of flakes, resulting in poor adhesion and gloss. On the other hand, polarity conversion electrolytic activity has a high reversal ratio of 86 to 95.
%, a bright and highly adhesive chromium plating is obtained directly on aluminum and its alloys without the formation of an interlayer.

Aluminum and its alloys can be activated by polarity conversion electrolysis even in acids, alkaline electrolytes, and various salt-added electrolytes other than these examples, and it is a breakthrough that allows direct plating with ease for plating other than those in the examples. This is a method of targeting.

(Effects of the Invention) According to the present invention configured as described above, the following effects can be obtained.

(1) Plating adhesion can be significantly improved by electrolytic activation, which is easy to perform by repeating anodic dissolution and cathodic reduction by polarity change, and direct plating can be performed without using an intermediate layer.

(2) When an alkaline degreasing solution is used, degreasing and activation can be performed simultaneously, and the gloss of the plating is improved because the metal to be treated is polished by anodic dissolution.

(3) The plating process is significantly shortened, reducing equipment costs, improving productivity, and reducing costs.゛(4) subtG
The burden of wastewater treatment is reduced because high-temperature treatment liquids, such as those used for displacement, are not used.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a plating process according to the present invention, FIG. 2 shows a voltage waveform of polarity conversion electrolytic activation according to the present invention, and FIG. 3 shows a conventional plating process on aluminum. −゛J−”・Eco・Figure 1Figure 2Figure 8Weigher

Claims (2)

[Claims] (1) Aluminum and an insoluble counter electrode are immersed in an alkaline or acidic electrolyte, respectively, and positive and negative voltages are applied alternately between the aluminum and the counter electrode to activate the surface of the metal to be treated. A method of plating on aluminum by electrolytic activation, which is characterized by directly plating without forming a layer and obtaining smooth plating with excellent adhesion. (2) Claims characterized in that activation is performed in a manner appropriate for the alloy to be treated and the type of plating by changing the inversion ratio of applying a negative voltage to the metal to be treated to which a positive voltage has been applied. 2. The method of plating on aluminum by electrolytic activation as described in item 1.