JPH0573835B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a chromium plating method, and particularly to a chromium plating method using a so-called non-fluoride chromium plating bath that contains sulfate radicals and does not contain fluoride ions. Regarding improvements.

Chrome plating is useful as an industrial plating because of its high hardness and excellent wear resistance and corrosion resistance. However, due to the use of chromic acid, it is not necessarily considered to be a desirable plating due to problems such as air pollution, difficulty in treating wastewater, and low energy balance.

Incidentally, with the recent development of pollution treatment technology, the problem of wastewater treatment can be said to be established as a treatment technology, although it is more complicated than other types of wastewater treatment. However, the low energy balance is quite fateful and is unavoidable to some extent as long as the current sulfuric acid-containing non-fluoride chromium plating bath based on a sergeant bath is used. Conventionally, the current efficiency in chrome plating was 10 to 20.
It is well known that the main reason for the low plating rate is hydrogen generated during plating. Although various proposals have been made to improve current efficiency, conventional high current efficiency chrome plating methods are still unsatisfactory in terms of the gloss of the plating film.

By the way, the main reason why chrome plating has high hardness and excellent gloss is that hydrogen is generated during plating. Therefore, improvements in current efficiency and improvements in gloss are contradictory requirements.

In conventional sergeant baths, the current efficiency is 20~20 in the low temperature and low current density area outside the practical range.
It is known that the percentage is 35%. however,
It is also well known that chrome plating in this area is gray and lacks luster.

Hitherto, chrome plating has not required the addition of brighteners as in other platings. This is because, as mentioned above, the generation of hydrogen during plating contributes to gloss, and also because there is no brightening agent that is stable against the high chromic acid concentration and extremely low pH in the plating bath. evening. In other words, chrome plating up until now has only been carried out under optimal electrolytic conditions within a narrow range of natural gloss that does not require brighteners, and the development of brighteners can be said to be a completely unknown field.

The present inventors focused on hydrogen generation, which is the cause of low current efficiency in conventional chrome plating.

That is, as shown in Figure 1, if we take the cathodic polarization curve (cathode potential-current density curve) of chromium electrodeposition using a sergeant bath containing chromic acid as the main component (indicated by the solid line in Figure 1), , in front of the 4th branch corresponding to the chromium precipitation region (the current rapidly increasing region that appears at -1.2 V or higher (versus saturated calomel electrode (SCE); the same applies hereinafter: the region where the chromium deposition current flows and the precipitation of metallic chromium is observed). In the range of −0.7 to −1.2V,
It shows peaks indicating the rise and fall of the current, and this is called the third branch. This region is a region where hydrogen is generated, and at the same time, it is also a region where hexavalent chromium is reduced to trivalent chromium. Due to the overvoltage, hydrogen generation starts from around -0.3V rather than 0V, and this is called the first branch. Next, there is an increase in current that is said to be caused by the reduction of oxygen in the electrode and the discharge of hexavalent chromium, and this is called the second branch. The current values of the first branch and the second branch are often small and cannot be clearly distinguished. On the other hand, the third
The branch and the fourth branch are considered to be important regions for the chromium electrodeposition mechanism.

Therefore, the present inventors conducted extensive research in order to find a substance that can influence the large potential peak corresponding to the third branch of the potential-current density curve of the surgent bath.

As a result, the present inventors found that by adding a rare earth element or its compound to the chromium plating bath, the above peak becomes significantly smaller or loses its shape (see Figure 1). , shown by the dotted line). As a result, hydrogen generation during electrolysis did not seem to have significantly decreased (as the current efficiency remained almost unchanged), but the reduction of hexavalent chromium to trivalent chromium progressed smoothly, and Electrolytic conditions (for example, metal surface technology handbook (revised new edition) 2nd edition (Nikkan Kogyo Shimbun,
It was found that the gloss of the chromium precipitates was extremely high even at 30° C. and 10 to 50 A/dm ² ) as shown in Figures 4 and 33 on page 295 (published on December 25, 1978).

By the way, composite plating, which is well known for electrolytic plating of nickel, copper, gold, silver, etc.
It was considered difficult to put it into practical use with chromium. This is mainly due to the above-mentioned generation of hydrogen interfering with the precipitation of the composite plating. However, as mentioned above, we have also discovered that chromium composite plating becomes possible by adding a rare earth element or a compound thereof to the chromium plating bath.

Therefore, the object of the present invention is to provide a chrome plate that can improve the gloss in ordinary chrome plating using a non-fluoride chromium plating bath and also enables composite plating. The goal is to provide a method for achieving this goal.

That is, the chromium plating method of the present invention uses a non-fluoride chromium plating bath that contains sulfate radicals and does not contain fluoride ions, and that contains rare earth elements and their compounds. It is characterized by using a non-fluoride chromium plating bath containing at least one rare earth material selected from among them as a metal in a proportion of at least about 0.01 g/liter.

When performing composite plating, fine ceramic powder is dispersed and contained in the plating bath.

To explain this invention in more detail below, the rare earth substances added to the non-fluoride chromium plating bath include scandium, yttrium, cerium, lanthanum, praseodymium, neodymium, promethium, samarium, europysium, and gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and/or lutetium, and compounds thereof. Compounds of rare earth elements include inorganic acid salts such as sulfates, hydrochlorides, and nitrates; organic acid salts such as acetates and tartrates; oxides; nitrides; carbides; and borides. These rare earth materials may be used in the form of a mixture or in the form of an alloy, for example, so-called Mitsushimetal.

The amount of the rare earth compound added is at least about 0.01 g/liter (bath) in terms of metal. The preferred amount added is 0.1 g/liter to 10 g/liter. If the amount of the rare earth compound is less than 0.01 g/liter, the gloss of the chromium precipitate cannot be obtained under low temperature, high current density conditions.

As the aqueous non-fluoride chromium plating bath to which the above-mentioned rare earth substances are added, any publicly known aqueous plating bath can be used, and typical baths are those mainly containing hexavalent chromium (chromic anhydride) such as Sargent bath. It is.
However, it may be made mainly of trivalent chromium. Such chromium plating baths include baths containing chromium sulfate as a main component and ammonium sulfate, urea, etc., known baths containing trivalent chromium chloride as a main component, or baths containing chromium plating baths containing trivalent chromium chloride as a main component, or baths containing chromium plating baths containing trivalent chromium chloride as a main component, or chromium plating baths containing chromium sulfate as a main component and containing ammonium sulfate, urea, etc.
There are things listed in the issue. This latter chromium plating bath consists of a chromic anhydride solution containing sulfur-containing salts such as hydrogen sulfide, sulfites, hyposulfites or thiosulfates in proportions of 5 to 15% by weight of chromic anhydride. It is.

Note that when fluoride is added to the plating bath used in the present invention, the rare earth element reacts with fluorine to produce insoluble fluoride in most cases, making it impossible to achieve the effects of the present invention sufficiently.

When chromium composite plating is performed in the present invention, fine ceramic powder is dispersed in the chromium plating bath and the plating process is performed while stirring. Ceramics include oxides, carbides, nitrides of silicon, aluminum, tungsten, etc.
These include borides, silicides, and sulfides. The finer the particle size of these ceramics, the better depending on the purpose, but the ones currently available usually have a particle size of
It has a diameter of 0.2 μm to 250 μm, and is preferably added to the bath at a rate of 1 to 800 g/liter. Through this plating process, a composite plating film is obtained in which fine ceramic powder is dispersed and fixed in the chrome plating film.

The plating conditions follow the commonly used chrome plating conditions. Typically, the current density is between 5 and 100 A/ ^dm2 , and the bath temperature is between -10°C and 70°C.
It is ℃.

As already mentioned, according to the plating method of the present invention, the gloss of the plating film is remarkable even under electrolytic conditions in the conventional non-glossy range in normal chrome plating. Although the principle has not yet been fully elucidated, the above-mentioned rare earth materials react in the plating bath to form rare earth chromates, which become fine floating particles and become precipitation nuclei, which physically act as a cathode. It is predicted that it contributes to the fine structure of the precipitated chromium and precipitates bright chromium precipitates, or that slightly dissolved rare earth ions act on the cathode reaction and have an electrochemical effect on the hydrogen overvoltage and chromium precipitation reaction. be done.

In addition, in the case of composite plating, as a result of eutectoid ceramic fine powder, the excellent physical properties of chromium, such as high hardness, wear resistance, corrosion resistance, and heat resistance, can be further improved by these ceramics. can. Therefore, it can be said to be particularly useful industrially because it further improves the physical properties that are considered to be close to the limit for plating and is also considered to be the functional limit of chrome plating, and can meet more advanced requirements.

This invention will be further explained below with reference to Examples.

Example 1 Normal sergeant bath, i.e. chromic anhydride
To a bath consisting of 250 g/liter and 2.5 g/liter of sulfuric acid, lanthanum sulfate was added at a rate of 0.1 to 10 g/liter, and brass and sulfuric acid were added as cathodes.
Using lead as the anode, current density 10-50A/d
m ² , electrolytic treatment was performed for 10 minutes at a bath temperature of 30 to 40°C.

In the case of a normal bath without addition of lanthanum sulfate, a glossy gray chrome plating film was obtained only around the cathode when the current density was 50 A/dm ² . Under other conditions, it resulted in a rough plated coating of gray to black-gray color.

On the other hand, when using the bath of this invention, in a bath to which lanthanum sulfate was added at a rate of 0.1 g/liter, bright chrome plating was almost completely formed under all conditions except for the condition of current density of 10 A/ ^dm2 . A coating was obtained. In the bath containing 0.2 g/liter or more of lanthanum sulfate, a completely bright chrome plated film could be obtained under all electrolytic conditions.
The current efficiency was 27.0 to 33.5%, which was about the same as or slightly higher than that of the normal bath.

In addition, the hardness of the chromium plating film was 680 to 740 Hv (Vickers hardness) when a normal bath was used, but when the bath of this invention was used, 0.1 g/liter of lanthanum sulfate was used. Including Hv
Hv is 850 to 1240, especially in those containing 0.5 g/liter or more of lanthanum sulfate, Hv is 1450.
We were able to obtain a hard chromium plating film that reached the desired level.

Example 2 A bath in which lanthanum oxide was added at a rate of 0.1 to 10 g/liter to the normal bath of Example 1 was electrolytically treated in the same manner under the conditions of a bath temperature of 10 to 80°C and a current density of 10 to 120 A/ ^dm2. As a result, a chrome-plated film with excellent gloss was obtained under all conditions.

Example 3 A chromium plating bath was prepared by adding commercially available Mitsushimetal (33% cerium) powder at a rate of 1 gram/liter to the normal bath of Example 1 while stirring. Using this bath, bath temperature 20-60℃, current density 20-50A/
When electrolytically treated at dm ² , a chromium plated film with excellent gloss was obtained under the entire range of conditions. The current efficiency was in the range of 15.5-35.5%, and the lower the bath temperature, the higher the efficiency.

Example 4 Using various baths in which 50 to 1000 grams/liter of chromic acid anhydride was added with 1/100 to 1/200 of sulfuric acid at a sergeant bath ratio (to chromic acid), the bath temperature was 40°C.
When electrolytic treatment was carried out at a current density of 15 A/dm ² , all precipitates were gray in color. When electrolytic treatment was carried out by adding 1/100 the amount of cerium sulfate to chromic acid to these baths, a bright chrome plating film could be obtained under all conditions.

Example 5 A chromium composite plating bath was prepared by gradually adding silicon carbide powder having a particle size of 2.0 μm to the bath of Example 3 at a rate of 100 to 800 grams/liter while stirring.
Using this bath, the bath temperature is 20 to 40℃ and the current density is 20A/d.
Electrolytic treatment was carried out for 1 hour under conditions of m ² .

As the amount of silicon carbide increased, the Hv of the composite plating coating changed from 1100 to 1450, but when the amount of silicon carbide was more than 150 g/liter, the Hv decreased.
It remained almost constant at 1450. When these composite plating films were treated with hydrochloric acid and the amount of silicon carbide was analyzed from the residue, it was found that 1.2% of silicon carbide was present in the composite plating films.
It was found that eutectoid was present at a rate of ~3.0% by weight.

In the same way, 50% alumina powder (particle size 0.5μm)
When added at a rate of ~200 grams/liter,
It was possible to eutectoid alumina at a ratio of 2.1 to 2.5% by weight. The Hv of these composite plating films is 1200
~1350, and stable hardness could be obtained.

Regarding the composite plating film obtained in this way, Tabar
By using a wear tester and a wear ring (CS-10)
The weight loss of the coating after 10,000 rotations was measured. the result,
The weight loss of the composite plated coating was 1/2 of that of the chrome plated coating, and the wear resistance was significantly improved.

Example 6 Silicon carbide powder with a particle size of 2 μm was placed in a bath consisting of 200 grams/liter of chromium sulfate, 450 grams/liter of ammonium sulfate, 240 grams/liter of urea, and 0.2 grams/liter of Mitsushimetal.
Added at a rate of 300 grams/liter (bath pH 2.2)
~2.5), the electrolytic treatment was carried out under the conditions of a bath temperature of 45° C. and a current density of 15 A/dm ² . In this way, a chromium-plated film in which 1% by weight of silicon carbide was eutectoid was obtained.

Example 7 Using a bath consisting of 150 g/liter of chromic anhydride, 15 g/liter of sodium hyposulfite, and 0.5 g/liter of lanthanum sulfate, the bath temperature was
When electrolysis was carried out for 30 minutes at 35° C. and a current density of 80 A/dm ² , a chromium-plated film with excellent gloss was obtained. The Hv of this coating was 1500.

[Brief explanation of the drawing]

FIG. 1 is a cathode potential-current density curve diagram in chromium plating using a sergeant bath.

Claims (1)

[Claims] 1. Contains a sulfate group for chromium plating treatment,
A non-fluoride chromium plating bath containing no fluoride ions containing at least one rare earth material selected from the group consisting of rare earth elements and their compounds at a rate of at least about 0.01 g/liter as metal. A chromium plating method characterized by using a fluoride-free chromium plating bath. 2. A patent claim in which the rare earth compound is at least one compound selected from the group consisting of sulfates, hydrochlorides, nitrates, acetates, tartrates, oxides, nitrides, carbides, and borides of rare earth elements. Chromium plating method according to item 1. 3. Claim 1 or 2, wherein the plating bath contains fine ceramic powder dispersed therein.
Chrome plating method described in section.