CHROMIUM-PLATED COMPONENT AND PRODUCTION METHOD FOR SAME

TECHNICAL FIELD

[0001]The present invention relates to a chromium-plated part, specifically a nickel chromium-plated part, and a method for manufacturing the same.

BACKGROUND

[0002]Traditionally, chromium plating has been used for purposes such as decorative covering of substrates, providing corrosion resistance for substrates, and providing electrical conductivity for substrates. Chromium plating usually provides silvery-white appearance and thus provides useful and beautiful, decorative coatings. The chromium plating layer has self-passivation ability and thus can form a passivation film on its surface, which provides high corrosion resistance. Specifically, resin-based products with chromium-plated surfaces are lighter and less expensive than metal-based products and thus used for a variety of parts, including automotive parts.

[0003]Some chromium-plated parts (products) have a nickel plating layer as a foundation (nickel chromium-plated products). Such products have particularly high decorative performance and corrosion resistance. Such chromium-nickel plating is used on a variety of substrates including resins. The high corrosion resistance of nickel-chromium plating is considered due to the sacrificial corrosion of the underlying nickel layer, which prevents the corrosion of the chromium plating layer.

[0004]In the nickel chromium-plated product, the nickel film can have improved corrosion resistance when it has a two- or three-layer structure including a combination of: a semi-bright nickel plating layer substantially free of sulfur; a sulfur-containing, bright nickel plating layer; and optionally a nickel plating layer with another composition (see, for example, Japanese Unexamined Patent Application, Publication No. H5-171468, Japanese Unexamined Patent Application, Publication No. H6-146069, Japanese Unexamined Patent Application, Publication No. 2007-275750, and Japanese Unexamined Patent Application, Publication No. 2010-185116). In such a structure, the bright nickel plating layer has a lower electric potential and thus can undergo sacrificial corrosion to provide improved corrosion resistance. Chromium-plated parts with such a nickel plating layer can have higher corrosion resistance. These days, there has been an increasing demand for higher corrosion resistance for chromium-plated parts, and measures for further improvement in corrosion resistance, such as underlying nickel plating layer structures, have been investigated.

[0005]For example, Japanese Unexamined Patent Application, Publication No. H5-171468 and Japanese Unexamined Patent Application, Publication No. H6-146069 disclose a nickel chromium-plated product including: a base; a stack of a semi-bright nickel plating layer, a bright nickel plating layer, and a co-deposition nickel plating layer, which are provided in this order on the base; and a chromium plating layer provided on the stack. In this structure, the co-deposition nickel plating layer is a layer that contains fine particles of silica or any other material co-deposited with nickel and has a microporous structure. In such a product, many pores can disperse corrosion current and thus protect the bright nickel plating layer from corrosion. The inventions described in Japanese Unexamined Patent Application, Publication No. H5-171468 and Japanese Unexamined Patent Application, Publication No. H6-146069 will respectively improve corrosion resistance by adjusting the electric potential difference between the nickel plating layers and by forming the nickel plating layers with different concentrations of high electrode potential metal ions.

[0006]Japanese Unexamined Patent Application, Publication No. 2007-275750 discloses a surface modification method that includes oxidizing the surface of a chromium-plated part with a structure similar to that disclosed in Japanese Unexamined Patent Application, Publication No. H5-171468 and Japanese Unexamined Patent Application, Publication No. H6-146069 to form a chromium oxide film on the surface of the chromium plating film. Japanese Unexamined Patent Application, Publication No. 2010-185116 discloses a chromium-plated part including: a resin substrate; and a copper plating, a sulfur-free nickel plating (semi-bright nickel plating), a bright nickel plating, a noble-potential nickel plating, and a trivalent chromium plating with a microporous or microcrack structure, which are provided in this order on the resin substrate. Patent Document 4 also discloses, as the noble-potential nickel plating, a co-deposition nickel plating with a microporous structure (MP nickel plating). The inventions of Japanese Unexamined Patent Application, Publication No. 2007-275750 and Japanese Unexamined Patent Application, Publication No. 2010-185116 provide techniques to form a modified chromium plating layer for the production of plated parts with improved corrosion resistance. In both of these inventions, the nickel plating film has a three-layer structure with a semi-bright nickel plating layer provided as the lowermost layer.

SUMMARY

[0007]Unfortunately, chromium-plated parts with an underlying nickel layer still have room for improvement in terms of corrosion resistance. In recent years, in consideration of the environment, trivalent chromium plating is often used instead of hexavalent chromium plating, but trivalent chromium plating on a nickel plating layer does not necessarily provide the same level of corrosion resistance as hexavalent chromium plating. Both Japanese Unexamined Patent Application, Publication No. H5-171468 and Japanese Unexamined Patent Application, Publication No. H6-146069, disclose the production of plated products using hexavalent chromium plating. If trivalent chromium plating is used instead in such production, the resulting chromium plating layer may have insufficient corrosion resistance. Moreover, the invention described in Publication No. 2007-275750, and Japanese Unexamined Patent Application including a method of passivating the surface of a chromium plating layer may require processes and equipment for surface oxidation, which may lead to an increase in manufacturing cost.

[0008]In the invention described in Japanese Unexamined Patent Application, Publication No. 2010-185116, it will be generally difficult to obtain sufficient corrosion resistance unless non-conductive fine particles are used to form a microporous structure in a trivalent chromium plating layer.

[0009]In addition, many nickel-chromium plated products have a three-layer nickel plating structure (the plated products described in Japanese Unexamined Patent Application, Publication No. H6-146069 and Japanese Unexamined Patent Application, Publication No. 2010-185116 essentially having two types of nickel plating layers also have such a structure), which includes a semi-bright nickel plating layer (sulfur-free nickel plating layer) on the base side; and a bright nickel plating layer on top of the semi-bright nickel plating layer. Such a semi-bright nickel plating layer, which has a higher electric potential than the bright nickel plating layer, covers an underlying copper or copper alloy portion to prevent the corrosion of the underlying portion. At this time, however, the present inventors have found that if such a higher potential nickel plating layer in contact with the base corrodes to the base side, the corrosion may tend to progress rapidly. In this regard, as a result of investigation, the present inventors have found that the semi-bright nickel plating layer on the base side should be replaced by a bright nickel plating layer with an electric potential lower than that of the semi-bright nickel plating layer so that such corrosion can be prevented.

[0010]However, the present inventors have also found that even if such a bright nickel plating layer is provided on the base side, the copper or copper alloy portion may corrode and undergo swelling and other damage.

[0011]It is an object of the present invention to provide a solution to the problems described above and to provide a chromium-plated part that has a copper or copper alloy portion protected from corrosion and exhibits high corrosion resistance and good appearance even when provided with a trivalent chromium plating layer as an upper layer; and a method for manufacturing such a chromium-plated part.

Means for Solving the Problems

[0012]The present inventors have created a technique that is different from commonly used nickel-chromium plating and includes forming two low-potential nickel plating layers on a base in such a way that each of the two low-potential nickel plating layers has an electric potential within a specific range. The present inventors have completed the present invention based on findings that when produced using such a technique, chromium-plated parts can have a copper or copper alloy portion protected from corrosion and exhibit high corrosion resistance and good appearance.

Effects of the Invention

[0023]The chromium-plated part of the present invention exhibits high corrosion resistance despite having a trivalent chromium plating layer as an upper layer. In the chromium-plated part of the present invention, the copper or copper alloy portion is notably protected from corrosion. Moreover, the chromium-plated part of the present invention has good appearance. The method of the present invention for manufacturing a chromium-plated part can provide a chromium-plated part having high corrosion resistance and good appearance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a schematic cross-sectional view of a chromium-plated part according to an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0025]Hereinafter, the present invention will be described with reference to embodiments, which are not intended to limit the present invention.

Chromium-Plated Part

[0026]The present invention is directed to a chromium-plated part including: a base having a surface layer including copper or a copper alloy; a first nickel plating layer formed on the surface layer of the base; a second nickel plating layer formed on the first nickel plating layer; and a trivalent chromium plating layer formed on the second nickel plating layer, the second nickel plating layer having an anode potential of −215 to −290 mV at a current density of 0.1 mA/cm², the first nickel plating layer having an electric potential 15 to 150 mV lower than that of the second nickel plating layer.

[0027]FIG. 1 is a schematic cross-sectional view of a chromium-plated part according to an embodiment of the present invention. As shown in FIG. 1, the chromium-plated part 1 according to an embodiment includes, in order, a base 2; a first nickel plating layer 3 provided on and in contact with a surface 2A of the base 2; a second nickel plating layer 4 provided on and in contact with the first nickel plating layer 3; and a trivalent chromium plating layer 5 provided on and in contact with the second nickel plating layer 4. Hereinafter, each of the elements of the chromium-plated part will be described.

Base

[0028]In the chromium-plated part 1, the base 2 is an object having a surface 2A provided with the plating layers described below. The base 2 has a surface layer 22 including copper or a copper alloy at least on the surface 2A side on which the plating layers are provided.

[0029]Specifically, the base 2 includes a substrate 21 and the surface layer 22 including copper or a copper alloy. In the embodiment shown in FIG. 1, the base 2 is composed of different components including the substrate 21 and the surface layer 22. However, this is a non-limiting example, and alternatively, the base 2 may be a substrate 21 itself composed of copper or a copper alloy to have the surface layer 22 that is continued therefrom and is integrated therewith.

Substrate

[0030]The substrate 21 corresponds to the main body of the base 2, which is the plating target. The substrate 21 may be made of any material with any shape. In the embodiment shown in

[0031]FIG. 1, as a non-limiting example of the present invention, the chromium-plated part includes: a flat sheet-shaped base 2 including a substrate 21 and a surface layer 22 provided thereon; and plating layers 3, 4, and 5 provided on one side of the base 2. The substrate 21 may be any material with any shape and for any application, examples of which include automotive parts, such as wheels, grilles, moldings, and emblems, in various shapes, outboard motor parts, faucet fittings, building parts, such as doorknobs and window frames, and home appliance parts.

[0032]As a non-limiting example, the base 2 preferably includes: a substrate 21 including one or more materials selected from the group consisting of a resin, a ceramic, and a metal; and a surface layer 22 provided on the substrate 21 and including copper or a copper alloy. The substrate 21 may be a composite material including any combination of various resins, elastomers, ceramics, metals, and carbon materials.

[0033]Alternatively, the substrate 21 itself may include copper or a copper alloy and form the base 2 without being provided with any surface layer as another component. In this case, the base 2 naturally has a surface layer including copper or a copper alloy. It should be noted that the phrase “including (comprising) copper or a copper alloy” as used herein means that the surface layer is composed mainly of copper or a copper alloy and is not intended to exclude the presence of a small amount of additives or the presence of unavoidable impurities.

[0034]The substrate 21 may include a metallic material. In such a case, the metallic material may have any composition. Examples of the metallic material include, but are not limited to, copper, copper alloys, nickel, nickel alloys, iron, stainless steel, and zinc. These metals may have undergone activation or strike plating for imparting adhesion properties as needed. The substrate 21 may be subjected to strike plating or any other treatment with copper or a copper alloy. The resulting treated substrate 21 may be directly used as the base 2 for the chromium-plated part 1.

[0035]The substrate 21 may include any type of resin. Examples of resin include, but are not limited to, ABS (acrylonitrile-butadiene-styrene resin), PC (polycarbonate), PC-containing ABS, SBS (styrene-butadiene-styrene copolymer), acrylic resin, polyolefin resin such as polypropylene and polyethylene, polyphenylene oxide, polyphenylene sulfide, polyacetal, polyamide, polyimide, polyester, polyvinyl acetate, polyurethane, epoxy resin, phenolic resin, CFRP (carbon fiber-reinforced plastic), and CNF (cellulose nanofiber)-containing resin. In particular, the substrate is preferably based on ABS resin. Such an ABS resin-based substrate can easily be subjected to plating for the formation of a copper or copper alloy surface layer and thus is suitable for use as the substrate for the chromium-plated part 1.

Plating on Substrate

[0036]When made of a ceramic or a resin as mentioned above, the substrate 21 is preferably subjected to a metal treatment for making it electrically conductive so that it can be easily plated. The metal treatment for making it electrically conductive may be any of various methods, such as electroless plating, electrolytic plating, metal sputtering, and metal vapor deposition. In particular, the metal treatment is preferably electroless plating with copper, copper alloy, nickel, or nickel alloy. Such electroless plating may be performed using any suitable method and conditions, such as a commonly used method and conditions.

[0037]After being subjected to electroless plating, the substrate 21 is preferably subjected to electrolytic copper plating or electrolytic copper alloy plating (electrolytic copper-based plating). The electrolytic copper-based plating can form a surface layer 22 having particularly good properties, such as high adhesion properties. Even after electroless copper or copper alloy plating is performed to form a copper-based layer, electrolytic copper-based plating is preferably performed on the copper-based layer. The electrolytic copper-based plating may also be performed using any suitable method and conditions, such as a commonly used method and conditions. The process described above forms the surface layer 22 of the base 2.

Surface Layer

[0038]In the base 2, the surface layer 22 may be provided on the surface of the substrate 21 or may correspond to an upper portion of the substrate 21. The surface layer 22 provides a surface on which the plating layers are to be formed as described below. The surface layer 22 includes copper or a copper alloy. A plating layer, specifically a first nickel plating layer 3 as described below is formed in contact with the surface 2A of the surface layer 22 including copper or a copper alloy.

Nickel Plating Layers

[0039]The chromium-plated part 1 includes the first nickel plating layer 3 on the surface 2A of the base 2; the second nickel plating layer 4 on the first nickel plating layer 3; and the trivalent chromium plating layer 5 on the second nickel plating layer 4. In the specification, the first and second nickel plating layers 3 and 4 are collectively referred to as the “nickel plating layers”.

[0040]Of the nickel plating layers, the second nickel plating layer 4 (on the trivalent chromium plating layer 5 side) has an anode potential of −215 to −290 mV at a current density of 0.1 mA/cm². The first nickel plating layer 3, which is in contact with the base 2, has an electric potential lower than that of the second nickel plating layer 4. In the chromium-plated part 1, therefore, the first nickel plating layer 3, which has a lower electric potential, is provided directly on the base 2 and in contact with the surface layer 22 without any other layer with a higher electric potential, such as a semi-bright nickel layer, between the base 2 and the first nickel plating layer 3.

[0041]Hereinafter, the nickel plating layers will be described in detail. First, the second nickel plating layer 4 will be described, which has an electric potential within the specified value range, and the electric potential of the second nickel plating layer 4 will be described.

Second Nickel Plating Layer

[0042]The second nickel plating layer 4 is a nickel plating layer having an anode potential of −215 to −290 mV at a current density of 0.1 mA/cm². To provide enhanced corrosion resistance for the chromium-plated part 1, the second nickel plating layer 4 preferably has an anode potential of −220 to-285 mV, more preferably −220 to −280 mV.

[0043]The anode potential is a measure of equilibrium potential for clearly indicating electric potential level (high or low). The anode potential can be measured using chronopotentiometry, for example, which is performed with a working electrode (a sample for electric potential measurement) obtained by removing the chromium plating layer 5 from the chromium-plated part 1. Specifically, the measurement method may include placing a silver-silver chloride (saturated KCl) reference electrode, a platinum counter electrode, and the working electrode in an electrolytic solution containing 300 g/L of nickel chloride hexahydrate, 50 g/L of sodium chloride, and 25 g/L of boric acid; and measuring the voltage of the working electrode by chronopotentiometry. During the measurement, the voltage of the working electrode is measured at a current density of 0.1 mA/cm² and determined to be the “anode potential” of the second nickel plating layer 4.

[0044]In general, the electric potential level (high or low) is evaluated based on equilibrium potential as described later. At this time, however, the present inventors have found that the anode potential is several times more sensitive to reflect the electric potential level (high or low) than the equilibrium potential. The anode potential is also advantageous in that it can be measured by a method simpler than that for the equilibrium potential. In the present invention, therefore, the anode potential of the second nickel plating layer 4 is defined as described above for the management of the physical properties of the chromium-plated part 1.

[0045]Although the anode potential of nickel plating layers depends on conditions such as plating type, average bright nickel plating layers usually have an anode potential of around-250 mV, and the top layer (e.g., a co-deposition nickel plating layer) of the three-layer nickel plating structure in general-purpose nickel-chromium plating layers has an anode potential higher than that of the average bright nickel plating layers, and average semi-bright nickel plating layers have an anode potential of around-150 mV, which is higher than that of the top layer. Therefore, the second nickel plating layer 4 may include, for example, a general-purpose bright nickel plating layer, a satin nickel plating layer with an electric potential similar thereto, or a nickel plating layer with an electric potential adjusted to a lower or higher level with a potential adjusting agent or any other chemical.

Equilibrium Potential

[0046]The second nickel plating layer 4 having an anode potential as specified above generally has an equilibrium potential of about −402 to about −414 mV. The equilibrium potential can be measured using, for example, a silver-silver chloride (saturated KCl) reference electrode, a platinum counter electrode, and an electrolytic solution containing 300 g/L of nickel chloride hexahydrate, 50 g/L of sodium chloride, and 25 g/L of boric acid. In such a system, for example, the voltage of a working electrode (a sample for electric potential measurement) obtained by removing the chromium plating layer 5 from the chromium-plated part 1 may be measured at each of the current densities 0.01, 0.1, 1, −0.01, −0.1, and −1 mA/cm² by chronopotentiometry. The equilibrium potential of the sample can be determined from Tafel plots of the measured voltages.

Bright Nickel Plating Layer

[0047]As a typical example, the second nickel plating layer 4 may be a bright nickel plating layer, which may be a sulfur-containing nickel plating film. The bright nickel plating layer may be any type.

[0048]The bright nickel plating layer can be formed by electroplating using a known nickel plating solution containing a primary brightening agent, such as an agent including a sulfur compound. Specifically, the nickel plating solution is typically, but not limited to, a Watts bath, a sulfamic acid bath, a citric acid bath, or a Weisberg bath.

[0049]Examples of the primary brightening agent for use in the nickel plating solution for forming the bright nickel plating layer include sodium 1, 5-naphthalenedisulfonate, sodium 1, 6-naphthalenedisulfonate, sodium 2, 5-naphthalenedisulfonate, sodium 1, 3, 6-naphthalenetrisulfonate, sodium benzene sulfonate, sodium benzene sulfinate, aromatic sulfonimides or sulfinic acids, such as o-sulfobenzimide (saccharin) sodium salt, and ethylenically unsaturated sulfonic acid salts, such as sodium vinyl sulfonate and sodium allyl sulfonate. One, two, or more of these brightening agents may be used alone or in combination.

[0050]The nickel plating solution may contain a brightening and leveling agent (secondary brightening agent) (for the purpose of brightening and leveling) in addition to or instead of the primary brightening agent. Examples of the brightening and leveling agent include acetylenically unsaturated alcohols and derivatives thereof, such as 1, 4-butynediol, hexinediol, and propargyl alcohol, and sodium salts of pyridine sulfonic acid. One, two, or more of these agents may be used alone or in combination.

[0051]The primary brightening agent and the brightening and leveling agent (secondary brightening agent) may also be commercially available products, such as #81, #83, and #810 for use in HI-BRITE #88 Process (manufactured by JCU CORPORATION).

[0052]The nickel plating solution may contain the primary brightening agent at a concentration of typically about 0.1 to about 10 g/L, preferably about 1 to about 5 g/L, more preferably about 1.5 to about 4 g/L. The nickel plating solution may also contain the brightening and leveling agent at a concentration of typically about 0.5 to about 300 ppm, preferably about 10 to about 200 ppm, more preferably about 20 to about 200 ppm.

[0053]The plating solution for use in bright nickel plating preferably contains a wetting agent. The wetting agent may be, for example, a surfactant. Examples of the surfactant include, but are not limited to, nonionic surfactants, such as polyethylene glycol, and anionic surfactants, such as sodium polyoxyethylene alkyl ether sulfate. One, two, or more of these surfactants may be used. The wetting agent may also be a commercially available product, such as #82, #82-A, or #82-K, for use in HI-BRITE #88 Process (manufactured by JCU CORPORATION). The nickel plating solution may contain the wetting agent at a concentration of typically about 10 to about 1,000 ppm, more preferably about 100 to about 500 ppm.

[0054]The bright nickel plating layer may be formed under any suitable electroplating conditions, such as commonly used conditions. For example, the bright nickel plating layer may be formed under conditions including a plating bath temperature of 40 to 60° C., preferably 45 to 55° C., and a current density of 1 to 10 A/dm², preferably 2 to 5 A/dm².

[0055]Using the process described above, the bright nickel plating layer may be prepared as the second nickel plating layer 4 on the first nickel plating layer. The anode potential of the bright nickel plating layer can be shifted to a lower or higher level within the range of −215 to −290 mV by controlling the temperature of the plating solution or by controlling the amount of the primary brightening agent or the brightening and leveling agent (secondary brightening agent) in the plating solution. The electric potential of the bright nickel plating layer can also be controlled by the addition of a potential adjusting agent to the plating solution.

Potential Adjusting Agent

[0056]The nickel plating solution may contain a known potential adjusting agent. Examples of the potential adjusting agent include, but are not limited to, agents for adjusting electric potential to lower level, such as sodium saccharin, sodium benzenesulfinate, S-ethyl isothiourea hydrobromide, 3-[[amino (imino) methyl]thio] propanoic acid, 2,4-thiazolidinedione, 5-chloro-2-methyl-4-isothiazolin-3-one, 2-methyl-4-isothiazolin-3-one; and agents for adjusting electric potential to higher level, such as butynediol, hexinediol, propargyl alcohol, sodium allyl sulfate, formalin, chloral hydrate (2,2, 2-trichloro-1, 1-ethanediol), and bromal hydrate (2, 2, 2-tribromo-1, 1-ethanediol).

[0057]The potential adjusting agent may also be a commercially available product, such as TRI-STRIKE (manufactured by JCU CORPORATION) (agent for adjusting electric potential to lower level) and ADDITIVE-E (manufactured by JCU CORPORATION) (agent for adjusting electric potential to higher level).

[0058]The second nickel plating layer 4 having an electric potential as defined above can be formed by plating using a nickel plating solution with appropriately adjusted concentrations of the primary brightening agent, the secondary brightening agent, and the potential adjusting agent described above. Alternatively, of course, the second nickel plating layer 4 with an anode potential of −215 to −290 mV may be formed using any suitable plating solution other than the bright nickel plating solution. For example, the second nickel plating layer 4 may be formed using satin nickel plating as described below. Such satin nickel plating may be performed under the same conditions as those for the bright nickel plating.

Satin Nickel Plating Layer

[0059]Satin nickel plating is a plating technique for forming a plating film with fine surface irregularities, which provides a semi-bright or non-bright matte appearance. The satin nickel plating solution may contain two or more surfactants for forming an emulsion, which can repeatedly adsorb onto and desorb from the plating surface. Plating deposition is inhibited at sites where the emulsion adsorbs, and at such sites, pits are left on the plating surface layer after the emulsion desorbs, which results in a matte appearance. The surfactants are typically, but not limited to, cationic or anionic surfactants. In such a way, the satin nickel plating layer can be formed using a nickel plating solution containing such surfactants dispersed therein. The satin nickel plating layer can also be formed using a plating solution containing non-conductive fine particles, such as fine particles of silica, kaolin, or barium sulfate. Such satin nickel plating is useful for the production of the chromium-plated part 1 having a matte appearance.

[0060]The satin nickel plating layer may also be formed using a commercially available product, such as the DOUBLET SATIN Process (manufactured by JCU CORPORATION).

[0061]Preferably, the nickel plating solution is free of non-conductive fine particles such as silica particles so that the second nickel plating layer 4 can be formed free of non-conductive fine particles. Even if non-conductive fine particles are used, the resulting second nickel plating layer 4 should preferably contain about at most 5 mass %, more preferably at most 1 mass % of the non-conductive fine particles based on the total mass of the second nickel plating layer 4. The second nickel plating layer 4 containing no non-conductive fine particles or about 5 mass % or less of non-conductive fine particles will provide better appearance for the chromium-plated part 1 with being less likely to suffer from precipitation of the fine particles. The non-conductive fine particle-free nickel plating solution is easy to manage for plating and advantageous for simplifying the plating process. As used herein, the phrase “free of non-conductive fine particles” means that the solution or the layer does not contain any non-conductive fine particles other than unavoidable impurities. The phrase is intended to mean not only that the content of non-conductive fine particles is 0 but also that the content of non-conductive fine particles mixed in with the layer is at most about 0.1 mass %.

[0062]The second nickel plating layer formed as described above on the first nickel plating layer 3 has an electric potential lower than that of the uppermost nickel plating layer of a common three-layer nickel plating structure in conventional nickel chromium-plated parts.

First Nickel Plating Layer

[0063]The first nickel plating layer 3 is a nickel plating layer having an electric potential 15 to 150 mV lower than that of the second nickel plating layer 4. The first nickel plating layer 3 has an electric potential much lower than that of a semi-bright nickel plating layer, which is widely provided directly on the base in conventional nickel chromium-plated parts.

[0064]The first nickel plating layer 3 may be formed using a modification of the bright nickel plating solution, which may contain a different type or amount of primary brightening agent, brightening and leveling agent (secondary brightening agent), or potential adjusting agent from that described above. The first nickel plating layer 3 may also be formed using a commercially available low-potential nickel plating solution, such as TRI-STRIKE Process (manufactured by JCU CORPORATION).

[0065]The first nickel plating layer 3 may be formed under any suitable electroplating conditions, such as commonly used conditions. For example, the first nickel plating layer 3 may be formed under conditions including a plating bath temperature of 40 to 60° C., preferably 45 to 55° C., and a current density of 1 to 10 A/dm², preferably 2 to 5 A/dm².

Measurement of Electric Potential Difference and Electric Potential

[0066]As mentioned above, the first nickel plating layer 3 has an electric potential 15 to 150 mV lower than that of the second nickel plating layer 4. To provide enhanced corrosion resistance for the chromium-plated part 1, the first nickel plating layer 3 preferably has an electric potential 30 to 150 mV lower, more preferably 40 to 140 mV lower, even more preferably 50 to 120 mV lower, furthermore preferably 70 to 100 mV lower than that of the second nickel plating layer 4.

[0067]Such an electric potential difference can be measured, for example, by the STEP test in accordance with ASTM B764: “Standard test method for simultaneous thickness and electrode potential determination of individual layers in multilayer nickel deposit”. More specifically, the electric potential difference may be measured by a process including: removing the trivalent chromium plating layer 5 from the chromium-plated part 1 to form a sample (for electric potential measurement); placing the sample and a silver-silver chloride reference electrode (reference electrode) in an electrolytic solution (at 20° C.) containing 300 g/L of NiCl₂·6H₂O, 50 g/L of NaCl, and 25 g/L of H₃BO₃; and measuring the electric potential difference using a commercially available device, such as a multilayer nickel plating corrosion resistance meter. It should be noted that the electric potential difference is measured under conditions different from those for the anode or equilibrium potential described above and that the anode or equilibrium potential of the first nickel plating layer 3 cannot be calculated by subtracting the electric potential difference from the anode or equilibrium potential of the second nickel plating layer 4.

[0068]As mentioned above, the chromium-plated part 1 includes the first nickel plating layer 3 directly on the surface layer 22 of the base 2; and the second nickel plating layer 4 provided directly on the first nickel plating layer 3 and on the chromium plating layer 5 side, in which the second nickel plating layer 4 has an anode potential of −215 to −290 mV at a current density of 0.1 mA/cm², and the first nickel plating layer 3 has an electric potential 15 to 150 mV lower than that of the second nickel plating layer 4. The chromium-plated part 1 with such features exhibits high corrosion resistance and good appearance.

[0069]The reason for the above advantage of the chromium-plated part 1 remains to be clarified and should not be bound to a specific theory. A reason for that, however, may be that the first nickel plating layer 3 with the lower electric potential can effectively suppress the corrosion of the adjacent surface layer 22 including copper or a copper alloy and thus make less likely the formation of holes and other defects, so that swelling can also be prevented around the plating layer. In addition, probably, the first nickel plating layer 3 with the lower electric potential can undergo sacrificial corrosion to suppress the corrosion of the second nickel plating layer 4 so that the chromium-plated part 1 can have improved corrosion resistance as a whole.

Thicknesses of Nickel Plating Layers and Thickness Ratio Therebetween

[0070]In the chromium-plated part 1, the nickel plating layers (the first nickel plating layer 3 and the second nickel plating layer 4) may each have any suitable thickness depending on purpose, such as a thickness of 100 μm or less or a thickness of about 1 to about 50 μm. For higher corrosion resistance and for nickel plating cost reduction, the total thickness of the first and second nickel plating layers 3 and 4 (nickel film thickness) is preferably about 1 to about 30 μm, more preferably about 2 to about 20 μm, even more preferably about 5 to about 15 μm.

[0071]Moreover, the ratio of the thickness of the first nickel plating layer 3 to the thickness of the second nickel plating layer 4 (the first nickel plating layer 3 thickness: the second nickel plating layer 4 thickness) is preferably 1:10 to 30:1, more preferably 1:4 to 14:1. The chromium-plated part with such a feature tends to exhibit higher corrosion resistance. The chromium-plated part 1 with the thickness ratio falling within the range of 1:4 to 4:1, particularly within the range of 1:2 to 4:1 will have significantly higher corrosion resistance.

Trivalent Chromium Plating Layer

[0072]In the chromium-plated part 1, the trivalent chromium plating layer 5 is provided in contact with the second nickel plating layer 4. The trivalent chromium plating layer 5 provides high corrosion resistance and beautiful decorative properties for the plated part and makes the plated part useful for decorative applications and other applications.

[0073]The trivalent chromium plating layer 5 may be formed using any suitable method, such as a commonly used plating method under desired conditions. For example, the trivalent chromium plating layer 5 may be formed by electroplating using a known trivalent chromium plating solution containing a trivalent chromium compound, a complexing agent, a conductive salt, and a pH buffer.

[0074]The trivalent chromium plating solution for the formation of the trivalent chromium plating layer 5 may contain any suitable trivalent chromium compound. Examples of such a compound include basic chromium (III) sulfate (Cr(OH)SO₄), chromium (III) sulfate, chromium (III) chloride, chromium (III) sulfamate, and chromium (III) acetate. In particular, the trivalent chromium compound is preferably basic chromium sulfate and/or chromium sulfate. One, two, or more of these trivalent chromium compounds may be used alone or in combination. The trivalent chromium plating solution may contain about 1 to about 25 g/L of chromium metal from the chromium compound(s).

[0075]The complexing agent may be any type. Examples of the complexing agent include aliphatic monocarboxylic acids and salts thereof, such as formic acid, ammonium formate, and potassium formate; aliphatic dicarboxylic acids such as succinic acid, maleic acid, and malic acid, and salts thereof; aliphatic tricarboxylic acids and salts thereof, such as citric acid and triammonium citrate; carboxylic acids having two or more hydroxyl groups and two or more carboxy groups and salts thereof, such as tartaric acid, diammonium tartrate, and sodium tartrate; and amino carboxylic acids, such as glycine. One, two, or more of these complexing agents may be used alone or in combination. The trivalent chromium plating solution may contain, for example, about 0.1 to about 50 g/L of the complexing agent.

[0076]The conductive salt may also be any type. Examples of the conductive salt include sulfates, such as potassium sulfate, ammonium sulfate, and sodium sulfate; chlorides, such as potassium chloride, ammonium chloride, and sodium chloride; and sulfamates, such as potassium sulfamate, ammonium sulfamate, and sodium sulfamate. One, two, or more of these conductive salts may be used alone or in combination. The trivalent chromium plating solution may contain, for example, about 100 to about 500 g/L of the conductive salt.

[0077]The pH buffer agent may also be any type. Examples of the pH buffer include boric acid, sodium borate, potassium borate, phosphoric acid, and dipotassium hydrogen phosphate. One, two, or more of these pH buffers may be used alone or in combination. The trivalent chromium plating solution may contain, for example, about 25 to about 200 g/L of the pH buffer.

[0078]The trivalent chromium plating solution may further contain a blackening agent, such as sodium thiocyanate, methionine, or cysteine; ascorbic acid, sodium ascorbate, hydrogen peroxide, polyethylene glycol, a tin salt, such as tin sulfate or tin chloride, iron chloride, sodium saccharin, sodium allylsulfonate, or sodium vinylsulfonate.

[0079]The trivalent chromium plating solution containing the complexing agent, the conductive salt, and the pH buffer may be a commercially available product, such as JCU TRICHROM JTC series (manufactured by JCU CORPORATION), TOP FINE CHROME series (manufactured by Okuno Chemical Industries, Co., Ltd.), EARSUS CHROME series (manufactured by SurTec), TRICHROME series (manufactured by Atotech), and Envirochrome Process and Twilight Process (each manufactured by MacDermid Performance Solutions). The chromium plating layer generally has a silvery-white appearance. Alternatively, the chromium plating layer may be a black plating layer formed using a plating solution containing, for example, the blackening agent.

[0080]The trivalent chromium plating layer 5 may be formed under any suitable electroplating conditions, such as commonly used conditions. For example, the trivalent chromium plating layer 5 may be formed using a carbon or iridium oxide anode under conditions including a plating bath temperature of 30 to 60° C. and a cathode current density of 5 to 20 A/dm².

[0081]As will be shown in the Examples section below, the chromium-plated part 1 can exhibit high corrosion resistance and good appearance regardless of the type and thickness of the trivalent chromium plating layer 5. Therefore, the trivalent chromium plating layer 5 may have any suitable thickness, such as a thickness of at least 0.05 μm (a common plating layer thickness for chromium-plated products) or a thickness of about 0.1 to about 1.0 μm, specifically a thickness of about 0.15 to about 0.50 μm.

Electrolytic-Chemical Conversion and Immersion-Chemical Conversion

[0082]The chromium-plated part 1 preferably further includes an electrolytic-chemical conversion coating and/or a immersion-chemical conversion coating on the trivalent chromium plating layer 5. In this case, the chromium-plated part 1 may have further enhanced corrosion resistance. The surface of the trivalent chromium plating layer 5 may be subjected to any type of electrolytic-chemical conversion or immersion-chemical conversion, in which commonly used treatment methods may be used as desired. Examples of electrolytic-chemical conversion or immersion-chemical conversion include, but are not limited to, chromate treatment, wax treatment, treatment with a benzotriazole or triazine thiol solution, treatment with a solution of an amino or imino group-containing compound, and heat treatment. Such post-treatment can further enhance the corrosion resistance of the chromium-plated part or more effectively prevent discoloration, hydrogen embrittlement, or other damage.

Chromate Treatment

[0083]The electrolytic-chemical conversion or the immersion-chemical conversion is preferably treatment with hexavalent chromium ions, which is more preferably what is called chromate treatment. Chromate treatment forms a chromate coating, which is self-restoring and thus can further enhance the corrosion resistance of the chromium-plated part 1.

[0084]For example, the chromate treatment includes a known electrolytic-chemical conversion or immersion-chemical conversion using hexavalent chromium ions from chromic anhydride, bichromates, or other sources. The electrolytic-chemical conversion may be performed by a commonly used method using a commercially available process or a commercially available treatment agent for such a process, such as EBACHRO-500 Process or EBACHRO-900 Process (manufactured by JCU CORPORATION). A treatment alternative to the chromate treatment may also be utilized, such as chemical treatment with trivalent chromium ions or chromium-free chemical conversion coating using phosphoric acid, permanganic acid, molybdenum, vanadium, iron, or any other metal.

Chromium-Plated Part and Its Applications

[0085]Despite having the trivalent chromium plating layer 5 as an upper layer, the chromium-plated part 1 has high corrosion resistance and good appearance. For example, when evaluated by the CASS test (the corrosion resistance evaluation test according to JIS H 8502), the chromium-plated part 1 can provide a good evaluation result, such as a rating number (R.N.) of about 9 or more, typically 9.3 or more. Therefore, the chromium-plated part 1 can be used as various parts, such as automotive parts, outboard motor parts, faucet fittings, building material parts, and home appliance parts.

Method for Manufacturing Chromium-Plated Part

[0086]As described above, the chromium-plated part 1 may be manufactured by a method including: forming the first nickel plating layer 3 on the surface layer 22 of the base 2, the surface layer comprising copper or a copper alloy; forming the second nickel plating layer 4 on the first nickel plating layer 3; and forming the trivalent chromium plating layer 5 on the second nickel plating layer 4, the second nickel plating layer 4 having an anode potential of −215 to −290 mV at a current density of 0.1 mA/cm², the first nickel plating layer 3 having an electric potential 15 to 150 mV lower than that of the second nickel plating layer 4.

[0087]Each of the steps of forming the first nickel plating layer 3, the second nickel plating layer 4, and the trivalent chromium plating layer 5 may be performed using methods and conditions described above for each of the plating layers. As described above, the electrolytic-chemical conversion coating and/or the immersion conversion coating may be further formed on the surface of the trivalent chromium plating layer 5. By the manufacturing method described above, the chromium-plated part 1 can be manufactured with a smaller number of steps since it only has to form two nickel plating layers.

EXAMPLES

[0088]Hereinafter, the present invention will be described in more detail with reference to examples, which are not intended to limit the present invention at all.

Example 1

[0089]A chromium-plated part according to the present invention was produced as follows.

Preparation of Base

[0090]The whole surface of an ABS resin molding (flat sheet-shaped) was etched with chromic acid. The etched molding was subjected to reduction treatment, catalization, and activation in a common manner, and then subjected to electroless nickel plating. The chromic acid etching was performed using a treatment solution containing 400 g/L of chromic anhydride, 400 g/L of sulfuric acid, and 10 g/L of trivalent chromium. The electroless nickel plating was performed at 40° C. using ENILEX NI-5 Process (manufactured by JCU CORPORATION). Subsequently, the electroless plated product was subjected to electrolytic copper plating under the conditions of room temperature and 3 A/dm² using CU-BRITE EP-30 Process (manufactured by JCU CORPORATION) to form a base.

First Nickel Plating

[0091]The resulting base was immersed in a first nickel plating solution under the conditions of 50° C., 3 A/dm², and 15 minutes so that a 6 μm-thick first nickel plating layer was formed on the base. A stock solution shown below for low potential nickel plating was directly used as the first nickel plating solution.

Second Nickel Plating

[0100]Next, the nickel-plated base was immersed in a second nickel plating solution (shown below) under the conditions of 50° C., 3 A/dm², and 15 minutes so that a 6 μm-thick second nickel plating layer was formed on the first nickel plating layer (the total thickness of the first and second nickel plating layers was 12 μm, and the first nickel plating layer thickness: the second nickel plating layer thickness ratio was 1:1). The second nickel plating solution was prepared by adding 1 mL/L of a potential adjusting agent (TRI-STRIKE manufactured by JCU CORPORATION) to the stock solution below for bright nickel plating. The resulting second nickel plating layer had an electric potential lower than that of an average bright nickel plating layer. The electric potential of the second nickel plating layer was adjusted to have an electric potential difference of +60 mV from the first nickel plating layer (the first nickel plating layer had an electric potential 60 mV lower than that of the second nickel plating layer).

Stock Solution for Bright Nickel Plating

Trivalent Chromium Plating

[0109]The resulting nickel-plated base was then immersed in a trivalent chromium plating solution (shown below) under the conditions of 55° C., 10 A/dm², and 4 minutes so that a white trivalent chromium plating layer was formed in contact with the second nickel plating layer. The resulting chromium-plated part sample had a good appearance.

Trivalent Chromium Plating Solution

[0116]The resulting chromium-plated part sample was evaluated for appearance, measured for layer thickness, and subjected to a corrosion resistance evaluation test. The test methods are as shown below. The test results are shown in Table 1 below.

Measurement of Layer Thickness

[0117]The thickness of each of the nickel plating layers was measured from cross-sectional micrographs. The thickness of the chromium plating layer (Cr film thickness) was measured using a fluorescent X-ray analyzer (FT-150H manufactured by Hitachi High-Tech Science Corporation).

Anode Potential Measurement

[0118]The chromium-plated part sample was immersed in 1:1 hydrochloric acid so that the chromium plating layer was removed. Masking was then performed on the part sample so that an electric potential measurement part sample was obtained having a 6 mm-diameter circular portion of the second nickel plating layer exposed on the surface. The resulting electric potential measurement sample was used as a working electrode. Chronopotentiometry was performed in which the voltage of the working electrode was measured using a silver-silver chloride (saturated KCl) reference electrode and a platinum counter electrode. In the chronopotentiometry, the electrolytic solution was an aqueous solution (at a pH of 2.75) containing 300 g/L of nickel chloride hexahydrate, 50 g/L of sodium chloride, and 25 g/L of boric acid. The measurement was performed using the measurement system HZ-7000 manufactured by Hokuto Denko Co., Ltd. under the conditions of 25° C., 600 seconds, and no stirring. The measurement was performed three times (n=3), and the average value was adopted.

[0119]At a current density of 0.1 mA/cm², the voltage of the working electrode was determined to be the “anode potential” of the second nickel plating layer when the voltage became stable.

[0120]At each of the current densities 0.01, 0.1, 1, −0.01, −0.1, and −1 mA/cm², the voltage of the working electrode was measured when the voltage became stable. The equilibrium potential of the second nickel plating layer was determined from Tafel plots of the measured voltages. As a result, the second nickel plating layer had an equilibrium potential of −402 to −414 mV when it had an anode potential of −215 to −290 mV.

Electric Potential Difference Measurement

[0121]The electric potential difference from the second nickel plating layer to the first nickel plating layer was measured by the STEP test in accordance with ASTM B764: “Standard test method for simultaneous thickness and electrode potential determination of individual layers in multilayer nickel deposit”. Before the measurement, an electrolytic solution (20° C.) was prepared containing 300 g/L of NiCl₂·6H₂O, 50 g/L of NaCl, and 25 g/L of H₃BO₃. The electric potential measurement sample was placed in the electrolytic solution and measured for electric potential difference using a multilayer nickel plating corrosion resistance meter (ED-3 manufactured by Chuo Seisakusho Ltd.) equipped with a silver-silver chloride reference electrode.

Corrosion Resistance Evaluation

Example 2

[0134]A chromium-plated part sample was obtained using the same procedure as in Example 1. The surface layer of the chromium-plated part sample was then subjected to electrolytic chromate treatment. The electrolytic chromate treatment was performed under the conditions of 40° C., 0.1 A/dm², and 1 minute using EBACHRO-500 Process (a hexavalent chromium-based treatment solution containing 100 mL/L of ECR-500 manufactured by JCU CORPORATION). The resulting chromium-plated part sample had a good appearance. The sample was tested as in Example 1. The test results are shown in Table 1 below.

Examples 3 to 8 and Comparative Examples 1 to 4

[0135]Chromium-plated part samples were prepared using the same procedure as in Example 1 or 2 except that the first and second nickel plating solutions had different compositions as described below. The first nickel plating solution was a lower-potential plating solution (in Examples 3 to 8) prepared by adding 1 to 5 mL/L of a potential adjusting agent (TRI-STRIKE manufactured by JCU CORPORATION) to the stock solution for bright nickel plating or a higher-potential plating solution (in Comparative Examples 1 to 4) prepared by adding 0 to 0.1 mL/L of ADDITIVE-E (manufactured by JCU CORPORATION) to the stock solution for bright nickel plating. The second nickel plating solution was the stock solution for bright nickel plating (in Examples 3 to 6) or a higher-potential plating solution (in Examples 7 and 8 and Comparative Examples 1 to 4) prepared by adding 0.1 to 1 mL/L of ADDITIVE-E (manufactured by JCU CORPORATION) to the stock solution for bright nickel plating.

[0136]In these examples and comparative examples, the second nickel plating layer was formed to have an electric potential difference of +60 mV from the first nickel plating layer (namely the electric potential of the first nickel plating layer was 60 mV lower than that of the second nickel plating layer) and to have an electric potential higher than that of the second nickel plating layer in Example 1 or 2. The resulting chromium-plated part samples all had a good appearance. The results of testing the samples are shown in Table 1 below.

Comparative Example 5

[0137]A chromium-plated part sample having a three-layer nickel plating structure was prepared under the conditions below according to Japanese Unexamined Patent Application, Publication No. 2010-185116 and evaluated as in Example 1.

Semi-Bright Nickel Plating

Bright Nickel Plating

[0146]Next, the semi-bright nickel-plated base was immersed in the bright nickel plating solution (shown above) under the conditions of 50° C., 3 A/dm², and 12 minutes so that a bright nickel plating layer was formed on the semi-bright nickel plating layer. The bright nickel plating layer had an electric potential 145 mV lower than that of the underling semi-bright nickel plating layer.

MP Nickel Plating

Preparation of Chromium-Plated Part Sample

[0158]Next, the MP nickel-plated product was subjected to trivalent chromium plating under the same conditions as in Example 1 to form a chromium-plated part sample, which was evaluated for the physical properties. The evaluation results are shown in Table 1 below.

Comparative Example 6

[0159]A chromium-plated part sample was prepared having two nickel plating layers in which the nickel plating layer (lower layer) on the base side had a higher electric potential. Specifically, the chromium-plated part sample was prepared by subjecting the base (prepared as in Example 1) to semi-bright nickel plating, bright nickel plating, and trivalent chromium plating in this order and then subjected to the physical property test. Each of the plating processes was performed under the same conditions as in Comparative Examples 1 and 5 except that the bright nickel plating was performed for 15 minutes. In the chromium-plated part sample of this comparative example, the lower semi-bright nickel plating layer had an electric potential 145 mV higher than that of the upper bright nickel plating layer. The resulting chromium-plated part sample was tested, and the test results are shown in Table 1.

[0160]The chromium-plated part samples of Examples 1 to 8 and Comparative Examples 1 to 4, in which the first nickel plating layer having an electric potential 60 mV lower than that of the second nickel plating layer was provided on the base side, all exhibited a rating number (R.N.) of at least 9.0 in the CASS test, which was higher than that for the sample of Comparative Example 5 having a three-layer nickel plating structure. This suggests the importance of the lower electric potential of the nickel plating layer on the base side. In particular, the chromium-plated part samples of Examples 1 to 8 according to the present invention, in which the anode potential of the second nickel plating layer was in the range of −215 to −290 mV at a current density of 0.1 mA/cm², exhibited the best value as a result of the swelling evaluation and were found to have a high level of net corrosion resistance. It has also been found that despite having one less nickel plating step than the conventional nickel-chromium plating process, the process of the present invention enables the production of chromium-plated parts with very high corrosion resistance.

[0161]In Comparative Examples 1 to 4 where the anode potential of the second nickel plating layer was outside the specified range according to the present invention, however, swelling portions were observed over the surface of the sample after the CASS test. This has revealed that for the prevention of swelling, the electric potential of the second nickel plating layer (upper nickel plating layer) should be adjusted within the specified range. The sample of Comparative Example 6, in which the lower nickel plating layer had an electric potential higher than that of the upper nickel plating layer, exhibited a low R.N. This suggests again that the lower electric potential of the nickel plating layer on the base side is important for enhanced corrosion resistance of the chromium-plated part.

[0162]Notably, the sample of Example 5 or 6, having a chromium plating layer thickness more than three times that of the sample of Example 3 or 4 with the same other conditions, showed a similar CASS test result to that in Example 3 or 4. This suggests that the chromium-plated part of the present invention may exhibit high corrosion resistance regardless of the thickness of the trivalent chromium plating layer. Moreover, despite having a nickel film thickness of 12 μm, the chromium-plated part samples of Examples 1 to 8 exhibited a R.N. of at least 9.0, which is equal to or higher than that of chromium-plated parts with a nickel film thickness of 25 μm shown below. The fact that the nickel film with such a half thickness or less can provide a similar level of corrosion resistance shows that the present invention can improve corrosion resistance without using an increased amount of nickel. This means that the manufacturing method of the present invention can contribute to a reduction in the usage of nickel and thus allow the nickel plating process to be completed in a shorter time at a lower cost.

Examples 9 to 18

[0163]Chromium-plated part samples were prepared using the same procedure as in Example 7 or 8 except that the first nickel plating solution had a different composition as described below. The first nickel plating solution was the stock solution for bright nickel plating (in Examples 9 to 14) or a plating solution (in Examples 15 to 18) prepared by adding 0 to 5 mL/L of TRI-STRIKE (manufactured by JCU CORPORATION) to the stock solution for low potential nickel plating. In Examples 9 and 10, the electric potential difference from the second nickel plating layer to the first nickel plating layer was adjusted by adjusting the plating bath temperature to 60° C. The resulting chromium-plated part samples all had a good appearance. The results of testing the samples are shown in Table 2 below.

Comparative Example 7

[0164]A chromium-plated part sample was prepared using the same procedure as in Example 3 except that the first and second nickel plating solutions were the stock solution for bright nickel plating and the stock solution for low potential nickel plating, respectively. The resulting chromium-plated part sample was tested, and the test results are shown in Table 2 below.

Comparative Example 8

[0165]A chromium-plated part sample was prepared using the same procedure as in Example 3 except that the temperature of the first nickel plating solution bath was raised to 52° C. The resulting chromium-plated part sample was tested, and the test results are shown in Table 2 below.

Comparative Example 9

Comparative Example 10

[0172]A chromium-plated part sample was prepared using the same procedure as in Example 3 except that the trivalent chromium plating was replaced by the same hexavalent chromium plating as in Comparative Example 9. The resulting chromium-plated part sample had an extremely poor appearance. Thus, the R.N. was not evaluated in the CASS test. Table 2 shows the result of the swelling evaluation.

[0173]The chromium-plated part samples of Examples 3, 4, and 7 to 18 according to the present invention, in which the second nickel plating layer had an anode potential in the range of −215 to −290 mV at a current density of 0.1 mA/cm² and the first nickel plating layer had an electric potential 15 to 150 mV lower than that of the second nickel plating layer, all exhibited a R.N. of at least 9.0 and a swelling evaluation rating of at least 4, which means high corrosion resistance. In particular, the chromium-plated part samples of Examples 13 to 18, in which the first nickel plating layer had an electric potential at least 70 mV lower than that of the second nickel plating layer, exhibited a R.N. of at least 9.5 and a swelling evaluation rating of 5, which means particularly high corrosion resistance.

[0174]On the other hand, the samples of Comparative Examples 7 and 8, in which the electric potential difference between the first and second nickel plating layers was outside the specified range, exhibited a low R.N. although they showed a good result in the swelling test. The samples of Comparative Examples 9 and 10 having a hexavalent chrome plating showed similar results. It has also been demonstrated that not only the electric potential of the second nickel plating layer but also the electric potential difference between the two nickel plating layers should be adjusted to fall within the specified ranges and that the chromium-plated part samples according to the present invention exhibited higher corrosion resistance than the hexavalent chromium-plated part sample.

Examples 19 to 70 and Comparative Examples 11 to 34

[0175]The electric potential difference from the second nickel plating layer to the first nickel plating layer was adjusted to −60 mV, and the nickel film thickness was adjusted to 12 μm. Under these conditions, it was investigated how the thickness ratio affected the corrosion resistance. Chromium-plated part samples were prepared using the same procedure as in Examples 1 to 8 and Comparative Examples 1 to 4 except that the first nickel plating time and the second nickel plating time were varied between 1 to 29 minutes and between 1 to 29 minutes, respectively, for formation of the nickel plating layers with different thickness ratios. The measurement from cross-sectional micrographs (described above) was used to confirm that the plating time correlated with the thickness of each resulting nickel plating layer. The resulting chromium-plated part samples all had a good appearance immediately after being prepared. The results of testing the samples are shown in Tables 3 and 4 together with the results of Examples 1 to 4 and Comparative Examples 1 to 4.

[0176]The chromium-plated part samples of Examples 19 to 70 according to the present invention, in which the second nickel plating layer had an anode potential of −215 mV or lower at a current density of 0.1 mA/cm², all exhibited a R.N. of at least 8.0 and a swelling evaluation rating of at least 4, which means high corrosion resistance. In particular, the chromium-plated part samples of Examples 1, 2, and 19 to 30, in which the second nickel plating layer had a low anode potential, specifically a low anode potential of −273 mV, exhibited very high corrosion resistance.

[0177]On the other hand, the chromium-plated part samples of Comparative Examples 11 to 34, in which the second nickel plating layer had an anode potential higher than-215 mV, exhibited a swelling evaluation rating of at most 3 although they had a good R.N. value. In particular, the chromium-plated part samples of Comparative Examples 3, 4, and 23 to 34, in which the anode potential is as high as-194 mV, all exhibited a swelling evaluation rating of 1, which means very poor corrosion resistance. This shows the importance of the lower electric potential of the nickel plating layer.

[0178]The samples with lower electric potentials of the two nickel plating layers or the samples with a first nickel plating layer thickness: second nickel plating layer thickness ratio of 1:9 to 29:1, in particular 1:4 to 14:1, especially 1:4 to 4:1 exhibited good R.N. results.

Examples 71 and 72

[0179]Chromium-plated part samples were prepared using the same procedure as in Example 3 or 4 except that the first and second nickel plating layers were formed with a total thickness of 25 μm (the electric potential difference between the two nickel plating layers was 60 mV, and the thickness ratio between them was 1:1). The resulting chromium-plated part samples all had a good appearance. The results of testing the samples are shown in Table 5 below.

Comparative Examples 35 to 38

[0180]Chromium-plated part samples were prepared using the same procedure as in Comparative Example 5 except that the semi-bright nickel plating layer thickness: the bright nickel plating layer thickness: the MP nickel plating layer thickness ratio was 9:6:1, the nickel film thickness was 25 μm, and the electric potential difference from the bright nickel plating layer to the MP nickel plating layer was +30 mV or +70 mV. In Comparative Examples 36 and 38, chromate treatment was also performed as in Example 4. The resulting samples were tested, and the test results are shown in Table 5 below.

Comparative Example 39

[0181]A chromium-plated part sample was prepared using the same procedure as in Comparative Example 35 except that the trivalent chromium plating was replaced by hexavalent chromium plating. The hexavalent chromium plating was performed as in Comparative Example 9. The resulting sample was tested, and the test results are shown in Table 5.

[0182]Despite having only two nickel plating layers, the chromium-plated part samples of Examples 71 and 72 according to the present invention exhibited a similar level of corrosion resistance to that of the samples of Comparative Examples 35 to 39 having a three-layer nickel plating structure.

Examples 73 and 74

Comparative Examples 40 and 41

[0191]Chromium-plated part samples were prepared using the same procedure as in Comparative Example 35 or 36 except that the chromium plating was black trivalent chromium plating and that the electric potential difference from the bright nickel plating layer to the MP nickel plating layer was +35 mV. The black trivalent chromium plating was performed as in Example 73 or 74. The resulting samples were tested, and the test results are shown in Table 6.

[0192]The chromium-plated part samples of Examples 73 and 74 according to the present invention exhibited a similar or higher level of corrosion resistance as compared to the sample of Comparative Example 40 or 41 with a three-layer nickel plating structure. The present invention has been found to be advantageous regardless of whether the trivalent chromium plating layer is a white or black trivalent chromium plating layer or any other type.

Examples 75 and 76

[0193]Chromium-plated part samples were prepared using the same procedure as in Example 3 or 4 except that the second nickel plating layer was formed under the conditions of 52° C., 3 A/dm², and 15 minutes using the satin nickel plating solution (pH 4.2) below as the second nickel plating solution. The resulting chromium-plated part samples all had a good appearance. The results of testing the samples are shown in Table 7.

Satin Nickel Plating Solution

[0203]It has been demonstrated that the chromium-plated part of the present invention can exhibit high corrosion resistance even when the second nickel plating layer is a satin nickel plating layer.

[0204]The above examples show that despite having a trivalent chromium plating layer as an upper layer, chromium-plated parts produced according to the present invention exhibit high corrosion resistance and good appearance and that such chromium-plated parts can be produced by a more simplified process according to the present invention.