JPH0240751B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for plating aluminum by electrolytic activation, which enables direct plating on aluminum and its alloys without using an intermediate film layer. The metal to be treated 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 to be treated at the anode, and destroy the oxide film by generating hydrogen at the cathode. This method involves repeating activation in a reducing atmosphere to clean and activate the surface of aluminum, and then directly plating it 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 equipment 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. limited. However, recently the requirements for aluminum surface treatment have become more diverse, such as solderability, magnetic properties, reduction of electrical resistance, hardness, abrasion resistance, and metallic luster. This cannot be achieved through treatment, 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.As with steel and copper alloys, it is difficult to achieve practical plating with only normal pretreatment. There is a big drawback that it cannot be done. 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. The plating process involves creating a film or porous oxide film and using it as a foothold for plating, making the plating process extremely complicated, resulting in high costs and low reliability due to the large number of steps. It is difficult to obtain a reliable finish. For example, in the plating process using the generally widely used zinc substitution method, as shown in Figure 3, before plating, 11 steps are performed as shown by ~.
It required a process. (Means for Solving the Problems) The purpose of the present invention is to eliminate the complicated intermediate treatments of zinc substitution coating, tin substitution coating, and anodic oxidation coating for plating on aluminum, and directly apply plating after degreasing and activation. Plating on aluminum using electrolytic activation to achieve good adhesion on aluminum to improve quality, simplify the plating process, reduce production equipment costs, improve productivity, and reduce costs. The present invention provides a method. That is, in order to achieve the above-mentioned object, the present invention provides aluminum to be plated and its alloy in an alkaline or acidic electrolyte to stainless steel,
Using an insoluble counter electrode such as a carbon plate, positive and negative voltages are applied alternately between the counter electrode, and the surface of the metal to be treated is melted by a positive current at the anode, and hydrogen is generated and oxidized by a negative current at the cathode. The structure is such that the film is repeatedly destroyed and activated in a reducing atmosphere to obtain excellent adhesion, gloss, and other properties. Furthermore, 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. That is, as time T ₂ becomes longer in FIG. 2, the reversal ratio becomes larger and the rate at which negative current flows becomes larger, more hydrogen gas is generated, and the reducing atmosphere becomes stronger. In the opposite case, dissolution increases, or if it does not increase, oxygen gas is generated and promotes oxidation of the metal surface. Controlling both of these reactions is important, and appropriate conditions vary depending on the material and type of plating, so the inversion ratio is also varied to obtain plating with excellent properties. As shown in Fig. 1, the plating process according to the present invention consists of solvent degreasing → electrolytic activation → water washing → plating,
It can be shortened to less than 1/3 compared to the zinc substitution method and is simplified. The major features of the present invention are (1) easy electrolytic activation with repeated operations of anodic dissolution and cathodic reduction through polarity change, which significantly improves plating adhesion, allowing direct plating without using an intermediate layer; be able to. (2) Degreasing and activation using an alkaline degreasing solution 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) It is significantly superior to conventional methods, as it does not use high-concentration treatment solutions such as those used for zinc substitution, so the burden of wastewater treatment is reduced.
This is a groundbreaking invention. Next, the present invention will be explained in more detail with reference to Examples. Example 1 Sodium carbonate 100g/, as degreasing liquid composition
Using an alkaline solution containing 5 g of sodium hydroxide, 1080 pure aluminum and ADC12 die-gast alloy were subjected to strike nickel and bright nickel plating by immersion degreasing only. After immersion degreasing, nitric acid immersion was performed, and 500 g of sodium hydroxide was oxidized. zinc 100
g/, potassium sodium tartrate 10 g/, ferric chloride 1 g/, zinc-substituted and plated in a bath containing stainless steel as the counter electrode in the alkaline degreasing solution of the present invention, frequency 13.3
Table 1 shows the plating characteristics of the samples which were plated after activation for 1 minute under the conditions of Hz, reversal ratio of 95%, and current density of 10 A/dm ² .

[Table] As is clear from this result, blistering occurs when the plating is finished with only immersion degreasing, which is used for general plating, and blistering occurs even with ABC12 when heated to 300℃. , adhesion was significantly poor. If a zinc-substituted film is formed as an intermediate layer to prevent the formation of an oxide film and then plated, the adhesion will be improved, but if heated to 400°C or
Poor adhesion occurs under severe conditions such as heating at ℃ and then cooling with water.
On the other hand, in this method, which performs electrolytic activation by periodically changing the polarity, good adhesion is achieved even under the harsh conditions of heating at 400°C and cooling with water. Another major feature of this method 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 with this method, degreasing is performed efficiently at the same time as activation, resulting in a clean surface.In other methods, the surface becomes rough due to etching, whereas with this method, activation occurs through dissolution during anodization. This is because electrolytic polishing progresses at the same time. Example 2 The ADC12 was electrolytically activated for 5 minutes by changing the polarity conversion frequency under the conditions of Example 1. The results are shown in Table 2. At frequency 0, that is, cathode direct current electrolytic activity, both adhesion and gloss are poor, but with polarity conversion electrolytic activity,
Excellent adhesion and gloss were obtained from a low frequency range of 0.2 to a high frequency range of 100Hz or higher. In a commonly used degreasing solution, you can degrease and activate by simply changing the polarity and passing positive and negative currents alternately.
Polishing is possible, and this method is extremely effective for direct plating onto aluminum.

[Table] Example 3 Trisodium phosphate 100g/as degreasing liquid composition
, using an alkaline solution containing 5 g of sodium hydroxide, electrolytically activated the ADC12 for 1 minute at 50°C, 13.3Hz, and 10A/ ^dm2 while changing the inversion ratio.
The results for strike nickel and bright nickel plating are shown in Table 3. As is clear from this result, the anodic DC electrolysis with a reversal ratio of 0% does not activate the product and the adhesion is poor, and even with the cathode DC electrolysis with a reversal ratio of 100%, heating at 400℃ and water cooling results in no activation.

[Table] Peeling occurs and the activation effect is small. On the other hand, when the polarity is changed periodically, the reversal ratio is changed, and the ratio of positive and negative currents is changed, no blistering or peeling occurs in the entire reversal ratio range, and strong adhesion occurs. I got the answer. However, the degree of gloss after plating differs depending on the reversal ratio. This tendency differs depending on the material and type of plating, so by changing the inversion ratio, activation processing suitable for the material and plating can be performed. Example 4 After electrolytically degreasing ADC12 in an alkaline degreasing solution containing 100 g of sodium carbonate and 5 g of sodium hydroxide for 1 minute, it was degreased at 13.3 Hz and 10 A/dm ² in a 10% hydrochloric acid solution at room temperature with a carbon plate as the counter electrode. The inversion ratio was changed depending on the conditions, electrolytic activation was performed for 30 seconds, and 40A/
Table 4 shows the results of chromium plating for 15 minutes at a current density of ^dm2 .

[Table] Unlike nickel plating, chrome plating has extremely high internal stress, so it is extremely difficult to obtain plating with good adhesion on aluminum. Even if the same pretreatment as in Example 2 is performed, chromium does not precipitate, or even if it precipitates, it becomes flaky and 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 activity, chromium plating is hardly deposited in anodic DC electrolysis with a reversal ratio of 0%, and chromium is deposited in cathodic DC electrolysis with a reversal ratio of 100%, but it peels off in scales and has poor adhesion and gloss. . On the other hand, in polarity conversion electrolytic activation, at a high reversal ratio of 86 to 95%, chrome plating with high gloss and excellent adhesion can be obtained directly on aluminum and its alloys without forming an intermediate layer. Aluminum and its alloys can be activated by polarity conversion electrolysis in acids, alkaline electrolytes, and various salt-added electrolytes other than those in the examples, and plating other than those in the examples can be easily plated directly. This is a new plating method. (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 through easy electrolytic activation, which involves repeated operations of anodic dissolution and cathodic reduction through 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 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) The burden of wastewater treatment is reduced because high-concentration treatment solutions such as zinc replacement are not used. (5) Furthermore, plating with excellent adhesion and gloss can be performed. (6) Activation suitable for the type of aluminum material and type of plating can be easily obtained.

[Brief explanation of drawings]

FIG. 1 shows the plating process according to the present invention, FIG. 2 shows the voltage waveform of polarity conversion electrolytic activation according to the present invention, and FIG. 3 shows the conventional plating process on aluminum.

Claims (1)

[Claims] 1 Aluminum and an insoluble counter electrode are immersed in an alkaline or acidic electrolytic solution, 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, and to Applying a negative voltage to the metal to be processed and applying a voltage to the metal to be processed, changing the inversion ratio to perform activation appropriate to the alloy to be processed and the type of plating, and directly plating without forming an intermediate layer. A method for plating on aluminum by electrolytic activation, characterized by: