U.S. patent application number 11/570172 was filed with the patent office on 2008-11-06 for engine part.
This patent application is currently assigned to YAMAHA HATSUDOKI KABUSHIKI KAISHA. Invention is credited to Naohisa Takahashi.
Application Number | 20080274373 11/570172 |
Document ID | / |
Family ID | 36202923 |
Filed Date | 2008-11-06 |
United States Patent
Application |
20080274373 |
Kind Code |
A1 |
Takahashi; Naohisa |
November 6, 2008 |
Engine Part
Abstract
An engine part according to the present invention includes: a
metal substrate; a chromium plating layer covering at least a
portion of a surface of the metal substrate, the chromium plating
layer being formed from a trivalent chromium plating solution. The
chromium plating layer includes a boron content of no less than
0.05 mass % and no more than 0.3 mass %, and the chromium plating
layer has a thickness or 0.7 .mu.m or less.
Inventors: |
Takahashi; Naohisa;
(Shizuoka, JP) |
Correspondence
Address: |
YAMAHA HATSUDOKI KABUSHIKI KAISHA;C/O KEATING & BENNETT, LLP
1800 Alexander Bell Drive, SUITE 200
Reston
VA
20191
US
|
Assignee: |
YAMAHA HATSUDOKI KABUSHIKI
KAISHA
Shizuoka-ken
JP
|
Family ID: |
36202923 |
Appl. No.: |
11/570172 |
Filed: |
October 17, 2005 |
PCT Filed: |
October 17, 2005 |
PCT NO: |
PCT/JP2005/019037 |
371 Date: |
December 7, 2006 |
Current U.S.
Class: |
428/666 |
Current CPC
Class: |
C25D 3/06 20130101; F01N
13/16 20130101; Y10T 428/12847 20150115; F01N 2530/02 20130101;
C25D 3/56 20130101 |
Class at
Publication: |
428/666 |
International
Class: |
C25D 5/10 20060101
C25D005/10 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 18, 2004 |
JP |
2004-302479 |
Claims
1: An engine part comprising: a metal substrate; a chromium plating
layer covering at least a portion of a surface of the metal
substrate, the chromium plating layer being formed from a trivalent
chromium plating solution, wherein, the chromium plating layer
includes a boron content of no less than 0.05 mass % and no more
than 0.3 mass %, and the chromium plating layer has a thickness or
0.7 .mu.m or less.
2: The engine part of claim 1, wherein the boron content in the
chromium plating layer is no less than 0.05 mass % and no more than
0.1 mass %.
3: The engine part of claim 2, wherein the chromium plating layer
includes an iron content of 2 mass % or less.
4: The engine part of claim 3, wherein the chromium plating layer
covers a region of the metal substrate surface that is heated to a
temperature of 350.degree. C. or more.
5: The engine part of claim 4, further comprising an underlying
plating layer provided between the metal substrate surface and the
chromium plating layer, wherein, the chromium plating layer has a
thickness of no less than 0.2 .mu.m and no more than 0.7 .mu.m in
the region that is heated to a temperature of 350.degree. C. or
more.
6: The engine part of claim 5, wherein the underlying plating layer
includes at least one of C and S.
7: The engine part of claim 6, wherein the underlying plating layer
further includes Ni.
8: The engine part of claim 5, wherein the underlying plating layer
is formed of a metal having a hardness lower than that of the Cr
composing the chromium plating layer.
9: The engine part of claim 5, wherein the underlying plating layer
is formed from Ni plating.
10: The engine part of claim 3, wherein the chromium plating layer
has a color tone such that an L* value measured according to CIE
(Commission Internationale de l'Eclairage) 1976 is in a range of no
less than 68 and no more than 80.
11: The engine part of claim 5, wherein the metal substrate is
composed of a material containing Fe, Al, Zn, or Mg as a main
component.
12: The engine part of claim 1, wherein the metal substrate is a
metal tube defining a passage through which an exhaust gas from an
engine travels.
13: An engine comprising the engine part of claim 1.
14: A transportation apparatus comprising the engine part of claim
12.
15: A transportation apparatus comprising the engine of claim 13.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an engine part, and more
particularly to an engine part which is subjected to a high
temperature due to a high-temperature exhaust gas discharged from
an engine.
[0003] 2. Description of the Related Art
[0004] In a vehicle such as a motorcycle or an all-terrain
four-wheeled vehicle, not only the performance of the vehicle
itself, but a good design is an important issue. FIG. 9 is a side
view showing an example of a sports-type motorcycle. A motorcycle
200 shown in FIG. 9 includes a V-type engine 201 and an exhaust
pipe 202 for guiding along exhaust gas. The V-type engine 201
includes cylinders 203, cylinder heads 204, and head covers 205.
The aesthetically excellent V-type engine 201 is likely to be
mounted to the motorcycle such that the engine is exposed on the
outside, and is highly influential to the exterior appearance of
the entire motorcycle.
[0005] Exhaust pipes extend from the two cylinders 203 of the
V-type engine 201 and are united into the single exhaust pipe 202,
which extends toward and above the rear wheel so as to allow
exhaust gas to be discharged at the rear portion of the body. The
exhaust pipe 202 must have a certain thickness for allowing the
exhaust gas generated in the engine 201 to be efficiently
discharged. Moreover, the portion constituting a muffler 202a has
an increased diameter in order to accommodate the structure for
muffling. For these reasons, the exhaust pipe accounts for a
relatively large part of the exterior appearance of the entire
motorcycle, and thus the shape and color of the exhaust pipe are
highly influential to the entire motorcycle design.
[0006] In the present specification, engine components such as the
cylinders 203, the cylinder heads 204, the head covers 205, and the
exhaust pipe 202 for guiding the exhaust gas from the engine as
well as a cover thereof, will be referred to as "engine parts". For
the aforementioned reasons, the shape and color of engine parts are
important factors in determining the entire motorcycle design.
[0007] Conventionally, those engine parts which show up on the
exterior appearance are subjected to surface treatments such as
plating to acquire a lustrous metallic color, thus to enhance the
engine part design. Above all, decorative chromium plating has been
widely used for engine parts because it is possible to give the
plated material a characteristic, lustrous silver-gray color (see,
for example, Patent Document 1).
[0008] Since decorative chromium plating provides an excellent
metallic luster, and also excels in anticorrosiveness, it is also
used in various fields other than engine parts. In order to obtain
excellent exterior appearance and anticorrosiveness, it is
unnecessary to form the decorative chromium plating so as to be
thick. In fact, when formed thick, decorative chromium plating will
result in a poor color tone and surface finish. Therefore, in
general, decorative chromium plating is preferably used at a
thickness in the range from 0.1 .mu.m to 0.15 .mu.m.
[0009] Note that, as plating using chromium, hard chromium plating
(industrial chromium plating) is also widely used in industrial
products. Since hard chromium plating has a low friction
coefficient and an excellent abrasion resistance, it is used for
sliding sections of various machine parts, for example. Since
abrasion resistance is a requirement, hard chromium plating is
usually formed to a thickness of several .mu.m or more. Moreover,
hard chromium plating does not have a decorativeness surface as
does decorative chromium plating. Usually, decorative chromium
plating has a surface roughness (Ra) of 1 .mu.m or less (typically,
0.2 .mu.m or less) after the plating, while hard chromium plating
has a surface roughness of 1 .mu.m or more.
[0010] Decorative chromium plating is usually formed by using a
plating solution of chromate, including hexavalent chromium
(Cr.sup.6+). Hexavalent chromium is inexpensive, and a chromium
plating layer formed from a plating solution containing hexavalent
chromium (hereinafter referred to as a "hexavalent chromium plating
solution") shows good contact with a base substrate, and has
excellent anticorrosiveness and abrasion resistance. Furthermore, a
chromium plating layer formed from a hexavalent chromium plating
solution has a silver-gray color with a characteristically metallic
luster. For these reasons, a hexavalent chromium plating solution
is widely used for engine parts for motorcycles.
[0011] However, hexavalent chromium is known for its highly
biotoxic nature. When performing plating with a hexavalent chromium
plating solution, it has become a requirement that safety of the
workers is ensured and that environmental pollution is
prevented.
[0012] In order to solve such problems of hexavalent chromium, it
is desired to use trivalent chromium (Cr.sup.3+), which is less
toxic than hexavalent chromium, to perform decorative chromium
plating. Various plating solutions containing trivalent chromium
have been proposed (Patent Documents 2 and 3).
[0013] Patent Document 1: Japanese Laid-Open Patent Publication No.
2003-41933
[0014] Patent Document 2: Japanese Laid-Open Patent Publication No.
52-065138
[0015] Patent Document 3: Japanese Laid-Open Patent Publication No.
52-092834
[0016] Patent Document 4: Japanese Laid-Open Patent Publication No.
9-95793
[0017] Patent Document 5: Japanese Laid-Open Patent Publication No.
9-228069
[0018] From environmental and safety standpoints, it is preferable
to use a plating solution containing trivalent chromium
(hereinafter referred to as a "trivalent chromium plating
solution"). However, a chromium plating layer which is formed by
using a conventional trivalent chromium plating solution has a
problem of being inferior to a plating layer which is formed from a
hexavalent chromium plating solution, in terms of stability of
growth of the chromium plating layer and the plating film-forming
characteristics of the resultant chromium plating layer, e.g.,
anticorrosiveness and abrasion resistance.
[0019] Moreover, an engine part is heated to a high temperature due
to heat generation associated with fuel combustion and a
high-temperature exhaust gas. When a chromium plating layer is
formed on an engine part by using a conventional trivalent chromium
plating solution, there is a problem in that the chromium plating
layer is likely to have cracks responsive to heating. If a crack
occurs in a chromium plating layer, rust will occur from that
portion, thus severely degrading the exterior appearance.
[0020] Therefore, trivalent chromium plating solutions of various
compositions have been proposed since twenty or more years ago.
However, trivalent chromium plating solutions have hardly been used
for engine parts, in particular.
[0021] The present invention solves such conventional problems, and
intends to provide an engine part having a chromium plating layer
which is formed by using a trivalent chromium plating solution;
which has as good film-forming characteristics as those of a
plating layer that is formed from a hexavalent chromium plating
solution; and which has excellent thermal resistance.
SUMMARY OF THE INVENTION
[0022] An engine part according to the present invention is an
engine part comprising: a metal substrate; a chromium plating layer
covering at least a portion of a surface of the metal substrate,
the chromium plating layer being formed from a trivalent chromium
plating solution, wherein, the chromium plating layer includes a
boron content of no less than 0.05 mass % and no more than 0.3 mass
%, and the chromium plating layer has a thickness or 0.7 .mu.m or
less.
[0023] In a preferred embodiment, the boron content in the chromium
plating layer is no less than 0.05 mass % and no more than 0.1 mass
%.
[0024] In a preferred embodiment, the chromium plating layer
includes an iron content of 2 mass % or less.
[0025] In a preferred embodiment, the chromium plating layer covers
a region of the metal substrate surface that is heated to a
temperature of 350.degree. C. or more.
[0026] In a preferred embodiment, the engine part further comprises
an underlying plating layer provided between the metal substrate
surface and the chromium plating layer, wherein, the chromium
plating layer has a thickness of no less than 0.2 m and no more
than 0.7 .mu.m in the region that is heated to a temperature of
350.degree. C. or more.
[0027] In a preferred embodiment, the underlying plating layer
includes at least one of C and S.
[0028] In a preferred embodiment, the underlying plating layer
further includes Ni.
[0029] In a preferred embodiment, the underlying plating layer is
formed of a metal having a hardness lower than that of the Cr
composing the chromium plating layer.
[0030] In a preferred embodiment, the underlying plating layer is
formed from Ni plating.
[0031] In a preferred embodiment, the chromium plating layer has a
color tone such that an L* value measured according to CIE
(Commission Internationale de l'Eclairage) 1976 is in a range of no
less than 68 and no more than 80.
[0032] In a preferred embodiment, the metal substrate is composed
of a material containing Fe, Al, Zn, or Mg as a main component.
[0033] In a preferred embodiment, the metal substrate is a metal
tube defining a passage through which an exhaust gas from an engine
travels.
[0034] An engine according to the present invention comprises any
of the aforementioned engine parts.
[0035] A transportation apparatus according to the present
invention comprises any of the aforementioned engine parts or
engine.
[0036] According to the present invention, a boron content in the
chromium plating layer is adjusted to no less than 0.05 mass % and
no more than 0.3 mass %, and the chromium plating layer has a
thickness or 0.7 .mu.m or less. Therefore, by using a trivalent
chromium plating solution, a chromium plating layer which has as
good film-forming characteristics as those of a plating layer that
is formed from a hexavalent chromium plating solution and which has
good thermal resistance can be obtained. Thus, an engine part
having good exterior appearance can be obtained by using a
trivalent chromium plating solution.
[0037] Moreover, by prescribing a boron content in the chromium
plating layer to be 0.1 mass % or less and an iron content to be 2
mass % or less, a silver-gray color tone similar to that of a
chromium plating layer which is formed from a hexavalent chromium
plating solution can be obtained. Furthermore, by prescribing the
thickness of the chromium plating layer in a range of no less than
0.2 m and no more than 0.7 .mu.m, an engine part which is not
likely to experience thermal discoloration can be obtained.
BRIEF DESCRIPTION OF DRAWINGS
[0038] FIG. 1 is a diagram schematically showing the structure of
an engine part according to the present invention.
[0039] FIG. 2(a) is a graph showing X-ray diffraction analysis
results of a chromium plating layer formed by using trivalent
chromium; and (b) is a graph showing X-ray diffraction analysis
results of a chromium plating layer formed by using hexavalent
chromium.
[0040] FIG. 3 is a diagram schematically showing the generation of
a "C--S thickened layer" or a "C--S--Ni thickened layer", obtained
by heating, near an interface between a chromium plating layer and
an underlying plating layer.
[0041] FIG. 4(a) is a schematic diagram for explaining prevention
of discoloration of a chromium plating layer by increasing the
thickness of the chromium plating layer; and (b) is a diagram
schematically showing a portion of a conventional chromium-nickel
plating layer.
[0042] FIG. 5 is a side view showing a motorcycle in which an
engine part according to the present invention is used.
[0043] FIG. 6(a) is a diagram schematically showing a portion of an
exhaust pipe which is directly connected to an engine; (b) is a
diagram schematically showing a cross section of a catalyst
accommodating section of an exhaust pipe; and (c) is a diagram
schematically showing a cross section of a manifold section.
[0044] FIG. 7 is a diagram showing an example of a chromium plating
apparatus used in the present invention.
[0045] FIG. 8(a) is a diagram schematically showing an arrangement
in which a minimum distance exists between a curved portion of a
metal substrate and an electrode; and (b) is a diagram
schematically showing an arrangement in which a long distance
exists between a curved portion of a metal substrate and an
electrode.
[0046] FIG. 9 is a side view showing the exterior appearance of a
motorcycle.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0047] As a result of studying the composition of trivalent
chromium plating solutions and the composition within resultant
chromium plating layers, the inventor has found that the problem of
cracks occurring in a chromium plating layer is related to the
boron content within the chromium plating layer. The inventor has
also found that the stability of growth of a chromium plating layer
and the anticorrosiveness and abrasion resistance of the resultant
chromium plating layer are also related to the boron content within
the chromium plating layer and the boron concentration within the
trivalent chromium plating solution.
[0048] Moreover, a chromium plating layer which is formed from a
conventional trivalent chromium plating solution has a blackish
color tone as compared to a chromium plating layer which is formed
from a hexavalent chromium plating solution. This has been found to
be related to the iron content in the resultant chromium plating
layer.
[0049] Furthermore, when providing on an engine part a chromium
plating layer which is formed from a trivalent chromium plating
solution, the surface color tone may change due to heating, so that
the chromium plating layer may appear blue-violet. Such
discoloration will degrade the exterior appearance of a motorcycle
or the like in which the engine part is mounted. The inventor has
found that the discoloration of a chromium plating layer due to
heating depends on the thickness of the chromium plating layer.
1. STRUCTURE OF THE ENGINE PART
(1) Basic Structure
[0050] Hereinafter, with reference to FIG. 1, the structure of an
engine part according to the present invention will be described.
As shown in FIG. 1, the engine part according to the present
invention includes a metal substrate 1, a chromium plating layer 3
covering at least a portion of the surface of the metal substrate
1, and an underlying plating layer 2 provided between the metal
substrate 1 and the chromium plating layer 3. Hereinafter, these
constituent elements will be specifically described.
(i) Metal Substrate
[0051] The metal substrate 1, which has a mechanical strength
suitable to its purpose and a necessary level of anticorrosiveness
and the like, can be formed from a material which is usually used
for an engine part. A typical example would be an Fe-type material.
Otherwise, the metal substrate 1 may be formed from any non-Fe
material, such as an Al-type material, a Zn-type material, an
Mg-type material, or a Ti-type material.
[0052] Examples of Fe-type materials include Fe or steels whose
main component is Fe, such as: steel tubes for machine structural
purposes (e.g., carbon steel tubes for machine structural purposes
(STKM) or alloy steels for machine structural purposes); stainless
steel (e.g., ferrite-type stainless steel, austenite-type stainless
steel, or austenite/ferrite-type stainless steel); and mild steel
(e.g., SPCC or SPHC). Examples of Al-type materials include: Al;
and Al alloys such as Al--Si alloys or Al--Si--Mg--type alloys.
Examples of Zn-type materials include: Zn; Zn-plated steel plates,
on which Zn plating is provided; and Zn alloy-plated steel plates,
on which Zn alloy plating whose main component is Zn and which
includes alloying elements such as Ni, Co, Cr, or Al is provided.
Examples of Mg-type materials include Mg--Al type alloys and Mg--Zn
type alloys. Examples of Ti-type materials include: Ti; and Ti
alloys whose main component is Ti and which includes elements such
as Al, V, or Si.
[0053] These materials have different characteristics depending on
their types. For example, Al is light-weight and shiny, whereas Ti
is light-weight and has excellent mechanical strength. Therefore,
any appropriate material may be selected depending on the purpose,
required characteristics, and the like.
(ii) Underlying Plating Layer
[0054] The underlying plating layer 2 formed on the metal substrate
1 defines an underlying plating for the chromium plating layer 3.
From the standpoint of preventing cracks from occurring in the
chromium plating layer and allowing a chromium plating layer having
good anticorrosiveness and abrasion resistance to stably grow, the
underlying plating layer 2 does not need to be provided. In the
case of chromium plating, an underlying plating layer is usually
formed under the chromium plating layer with the purpose of
providing enhanced contact with the base substrate, for example.
Therefore, it is preferable that the underlying plating layer 2
exhibits good contact with various metal substrates, as well as
good contact with the chromium plating layer. There might be other
preferable characteristics, e.g., anticorrosiveness.
[0055] The metal composing the underlying plating layer 2 used for
the present invention can be defined in terms of a relationship
with the hardness (Vickers hardness) of chromium, which is used for
forming the chromium plating layer. Specifically, it is preferable
that the metal composing the underlying plating layer 2 is formed
of a metal having a hardness lower than the hardness of chromium
(about 350 to 1200 Hv). The presence of a layer composed of a
low-hardness metal interposed between the chromium plating layer
and the metal substrate reduces the stress applied to the chromium
plating layer due to heat cycles. As a result, the generation of
cracks and the like is prevented, and a chromium plating layer
having good surface characteristics can be obtained. Examples of
metals having a lower hardness than that of chromium include nickel
(Ni) (hardness: about 150 to 350 Hv), Copper (Cu) (hardness: about
40 to 250 Hv), Tin (Sn) (hardness: about 20 to 200 Hv), and lead
(Pb) (whose hardness is immeasurable).
[0056] As an underlying plating layer including such metals, for
example, a layer composed of nickel plating, copper plating, tin
plating, lead plating, zinc-nickel plating, or the like is used. A
single such plating layer may be formed, or two or more kinds may
be combined to result in there being a plurality of underlying
plating layers. Alternatively, a plurality of underlying plating
layers of the same kind but containing different types of
additives, etc., may be formed. A typical underlying plating layer
to be used as the underlying treatment for a chromium plating layer
is nickel plating, which further enhances anticorrosiveness,
luster, and the like.
[0057] The underlying plating layer 2 includes elements which
compose various additives. With the purpose of enhancing the luster
of the chromium plating layer, such additives are added in a
plating solution for forming the underlying plating layer 2.
Specifically, a primary brightener (a non-butyne-type brightener,
e.g., saccharin sodium, naphthalene-1,3,6-trisodium trisulfonate,
or benzene sulfonic acid), a secondary brightener (e.g.,
2-butyl-1,4-diol, sodium allylsulfonate), and the like are used.
Any such additive includes C and/or S as its component elements.
Although depending on the type of underlying plating layer and the
type of additive, a total of about 0.001 to 1.0 mass % of C and/or
S is included in the underlying plating layer. As will be
specifically described later, these elements will thicken to about
0.1 to 10 mass % responsive to heating, thus causing surface
discoloration of the chromium plating layer. Furthermore, in the
case where the underlying plating layer 2 is a nickel plating
layer, the Ni contained in the nickel plating layer also
substantially affects surface discoloration.
[0058] Hereinafter, nickel plating will be specifically described
as a typical example of an underlying plating layer. Nickel plating
is generally classified into lusterless nickel plating, semigloss
nickel plating, and gloss nickel plating, mainly depending on the
type of brightener added in the plating solution, whether such an
addition is made or not, etc. These types of plating can be
appropriately combined in accordance with the required
characteristics, purpose, and the like, whereby the desired
exterior appearance can be obtained.
[0059] Among others, gloss nickel plating is obtained by adding a
brightener such as saccharin or benzene sulfonic acid to the
plating solution. Gloss nickel plating provides an excellent
surface leveling (planarize) action and exhibits good contact with
the chromium plating layer, and therefore is widely used as an
underlying layer to be formed directly under a chromium plating
layer. A brightener for gloss nickel plating is usually used in an
amount such that a total of about 0.001 to 1.0 mass % of at least
one of C and S is included in the plating layer. Anticorrosiveness
tends to decrease as the S content increases.
[0060] Lusterless nickel plating differs from gloss nickel plating
in that no brightener is contained in the plating solution.
Although providing less luster than gloss nickel plating,
lusterless nickel plating is excellent in terms of throwing power
(adhesion), anticorrosiveness, discoloration prevention, etc., of
the plating layer.
[0061] Semigloss nickel plating is obtained by adding a
non-coumarin-type semi-brightener to the plating solution. Unlike
the aforementioned brighteners, semi-brighteners have little C
and/or S content. Therefore, it provides better anticorrosiveness
but poorer luster than those provided by gloss nickel plating.
[0062] In general, when a number of nickel plating layers
containing different amounts of S are formed on top of one another,
potential differences will emerge between the plating layers, so
that coatings having greater S contents are corroded first. By
utilizing this property, it is common to use two or more layers of
nickel plating to provide improved anticorrosiveness. For example,
in the case of a two-layer plating obtained by sequentially forming
a semigloss nickel plating layer and a gloss nickel plating layer
upon a base metal, the gloss plating layer is corroded first
because of having a lower potential than that of the semigloss
plating layer. As a result, the base metal under the semigloss
plating layer is protected without being corroded. In order to
further enhance anticorrosiveness in the aforementioned two-layer
plating structure, a tri-nickel plating layer (which is a type of
gloss nickel plating layer) having a large S content may be formed
between the semigloss nickel plating layer and the gloss nickel
plating layer, thus obtaining a three-layer plating structure. The
S contained in the tri-nickel plating layer is most often supplied
from an additive other than a brightener. In this case, the
uppermost gloss nickel plating layer is corroded first, and then
the intermediate tri-nickel plating layer is corroded, whereby both
the semigloss nickel plating layer and the base metal are
protected.
[0063] In order to effectively utilize such actions of nickel
plating, the nickel plating layer(s) preferably has a total
thickness of about 10 to 30 .mu.m, and more preferably about no
less than 15 .mu.m and no more than 25 .mu.m.
[0064] In the case where an underlying plating layer other than a
nickel plating layer is to be formed, the underlying plating layer
is preferably controlled to a thickness of about 10 to 30
.mu.m.
(iii) Chromium Plating Layer
[0065] On the underlying plating layer 2, the chromium plating
layer 3 is formed. As will be specifically described later, the
chromium plating layer 3 is a decorative chromium plating layer
which is formed by electroplating using a trivalent chromium
plating solution.
[0066] Whether a chromium plating layer has been formed by using a
trivalent chromium plating solution or by using a hexavalent
chromium plating solution can be determined by measuring the
crystal state of the chromium plating layer. Specifically, the
determination can be easily made by subjecting the chromium plating
layer to an X-ray diffraction analysis. FIGS. 2(a) and (b) each
show X-ray diffraction analysis results of a chromium plating
layer. The detailed measurement method was as follows.
[0067] Analysis equipment: X-ray diffraction apparatus RAD-3C type
(Rigaku Corporation).
[0068] Measurement conditions: A Cu anticathode was used, and power
was supplied at 40 kV/40 mA.
[0069] X-ray diffraction results of a chromium plating layer which
has been formed by using a hexavalent chromium plating solution are
shown in FIG. 2(b). Near diffraction angles of about 40.theta. to
50.theta., a very large diffraction peak of about 1200 cps is
observed, and large diffraction peaks of about 200 cps are observed
near diffraction angles of about 65.theta. and about 83.theta.. In
the ascending order of diffraction angles, these peaks correspond
to (111) orientation crystal, (200) orientation crystal, and (211)
orientation crystal, respectively.
[0070] On the other hand, X-ray diffraction results of a chromium
plating layer which has been formed by using a trivalent chromium
plating solution are shown in FIG. 2(a). Only a small diffraction
peak of about 100 cps is observed near diffraction angles of about
40 to 50.theta., this corresponding to (111) orientation crystal. A
value obtained by dividing the half-width of the peak corresponding
to (111) orientation crystal by the peak intensity (half-width/peak
height) is about 0.6 rad/cps, which is much broader than the value
of (111) orientation crystal (about 7.9.times.10.sup.-4 rad/cps)
which is observed when using hexavalent chromium.
[0071] From these figures, it can be seen that the chromium plating
layer obtained by using hexavalent chromium has a crystal structure
composed of polycrystals, whereas the chromium plating layer
obtained by using trivalent chromium plating solution has a
substantially amorphous structure. The determination as to whether
a plating layer has a crystalline structure or an amorphous
structure can be made based on, for example, whether a diffraction
peak associated with a (half-width/peak height) value of about
0.001 rad/cps or less is observed or not near diffraction angles of
about 40 to 500.
[0072] The amount of boron contained in the chromium plating layer
3 is no less than 0.05 mass % and no more than 0.3 mass %. If the
boron content is more than 0.3 mass %, cracks may occur in the
chromium plating layer 3 responsive to heating to 400.degree. C. or
above. On the other hand, if the boron content is less than 0.05%,
the anticorrosiveness and abrasion resistance of the chromium
plating layer 3 become lowered. Moreover, stability of plating
growth when forming the chromium plating layer 3 becomes lower. The
boron content is preferably no less than 0.05 mass % and no more
than 0.2 mass %, and more preferably no less than 0.05 mass % and
no more than 0.1 mass %. Crack generation can be prevented with
more certainty and better anticorrosiveness and abrasion resistance
can be obtained as there is less boron content.
[0073] Herein, "stability of plating growth" means that the
throwing power (adhesion) of a chromium plating layer in a plating
process is constant over time. Specifically, it means that, when a
Hull cell test is conducted, poor exterior appearance, e.g.,
inadequate plating thickness, scorching, or streaks, is prevented
over a large area of current density, or that a plating layer with
a predetermined thickness is formed also on portions of the
material to be plated that are difficult to be plated (e.g., a
surface to be plated that is located opposite from an
electrode).
[0074] Moreover, "plating anticorrosiveness" means an
anticorrosiveness that is imparted through a plating process.
Specifically, if a rating number of 7.0 or more is satisfied when
performing a CASS test (which is a plating anticorrosiveness test
method defined in JIS H8502) by using a sample that has experienced
a plating process, it is said that "there is good plating
anticorrosiveness".
[0075] "Plating abrasion resistance" means an abrasion resistance
(hardness) that is imparted through a plating process.
Specifically, if the Vickers hardness is in the range of 350Hv0.1
(test power: 0.9807N) or more when a Vickers hardness as defined in
JIS Z 2244 is measured by using a sample that has experienced a
plating process, it is said that "there is good plating abrasion
resistance".
[0076] Thus, since the chromium plating layer 3 contains boron, a
chromium plating layer having good anticorrosiveness and abrasion
resistance can be obtained. Moreover, the chromium plating layer
can be allowed to stably grow. However, boron acts to harden the
plating layer, and, if its content exceeds the aforementioned
range, the hardness of the chromium plating layer will increase
especially when heated to a high temperature (esp., 400.degree. C.
or more), so that the stress on the chromium plating layer due to
heat cycles will be increased. As a result, cracks and the like are
likely to occur on the surface. If cracks occur, rust is likely to
occur from the cracks. If a large number of cracks occur, coating
may peel, etc., thus degrading the exterior appearance. Moreover,
the color tone will be degraded when the content exceeds the
aforementioned range.
[0077] As described above, since boron causes the chromium plating
layer to be hardened, cracks become likely to occur when a thick
chromium plating layer containing boron is formed. Therefore, the
thickness of the chromium plating layer 3 is preferably 0.7 .mu.m
or less. If the chromium plating layer 3 is thicker than 0.7 .mu.m,
it becomes difficult to form a chromium plating layer without
cracks even if the boron content is within the aforementioned
range. In order to more certainly prevent crack generation, it is
preferable that the thickness of the chromium plating layer 3 is
0.5 .mu.m or less.
[0078] From the standpoint of preventing crack generation due to
heating, it will suffice that the boron content in the chromium
plating layer is kept at 0.3 mass % or less only in regions of the
engine part that will rise to a high temperature. However, in any
chromium plating layer which is to continuously cover the surface
of an engine part through plating, it is difficult to vary the
boron content in a portion thereof. Therefore, it is preferable
that the boron content in the chromium plating layer remains within
the aforementioned range in any region of the chromium plating
layer.
[0079] However, just because the boron content remains within the
aforementioned range in any region of the chromium plating layer,
it is not necessary for the engine part according to the present
invention to be used in an environment where its entirety is heated
to a high temperature. The present invention is suitably used for
an engine part at least a portion of which is exposed to a high
temperature.
[0080] So long as the chromium plating layer 3 has a boron content
as described above, it is possible, by using a trivalent chromium
plating solution, to form a chromium plating layer which has as
good film-forming characteristics as those of a plating layer
formed from a hexavalent chromium plating solution and which has
good thermal resistance. However, a chromium plating layer which is
formed from a trivalent chromium plating solution has a blackish
color tone as compared to a chromium plating layer which is formed
from a hexavalent chromium plating solution. The causes for the
blackish color tone are related to the concentrations of iron and
boron that are contained in the chromium plating layer. The color
tone becomes blackish particularly when the iron content increases.
Although not clearly known, it is believed that, when growing a
chromium plating layer from a trivalent chromium plating solution,
the iron within a chromium plating layer binds with various
elements within the plating solution, thus generating a deposit
containing black iron.
[0081] Therefore, in order to a silver-gray color tone similar to a
chromium plating layer which is formed from a hexavalent chromium
plating solution, the boron content is preferably 0.1 mass % or
less and the iron content is 2 mass % or less in the chromium
plating layer 3. The iron content should be as little as possible,
and is more preferably 1 mass % or less, and further more
preferably 0.5 mass % or less.
[0082] However, in the case where the engine part according to the
present invention is not to be used in a manner of being compared
against a chromium plating layer which is formed from a hexavalent
chromium plating solution, the boron content may be prescribed to
be 0.3 mass % or less and the iron content to be 7 mass % or less,
whereby a chromium plating layer with a color tone that qualifies
for chromium plating can be obtained, although its color tone may
be somewhat muddier.
[0083] The boron and iron contents in the chromium plating layer
are to be each expressed as a maximum value when the content of
each element, contained along the depth direction of the chromium
plating layer, is analyzed through GDS analysis.
[0084] In the case where the boron content is 0.1 mass % or less
and the iron content is 2 mass % or less in the chromium plating
layer 3, the chromium plating layer has a color tone such that an
L* value measured according to CIE (Commission Internationale de
l'Eclairage) 1976 is in a range from 68 to 80. The L value is to be
measured by using a spectrometric color difference meter (e.g.,
color analyzer TC-1800MK-II (Tokyo Denshoku)). This value is
similar to that of a chromium plating layer which is formed from a
hexavalent chromium plating solution. Therefore, even in a
side-by-side comparison between the chromium plating layer 3 whose
iron content is confined within the aforementioned range and a
chromium plating layer which is formed from a hexavalent chromium
plating solution, hardly any difference in color tone will be
noticed.
[0085] Since the engine part has such a structure, it is possible,
by using a trivalent chromium plating solution, to obtain a
chromium plating layer which has as good film-forming
characteristics as those of a plating layer which is formed from a
hexavalent chromium plating solution and which has good thermal
resistance. Moreover, by prescribing the boron content to be 0.1
mass % or less and the iron content to be 2 mass % or less in the
chromium plating layer, a silver-gray color tone similar to that of
a chromium plating layer which is formed from a hexavalent chromium
plating solution can be obtained.
(iv) Mechanism of Discoloration Prevention
[0086] The engine part according to the present invention can be
suitably used in parts of an engine; e.g., a cylinder, a cylinder
head, and a head cover; an exhaust pipe for guiding along exhaust
gas from the engine; and the like. However, in the case where the
engine part according to the present invention is used in a place
which is exposed to a heat of 350.degree. C. or above, the color
tone of the surface may change due to heating, so that the chromium
plating layer may appear blue-violet. In the case where the engine
part according to the present invention is used in such a place, it
is preferable that the chromium plating layer has a thickness of
0.2 .mu.m or more. Hereinafter, the reason why discoloration of the
chromium plating layer due to heating can be prevented by
controlling the thickness of the chromium plating layer will be
described with reference to FIG. 3 and FIG. 4.
[0087] FIG. 3 is a diagram schematically showing a manner in which
C and/or S gather near the interface between the chromium plating
layer and the underlying plating layer responsive to heating, thus
showing a generation mechanism of a "C--S thickened layer" or a
"C--S--Ni thickened layer" (hereinafter may simply be referred to
as a "thickened layer"), which is considered as responsible for the
surface discoloration of the chromium plating layer. FIG. 3
illustrates a typical structure according to the present invention,
in which a nickel plating layer is formed between an Fe substrate
and a chromium plating layer. The nickel plating layer is composed
of the following three layers, respectively from the Fe substrate
side: a semigloss nickel plating layer, a tri-nickel plating layer,
and a gloss nickel plating layer.
[0088] Note that, responsive to heating, various elements which
compose the additives contained in the chromium plating layer
and/or the nickel plating layer will gather near the interface
between the chromium plating layer and the nickel plating layer.
FIG. 3 only shows those elements which are considered as
contributive to the discoloration of the chromium plating layer,
i.e., the elements (at least one of C, S, and Ni) composing the
aforementioned thickened layer and elements (e.g., Fe or Cr) which
are likely to bind to these elements, while omitting any other
elements (e.g., O which gathers near the aforementioned interface
responsive to heating).
[0089] As shown in FIG. 3, mainly C or S moves from the nickel
plating layer side to the chromium plating layer side, thus
gathering at the aforementioned interface. As described earlier, C
or S is mainly the element which composes a non-butyne-type
brightener (e.g., benzene sulfonic acid) which is added to the
nickel plating solution. In particular, a large amount of S is
contained in the gloss nickel plating layer and the tri-nickel
plating layer. Therefore, near the aforementioned interface, a
"C--S thickened layer" in which a large amount of C or S has
gathered is formed. As used herein, a "C--S thickened layer" means
a layer in which at least one of C and S has gathered. Note that,
although C or S may also diffuse over from the chromium plating
layer side, such diffusion will account for a very small proportion
as compared to the diffusion from the nickel plating layer side,
and therefore is omitted from illustration.
[0090] In the case where the underlying plating layer is a nickel
plating layer, a "C--S--Ni thickened layer" containing Ni will be
formed. Similarly to C or S, Ni is also considered as contributive
to discoloration. As used herein, a "C--S--Ni thickened layer"
means a layer in which at least one of C or S and Ni has
gathered.
[0091] Although not clearly known, the reason why formation of such
a thickened layer causes surface discoloration of the chromium
plating layer may be that the Cr composing the chromium plating
layer may bind to the element (C or S, or Ni) composing the
thickened layer and change the refractive index of the chromium
plating layer, thus causing discoloration, for example. Iron is
also considered as a substance that contributes to discoloration.
In the case where the metal substrate is composed of an Fe-type
material, responsive to heating, Fe may diffuse from the Fe-type
material and become thickened in the area of the aforementioned
interface (not shown).
[0092] Surface discoloration of the chromium plating layer
associated with such a thickened layer can be prevented by forming
the thickness of the chromium plating layer to be 0.2 .mu.m or
more. Hereinafter, the reason thereof will be described with
reference to FIGS. 4(a) and (b).
[0093] FIG. 4(b) is a diagram schematically showing a portion of a
conventional Cr-nickel plating layer. As shown in FIG. 4(b), the
thickness of the conventional chromium plating layer is as small as
about 0.1 .mu.m or less. Therefore, incident light will be
transmitted to the vicinity of the interface between the chromium
plating layer and the nickel plating layer. Consequently, a portion
of the incident light will be absorbed by the thickened layer which
is generated near the interface, thus making the discoloration due
to heating more conspicuous.
[0094] On the other hand, by prescribing the thickness of the
chromium plating layer to be 0.2 .mu.m or more, as shown in FIG.
4(a), the incident light will for the most part be reflected near
the surface of the chromium plating layer, instead of being
transmitted over to the vicinity of the interface between the
chromium plating layer and the nickel plating layer. Therefore,
only the interference colors caused by the usually-occurring oxide
coating on the outermost surface of a chromium plating layer are
observed, and the influence due to the thickened layer can be
minimized.
[0095] In order to allow such action to be effectively exhibited,
the thickness of the chromium plating layer is set to be 0.2 .mu.m
or more. From the perspective of prevention of thermal
discoloration due to heating, the chromium plating layer should be
as thick as possible. Preferably, the thickness of the chromium
plating layer is 0.3 .mu.m or more, and more preferably 0.4 .mu.m
or more.
[0096] Thus, in order to prevent crack generation and discoloration
due to heating, the thickness of the chromium plating layer is
preferably no less than 0.2 .mu.m and no more than 0.7 .mu.m, and
more preferably no less than 0.3 .mu.m and no more than 0.5 .mu.m.
When the thickness of the chromium plating layer is no less than
0.4 .mu.m and no more than 0.5 .mu.m, crack generation can be
almost surely prevented, and discoloration due to heating can be
surely prevented.
[0097] The thickness of the chromium plating layer 3 may be
measured through observation with an optical microscope
(magnification: .times.400). Specifically, a cross section along
the thickness direction of the plating layer is mirror-polished and
etched. As a result, the chromium plating layer becomes clearly
distinguishable from the underlying plating layer. Note that the
chromium plating layer will have a surface roughness Ra of not more
than about 0.01 .mu.m, and therefore the influence of the surface
roughness Ra on the thickness of the chromium plating layer is
virtually negligible. Since the thickness of the chromium plating
layer will slightly vary depending on the measurement site, a total
of three measurements are to be taken in different measurement
sites within a given a field of observation, and an average value
thereof is to be defined as the "thickness of the chromium plating
layer".
[0098] Another means for preventing discoloration due to the
formation of a thickened layer might be to reduce the C or S
content, although this method would not be practical. For example,
in order to reduce the C or S content, the amount of brightener to
be contained in the underlying plating layer would mainly have to
be reduced. However, this would result in a reduced luster, and
thus considerably impair the design of the engine part. When good
design is considered as an important factor as in the case of the
present invention, any deterioration in design because of not using
the brightener must definitely be avoided.
[0099] In order to prevent discoloration due to heating, it will
suffice if a portion of the chromium plating layer 3 which is to be
heated to a temperature of 350.degree. C. or above satisfies the
aforementioned range of thickness. It is not necessary that the
entire area of the chromium plating layer 3 formed on the metal
substrate 1 satisfy the aforementioned range of thickness. In
regions which will only rise to a temperature of 350.degree. C. or
less, the thickness of the chromium plating layer 3 may be 0.2
.mu.m or less. In general, a chromium plating layer is tinted from
silver-gray to yellow to gold responsive to heating, and at a high
temperature of about 350 to 500.degree. C., becomes discolored from
gold to violet. Such changes in color tone will not be uniformly
observed over the entire area in which the chromium plating layer
is formed, but will be most noticeable in portions which are likely
to be exposed to a high-temperature exhaust gas.
[0100] Therefore, in order to prevent discoloration from gold to
violet, it will be sufficient to control the thickness of the
portion at which discoloration due to heating is most likely to
occur, i.e., the region of the chromium plating layer that is
exposed to a temperature of 350.degree. C. or above, to be in the
aforementioned range.
[0101] Examples of the "region which is to be heated to a
temperature of 350.degree. C. or above" may include a portion of an
engine part composing an engine, e.g., a cylinder, a cylinder head,
a head cover, as well as a portion of an exhaust pipe defining a
channel for guiding the exhaust gas discharged from the engine, or
a cover of the exhaust pipe. As used herein, the exhaust pipe may
be an exhaust pipe which directly guides exhaust gas, or an exhaust
pipe (a double tube) which is indirectly heated by exhaust gas. An
exhaust pipe includes a manifold section for guiding along the
exhaust gas from each cylinder, a catalytic apparatus accommodating
section covering a catalytic apparatus, a muffler, and the
like.
(2) Specific Structure and Application to Motorcycle
[0102] With reference to FIG. 5, a specific structure of the engine
part according to the present invention will be described. FIG. 5
shows a motorcycle 100 incorporating an exhaust pipe which is an
engine part according to the present invention. As shown in FIG. 5,
the motorcycle 100 includes an engine 30 which is composed of a
4-cycle internal combustion engine, and an exhaust pipe 4 for
guiding the exhaust gas generated in the engine 30 so as to be
discharged at the rear portion of the body. The exhaust pipe 4
includes an exhaust pipe congregation section 4a, which is
connected to the engine 30 and constitutes a substantially bent
exhaust path for allowing the exhaust gas having been discharged at
the front of the engine 30 to be guided toward the rear, and a
muffler 4b. The exhaust pipe congregation section 4a may be
integrally formed of a single part, or composed of a plurality of
parts which are connected with one another. In the present
embodiment, the exhaust pipe 4 is entirely exposed so as to appear
on the exterior of the motorcycle 100, thus constituting a part of
the design of the motorcycle 100 as a whole. As will be
specifically described below, the effect of the present invention,
i.e., cracks or rust does not occur in the chrome plating layer of
the exhaust pipe 4, discoloration is prevented, and the fresh
exterior appearance of a brand-new motorcycle is retained for long
periods of time, is more clearly enhanced in the exterior
appearance in the case where the entire exhaust pipe 4 is exposed.
However, as long as the exhaust pipe 4 at least partially appears
on the exterior, a part of the exhaust pipe 4 may be covered by a
cowl or a protector, depending on the design of the motorcycle.
Moreover, the shape of the motorcycle for which the exhaust pipe is
used is not limited to that shown in FIG. 5. For example, the
exhaust pipe according to the present invention may be adopted in a
motorcycle having a structure as shown in FIG. 9.
[0103] Next, with reference to FIGS. 6(a), (b), and (c), the
"region of the metal substrate surface that is to be heated to a
temperature of 350.degree. C. or above", on which the chromium
plating layer preferably has a thickness of 0.2 .mu.m or more, will
be described. Each of these figures is a cross-sectional view
showing a part of the exhaust pipe 4.
[0104] FIG. 6(a) shows an exhaust pipe congregation section 4a of
the exhaust pipe 4, which is directly connected to an engine. As
shown in FIG. 6(a), the exhaust pipe congregation section 4a, which
is connected to an engine (not shown), includes a metal tube 5
defining a passage 6 in which exhaust gas travels through, and a
plating layer 10 covering the outer side surface of the metal tube
5. The metal tube 5 includes a bent portion 9. The bent portion 9
is a portion at which the passage 6 is bent, or a portion at which
the longitudinal direction of the passage 6 changes.
[0105] As described above, the plating layer 10 includes an
underlying plating layer and a chromium plating layer. The metal
tube 5 simply needs to define the passage 6, and may have a
double-tube structure composed of an inner tube defining the
passage 6 and an outer tube covering the inner tube from the
outside.
[0106] The exhaust gas that comes through the exhaust pipe
congregation section 4a, which is directly connected to the engine
(not shown), rapidly travels through the passage 6, and therefore
collides against the metal tube 5 at the bent portion 9. The
exhaust gas collides especially intensely against an inner side
surface 9b of the metal tube 5 that is located at the convex
surface portion 9a, which in itself is formed as a result of the
bending. Therefore, the outer convex surface portion 9a is heated
by the high-temperature exhaust gas to a temperature of about
350.degree. C. or more (e.g., about 400 to 500.degree. C.).
[0107] In the case where the metal tube 5 has a double-tube
structure, a single-tube structure is often adopted for a link
section 21 at which another exhaust pipe member 23 is to be
connected. The reason is that, if a double-tube structure were
adopted at the link section 21, the metal tube 5 might deform or be
destroyed due to a difference in thermal expansion between the
outer tube and the inner tube during the welding to the other
exhaust pipe member 23. If the metal tube 5 is of such a structure,
the link section 21 will be heated to a temperature of about
350.degree. C. or more (e.g., about 400 to 500.degree. C.) as the
high-temperature exhaust gas comes in direct contact with the inner
side surface of the link section 21.
[0108] FIG. 6(b) schematically shows a cross section of a catalyst
accommodating section 22 of the exhaust pipe 4, in which a
catalytic apparatus 8 is accommodated. The catalytic apparatus 8
which is provided within the catalyst accommodating section 22,
decomposes at least one component contained in the exhaust gas when
the exhaust gas travel therethrough. Since the catalytic apparatus
8 is heated responsive to the aforementioned decomposition, the
catalyst accommodating section 22 is heated to a temperature of
about 350.degree. C. or more (e.g., about 400 to 500.degree.
C.).
[0109] In the case where the engine 30 (FIG. 5) has a plurality of
cylinders, the exhaust pipe 4 may include a manifold section for
combining the exhaust gas generated in the respective cylinders so
as to allow such exhaust gas to be guided to the rear portion of
the motorcycle 100. FIG. 6(c) shows an exhaust pipe 4 having a
manifold section 15 at which branch pipes 4d and 4e (each of which
is connected to a cylinder) come together, such that discharge
takes place through a unified exhaust pipe member 4f. At such a
manifold section 15 of the exhaust pipe congregation section 4a,
exhaust gas comes together through the plurality of branch pipes 4d
and 4e, whereby the flow rate of the exhaust gas is increased, and
the flow paths are deflected. As a result, the exhaust gas collides
against the inner side surface of the manifold section 15.
Therefore, the manifold section 15 is heated by the exhaust gas to
a temperature of about 350.degree. C. or more (e.g., about 400 to
500.degree. C.).
[0110] In the motorcycle of the present embodiment, those portions
of the exhaust pipe which are exemplified as being heated to a high
temperature in FIGS. 6(a), (b), and (c) are covered from the
outside by a chromium plating layer of 0.2 .mu.m or more. As a
result, even when exposed to a high-temperature exhaust gas,
discoloration of the chromium plating layer due to the heating can
be prevented.
[0111] In the case of using a chromium plating layer which is
formed by using trivalent chromium, the Fe content in the chromium
plating layer is reduced. As a result, an excellent color tone
similar to that which is obtained by using hexavalent chromium can
be expressed.
[0112] The present invention also encompasses a transportation
apparatus incorporating the above-described engine part(s).
Examples of transportation apparatus include a vehicle having an
engine (e.g., a motorcycle or an all-climate four-wheeled vehicle)
and a transportation apparatus having an engine (e.g., a marine
vessel or an airplane).
2. METHOD OF PRODUCING THE ENGINE PART
[0113] Next, a method for producing an engine part according to the
present invention will be described. The engine part according to
the present invention is produced by sequentially forming an
underlying plating layer and a chromium plating layer on a metal
substrate having a predetermined shape. Hereinafter, the production
method will be specifically described.
[0114] First, in order to degrease and clean the surface of a metal
substrate, a metal substrate is immersed in a bath such as a water
rinse bath, ultrasonic wave alkaline degreasing bath, electrolytic
degreasing bath, or acid treatment activation bath for a
predetermined period of time. As a result of this, the surface of
the metal substrate is sufficiently degreased, thus making it easy
to form an underlying plating layer and a chromium plating layer on
the metal surface.
[0115] Next, by using the metal substrate thus washed, an
electroplating is conducted to sequentially form an underlying
plating layer and a chromium plating layer at least on the outer
surface of the metal substrate. Both the underlying plating layer
and the chromium plating layer are to be formed through
electroplating, based on the same principle. Therefore, in the
following description, only the step of forming the chromium
plating layer will be specifically described by using the plating
apparatus shown in FIG. 7, whereas the step of forming the
underlying plating layer will be described without referring to any
figures.
[0116] The underlying plating layer is formed by immersing the
aforementioned metal substrate in a plating trough containing a
solution of a metal from which a plating layer is to be formed, and
applying power until reaching a desired thickness. For example, in
the case where three nickel plating layers, namely a semigloss
nickel plating layer, a tri-nickel plating layer, and a gloss
nickel plating layer, are to be formed as the underlying plating
layer, the metal substrate which has been washed in the
aforementioned manner is immersed in a semigloss nickel plating
solution, a tri-nickel plating solution, and a gloss nickel plating
solution, and power is applied until the respective desired plating
layers are obtained. The specific plating conditions will depend on
the metal substrate to be used, plating solution composition,
purpose, and the like. The conditions which are usually used for
Ni-chromium plating can be selected as appropriate. For example, in
the case where a semigloss nickel plating layer (5 to 15 .mu.m), a
tri-nickel plating layer (1 to 2 .mu.m), and a gloss nickel plating
layer (5 to 15 .mu.m) are to be sequentially formed on an Fe
substrate, it is preferable to control the temperature of the
plating solution to be about 40 to 65.degree. C., and the pH of the
plating solution to be about 2 to 5. The plating time is preferably
about 10 to 20 minutes for semigloss nickel plating as well as
gloss nickel plating, and about 1 to 5 minutes for tri-nickel
plating. Note that a strike nickel layer may be further provided
between the underlying plating layer and the metal substrate.
[0117] Next, a chromium plating layer is formed on the metal
substrate on which the underlying plating layer has been formed. As
shown in FIG. 7, a chromium plating apparatus 20 includes a
chromium plating trough 11 in which to perform chromium plating, a
pump 12 for pumping up a plating solution which has been introduced
to the chromium plating trough 11, a percolator 13 for removing
impurities which are suspended in the plating solution, an
adjustment valve 14 for adjusting the flow rate of the plating
solution, and a flowmeter 15 for monitoring the flow rate of the
plating solution. On the downstream end of the chromium plating
apparatus 20, an ion exchange apparatus 16 for removing the metal
ions (such as Fe) contained in the plating solution is provided.
The chromium plating apparatus 20 and the ion exchange apparatus 16
are connected to each other via a metal tube (not shown).
[0118] The chromium plating trough 11 is filled with a trivalent
chromium plating solution. As a trivalent chromium ion source, the
plating solution contains basic chromium sulfate which converts to
30 to 40 g/l of chromium.
[0119] In order for the amounts of boron and iron contained in the
chromium plating layer to fall within the aforementioned ranges, it
is necessary to ensure that the concentrations of boron and iron
within the plating solution that is used fall within predetermined
ranges. Specifically, the trivalent chromium plating solution has a
boric acid content in a range from 1 to 5 g/l as converted to a
boron amount, and an Fe content of 0.5 mg/l or less.
[0120] The boric acid content is reduced to about 1/15 to as
compared to the amount of boron that is added to a typical
conventional trivalent chromium plating solution (about 6 to 15
g/l). In recent years, regulations on the amounts of boron to be
discharged to the environment have been introduced. Using a plating
solution with a low boron concentration also complies with such
environmental regulations.
[0121] Moreover, a typical conventional trivalent chromium plating
solution contains about 0.0001 to 0.0003 mass % of ferrous sulfate
as an additive, in order to enhance the throwing power and the like
of the plating layer. Therefore, when using a conventional
trivalent chromium plating solution, the chromium plating layer
contains about 2 to 20 mass % of Fe. However, it is preferable that
the trivalent chromium plating solution to be used in the present
invention does not contain such an additive, and the Fe content is
restricted to 0.5 mg/l or less.
[0122] Boric acid is used to adjust pH and to allow the chromium
plating layer to acquire a silver-gray color tone. Therefore, in
order to reduce the boric acid amount and maintain appropriate pH,
the trivalent chromium plating solution to be used in the present
invention contains citric acid or a citrate compound, as converted
to a citric acid amount in a range from 5 to 30 g/l. As citrate
compounds, citrates such as potassium citrate or metal citrate
compounds such as nickel citrate can be used. In order to obtain a
chromium plating layer with good color tone, it is more preferable
to use citric acid. The amount of citric acid to be added is
preferably no less than 10 g/l and no more than 25 g/l, and more
preferably no less than 20 g/l and no more than 25 g/l.
[0123] The chromium plating is to be carried out via
electroplating. The chromium plating trough 11 is filled with the
aforementioned trivalent chromium plating solution, and a metal
substrate 17 which is to receive chromium plating is used as a
cathode. Since chromium plating is to be performed while supplying
chromium ions from the plating solution, an insoluble anode 18
which does not dissolve in a chromium plating solution is used as
an anode.
[0124] Next, the DC power source 19 is connected between the
electrodes, and power is supplied therefrom. The chromium ions
contained in the chromium plating solution move toward the cathode
side, i.e., the metal substrate 17, where the ions are reduced to
metal Cr and deposit.
[0125] In order to form a chromium plating layer of 0.2 .mu.m or
more in a region which is to be heated to a temperature of
350.degree. C. or above, the plating is preferably performed while
placing the metal substrate in a position which will allow a
chromium plating layer having a desired thickness to be formed. In
particular, in the case where a metal substrate such as a curved
exhaust pipe is to be plated, it is preferable to locate the metal
substrate so that the distance between the electrode and the
material to be plated (metal substrate) is as short as
possible.
[0126] For example, as shown in FIG. 8(a), if the metal substrate
17 is arranged so that the distance between the convex portion 9a
of the bent portion of the metal substrate 17 and the electrode 18
becomes smallest, a Cr layer can be efficiently formed at the
convex portion 9a, which will be heated to a high temperature.
[0127] On the other hand, as shown in FIG. 8(b), if the metal
substrate 17 is arranged so that a long distance exists between the
convex portion 9a of the metal substrate 17 and the electrode 18, a
chromium plating layer is likely to be formed at the concave
portion opposite from the convex portion 9a, and not at the convex
portion 9a itself, thus resulting in a poor plating efficiency.
[0128] Otherwise, by controlling the amount of charge
(current.times.time) traveling through the electrode surface,
current density, or the like, for example, the thickness of the
chromium plating layer can be controlled to be within a
predetermined range. In particular, the electrode positioning may
be controlled, or an auxiliary electrode may be attached to control
the current density or the like, for example, thus making it easier
to form a predetermined chromium plating layer in regions which are
to be heated to a temperature of 350.degree. C. or above. In the
specific controlling method, appropriate conditions may be selected
according to the type and shape of the metal substrate which is
used, the constitution of the plating solution, the thickness of
the chromium plating layer, or the like. Especially in the case
where no ferrous sulfate is contained in the plating solution, the
throwing power of a plating layer will generally worsen, and
therefore it is necessary to ensure a uniform current density by
optimizing the electrode location, for example.
[0129] The boron contents in the chromium plating layer to be
formed depends not only on the boron concentration in the plating
solution but also on the temperature of the plating solution,
plating time, stirring rate of the plating solution, etc., and in
particular on the temperature of the plating solution. Generally
speaking, the boron content in the chromium plating layer increases
as the temperature of the plating solution increases. Therefore, in
order to keep the boron amount in the chromium plating layer within
the aforementioned range, it is preferable to control the
temperature of the plating solution to be in a range from
25.degree. C. to 30.degree. C.
[0130] In order to ensure that the iron content in the chromium
plating layer falls in the aforementioned range, it is necessary to
pay attention to the iron which elutes from the metal substrate
into the plating solution. Moreover, even if the metal substrate
does not include iron as its main component, Fe may inevitably
gather at the chromium plating layer in some cases. As a result,
even if ferrous sulfate is not added to the plating solution, the
iron concentration within the plating solution may increase so that
iron may be contained in the chromium plating layer.
[0131] In order to eliminate such possibilities, it is necessary to
regularly monitor the iron concentration in the plating solution
during plating, and remove the iron when the iron concentration
reaches a predetermined value or more. The iron ions which stray
into the plating solution are to be removed by using the ion
exchange apparatus 16, which includes a cation exchange resin. The
cation exchange resin to be used for the present invention may be
any resin that permits easy exchange with divalent metal cations
such as Fe, without any particular limitations.
[0132] The specific removal method may be as follows. First, during
the plating, the plating solution is regularly pumped up from the
plating trough 11 by using the pump 12, and suspended matter is
removed by using the percolator 13. Next, the plating solution from
which the suspended matter has been removed is introduced into the
ion exchange apparatus 16, while adjusting the flow rate via the
adjustment valve 14, and metal cations such as Fe ions are removed
by using the cation exchange resin. The flow rate of the plating
solution is monitored with the flowmeter 15. The plating solution
which has been processed by the ion exchange apparatus 16 is
regularly collected in order to check for Fe concentration. In
order to reduce the Fe concentration within the chromium plating
layer to 2 mass % or less as in the present invention, it is
necessary to control the Fe concentration in the plating solution
to be about 0.0001 mass % or less. Thus, removal by the ion
exchange apparatus 16 is performed until the aforementioned range
is satisfied.
[0133] The plating solution (recovered plating solution) from which
Fe ions are thus removed exits the outlet of the ion exchange
apparatus 16, and is led through the tube path 24, thus to be
circulated to the plating trough 11. The recovered plating solution
may be stored in an appropriate storage container (not shown), for
example.
[0134] Note that a technique of removing metal cations within a
plating solution by using the ion exchange apparatus 16 is
specifically described in Patent Document 4, for example, and such
a technique is applicable to the method of the present invention.
Moreover, various alterations thereof have also been proposed
(e.g., Patent Document 5), which are also applicable to the method
of the present invention.
[0135] The chromium plating layer after the plating has a surface
roughness (Ra) of about 1 .mu.m or less, and preferably a surface
roughness of 0.2 .mu.m or less. Therefore, after the plating, the
chromium plating layer has sufficient luster, without the need to
particularly finish up the surface.
3. EXPERIMENTAL EXAMPLE
Experimental Example 1
[0136] In this Experimental Example, the following experiment was
performed in order to prove that stability of plating growth is
ensured by setting the lower limit of the boron content in the
chromium plating layer to be 0.05 mass %.
[0137] First, metal tubes composed of STKM were prepared, and a Ni
plating layer composed of a semigloss Ni plating layer, a tri-Ni
plating layer, and a gloss Ni plating layer was formed by the
following method. The compositions of the plating solutions used
for forming these plating layers are shown in Table 1. Note that
the C and/or S contained in the tri-Ni plating solution was
supplied from an additive other than a brightener.
[0138] Semigloss Ni plating layer (thickness: about 5 to 15
.mu.m)
[0139] plating conditions: power was supplied at 10 to 12V (volts),
1800 to 2800 A (amperes).
[0140] Tri-Ni plating layer (thickness: about 1 to 5 .mu.m)
[0141] plating conditions: power was supplied at 3 to 3.5V (volts),
20 to 40 A (amperes).
[0142] Gloss Ni plating layer (thickness: about 5 to 15 .mu.m)
[0143] plating conditions: power was supplied at 10 to 12V (volts),
1800 to 2800 A (amperes).
TABLE-US-00001 TABLE 1 Semigloss Gloss Component Nickel Tri-Nickel
Nickel Nickel Sulfate 200-260 180-230 200-400 (g/l) Nickel Chloride
30-60 30-60 30-60 (g/l) Boric Acid 35-60 35-60 35-60 (g/l)
Brightener Added None Added Amounts of 0.0001 0.1-0.2 0.05 carbon
and sulfur (%) pH 4-4.4 2-2.4 4-4.4
[0144] Next, by using a chromium plating apparatus having an ion
exchange apparatus as shown in FIG. 6, a chromium plating layer was
formed on the underlying plating layer (the chromium plating layer
had a thickness of 0.3 .mu.m). As the chromium plating solution,
four types of trivalent chromium plating solutions as described in
Table 2 were used (Nos. 1 to 4). The components of the trivalent
chromium plating solutions of Sample 1 to Sample 4 were identical
except for different ferrous sulfate and boric acid contents.
Specifically, each of these trivalent chromium plating solutions
contained citric acid, and ferrous sulfate was added in amounts of
0 (Sample 1), 2.5 mg/l (Sample 2), mg/l (Sample 3), and 10 mg/l
(Sample 4); or, as converted to Fe amounts, 0 (Sample 1), 0.05 mg/l
(Sample 2), 0.1 mg/l (Sample 3), and 0.3 mg/l (Sample 4). On the
other hand, boric acid was added in amounts of 5 g/l (Sample 1), 5
g/l (Sample 2), 30 g/l (Sample 2), and 60 g/l (Sample 3).
TABLE-US-00002 TABLE 2 Sample 1 Sample 2 Sample 3 Sample 4 (Present
(Present (Comparative (Comparative Component Invention) Invention)
Example) Example) Basic Chromium Sulfate 95-115 95-115 95-115
95-115 Cr(OH)SO.sub.4 (g/l) Boric Acid 5 5 30 60 H.sub.3BO.sub.3
(g/l) Citric Acid* 30 30 30 30 Ammonium Formate 200 200 200 200
HCOONH.sub.4 (g/l) Surfactant**(mg/l) 6.5 6.5 6.5 6.5 Ferrous
Sulfate 0 2.5 5 10 FeSO.sub.4.cndot.7H.sub.2O (mg/l) (Note)
*HOOCCH.sub.2C(OH) (COOH)CH.sub.2COOH **Sodium Sulfosuccinate
2-Ethylhexyl Sodium Sulfate
[0145] Note that, in order to adjust the boron content in the
plating layer in various ranges, the temperature of the plating
solution was varied in a range from about 25 to 60.degree. C., and
the current density was varied in a range from about 10 to 30
A/dm.sup.2, while also adjusting the air agitation amount of the
plating solution.
[0146] In the case where the trivalent chromium plating solution of
Sample 1 (Present Invention) was used, the Fe ions which strayed
into the plating were removed by using an ion exchange apparatus
having a cation exchange resin. Specifically, a plating solution
was regularly fed to the ion exchange apparatus, and the Fe
concentration within the plating solution was controlled to be
within the range from 0 to 0.0001 mass %. As a result, in the case
where the trivalent chromium plating solution of Sample 1 (Present
Invention) was used, the Fe content in the chromium plating layer
was 0.2 mass % at the most, when being measured along the thickness
of the plating layer. On the other hand, in the case where the
trivalent chromium plating solutions of Sample 2 to Sample 4 were
used, the respective chromium plating layers had Fe contents
(maximum values) of 2.0 mass %, 7.0 mass %, and 15.0 mass % because
Fe ion removal with an ion exchange apparatus was not
performed.
[0147] With respect to each sample, stability of plating growth was
measured by the aforementioned method, and evaluations were made
according to the following standards, based on the thickness of the
chromium plating layer at each measurement site.
<Stability of Plating Growth>
[0148] Evaluation Standards (where .circleincircle. to .DELTA.
correspond to Present Invention)
[0149] .circleincircle.: chromium plating layer has a thickness in
the range of no less than 0.25 mm but less than 0.5 mm.
[0150] .largecircle.: chromium plating layer has a thickness in the
range of no less than 0.20 mm but less than 0.25.
[0151] .DELTA.: chromium plating layer has a thickness in the range
of no less than 0.05 mm but less than 0.20 mm.
[0152] X: chromium plating layer has a thickness less than 0.05
mm.
[0153] The obtained results are shown in Table 3.
TABLE-US-00003 TABLE 3 Concentration of Iron added Concentration to
Plating of Iron in Boron Content in Chrome Sample Solution Plating
Layer Plating Layer (mass %) No. (mg/l) (mass %) 0 0.05 0.1 0.2 0.3
Sample 1 0 0.2 X .largecircle. .circleincircle. .circleincircle.
.circleincircle. Sample 2 2.5 2 X .largecircle. .circleincircle.
.circleincircle. .circleincircle. Sample 3 5 7.0 X .largecircle.
.circleincircle. .circleincircle. .circleincircle. Sample 4 10 15.0
X .largecircle. .circleincircle. .circleincircle.
.circleincircle.
[0154] It can be seen from Table 3 that, when any of the trivalent
chromium plating solutions of Sample 1 to Sample 4 is used, plating
growth becomes considerably unstable in the case where no boron is
contained in the chromium plating layer (zero content), but plating
can stably grow in the case where plating conditions are set so
that at least 0.05 mass % or more of boron is contained in the
chromium plating layer. As far as stable plating growth is
concerned, the boron content in the chromium plating layer is
preferably as much as possible.
[0155] Thus, it has been proven that boron is an indispensable
component for ensuring stability of plating growth, and that good
plating characteristics cannot be obtained from merely adding
citric acid instead of boric acid. This effect, i.e., stable growth
of plating which is realized by the addition of boron, was
similarly observed irrespective of the Fe content in the plating
layer.
[0156] Next, in order to further investigate in the aforementioned
effects of boron, chromium plating layers were formed in a manner
similar to the aforementioned method by using the trivalent
chromium plating solution of Sample 1, and their plating
anticorrosiveness and plating abrasion resistance were measured
according to the aforementioned method, based on the following
standards.
<Plating Anticorrosiveness>
[0157] Evaluation Standards (where .circleincircle. to .DELTA.
correspond to Present Invention)
[0158] .circleincircle.: rating number is no less than 9.0.
[0159] .largecircle.: rating number is no less than 8.0 but less
than 9.0.
[0160] .DELTA.: rating number is no less than 7.0 but less than
8.0.
[0161] X: rating number is less than 7.0.
<Plating Abrasion Resistance>
[0162] Evaluation Standards (where .circleincircle. to .DELTA.
correspond to Present Invention)
[0163] .circleincircle.: Vickers hardness is 500 Hv or more.
[0164] .largecircle.: Vickers hardness is no less than 450 Hv but
less than Hv.
[0165] .DELTA.: Vickers hardness is no less than 350 Hv but less
than Hv.
[0166] X: Vickers hardness is less than 350 Hv.
[0167] The obtained results are shown in Table 4. Table 4 also
shows results of plating growth stability.
TABLE-US-00004 TABLE 4 Boron Content in Chrome Plating Layer (mass
%) Characteristics 0 0.05 0.1 0.2 0.3 Stability of Plating Growth X
.largecircle. .circleincircle. .circleincircle. .circleincircle.
Plating Anticorrosiveness X .circleincircle. .circleincircle.
.largecircle. .DELTA. Plating Abrasion Resistance X
.circleincircle. .circleincircle. .largecircle. .DELTA.
[0168] It can be seen from Table 4 that plating anticorrosiveness
and plating abrasion resistance are considerably degraded in the
case where no boron is contained in the chromium plating layer
(zero content), but these properties are improved in the case where
plating conditions are set so that at least 0.05 mass % or more of
boron is contained in the chromium plating layer. These results are
similar to the aforementioned results of stability of plating
growth. Thus, it can be seen that boron is an indispensable
component not only for the enhancement of stability of plating
growth but also for the enhancement of plating anticorrosiveness
and plating abrasion resistance.
[0169] However, as shown in Table 4, plating anticorrosiveness and
plating abrasion resistance are gradually degraded once the boron
content in the chromium plating layer exceeds 0.1 mass %. These
results differ from the aforementioned results of stability of
plating growth.
[0170] From the above results, in order to improve the plating
film-forming characteristics which are to be evaluated in a
totalistic manner based on the three properties of plating growth
stability, plating anticorrosiveness, and plating abrasion
resistance, it is preferable to control the boron content in the
chromium plating layer to be in the range from 0.05 to 0.3 mass
%.
Experimental Example 2
[0171] In this Experimental Example, the following experiment was
performed in order to prove that the color tone of a chromium
plating layer immediately after plating is improved by controlling
the boron content and Fe content in the chromium plating layer.
[0172] Specifically, four kinds of trivalent chromium plating
solutions, i.e., Sample 1 to Sample 4 in Experimental Example 1,
were used to form chromium plating layers in a manner similar to
Experimental Example 1 described above.
[0173] With respect to each sample thus obtained, a color tone
immediately after plating was measured by the following method, and
evaluated based on the following standards.
<Color Tone Immediately After Plating>
[0174] By using a spectrometric color difference meter (color
analyzer TC-1800MK-II (Tokyo Denshoku)), the L* value was measured
according to the method described in CIE 1976. The color tone of a
chromium plating layer which is obtained by using a hexavalent
chromium plating solution is in a range from 70 to 80 in L* value.
In this Experimental Example, it is regarded that a color tone
similar to that of a chromium plating layer which is obtained from
a hexavalent chromium plating solution has been obtained so long as
the L* value is 68 or more.
[0175] Evaluation Standards:
[0176] .circleincircle.: A good color tone similar to that which is
obtained by using hexavalent chromium is obtained.
[0177] (L* value=no less than 70 and no more than 80)
[0178] .largecircle.: A color tone similar to that which is
obtained by using hexavalent chromium is obtained, with a slightly
lower metallic luster.
[0179] (L* value=no less than 68 but less than 70)
[0180] .DELTA.: a Slightly Blackish Color Tone is Obtained.
[0181] (L* value=no less than 65 but less than 68)
[0182] X: A blackish color tone is obtained.
[0183] (L* value=less than 65)
[0184] These results are shown in Table 5.
TABLE-US-00005 TABLE 5 Concentration of Iron in Boron Content in
Chrome Sample Plating Layer Plating Layer (mass %) No. (mass %)
0.05 0.1 0.2 0.3 0.5 0.8 1.0 1.5 Sample 1 0.2 .circleincircle.
.circleincircle. .DELTA. .DELTA. X X X X Sample 2 2 .largecircle.
.largecircle. .DELTA. .DELTA. X X X X Sample 3 7.0 .DELTA. .DELTA.
.DELTA. .DELTA. X X X X Sample 4 15.0 X X X X X X X X
[0185] As is clear from Table 5, when the trivalent chromium
plating solution of Sample 4 was used and the Fe content in the
chromium plating layer was controlled to 15.0 mass %, a chromium
plating layer with a blackish color tone was obtained, irrespective
of the boron content in the chromium plating layer.
[0186] When the trivalent chromium plating solution of Sample was
used and the Fe content in the chromium plating layer was
controlled to 7.0 mass %, a chromium plating layer with a slightly
blackish color tone was obtained by controlling the boron amount to
0.3 mass % or less.
[0187] On the other hand, when the trivalent chromium plating
solution of Sample 2 was used and the Fe content in the chromium
plating layer was reduced to 2.0 mass %, a chromium plating layer
with a slightly blackish color tone was obtained by controlling the
boron amount to 0.3 mass % or less, and a chromium plating layer
having substantially the same color tone as that of a chromium
plating layer which is obtained from a hexavalent chromium plating
solution was obtained by further controlling the boron amount to
0.1 mass % or less. Moreover, similar results were also obtained
when the trivalent chromium plating solution of Sample 1 was used
and the Fe content in the chromium plating layer was reduced to 0.2
mass %, with the boron amount being controlled to 0.3 mass % or
less. By reducing the Fe content in the chromium plating layer to
0.2 mass % and controlling the boron amount to 0.1 mass % or less,
a chromium plating layer having a color tone which is almost
indistinguishable from that of a chromium plating layer which is
obtained from a hexavalent chromium plating solution was
obtained.
[0188] Accordingly, it will be seen that a trivalent chromium
plating layer having a color tone similar to that obtained by using
hexavalent chromium can be obtained by reducing the Fe amount in
the trivalent chromium plating layer to 2.0 mass % or less and
reducing the boron amount to 0.1 mass % or less. Note that, so far
as color tone is concerned, better characteristics are obtained as
the Fe content and boron content in the chromium plating layer are
smaller.
Experimental Example 3
[0189] In this Experimental Example, it was examined as to how
cracks would occur before and after heating and how the color tone
of a chromium plating layer would change depending on the thickness
of the chromium plating layer.
[0190] Specifically, by using the trivalent chromium plating
solution of Sample 1 in Experimental Example 1, chromium plating
layers were formed in a manner similar to Experimental Example 1.
The thickness of a chromium plating layer can be adjusted by
appropriately controlling the plating time depending on the size of
the plating material used, and the like. In this Experimental
Example, plating time was varied in a range of 0.3 to 5 minutes,
thus changing the thickness of the chromium plating layer from 0.1
to 1.5 .mu.m.
[0191] With respect to each sample thus obtained, occurrence of
cracks that are observed immediately after plating (before heating)
was measured by the following method, and evaluated according to
the following standards. Furthermore, according to the
aforementioned method, the color tone of the chromium plating layer
immediately after plating was measured and evaluated.
[0192] Next, each of the aforementioned samples was placed in an
atmospheric furnace, and after being heated under the conditions of
400.degree. C..times.8 hours, occurrence of cracks after heating
was examined by the same measurement method and evaluation
standards as those used when examining occurrence of cracks
observed immediately after plating.
<Occurrence of Cracks>
[0193] Measurement method: By using an optical microscope
(magnification: .times.400), cracks occurring in the chromium
plating layer surface (about 10 mm.times.10 mm) were observed.
[0194] Evaluation Standards (.circleincircle. to .DELTA. correspond
to Present Invention):
[0195] .circleincircle.: There are no cracks.
[0196] .largecircle.: A few discontinuous cracks are observed.
[0197] .DELTA.: Some slightly continuous cracks have occurred.
[0198] X: A large number of continuous cracks have occurred.
[0199] Occurrence of cracks is shown altogether in Table 6. Also,
the color tone of each chromium plating layer immediately after
plating is shown in Table 7.
TABLE-US-00006 TABLE 6 Thickness of Chromium Boron Content in
Chrome Sample Plating Layer Plating Layer (mass %) No. (.mu.m) 0.05
0.1 0.2 0.3 0.5 0.8 1.0 1.5 Before 0.1 .circleincircle.
.circleincircle. .circleincircle. .circleincircle. .circleincircle.
.largecircle. .largecircle. .largecircle. Heating 0.3
.circleincircle. .circleincircle. .circleincircle. .circleincircle.
.circleincircle. .largecircle. .largecircle. .largecircle. 0.5
.circleincircle. .circleincircle. .largecircle. .largecircle.
.DELTA. X X X 0.7 .largecircle. .largecircle. .largecircle.
.largecircle. .DELTA. X X X 1.5 X X X X X X X X After 0.1
.circleincircle. .circleincircle. .largecircle. .DELTA. X X X X
Heating 0.3 .circleincircle. .circleincircle. .largecircle. .DELTA.
X X X X 0.5 .circleincircle. .circleincircle. .largecircle. .DELTA.
X X X X 0.7 .circleincircle. .largecircle. .DELTA. .DELTA. X X X X
1.5 X X X X X X X X
[0200] As shown in Table 6, it can be seen that cracks are more
likely to occur when the thickness of the chromium plating layer
increases and the boron content in the chromium plating layer
increases.
[0201] For example, the experimental results before heating
(immediately after plating) indicate that hardly any cracks
occurred while the chromium plating layer had a thickness in the
range from 0.1 to 0.3 .mu.m, even if the boron content in the
chromium plating layer was increased to 1.5 mass %. Cracks were
more likely to occur as the thickness of the chromium plating layer
increased to 0.5 .mu.m or above, and when the chromium plating
layer had a thickness of 1.5 .mu.m, cracks occurred irrespective of
the boron content in the chromium plating layer.
[0202] On the other hand, there was a tendency that cracks were
more likely to occur in the case where a heat treatment was
performed at 400.degree. C. for 8 hours, as compared to before
heating. Specifically, even if the thickness of the chromium
plating layer was in the range from 0.1 to 0.7 .mu.m, it was not
possible to effectively prevent crack generation unless the boron
content in the chromium plating layer was controlled to 0.05 to 0.3
mass %. An even more preferable boron content would be 0.05 to 0.2
mass %. Note that cracks were likely to occur when the thickness of
the chromium plating layer exceeded 0.7 .mu.m, and cracks occurred
irrespective of the boron content in the chromium plating layer
when the thickness of the chromium plating layer was 1.5 .mu.m.
[0203] It can be seen from these experimental results that crack
generation before and after heating can be effectively prevented by
controlling the thickness of the chromium plating layer to 0.7
.mu.m or less, and further controlling the boron content in the
chromium plating layer to be within a range from 0.05 to 0.3 mass
%.
[0204] Next, with respect to samples whose chromium plating layers
had boron contents of 0.05 mass % and 0.1 mass %, the thickness of
the chromium plating layer was varied in a range from 0.05 .mu.m to
0.7 .mu.m, and the degree of discoloration due to heating was
evaluated.
<Thermal Discoloration>
[0205] Measurement method: By using a spectrometric color
difference meter (color analyzer TC-1800MK-II (Tokyo Denshoku)),
the L* value, a* value, and b* value were measured, before and
after heating, according to the method described in CIE 1976. The
values before heating are labeled as "L0* value", "a0* value", and
"b0* value", whereas the values after heating are labeled as "L1*
value", "a1* value", and "b1* value". Thus, a color difference
.DELTA.E* value after heating was measured as follows.
.DELTA. E * value = ( L 1 * value - L 0 * value ) 2 + ( a 1 * value
- a 0 * value ) 2 + ( b 1 * value - b 0 * value ) 2 [ Equation 1 ]
##EQU00001##
[0206] Evaluation Standards (where .circleincircle. to .DELTA.
correspond to Present Invention):
[0207] .circleincircle.: .DELTA.E* value <1
[0208] .largecircle.: 1.ltoreq..DELTA.E* value <3
[0209] .DELTA.: 3.ltoreq..DELTA.E* value <4
[0210] X: 4.ltoreq..DELTA.E* value
TABLE-US-00007 TABLE 7 Boric Acid Content Thickness of Chromium
Plating Layer (.mu.m) (mass %) 0.05 0.10 0.15 0.20 0.25 0.30 0.40
0.50 0.60 0.70 0.05 X X X .DELTA. .DELTA. .largecircle.
.circleincircle. .circleincircle. .circleincircle. .circleincircle.
0.1 X X X .DELTA. .DELTA. .largecircle. .circleincircle.
.circleincircle. .circleincircle. .circleincircle.
[0211] As is clear from Table 7, there was little change in color
tone before and after heating when the thickness (.mu.m) of the
chromium plating layer was 0.2 .mu.m or more, and the color tone
almost never changed when the thickness (.mu.m) of the chromium
plating layer was 0.3 .mu.m or more.
[0212] The present invention is broadly applicable to a vehicle
having an engine (e.g., a motorcycle or an all-terrain four-wheeled
vehicle) and a transportation apparatus having an engine (e.g., a
ship or an airplane).
* * * * *