JPS6024200B2 - How to improve and maintain trivalent chrome plating solution - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to chromium electroplating, and more particularly to preserving trivalent chromium electroplating standards.

Although the potential benefits of electroplating from solutions containing chromium in the trivalent state were known,
Until recently, this solution was not commercially available due to various practical problems.

Therefore, methods containing chromium in the hexavalent state have been used despite certain serious deficiencies when electroplating chromium. Recently, however, various proposals have been made to solve at least some of the aforementioned problems.

An example of such a partially improved electroplating bath is one containing trivalent chromium salts, formates, bromides, and ammonia, as described in my U.S. Pat. No. 39,574. Some substances are included as essential ingredients. The use of such electric finishing baths has recently been introduced commercially and has already received wide praise as an alternative to hexavalent chromium finishing baths. However, it has been discovered that while trivalent chromium plating works well under laboratory or test conditions, it sometimes exhibits defects after commercial installation.

In particular, at least one of the following defects or a combination thereof occurs. A At high current densities, a white haze is produced, which in severe cases gradually spreads and reduces the current density.

B. A white band forms at the lower end of the plating range, and adhesion is lost at high current densities.

C.O. Current densities around 0.54 amps per square foot (50 amps per square foot) result in a white precipitate.

D Approximately 0.107~0.215 ampere/enemy (100~
Current densities of 200 amps/sq ft) result in brown or black stains.

The inventors have discovered that the aforementioned defects are often eliminated by adding a small amount of water-soluble ferrocyanide to the plating solution when the defect is observed.

The object of the present invention is therefore to prepare aqueous trivalent compounds in which at least one of the aforementioned defects has begun to occur.

The object of the present invention is to eliminate the aforementioned defects by adding a sufficient amount of water-soluble fluorocyanide to the electroplated film in an amount sufficient to substantially prevent the defects. It has been discovered that the method of the present invention is generally utilized to maintain good quality of trivalent chromium electroplating. For example, those of the type described in applicant's above-mentioned U.S. patents or U.S. Pat.
6636-3706643 containing glycolic acid. The method of the present invention is also described in British Patent No. 1,144,913 and US Pat.
The types described in US Pat. No. 0,212,67, US Pat. No. 3,006,823, US Pat. No. 3,069,333 and US Pat. These electroplating baths generally contain trivalent chromium, such as chromium chloride, chromium sulfate or chromium fluoride, and a carboxylic acid, preferably a formate, or a softening agent such as an acetate, a glycolate or an oxalate. contains.

Preferably halides, especially bromides, are also present. The plating solution preferably contains alkali metal ions, such as sodium and/or potassium, and sulfate ions. A dipolar aprotic solvent such as dimethylformamide may also be included, but is preferably absent. The pH of the electroplating grade is usually 1-7, for example 1.5-5. Furocyanides are those soluble in the electroplating solution, such as alkali metal or ammonium ferrocyanides, such as sodium or potassium ferrocyanides.

Ferrocyanide is conveniently added to electroplating as an aqueous solution.

The concentration of the ferrocyanide solution is not critical and is usually selected depending on the solubility of the ferrocyanide used.
For example, when using potassium ferrocyanide,
Preferably, a solution containing about 20% by weight of ferrocyanide is used. Addition of ferrocyanide in excess of the amount required to eliminate the aforementioned defects adversely affects the properties of the electroplated grade.

One way to overcome this problem is to add a solution of ferrocyanide in small portions when the aforementioned defects begin to occur until good chromium precipitation is achieved. If too much ferrocyanide is added to cause adverse effects, the excess can be removed by adding small amounts of soluble cations such as copper, nickel, iron or zinc. This operation is conveniently accomplished in most plating shops by adding a small amount of nickel plating solution to the electroplating plate. This addition of metal ions should be done within 18 minutes, preferably within 10 minutes, after adding the ferrocyanide to be fully effective.
This is because if left for a long period of time, excess ferrocyanide forms a complex with chromium, making it difficult or impossible to form a precipitate with subsequently added metals. It is believed that the aforementioned defects are caused by contamination of the electroplating solution with a significant amount of metal cations that eutectoid with chromium.

As a result of experiments, the present inventor found that the aforementioned defect A is caused by adding copper to the electroplating layer, similarly, defect B is caused by the presence of zinc, defect C is caused by lead, and defect D is caused by the presence of zinc. They discovered that this is caused by the presence of iron or nickel. Surprisingly, ferrocyanide is capable of precipitating virtually all of the potentially harmful metals that are normally present in very low concentrations as contaminants when used commercially, making it possible to reduce the Chromium, the main cationic component, does not precipitate. Therefore, the present invention provides a method for maintaining a trivalent chromium plating solution in a good condition by eliminating defects in plating operations due to eutectoid deposition of chromium and other metal contaminants. The method consists of analyzing the electroplating solution to determine the concentration of metal contaminants and adding a sufficient amount of water-drop ferrocyanide to precipitate said contaminants.

That is, the present invention provides a method for improving and maintaining in good condition a trivalent chromium electroplating solution that produces defective plating due to the presence of certain metal contaminants. An object of the present invention is to provide an improved method for maintaining a trivalent chromium electroplating solution, which comprises adding the trivalent chromium electroplating solution to the plating solution in an amount sufficient to remove the trivalent chromium.

When carrying out the present invention, preferably, the plating solution or the sample used for analysis should be filtered to remove precipitated metals before analysis.

Analysis of the plating solution can be performed by conventionally known quantitative analysis techniques.

Typically, plating solutions can be analyzed by spectrophotographic techniques, such as spark ionization techniques or atomic absorption techniques. Alternatively, polarographic techniques may be used. Judging from experience, copper poses a practical problem.
Only zinc, iron and nickel, so only these metals need to be tested.

However, the present invention also overcomes deficiencies in plating operations due to the presence of precipitating metals such as lead, cadmium, silver, gold, etc., although they rarely exist in practice. The amount of ferrocyanide added is stoichiometric or slightly less than the amount of metal contaminant present.

Use of a substantial excess of ferrocyanide should be avoided. On the other hand, if an amount considerably smaller than the stoichiometric amount is added, defects cannot be completely eliminated. A simple and good guideline is egg-stick plating solution 11 with metal contaminants 5.
Add one solution of 20% potassium ferrocyanide per kite. In this way, at least commonly present contaminants can be addressed. Also, before adding ferrocyanide, it is preferable to reduce free halogens to halides by adding a reducing agent that can convert halogens into halides and does not have any negative effect on the performance of the plating solution. .

A particularly suitable reducing agent for this purpose is ammonium formate. The formate salt is preferably used in an amount sufficient to reduce all free halogen in the plating solution. Usually, preferably 2 to 3 portions of ammonium formate are added as an aqueous solution per 11 parts of the plating solution. The formate salt is preferably added by trickling about 10 minutes before adding the ferrocyanide. Halogens are usually present in the plating solution immediately after plating. Also, in practicing the present invention, methods for quickly detecting the presence of harmful excess ferrocyanide or metal impurities, for example, can be used so that appropriate action can be taken quickly.

The above-described method for testing an aqueous trivalent chromium plating solution used in the present invention involves contacting the plating solution with a water-permeable medium, permeating the medium upwardly, and dispersing the plating solution into the medium. It consists of contacting the separated parts by osmotic diffusion with a water-soluble ferrocyanide and a water-soluble iron salt, respectively.

Normally, contact between the diffused plating solution and the two types of salts is achieved by separately soaking the two types of salts in the medium and arranging the impregnated portions to intersect with different portions of the plating solution. It is done.

Preferably, the impregnated portion is one that can be easily visually observed by an outside observer to quickly detect a color change. The water-permeable medium is preferably a cellulosic material such as filter paper or chromatography paper.

However, in principle, any medium may be used as long as the medium can diffuse the plating solution upward by capillary action or the like when the lower end is immersed in the plating solution. Preferably, the medium is substantially colorless so that even slight color changes are observable. The medium is preferably a water-permeable paper.

A particularly convenient form of test paper for use in the detection method described above consists of a strip of rectangular or similarly shaped filter paper or the like, impregnated at one end with ferrocyanide and the other end with iron salt, and then The dark central part of the impregnated area is left unimpregnated. Next, when actually using it, bend it at the central part that was left unimpregnated, immerse the unimpregnated central part in the plating solution, and replace the impregnated parts at both ends. will not be immersed in the plating solution. The plating solution then rises and diffuses, and a color change can be easily detected at each end. The test paper used in the above-described detection method is conveniently obtained by cutting water-permeable paper into rectangular strips, each end of which is immersed in a solution of the two salts for a sufficient period of time. Diffuse the two types of solutions, but do not let them penetrate into the center.

The paper may then be dried in an oven. Alternatively, two separate pieces of test paper may be used to impregnate each solution.

A suitable support may be used as long as the medium is non-cohesive and brittle. For example, a thin storm of silica gel supported on a plate may be used. Alternatively, in the case of powdered or gelatinous media, the media may be enclosed in a column (preferably glass or similar transparent material). The ferrocyanide is preferably ammonium ferrocyanide or an alkali metal ferrocyanide salt, such as tetrapotassium ferrocyanide or tetrasodium ferrocyanide.

The iron salt may be ferric, but is preferably a ferrous salt,
Preference is given to inorganic acid salts, such as chlorides, nitrates or sulfates. If a blue stain appears at the contact between the plating solution and the ferrocyanide being tested, it means that the plating solution contains metal impurities, while a blue stain appears at the contact between the plating solution and the iron salt. It can be seen that ferrocyanide is present in excess in the plating solution.

Preferably, the trivalent plating solution is taken out, and ferrocyanide is dropped in stages. The plating solution is inspected with test paper after being dropped at each stage. The end point is expressed in terms of the number of cyanide added per liter of plating solution, and in practice it is preferable to first add 50% of the amount of cyanide added to reach the end point to the plating solution, and then add as needed (e.g. 2
5%) Continue to add more. The present invention will be explained in more detail in the following Examples, which are referred to merely as examples unless otherwise specified.

Example 1 Trivalent chromium plating solution that was working well was 100 to 20
Plating defects occurred in the 0 amp/sq. ft. range and black stains began to form.

A sample of this plating solution was added to my Hull cell (Hulcel).
1Cell) plating bath, and cooled for 10 minutes while circulating.
Panel plating was carried out for 3 minutes at amperage. This panel is 10
At current densities of 0 to 200 amperes/square foot, black streaks appeared, and experience determined that this was due to contamination of the plating solution with nickel and possibly iron. This was because the nickel-plated components were lost from the plating jig and dissolved in the electrolyte. Spectroscopic analysis of metal impurities in this plating solution revealed the following. Prepare a solution of potassium ferrocyanide & Fe (CN) 8.2% by weight/volume,
This solution was added at a ratio of 1/4 melon of the total amount of metal impurities, or 7M per 11 parts of the plating solution.

This mixture was allowed to stand for 30 minutes and then used as a glazing solution. Black streaks completely disappeared, and normal plating performance was again obtained. Thereafter, this plating solution was analyzed and the results were as follows.

*This number includes the increase due to excess reagent.

Example 2 Plating solutions contaminated with iron, nickel, copper and zinc produced similar plating defects.

No analysis of metal impurities was performed in this example. A 55% w/w ammonium formate solution was added to the plating solution at a ratio of 5/1 to reduce the halogen.

The plating solution was allowed to stand for 10 minutes, and then a 20% solution of potassium ferrocyanide was added stepwise. Plating solution 11
Start adding at a rate of 1 part per hour, and then gradually add 1 part per part.
Add at a ratio of 1 to 1, and add 30% between each addition and the next addition.
Wait a few minutes to complete the reaction. Improvement began to be seen after addition of 3/1, and sufficiently good performance was obtained when 5/1 was added.

The precipitated metal salt did not interfere with the plating operation even if it was left in the plating solution, but the precipitated metal was removed by filtering it in an oven at the next stop of operation. Example 3 (Reference Example: Preparation of Test Paper for Metal Contaminant Detection) One end of a rectangular strip of filter paper is immersed in a 2% wt/vol. Test paper was prepared by soaking it in a dichloride solution.

Each of these solutions was allowed to permeate and diffuse until reaching the center of the strip, and then dried in an oven. Example 4 (Reference example) After the trivalent chromium plating solution has been working well for several weeks,
Black stains began to form at flow densities between 0 and 200 amps per square foot.

The test paper prepared in Example 3 was folded in two at the central part not impregnated with the reagent, and the central bent part was immersed in the plating solution. The plating solution spread toward both ends of the test paper, and when it came into contact with the ferrocyanide-coated area, a blue stain appeared, indicating the presence of metallic impurities in the plating solution. Therefore, a 10% by weight/volume aqueous solution of tetrapotassium ferrocyanide was added to
The test was repeated after each addition was allowed to stand for 3 minutes.

After the second addition, a blue stain was observed on the ferrous chloride impregnated side of the test paper. No blue stain was observed when 2% w/v ferrous chloride solution 2' was added.

Claims (1)

[Claims] 1 In a method for improving and maintaining in good condition a trivalent chromium electroplating solution which produces defective plating due to the presence of trace metal contaminants, water-soluble ferrocyanide is added to a solution sufficient to remove said metal contaminants. An improved method for maintaining a trivalent chromium electroplating solution, characterized in that the amount of the electroplating solution is added to the plating solution.