U.S. patent application number 15/576892 was filed with the patent office on 2020-02-06 for plating solution and method for producing plated product.
The applicant listed for this patent is FUSO COMPANY, LTD., TECHNO ROLL CO., LTD.. Invention is credited to Yoshiyuki KAMEGAWA, Toshikazu MURATA, Hidemi NAWAFUNE, Hiroshi NISHIWAKI, Hidehiro YOSHIOKA.
Application Number | 20200040477 15/576892 |
Document ID | / |
Family ID | 63107327 |
Filed Date | 2020-02-06 |
United States Patent
Application |
20200040477 |
Kind Code |
A1 |
NAWAFUNE; Hidemi ; et
al. |
February 6, 2020 |
PLATING SOLUTION AND METHOD FOR PRODUCING PLATED PRODUCT
Abstract
The present invention provides a plating solution containing
chromium sulfate and formic acid at a concentration of Cr.sup.3+
ions of 0.1 mol/L or more and 1 mol/L or less and a concentration
of formic acid of 0.05 mol/L or more and 0.2 mol/L or less.
Inventors: |
NAWAFUNE; Hidemi; (Osaka,
JP) ; NISHIWAKI; Hiroshi; (Osaka, JP) ;
MURATA; Toshikazu; (Hyogo, JP) ; YOSHIOKA;
Hidehiro; (Hyogo, JP) ; KAMEGAWA; Yoshiyuki;
(Osaka, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TECHNO ROLL CO., LTD.
FUSO COMPANY, LTD. |
Osaka
Hyogo |
|
JP
JP |
|
|
Family ID: |
63107327 |
Appl. No.: |
15/576892 |
Filed: |
August 25, 2017 |
PCT Filed: |
August 25, 2017 |
PCT NO: |
PCT/JP2017/030445 |
371 Date: |
November 27, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25D 3/10 20130101; C25D
3/06 20130101 |
International
Class: |
C25D 3/06 20060101
C25D003/06; C25D 3/10 20060101 C25D003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 8, 2017 |
JP |
2017-021006 |
Claims
1. A plating solution used for trivalent chromium plating, the
plating solution comprising chromium sulfate and formic acid at a
concentration of Cr.sup.3+ ions of 0.1 mol/L or more and 1 mol/L or
less and a concentration of formic acid of 0.05 mol/L or more and
0.2 mol/L or less.
2. The plating solution according to claim 1 used for trivalent
chromium plating with a plating thickness of 5 .mu.m or more.
3. A method for producing a plated product, the method comprising a
plating step of performing electric plating in a plating bath that
contains a plating solution comprising chromium sulfate so as to
produce the plated product to which trivalent chromium plating is
applied by the plating step, wherein the plating solution used in
the plating step has a concentration of Cr.sup.3+ ions of 0.1 mol/L
or more and 1 mol/L or less, the plating bath has a temperature of
20.degree. C. or more and less than 40.degree. C., and the electric
plating is performed at a current density of 2 A/dm.sup.2 or more
and 20 A/dm.sup.2 or less.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to Japanese Patent
Application No. 2017-21006, the disclosure of which is incorporated
herein by reference in its entirety.
FIELD
[0002] The present invention relates to a plating solution and a
method for producing a plated product, more specifically, to a
plating solution used for trivalent chromium plating and a method
for producing a plated product by applying trivalent chromium
plating.
BACKGROUND
[0003] Conventionally, plated products obtained by applying
chromium plating to metal products or plastic products are widely
used. In producing the plated products of this type, the cases of
using plating solutions containing trivalent chromium, which is
environmentally friendly, instead of plating solutions containing
hexavalent chromium are increasing (see Patent Literature 1
below).
CITATION LIST
Patent Literature
[0004] Patent Literature 1: JP 2000-249340 A
SUMMARY
Technical Problem
[0005] There has been a strong demand for employing trivalent
chromium plating instead of hexavalent chromium plating due to a
recent increase in environmental awareness. However, the cases of
employing trivalent chromium plating are limited in the current
situation since the covering power on the product serving as an
object to be plated is poorer than in the cases of employing
hexavalent chromium plating. Further, it is difficult to obtain a
linear relationship between the current density and the thickness
of plating deposition and to exert throwing power in trivalent
chromium plating. Therefore, trivalent chromium plating is rarely
employed in the cases of producing plated products having complex
shapes such as dies and thick plated products, though there are
some cases of employing trivalent chromium plating in decorative
plating or the like where the plating thickness is several .mu.m.
It is therefore an object of the present invention to provide a
plating solution for trivalent chromium plating with which plated
products equivalent to those produced by hexavalent chromium
plating can be produced, so as to enlarge the application range of
environmentally friendly plated products.
Solution to Problem
[0006] As a result of diligent studies in order to achieve the
aforementioned object, the inventors have found that a plating
solution containing chromium sulfate in which the concentration of
Cr.sup.3+ ions is adjusted to a specific range exhibits a covering
power equivalent to that of a plating solution containing
hexavalent chromium, so as to accomplish the present invention.
[0007] That is, the present invention provides a plating solution
used for trivalent chromium plating, the plating solution
containing chromium sulfate and formic acid at a concentration of
Cr.sup.3+ ions of 0.1 mol/L or more and 1 mol/L or less and a
concentration of the formic acid of 0.05 mol/L or more and 0.2
mol/L or less.
[0008] Further, the present invention provides a method for
producing a plated product, the method including a plating step of
performing electric plating in a plating bath that contains a
plating solution containing chromium sulfate, so as to produce the
plated product to which trivalent chromium plating is applied by
the plating step, wherein the plating solution used in the plating
step has a concentration of Cr.sup.3+ ions of 0.1 mol/L or more and
1 mol/L or less, the plating bath has a temperature of 20.degree.
C. or more and less than 40.degree. C., and the electric plating is
performed at a current density of 2 A/dm.sup.2 or more and 20
A/dm.sup.2 or less.
BRIEF DESCRIPTION OF DRAWINGS
[0009] FIG. 1 is a relationship graph between current density and
plating thickness in hexavalent chromium plating and trivalent
chromium plating disclosed in general literatures or the like.
[0010] FIG. 2 is a graph showing the relationship between current
density and plating thickness in trivalent chromium plating using
the plating solution of the present invention.
[0011] FIG. 3 is a graph showing the results of a Hull cell test
using a plating solution of an embodiment.
[0012] FIG. 4 is a graph showing the measurement results of plating
thickness using a fluorescent x-ray film thickness meter.
[0013] FIG. 5 is a graph showing the results of a Hull cell test
using a plating solution of another embodiment.
[0014] FIG. 6 is a graph showing the measurement results of plating
thickness using a fluorescent x-ray film thickness meter.
[0015] FIG. 7 is a graph showing the results of a Hull cell test
using a plating solution of another embodiment.
[0016] FIG. 8 is a graph showing the results of a Hull cell test
using a plating solution of another embodiment.
[0017] FIG. 9 is a graph showing the results of a Hull cell test
using a plating solution of another embodiment.
[0018] FIG. 10 is a graph showing the results of a Hull cell test
using a plating solution of another embodiment.
[0019] FIG. 11 is a graph showing the results of a Hull cell test
using a plating solution of another embodiment.
DESCRIPTION OF EMBODIMENTS
[0020] Hereinafter, a preferable embodiment of the present
invention will be described. First, a method for producing a plated
product will be described. In the method for producing a plated
product of this embodiment, a pretreatment step of adjusting the
surface properties of a product that serves as an object to be
plated (which will be hereinafter referred to also as "original
product") and a plating step of applying trivalent chromium plating
to the pre-treated original product (which will be hereinafter
referred to also as "pre-treated product") are performed. In the
method for producing a plated product of this embodiment, a base
plating step of applying base plating to the pre-treated product or
an intermediate plating step of further applying intermediate
plating to the pre-treated product to which base plating has been
applied (which will be hereinafter referred to also as "base plated
product") may be performed between the pretreatment step and the
plating step, as needed. In such a case, trivalent chromium plating
is performed as finish plating to the product to which the
intermediate plating has been applied (which will be hereinafter
referred to also as "intermediate plated product").
[0021] In the method for producing a plated product of this
embodiment, chemical surface treatment, heat treatment, or the like
may be further applied to the product to which trivalent chromium
plating has been applied in the plating step. Further, coating such
as clear coating may be applied to the plated product, as
needed.
[0022] Examples of the original product to which the pretreatment
step is applied include resin products, ceramic products, metal
products, composite products combining resin parts and metal parts,
and composite products obtained by coating metal parts with
ceramics. Examples of the resins forming the original product
include common thermoplastic resins and common thermosetting
resins. The resins may be fiber reinforced plastic (FRP). Examples
of the ceramics forming the original product include common
ceramics mainly containing silicon oxide, aluminum oxide, or the
like. Examples of the ceramics also include glassy materials such
as enamels. Examples of the metals forming the original product
include common metals such as iron and copper. The metals may be
alloys. Examples of the pretreatment applied to the original
product include polishing such as mechanical polishing, honing, and
blasting, and degreasing such as alkali degreasing. In the base
plating step and the intermediate plating step, various types of
plating such as nickel plating, copper plating, and iron plating
with a thickness of several .mu.m can be applied to the pre-treated
product or the base plated product for purposes such as improving
the aesthetic appearance and the corrosion resistance of the finish
plated product.
[0023] In the plating step of applying trivalent chromium plating,
electric plating is performed using the pre-treated product, the
base plated product, the intermediate plated product, or the like
as a workpiece. In the electric plating, trivalent chromium plating
is applied to the workpiece using a plating bath that contains a
plating solution containing chromium sulfate. Hereinafter, the
plating solution used in the plating step will be described in
detail.
[0024] For plating the workpiece with good covering power, it is
important to use a plating solution having a concentration of
Cr.sup.3+ ions of 0.1 mol/L or more and 1 mol/L or less as the
plating solution in the plating step. The plating solution can
contain a complexing agent, a pH buffer, a conductive agent, a
surfactant, or the like, in addition to chromium sulfate as the
main component. Examples of water that serves as the solvent of the
plating solution include industrial water, tap water, deionized
water, distilled water, and pure water.
[0025] As described above, the chromium sulfate is contained in the
plating solution so that the concentration of Cr.sup.3+ ions in the
plating bath is 0.1 mol/L or more and 1 mol/L or less. The chromium
sulfate is preferably contained in the plating solution so that the
concentration of Cr.sup.3+ ions in the plating bath is 0.1 mol/L or
more and 0.3 mol/L or less. Use of such a preferable plating
solution can give better covering power in trivalent chromium
plating on the workpiece in the plating step.
[0026] Although it is also possible to partially replace the
chromium sulfate contained in the plating solution of this
embodiment as a supply source of the Cr.sup.3+ ions with one or
more selected from the group consisting of chromium chloride, basic
chromium sulfate, chrome alum, and chromium nitrate, there is a
possibility of generating chlorine gas at the anode during plating
when chromium chloride is contained in a large amount, and there is
a possibility of reducing the current density during plating when
chromium chloride is contained in a large amount. Accordingly, the
ratio of chromium sulfate in the supply source of the Cr.sup.3+
ions in the plating solution is preferably 90 mol % or more. The
ratio of chromium sulfate is more preferably 95 mol % or more,
further preferably 99 mol % or more. It is particularly preferable
that the supply source of the Cr.sup.3+ ions in the plating
solution be substantially only chromium sulfate.
[0027] As the complexing agent contained in the plating solution of
this embodiment, organic acids and salts thereof can be used.
Examples of the organic acids include oxalic acid, citric acid,
formic acid, maleic acid, malonic acid, tartaric acid, malic acid,
acetic acid, phthalic acid, propionic acid, and ethylenediamine
tetraacetic acid. Examples of the salts thereof include alkali
metal salts such as lithium salts, potassium salts, and sodium
salts, and alkaline earth metal salts such as magnesium salts and
calcium salts. Among these, the formic acid is a particularly
effective component as the complexing agent, and it is important
that the formic acid be contained in the plating solution at a
concentration of 0.05 mol/L or more and 0.2 mol/L or less. The
concentration of the formic acid in the plating solution is more
preferably 0.08 mol/L or more and 0.12 mol/L or less.
[0028] The aforementioned organic acids and the salts thereof exert
at least a certain effect also as the pH buffer. Further, as the
complexing agent, amino carbonyl compounds such as urea and
carbamic acid also may be employed. In particular, urea functions
not only as the pH buffer but also as a supply source of nitrogen
to the plating film, which is effective in hardening the film.
Further, urea can be expected to have an effect of suppressing
generation of a precipitate such as chromium hydroxide in the
plating solution. In view of such a point, the urea is preferably
contained in the plating solution at a concentration of 0.1 mol/L
or more and 1 mol/L or less. The concentration of the urea in the
plating solution is more preferably 0.2 mol/L or more and 0.8 mol/L
or less, particularly preferably 0.4 mol/L or more and 0.6 mol/L or
less.
[0029] Other than the above, examples that are usable as the pH
buffer include boric acid and borate. In the case where the boric
acid is contained in the plating solution, the boric acid is
generally contained in the plating solution at a concentration of
0.5 mol/L or more and 1 mol/L or less, though it also depends on
the amount of the organic acids or the urea contained as the pH
buffer. The pH of the plating solution is preferably adjusted to 1
or more and 2 or less, more preferably 1.3 or more and 1.7 or less,
using the pH buffer or the like.
[0030] Examples of the conductive agent include ammonium chloride,
sodium chloride, potassium chloride, ammonium sulfate, sodium
sulfate, potassium sulfate, ammonium nitrate, sodium nitrate, and
potassium nitrate. Further, examples of the surfactant include
sodium lauryl sulfate, sodium dodecyl sulfate, polyethylene glycol,
diisohexyl sulfosuccinate, 2-ethylhexyl sulfate, diisobutyl
sulfosuccinate, diisoamyl sulfosuccinate, and isodecyl
sulfosuccinate.
[0031] In the plating solution, various additives including a film
forming agent such as polyethylene glycol, polyvinyl alcohol, and
gelatin, and a defoamer can be contained as additional
additives.
[0032] In the plating step using the plating solution containing
such components, it is important to set the temperature of the
plating bath to 20.degree. C. or more and less than 40.degree. C.
The lower the temperature of the plating bath in the plating step,
the better the covering power in trivalent chromium plating on the
workpiece can be obtained. Meanwhile, when the temperature of the
plating bath is a certain temperature or higher, the deposition of
the components contained in the plating solution can be suppressed,
and the occurrence of roughness on the plated surface can be
reduced. In view of such a point, the temperature of the plating
bath in the plating step is more preferably 23.degree. C. or more
and 29.degree. C. or less, particularly preferably 24.degree. C. or
more and 28.degree. C. or less. A plating film with uniform
thickness and excellent surface brightness can be provided on the
finish plated product by performing the plating step under such
preferable temperature conditions.
[0033] In the electric plating in the plating step, it is important
to set the current density to 2 A/dm.sup.2 or more and 20
A/dm.sup.2 or less. The current density is more preferably 2
A/dm.sup.2 or more and 15 A/dm.sup.2 or less, particularly
preferably 2 A/dm.sup.2 or more and 13 A/dm.sup.2 or less. The
covering power in trivalent chromium plating can be further
enhanced by plating the workpiece at such a preferable current
density.
[0034] In the plating step, air bubbles are generated in the
plating solution due to the hydrogen gas. Therefore, in order to
prevent adhesion of the air bubbles to the workpiece, the workpiece
may be vibrated during the plating or may be subjected to bubbling
to generate air bubbles by an inert gas or the like from below the
workpiece.
[0035] Although the aforementioned various conditions in the
plating step such as the concentration of the components of the
plating solution, the bath temperature of the plating solution, and
the current density applied to the workpiece are not always
necessarily maintained within the aforementioned ranges during the
period from the very start of the plating step to the completion of
the plating step in order to exert the effects thereof, it is
preferable that almost the same conditions as those at the start of
the plating be maintained over the entire period.
[0036] In the plating step of this embodiment, chromium plating
with the same thickness as in flat portions is applied also to
portions that conventionally tend to have different plating
thickness from the flat portions, such as corner portions and fine
uneven portions of the workpiece, and the same covering power and
throwing power as in hexavalent chromium plating are exerted even
in trivalent chromium plating. The plating thickness of the plated
product produced in this embodiment can be appropriately set
corresponding to the application or the like of the plated product,
but portions with trivalent chromium plating alone, excluding base
plating, preferably have a plating thickness of 5 .mu.m or more and
600 .mu.m or less, in order to exert the effects of the present
invention more significantly. The plating thickness is more
preferably 50 .mu.m or more, particularly preferably 100 .mu.m or
more. The plating thickness may be measured, for example, using a
fluorescent x-ray film thickness meter or the like, as needed.
However, the measurement using the fluorescent x-ray film thickness
meter is difficult when the plating thickness is 50 .mu.m or more,
and therefore the cross sections of the plated product may be
observed using a scanning electron microscope (SEM) in such a case.
Then, the plating thickness can be determined by measuring the
plating thickness of the plated product at several sites selected
at random and arithmetically averaging the measurement results
excluding abnormal values.
[0037] Detailed descriptions are not further repeated herein, but a
conventionally known technical matter that is not specifically
shown in the aforementioned examples can be appropriately employed
for the method for producing a plated product and the plating
solution used for the plating in this embodiment, without
significantly impairing the effects of the present invention. That
is, the present invention is not limited to those in the ranges of
the aforementioned examples.
EXAMPLES
[0038] Next, the present invention will be described further in
detail by way of examples. However, the present invention is not
limited to these examples.
(Summary of the Present Invention: Difference from Conventional
Trivalent Chromium Plating)
[0039] FIG. 1 shows the relationship between current density and
plating thickness in hexavalent chromium plating and trivalent
chromium plating disclosed in general literatures or the like. It
is understood from this graph that it is difficult to exert
throwing power in conventional trivalent chromium plating since the
slope of the straight line that shows the relationship between
current density and plating thickness rapidly increases at a
current density of about 5 A/dm.sup.2. Further, it is understood
from this graph that it is difficult to perform plating at a
current density of 5 A/dm.sup.2 or less and to perform plating with
excellent covering power in conventional trivalent chromium
plating.
[0040] In contrast, in the case of using the plating solution of
the present invention, the relationship between current density and
plating thickness is as shown in FIG. 2, where trivalent chromium
plating at a low current density of 5 A/dm.sup.2 or less is made
possible as in hexavalent chromium plating. Further, in the case of
using the plating solution of the present invention, the
relationship between current density and plating thickness is made
linear from the region at a low current density of 5 A/dm.sup.2 or
less to the region at a high current density of about 30
A/dm.sup.2, and thus plating can be performed with excellent
throwing power. Accordingly, it can be seen also from this graph
that the present invention provides a plating solution for
trivalent chromium plating that is excellent in covering power and
throwing power. The results of the study on this point will be
described in detail below.
Evaluation 1: Bath Temperature 1
[0041] A plating solution was prepared so as to contain the
ingredients shown in Table 1. That is, a plating solution
containing chromium sulfate and having a concentration of Cr.sup.3
ions of 1 mol/L was prepared. Formic acid and urea were added to
the plating solution so that each has a concentration of 0.5 mol/L.
Further, the plating solution was adjusted to have a pH of 1.5.
TABLE-US-00001 TABLE 1 Reference Complexing agent concentration 1-1
1-2 1-3 Chromium sulfate 1 mol/L 1 1 1 Formic acid 0.5 mol/L 0.5
0.5 0.5 Urea 0.5 mol/L 0.5 0.5 0.5 Boric acid 0.5 mol/L 0.5 0.5 0.5
Aluminum sulfate 0.15 mol/L 0.15 0.15 0.15 Sodium sulfate 0.3 mol/L
0.3 0.3 0.3 Sodium dodecyl 50 mg/L 50 50 50 sulfate Bath 30 35 40
temperature (.degree. C.) Plating condition at 1 A/dm.sup.2 x x x
each current density 2 A/dm.sup.2 .smallcircle. x x 5 A/dm.sup.2
.smallcircle. .smallcircle. x 10 A/dm.sup.2 .smallcircle.
.smallcircle. x 15 A/dm.sup.2 .smallcircle. .smallcircle. x 20
A/dm.sup.2 .smallcircle. .smallcircle. x 25 A/dm.sup.2
.smallcircle. .smallcircle. x Conditions: pH 1.5, Hull cell total
current amount of 5 A, Plating time of 10 mins Evaluation Symbol:
.smallcircle.: Normal plating x: No plating
[0042] Using the aforementioned plating solution, a Hull cell test
was conducted with a current value of 5 A. The test was performed
on three patterns at a bath temperature of 30.degree. C.,
35.degree. C., and 40.degree. C., and the test was conducted for 10
minutes. The results are shown in FIG. 3 and Table 1 together.
[0043] Further, the plating thickness of the sample after the test
was measured using a fluorescent x-ray film thickness meter. The
results are shown in FIG. 4. It was seen from the results shown in
Table 1 and FIG. 4 that at least a certain plating thickness could
be ensured when the current density was low due to the
concentration of Cr.sup.3+ ions being 1 mol/L or less. Further, it
was proved from the results shown in Table 1 and FIG. 4 that the
bath temperature of less than 40.degree. C. was advantageous in
order to perform trivalent chromium plating with excellent covering
power.
Evaluation 2: Bath Temperature 2
[0044] Based on the results of "Evaluation 1", the Hull cell test
was performed on five patterns at a bath temperature of 24.degree.
C., 26.degree. C., 28.degree. C., 30.degree. C., and 32.degree. C.
by further subdividing the bath temperature, and the test results
were evaluated in the same manner as in "Evaluation 1". The results
are shown in Table 2 and FIGS. 5 and 6.
TABLE-US-00002 TABLE 2 Reference Complexing agent concentration 2-1
2-2 2-3 2-4 2-5 Chromium sulfate 1 mol/L 1 1 1 1 1 Formic acid 0.5
mol/L 0.5 0.5 0.5 0.5 0.5 Urea 0.5 mol/L 0.5 0.5 0.5 0.5 0.5 Boric
acid 0.5 mol/L 0.5 0.5 0.5 0.5 0.5 Aluminum sulfate 0.15 mol/L 0.15
0.15 0.15 0.15 0.15 Sodium sulfate 0.3 mol/L 0.3 0.3 0.3 0.3 0.3
Sodium dodecyl 50 mg/L 50 50 50 50 50 sulfate Bath 24 26 28 30 32
temperature (.degree. C.) Plating condition at 1 A/dm.sup.2 x x x x
x each current density 2 A/dm.sup.2 .smallcircle. .smallcircle. x x
x 5 A/dm.sup.2 .smallcircle. .smallcircle. x x x 8 A/dm.sup.2
.smallcircle. .smallcircle. .smallcircle. x x 10 A/dm.sup.2
.smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. 15 A/dm.sup.2 .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle. 20 A/dm.sup.2
.smallcircle. .smallcircle. .smallcircle. .smallcircle. x 25
A/dm.sup.2 x x x x x Conditions: pH 1.5, Hull cell total current
amount of 5 A, Plating time of 10 mins Evaluation Symbol:
.smallcircle.: Normal plating x: No plating or abnormal
deposition
[0045] It is seen also from these table and graphs that deposition
of the plating film at each current density can be performed at a
substantially constant speed when the bath temperature falls within
the range from 24.degree. C. to 28.degree. C. Further, it turned
out that the covering power was most improved at a bath temperature
of 24.degree. C., and preferable results could be obtained by
performing the plating at a lower bath temperature. Then, it was
also considered to further decrease the bath temperature, but when
the bath temperature was excessively decreased, the bath
composition partially crystallized to make the plating surface
rough, or the bath viscosity increased to make it difficult to
remove hydrogen at the cathode. Therefore, it was determined that
the bath temperature was optimally within the range of 24.degree.
C. to 28.degree. C. It was confirmed also in a bent cathode test,
which was separately performed, that good plating tends to be made
possible, even at a low current density, by employing a bath
temperature of 24.degree. C. to 28.degree. C.
Evaluation 3: Bath Concentration
[0046] A Hull cell test was conducted with a reduced concentration
of the bath composition itself, in order to improve the covering
power by suppressing decrease in the bath viscosity and
crystallization of the bath liquid. Specifically, the Hull cell
test was conducted at a bath temperature of 25.degree. C. with the
bath composition reduced to 4/5 (at a concentration of Cr.sup.3+
ions of 0.8 mol/L and a concentration of each of formic acid and
urea of 0.4 mol/L), 3/5 (at a concentration of Cr.sup.3+ ions of
0.6 mol/L and a concentration of each of formic acid and urea of
0.3 mol/L), and 1/2 (at a concentration of Cr.sup.3+ ions of 0.5
mol/L and a concentration of each of formic acid and urea of 0.25
mol/L). The results are shown in Table 3 and FIG. 7. As is obvious
also from Table 3 and FIG. 7, it turned out in this "Evaluation 3"
that a better plating film was obtained at a lower bath
concentration. It was confirmed also in a bent cathode test, which
was separately performed, that the best results were obtained by
using a plating solution containing 0.5-fold components.
TABLE-US-00003 TABLE 3 Reference Complexing agent concentration 3-1
3-2 3-3 Chromium sulfate 1 mol/L 0.8 0.6 0.5 Formic acid 0.5 mol/L
0.4 0.3 0.25 Urea 0.5 mol/L 0.4 0.3 0.25 Boric acid 0.5 mol/L 0.4
0.3 0.25 Aluminum sulfate 0.15 mol/L 0.12 0.09 0.075 Sodium sulfate
0.3 mol/L 0.24 0.18 0.15 Sodium dodecyl 50 mg/L 40 30 25 sulfate
Plating condition at 1 A/dm.sup.2 x x x each current density 2
A/dm.sup.2 x .smallcircle. .smallcircle. 5 A/dm.sup.2 .smallcircle.
.DELTA. .DELTA. 10 A/dm.sup.2 .smallcircle. .DELTA. .DELTA. 15
A/dm.sup.2 .DELTA. .DELTA. .DELTA. 20 A/dm.sup.2 .DELTA. .DELTA.
.DELTA. 25 A/dm.sup.2 .DELTA. x x Conditions: pH 1.5, Bath
temperature of 25.degree. C., Hull cell total current amount of 5
A, Plating time of 10 mins Evaluation Symbol: .smallcircle.: Normal
plating .DELTA.: Partly faulty x: No plating or abnormal
deposition
Evaluation 4: Added Amount 1 of Formic Acid and Urea
[0047] Based on the results of "Evaluation 3", attempts to further
reduce the bath concentration were made. Specifically, a Hull cell
test was conducted with the bath composition reduced to 1/4 and
1/10. As a result, the covering power was further improved, but
abnormal plating deposition was found. Therefore, while the
chromium ion concentration was set to 1/4 (0.25 mol/L) and 1/10
(0.1 mol/L), the concentration of formic acid and urea was reset to
the initial value (0.5 mol/L). As a result, the abnormal deposition
was eliminated as shown in Table 4 and FIG. 8, but the covering
power decreased.
TABLE-US-00004 TABLE 4 Reference Complexing agent concentration 4-1
4-2 4-3 4-4 Chromium sulfate 1 mol/L 0.25 0.25 0.1 0.1 Formic acid
0.5 mol/L 0.125 0.5 0.05 0.5 Urea 0.5 mol/L 0.125 0.5 0.05 0.5
Boric acid 0.5 mol/L 0.125 0.5 0.05 0.5 Aluminum sulfate 0.15 mol/L
0.0375 0.15 0.015 0.15 Sodium sulfate 0.3 mol/L 0.075 0.3 0.03 0.3
Sodium dodecyl 50 mg/L 12.5 50 5 5 sulfate Plating condition at 1
A/dm.sup.2 x x x x each current density 2 A/dm.sup.2 x x x x 5
A/dm.sup.2 x .smallcircle. x .DELTA. 10 A/dm.sup.2 x .smallcircle.
x .DELTA. 15 A/dm.sup.2 x .smallcircle. x .DELTA. 20 A/dm.sup.2
.smallcircle. .smallcircle. x x 25 A/dm.sup.2 .smallcircle. x
.smallcircle. x Conditions: pH 1.5, Bath temperature of 25.degree.
C., Hull cell total current amount of 5 A, Plating time of 10 mins
Evaluation Symbol: .smallcircle.: Normal plating .DELTA.: Partly
faulty x: No plating or abnormal deposition
Evaluation 5: Added Amount 2 of Formic Acid and Urea
[0048] Based on the results of "Evaluation 4", attempts to reduce
the abnormal deposition by varying the amounts of formic acid and
urea were made. As a result of the Hull cell test (see Table 5 and
FIG. 9), it turned out that the abnormal deposition was suppressed
and the covering power was good by setting the concentration of
formic acid to a low value of 0.1 mol/L and the concentration of
urea to the initial value (0.5 mol/L).
TABLE-US-00005 TABLE 5 Reference Complexing agent concentration 5-1
5-2 5-3 5-4 Chromium sulfate 1 mol/L 0.1 0.1 0.25 0.25 Formic acid
0.5 mol/L 0.5 0.1 0.5 0.1 Urea 0.5 mol/L 0.1 0.5 0.1 0.5 Boric acid
0.5 mol/L 0.5 0.5 0.5 0.5 Aluminum sulfate 0.15 mol/L 0.15 0.15
0.15 0.15 Sodium sulfate 0.3 mol/L 0.3 0.3 0.3 0.3 Sodium dodecyl
50 mg/L 50 50 50 50 sulfate Plating condition at 1 A/dm.sup.2 x x x
x each current density 2 A/dm.sup.2 x x x x 5 A/dm.sup.2 x x x
.smallcircle. 10 A/dm.sup.2 x x x .smallcircle. 15 A/dm.sup.2 x x x
.smallcircle. 20 A/dm.sup.2 x .smallcircle. .smallcircle.
.smallcircle. 25 A/dm.sup.2 .smallcircle. .smallcircle.
.smallcircle. x Conditions: pH 1.5, Bath temperature of 25.degree.
C., Hull cell total current amount of 5 A, Plating time of 10 mins
Evaluation Symbol: .smallcircle.: Normal plating .DELTA.: Partly
faulty x: No plating or abnormal deposition
[0049] Subsequently, whether the covering power was improved was
checked, with the concentration of urea fixed to 0.5 mol/L and the
concentration of formic acid varied from 0.1 mol/L to 0.22 mol/L.
The results are shown in Table 6 and FIG. 10.
TABLE-US-00006 TABLE 6 Reference Complexing agent concentration 5-5
5-6 5-7 5-8 5-9 5-10 5-11 Chromium sulfate 1 mol/L 0.25 0.25 0.25
0.25 0.25 0.25 0.25 Formic acid 0.5 mol/L 0.1 0.12 0.14 0.16 0.18
0.2 0.22 Urea 0.5 mol/L 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Boric acid 0.5
mol/L 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Aluminum sulfate 0.15 mol/L 0.15
0.15 0.15 0.15 0.15 0.15 0.15 Sodium sulfate 0.3 mol/L 0.3 0.3 0.3
0.3 0.3 0.3 0.3 Sodium dodecyl 50 mg/L 50 50 50 50 50 50 50 sulfate
Plating condition at 1 A/dm.sup.2 x x x x x x x each current
density 2 A/dm.sup.2 .smallcircle. .smallcircle. .smallcircle. x x
x x 5 A/dm.sup.2 .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. x x 10 A/dm.sup.2 .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. 15 A/dm.sup.2 .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. 20 A/dm.sup.2 .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. 25 A/dm.sup.2 x x x x x x x Conditions:
pH 1.5, Bath temperature of 25.degree. C., Hull cell total current
amount of 5 A, plating time of 10 mins Evaluation Symbol:
.smallcircle.: Normal plating .DELTA.: Partly faulty x: No plating
or abnormal deposition
[0050] It turned out from these table and graph that good results
were obtained when the concentration of formic acid was 0.2 mol/L
or less.
[0051] Further, the covering power was evaluated by further
reducing the concentration of formic acid. The results are shown in
Table 7 and FIG. 11.
TABLE-US-00007 TABLE 7 Reference Complexing agent concentration
5-12 5-13 5-14 5-15 Chromium sulfate 1 mol/L 0.25 0.25 0.25 0.25
Formic acid 0.5 mol/L 0 0.04 0.06 0.08 Urea 0.5 mol/L 0.5 0.5 0.5
0.5 Boric acid 0.5 mol/L 0.5 0.5 0.5 0.5 Aluminum sulfate 0.15
mol/L 0.15 0.15 0.15 0.15 Sodium sulfate 0.3 mol/L 0.3 0.3 0.3 0.3
Sodium dodecyl 50 mg/L 50 50 50 50 sulfate Plating condition at 1
A/dm.sup.2 x x x x each current density 2 A/dm.sup.2 .smallcircle.
.smallcircle. .smallcircle. .smallcircle. 5 A/dm.sup.2
.smallcircle. .smallcircle. .smallcircle. .smallcircle. 10
A/dm.sup.2 x .smallcircle. .smallcircle. .smallcircle. 15
A/dm.sup.2 x x .smallcircle. .smallcircle. 20 A/dm.sup.2 x x
.smallcircle. .smallcircle. 25 A/dm.sup.2 x x x .smallcircle.
Conditions: pH 1.5, Bath temperature of 25.degree. C., Hull cell
total current amount of 5 A, plating time of 10 mins Evaluation
Symbol: .smallcircle.: Normal plating .DELTA.: Partly faulty x: No
plating or abnormal deposition
[0052] It turned out from these table and graph that good results
were obtained when the concentration of formic acid was 0.05 mol/L
or more. As a result of a bent cathode test conducted under the
same conditions as shown in Tables 6 and 7, using a cathode with a
recessed portion of a "C-shape" in side view obtained by folding a
strip-shaped metal plate, it was confirmed that plating was applied
also to a range of 60% or more of the recessed portion when the
concentration of formic acid falls within the range of 0.05 mol/L
or more and 0.1 mol/L or less.
[0053] As described above, it was able to be confirmed that good
covering power was exhibited in the plating solution for trivalent
chromium plating containing chromium sulfate when the concentration
of Cr.sup.3+ ions was set to the range of 0.1 mol/L or more and 1
mol/L or less, and the concentration of formic acid was set to the
range of 0.05 mol/L or more and 0.2 mol/L or less. Further, it was
able to be confirmed that particularly good covering power was
exhibited in the plating solution when the concentration of
Cr.sup.3+ ions was set to the range of 0.1 mol/L to 0.3 mol/L.
Further, it was able to be confirmed from the aforementioned
evaluation that it was effective to set the temperature of the
plating bath to 20.degree. C. or more and less than 40.degree. C.
Also from these facts, it is understood that the present invention
can provide a plating solution for trivalent chromium plating with
excellent covering power so as to enlarge the application range of
environmentally friendly plated products.
* * * * *