JPH0445598B2 - - Google Patents

Abstract

Chromium is electroplated from a bath which includes a trivalent chromium salt and chloride ions. Chlorine gas production at the anode is reduced through the incorporation in the plating bath of at least one salt having a nitrogen containing cation and at least one soluble salt yielding from about 0.001 mole per mole liter to about 0.01 mole per liter of bromide ion. The bath can further include sulfate ions, formate and boric acid or borate ions.

Description

[Detailed description of the invention]

The present invention relates to a method for electroplating chromium from an aqueous plating bath, and more particularly to a method for producing chromium electrodeposit from a plating bath containing trivalent chromium. The present invention also relates to a novel plating bath containing trivalent chromium, and further relates to a method for suppressing the formation of chlorine gas and hexavalent chromium at the anode during use of the plating bath. For decorative purposes primarily, chrome is electroplated from a trivalent plating bath. Compared to the adhesion of chromium electroplating produced from plating baths containing hexavalent chromium, trivalent plating baths have weaker adhesion to substrates, so chromium electroplating is generally used for functional purposes. It was never used. The adhesion obtained in the trivalent chromium electroplating operation is sufficient for decorative purposes and when compared with the plating obtained from the hexavalent chromium plating bath.
There are several advantages such as: 1. The coating is thick and does not burn even at high current density. 2 Good coatings can be obtained even at low current densities. 3. Insensitive to current interruption. 4. There are fewer problems associated with waste treatment. However, in addition to the adhesion difficulties discussed above, there are several other problems associated with the use of trivalent chromium plating baths. First, there are only a few
Even when ppm of hexavalent chromium is formed, the quality of the electrodeposit is rapidly degraded. The presence of hexavalent chromium is extremely unfavorable to the quality of chrome plating and tends to cause black streaks, which impairs the appearance of the plating. Furthermore, conventional trivalent chromium plating baths typically contain sulfate ions as conductive anions. Since the commonly used anode is graphite, oxygen is released during the plating operation and attacks the anode, eroding it and releasing carbon particles into the bath. The generated carbon particles are likely to be included in the chromium electrodeposit, resulting in defects in plating. It has already been taught that the introduction of salts containing halogen anions into the bath suppresses the formation of oxygen, resulting in better quality electrodeposit and extending the life of the bath. For example, U.S. Pat. No. 3,833,485 shows that the use of relatively high concentrations (0.05 to 4 moles) of chloride or bromide ions results in less erosion of the anode. The minimum effective concentration is
It is stated that it is 0.01 mole. Generally, due to cost factors, the halogen ion of choice has been chloride. However, when this type of plating bath is used at an effective concentration of chlorine ions, toxic chlorine gas is generated at the anode. It has been found that the use of a combination of ammonium ions and low concentrations of bromine ions in a chlorine-containing trivalent chromium plating bath can suppress the generation of chlorine and other toxic gases at the anode during plating operations. According to the invention, significant amounts of chloride ions, ammonium ions, and 0.001 mol/~
By performing electroplating of chromium from a trivalent chromium plating bath consisting of a combination of 0.008 mol/bromine ion, the formation of chlorine gas and hexavalent chromium at the anode during the electroplating process is suppressed. The bromide ion concentration that provides the best balance between cost and efficiency is in the range of 0.004 to 0.008 mole/mol/. It has been determined that the plating action of the bath is not dependent on this bromine concentration, and the above composition is much more cost-effective than the prior art. The trivalent chromium electroplating bath and method according to the present invention is a bath and method useful for producing decorative bright chromium electrodeposited films. The electrodeposited film according to the present invention is
Although they can be formed on any type of metal substrate material, ferrous metal substrates are commonly used. Although not necessarily a requirement, standard commercial practice is to first form a bright nickel electroplating onto the substrate material.
The first coating material applied on the substrate includes cobalt,
A bright electrodeposited film of nickel-iron alloy, cobalt-iron alloy, nickel-cobalt-iron alloy, copper, or the like may also be employed. The surface is made smooth by the electrodeposited nickel film,
Chromium is deposited on top of it. Without such a smooth surface, it would be difficult for thin chromium deposits to have sufficient shine to be acceptable to customers. Electrodeposited nickel films also serve to provide corrosion resistance to the substrate material, which is particularly useful when manufactured articles are to be used outdoors. This nickel electrodeposit is not to be confused with a nickel "strike," which actually deposits a significant amount of material. Generally, the thickness is about 0.0025 to about 0.0127 mm (0.1 to 0.5 mil) for indoor use, and about 0.025 to about 0.038 mm for outdoor use.
(1 to 1.5 mils) thick nickel is deposited. The trivalent chromium plating bath according to the invention is the same as that of the conventional process, except for the presence of ammonium ions in combination with bromide ions.
The details of the process also follow conventional methods used on a commercial scale. Details of the bath and process can be found in the above-cited U.S. Patent No.
No. 3,833,485, and the relevant portions of the above-mentioned prior patent specification are hereby cited as references. Each parameter of the bath and process will be described later. US Pat. No. 3,833,485 discloses the use of either chlorine or bromine as the halogen to suppress oxygen release at the anode.
In contrast, in the present invention, chlorine and bromine are not in the relationship of arbitrarily selected substitutes, but both chlorine and bromine are essential additives. Chlorine is an essential conductive anion and prevents the formation of hexavalent chromium at the anode. 6 at a concentration of several ppm
Even the presence of valent chromium can poison the bath. Bromine is essential to prevent chlorine generation at the carbon anode. In short, U.S. Patent No.
No. 3,833,485 uses either chlorine or bromine as the halogen to suppress the generation of oxygen at the anode. In contrast, the present invention requires the use of both chlorine and bromine. In the present invention, bromine is necessary to suppress the generation of chlorine gas at the anode and provide a high quality chromium electrodeposit, while chlorine is an essential conductive anion and the formation of hexavalent chromium at the anode. necessary to prevent Furthermore, in the present invention, the presence of ammonium ions is necessary for the effective action of bromide ions. Generally, any water-soluble salt in which chromium is present in the trivalent state can be used in the plating bath. Generally, chromic chloride, chromic sulfate, or mixtures thereof are used because they are cheap and easily available. The inventors have found that it is advantageous to utilize a substance known under the name "chromium tanning salt." This is the commonly available form of chromium sulfate, often represented by the chemical formula Cr(OH)SO ₄ .xH ₂ O. However, it is believed that this material is actually a mixture of hydrated basic chromium sulfates, the exact composition of which varies depending on the pH value of the solution from which it crystallizes. The concentration range of the chromium salt present can be relatively wide. Generally, the range of possible chromium ion concentrations is from about 0.001 mol/mole/mole to the critical solubility of the chromium salt. A preferred range is from about 0.01 mole/ to about 1 mole/. For commercial scale operations, the initial chromium ion concentration is typically in the middle of the preferred range. In this way, it is possible to reliably prevent the plating product from being burnt due to excessive reduction in concentration during the plating process. Furthermore, too high a chromium ion concentration is commercially undesirable because it can cause waste disposal problems. Of course, the conductivity of the bath can be greatly increased by including large amounts of various salts. These salts and their component ions are generally considered to be salts other than chromium salts. As the total amount of conductive salt,
Generally, from about 2 moles per to about 6 moles per salt are used. A commercially preferred range is from about 3.5 to about
It is 5.5 mol/. Here, the terms "conductive cation" and "conductive anion" respectively mean various cations and various anions constituting the conductive salt. Conductive anions include anionic species introduced with chromium cations. Although not essential, generally the most important conducting anion is sulfate. Of course, when chromium sulfate is used as the chromium ion source, sulfate ions are always present. When it is considered desirable to add sulfate ions, the sulfate ions are introduced in water-soluble form. Most preferably, it is introduced in the form of sulfuric acid, which helps adjust the pH, and may also be introduced in the form of ammonium sulfate, for reasons discussed below. In any case, the use of sulfate ions in the plating bath is particularly convenient, although not a requirement. The maximum amount is approximately 90% of the total conductive anions in the bath (percentages are on a molar basis, the same applies hereinafter)
It can contain up to sulfate ions. In order to obtain good chromium plating from the trivalent chromium plating bath of the present invention, the presence of chlorine ions is essential. Preferably, at least 5% of the total conductive anions in the plating bath are chloride ions. Depending on the application, all of the conductive anions present in the bath may be chloride ions. Most preferably, chloride ions are about 10% to about 90% of the total conductive anions present in the plating bath. Commercially, it is commonly used in proportions of 50% or higher. Chlorine ions may be introduced in the form of any water-soluble substance that is convenient to use. Conveniently it is introduced in the form of chlorine, ammonium chloride, alkali metal chlorides or mixtures thereof. Of course, it can also be added in the form of chromium chloride. It is essential that ammonium ions are present in an amount equal to at least 5% of the total conductive cations present in the bath. The preferred abundance of this type of cation is ~20% of the total conductive cations.
This amount corresponds to approximately 90%. Most conveniently, ammonium ions are added to the bath in the form of ammonium salts, such as ammonium sulfate or ammonium chloride as mentioned above. It is believed that the ammonium ions themselves are oxidized at the anode, but this process is not sufficient to prevent the generation of chlorine gas at the anode. However,
It is believed that ammonium ions can reduce bromine produced at the anode and regenerate bromine ions. Thus, while ammonium ions are consumed during the plating operation, bromide ions are not. The presence of ammonium ion is essential at the very low concentrations of bromide used in the present invention to perform its function. The preferred amount of bromide ion in the present invention is
It is in the range of 0.001 mol/~0.008 mol/. The most preferred concentration for optimal effectiveness is at least about 0.004 mol/. Commercially, it is common to use amounts of about 0.0075 mole/bromide to ensure that at least an optimal concentration of bromide ion is present throughout the bath. If the amount of bromide ions present is insufficient, gas generation at the anode cannot be sufficiently suppressed, making it impossible to maintain commercial significance. Even if the concentration of bromide ion used is too high, the effect will not be further enhanced, and bromide salts are generally extremely expensive, so that from a commercial standpoint, they lose their economic significance. As described above, in the present invention, a small amount of bromine ion acts catalytically. Therefore, the plating process can be continued continuously for quite a long time without newly replenishing bromide ions. The bromide ion concentration may exceed 0.008 mol/l, but
unnecessary from an economic point of view. However, if it is lower than 0.001 mol/l, it is no longer possible to effectively suppress the generation of chlorine gas at the anode. Bromine produced at the anode is reduced by ammonium ions. Therefore, ammonium compounds are consumed and need to be replenished, but bromine circulates during the reaction or acts as a catalyst, so in any case, replenishment is not necessary even during continuous operation for a considerable period of time. should be in a minimum amount. As in the case of chlorine ions, bromine ions may be added by any convenient method. Typically, the plating baths of the present invention also contain boric acid or borates. The presence of boric acid or borate or both serves to help control the PH value of the system. The typical total amount of boron compounds used is about 0.1 mole/mole/mole/mole/mole/mole/mole/mole/mole/mole of boron. Furthermore, it is generally advantageous to include formates in order to improve the appearance of the plated products and to increase the covering power of the bath. The general usage amount of formate ion is about 0.1 mol/~about 1.5 mol/
is within the range of Formate has a tendency to oxidize at the anode, so it needs to be replenished from time to time over time. As an auxiliary means to prevent the formation of hexavalent chromium,
Optionally, acetate ions can also be introduced into the bath. Of course, other known additives may be added to the trivalent chromium plating bath according to the invention. The pH value of the bath of the present invention may range from about 1 to 6, preferably from 2 to 5. For baths operating on a commercial scale, typically about 2.5 to about 4.5
All you have to do is operate at the PH value. The temperature at which the plating operation of the present invention is carried out is not critical. Generally about 15°C to about 50°C, preferably about
It is best to keep the temperature between 15°C and about 30°C. The anodes commonly used in trivalent chromium plating processes are carbon or graphite or other anode materials with low overpotentials. These anode materials are called inert anodes. High overpotential anode materials, such as normally insoluble metal anodes such as lead, are not commonly used because they tend to promote oxidation of trivalent chromium ions to the undesirable hexavalent state. Similarly, soluble chromium anodes are also typically not used. Example 1 A plating bath was prepared by dissolving the following materials in water and diluting the solution to 1 liter.

[Table] Among the above materials, "chromium tannin salt" is
Chromium sulfate () used in leather tanning,
Manufactured by reducing sodium dichromate with sulfur dioxide. "Basicity" of those used in this example
is 33%. Basicity is a value indicating the degree of substitution of sulfate ions by hydroxyl ions in the reduction product. The equilibrium PH value of the bath after standing overnight was 3.4. The bath prepared as described above was divided into two equivalent sections: Section A and Section B. to category A
6 of a solution containing 100 g of potassium bromide
ml/ml to adjust the bromide concentration in the bath.
It was set to 0.005 mol/. No bromide was added to Category B. Both solutions were placed in a beaker equipped with a magnetic stirrer and circulated for cooling. A carbon anode and a copper cathode were inserted into each solution. Anode area to cathode area 2:
I set it to 1. All electrodes were completely immersed in the bath.
Both baths were connected in series to a DC power source and a current of 1.2 amps (2.4 amps/liter for each bath) was applied for 1 hour. The current density is approximately 50 amps per square foot (ASF) at the anode and approximately 50 amperes per square foot (ASF) at the cathode.
It was 100ASF. The temperature of both solutions was maintained between 25°C and 27°C. Initially, some odor was emitted from category A, while the odor emitted from category B was strong. After 5 minutes, the odor from Segment B became slightly milder, while the odor from Segment A remained much milder. The odor intensity from both solutions varied during the remainder of the test period, but the odor from Segment A was always at a significantly lower level than the odor from Segment B. Example 2 A 1 liter plating bath was prepared according to the following recipe.

[Table] The wetting agent used was an aqueous solution of sodium alkyl sulfate. After reaching equilibrium, the pH of the bath was adjusted to 3.3 with concentrated aqueous ammonia. The solution was divided into equal volumes, Bath A and Bath B. To Bath A, 2 ml of a solution containing 49 g of ammonium bromide was added to bring the bromide concentration in Bath A to 0.001 mol/. No bromide was added to Bath B. Using the same type of equipment, both of the above solutions were electrolyzed in series under the same conditions as in Example 1. Initially, both baths emitted unpleasant odors, but the odor from Bath A rapidly decreased within 1 minute after the start of energization. Two samples were heated at 9.6 amps for 4 hours under the same conditions.
It was subjected to electrolysis at 1 hour/liter. The odor from Bath B remained very strong, while the odor from Bath A was very mild. Example 3 The composition of the bath is as follows.

[Table] The hydrochloric acid used was standard laboratory concentrated hydrochloric acid.
After standing overnight to reach equilibrium, the pH of the bath was 3.4. A current of 0.63 amperes/hour was applied to this bath for 5 hours, and the current density of the carbon anode completely immersed was determined.
Electrolysis was performed at a current density of 50 ASF and a current density of 100 ASF for the copper cathode, which was completely immersed. During the above operation, circulate cooling to keep the bath temperature from 17℃ to 22℃.
It was kept at ℃. During plating, an extremely unpleasant odor was constantly emanating from the bath. Potassium bromide was added to the bath at 0.89 g/mole to give a bromide ion concentration of 0.0075 mol/mole.
The bath was air stirred for 20 minutes to double the total anode and cathode areas. The power was turned on again. In this case, the amount of current is 1.25 ampere/, but the current density at the anode is 50ASF and the current density at the cathode is 100ASF.
was held. The odor is alleviated within 4 minutes,
The total current is almost 10 amperes.
It remained at an extremely low level after energizing for / hours. A comparative plating test in a bath before and after addition of bromide ions and electrolysis revealed that the cathode electrodeposit was not affected and the luster was not lost even when the current density was varied over a wide range. The solution was always kept free of hexavalent chromium, as indicated by the plating effect. Example 4 A bath having the composition of Example 3 was placed in a 267 ml Hull cell.
A carbon anode was inserted, the solution was stirred magnetically, and the brass panels with new nickel plates were chromed for 10 minutes at a pH of 3.3 and an average cathode current density of 5.92 ASD. The anode is not completely immersed, its current density is 4.84 ASD, and the bath temperature is 18℃ to 23℃ at the time of fitting.
It rose to During the plating process, a very strong odor mainly of chlorine gas was generated. Add 0.90 g of potassium bromide to this cell to bring the bromide ion concentration in 1 liter of bath to 0.0075.
The solution was stirred magnetically and identical brass panels with new nickel plates were chromed for 10 minutes at pH 3.3 and average cathodic current density 5.92 ASD. During the first 3 minutes of the plating process, almost no odor of chlorine gas was observed. After 10 minutes of plating treatment, the anode current density was increased from 4.84 ASD to 5.9 ASD, and this state was maintained for an additional 5 minutes, but no increase in the odor generated from the bath could be detected. The bath contained no hexavalent chromium either before or after the addition of bromide, and all of the plated samples were visually indistinguishable in appearance gloss and free of objectionable defects.

Claims (1)

[Claims] 1. When electroplating high quality chromium from a bath containing trivalent chromium salts and chlorine ions, the generation of chlorine gas at the anode is suppressed even during long plating processing, In the method for preventing the production of hexavalent chromium ions, the bath contains ammonium ions and 0.001 to
A method characterized by containing an inhibitor mixture consisting of 0.008 mol/1 bromide ion, plating continuously for a long time without replenishing bromide ions, and providing high quality chrome plating. 2. The method according to claim 1, wherein the bromide ion concentration is 0.005 mol/1 and the chromium ion concentration is 0.67 mol/1. 3. The method according to claim 1, wherein the bromide ion concentration is 0.0075 mol/1 and the chromium ion concentration is 0.43 mol/1.