PLATING FILM MANUFACTURING METHOD

Description

TECHNICAL FIELD

The present invention relates to a method for producing a plating film.

BACKGROUND ART

Hard chromium plating, which provides excellent hardness and wear resistance, has been widely used in automotive parts, molding dies, mill rolls, printing rolls, and the like.

A trivalent-chromium plating solution for chromium plating and a trivalent-chromium plating method have been proposed (see PTL 1).

However, a plating film formed by using a trivalent-chromium plating bath is prone to cracks that penetrate through the base material. A plating film formed by using a hexavalent-chromium plating bath develops fine microcracks inside the film. Thus, hard trivalent-chromium plating films are disadvantageous in terms of corrosion resistance as compared to hexavalent-chromium plating.

Additionally, because of the applications as stated above, chromium plating is required to provide brightness. Therefore, there is demand for a plating film production method that can form a trivalent-chromium plating film with excellent corrosion resistance and brightness.

CITATION LIST

Patent Literature

• • PTL 1: JP 2006-249518A

SUMMARY OF INVENTION

Technical Problem

An object of the present invention is to provide a method for producing a plating film that can impart excellent corrosion resistance to an object on which a trivalent-chromium plating film is formed and that can form a trivalent-chromium plating film with excellent brightness.

Solution to Problem

As a result of extensive research, the present inventors found that the object can be achieved by a method for producing a plating film that includes step 1 of forming an electrolytic nickel plating film and step 2 of forming an electrolytic trivalent-chromium plating film on the electrolytic nickel plating film, wherein the stress in electrodeposits of the electrolytic nickel plating film is compressive stress, and the compressive stress is within a specific range. Then, the inventors completed the present invention.

Specifically, the present invention relates to the following method for producing a plating film.

Item 1

A method for producing a plating film, comprising

• • step 1 of forming an electrolytic nickel plating film, and
  • step 2 of forming an electrolytic trivalent-chromium plating film on the electrolytic nickel plating film, wherein the electrolytic nickel plating film has a compressive stress of 0 to 100 MPa.

Item 2

The method according to Item 1, wherein the electrolytic trivalent-chromium plating film has an arithmetic mean roughness Ra of 0.080 μm or less.

Item 3

The method according to Item 1 or 2, wherein the electrolytic trivalent-chromium plating film has a thickness of 2 μm or more.

Item 4

The method according to any one of Items 1 to 3, wherein the electrolytic trivalent-chromium plating film has a Vickers hardness of 750 HV or more.

Item 5

The method according to any one of Items 1 to 4, wherein step 2 is a step of intermittently forming an electrolytic trivalent-chromium plating film.

Advantageous Effects of Invention

The method for producing a plating film according to the present invention imparts excellent corrosion resistance to an object on which a trivalent-chromium plating film is formed, and can produce a trivalent-chromium plating film with excellent brightness.

DESCRIPTION OF EMBODIMENTS

The method for producing a plating film of the present invention includes step 1 of forming an electrolytic nickel plating film, and step 2 of forming an electrolytic trivalent-chromium plating film on the electrolytic nickel plating film, wherein the electrolytic nickel plating film has a compressive stress of 0 to 100 MPa. In the production method of the present invention, the stress in electrodeposits of the underlying electrolytic nickel plating film is compressive stress, and the compressive stress is 0 to 100 MPa. Thus, the stress in electrodeposits of the electrolytic trivalent-chromium plating film is tensile stress. This relaxes the stress in electrodeposits and reduces the development of cracks, thus imparting excellent corrosion resistance to an object on which a trivalent-chromium plating film is formed. Because the production method of the present invention reduces the development of cracks in a plating film as described above, the method can produce an electrolytic trivalent-chromium plating film with excellent brightness. Specifically, because the production method of the present invention includes step 1 and step 2 described above in combination with an electrolytic nickel plating film having a compressive stress of 0 to 100 MPa, the method can impart excellent corrosion resistance to an object on which an electrolytic trivalent-chromium plating film is formed, and can produce an electrolytic trivalent-chromium plating film with excellent brightness.

Step 1

Step 1 is a step of forming an electrolytic nickel plating film.

The electrolytic nickel plating solution for forming an electrolytic nickel plating film is not particularly limited as long as the film can have a compressive stress of 0 to 100 MPa. Specific examples of such electrolytic nickel plating solutions include a “Watts bath” containing, for example, nickel sulfate, nickel chloride, or boric acid, a nickel sulfamate bath containing nickel sulfamate, and a “strike bath” (Wood's bath) containing nickel chloride. Various nickel baths containing nickel monohydroxide, nickel carbonate, nickel acetate, or the like may also be used. In the production method of the present invention, the electrolytic nickel plating solution is preferably a Watts bath or a sulfamic acid bath, and more preferably a sulfamic acid bath from the viewpoint of further improved wear resistance of the plating film produced according to the production method of the present invention.

Specifically, the electrolytic nickel plating solution for use may be an electrolytic nickel plating solution containing a water-soluble nickel compound. The electrolytic nickel plating solution may also contain, for example, a primary brightener, a secondary brightener, a potential adjuster, and a pitting inhibitor.

Water-Soluble Nickel Compound

The water-soluble nickel compound is not particularly limited as long as the compound is soluble in water. Examples of such water-soluble nickel compounds include nickel sulfate, nickel chloride, nickel sulfamate, nickel carbonate, and hydrates of these. More specifically, examples of water-soluble nickel compounds include nickel sulfate hexahydrate, nickel chloride hexahydrate, nickel sulfamate, and nickel carbonate tetrahydrate. Of these, nickel sulfamate, nickel chloride, and hydrates of these are preferred, and a mixture of nickel sulfamate and nickel chloride or a hydrate of the mixture is more preferred because these compounds make it easy to adjust the compressive stress of the electrolytic nickel plating film to 0 to 100 MPa, further reduce the development of cracks, and impart greater corrosion resistance to an object on which a trivalent-chromium plating film is formed.

The water-soluble nickel compounds may be used alone or in a combination of two or more.

The content of nickel sulfamate and a hydrate of nickel sulfamate in the electrolytic nickel plating solution is preferably 200 to 500 g/L, more preferably 220 to 400 g/L, and even more preferably 230 to 350 g/L. A content of nickel sulfamate and a hydrate of nickel sulfamate within these ranges makes it easy to adjust the compressive stress of the electrolytic nickel plating film to 0 to 100 MPa, further reduces the development of cracks, and imparts greater corrosion resistance to an object on which a trivalent-chromium plating film is formed.

The content of nickel sulfate and a hydrate of nickel sulfate in the electrolytic nickel plating solution is preferably 200 to 500 g/L, more preferably 220 to 400 g/L, and even more preferably 230 to 350 g/L. Setting the upper limit of the content of nickel sulfate and a hydrate of nickel sulfate within these ranges can further reduce pits formed due to an increase in the specific gravity of the plating solution. Setting the lower limit of the content of nickel sulfate and a hydrate of nickel sulfate within these ranges can reduce the development of cloudiness and burning in the appearance of the plating film.

The content of nickel chloride and a hydrate of nickel chloride in the electrolytic nickel plating solution is preferably 30 to 70 g/L, more preferably 35 to 60 g/L, and even more preferably 40 to 55 g/L. Setting the content of nickel chloride and a hydrate of nickel chloride within these ranges makes it easy to adjust the compressive stress of the electrolytic nickel plating film to 0 to 100 MPa, further reduces the development of cracks, and imparts greater corrosion resistance to an object on which a trivalent-chromium plating film is formed.

Primary Brightener

The electrolytic nickel plating solution preferably contains a primary brightener. Due to the primary brightener contained in the electrolytic nickel plating solution, it becomes easier to adjust the compressive stress of the electrolytic nickel plating film to 0 to 100 MPa.

The primary brightener for use may be a sulfur-based compound, a sulfonic acid-based compound, or the like. Sulfur-based compounds are preferred in that sulfur-based compounds make it easy to adjust the compressive stress of the electrolytic nickel plating film to 0 to 100 MPa, further reduce the development of cracks, and can impart greater corrosion resistance to an object on which a trivalent-chromium plating film is formed.

Examples of sulfur-based compounds include heterocyclic compounds containing sulfur and nitrogen as heteroatoms and sulfur-containing heterocyclic compounds. Of these, heterocyclic compounds containing sulfur and nitrogen as heteroatoms and sulfur-containing heterocyclic compounds are preferred, and heterocyclic compounds containing sulfur and nitrogen as heteroatoms are more preferred, in that these compounds make it easy to adjust the compressive stress of the electrolytic nickel plating film to 0 to 100 MPa, further reduce the development of cracks, and impart greater corrosion resistance to an object on which a trivalent-chromium plating film is formed.

The primary brightener is preferably a heterocyclic compound represented by the following formula (1):

Using the heterocyclic compound represented by formula (1) as a primary brightener makes it easy to adjust the compressive stress of the electrolytic nickel plating film to 0 to 100 MPa, further reduces the development of cracks, and imparts greater corrosion resistance to an object on which a trivalent-chromium plating film is formed.

In formula (1), R¹ is S or SO₂. Of these, SO₂ is preferred in that the compressive stress of the electrolytic nickel plating film can be easily adjusted to 0 to 100 MPa, the development of cracks is further reduced, and greater corrosion resistance can be imparted to an object on which a trivalent-chromium plating film is formed.

In formula (1), R² is H, Cl, Br, I, or CH₃. Of these, H or CH₃ is preferred in that the compressive stress of the electrolytic nickel plating film can be easily adjusted to 0 to 100 MPa, the development of cracks is further reduced, and greater corrosion resistance can be imparted to an object on which a trivalent-chromium plating film is formed.

More specifically, examples of primary brighteners include 1,2-benzisothiazol-3(2H)-one, saccharin, N-methylsaccharin, N-chlorosaccharin, N-bromosaccharin, and N-iodosaccharin. Of these, 1,2-benzisothiazol-3(2H)-one, saccharin, N-methylsaccharin, and N-chlorosaccharin are preferred, and 1,2-benzisothiazol-3(2H)-one, saccharin, and N-methylsaccharin are more preferred, in that the compressive stress of the electrolytic nickel plating film can be easily adjusted to 0 to 100 MPa, the development of cracks is further reduced, and greater corrosion resistance can be imparted to an object on which a trivalent-chromium plating film is formed.

The primary brighteners may be used alone or in a combination of two or more.

The content of the primary brightener in the electrolytic nickel plating solution is preferably 0.1 to 10 g/L, more preferably 0.2 to 5 g/L, and even more preferably 0.2 to 2 g/L. Setting the lower limit of the content of the primary brightener within these ranges makes it easy to adjust the compressive stress of the electrolytic nickel plating film to 0 to 100 MPa, further reduces the development of cracks, and imparts greater corrosion resistance to an object on which a trivalent-chromium plating film is formed. Setting the upper limit of the content of the primary brightener within these ranges limits the sulfur content in a bright nickel plating film, and further limits the deterioration of corrosion resistance.

Secondary Brightener

The electrolytic nickel plating solution preferably contains a secondary brightener. The secondary brightener contained in the electrolytic nickel plating solution makes the surface of the electrolytic nickel plating film smoother, and further improves the brightness of the trivalent-chromium plating film formed in step 2.

The secondary brightener for use may be a multiple-carbon-carbon-bond compound, a multiple-carbon-heteroatom-bond compound, or the like. The secondary brightener is preferably a multiple-carbon-carbon-bond compound in that such a secondary brightener makes the surface of the electrolytic nickel plating film smoother and further improves the brightness of the trivalent-chromium plating film formed in step 2.

Examples of secondary brighteners include unsaturated compounds having a carbon-carbon double bond and/or a carbon-carbon triple bond. Of these, unsaturated compounds having a carbon-carbon triple bond are preferred from the viewpoint of a smoother surface of the electrolytic nickel plating film and improved brightness of the trivalent-chromium plating film formed in step 2.

The secondary brightener is preferably sodium propynesulfonate, butynediol diethoxylate, hexynediol, an unsaturated compound represented by the following formula (2), and an unsaturated compound represented by the following formula (3).

Using these unsaturated compounds as a secondary brightener makes the surface of the electrolytic nickel plating film smoother and further improves the brightness of the trivalent-chromium plating film formed in step 2.

In formulas (2) and (3), R³ and R⁵ are H, OH, or CH₃. R³ and R⁵ are preferably H or CH₃ from the viewpoint of a smoother surface of the electrolytic nickel plating film and further improved brightness of the trivalent-chromium plating film formed in step 2.

In formula (2), R⁴ is an alkyl group having 1 to 7 carbon atoms or a propargyl group. R⁴ is preferably an alkyl group from the viewpoint of a smoother surface of the electrolytic nickel plating film and further improved brightness of the trivalent-chromium plating film formed in step 2. The number of carbon atoms of R⁴ is preferably 1 to 5, and more preferably 1 to 3.

More specifically, examples of secondary brighteners include 2-propyn-1-ol (propargyl alcohol), sodium propynesulfonate, sodium propenesulfonate, 1,4-butynediol, 2-butyn-1-ol, 2-pentyn-1-ol, butynediol diethoxylate, 2-hexyn-1-ol, hexynediol, 2-heptyn-1-ol, 2-octyn-1-ol, 2-nonyn-1-ol, 2-decyn-1-ol, and 2,4-hexadiyn-1,6-diol. Of these, 2-propyn-1-ol (propargyl alcohol), 1,4-butynediol, 2-butyn-1-ol, butynediol diethoxylate, and hexynediol are preferred from the viewpoint of a smoother surface of the electrolytic nickel plating film and further improved brightness of the trivalent-chromium plating film formed in step 2.

These secondary brighteners may be used alone or in a combination of two or more.

The content of the secondary brightener in the electrolytic nickel plating solution is preferably 0.005 to 3 g/L, more preferably 0.02 to 1.8 g/L, and even more preferably 0.04 to 1.5 g/L. Setting the lower limit of the content of the secondary brightener within these ranges makes the surface of the electrolytic nickel plating film smoother and further improves the brightness of the electrolytic trivalent-chromium plating film formed in step 2.

Potential Adjuster

The electrolytic nickel plating solution preferably contains a potential adjuster. A potential adjuster contained in the electrolytic nickel plating solution makes the potential of the electrolytic nickel plating film more noble and improves the corrosion resistance of the electrolytic nickel plating film.

Examples of potential adjusters for use include aldehyde compounds and diol compounds. Of these, diol compounds are preferred in that diol compounds can further increase the carbon content while further decreasing the sulfur content in the electrolytic nickel plating film to thereby further improve the corrosion resistance of the electrolytic nickel plating film, and can also make the potential of the electrolytic nickel plating film more noble.

The potential adjuster is preferably the following: an aldehyde compound represented by the following formula (4):

R⁶—R⁷—CHO  (4)

an aldehyde compound represented by the following formula (5):

R⁸—CHO  (5)

a diol compound represented by the following formula (6):

R⁹—R¹⁰—CH(OH)₂  (6)

and a diol compound represented by the following formula (7):

R¹¹—CH(OH)₂  (7)

Using the aldehyde compound represented by formula (4) or (5) or the diol compound represented by formula (6) or (7) as a potential adjuster can further improve the corrosion resistance of the electrolytic nickel plating film.

In formulas (4) to (7), R⁶, R⁸, R⁹, and R¹¹ are H, Cl, Br, CH₃, CBr₃, or CCl₃. Of these, H, CH₃, Cl, and CCl₃ are preferred from the viewpoint of further improved corrosion resistance of the electrolytic nickel plating film.

In formulas (4) and (6), R⁷ and R¹⁰ are an alkyl group having 1 to 7 carbon atoms or a vinyl group. R⁷ and R¹⁰ are preferably an alkyl group from the viewpoint of further improved corrosion resistance of the bright nickel plating film. The number of carbon atoms of R⁷ and R¹⁰ is preferably 1 to 5, and more preferably 1 to 3.

More specifically, examples of potential adjusters include acetaldehyde, formaldehyde, propanal, butanal, hexanal, 3-chloropropanal, chloroacetaldehyde, chloral hydrate, bromal hydrate, 3-methylbutanal, 2-propenal, and 2-butenal. Of these, chloroacetaldehyde, chloral hydrate, 2-propenal, and 2-butenal are preferred from the viewpoint of further improved corrosion resistance of the electrolytic nickel plating film.

The potential adjusters may be used alone or in a combination of two or more.

The content of the potential adjuster in the electrolytic nickel plating solution is preferably 0.001 to 1 g/L, more preferably 0.002 to 0.1 g/L, and even more preferably 0.005 to 0.05 g/L. Setting the content of the potential adjuster within these ranges can further increase the carbon content while further decreasing the sulfur content in the electrolytic nickel plating film to thereby further improve the corrosion resistance of the electrolytic nickel plating film, and can also make the potential of the electrolytic nickel plating film more noble.

Other Additives

The electrolytic nickel plating solution preferably contains the water-soluble nickel compound described above optionally with the primary brightener, secondary brightener, and potential adjuster described above, and more preferably is an aqueous solution containing these components.

The electrolytic nickel plating solution may contain other components, such as a pitting inhibitor and a pH buffer, in addition to the above components.

Examples of pitting inhibitors include surfactants, such as nonionic surfactants, anionic surfactants, and cationic surfactants. The content of the pitting inhibitor is not particularly limited, but may be, for example, about 0.01 to 10 g/L.

Examples of pH buffers include boric acid, phosphoric acid, phosphorous acid, carbonic acid, sodium salts thereof, potassium salts thereof, and ammonium salts thereof. The amount of the pH buffer to be added is not particularly limited, but may be, for example, about 0.1 to 200 g/L.

The pH of the electrolytic nickel plating solution may be typically about 3.5 to 5.0, and preferably about 3.8 to 4.8. To adjust the pH, the following may be used: for example, inorganic acids, such as sulfuric acid and hydrochloric acid, metal carbonates, such as nickel carbonate, sodium hydroxide, and aqueous ammonia.

To perform electrolytic nickel plating by using the electrolytic nickel plating solution described above, the electrolytic nickel plating solution may be brought into contact with an object to be plated in accordance with a commonly used method. Typically, an electrolytic nickel plating film can be efficiently formed by immersing an object to be plated in an electrolytic nickel plating solution and performing electrolytic plating.

The liquid temperature of the electrolytic nickel plating solution is typically about 45 to 65° C., and preferably about 50 to 60° C. Optionally, the electrolytic nickel plating solution can be stirred, and the object to be plated can be rocked.

The current density during electrolytic nickel plating may be about 1 to 6 A/dm², and preferably about 2 to 4 A/dm².

The plating time can be appropriately determined according to the target thickness of the electrolytic nickel plating film, and may be about 10 to 30 minutes.

The material of the object to be plated is not particularly limited as long as it can be electrolytically plated. Examples of materials for the object include metals, such as iron, copper, zinc, and aluminum, alloys of these metals, and resins with a base plating.

In the production method of the present invention, the stress in electrodeposits of the electrolytic nickel plating film formed in step 1 is compressive stress. Specifically, the electrolytic nickel plating film has a compressive stress of 0 to 100 MPa. A compressive stress of less than 0 MPa results in tensile stress; since the stress in electrodeposits of the trivalent-chromium plating film formed in step 2, described below, is also tensile stress, the stress in electrodeposits is not relaxed, and the development of cracks is not reduced. This makes it impossible to impart excellent corrosion resistance to an object on which a trivalent-chromium plating film is formed. A compressive stress exceeding 100 MPa makes it likely for cracks to form in the nickel plating film, and results in a trivalent-chromium plating film with poor corrosion resistance. The electrolytic nickel plating film preferably has a compressive stress of 0 to 50 MPa.

In the present specification, the compressive stress of the electrolytic nickel plating film is measured according to the method described in the Examples below.

The electrolytic nickel plating film has an arithmetic mean roughness Ra of preferably 0.080 μm or less, more preferably 0.050 μm or less, and even more preferably 0.040 μm or less. Setting the upper limit of arithmetic mean roughness Ra within these ranges further improves the brightness of the electrolytic trivalent-chromium plating film formed on the electrolytic nickel plating film, in step 2 described below. The lower the lower limit of arithmetic mean roughness Ra, the better. The lower limit of arithmetic mean roughness Ra may be, for example, about 0.020 μm.

In the present specification, arithmetic mean roughness Ra of the electrolytic nickel plating film is measured according to a measurement method conforming to JIS B0601-2001.

The electrolytic nickel plating film has a Vickers hardness of preferably 300 HV or more, more preferably 350 HV or more, and even more preferably 400 HV or more. The electrolytic nickel plating film also has a Vickers hardness of preferably 1000 HV or less, more preferably 900 HV or less, and even more preferably 800 HV or less. A Vickers hardness within these ranges further reduces the development of cracks in the plating film produced by the production method of the present invention and further improves corrosion resistance.

In the present specification, the Vickers hardness of the electrolytic nickel plating film is measured according to a measurement method conforming to JIS Z2244-1:2020.

The electrolytic nickel plating film has a thickness of preferably 5 μm or more, and more preferably 10 μm or more. The electrolytic nickel plating film also has a thickness of preferably 50 μm or less, and more preferably 30 μm or less. A thickness of the electrolytic nickel plating film within the these ranges, combined with the fact that the electrolytic trivalent-chromium plating film formed in step 2 on the electrolytic nickel plating film described below has tensile stress, further reduces the development of cracks in the plating film and further improves the corrosion resistance of the plating film.

Step 2

Step 2 is a step of forming an electrolytic trivalent-chromium plating film on the electrolytic nickel plating film.

The electrolytic trivalent-chromium plating solution for forming the electrolytic trivalent-chromium plating film is not particularly limited, and any known electrolytic trivalent-chromium plating solution can be used.

The electrolytic trivalent-chromium plating solution for use may be, for example, an electrolytic trivalent-chromium plating solution containing a trivalent chromium compound, a complexing agent, a conductive salt, and a pH buffer.

Trivalent Chromium Compound

The trivalent chromium compound for use may be any water-soluble trivalent chromium compound, and examples include chromium sulfate, chromium chloride, and basic chromium sulfate. These trivalent chromium compounds may be used alone or in a combination of two or more.

The concentration of the trivalent chromium compound in the electrolytic trivalent-chromium plating solution is not particularly limited; however, for example, the chromium metal concentration is preferably about 20 to 60 g/L, and more preferably about 30 to 45 g/L.

Complexing Agent

The complexing agent is not particularly limited as long as it is a compound capable of complexing a trivalent chromium. The complexing agent for use is preferably at least one compound selected from the group consisting of water-soluble aliphatic carboxylic acids and salts thereof.

The type of water-soluble aliphatic carboxylic acid is not particularly limited, and any carboxylic acid that can be made into an aqueous solution of a predetermined concentration can be used. Examples of water-soluble aliphatic carboxylic acid for use include aliphatic monocarboxylic acids, such as formic acid, acetic acid, and glycine; aliphatic dicarboxylic acids, such as oxalic acid, malonic acid, and succinic acid; aliphatic hydroxymonocarboxylic acids, such as gluconic acid; aliphatic hydroxydicarboxylic acids, such as malic acid; and aliphatic hydroxytricarboxylic acids, such as citric acid.

The salt of a water-soluble aliphatic carboxylic acid for use may be any water-soluble salt of various kinds of carboxylic acids described above. Examples include alkali metal salts, such as sodium salts and potassium salts, alkaline earth metal salts, such as calcium salts and magnesium salts, and ammonium salts.

These complexing agents may be used alone or in a combination of two or more.

The amount of the complexing agent for use is preferably about 0.1 to 0.8 mol per mole of trivalent chromium ions contained in the electrolytic trivalent-chromium plating solution. In particular, the complexing agent in an amount of about 0.1 to 0.3 mol per mole of trivalent chromium ions can increase the deposition rate of the electrolytic trivalent-chromium plating film.

Conductive Salt

Examples of conductive salts for use include sulfates, such as potassium sulfate, sodium sulfate, and ammonium sulfate; chlorides, such as potassium chloride, sodium chloride, and ammonium chloride; and bromides, such as potassium bromide, sodium bromide, and ammonium bromide. Of these, sulfates, such as potassium sulfate, sodium sulfate, and ammonium sulfate, are preferable for use.

These conductive salts may be used alone or in a combination of two or more.

The concentration of the conductive salt in the electrolytic trivalent-chromium plating solution is not particularly limited, and is preferably about 50 to 200 g/L, and more preferably about 70 to 180 g/L.

pH Buffer

The pH buffer for use may be, for example, aluminum sulfate, and boric acid compounds, such as boric acid, sodium borate, potassium borate, and boron oxide. The use of both aluminum sulfate and a boric acid compound is preferable in that use of both can further improve the deposition properties of the plating film in a low current density region and can provide better throwing power.

These pH buffers may be used alone or in a combination of two or more.

The concentration of aluminum sulfate in the electrolytic trivalent-chromium plating solution is preferably 10 to 50 g/L, and more preferably 20 to 40 g/L.

The concentration of the boric acid compound in the electrolytic trivalent-chromium plating solution is preferably 10 to 80 g/L, and more preferably 20 to 60 g/L.

The electrolytic trivalent-chromium plating solution preferably contains the trivalent chromium compound, complexing agent, conductive salt, and pH buffer described above, and more preferably is an aqueous solution containing these components.

The pH of the electrolytic trivalent-chromium plating solution is typically preferably 1.0 to 3.0, and more preferably 1.5 to 2.5. A pH within these ranges further improves deposition properties in a low current density region and provides better throwing power.

The method of electrolytic trivalent-chromium plating is not particularly limited, and a method similar to a commonly used electrolytic trivalent-chromium plating method may be used by using the electrolytic trivalent-chromium plating solution described above.

The anode for use in the electrolytic trivalent-chromium plating is not particularly limited, and a known insoluble anode, such as a Ti—Pt electrode, can be used. In particular, a Ti electrode coated with a thin film of Ir—Ta composite oxide is advantageous in suppressing the formation of hexavalent chromium.

In regards to the liquid temperature of the electrolytic trivalent-chromium plating solution, there is the following trend: a low bath temperature during plating improves throwing power, whereas a high bath temperature decreases throwing power at a low current density. Additionally, a bath temperature too low may crystallize the components. Considering this point, the bath temperature is preferably about 25 to 50° C., and more preferably about 30 to 40° C.

The current density during electrolytic trivalent-chromium plating is preferably 5 to 40 A/dm², and more preferably 10 to 20 A/dm².

The plating time may be appropriately determined according to the target thickness of the electrolytic trivalent-chromium plating film, and may be about 10 to 90 minutes.

In the production method of the present invention, the electrolytic trivalent-chromium plating film formed in step 2 preferably has a tensile stress of 0 to 50 MPa, and more preferably 0 to 30 MPa. The stress in electrodeposits of the electrolytic trivalent-chromium plating film is tensile stress within these ranges, and the stress in electrodeposits of the electrolytic nickel plating film formed in step 1 is compressive stress. This further relaxes the stress in electrodeposits, and thereby further reduces the development of cracks, thus imparting greater corrosion resistance to an object on which a trivalent-chromium plating film is formed.

In the present specification, the tensile stress of the electrolytic trivalent-chromium plating film is measured according to the method described in the Examples below.

The electrolytic trivalent-chromium plating film has an arithmetic mean roughness Ra of preferably 0.080 μm or less, more preferably 0.050 μm or less, and even more preferably 0.040 μm or less. Setting the upper limit of arithmetic mean roughness Ra within the above ranges further improves the brightness of the electrolytic trivalent-chromium plating film and also improves the wear resistance of the electrolytic trivalent-chromium plating film. The lower the lower limit of arithmetic mean roughness Ra, the better. The lower limit of arithmetic mean roughness Ra may be, for example, about 0.020 μm.

In the present specification, arithmetic mean roughness Ra of the electrolytic trivalent-chromium plating film is measured according to a measurement method conforming to JIS B0601-2001.

The electrolytic trivalent-chromium plating film has a Vickers hardness of preferably 750 HV or more, more preferably 800 HV or more, and even more preferably 850 HV or more. The electrolytic trivalent-chromium plating film also has a Vickers hardness of preferably 1500 HV or less, more preferably 1300 HV or less, and even more preferably 1200 HV or less. A Vickers hardness within these ranges further reduces the development of cracks in the electrolytic trivalent-chromium plating film and further improves the corrosion resistance of the plating film.

In the present specification, the Vickers hardness of the electrolytic trivalent-chromium plating film is measured according to a measurement method conforming to JIS Z2244-1:2020.

The electrolytic trivalent-chromium plating film has a thickness of preferably 2 μm or more, and more preferably 5 μm or more. The electrolytic trivalent-chromium plating film also has a thickness of preferably 200 μm or less, and more preferably 100 μm or less. A thickness of the electrolytic trivalent-chromium plating film within these ranges, combined with the fact that the electrolytic nickel plating film formed in step 1 above has compressive stress, further reduces the development of cracks in the plating film and further improves the corrosion resistance of the plating film.

Step 2 may be a step of intermittently forming an electrolytic trivalent-chromium plating film. Step 2 in this manner forms the electrolytic trivalent-chromium plating film like a multilayer and prevents cracks that form on the lower side of the electrolytic trivalent-chromium plating film (the side adjacent to the electrolytic nickel plating film) from reaching the upper side, thereby further improving the corrosion resistance of the trivalent-chromium plating film.

The method for intermittently forming an electrolytic trivalent-chromium plating film may be, for example, a method of repeating a cycle of (1) passing electric current and (2) stopping electrolysis, or a method of repeating a cycle of (1) passing electric current, (2) stopping electrolysis, and (3) washing with water.

The conditions for (1) to (3) above may be appropriately set according to the target thickness of the electrolytic trivalent-chromium plating film. For example, the time for passing electric current in (1) is preferably 5 to 20 minutes, and more preferably 10 to 15 minutes. If the method is the repetition of a cycle of (1) and (2), it is preferable to perform (1) immediately after stopping electrolysis of (2). The time period for stopping electrolysis (2) is preferably within 60 seconds. The step of washing with water of (3) may be performed immediately after stopping electrolysis of (2). The time period of washing with water of (3) is preferably 5 to 60 seconds, and more preferably 10 to 20 seconds.

Heat Treatment

In the production method of the present invention, after step 2, heat treatment may be performed. Heat treatment can further suppress hydrogen embrittlement that may occur due to hydrogen absorbed into the produced plating film. The temperature of heat treatment is not particularly limited, and is preferably 100 to 500° C., and more preferably 100 to 200° C. The time of heat treatment is preferably 15 to 180 minutes, and more preferably 30 to 60 minutes.

Article with Plating Film

The plating film produced according to the production method of the present invention can be used for various purposes, such as industrial chromium plating. In particular, the production method of the present invention can impart excellent corrosion resistance to an object on which a trivalent-chromium plating film is formed, and can also produce a trivalent-chromium plating film with excellent brightness. Thus, the plating film produced according to the production method of the present invention is particularly suitable for industrial chromium plating.

Industrial chromium plating is used in various industrial areas, taking advantage of the properties of chromium plating films, such as a high level of hardness, wear resistance, corrosion resistance, adhesion, and mold releasability, and is used, for example, in the manufacture of dies, mill rolls, and printing rolls. In industrial chromium plating, metal materials, such as iron and steel, stainless steel, brass, and zinc die-cast products, are mainly used as an object to be plated.

EXAMPLES

The present invention is described in detail below with reference to Examples and Comparative Examples. However, the present invention is not limited to the Examples.

An iron plate (6 cm×5 cm, thickness: 0.3 mm) was prepared as the object to be plated.

Step 1

The components of amounts shown below were sequentially added to water and mixed to prepare a matte nickel plating bath (Watts bath).

Composition of Matte Nickel Plating Bath (Watts Bath)

• • Nickel sulfate hexahydrate: 280 g/L
  • Nickel chloride hexahydrate: 45 g/L
  • Boric acid: 40 g/L
  • Additives: the additives listed in Table 1 were added as shown in Table 2.

The components of amounts shown below were sequentially added to water and mixed to prepare a nickel sulfamate plating bath.

Composition of Nickel Sulfamate Plating Bath

• • Nickel sulfamate: 300 g/L
  • Nickel chloride hexahydrate: 10 g/L
  • Boric acid: 40 g/L
  • Additives: the additives listed in Table 1 were added as shown in Table 2.

Table 1 shows the nickel plating additives used in the Watts bath and nickel sulfamate plating bath.

TABLE 1
TOP DuNC SB-XE-M	Semi-bright Nickel Additive	10 ml/L
	(Multiple-bond Compound)	(2 g/L on the Basis of Semi-bright Nickel Additive)
TOP DuNC SB-XE-R	Semi-bright Nickel Additive	1 ml/L
	(Multiple-bond Compound)	(0.2 g/L on the Basis of Semi-bright Nickel Additive)
TOP DuNC BN-1	Primary Brightener	10 ml/L
	(Sulfur Compound)	(0.8 g/L on the Basis of Primary Brightener)
TOP DuNC BN-2	Secondary Brightener	1 ml/L
	(Multiple-bond Compound)	(0.3 g/L on the Basis of Secondary Brightener)
TOP DuNC SB-S	Potential Adjuster	0.1 ml/L
		(0.01 g/L on the Basis of Potential Adjuster)
TOP SELENA DDX-1	Primary Brightener (Sulfur Compound)	5 ml/L
		(0.2 g/L on the Basis of Primary Brightener)
TOP SELENA DDX-2	Secondary Brightener (Multiple-bond Compound)	5 ml/L
		(1.2 g/L on the Basis of Secondary Brightener)
TOP SELENA 73X	Primary + Secondary Brightener (Sulfur Compound)	5 ml/L
		(0.2 g/L on the Basis of Primary Brightener)
		(0.2 g/L on the Basis of Secondary Brightener)
MU-2	Primary Brightener (Sulfur Compound)	5 ml/L
		(0.8 g/L on the Basis of Primary Brightener)

As shown in Table 2, Watts baths and nickel plating baths using a base bath (sulfamic acid bath) and a brightener were prepared. Then, nickel plating films were formed under the following nickel plating electrolysis conditions.

Nickel Plating Electrolysis Conditions

• • pH: 4.2
  • Bath temperature: 55° C.
  • Cathode current density: 3 A/dm²
  • Electrolysis time: 20 minutes
  • Film thickness (target): 10 μm
  • Anode: Nickel plate
  • Agitation: Air stirring (1 L/min)

Measurement of Stress in Electrodeposits of Nickel Plating Film

Nickel films were formed under the nickel plating electrolysis conditions described above, and the stress in electrodeposits was measured with a test strip (manufactured by Fuji Kasei Co., Ltd.) under a measurement condition of an effective area of 0.077 dm². The stress in electrodeposits was calculated according to the following formula.

Stress ⁢ in ⁢ Electrodeposits ⁢ ( MPa ) = 58.2 × UK / T

U=opening width (mm), K=coefficient (K=0.2949 for tensile stress, K=0.1918 for compressive stress), T=film thickness (μm) (calculated from the weight difference of the test specimen before and after plating)

Table 2 shows the stress in electrodeposits of the measured nickel plating films.

Step 2 The components of amounts shown below were sequentially added to water and mixed to prepare a chromium plating bath (Top Fine Chrom SP-A, manufactured by Okuno Chemical Industries Co., Ltd.).

Composition of Chromium Plating Bath

• • Top Fine Chrom SP-A Conc (Cr³⁺ source, complexing agent): 330 ml/L (Cr³⁺: 40 g/L)
  • Top Fine Chrom SP-A Conductor 1 (conductive salt): 90 g/L
  • Top Fine Chrom SP-A Conductor 2 (conductive salt): 90 g/L
  • Top Fine Chrom SP-A Base 1 (pH buffer): 30 g/L
  • Top Fine Chrom SP-A Base 2 (pH buffer): 40 g/L

A trivalent-chromium plating film was formed on the nickel plating film formed in step 1 above by using the chromium plating bath under the following conditions for chromium plating electrolysis to produce a plating film.

Chromium Plating Electrolysis Conditions

• • pH: 1.7
  • Bath temperature: 35° C.
  • Cathode current density: 15 A/dm²
  • Electrolysis time: 60 minutes
  • Film thickness (target): 8 μm
  • Anode: Iridium oxide anode
  • Agitation: Air stirring (0.1 L/min)

In forming chromium plating films by intermittent plating, the following conditions were applied.

Electrolysis was suspended every 10 minutes of passing electric current, followed by washing with water for 10 seconds.

Measurement of Surface Roughness Ra after Formation of Chromium Plating Film

Arithmetic mean roughness Ra (μm) of the plating film produced by performing steps 1 and 2 was measured on the side of the trivalent-chromium plating film according to a measurement method conforming to JIS B0601-2001.

Table 2 shows arithmetic mean roughness Ra (pnm) of the plating films measured on the side of the trivalent-chromium plating films.

Salt Spray Test

A plating film was produced on an object by performing steps 1 and 2. The plating film was subjected to a salt spray test under conditions of a temperature of 35° C., a humidity of 95%, and a time of 200 hours in accordance with a measurement method conforming to JIS Z2371, and was evaluated according to the following evaluation criteria. A rating of C or higher was considered to be satisfactory for practical use.

• • A: No red rust was formed.
  • B: A small amount of red rust was formed.
  • C: Red rust was formed in several parts.
  • D: Red rust was formed on almost the entire surface.

	TABLE 2
					Surface Roughness	Salt Spray
			Stress in Electrodeposits	Cr Plating	Ra after Cr Plating	Test
	Base Bath	Brightener	of Ni Plating (MPa)	Method	(μm)	(200 hr)

Comparative	Watts Bath (Matte)	—	116.6 (Tensile Stress)	Continuous	0.123	A
Example 1
Comparative	Watts Bath (Matte)	—	47.2 (Tensile Stress)	Continuous	0.144	D
Example 2	Nickel Chloride Hexahydrate
	5 g/L
Comparative	Watts Bath (Semi-bright)	TOP DuNC SB-XE-M	57.8 (Tensile Stress)	Continuous	0.184	D
Example 3		TOP DuNC SB-XE-R
		TOP DuNC SB-S

Comparative

No Ni Plating Film

5.2 (Cr) (Tensile Stress)

Continuous

0.087

D

Example 4
Example 1	Sulfamic Acid Bath (Bright)	TOP DuNC BN-2	4.8 (Compressive Stress)	Continuous	0.056	A
Example 2	Sulfamic Acid Bath (Bright)	TOP DuNC BN-1	12.4 (Compressive Stress)	Continuous	0.041	A
		TOP DuNC BN-2
Example 3	Sulfamic Acid Bath (Bright)	TOP DuNC BN-1	25.6 (Compressive Stress)	Continuous	0.051	A
		TOP DuNC BN-2
		TOP DuNC SB-S
Example 4	Watts Bath (Bright)	TOP DuNC BN-1	40.2 (Compressive Stress)	Continuous	0.043	B
		TOP DuNC BN-2
Example 5	Watts Bath (Bright)	TOP SELENA DDX-1	47.7 (Compressive Stress)	Continuous	0.073	B
		TOP SELENA DDX-2
Example 6	Watts Bath (Bright)	TOP SELENA 73X	56.1 (Compressive Stress)	Continuous	0.080	B
		MU-2
Example 7	Watts Bath (Bright)	TOP SELENA DDX-1	67.1 (Compressive Stress)	Continuous	0.080	A
	Nickel Chloride Hexahydrate	TOP SELENA DDX-2
	5 g/L
Example 8	Watts Bath (Bright)	TOP DuNC BN-1	40.2 (Compressive Stress)	Intermittent	0.043	A
		TOP DuNC BN-2
Example 9	Watts Bath (Bright)	TOP SELENA DDX-1	47.7 (Compressive Stress)	Intermittent	0.073	A
		TOP SELENA DDX-2
Example 10	Watts Bath (Bright)	TOP SELENA 73X	56.1 (Compressive Stress)	Intermittent	0.080	A
		MU-2

Wear Resistance Test

The plating films produced in Example 3 and Comparative Example 3 were subjected to a wear resistance test under the following measurement conditions in accordance with a measurement method conforming to the reciprocating wear test method of JIS H8503. With 500 reciprocations counted as one test, the number of tests was counted until the chromium plating film was confirmed to have been stripped, and evaluation was made. For comparison, evaluation was also performed on a sample prepared by forming a trivalent-chromium plating film directly on an iron plate without forming a nickel plating film (Reference Example 1).

• • Equipment: Suga Abrasion Tester NUS-ISO-3
  • Abrasive paper: #600
  • Load: 500 gf
  • Frequency of reciprocation: 500

Wear Amount Measurement

The weight of the object on which the plating film was formed was measured before and after the wear resistance test. The weight after the wear resistance test was subtracted from the weight before the wear resistance test, and the difference was taken as the amount of wear. The wear resistance test was performed three times for the same part to measure the amount of wear, and the average was taken as the measured value.

Table 3 shows the results.

					Surface	Wear Resistance
			Stress in		Roughness	(Number of Tests
			Electrodeposits		Ra after Cr	until Plating	Amount
			of Ni Plating	Cr Plating	Plating	Film was Stripped)	of Wear
	Base Bath	Brightener	(MPa)	Method	(μm)	(frequency)	(mg)

Reference

No Ni Plating Film

—

Continuous

0.056

7

0.6

Example 1
Example 3	Sulfamic Acid Bath	TOP DuNC BN-1	25.6 (Compressive	Continuous	0.051	9	0.4
	(Bright)	TOP DuNC BN-2	Stress)
		TOP DuNC SB-S
Comparative	Watts Bath (Semi-	TOP DuNC SB-XE-M	57.8 (Tensile	Continuous	0.184	5	1.3
Example 3	bright)	TOP DuNC SB-XE-R	Stress)
		TOP DuNC SB-S

The results shown in Table 3 indicate that the plating film having a nickel plating film formed in a sulfamic acid bath as a base in Example 3 was superior in terms of wear resistance of the chromium plating film to the plating film having a nickel plating film formed in a Watts bath (semi-brightness) as a base in Comparative Example 3. This is probably because the plating film of Example 3 had low stress in electrodeposits for the nickel plating film. Moreover, the results indicate that the plating film of Example 3 had a smaller amount of wear than the plating film of Comparative Example 3 because the plating film of Example 3 had smaller surface roughness and higher smoothness.