JPH09209199A - Basic chromium sulfate for chromium plating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain basic chromium sulfate for chromium plating which is capable of forming a chromium plated film having uniform and firm covering power and excellent lustrous appearance. SOLUTION: This basic chromium sulfate is obtained by reducing chromic acid anhydride or/and sodium bichromate and has characteristics of which content of reducing agent remaining in the basic chromium sulfate is <=500ppm and content of chromium (VI) is <=0.24wt.% as Cr and in which organic substance does not substantially exist. Preferably, the basic chromium sulfate has <=55% basicity and content of sodium sulfate which is inevitably incorporated when sodium bichromate is used as a raw material is 25wt.% at most.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a basic chromium sulphate for chrome plating which can impart a uniform and strong covering power and an excellent glossy appearance.

[0002]

2. Description of the Related Art Industrial chromium electroplating work has conventionally been carried out using a hexavalent chromium bath. Recently, from the viewpoint of the safety of hexavalent chromium toxicity, handling of chemicals,
It is necessary to pay close attention to treatment measures and management in a series of process fields such as storage, plating environment, and waste liquid treatment, which inevitably leads to a large increase in capital investment and treatment cost. For this reason, the electroplating method using trivalent chromium, which is not toxic as a chemical agent replacing hexavalent chromium, has been reviewed and partly put to practical use.

The electrolytic plating method using a trivalent chromium bath such as chromium sulfate (hereinafter referred to as "trivalent chromium plating method") is a chromium plating technique that has been known for a long time. For example, chromium sulfate and oxalic acid (salt) are essential. A method using a chromium plating bath as a component (JP-A-52-125427), a method using a chromium plating bath containing chromium sulfate and an amino acid or an amino acid salt, sulfamic acid or sulfamic acid and ammonium ion (JP-A-55). No. 31122), a method of using a chromium plating bath containing chromium sulfate having a specific concentration or less, a complexing agent, a buffer and SCN (JP-A-58-872915), A method using a chromium plating bath containing a complexing agent having a limited stability constant, a buffering agent and an S-containing species consisting of sulfite or dithionous acid (JP-A-58) 87292 JP) and the like have been proposed.

However, in the trivalent chromium plating method, it is 6
Since it is difficult to obtain a hard chrome film such as that obtained by electrolytic plating using valent chromium, in many cases it has been studied for practical use for many years for decorative items, and by the addition of various auxiliaries, etc. Although some improvements have been made, many problems still remain to be solved in terms of the glossiness, uniformity, adhesion, etc. of the obtained plating film. Recently 3
Proposal for chromium sulphate with improved peel resistance of the plating film in the valent chromium plating method (JP-A-6-173098)
Japanese Patent Laid-Open No. 6-158398 and Japanese Patent Laid-Open No. 6-173100), but does not disclose the use of basic chromium sulfate as a chromium source for plating.

By the way, basic chromium sulfate is an industrial chemical which has been conventionally used as a leather tanning agent. For example, chromic anhydride is reduced by using an organic reducing agent such as sucrose, starch syrup, glucose and alcohol. , Sodium dichromate is reduced by sulfurous acid gas.
Therefore, there is a possibility that this basic chromium sulfate can be used for electrolytic plating as a trivalent chromium source, but in reality, the electrolytic chromium plating film formed by using the basic chromium sulfate has a remarkably poor appearance and is extremely poor. It cannot be put to practical use.

[0006]

DISCLOSURE OF INVENTION Problems to be Solved by the Invention The inventors of the present invention have conducted extensive studies to apply harmless and relatively inexpensive basic chromium sulfate as a chromium source for electrolytic plating. The surplus reducing agent, hexavalent chromium ions and organic substances remaining in the inside adversely affect the properties of the plating film, and when the content of these remaining components is suppressed below a certain amount, a chromium plating exhibiting an excellent gloss appearance. The fact that the layer can be formed with good adhesion was clarified.

The present invention was developed on the basis of such findings, and an object of the present invention is to form a chromium plating film having a uniform and excellent gloss appearance with good coverage and reproducibility. The object is to provide a base chromium sulfate capable of forming chromium.

[0008]

The basic chromium sulfate for chromium plating according to claim 1 of the present invention for achieving the above object is obtained by reducing chromic anhydride or / and sodium dichromate. Basic chromium sulfate,
The content of the reducing agent remaining in the basic chromium sulfate is 500.
It is a structural feature that the content of hexavalent chromium is 0.24% by weight or less as Cr and the organic substance is substantially absent at ppm or less.

According to the invention of claim 2, the basicity is 55% or less in the above constitution, and the invention of claim 3 is
The content of Na ₂ SO ₄ inevitably mixed in basic chromium sulfate when sodium dichromate is used as a raw material is 25% by weight.

The basic chromium sulfate to which the present invention is directed is that an OH ion is coordinated with a part of a ligand (ligand) constituting a complex of chromium sulfate formed as a trivalent chromium salt. It has a composition and the coordination amount of OH is
Is an OH group. ) Is defined as representing the basicity. Basicity (BD) = x / 3 × 100 (%)

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The basic chromium sulfate according to the present invention is obtained by adding a reducing agent to chromic anhydride, sodium dichromate, or a mixture thereof to reduce to trivalent chromium. . Therefore, basically, there is no difference from the basic chromium sulfate that is generally used as a leather tanning agent. The basicity of the basic chromium sulfate is not necessarily limited as long as it is soluble in the plating bath, but it is preferably adjusted to 55% or less. When the basicity exceeds 55%, the gelation of chromium hydroxide is promoted, which causes uneven plating.
If the basicity is low, it does not generate chromium hydroxide, but the chromium composition in the plating bath tends to change,
As a result, the physical properties of the plating film may be impaired. A more preferred basicity is in the range of 10-40%. The basicity can be measured by a pH titration method using an aqueous sodium hydroxide solution.

In the present invention, a coating excellent in that the content of the reducing agent remaining in the basic chromium sulfate is 500 ppm or less, the content of hexavalent chromium is 0.24% by weight or less, and the organic substance is substantially absent. It is the most important requirement for obtaining a chrome plated layer that has both strength and appearance. Of these, in order to eliminate the remaining of the reducing agent, it is sufficient to carry out the reduction reaction with a theoretical reduction amount that does not generate an excess reducing agent, but in such a reaction system, the removal of hexavalent chromium must be incomplete. I don't get it. Therefore, the excess reducing agent component that is inevitably mixed in when the reaction is carried out under the conditions exceeding the theoretical reduction amount is removed as much as possible, and the amount of the basic chromium sulfate finally obtained is 500 ppm. Below, it is preferably adjusted to 100 ppm or less. Remaining reducing agent content is 500
When it exceeds ppm, the coating power of the plating film decreases and the appearance of the plating deteriorates significantly.

On the other hand, the reason for setting the content of hexavalent chromium remaining in the basic chromium sulfate as 0.24% by weight or less as Cr is that the presence of hexavalent chromium in the plating bath causes Cr ^{6+ in the} reaction process. Is not completely reduced, which causes a slight influence on the formation of the plating film and leads to a decline in plating efficiency and deterioration of the physical properties of the plating film, but the residual content is 0.24% by weight.
If the following, the above-mentioned inconvenient phenomenon is effectively suppressed,
This is because it is possible to form a chromium plating layer having good adhesion and an excellent appearance under an efficient plating process. The more preferable upper limit of the residual content of hexavalent chromium is 0.01%.
It is.

Further, in the present invention, the organic substances are not substantially present in the basic chromium sulfate. The term "substantially free of organic matter" means a state in which there is no excess organic reducing agent in the basic chromium sulfate when the organic reducing agent described below is used as the reducing agent. The reason for this is that if an organic substance is present in the basic chromium sulfate, the organic substance is mixed in the plating film to lower the plating adhesion and significantly deteriorate the plating appearance.

The basic chromium sulfate for chromium plating of the present invention having the above-mentioned composition is produced by a method in which chromic anhydride or / and sodium dichromate is subjected to a reduction reaction using an appropriate reducing agent. When sodium dichromate is used as the raw material, Na ₂ S 0 ₄
However, since this sodium sulfate functions as a conductive salt for plating, it is more advantageous in terms of composition than the case of using chromic anhydride as a raw material, which is not accompanied by this contamination. However, when sodium dichromate is used as a raw material, the content of Na ₂ SO ₄ inevitably mixed is 25% by weight at most.
It is preferable that the content be in the range of up to, and if the content exceeds the above amount, the smoothness of the plating process is hindered.

Examples of the reducing agent include organic reducing agents such as sucrose, starch and alcohol, and inorganic reducing agents such as hydrogen peroxide, sulfurous acid gas and sulfurous acid. However, among these, it is preferable to use an inorganic reducing agent that can easily remove the excess reducing agent component by heating degassing even if the amount of hexavalent chromic acid added is larger than the theoretical reduction amount. Application of sulfurous acid gas is preferred.

When the residual reducing agent content is 500 ppm, 6
A preferred embodiment for obtaining basic chromium sulfate having a valent chromium content of 0.24% by weight or less as Cr and substantially free of organic substances is as follows. When an organic reducing agent is used, a reducing agent in an amount less than the theoretical amount is added to hexavalent chromium to cause a reduction reaction, and then the remaining hexavalent chromium is treated with an inorganic reducing agent such as sulfurous acid gas. It is substantially completely reduced, and the excess inorganic reducing agent is heated and deaerated. When only an inorganic reducing agent is used, the hexavalent chromium is reduced by a reducing agent in a stoichiometric amount or more, and the excess reducing agent component remaining after the reaction is degassed by heating. It should be noted that the reducing agent in which a trace amount remains in the reduction reaction of hexavalent chromium and the reduction of the amount of hexavalent chromium are contradictory contradictory factors,
As noted above, pay close attention to the final stage of the reduction reaction,
Moreover, by adding a post-treatment to the reaction, it is possible to simultaneously control the amounts of both of these components without containing a substantial organic substance.

For example, the reaction mode for reducing sodium dichromate with sulfurous acid gas to obtain basic chromium sulfate is as follows. Na ₂ Cr ₂ O ₇ + 3SO ₂ + H ₂ O → 2Cr (OH)
SO ₂ + Na ₂ SO ₄

This reaction is extremely simple with sodium dichromate, sulfurous acid gas and water, the amount of sulfurous acid gas as a reducing agent can be easily adjusted, and sodium sulfate as a by-product is used as it is in the electroconductivity of the plating bath. Since it can be applied as a basic salt, it is the most industrially effective means for obtaining the basic chromium sulfate of the present invention.

The reaction product thus obtained is adjusted to basicity as necessary to obtain basic chromium sulfate for chromium plating. To form the basic chromium sulfate as a chromium plating bath, if necessary, for example, a complexing agent such as formate and acetate, a buffering agent such as boric acid and sodium borate, an alkali metal salt, and an alkaline earth metal. Soluble metal salts such as metal salts,
An adjusting agent such as a pH adjusting agent and a surfactant can be contained. The bath concentration is not particularly limited, but preferably 0.01 to 1 mol / l of basic chromium sulfate and 0.01 as a complexing agent.
~ 3 mol / l, buffering agent 0.1 to 1 mol / l, conductive salt 2
It is set in the range of 6 mol / l.

The plating conditions are pH 2.3 to 3.8, temperature 27 to 60 ° C., current density 0.3 to 50 A / dm ² , preferably 3 to 11 A / dm, depending on the composition of the plating bath used. ²
It is preferable to set

The chromium plating layer formed by using the basic chromium sulfate of the present invention as a chromium source is always uniform and bright because excess reducing agent, hexavalent chromium, and organic substances are not mixed in the plating bath beyond the allowable amount. It has a high-quality property of exhibiting an excellent appearance and adhering to a substrate.

[0023]

EXAMPLES Hereinafter, examples of the present invention will be specifically described in comparison with comparative examples, but the scope of the present invention is not limited to these examples. In each example, the amount of SO ₂ remaining in basic chromium sulfate was measured by oxidizing SO ₂ with iodine and back titrating excess iodine with a sodium thiosulfate solution. Further, the amount of residual hexavalent chromium was measured by a colorimetric analysis method.

Example 1 Sulfurous acid gas (105% of theoretical reduction amount) was gradually introduced into a 60% by weight aqueous solution of sodium dichromate to carry out a reduction treatment, and basic chromium sulfate having a basicity of 33.3% was added. Obtained.

A brass plate having a width of 67 mm and a length of 100 mm, which was prepared by bathing a chromium plating solution having the composition shown in Table 1 in a Hansen bath in advance and using the obtained basic chromium sulfate as a main agent, was tested (test). The plate) was electrolytically chromium plated for 3 minutes under the conditions of a bath temperature of 30 ° C. and 5 amperes. The covering power and plating gloss of the obtained chromium plating film are measured and evaluated,
The results are shown in Table 2 in comparison with the residual content in the basic chromium sulfate. In addition, the covering power of the chrome plating film is indicated by the minimum deposition current density (index of the ease of plating),
The plating gloss (appearance) was evaluated by visual evaluation.

Example 2 98 parts of 100 parts by weight of a 60% by weight aqueous solution of chromic anhydride were added.
60 parts by weight of sulfuric acid having a concentration of 1% by weight and 13 parts by weight of sucrose (100% of the theoretical reduction amount) were added and mixed with stirring for 1 hour,
Further, 5 parts by weight of sulfurous acid gas was introduced and left for 2 hours. Then, in order to remove the excess sulfurous acid gas, the temperature is 110 ° C.
After heating for 5 hours, basic chromium sulfate having a basicity of 33.3% was obtained. Using the obtained basic chromium sulfate as the main agent,
Chromium plating was performed under the same conditions as in Example 1 using a chromium plating bath having the composition shown in Table 1. The formed chromium plating film was measured and evaluated in the same manner as in Example 1, and the results are also shown in Table 2.

Example 3 100 parts by weight of a 60% by weight aqueous solution of chromic anhydride and 197 parts by weight of a 40% by weight aqueous solution of sodium dichromate were mixed,
To this mixture was added 150 parts by weight of 98% by weight sulfuric acid and 26 parts by weight of sucrose (100% of the theoretical reduction amount) to obtain 1
After stirring and mixing for 11 hours, 11 parts by weight of sulfurous acid gas was introduced and the mixture was allowed to stand for 1 hour. Then, in order to remove excess sulfurous acid gas, heat treatment was performed at a temperature of 110 ° C. for 5 hours to adjust the basicity to 3
3.3% basic chromium sulphate was obtained. Using the obtained basic chromium sulfate as a main agent, a chromium plating bath having the composition shown in Table 1 was subjected to a chromium plating treatment under the same conditions as in Example 1. The formed chromium plating film was measured and evaluated in the same manner as in Example 1, and the results are also shown in Table 2.

Example 4 Sulfurous acid gas (110% of theoretical reduction amount) was gradually introduced into a 40% by weight aqueous solution of sodium dichromate to carry out a reduction treatment, and at a temperature of 110 ° C. for removing excess sulfurous acid gas.
After heat treatment for an hour, basic chromium sulfate having a basicity of 33.3% was obtained. The obtained basic chromium sulfate was used as the main agent, and Table 1
Chromium plating treatment was performed under the same conditions as in Example 1 with a chromium plating bath having the composition shown in. The formed chromium plating film was measured and evaluated in the same manner as in Example 1, and the results are also shown in Table 2.

Comparative Example 1 Sulfurous acid gas (110% of theoretical reduction amount) was gradually introduced into a 40% by weight aqueous solution of sodium dichromate to carry out a reduction treatment, and the basicity was 33. Three
% Basic chromium sulphate was obtained. Using the obtained basic chromium sulfate as a main agent, a chromium plating bath having the composition shown in Table 1 was subjected to a chromium plating treatment under the same conditions as in Example 1. The formed chromium plating film was measured and evaluated in the same manner as in Example 1, and the results are also shown in Table 2.

Comparative Example 2 Sulfurous acid gas (100% of theoretical reduction amount) was gradually introduced into a 40% by weight aqueous solution of sodium dichromate to carry out a reduction treatment, and the basicity was 33. Three
% Basic chromium sulphate was obtained. Using the obtained basic chromium sulfate as a main agent, a chromium plating bath having the composition shown in Table 1 was subjected to a chromium plating treatment under the same conditions as in Example 1. The formed chromium plating film was measured and evaluated in the same manner as in Example 1, and the results are also shown in Table 2.

Comparative Example 3 100 parts by weight of a 60% by weight aqueous solution of chromic anhydride was added with 98 parts by weight.
60 parts by weight of sulfuric acid having a concentration of 1% by weight and 13 parts by weight of sucrose (110% of the theoretical reduction amount) were added, and the mixture was stirred and mixed for 1 hour for reduction treatment to obtain basic chromium sulfate having a basicity of 33.3%. . Using the obtained basic chromium sulfate as a main agent, a chromium plating bath having the composition shown in Table 1 was subjected to a chromium plating treatment under the same conditions as in Example 1. The formed chromium plating film was measured and evaluated in the same manner as in Example 1, and the results are also shown in Table 2.

[0032]

[Table 1] [Table Note] * Na ₂ SO in Examples 1 and 4 and Comparative Examples 1 and ₂
Contains less than 25% by weight of ₄ .

[0033]

[Table 2]

From the results shown in Table 2, the chromium plating films according to the examples using the basic chromium sulfate of the present invention were homogeneous, had good covering power, and had an excellent gloss appearance.
On the other hand, the plating films of Comparative Examples all have a poor appearance. In Comparative Example 2, the amount of hexavalent chromium exceeds 0.24% by weight, and in Comparative Example 3, the organic substances are mixed in the plating bath, so that the color of trivalent chromium plating is different. It exhibited different plating layer tones. Also,
In Comparative Example 1, since a large amount of the excess reducing agent remained, not only the appearance deterioration but also the covering power was inferior.

[0035]

As described above, according to the present invention, the content of the reducing agent and hexavalent chromium remaining in the production process is suppressed to the allowable value or less, and the organic substance is substantially absent. It is possible to provide a basic chromium sulfate for chromium plating, which can form a chromium plating film having excellent gloss appearance with good reproducibility by using trivalent basic chromium sulfate which could not be used. . Therefore, it can be expected to be industrially useful as a chromium source for chromium plating that has excellent plating properties in a harmless environment.

Claims (3)

[Claims] 1. A basic chromium sulfate obtained by reducing chromic anhydride or / and sodium dichromate,
The content of the reducing agent remaining in the basic chromium sulfate is 500.
A basic chromium sulfate for chromium plating, which is characterized by having a hexavalent chromium content of 0.24% by weight or less as Cr and having substantially no organic substance at ppm or less. 2. The basic chromium sulfate for chromium plating according to claim 1, which has a basicity of 55% or less. 3. The chromium plating according to claim 1 or 2, wherein the content of Na ₂ SO ₄ which is unavoidably mixed in the basic chromium sulfate when sodium dichromate is used as a raw material is 25% by weight. Basic chrome sulfate for use.