JPS5856039B2 - Zinc-trivalent chromium electroplating method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a zinc-trivalent chromium electroplating method, and more specifically, by adding a trivalent chromium salt to an acidic zinc electroplating bath, uniform and good surface color tone and gloss can be achieved. The present invention relates to a zinc-trivalent chromium electroplating method that provides a zinc plating film with excellent corrosion resistance.

Conventionally, as electrogalvanizing baths, cyanizing baths, zincate baths, acid baths, etc. are known.

However, when these zinc baths are used as they are and normal plating methods are applied, the resulting plating film has extremely poor gloss, so for applications that require high gloss, a special plating method must be used or Addition of brightener is required.

In addition, zinc plating is often applied mainly to areas that require corrosion resistance, but as it is plated, it has very poor corrosion resistance (zinc spots rust), so it is used for applications that require corrosion resistance. must be treated with chromate and, in some cases, with alkali.

Furthermore, in applications where adhesion is required, such as when painting over it, it is necessary to apply a chemical conversion coating treatment such as phosphate treatment, which may complicate the galvanizing process or subsequent treatment process. was there.

In view of the above circumstances, the present inventors conducted extensive research to obtain a galvanized film with good appearance and excellent corrosion resistance. As a result, the present inventors found that chromium sulfate, Soluble trivalent chromium salts such as chromium chloride are
By dissolving 3 g/1 or more of chromium as +, it is possible to obtain a plating film having a good appearance with a uniform surface color and pearl-like luster, and approximately 10 to 100 ppm of chromium is added to the plating film. contains,
This film has excellent corrosion resistance even without chromate treatment, and compared to films obtained by conventional zinc plating methods or zinc plating methods in which solid fine particles are eutectoid, this film contains trivalent chromium salts. It was discovered that the galvanized film of 1999 has various excellent properties, and the present invention was developed based on this finding.

That is, in the present invention, the trivalent chromium salt is 3g/1 as Cra+.
The present invention provides a zinc-trivalent chromium electroplating method characterized by electroplating an object to be plated in an acidic zinc electroplating bath containing the above-dissolved zinc.

The present invention will be explained in detail below.

The acidic zinc plating bath used in the present invention includes:
Known baths include zinc sulfate baths, zinc chloride baths, zinc sulfate-zinc chloride baths, and the like.

These acidic zinc plating baths usually have a pH of 2.5 to 5.5.
The zinc source is mainly zinc sulfate and/or zinc chloride, and if necessary, ammonium sulfate,
Conductivity salts such as sodium sulfate, ammonium chloride, sodium chloride, pH buffers such as boric acid, sodium acetate,
Furthermore, additives such as aluminum sulfate, magnesium sulfate, dextrin, and glucose are added.

Brighteners can also be added, especially if a significant shine is required in the deposited film.

The concentration of these fractions can be varied over a wide range depending on the use, plating conditions, etc. For example, 300 to 500 g of zinc sulfate.
A high-concentration zinc bath with a concentration of 20 to 309 zinc sulfate/l can be used, or a low-concentration zinc bath with a concentration of 20 to 309 zinc sulfate/l can be used.

In the present invention, a soluble trivalent chromium salt is dissolved in the acidic zinc plating bath.

The trivalent chromium salt may be any salt that can be dissolved in the acidic zinc plating bath, regardless of whether it is a single salt or a double salt.

Specifically, one or more of chromium chloride, chromium nitrate, chromium sulfate, chromium potassium sulfate, etc. is used.

The amount of trivalent chromium salt added varies depending on the composition, temperature, etc. of the plating bath, but the solubility of the trivalent chromium salt varies depending on the composition, temperature, etc.
The s+ value is 3 g/1 or more, and as a result, a plated film with good corrosion resistance can be obtained even without chromate treatment.

Further, the upper limit of the trivalent chromium salt is preferably 809/11.

As described above, the zinc-trivalent chromium plating bath according to the present invention is one in which trivalent chromium salt is dissolved in an acidic zinc electroplating bath, but insoluble fine particles, water-soluble polymers, etc. may be added depending on the application. It is also possible.

For example, when plating for purposes such as painting or lining, thermoplastic or thermosetting organic polymer compounds such as phenol, polyamide, epoxy, or tetrafluoroethylene resin, or natural or synthetic rubber latex are used. It is also possible to contain elastic polymers such as Al2O3, Si
It is also possible to add particles of insoluble metals such as O2, SlC2MoS2, TiO2, BN, tungsten or their oxides, fluorides, silicates, carbonates, carbides, nitrides, glass beads, inorganic fibers, etc.

Furthermore, if corrosion resistance and excellent gloss are required, it is also possible to add water-soluble polymers such as polyvinyl alcohol, polyvinylpyrrolidone, sodium polyacrylate, polyethylene oxide, polyethyleneimine, etc.

In addition, these insoluble fine particles usually have a content of 1 to 200 g/l.
The water-soluble polymer can be added in a range of 0.1 to 209/l.

The zinc-trivalent chromium plating bath of the present invention is used under the same plating conditions as those in known acidic zinc plating baths, and there is no need to adopt special plating conditions due to the dissolution of trivalent chromium salts.

The dampened film obtained from the bath of the present invention is mainly composed of zinc even when a fairly large amount of trivalent chromium salt is dissolved.
Chromium is uniformly precipitated in a range of 30 ppm.

This film is produced without the addition of special brighteners, in contrast to the dull, dark, matte appearance of the matte films obtained from baths of the same composition except that the trivalent chromium salts are not dissolved. Both have a pearl-like bright zinc luster and a good appearance with a uniform color tone.

In addition, the plating film from the bath of the present invention shows only slight white rust, whereas the film from the conventional galvanizing bath, without chromate treatment as a post-treatment, develops white rust on the entire surface within 24 hours of the salt spray test. Corrosion resistance is significantly improved even if only a few spots of rust are seen on the surface.

Therefore, after plating with the bath of the present invention, it is possible to use the product as it is for various purposes without performing chromate treatment or alkali treatment.

Furthermore, when a composite plating film is obtained from a plating bath containing insoluble fine particles and water-soluble polymers, the film from a trivalent chromium salt solution bath has a lower temperature than the film from a bath containing no trivalent chromium salt. In addition, as mentioned above, the film obtained from the bath of the present invention has excellent properties such as corrosion resistance and adhesion by itself. and
In some cases, it is possible to omit chemical conversion coating treatments such as chromate treatment and phosphate treatment as post-treatments, but even if these chemical conversion coating treatments are applied, there is no problem with inspection, and chemical conversion coating treatments can provide corrosion resistance and It has characteristics such as adhesion and abrasion resistance.

Furthermore, the above-mentioned characteristics of the dazzling film obtained by the plating method of the present invention can only be obtained when a trivalent chromium salt is dissolved in an acidic zinc plating bath.
Even if trivalent chromium salt is added to an alkaline zinc plating bath such as a zincate bath, it is difficult to obtain a plating film with the characteristics described above. Even if hexavalent chromium compounds such as the above are dissolved, the object of the present invention is not remotely related.

As explained above, in the zinc-trivalent chromium plating method according to the present invention, a trivalent chromium salt is added to an acidic zinc plating bath.
Since we decided to dissolve more than 3 g/IJ by adding +,
It has the feature of being able to deposit plating films with various excellent properties in terms of plating appearance, corrosion resistance, etc.

Furthermore, the plating method of the present invention does not require any special plating conditions or plating equipment, and can be adopted without changing the conventional plating process. It is also advantageous from the viewpoint of wastewater treatment, since there is no need to carry out treatments such as reduction to valent chromium, and ordinary neutralization methods can be employed.

EXAMPLES The present invention will be specifically explained below with reference to Examples.

Example 1 Bath composition Zinc sulfate (Znso, ・7H20) 400 g/l Aluminum sulfate (A12■04) 3) 5 Sodium sulfate (Na2SO4・10H20) 5 Chromium sulfate (Cr2(SO4)3・xH2O) 0 H 4, 5 Plating conditions Temperature: 0 C Cathode current density: 5 A/di" Stirring temperature: Using a zinc-trivalent chromium plating bath with the above bath composition, predetermined pretreatment (electrolytic degreasing, pickling) was performed under the above plating conditions. A plated steel plate (7XL5crft) was plated with a thickness of 5μ.

The appearance of the resulting frosting had a shiny, uniform zinc color.

In addition, this plating sample was subjected to a salt spray test (TIS Z
2371) and examined its corrosion resistance, the results showed that 10% of white spot rust occurred.

As a comparison, a conventional electrogalvanizing bath (
5μ using the above bath composition excluding chromium sulfate)
A comparison sample with plating on the door was similarly subjected to a salt spray test, and white rust appeared on the entire surface, and 30% spot white rust was observed on the sample treated with phosphate. .

Therefore, from the above results, it was confirmed that by including trivalent chromium salt in the acidic zinc plating bath, the corrosion resistance of the plating film was very good even when no chemical conversion coating treatment was performed after plating. .

Example bath composition Zinc sulfate (ZnSO4, 7H20) 300 g/l Sodium sulfate (Na2SO4.10H20) 0 Ammonium sulfate 0 Boric acid 5 Potassium chromium sulfate (K2Cr2(SO4) 4.24H20) 0 SiO2 (('3 μm after homogenization ) 0 H 4,0 Plating conditions Temperature 0°C Cathode current density 10A/d77+2 Stirring propeller-controlled agitation Polishing pretreated under the above plating conditions using a zinc-trivalent chromium plating bath with the above composition in which SiO2 was suspended. A steel plate was plated with a thickness of 5 μm.

Next, after performing chromate treatment (25°C, iO seconds) in a 10ra4/1 nitric acid bath, acrylic paint was applied to a film thickness of 30 μm. The hardness was measured and a weight resistance test was conducted.

The hardness was evaluated by pencil hardness using a Mitsubishi Uni-Pencil, and the impact test was performed using a Du Pont impact tester (using a 500 gr weight).

For comparison, sample A1 was similarly plated, chromate-treated, and painted using a conventional electrogalvanizing bath (with chromium potassium sulfate and S io 2 removed from the above composition), and sample A1 was phosphate-treated. Thereafter, hardness and impact tests were conducted on Sample B, which was subjected to chromate treatment and then painted.

The results are shown in Table 1.

In addition, the plating appearance of Example 2 obtained with the above bath composition showed white spots due to SiO2, but the zinc in the metal part exhibited grayish-white luster.

Example 3 Bath composition Zinc sulfate 4 Ammonium chloride borate chromium sulfate phenolic resin particles (average particle size 27s) H 50g/l 15 〃 011 30 〃 30 〃 4.7 Plating condition temperature 50 °C Cathode current density 30A/di” Stirring
Liquid Circulation Method Using a zinc trivalent chromium plating bath with the above composition in which phenol resin particles are suspended, a polished steel plate pretreated under the above plating conditions is plated with a thickness of 5 μm, and then a 20 μm film of melamine paint is applied. Painted thickly.

The obtained samples were subjected to a cross-cut test and an Erichsen test (9 mm extrusion) to evaluate adhesion, and a salt spray test was conducted for 120 hours to examine their corrosion resistance.

The results of the salt spray test were evaluated based on the presence or absence of blistering at the cross-cut portion.

For comparison, a similar test was also conducted on Comparative Sample C, which was coated after phosphate treatment.

The results are shown in Table 2.

The appearance of the plating obtained from the bath composition of Example 3 is as follows:
Example 1: Surface color with pearl-like luster 4 Bath composition Zinc chloride Ammonium chloride Chromium chloride Polyvinylpyrrolidone H 40.El/1 200 〃 60 // tt 5.0 Plating conditions Temperature Cathode current density Stirring 30 0C 3A /dm' Cathode locker Using a zinc-trivalent chromium plating bath with the above bath composition, plate a 5μ door on a polished steel plate pretreated under the above plating conditions, then apply acrylic paint to a coating thickness of 35μ. The gloss was measured using a GM-5 type gloss tester manufactured by Murakami Color Research Co., Ltd. (60°C mirror surface), and the corrosion resistance was evaluated by conducting a salt spray test for 240 hours.

For comparison, the glossiness and corrosion resistance of Comparative Sample D, which was painted after phosphate treatment, were similarly tested.

Table 3 shows the results.

The appearance of the plating obtained from the bath composition of Example 4 is as follows:
It had a slightly yellowish luster.

Example 5 Bath composition Zinc chloride 120 g/11 Ammonium chloride 180//Chromium chloride
8011 Brightener Asahijinko → Shi ST (Jozai Kogyo tJ) 30ml/
1 Asahidincol A-4 (1/) 3
//pH 4・7 Plating conditions Temperature 30 ℃ Cathode current density 4A/dm" Stirring
The appearance of the plating obtained from the zinc-trivalent chromium plating bath with the above-mentioned air conditioner had excellent gloss and good leveling.

In addition, its corrosion resistance was greatly improved, and good corrosion resistance was exhibited even without chromate treatment.

Claims (1)

[Claims] 1. A method for zinc-trivalent chromium electroplating, which comprises electroplating an object to be plated in an acidic zinc electroplating bath in which 3 g/1 or more of trivalent chromium salt as Cr3+ is dissolved. 2. The zinc trivalent chromium electroplating method according to claim 1, wherein the acidic zinc electroplating bath uses zinc sulfate and/or zinc chloride as a zinc source and has a pH in the range of 2.5 to 5.5. 3. Zinc-trivalent chromium electroplating according to claim 1 or 2, wherein the trivalent chromium salt is one or more selected from the group consisting of chromium chloride, chromium nitrate, chromium sulfate, and chromium potassium sulfate. Method. 4. The zinc trivalent chromium electroplating method according to any one of claims 1 to 3, wherein the upper limit of the trivalent chromium salt is 80 g/l.