Zinc plating method

Abstract

The present invention provides a zinc plating method that enables the formation of zinc plating with good corrosion resistance at a reasonable cost. [Solution] A zinc plating method comprising: a desmutting step of ultrasonically treating the surface of a metal substrate with a treatment solution containing an alkaline agent and a chelating agent; and a plating step of forming a zinc plating on the surface of the metal substrate using a zinc plating solution containing zinc ions, chloride ions, a buffer, and a brightener after the desmutting step.

Description

Technical Field

[0001] The present invention relates to a zinc plating treatment method.

Background Art

[0002] Zinc chloride plating is often used for cast iron (brake calipers) that are difficult to electroplate because of its very high electroplating deposition rate. Also, zinc chloride plating is roughly divided into two types, namely an ammonium chloride bath and a potassium chloride bath as described in Patent Document 1. Generally, most of the plating solutions used for cast iron are ammonium chloride baths.

[0003] However, different from ordinary steel products, brake calipers treated in a chloride bath are difficult to be plated, prone to poor appearance, and have low plating leveling property, making it difficult to achieve corrosion resistance. Therefore, improvement is required.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The reasons for the reduction in the corrosion resistance of brake calipers are considered as follows. If there is smut (carbon smut: observed as black spots in surface observation) on the surface of the material, it will inhibit electroplating deposition, making it difficult for the plating film to become dense, resulting in a decrease in plating leveling property and a consequent reduction in corrosion resistance. Especially in low-current areas, it is difficult for the brightener to take effect due to the presence of smut on the surface, making the plating more likely to be rough, and rust progresses from the parts that tend to be in the shade (low-current areas). [[ID=四十三]]

[0006] The following methods should be considered to improve the corrosion resistance of brake calipers using a chloride bath. 1. Increase the plating current density. 2. Add an excessive amount of plating brightener to improve leveling and coating properties. 3. Improve the performance of chemical treatment. 4. Apply coating treatment.

[0007] 1. Increase the plating current density. Increasing the current density makes it easier for the plating brightener to work and improves plating leveling, but if the current density is too high, the problem of high-current scorching occurs, requiring the addition of a scorching-preventing brightener. Also, it can lead to excessive plating thickness, making it difficult to ensure plating adhesion. Increasing the current density increases electricity costs and does not reduce overall costs. Furthermore, it is not very effective in areas that are easily shaded by the plating (low-current areas), and it is difficult to obtain an effect without devising the way the jig is attached and the plating bath composition (zinc concentration, total chlorine concentration, brightener), resulting in poor cost-effectiveness and difficulty in on-site management.

[0008] 2. Add an excessive amount of plating brightener to improve leveling and coating properties. By using excessive amounts of smoothing and brightening agents, the leveling properties of the plating, including in low-current areas, are improved, thereby enhancing corrosion resistance. However, this alone results in very poor plating durability, and it is difficult to achieve the desired effect without continuously adding significantly more brightening agents than in chloride baths used in rotary baths, leading to increased costs. Furthermore, due to the presence of smut on the material surface, the effectiveness of the smoothing and brightening agents is less than expected, and they are particularly ineffective in low-current areas.

[0009] 3. Improve the performance of chemical treatment. While a two-layer Cr·Si coating can improve corrosion resistance, it often depends on the leveling properties of the plating film. Furthermore, in low-current areas with poor leveling properties, the corrosion resistance is largely due to the plating itself and cannot be improved.

[0010] 4. Apply coating treatment. While corrosion resistance is significantly improved compared to methods 1-3 above, it becomes more dependent on the thickness of the coating film. If the coating becomes too thick, the coating film may peel off due to changes in internal stress over time. Also, because the coating process is immersion coating, there will inevitably be areas where the coating film becomes thicker due to the shape. Furthermore, it is not possible to apply the coating treatment to all items depending on the requirements, so it cannot solve all problems. In addition, the total cost is higher than methods 1-3 above because it includes a coating treatment process. Thus, it is difficult to achieve both improved corrosion resistance and cost reduction with any of these methods.

[0011] The object of this invention is to provide a zinc plating method that can form a zinc plating with good corrosion resistance at a good cost. [Means for solving the problem]

[0012] As a result of diligent research, the inventors have found that the above problem can be solved by the zinc plating method specified below.

[0013] (1) A desmutting process in which the surface of a metal substrate is subjected to ultrasonic treatment with a treatment solution containing an alkaline agent and a chelating agent, After the desmutting step, a plating step is performed in which zinc plating is formed on the surface of the metal substrate using a zinc plating solution containing zinc ions, chloride ions, a buffer, and a brightener. A zinc plating method, including the following: (2) The zinc plating method according to (1), wherein the zinc plating solution contains a β-naphthol-based anionic surfactant as the brightener. (3) The zinc plating method according to (1) or (2), wherein the chelating agent is a water-soluble carboxylic acid or a water-soluble amine. (4) The zinc plating method according to any one of (1) to (3) above, wherein the buffer is one or more of ammonia, boric acid, formic acid, acetic acid, gluconic acid, citric acid, malic acid, and amines. (5) After the plating process, there is a trivalent chromium conversion treatment step of treating the plated surface of the metal substrate with a hexavalent chromium-free conversion treatment solution containing trivalent chromium ions, nitrate ions, and colloidal silica, according to any one of the above (1) to (4). The zinc plating treatment method. (6) The zinc plating treatment method according to any one of the above (1) to (5), wherein the metal substrate is cast iron.

Effect of the Invention

[0014] According to the present invention, it is possible to provide a zinc plating treatment method capable of forming zinc plating with good corrosion resistance at a good cost.

Embodiments for Carrying Out the Invention

[0015] Next, the embodiments for carrying out the present invention will be described in detail. It should be understood that the present invention is not limited to the following embodiments, and design changes, improvements, etc. can be appropriately added based on the ordinary knowledge of those skilled in the art without departing from the gist of the present invention.

[0016] <Zinc Plating Treatment Method> The zinc plating treatment method according to an embodiment of the present invention includes a degreasing step of performing ultrasonic treatment on the surface of a metal substrate with a treatment solution containing an alkaline agent and a chelating agent, and after the degreasing step, a plating step of forming zinc plating on the surface of the metal substrate using a zinc plating solution containing zinc ions, chloride ions, a buffering agent, and a brightening agent. Further, in the zinc plating treatment method according to an embodiment of the present invention, as a pretreatment of the plating step, it is preferable to perform an alkaline degreasing step, a pickling step, and the above-described degreasing step in this order, and as a post-treatment of the plating step, it is preferable to perform a trivalent chromium conversion treatment step. Hereinafter, each step will be described in detail.

[0017] (Metal Substrate) In the zinc plating treatment method according to an embodiment of the present invention, the metal substrate to be treated is steel, which is iron containing carbon and may generate black dot stains during the zinc plating treatment. Further, although details will be described later, conventionally, when cast iron, which has been a problem for the generation of black dot stains, is used as the metal substrate, the generation of black dot stains is well suppressed after the zinc plating treatment, so the effects of the present invention can be more significantly confirmed. Examples of cast iron include gray cast iron and ductile cast iron.

[0018] (Alkaline degreasing process) In the alkaline degreasing process, the conditions are not particularly limited, and the treatment temperature and treatment time can be appropriately adjusted according to the state of the metal substrate. As the treatment liquid used for alkaline degreasing, a commercially available alkaline degreasing agent can be dissolved using sodium hydroxide as a solvent for preparation.

[0019] (Pickling process) In the pickling process, the metal substrate after alkaline degreasing is immersed in the treatment bath of the pickling treatment liquid for pickling. Pickling is preferably performed under the following conditions. · Type of acid: 35 mass% hydrochloric acid aqueous solution or 75 mass% sulfuric acid aqueous solution, preferably 35 mass% hydrochloric acid aqueous solution · Concentration of acid: 150 - 500 mL / L, preferably 250 - 400 mL / L · Inhibitor: A compound containing hexamethylenetriamine or a reaction product of epichlorohydrin and an amine compound · Concentration of inhibitor: 0.1 - 3.0 g / L, preferably 1.5 - 2.5 g / L · Treatment temperature: 20 - 40°C · Treatment time: 1 - 15 minutes, preferably 3 - 10 minutes

[0020] (Desmuttling process) In the smut removal process, the metal substrate after pickling is immersed in a treatment solution containing an alkaline agent and a chelating agent, and then subjected to ultrasonic treatment. Here, smut is a fine powdery substance consisting of carbon and metal oxides that adheres to the surface of the metal substrate. The presence of smut on the surface of the metal substrate inhibits plating electrodeposition, making it difficult for the plating film to become dense, thus reducing the leveling properties of the plating and leading to a decrease in corrosion resistance. In particular, in low-current areas, the effectiveness of the brightener is reduced and the presence of smut on the surface makes the plating more prone to becoming coarser, and rust progresses from areas that are easily shaded. In contrast, ultrasonic treatment with a treatment solution containing an alkaline agent and a chelating agent can effectively remove the smut. This normalizes the plating electrodeposition in the zinc plating process and makes the plating film denser.

[0021] Examples of alkaline agents used in the desmutting process include sodium hydroxide, potassium hydroxide, and sodium carbonate, with sodium hydroxide being preferred. The concentration of the alkaline agent is 40-90 g / L, preferably 50-80 g / L.

[0022] The chelating agent in the treatment solution used in the desmutting process is a water-soluble carboxylic acid such as gluconic acid, ethylenediaminetetraacetic acid (EDTA), citric acid, or tartaric acid, or a water-soluble amine such as triethanolamine or diethylenetriamine. Of these, gluconic acid, ethylenediaminetetraacetic acid (EDTA), citric acid, tartaric acid, and triethanolamine are preferred. The concentration of the chelating agent is 5 to 80 g / L, preferably 20 to 80 g / L.

[0023] The conditions for smut removal are as follows: Processing temperature: 25-70°C, preferably 40-60°C Processing time: 1-10 minutes, preferably 2-10 minutes. • Ultrasonic frequency: 28kHz~40kHz

[0024] Generally, anodic electrolysis is recommended for plating steel products, especially high-carbon steel. In anodic electrolysis, smut is removed by generating oxygen gas through the electrolysis of water and by electrolytic dissolution of the material. However, in the case of cast iron, because the material contains many impurities, smut is more likely to occur due to material dissolution than through electrolytic dissolution in anodic electrolysis. In addition, silicon contained in cast iron causes anodic oxidation, resulting in poor plating adhesion, so anodic electrolysis cannot effectively remove smut from cast iron. As a method of smut removal other than anodic electrolysis, there is immersion-type smut removal treatment. However, the chemicals used in immersion-type smut removal treatment are strong oxidizing agents, so when introduced into the plating solution, the plating solution is oxidized, leading to plating defects. Furthermore, because it contains oxidizing agents, it puts a burden on wastewater treatment, making its operation in actual processing impractical. For these reasons, it is reasonable to remove smut using a method that utilizes physical action other than anodic electrolysis. Furthermore, according to the zinc plating method according to the embodiment of the present invention, smut can be effectively removed by performing ultrasonic treatment with a treatment solution containing an alkaline agent and a chelating agent. As a result, the effectiveness (leveling) of the zinc plating brightener is improved, and overall corrosion resistance, including low-current areas, can be improved. In addition, with the normalization of the plating brightener, low-current plating, which was previously impossible, becomes possible, making it possible to reduce excessive film thickness and electricity costs. Furthermore, after the desmutting process, an acid activation process may be carried out in which the material is immersed in an aqueous hydrochloric acid solution for a predetermined time in order to neutralize the alkaline components on the material surface with acid and to activate the material surface.

[0025] (Plating process) In the plating process, the metal substrate is immersed in a zinc plating bath using a zinc plating solution, and electroplating is performed under the following plating conditions. The thickness of the resulting zinc plating is 1 to 50 μm. • Plating solution temperature: 20-50°C, preferably 25-40°C • pH of the plating solution: 4.5 to 6.0, preferably 5.0 to 5.8 (pH adjustment is preferably done with hydrochloric acid or potassium hydroxide). • Cathode current density of the plating bath: 1.0~4.0 A / dm 2 Preferably 1.5 to 4.0 A / dm 2

[0026] The zinc plating solution contains zinc ions, chloride ions, a buffer, and a brightener. Preferably, the brightener contains a β-naphthol-based anionic surfactant.

[0027] When a zinc plating solution contains a β-naphthol-based anionic surfactant as a brightener, the zinc plating film can be densified by electrodeposition along the surface of the metal substrate, thereby improving the plating current efficiency. As a result, potassium hydroxide is less likely to form, and the occurrence of black spots when potassium or sodium ions are present in the plating solution can be suppressed. Furthermore, after the plating process, an activation process may be carried out in which the plated surface is immersed in an aqueous solution of nitric acid or hydrochloric acid for a predetermined time in order to activate it.

[0028] (Zinc ions) Sources of zinc ions in zinc plating solutions include zinc chloride, zinc sulfate, and zinc oxide. Of these, zinc chloride is preferred. The zinc ion concentration in the zinc plating solution is preferably 20 to 100 g / L. A zinc ion concentration of 20 g / L or higher in the zinc plating solution has the effect of improving current efficiency. A zinc ion concentration of 100 g / L or lower in the zinc plating solution has the effect of improving the plating's spreadability. A zinc ion concentration of 30 to 60 g / L in the zinc plating solution is more preferable.

[0029] (Chloride ions) Sources of chloride ions in zinc plating solutions include potassium chloride, sodium chloride, and ammonium chloride. The chloride ion concentration in the zinc plating solution is preferably 160-250 g / L. A chloride ion concentration of 160 g / L or higher improves the plating's spreadability. A chloride ion concentration of 250 g / L or lower reduces the salt concentration in the plating bath, thus preventing salting out of the plating solution. A chloride ion concentration of 180-220 g / L is more preferable.

[0030] (buffering agent) Buffers in zinc plating solutions include one or more of the following: ammonia, boric acid, formic acid, acetic acid, gluconic acid, citric acid, malic acid, and amines. Of these, ammonia, boric acid, or acetic acid are more preferred.

[0031] The concentration of the buffering agent in the zinc plating solution is preferably 10 to 100 g / L. A buffering agent concentration of 10 g / L or higher in the zinc plating solution has the effect of preventing charring of the plating and improving the adhesion of the plating film. A buffering agent concentration of 100 g / L or lower in the zinc plating solution reduces the salt concentration in the plating bath, which prevents salting out of the plating solution and also improves current efficiency. A buffering agent concentration of 20 to 90 g / L in the zinc plating solution is more preferable.

[0032] (Gloss agent) The brightener in the zinc plating solution preferably contains a β-naphthol-based anionic surfactant. Examples of β-naphthol-based anionic surfactants include polyoxyethylene β-naphthol sulfonate and polyoxypropylene β-naphthol sulfonate. Among polyoxyethylene β-naphthol sulfonates, sodium polyoxyethylene β-naphthol sulfonate is particularly preferred.

[0033] The concentration of β-naphthol-based anionic surfactant as a brightener in the zinc plating solution is preferably 0.5 to 5 g / L. A concentration of β-naphthol-based anionic surfactant as a brightener in the zinc plating solution of 0.5 g / L or higher improves the adhesion of the plating and densifies the plating film. A concentration of β-naphthol-based anionic surfactant as a brightener in the zinc plating solution of 5 g / L or lower improves the cloud point (cloudiness) of the plating solution. A concentration of β-naphthol-based anionic surfactant as a brightener in the zinc plating solution of 1 to 3 g / L is more preferable.

[0034] In addition to β-naphthol-based anionic surfactants, the brightener in the zinc plating solution may also be one of the following: Polyoxyethylene alkyl ether sulfonates, aromatic carboxylates (benzoates, salicylates, cinnamic acid, m-chlorobenzoic acid, p-chlorobenzoic acid, etc.), aromatic aldehydes (vanillin, pyrogallol, anisaldehyde, benzaldehyde, o-chlorobenzaldehyde, etc.), aromatic ketones (benzylideneacetone, etc.), aromatic chlorides (benzoyl chloride, etc.), and the like.

[0035] The concentration of polyoxyethylene alkyl ether sulfonate in the zinc plating solution is preferably 1 to 10 g / L, and more preferably 3 to 9 g / L. The concentration of benzoates such as sodium benzoate in the zinc plating solution is preferably 1 to 10 g / L, and more preferably 1 to 8 g / L. The concentration of aromatic aldehydes, aromatic ketones, and aromatic chlorides in the zinc plating solution is preferably 0.01 to 1.0 g / L, and more preferably 0.01 to 0.4 g / L.

[0036] Furthermore, when forming a zinc alloy plating using a zinc plating solution, in addition to the above components, metal ions other than zinc and chelating agents may also be included. Alternatively, the brightener may be used in two parts: a primary brightener, which serves as a base agent for smoothing the plating, and a secondary brightener, which serves as a brightener for increasing the glossiness of the plating.

[0037] (Metal ions other than zinc) Other metal ions besides zinc include ions of metals that form alloy plating with zinc, such as nickel ions, aluminum ions, copper ions, magnesium ions, iron ions, lead ions, and tin ions, which are commonly used in zinc alloy plating. Sources of these metal ions include chlorides, sulfates, and oxides of these metals.

[0038] The concentration of metal ions other than zinc in the zinc plating solution is preferably 10 to 40 g / L. A concentration of metal ions other than zinc in the zinc plating solution of 10 g / L or more improves adhesion and makes it easier to control the co-deposition rate. A concentration of metal ions other than zinc in the zinc plating solution of 40 g / L or less also makes it easier to control the co-deposition rate. A concentration of metal ions other than zinc in the zinc plating solution of 12 to 25 g / L is more preferable.

[0039] (Chelating agent in zinc plating solution) The chelating agent in the zinc plating solution is preferably a water-soluble carboxylic acid or a water-soluble amine. Examples of water-soluble carboxylic acids include gluconic acid, citric acid, malic acid, and tartaric acid. Of these, gluconic acid or citric acid are particularly preferred. Examples of water-soluble amines include triethanolamine, ethanolamine, ethylenediamine, diethylenetriamine, triethylenetetramine, or ethylene oxide reaction products thereof. Of these, diethylenetriamine or ethylene oxide reaction products are particularly preferred.

[0040] The concentration of the chelating agent in the zinc plating solution is preferably 5 to 50 g / L. A concentration of 5 g / L or higher improves plating adhesion and makes it easier to control the co-deposition rate. A concentration of 50 g / L or lower improves current efficiency. A concentration of 10 to 40 g / L is more preferable.

[0041] (Trivalent chromium conversion treatment process) After the plating process, the plated surface of the metal substrate may be treated with a hexavalent chromium-free chemical conversion solution containing trivalent chromium ions, nitrate ions, and colloidal silica.

[0042] Sources of trivalent chromium ions in the chemical treatment solution include chromium nitrate, chromium chloride, chromium sulfate, chromium acetate, and chromium hydroxide. Of these, chromium nitrate, chromium sulfate, or chromium chloride are preferred. The concentration of trivalent chromium ions in the chemical treatment solution is 0.5 to 5.0 g / L, and preferably 1.0 to 4.0 g / L.

[0043] Sources of nitrate ions in the chemical treatment solution include nitrate compounds such as nitric acid, sodium nitrate, and ammonium nitrate. The nitrate ion concentration is 1.5 to 30 g / L, and preferably 3 to 20 g / L.

[0044] Other anions besides nitrate ions may be added to the chemical treatment solution as needed. Sources of anions other than nitrate ions may include sulfuric acid, sodium sulfate, ammonium sulfate, and other sulfate compounds; halogen compounds such as hydrochloric acid, sodium chloride, potassium chloride, and ammonium fluoride; phosphoric acid compounds; or organic sulfonic acid compounds. The concentration of anions other than nitrate ions is 0 to 15 g / L, preferably 0 to 10 g / L.

[0045] The colloidal silica contained in the chemical treatment solution can be commercially available general colloidal silica, and the particle size is not particularly limited. As colloidal silica, Nissan Chemical Corporation's Snowtex XS (hereinafter, Snowtex is a registered trademark), Snowtex OXS, Snowtex CXS, Snowtex C, Snowtex OX, Snowtex OS, Snowtex UP, Snowtex OUP, etc. can be used. The concentration of colloidal silica in the chemical treatment solution is 0.5 to 8.0 g / L as Si, and preferably 1.0 to 6.0 g / L.

[0046] The chemical treatment solution may contain cobalt ions from sources such as cobalt nitrate, cobalt sulfate, cobalt chloride, and cobalt hydroxide, as needed. The cobalt ion concentration is 0 to 5.0 g / L, and preferably 0 to 3.0 g / L.

[0047] The chemical treatment solution may contain other metal ions as needed. Examples of other metal ions include zinc ions, aluminum ions, titanium ions, zirconium ions, molybdenum ions, tungsten ions, vanadium ions, and cerium ions. It is preferable that these metal ions are water-soluble. Examples of sources for these metal ions include vanadium sulfate, vanadium chloride, vanadium oxalate, aluminum chloride, aluminum sulfate, Orgatix TC-310, Orgatix TC-300, Orgatix ZC-126 manufactured by Matsumoto Fine Chemical Co., Ltd., ammonium titanium fluoride, ammonium zirconium fluoride, and hydrozirconium fluoride. The concentration of other metal ions is 0 to 5.0 g / L, and preferably 0 to 3.0 g / L.

[0048] The chemical treatment solution may contain a chelating agent as needed. Examples of chelating agents include dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, glutaric acid, and adipic acid; oxycarboxylic acids such as citric acid, malic acid, tartaric acid, and gluconic acid; and aminocarboxylic acids such as nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA), and glycine. Of these, oxalic acid, malonic acid, succinic acid, tartaric acid, malic acid, and citric acid are preferred as chelating agents. The concentration of the chelating agent is 0 to 20 g / L as an organic acid, and preferably 0 to 10 g / L.

[0049] The trivalent chromium conversion treatment is carried out by immersing the plated metal substrate in the above conversion treatment solution and performing an electrolytic treatment under the following conditions. • pH of the chemical treatment solution: 1.5 to 5.0, preferably 1.8 to 4.5 Processing temperature: 20-50°C, preferably 25-40°C Processing time: 20-120 seconds, preferably 30-90 seconds

[0050] As described above, by treating the plated surface of a metal substrate with a hexavalent chromium-free chemical conversion treatment solution containing trivalent chromium ions, nitrate ions, and colloidal silica, a two-layer film consisting of a Cr layer and a Si layer can be formed on the plated surface of the metal substrate. Furthermore, by appropriately adjusting the conditions of the chemical conversion treatment solution, the Cr layer and Si layer can be formed as thick and dense films. [Examples]

[0051] The following are examples of the present invention, provided to better understand the invention and not intended to limit it.

[0052] (Examples 1-10, Comparative Examples 1-6) A disc brake cylinder (material: FCD-450 (cast iron), size: 10cm (depth) x 21cm (width) x 16cm (height)) was prepared as the material to be treated. As shown in Tables 1-3, the alkaline degreasing process, pickling process, desmutting process, acid activation process, plating process, activation process, post-treatment process (trivalent chromium conversion treatment process), and drying process were carried out on the material in this order. In Comparative Example 1, the desmutting process was not performed. A rinsing step was performed after each processing step. In addition, the pH of the plating solution was adjusted with hydrochloric acid or potassium hydroxide, and the pH of the post-treatment solution was adjusted with nitric acid or sodium hydroxide. Tables 1-3 also show the chemicals and processing conditions used in each step. Details regarding the raw material classification and raw material components of the drugs used in Tables 1-3 are shown in Table 4. Table 1 is common to Examples 1-10 and Comparative Examples 1-6, while details of the desmutting process, plating process, and post-treatment process are described in Tables 2 and 3.

[0053] [Table 1]

[0054] [Table 2-1]

[0055] [Table 2-2]

[0056] [Table 3]

[0057] [Table 4-1]

[0058] [Table 4-2]

[0059] [Table 4-3]

[0060] <Evaluation Test> • Appearance evaluation (1) Pre-plating evaluation In Examples 1-10 and Comparative Examples 1-6, the appearance of the samples immediately before the plating process was visually observed, and the presence or absence of smut was evaluated according to the following evaluation criteria. The evaluation results are shown in Table 5. A: No smut B: Some smut remains (the smut can be removed by rubbing the sample surface with a sponge). C: Smut present (The smut cannot be easily removed by rubbing the sample surface with a sponge) (2) Appearance of the finished plated product The samples prepared in Examples 1-10 and Comparative Examples 1-6 were visually inspected for appearance and color tone, and their gloss was evaluated according to the following criteria. The evaluation results are shown in Table 5. A: Has a glossy appearance (glossy appearance) B: Inferior to A, but has a glossy appearance (semi-gloss appearance). C: Inferior to B, with a partially dull appearance (matte finish).

[0061] • Corrosion resistance evaluation The samples prepared in Examples 1-10 and Comparative Examples 1-6 were left for 48 hours, after which a salt spray test was performed according to JIS Z 2731. The samples were then visually inspected, and the time at which white rust and red rust developed was evaluated. The evaluation results are shown in Table 5.

[0062] [Table 5]

[0063] <Consideration> In Examples 1 to 10, samples were subjected to a desmutting step in which the surface of a metal substrate was ultrasonically treated with a treatment solution containing an alkaline agent and a chelating agent, and a plating step in which, after the desmutting step, zinc plating was formed on the surface of the metal substrate using a zinc plating solution containing zinc ions, chloride ions, a buffer, and a brightener. As a result, zinc plating with good appearance and corrosion resistance could be formed at low cost. Comparative Example 1 did not undergo a smut removal process and therefore exhibited poor corrosion resistance. Comparative Example 2 had poor corrosion resistance because the desmutting process consisted only of washing with water. Comparative Example 3 showed poor corrosion resistance because the treatment solution used in the desmutting process did not contain a chelating agent. Comparative Example 4 showed poor corrosion resistance because the treatment solution used in the desmutting process did not contain an alkaline agent. Comparative Example 5 exhibited poor corrosion resistance because the desmutting process involved immersion treatment instead of ultrasonic treatment. Comparative Example 6 exhibited poor corrosion resistance because anodic electrolysis treatment was performed instead of ultrasonic treatment in the smut removal process.

Claims

1. A desmutting process involves ultrasonically treating the surface of a metal substrate with a treatment solution containing an alkaline agent and a chelating agent, After the desmutting step, a plating step is performed in which zinc plating is formed on the surface of the metal substrate using a zinc plating solution containing zinc ions, chloride ions, a buffer, and a brightener. A zinc plating method, including the following.

2. The zinc plating method according to claim 1, wherein the zinc plating solution contains a β-naphthol-based anionic surfactant as the brightener.

3. The zinc plating method according to claim 1, wherein the chelating agent is a water-soluble carboxylic acid or a water-soluble amine.

4. The zinc plating method according to claim 1, wherein the buffer is one or more of ammonia, boric acid, formic acid, acetic acid, gluconic acid, citric acid, malic acid, and amines.

5. The zinc plating method according to claim 1, further comprising a trivalent chromium conversion treatment step, in which the plated surface of the metal substrate is treated with a hexavalent chromium-free chemical conversion treatment solution containing trivalent chromium ions, nitrate ions, and colloidal silica, after the plating step.

6. The zinc plating method according to any one of claims 1 to 5, wherein the metal substrate is cast iron.

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