KR102699452B1 - Acidic zincate composition for aluminum or aluminum alloy and zincate treatment method using the same - Google Patents

Abstract

The present invention relates to an acidic zincate composition for aluminum or an aluminum alloy and a zincate treatment method using the same.
The acidic zincate composition for aluminum or an aluminum alloy according to the present invention contains 27 to 33 parts by weight of zinc acetate, 65 to 75 parts by weight of nickel sulfate, 3 to 7 parts by weight of cobalt sulfate, 27 to 33 parts by weight of ammonium acetate, 4 to 8 parts by weight of ammonium bifluoride, 27 to 33 parts by weight of malic acid, 0.5 to 1.5 parts by weight of bismuth oxide, 0.05 to 0.15 parts by weight of thiourea, and 0.5 to 1.5 parts by weight of an Fe compound to form a zincate composition, and the zincate composition has a pH in a range of 4.0 to 6.5.
According to the present invention, the acidic zincate composition for aluminum or an aluminum alloy, by using an acidic zincate composition that does not contain cyanide rather than an alkaline zincate containing cyanide, can realize plating adhesion equivalent to that of a conventional two-time alkaline zincate treatment with only one-time acidic zincate treatment, thereby shortening the process time and cost.

Description

{ACIDIC ZINCATE COMPOSITION FOR ALUMINUM OR ALUMINUM ALLOY AND ZINCATE TREATMENT METHOD USING THE SAME}

The present invention relates to an acidic zincate composition for aluminum or an aluminum alloy and a zincate treatment method using the same, and more specifically, to an acidic zincate composition for aluminum or an aluminum alloy and a zincate treatment method using the same, which enables a plating adhesion equivalent to that of a conventional two-time alkaline zincate treatment to be achieved with only a single acidic zincate treatment by using an acidic zincate composition that does not contain cyanide rather than an alkaline zincate containing cyanide, thereby shortening the process time and cost.

Aluminum-based metals have been widely used for industrial purposes for a long time. The aluminum-based metals have excellent corrosion resistance, thermal conductivity, and ductility and are easy to form and cut. In addition, they are lighter than iron, so the demand for electroplated aluminum or aluminum alloys is increasing for products and parts in industries that require weight reduction. Electroplating is widely used as a surface treatment method to improve the wear resistance, corrosion resistance, magnetic properties, etc. of such aluminum or aluminum alloys.

It is widely known in the art that the plating process for aluminum is much more difficult than the plating process for other metals such as steel or copper alloys. Aluminum-based metals are amphoteric metals, have high surface activity, and have a high affinity for oxygen, so their surfaces are easily oxidized to form an oxide film. Even if this oxide film is removed, it is easily regenerated.

Since the aluminum oxide film causes poor adhesion between the plating film and the plated material, thereby reducing the adhesion of the plating film, a special pretreatment process is required to remove the oxide film before applying a surface treatment process such as plating to aluminum-based metals.

In the past, when plating aluminum-based metals, zincate treatment, a zinc substitution treatment called immersion plating or substitution plating, was essential as a special pretreatment method, and after the zincate treatment, strike plating or electroless plating was performed to achieve the target final plating.

If we examine the treatment processes for plating these conventional aluminum-based metals by dividing them into (1) degreasing - (2) washing - (3) etching - (4) washing - (5) desmut - (6) washing - (7) primary zincate treatment - (8) washing - (9) pickling - (10) washing - (11) secondary zincate treatment - (12) washing - (13) strike plating or electroless plating - (14) washing - (15) electroless nickel plating or electrolytic strike plating - electrolytic copper plating - electrolytic nickel plating - electrolytic chrome plating - (16) washing, there were problems in that many treatment processes were required, making the production line long, taking a long time, and unavoidable use of many chemicals and hazardous substances.

Zincate is a process that substitutes zinc nuclei on the surface of aluminum or aluminum alloy by utilizing the potential difference between the surface of aluminum or aluminum alloy and zinc in a zincate solution. When washed aluminum or aluminum alloy is immersed in a zincate treatment solution, oxidation and reduction reactions occur simultaneously on the surface of aluminum or aluminum alloy, so that aluminum is substituted with Zn ²⁺ and zinc is precipitated on the aluminum structure.

In more detail, when zincate is treated, ^Zn2 + is precipitated on the aluminum surface by a chemical reaction and zinc is separated, and Al3 ⁺ is oxidized and separated from the aluminum or aluminum alloy. The zincate layer is a fugitive coating that disappears in the subsequent metal plating operation, and the process of removing and activating the oxide layer on the surface provides conductivity to the aluminum film.

Therefore, in order to provide a more fine and uniform zinc surface texture, a secondary zincate treatment is generally additionally performed. The first zincate treatment removes the oxide film and alloy inclusions on the aluminum surface to precipitate primary zinc crystals, and the second zincate treatment provides a more uniform, thinner, and denser zinc surface texture to the aluminum or aluminum alloy surface by performing the zincate treatment once more.

In the past, alkaline type zincate was generally used for such zincate treatment, and this procedure has long been considered the most economical and practical aluminum pretreatment method. The main advantage of applying a zincate layer for pretreatment is the relatively low cost of equipment and chemicals.

However, in the past, when zincate was used, there was a negative aspect to the use of cyanide, namely, the problem of extreme toxicity, and some non-cyanide alloy zincates developed in recent years contained hard complexing agents such as EDTA, NTA, ethylene, and diamine, which not only required disposal of the zincate solution, but also made cleaning more difficult.

Domestic registration patent No. 10-1022006 (registered on March 7, 2011) Domestic registration patent No. 10-1854195 (registered on April 26, 2018) Domestic registration patent No. 10-1751377 (registered on June 21, 2017)

The present invention provides an acidic zincate composition for aluminum or an aluminum alloy, which can realize plating adhesion equivalent to that of a conventional two-time alkaline zincate treatment by only one-time acidic zincate treatment, by using an acidic zincate composition that does not contain cyanide, rather than an alkaline zincate containing cyanide, thereby shortening the process time and cost, and a zincate treatment method using the same.

In addition, the present invention provides an acidic zincate composition for aluminum or an aluminum alloy, which can simplify the process by reducing the number of zincate treatment cycles, while forming a fine and uniform zinc layer and improving adhesion with a plated layer, and a zincate treatment method using the same.

The various problems to be solved by the present invention are not limited to the problems mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the description below.

The acidic zincate composition for aluminum or an aluminum alloy according to the present invention contains 27 to 33 parts by weight of zinc acetate, 65 to 75 parts by weight of nickel sulfate, 3 to 7 parts by weight of cobalt sulfate, 27 to 33 parts by weight of ammonium acetate, 4 to 8 parts by weight of ammonium bifluoride, 27 to 33 parts by weight of malic acid, 0.5 to 1.5 parts by weight of bismuth oxide, 0.05 to 0.15 parts by weight of thiourea, and 0.5 to 1.5 parts by weight of an Fe compound to form a zincate composition, and the zincate composition has a pH in a range of 4.0 to 6.5.

In addition, the acid zincate treatment method for aluminum or an aluminum alloy according to the present invention comprises etching, desmut and zincate processes, wherein the etching is performed using 80 ml/L of an acid etching solution at a temperature of 30°C for 10 minutes.

The above zincate is contained in a weight ratio of 27 to 33 parts by weight of zinc acetate, 65 to 75 parts by weight of nickel sulfate, 3 to 7 parts by weight of cobalt sulfate, 27 to 33 parts by weight of ammonium acetate, 4 to 8 parts by weight of ammonium bifluoride, 27 to 33 parts by weight of malic acid, 0.5 to 1.5 parts by weight of bismuth oxide, 0.05 to 0.15 parts by weight of thiourea, and 0.5 to 1.5 parts by weight of an Fe compound, and the pH may be in a range of 4.0 to 6.5.

The composition constituting the above zincate is treated to substitute a zinc alloy layer on aluminum or an aluminum alloy, and the aluminum or aluminum alloy substituted with the zinc alloy can be plated at a temperature of 90°C for 20 minutes using an electroless nickel plating solution.

The above etching solution may be composed of an etching solution composition containing a weight ratio of 40 to 50 parts by weight of sulfuric acid, 10 to 20 parts by weight of phosphoric acid, 5 to 10 parts by weight of a phosphate compound, 3 to 7 parts by weight of a thiourea compound, 1 to 5 parts by weight of a glycol ether compound, 2 to 4 parts by weight of benzotriazole, 1 to 5 parts by weight of a persulfate, 1 to 3 parts by weight of a glycol derivative, 0.5 to 1.5 parts by weight of an antistatic additive, 0.1 to 1 part by weight of a dispersant, and 20 to 40 parts by weight of deionized water.

The above dispersant may be a compound represented by the following [chemical formula 2].

[Chemical formula 2]

Specific details of other embodiments are included in the detailed description.

The acidic zincate composition for aluminum or an aluminum alloy according to the present invention uses an acidic zincate composition that does not contain cyanide, rather than an alkaline zincate containing cyanide, thereby enabling a plating adhesion equivalent to that of a conventional two-time alkaline zincate treatment to be achieved with only one-time acidic zincate treatment, thereby shortening the process time and cost.

In addition, the acidic zincate composition for aluminum or aluminum alloy according to the present invention can simplify the process by reducing the number of zincate treatment cycles, while forming a fine and uniform zinc layer and improving adhesion to the plated layer.

It will be fully appreciated that embodiments of the technical idea of the present invention can provide various effects not specifically mentioned.

Figure 1 is a comparative photograph showing the surface of an aluminum specimen after etching the composition of [Table 1] onto the aluminum specimen, taken with an electron microscope (550x).
Figure 2 is a comparative photograph showing the surface of an aluminum specimen taken with an electron microscope (550x) after the composition of [Table 2] was treated with a dismutase agent.
Figure 3 is a sample photograph showing a 90° bending test method for a plated sample treated with the composition shown in [Table 3].
Figure 4 is a photograph showing the gloss after plating on aluminum material.
Figure 5 is a photograph showing aluminum plating specimens according to the amount of zincate treatment.
Figure 6a is an SEM image after the first stage of alkaline zincate treatment, and Figure 6b is an SEM image after the second stage of alkaline zincate treatment.
Figure 7 is an SEM photograph after the first stage of acid zincate treatment.
Figure 8 is an SEM photograph after acid zincate optimization treatment.

The advantages and features of the present invention, and the method for achieving them, will become clear with reference to the embodiments described in detail below. However, the present invention is not limited to the embodiments described herein and may be embodied in other forms. Rather, the embodiments introduced herein are provided so that the disclosed content can be thorough and complete, and so that the spirit of the present invention can be sufficiently conveyed to those skilled in the art.

The terminology used in this application is only used to describe specific embodiments and is not intended to limit the present invention. The singular expression includes the plural expression unless the context clearly indicates otherwise.

Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Terms defined in commonly used dictionaries, such as those defined in common dictionaries, should be interpreted as having a meaning consistent with the meaning they have in the context of the relevant art, and shall not be interpreted in an idealized or overly formal sense, unless expressly defined in this application.

Hereinafter, preferred examples of an acidic zincate composition for aluminum or aluminum alloy according to the present invention will be described in detail.

The acidic zincate composition for aluminum or an aluminum alloy according to the present invention comprises zinc acetate, nickel sulfate, cobalt sulfate, ammonium acetate, ammonium bifluoride, malic acid, bismuth oxide, thiourea, and an Fe compound.

In addition, the acidic zincate composition for aluminum or an aluminum alloy according to the present invention contains 27 to 33 parts by weight of zinc acetate, 65 to 75 parts by weight of nickel sulfate, 3 to 7 parts by weight of cobalt sulfate, 27 to 33 parts by weight of ammonium acetate, 4 to 8 parts by weight of ammonium bifluoride, 27 to 33 parts by weight of malic acid, 0.5 to 1.5 parts by weight of bismuth oxide, 0.05 to 0.15 parts by weight of thiourea, and 0.5 to 1.5 parts by weight of an Fe compound, and the acidic zincate composition may have a pH in a range of 4.0 to 6.5.

Hereinafter, with reference to the attached drawings, a preferred embodiment of an acid zincate treatment method for aluminum or an aluminum alloy according to the present invention will be described in more detail.

The acid zincate treatment method for aluminum or aluminum alloy according to the present invention includes etching, desmut and zincate.

1. Etching

The above etching is a step for improving the adhesion (roughness) of aluminum materials by removing the natural oxide film and micron-wise corroding the surface of aluminum or aluminum alloy (hereinafter, aluminum).

Conventionally, alkaline etching has been performed using a dipping method using NaOH. The alkaline etching can dissolve aluminum well and remove the physically adsorbed oil through the generated hydrogen, while also making the surface smooth.

However, the hydrogen generated in the process gives a stinging sensation to the face of the worker, which reduces work efficiency. In particular, as the number of etching agents increases, the amount of dissolved aluminum (Al ³⁺ ) in the etching solution increases, which significantly reduces the etching effect. When the dissolved aluminum becomes supersaturated, it changes to aluminum oxide, forming hard scale that accumulates at the bottom of the work tank and is difficult to remove.

In addition, since the etching solution becomes insufficient in proportion to the amount of precipitated alumina lumps, there is a problem in that the etching solution replenishment cycle becomes shorter and the etching cost increases each time the insufficient etching solution is replenished.

In order to improve the etching function, phosphoric acid ( _m P ₂ O _5n H ₂ 0), alkyl sulfonate, potassium salts, etc. are used to penetrate the aluminum surface details, an inhibitor is used for uniform etching and rust prevention, ethylene glycol is used to form etch pits, and a surfactant is used to lower the surface tension of the etching solution, improve wettability, and achieve uniform etching. By manufacturing an acidic type etching solution that can be used at room temperature (25°C) using the materials described above, the problems of the conventional alkaline etching can be solved.

That is, acid etching does not require increasing the temperature, and even if it does, it requires little heating because the amount of heat required is small, and the amount of aluminum dissolved is 1/3 to 1/5 less than that of alkaline etching, so the consumption is small, and the generation of precipitates (alumina) is slow, so the solution is easy to manage and can be simply corrected.

In the case of alkaline etching, uniform etching cannot be achieved if temperature and concentration are not managed accurately, but acid etching can produce products of uniform quality because it is a room temperature process.

In the present invention, the etching solution can be manufactured by including the following etching solution composition.

The above etching composition comprises sulfuric acid, phosphoric acid, a phosphate compound, a thiourea compound, a glycol ether compound, benzotriazole, a persulfate, a glycol derivative, an antistatic additive, a dispersant, and deionized water.

In addition, the etching composition may contain a weight ratio of 40 to 50 parts by weight of sulfuric acid, 10 to 20 parts by weight of phosphoric acid, 5 to 10 parts by weight of a phosphate compound, 3 to 7 parts by weight of a thiourea compound, 1 to 5 parts by weight of a glycol ether compound, 2 to 4 parts by weight of benzotriazole, 1 to 5 parts by weight of a persulfate, 1 to 3 parts by weight of a glycol derivative, 0.5 to 1.5 parts by weight of an antistatic additive, 0.1 to 1 part by weight of a dispersant, and 20 to 40 parts by weight of deionized water.

The above sulfuric acid and phosphoric acid may be included to micron-wise corrode the aluminum surface, and the sulfuric acid may be included in a weight ratio of 40 to 50 parts by weight based on the total content of the etching solution composition, and the phosphoric acid may be included in a weight ratio of 10 to 20 parts by weight. If the sulfuric acid and phosphoric acid are included in an amount less than the lower limit range described above, a problem may arise in which it is difficult for the aluminum surface to be sufficiently corroded, and if the sulfuric acid and phosphoric acid are included in an amount exceeding the upper limit range described above, a problem may arise in which the aluminum surface is excessively corroded.

The above phosphate compound may be added to prevent the aluminum surface from being overcorroded and to control the etching rate so that etching proceeds uniformly. As the phosphate compound, at least one selected from the group consisting of monobasic sodium phosphate (NaH ₂ PO ₄ ), dibasic sodium phosphate (Na ₂ HPO ₄ ), tribasic sodium phosphate (Na ₃ PO ₄ ), monobasic potassium phosphate (KH ₂ PO ₄ ), dibasic potassium phosphate (K ₂ HPO ₄ ), monobasic ammonium phosphate ((NH ₄ )H ₂ PO ₄ ), dibasic ammonium phosphate ((NH ₄ ) ₂ HPO ₄ ), and tribasic ammonium phosphate ((NH ₄ ) ₃ PO ₄ ) may be used.

The above phosphate compound may be included in a weight ratio of 5 to 10 parts by weight of the total content of the etching solution composition. However, if the phosphate compound is included in an amount of less than 5 parts by weight, there is a problem in that it is difficult to sufficiently perform its role as an etching rate regulator for the aluminum surface, and if it is included in an amount of more than 10 parts by weight, the etching rate may be reduced, making it difficult to corrode the aluminum surface.

The above thiourea compound may be included to ensure that the etching rate of the aluminum surface is not reduced when etching the aluminum surface and to ensure that the etching solution composition is stably maintained.

The above thiourea compound may include thiourea (NH ₂ ) ₂ CS or a thiourea derivative represented by the general formula (R1R2N)(R3R4N)C=S (wherein, R1, R2, R3, and R4 are each independently selected from a hydrogen atom, an ethyl group, and a methyl group).

The above thiourea compound may be included in a weight ratio of 3 to 7 parts by weight of the total content of the etching composition. However, if the thiourea is included in less than 3 parts by weight, the content of the added thiourea is small, so the effect of reducing the etching speed is minimal. If the content exceeds 7 parts by weight, there is no further significant increase in the effect, and the relative content of other components is small, so the stability of the etching composition may be deteriorated.

The above glycol ether compound can reduce undercut and enable uniform etching by improving penetrability at the interface of the aluminum surface. The glycol ether compound may be at least one selected from the group consisting of ethylene glycol ethers, diethylene glycol ethers, and propylene glycol ethers.

For example, the ethylene glycol ethers may be at least one selected from the group consisting of ethylene glycol monopropyl ether, ethyl acetate, and ethyl vinyl ether, the diethylene glycol ethers may be at least one selected from the group consisting of diethylene glycol diethyl ether, diethylene glycol dimethyl ether, and diethylene glycol ethyl methyl ether, and the propylene glycol ethers may be at least one selected from the group consisting of propylene glycol dimethyl ether, propylene glycol monobutyl ether, and propylene glycol monobutyl ether.

The above benzotriazole can play a role in controlling the corrosion rate of the aluminum surface, and the benzotriazole can be included in a weight ratio of 2 to 4 parts by weight of the total content of the etching solution composition.

If the above benzotriazole is included in an amount of less than 2 parts by weight, it is difficult to sufficiently control the etching speed of the aluminum surface, which may cause excessive corrosion, and if it exceeds 4% by weight, the etching speed of the aluminum surface is reduced, which may lengthen the etching time and lower productivity.

The above persulfate is a substance used as an oxidizing agent for etching the aluminum surface, and the above persulfate can enable etching to be performed easily. As the above persulfate, at least one selected from ammonium persulfate ((NH ₄ ) ₂ S ₂ O ₈ ), sodium persulfate (Na ₂ S ₂ O ₈ ), and potassium persulfate (K ₂ O ₂ S ₈ ) can be used.

The above persulfate may be included in a weight ratio of 1 to 5 parts by weight based on the total content of the etching solution composition. If the persulfate is included in an amount less than 1 part by weight, the content of the persulfate may be too small to sufficiently function as an oxidizing agent, and if it is included in an amount exceeding 5 parts by weight, the relative content of the persulfate may be excessive, causing excessive corrosion of the aluminum surface.

The above glycol derivative acts as an oxidation stabilizer, and the glycol derivative can stabilize the substances used as oxidizing agents by preventing them from being decomposed in small amounts as etching progresses. The glycol derivative is represented by R ₁ (OR ₂ )OH, where R ₁ = C _n H _2n+1 (n = 1 to 4) and R ₂ = C _m H _2m (m = 2 to 3). For example, HP-3 (trade name), one of the HP series of 3M Company, can be used as the glycol derivative.

As the above-mentioned antistatic additive, poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrenesulfonate) can be used. The above-mentioned antistatic additive is a dark blue liquid, has a pH of 1.7 to 2.3, a flash point of 100°C or higher, and a vapor density of 0.98±0.1 (25°C).

The above dispersant can be a material having excellent dispersibility and stability in the etching solution composition. The above dispersant can be a compound represented by the following [chemical formula 1].

[Chemical Formula 1]

Here, X and X' are hydrogen atoms or benzoylamino groups, Z is a hydrogen atom, R1 to R4 may be the same or different and are methyl groups or ethyl groups, and n and m mean 2 or 3.

Specifically, the dispersant may be a dispersant represented by the following [chemical formula 2].

[Chemical formula 2]

The above deionized water is water from which ions have been removed, and it is preferable to use deionized water having a resistivity value of 18 MΩㆍcm or higher.

[Table 1] below is a table showing the etching solution composition and conditions.

division
Alkaline etching solution
Acid etchant
furtherance
Sodium hydroxide: 50~100g/L
Etching solution: 80ml/L
temperature
30℃
30℃
hour
10 minutes
10 minutes

In the above [Table 1], the etching solution was prepared by containing 45 parts by weight of sulfuric acid, 15 parts by weight of phosphoric acid, 7 parts by weight of a phosphate compound, 5 parts by weight of a thiourea compound, 3 parts by weight of a glycol ether compound, 3 parts by weight of benzotriazole, 3 parts by weight of a persulfate, 2 parts by weight of a glycol derivative, 1 part by weight of an antistatic additive, 0.5 parts by weight of a dispersant, and 30 parts by weight of deionized water.

Figure 1 is a comparative photograph showing the surface of an aluminum specimen after etching the composition of [Table 1] onto the aluminum specimen, taken with an electron microscope (550x).

At this time, Blank in Fig. 1 is a photograph showing the surface of an unetched aluminum specimen taken with an electron microscope (550x).

Referring to Figure 1, it can be seen that the alkaline etching solution causes more severe corrosion of the aluminum surface than the acidic etching solution, and when the weight of the specimen was measured after etching solution treatment, 259 mg was dissolved in the alkaline etching solution and 170 mg was dissolved in the acidic etching solution.

This shows that since alkaline etching solutions cause more corrosion of materials and have a greater amount of aluminum dissolved than acidic etching solutions, there is a difference in not only the life of the etching solution but also in the deterioration of performance as the processing amount increases.

In addition, in the case of alkaline etching, the surface of the aluminum specimen appears to be blunted, which means that the formation of unevenness due to over-etching is increasing, and when checking the adhesion test after actual plating, the result may be poor adhesion.

On the other hand, in the case of acidic etching solutions, it can be seen that the aluminum material surface is relatively less corroded.

In conclusion, it can be seen that acidic etching solution can reduce corrosion of aluminum surface and relatively maintain the gloss of the surface, and also slow down the supersaturation state according to the amount of dissolution, thereby reducing the amount of scale that changes into aluminum oxide.

2. Desmut

After etching, most of the irregular or multi-colored surfaces are attached to the material in black after etching due to the difference between the alloy components such as Mg, Cu, Si, and Mn on the surface and the etching speed of the base material, but do not have a strong adhesive and usually exist on the surface as a fine powder in the form of a hydrate ( _nH2O ), which causes poor adhesion of the plating layer and pitting, etc. This is called smut, and the process of removing it is called desmut.

In the above dismutase, the existing nitric acid and hydrofluoric acid are harmful to the human body, and the existing acid treatment work generates a large amount of gas (NO, _NO2 , HF), which worsens the working environment and causes problems with exhaust gas and wastewater treatment. In addition, Si is removed with fluoride and ammonium fluoride compounds, and hydrogen peroxide ( _H2O2 ) can be used as _an accelerator to promote oxidation.

[Table 2] below shows the composition and conditions of the dismutase.

division
Dismertje
Nitrate Free Dismutase
furtherance
Nitric acid and hydrofluoric acid mixture
Dismutase: 80ml/L
temperature
20℃
20℃
hour
1 minute
1 minute

Figure 2 is a comparative photograph showing the surface of an aluminum specimen taken with an electron microscope (550x) after the composition of [Table 2] was treated with a dismutase agent.

At this time, Blank in Fig. 2 is a photograph showing the surface of an aluminum specimen that has not been treated with a desmut agent, taken with an electron microscope (550x).

Referring to Figure 2, it can be seen that the nitrate-free desmut agent exhibits better or equal performance than the nitric acid desmut agent. As described above, the nitrate-free (ammonium fluoride series) desmut agent can solve the following problems: a pleasant environment without nitric acid gas, resolution of environmental regulations, ease of wastewater treatment, matting occurrence and residual smut phenomenon that occur when removing smut.

3. Zincate (Zinc replacement)

Due to the chemical reaction of the aluminum surface with the zincate solution, Zn ²⁺ is precipitated and zinc is separated on the aluminum surface, and at the same time, Al ³⁺ is oxidized and falls off from the aluminum material, and the zincate layer is removed during the subsequent metal plating operation. This is a fugitive coating that corresponds to the process of removing and activating the oxide layer on the surface, thereby imparting conductivity to the aluminum film.

That is, the zincate process uses the potential difference between the aluminum surface and zinc in the zincate solution to substitute zinc nuclei on the aluminum surface, thereby allowing other metals such as copper and nickel to be plated directly.

The conventional alkaline zincate treatment process performs a two-step zincate treatment to provide a uniform texture state to the zinc layer surface.

① Step 1 zincate treatment is a process to primarily precipitate zinc crystals by removing the oxide film and alloy contents on the aluminum surface, and ② Step 2 zincate treatment is a process to repeat the above treatment to provide a uniform, thin, and dense zinc surface texture on the aluminum surface.

The above alkaline zincate must necessarily use a cyanide series (CN) chemical to secure adhesion between the aluminum material and the plating layer. The advantages of the above alkaline zincate include that numerous prior studies have been conducted, making it easy to design the experimental process, a mass production system has been well established, and low equipment and chemical costs.

On the other hand, as a disadvantage, it is inevitable to use toxic cyanide substances, which poses a fatal problem from an environmental perspective, and the alkaline type multi-zincate process has more complex process steps than a single process, which increases the time required and cost.

Bicyanide alloy zincates developed in recent years contain hard complexing agents such as EDTA, NTA, ethylene, and diamine, which causes problems such as waste disposal of zincate solutions and difficult cleaning.

To solve this problem, it is necessary to unify the process steps (etching, desmut) of neutral and weakly acidic series, develop a non-cyanide zincate solution of acidic series excluding cyanide substances, and shorten the process using this.

(1) Alkaline zincate composition

Zinc oxide is mainly used as the metal ion, and nickel and copper ions are used as auxiliary metal ions.

The most commonly used chelators (complexing agents) include tartaric acid, Rochell's salt, acetic acid, citric acid, lactic acid, EDTA, NTA, and diethylamine. Occasionally, when a hard complexing agent such as EDTA is used, there may be difficulties in wastewater treatment when disposing of the treatment solution.

To increase the adhesion (bonding) with the post-plating during zincate deposition, NaCN (sodium cyanide) and KCN (potassium cyanide) are mainly used, and reaction stabilizers (inhibitors) are included. In addition, Fe ions and NO _X ions are used as auxiliary components.

In the case of alkaline zincate manufactured based on the composition described above, when aluminum and aluminum alloys are immersed, the amount of material dissolved is high due to the high pH, and it is difficult to secure satisfactory adhesion in post-plating with a single-step zincate treatment, so a two-step double-zincate process is performed. However, the two-step double-zincate process has the problem of forming a film of zinc and zinc alloy, and also has the problem of generating precipitates due to iron[III] chloride hexahydrate, which is widely used to supply Fe ions in the zincate treatment solution, when used or left for a long time.

(2) Acid zincate composition not containing cyanide

The use of zinc, copper, nickel and other metal ions as metal ions is not much different from the alkaline zincates described above.

However, it does not contain No _X and CN ions, which cause environmental problems.

[Table 3] below is a table showing compositions of various examples for acid zincate testing (contents of compositions are expressed in parts by weight).

division
#1
#2
#3
#4
#5
#6
#7
#8
#9
Zinc acetate
30
30
30
30
30
30
30
30
30
Nickel sulfate
70
70
70
70
70
70
70
70
70
Cobalt sulfate
5
5
5
5
5
5
5
5
5
Ammonium acetate
30
30
30
30
30
30
30
30
30
Ammonium bifluoride
6
6
6
6
6
6
6
6
6
Malic acid
30
30
30
30
30
30
30
30
30
2-Mercaptobenzothiazole
0.01
-
-
-
-
-
-
-
-
2,2-dipyridyl
-
0.01
-
-
-
-
-
-
-
Benzotriazole
-
-
0.05
-
-
-
-
-
-
Bismuth oxide
-
-
-
1
-
-
1
-
1
Thiourea
-
-
-
-
0.1
-
0.1
0.1
0.1
Fe compounds
-
-
-
-
-
1
-
1
1
pH
4.0~6.5
4.0~6.5
4.0~6.5
4.0~6.5
4.0~6.5
4.0~6.5
4.0~6.5
4.0~6.5
4.0~6.5

The compositions of Examples 1 to 9 (indicated as #1 to #9) including the compositions shown in the above [Table 3] were processed to substitute a zinc alloy layer on an aluminum alloy, and then the aluminum alloy substituted with the zinc alloy was plated with an electroless nickel plating solution at a temperature of 90°C for 20 minutes.

The above-mentioned plated sample was washed with water, dried, and then tested for adhesion through a 90° bending test as shown in Fig. 3, and the results are shown in [Table 4] below.

Figure 3 is a sample photograph showing a 90° bending test method for a plated sample treated with the composition shown in [Table 3].

division
Aluminum 5000 series
Aluminum 6000 series
Aluminum Diecasting
Example #1
titration
titration
titration
Example #2
titration
titration
titration
Example #3
titration
titration
titration
Example #4
titration
titration
titration
Example #5
titration
Good
titration
Example #6
titration
Good
Good
Example #7
Good
Good
Good
Example #8
Good
Good
Good
Example #9
Good
Good
Good

Referring to the above [Table 4], the plating adhesion test results for each example were conducted with the addition amounts of zinc acetate and nickel sulfate at a fixed concentration without change, and the remaining additive compositions were arbitrarily adjusted according to the test results.

The above composition was determined as the optimal combination of raw materials that supply metal ions with a pH range of 4.0 to 6.5 by confirming the reactions (hydrate formation, decomposition reaction, and inappropriate pH range, etc.) when metal salts are dissolved through preliminary tests.

In the case of zincate manufactured from a zinc/nickel alloy, the concentration of nickel ions is higher than that of zinc ions.

When aluminum material is immersed in an aqueous solution composed of the above composition, an etching reaction occurs due to fluoride ions, Al ³⁺ is separated (oxidized) from aluminum, and Zn ²⁺ is first substituted on the aluminum surface due to the ionization tendency, and then Ni ²⁺ is substituted.

Since nickel ions are precipitated on the aluminum surface in addition to zinc ions, the zinc/nickel alloy form has better adhesion (bonding) with the post-plating layer than the single zincate of zinc ions, and by adding cobalt ions to this, stronger adhesion is secured.

Fluoride ions used to etch the surface of aluminum materials include soluble fluoride ions such as ammonium acid fluoride, sodium fluoride, and ammonium borofluoride.

① Improvement of preprocessing process

The existing process is a mixing process of an alkaline aqueous solution and an acidic aqueous solution, and after each step of treatment, an oxide film is formed on the surface of the material during rinsing, which causes stains and poor plating adhesion due to neutralization. To solve this problem, uneconomical and unproductive processes such as increasing the rinsing water and overflow are performed.

By replacing this with the pretreatment chemical and acid zincate developed through prior research, the process was applied as a neutral or acidic aqueous solution type process of the pretreatment process, and the two-step zincate treatment was simplified into a one-step zincate treatment to conduct the experiment.

[Table 5] below is a table showing the applicability of the pretreatment process.

division
E ching
Dismute
Zincate
product
ALCO - 19 (70ml/L)
ALCO - 81 (100ml/L)
Acid Zinc (1 liquid)
Processing conditions
Room temperature, 1 minute
Room temperature, 1 minute
Room temperature, 1 minute

② Apply pretreatment and improve post-plating gloss

One problem that arose while applying the neutral or acid type process was the surface gloss after plating.

This is because the glossiness of the specimens made through the conventional process and the specimens made through the acid type process were different when confirmed with the naked eye.

Accordingly, experiments were conducted to increase the gloss of materials such as a polishing agent and surfactant in the acid zincate composition, but no significant results were achieved and instead, a problem occurred in which the performance of the zincate was reduced. Accordingly, the pretreatment conditions were changed and conducted.

Aluminum is an amphoteric metal, meaning it reacts with both alkalis and acids, but reacts more easily with alkalis.

In the case of the existing process, the main pretreatment process is an alkaline aqueous solution, so it is easy to form irregularities or remove smut on the material, which provides excellent gloss to the material after plating. On the other hand, in the case of an acidic aqueous solution, it is found that the formation of irregularities and removal of smut are relatively less than in the alkaline aqueous solution, so it does not provide excellent gloss to the material after plating. Therefore, the pretreatment conditions were changed and an experiment was conducted.

It was found that when the desmut treatment was prolonged or when the etching and desmut treatment were prolonged together, the effect was reduced, and when the zincate treatment was prolonged, the adhesion and gloss were reduced.

The optimal condition was to perform etching for 3 minutes to remove the remaining smut while forming the roughness of the aluminum material, and then perform zincate treatment to remove the fine smut in the desmut process, which resulted in the best gloss.

[Table 6] below is a table showing the optimal conditions for the pretreatment process, and Fig. 4 is a photograph showing the gloss after plating on aluminum material.

division
E ching
Dismute
Zincate
product
ALCO - 19 (70ml/L)
ALCO - 81 (100ml/L)
Acid Zinc (1 liquid)
Processing conditions
Room temperature, 3 minutes
Room temperature, 1 minute
Room temperature, 1 minute

Referring to the above [Table 6] and Fig. 4, it can be seen that the gloss measurement value was best in the optimized process where the etching conditions were changed.

When visually inspected, it was determined that the specimens made through the conventional process had better gloss than the specimens made through the acid process, but when looking at the result values, it was confirmed that they had almost the same or slightly better gloss.

③ Study on the reproducibility of the acid zincate application process

For field application of acid zincate, continuous plating was performed on a plating sample line to confirm the reproducibility of the plating solution.

Since acid zincate is a one-liquid type, an analysis method that can determine the amount of supplementation compared to the amount consumed is required prior to reproducibility testing. Accordingly, an analysis method was developed using a reverse calculation method that calculates the amount of zinc based on the amount of nickel, inspired by the nickel analysis method (when calculating the amount of supplementation in the one-liquid type, calculation is possible as a %).

a) Sample 5 ml of zincate solution and add it to a conical beaker along with 100 ml of distilled water.

A) Add 1:1 ammonia water until the sampling liquid turns blue, then add 0.2g of MX indicator.

d) Titrate with 0.05M EDTA standard solution until dark purple color appears.

A) Zincate (Zincate(%)) =〔(0.05M - EDTA titer ×0.59 ×F) ×0.69 〕×6.07

To verify the accuracy and reproducibility of the acid zincate analysis, aluminum specimens were treated with 50 dm, 100 dm, and 150 dm, and then zincate solution was analyzed and aluminum specimens were plated to compare with the reference specimen.

The aluminum specimen was ^1.3 dm2 and the plating conditions were etching for 3 minutes, desmut for 1 minute, zincate for 1 minute, and electroless nickel for 20 minutes. After plating to a thickness of approximately 6 to 6.5 ㎛, a pulling test was performed to check the gloss and adhesion.

[Table 7] below is a table showing analysis values according to the amount of zincate treatment, and Fig. 5 is a photograph showing aluminum plating specimens according to the amount of zincate treatment.

Acid zincate
EDTA appropriate amount (㎖)
Metal concentration (g)
% conversion value
Original
41.3
16.81
102
50% dilution
21.1
8.58
52
50dm ² /ℓ treatment solution
12.3
5
30
100dm ² /ℓ treatment solution
24.1
9.81
59
150dm ² /ℓ treatment solution
36.3
14.77
89

Referring to the above [Table 7] and Fig. 5, it can be seen that the analysis value (%) is almost the same as the reference value.

This shows that there is no problem with the zincate analysis method and that there is no problem with the amount of supplementation compared to the amount of consumption according to the amount of zincate processed.

In addition, there were no significant abnormalities in the pulling test of the plated specimens after 50-decy, 100-decy, and 150-decy zincate treatment, and the gloss was also maintained without significant difference.

The reproducibility test of zinc oxide liquid shows that there is no problem in operating for long periods of time while replenishing the amount consumed.

Zinc Process Optimization

In the aluminum plating process, pretreatment accounts for more than 70% of plating defects and is particularly essential for improving plating adhesion. Since the oxide film must be removed before plating, it can be considered even more important in the surface treatment of aluminum materials.

1) Shortening and optimizing the pretreatment process for aluminum materials

The zincate currently in use is a type that contains alkaline cyanide, so a two-step zincate treatment was required to obtain adhesion between the aluminum material and the plating layer.

The acidic nocyanide type zincate according to the present invention can replace this and secure adhesion of the same aluminum material and plating with a one-step zincate deposition, shorten the process, and improve productivity.

When conducting a plating test by applying the zincate according to the present invention, it was found that the gloss (roughness) of electroless nickel plating was lower than that of existing products when compared to the existing process. However, this could be improved by changing the pretreatment conditions (while keeping the other conditions the same, changing the etching conditions from 1 minute to 3 minutes), and a gloss higher than that of the electroless nickel plating of the existing process could be obtained.

This is because the acid process causes relatively less corrosion, or surface activation reaction, than the alkaline process, so there is less removal of smut and formation of unevenness on the surface of the aluminum, which affects the gloss. Accordingly, by giving more time to the etching process of the acid process, impurities and smut on the aluminum surface are removed, and by giving more unevenness formation, the gloss is improved. Basically, since the acid process causes less corrosion of the material than the alkaline process, a much better gloss can be obtained.

Zinc SEM-EDX Comparative Analysis

The surfaces of the alkaline zincate one-step treatment specimens and two-step treatment specimens, and the acidic zincate one-step treatment specimens and optimized treatment specimens were photographed and compared using SEM-EDX (scanning electron microscope) after zincate deposition.

Fig. 6a is an SEM image after a one-step alkaline zincate treatment, Fig. 6b is an SEM image after a two-step alkaline zincate treatment, Fig. 7 is an SEM image after a one-step acidic zincate treatment, and Fig. 8 is an SEM image after an optimized acidic zincate treatment.

Referring to FIGS. 6a to 8, it can be seen that a very rough surface is formed when the alkaline zincate is treated in the first stage, and that when the zincate is treated in the second stage, the roughness is formed densely and cleanly compared to the surface treated in the first stage.

In addition, when looking at the acid zincate 1-stage treatment and optimized treatment specimens, it can be seen that they are both similar to the alkaline zincate 2-stage treatment specimens.

As a result, it can be concluded that the two-stage alkaline zincate treatment and the one-stage acid zincate treatment have the same shape.

Zincate AFM Comparative Analysis

The results of comparing the alkaline zincate 1-stage treatment specimens, 2-stage treatment specimens, and acid zincate 1-stage treatment specimens showed that acid zincate produced better results than the alkaline zincate 2-stage treatment specimens.

AAS analysis of pretreatment solution

To confirm that the etching and desmut solutions of the pretreatment process developed through prior research have less aluminum dissolution than the etching and desmut solutions of the existing process, the analysis was conducted using an AAS analyzer (model number AA-7000) from SHIMADZU of Japan. The aluminum specimens were analyzed at 1.3 decimeters (d), and the dissolution amounts were calculated and compared after being analyzed at 50, 100, and 150 decimeters.

Etching

Table 8 below is a comparison table of etching solution dissolution amounts.

Etching (d)
After alkaline etching treatment
Dissolution analysis (ppm)
Dissolution rate per minute (ppm)
After acid etching treatment
Dissolution analysis (ppm)
Dissolution rate per minute (ppm)
50
268
7
50
1.3
100
769
10
96
1.25
150
1053
9.15
176
1.53

Desmut

Table 9 below is a comparison table of dissolution rates of dismutases.

Dismute (d)
Analysis of the amount of dissolved matter after desmut treatment (ppm)
Dissolution rate per minute (ppm)
Analysis of the amount of dissolved matter after treatment with Nitrate Free Dismutase (ppm)
Dissolution rate per minute (ppm)
50
76
2
30
0.8
100
169
2.2
69
0.9
150
260
2.26
110
0.96

Referring to the above [Table 8] and [Table 9], when comparing the elution amount of the etching solution or desmut agent, it can be seen that the elution amount of the developed drug is lower than that of the existing drug, and it can be seen that the elution amount increases further after 50 deci treatment.

This is due to overreaction as the activity of the drug increases with the amount processed, and at a certain point, it can lead to a decrease in the amount processed due to the accumulation of the amount of dissolved substances.

However, what is important here is that the drugs used in the existing process show relatively large amounts of dissolved substances, which means that the performance of the drugs, that is, the efficiency, is inferior to that of the developed drugs, and it is less productive in terms of cost compared to the time it takes to discard them and re-dry them at a certain point in time.

Although the preferred embodiment of the present invention has been described above, those skilled in the art will understand that the present invention can be implemented in other specific forms without changing the technical idea or essential features thereof. Therefore, it should be understood that the embodiment described above is exemplary in all respects and not restrictive.

Claims (6)

An acidic zincate composition for aluminum or an aluminum alloy, comprising 27 to 33 parts by weight of zinc acetate, 65 to 75 parts by weight of nickel sulfate, 3 to 7 parts by weight of cobalt sulfate, 27 to 33 parts by weight of ammonium acetate, 4 to 8 parts by weight of ammonium bifluoride, 27 to 33 parts by weight of malic acid, 0.5 to 1.5 parts by weight of bismuth oxide, 0.05 to 0.15 parts by weight of thiourea, and 0.5 to 1.5 parts by weight of an Fe compound, wherein the zincate composition has a pH in a range of 4.0 to 6.5.
In a method for acid zincate treatment for aluminum or aluminum alloy, including etching, desmut and zincate processes,
The above etching is performed using 80 ml/L of acidic etching solution at a temperature of 30°C for 10 minutes.
It is happening,
The above Zincate process is,
It comprises treating a composition constituting the above zincate to substitute a zinc alloy layer on aluminum or an aluminum alloy, and plating the aluminum or aluminum alloy substituted with the zinc alloy with an electroless nickel plating solution at a temperature of 90°C for 20 minutes.
A method for treating aluminum or an aluminum alloy with acid zincate, wherein the composition constituting the zincate contains 27 to 33 parts by weight of zinc acetate, 65 to 75 parts by weight of nickel sulfate, 3 to 7 parts by weight of cobalt sulfate, 27 to 33 parts by weight of ammonium acetate, 4 to 8 parts by weight of ammonium bifluoride, 27 to 33 parts by weight of malic acid, 0.5 to 1.5 parts by weight of bismuth oxide, 0.05 to 0.15 parts by weight of thiourea, and 0.5 to 1.5 parts by weight of an Fe compound, and has a pH in the range of 4.0 to 6.5.
In the second paragraph,
An acidic zincate treatment method for aluminum or an aluminum alloy, characterized in that the etching solution comprises an etching solution composition containing a weight ratio of 40 to 50 parts by weight of sulfuric acid, 10 to 20 parts by weight of phosphoric acid, 5 to 10 parts by weight of a phosphate compound, 3 to 7 parts by weight of a thiourea compound, 1 to 5 parts by weight of a glycol ether compound, 2 to 4 parts by weight of benzotriazole, 1 to 5 parts by weight of a persulfate, 1 to 3 parts by weight of a glycol derivative, 0.5 to 1.5 parts by weight of an antistatic additive, 0.1 to 1 part by weight of a dispersant, and 20 to 40 parts by weight of deionized water.

In the third paragraph,
A method for treating aluminum or an aluminum alloy with acid zincate, characterized in that the dispersant used is a compound represented by the following [chemical formula 1].
[Chemical Formula 1]

(Here, X and X' are hydrogen atoms or benzoylamino groups, Z is a hydrogen atom, R1 to R4 may be the same or different and are methyl groups or ethyl groups, and n and m mean 2 or 3.)
In paragraph 4,
A method for treating aluminum or an aluminum alloy with acid zincate, characterized in that the dispersant used is a compound represented by the following [chemical formula 2].
[Chemical formula 2]

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