Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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INYPROVED PROCESS FOR COLOR7NG
I,OW TEiMPERA'I'URE CARBURIZED AUSTENITIC STAINI.ESS STEEL
Related Applications
[0001] This application claims the benefit of United States Provisional patent
application serial number 60/773,497 filed on February 15, 2006 for Improved
Process
for Coloring Low Temperature Carburized Austenitic Stainless Steel, the entire
disclosure of which is fully incorporated herein by reference.
Background and Summary
[0002] U.S. Patent No. 5,792,282, as well as commonly assigned U.S. Patent No.
6,547,888 B 1, the disclosures of which are incorporated herein by reference,
describe
processes for increasing the hardness of austenitic stainless steel workpieees
by low
temperature carburization, i.e., carburization carried out in such a way that
a hardened
surface or "ease" is formed which is rich in diffused carbon but substantially
free of
corrosion-promoting carbide precipitates.
[0003] ivieanwhile, commonly assigned Provisional Application 60/653,147,
filed
February 15, 2005, and Non-Provisional Appiication 11/272,915 the disclosures
of
which are also ineorporated herein by reference, deseribes stainless steel
tube fittings
and ferrules which are color coded for easy identification. Color coding is
acconiplished by growing a eolored oxide coating on the workpiecc surfaces,
either
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thermally (i.e., by heating in the presence of an oxygen-containing gas) or
electrochemically.
[0004] This technology is described as being applicable to conventional or
"native"
stainless steels, as well as low temperature carburized austenitic stainless
steels such as
described in the above '282 and '888 patents. In practice, however, it has
been found
that colorizing low temperature carburized stainless steel by known
electrochemical
processes is essentially ineffective in that no substantial color change
occurs.
[0005] It has now been found that a low temperature carburized stainless steel
workpiece can be carburized to a wide spectrum of different intense colors by
cleaning
the workpiece so as to remove the porous oxide layer inherently formed during
low
temperature carburization and then subjecting the electropolished workpiece to
alternating current electrolysis in an electrolysis bath which contains a
metal having
multiple valence states and which further is maintaiined at a neutral to
siightly basic pH.
BRIEF DESCRIPTION OF THE DRAWING
[0006] The present invention may be more readily understood by reference to
the
Drawings and Detailed Description.
[00071 Fig. 1 illustrates a prior art fitting in a finger tight position;
[0008] Fig. lA illustrates the fitting ofFig. lA in a tightened position; and
[0009] Fig.2 is a schematic illustration of the waveform of the electrical
current
applied to the workpiece in accordance with one embodixnent of the invention.
DETAILED DESCRIPTION
Tube Fittings and Ferrules
[0010] Tube fittings are well known articles of commerce. In this application,
the
term "tube fitting" means any type of tube fitting, unless otherwise stated.
Examples of
tube fittings include, but are not limited to, ferru.le type fittings, such as
single ferrule
fittings, two ferruie fittings, and fittings that include more than two
ferrules, flared tube
end fittings, and other types of fittings. Examples of ferrule type fittings
are described,
for example, in comrnonly assigned TJS Patent Nos. 3,103,373, 6,629,708,
provisional
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application Serial No. US601652,631 (attorney docket no. 22188/06884), and PCT
application PCT/US06/03909 the disclosures of which are incorporated herein by
reference in their entirety. Typically, a fitting is composed of various
components
including body sections, nuts, ferrules or "gripping rings," and the like. A
ferrule ma,y
be designed so that, during pull-up, it plastically deforms, or its leading
edge bites into
the conduit being joined, or both. A ferrule may also be designed so that ,
during pull-
up, the ferrule does not bite into the conduit being joined. In accordance
with this
invention, such fittings (and/or component parts thereof) are color coded
electrolytically
for easy identification by growing a colored oxide coating on one or rnore
surfaces of
the fitting or part thereof.
[0011] Although tube fittings can be made from a wide variety of different
metals,
fittings of particular interest are made from steels containing 5 to 50,
preferably 10 to
40, wt.% Ni. Preferred alloys contain 10 to 40 wt.% Ni and 10 to 35 wt. fo Cr.
More
preferred are the stainless steels, especially the AISI 300 and 400 series
steels. Of
special interest are AISI 316, 316L, 317, 317L and 304 stainless steels, alloy
600, alloy
C-276 and alloy 20 Cb, to name a few examples. Fittings made from such steels,
and
particularly from austenitic stainless steels, find particular use in high
purity piping
systems, i.e., piping systems used for processing high purity liquids and
gases. See, the
above-noted U.S. Patent No. 6,547,888 B 1.
Low Temperature Carburized Stainless Steel
[0012] Case hardening is a widely used industrial process for enhancing the
surface
hardness of inetal articles. In a typical commercial process, the workpiece is
contacted
with a carburizing gas at 1700 F(950 C) or above whereby carbon atoms
diffuse into
the arrticle's surface. Hardening occurs through the formation of "carbide
precipitates,"
i.e., specific metal carbide compounds arranged in the form of discrete
particles separate
and apart frorn the metal matrix in which they are contained_
[0013] Stainless steel is rarely case hardened by conventional gas
carburization,
because the carbide precipitates produced promote corrosion.
[0014] To overcome this problem, a technique was developed for case hardening
stainless steel in which the workpiece is contacted with a carburizing gas
below 1000 F.
At these temperatures, and provided that carburization does not last too long,
carbon
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atoms diffuse into the workpiece surfaces with little or no formation of
corrosion-
promoting carbide precipitates. As a result, the workpiece surface not only
becorn.es
hardened but also the inherent corrosion resistance of the stainless steel is
maintained or
even improved.
[0015] Low temperature carburization produces a substantial amount of soot as
an
unwanted by-product. Normally, the amount of soot produced exceeds the amount
of
carbon taken up by the workpiece. Indeed, when the parts being carburized are
small,
such as in the case of ferrules or the like, the amount of soot created is
often large
enough to completely engulf adjacent parts, thereby forming an amalgamated
mass of
soot and carburized parts. In normal practice, this unwanted soot by-product
is almost
always removed from the workpiece such as by washing or the like prior to use.
[0016] In addition to soot, low temperature carburization also produces a
heavy oxide
film, at least when carbon monoxide is used as the carbon source. This heavy
oxide
film, which typically has a color ranging from light gold to dark gold-brown,
is
considerably different from the coherent chromium oxide film which makes
stainless
steel corrosion-resistant in that it is thicker and not coherent; i.e., the
heavy oxide film is
relatively porous. Therefore, this film is also removed before use to uncover
the
workpiece's carburized surface, thereby producing a"surface-cleaned"
carburized
workpiece. See, commonly assigned WO 02/063195 A(22188/06303), the disclosure
of
which is also incorporated herein by reference.
[0017] In practice, removing the heavy oxide fihn may be done mechanically.
However, it is most often done by anodic electropolishing in which the
workpiece is
immersed in an aqueous acidic bath and subjected to a direct electrical
current to cause
oxidation and dissolution of the outerrnost metal surface layer of the
workpiece and
removal of the heavy oxide film attached thereto. See, for example, U.S.
Patent No.
4,026,737, U.S. Patent No. 4,269,633, U.S. Patent No. 4,859,287 and U.S.
Patent No.
4,620,882, which disclose similar electropolishing treatments used to clean
native
stainless steels in preparation for coloring by conventional stainless steel
electrolysis
coloring processes. The disclosures of these patents are also incorporated
herein by
reference,
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[0018] Electropolishing of native stainless steels is normally done to remove
a
substantial proportion and preferably all of the so-called "Bi1by layer,"
which is the
surface layer of the native stainless steel containing contaminants as well as
fracture
grains. This layer is about 2.5 microns thick, and so ~electropolishing here
is normally
accomplished to remove at least this 2.5 micron surface layer and perhaps
more.
[0019] In contrast, electropolishing low temperature carburized stainless
steel is
carried out to remove a minimum amount of the workpiece's metal surface, only
about 1
micron or so. This is because the hardened "case" produced by low temperature
carburization only extends down to the f rst 10-25 microns or so of the
workpiece's
surface and, moreover, most of the diffused caxbon which forms this hardened
case is
located at or near the workpiece's outer surface. Therefore electropolishing
of low
temperature carburized stainless steel is normally carried out to remove only
a minimum
amount of the workpiece's metal surface, so that the carburized surface layer
of the
workpiece is lefft largely intact. For the same reason, electropolishing is
preferred over
other techniques for removing this heavy oxide layer such as mechanical
polishing or
the like, since electropolishing avoids removing too much of the workpiece's
surface
layer.
[0020] Once the heavy oxide film is removed, the low temperature carburized
workpiece is ready for use as is. Alternatively, the workpiece can be
subjected to still
additional, optional processing steps.
Alternating Current Electrolysis Coloring
(0021] In accordance with the invention, a low temperature carburized
stainless steel
workpiece which has been electropolished for removing the heavy oxide film
formed
during low temperature carburization is colorized by subjecting the workpiece
to
alternating current electrolysis in an electrolysis bath which contains a
metal having
multiple valence states and which is maintained at a neutral to slightly basic
pH.
[0022] Coloring stainless steel by alternating current electrolysis is already
known
and shown, for example, in the above-noted U.S. Patent No. 4,859,287. As
described
there, an alternating current is applied to the stainless steel workpiece to
be colorized in
such a way that the polarity of the electricity applied to the workpiece
alters between
positive and negative. The same approach is used in this invention except
that, in the
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inventive process, a neutral to rnildly basic electrolysis bath is used. In
addition, cycle
times are typically longer. Moreover, activation of the workpiece's surface by
treatment
with nitric, phosphoric or other acid, with or without accompanying anodic or
cathodic
treatment, which is an important feature of known stainless steel coloring
processes, is
unnecessary with this invention.
[0023] The workpiece can take a wide variety of different forms. For example,
the
workpiece may be a fitting component, including but not limited to a fitting
body, a nut,
a ferrule, a gripping ring, etc. One commercially available and highly
successful fitting
is illustrated in FIGS. 1 and lA. FIGS. 1 and lA, which taken from U.S. Patent
No.
6,629,708, the disclosure of which is incorporated herein by reference in its
entirety.
The workpiece may be any one or more of the components of the fitting
illustrated by
FIGS. 1 and lA. The workpiece is not limited to the components of the fitting
shown in
FIGS 1 and lA and may be a component of any type of fitting or a stainless
steel part of
any type of assembly.
[0024] FIG. 1 shows the fitting components in a finger tight position
preparatory to
final tightening, whereas FIG. lA shows the fitting affter final tightening.
As shown, the
fitting comprises a body 10 having a cylindrical opening 12 counterbored for
receiving
tube end 13. A tapered, frusto-conical cammiing mouth 14 is located at the
axial outer
end of the counterbore. A front ferrule 16 having a smooth, cylindricai inner
wall 18 is
closely received on the tube. The front ferrule has a frusto-conical outer
surface 20 to
be received in the camming mouth.
[0025] Associated with the front ferrule 16 and located axially outward
therefrom is a
rear ferrule 22 configured as shown with a tapered nose portion.24 and a rear
flange 26
having an inclined end surface 28. The inclined end surface of the rear
ferrule 22
provides a radial component as well a.s an axial component of the pull-up
forces acting
on the end surface as will be apparent to those skilled in the art. The
tapered nose 24
enters a tapered camming surface in the rear surface of the front ferrule.
[0026] The ferrules 16, 22 are enclosed by a drive nut member 30 threaded to
the
body 10. During tightening and make-up of the fitting, the inner end face,
flange, or
shoulder 32 of the nut acts against the rear wall end surface 28 of the rear
ferrule to
drive the ferrules forwardly into the fully engaged position shown in FIG. 1A.
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(i) Electrolysis Bath
[0027] The electrolysis bath used for the inventive coloring process contains
ions of a
metal having multiple valence states such as chromium, molybdenum, tungsten,
manganese and vanadiurn. Specific examples of such ions include chromates,
molybdates, tungstates, manganates and vanadates, for example, Cr+6, Cr04 2,
M003"2,
MnOa 2, V+5, V03" (metavanadate), V207'4 (pyrovanadate), and VO-4
(orthovanadate).
Mixtures of these ions can also be used. Specific compounds which can be used
to
supply such ions include, but are not limited to, ammonium dichromate,
ammonium
molybdate, ammonium metatungstate, lithium molybdate, sodium molybdate, sodium
vanadate, sodium manganate and the like.
[0028] The concentration of the multivalent metal ion can vary widely, and any
concentration can be used which will give the desired result. In general,
concentrations
ranging from about 0.01 to 1.0 moles/liter, more typically about 0.05 to 0.5
moles/liter,
or even about 0.1 to 0.3 moles/liter, have been found to be useful.
[0029] The pH of the electrolysis bath used in the inventive process is
normally
maintained between about 5-12, more typically about 6-11 or even 7-10. This
represents a significant departure from prior electrolytic processes for
coloring stainless
steel in which the electrolysis baths are maintained at strongly acidic or
strongly basic
pH's through the addition of strong acids such as sulfuric acid or nitric
acid, or strong
bases such as sodium hydroxide. Such pH adjusters are not normally used in the
electrolysis baths of the present invention and, indeed, are preferably
avoided.
(ii) Alternating Electric Current
[0030] In accordance with the invention, an electropolished low temperature
carburized stainless steel workpiece is colorized by subjecting the workpiece
to
alternating current electrolysis in an electrolysis bath as described above.
This is done
by alternating the polarity of the electrical current applied to the workpiece
in a similar
manner to that described in the above-noted U.S. Patent No. 4,859,287.
Preferably this
is done so that a plot of current density versus time assumes a generally
rectangular
wave form. Most preferably, this is done so that equal amounts of electrical
current are
applied in both parts of each cycle as illustrated, for example, in the
waveform of Fig. 2
of this disclosure. In this context, "equal" means that the absolute amount of
electrical
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current applied per unit of surface area of the part being colorized in the
positive pulse
of each cycle, as determined by integrating its current density/time waveform,
is equal
to the absolute amount of electrical current applied per unit of surface area
in the
negative pulse of each cycle.
[0031] This is most easily done by regulating the positive and negative pulse
of each
cycle to have the same rnagnitude and duration, as illustrated in Fig. 2. For
example, if
the amplitude of a positive pulse is +1 Amp and its duration is 100
rnilliseconds, the
axnplitude of the negative pulse of the same cycle should also be - 1 Amp and
its
duration should also be 100 milliseconds. Alternatively, the magnitudes and
durations
of the positive and negative pulses can be different, so long as the total
amount of
current supplied per unit of area is essentially the same in the positive and
negative
pulses.
[0032] The magnitude and duration of the applied electrical current varies
depending
on the composition of the electrolysis bath and can easily be determined by
routine
experimentation in light of the working exarnples presented below. In general,
the
magnitude of the applied current, in terms of current density, should
generally be
between about 0.01 to 3 A/in2. This rneans that the current density in each
positive
pulse should be between about +0.01 and +3 A lin2, while the current density
in each
negative pulse should be between about -0.01 and -3 /in2. More typically, the
magnitude of the applied current will be between about 0.02 to 1 A/ina or even
about
0.03 to 0.7 A/ina. Similarly, the duration of each pulse should normally be
about 15-
1000 milliseconds, more commonly about 50-500 milliseconds, or even 75-200
milliseconds. Pulses lasting about 100 milliseconds have been found to be
especially
convenient, although pulses lasting less than 15 and more than 1000
rnilliseconds can
also be used.
[0033] As indicated above, the most convenient way of carrying out alternating
current electrolysis in accordance with the invention is to adopt the wave
form
illustrated in Fig. 2 in which alternating pulses of positive and negative
current of equal
current densities and equal durations are applied to the workpiece immediately
following one another. Thus, Fig. 2 shows that the duration 12 of positive
pulse 14 is
equal to the duration 16 of negative pulse 18, the magnitude 20 of positive
pulse 14 is
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equal to the magnitude 22 of negative pulse 18, and no delay is inserted
between
adjacent positive and negative pulses.
[0034] Other approaches, however, can also be used. For example, a delay
(where
the workpiece is held at zero potential) can be inserted between successive
positive and
negative pulses. In addition, the magnitude and duration of the positive and
negative
pulses can be varied from cycle to cycle. Similarly, the magnitude and
duration of the
positive pulse can be different from the magnitude and duration of the
negative pulse in
a particular cycle, provided that the absolute amounts of electrical current
supplied in
both pulses is essentially equal, as indicated above. Finally, additional
positive and
negative pulses can be included in the pattern of electrical current provided
as described
in col. 9, lines 9-28 of the above-noted U.S. Patent No. 4,859,287, so long as
the last
applied electric current is an alternating current or negative pulse current
as described
there.
[0035] Finally, it should be appreciated that the different approaches
described above
can be applied over the entire course of treatment of the workpiece or,
alternatively,
over only a portion of this treatment.
. WORKING EXAMPLES
[0036] 7n order to describe the invention more thoroughly, the following
working
examples were conducted.
Examples 1-10
[0037] In these examples, stainless steel ferrules made from AISI 316
stainless steel
were low temperature carburized in general accordance with the above-noted
U.S.
Patent No. 6,547,888 B1. After washing to rernove the soot produced during
carburization, the ferrules were electropolished to remove the heavy oxide
coating that
had also formed during carburization. The electropolished ferrules were then
tumbled
for 8 minutes in the presence of detergents, bumishing compounds and tumbling
media
for enhancing surface smoothness and then rinsed with water and dried, thereby
producing electropolished ferrules each having a surface area of about 0.63
inz.
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[0038) The electropolished ferrules so obtained were then colorized by an
electrolytic
coloring process in accordance with the invention. This was done by subjecting
the
ferrules, which were mounted on a titanium anode, to alternating current
electrolysis
using an aqueous electrolysis bath containing 0.15M Na2MoO4. The pH of the
electrolysis bath was approximately 9.5, which was due solely to the Na2Mo04,
no
additional acid or base being present. Electrolysis was carried out using
altemating
pulses of positive and negative current, each pulse lasting 100 milliseconds
with no
delays between the pulses. Ten different experiments were conducted at
different
current densities ranging from 0.033-0.13 A/in2. Each experiment lasted about
22
minutes, with the color of the ferrules obtained being monitored and recorded
each
minute.
[0039] The current densities employed, and the results obtained, are set forth
in the
following Table 1. In this table, the following abbreviations are used: ~
G = Gold
B = Blue
DG = Dark Gold
T1 = Transition No. 1 constituting a reddish/blue rainbow effect
T2 = Transition No. 2 constituting a greenish/gold rainbow effect
T3 = Tra.nsition No. 3 constituting a bluish/gold rainbow effect
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Table 1
_Color of Tre_ated Ferrules as a Function of Current Density and Time
Ex 1_ T 2 ~ 3 T_ 4 5 6 7 8 9 10
Current
Density
A/in2 0.033 0.044 0.055 0.0_66 0.077 0.088 0.099 0.11 0.12 0.13
1 min G G G G ~ G G B B B B 2_min _ GG_ G_ _ T 1 B B B B B T3
3 min ~ GG T1 ~T_1 B ~ B B T3 G G
4 min G_ G Tl B~ B T3 T3 G G G
rnin T T1 T1 B B T3 T3 G G G
6_min TT 1 B _B T3 T3 T3 G G DG
_7_min _ TT 1_ B_ ~ BT3 G G G G DG
8 min T 1 i BB_ T3 G G G G DG
.-----
9 min T T 1 B B G G G G DG DG
...._._...__....... __. __......
----
min T T1 B B G G G G DG DG
11 min T B B G _G G G G DG DG
12 min B B T2 G -__G_ G G G DG DG
13 min B B T2 _G G~ G G G DG DG
_1_4 min - B B T2 _ G_ ~ GG G G DG DG
min B_ B _ T2 G_ _G G G G DG DG
16 min B B T2 ' G - G G G G _DG DG
17 min B B T2 G _G_ G G G DG DG
18 min B B T2 G - G_ G G G DG DG
19 min B B T2 G G_ G G G DG DG
2_0_min B T2 T2 G G_-~ G G G DG DG
21 min B T2 T2 GG__~ G G G DG DG
22 min B T2 T2 G - G ' G G G DG DG
[0040] From Table 1, it can be seen that significantly different colors,
specifically
blue, gold and dark gold, can be imparted to low ternperature carburized
stainless steel
articles by the technology of the present invention.
Examples 11-13
[0041] Examples 1-10 were repeated, except that the current densities ranged
from
0.44-0.64 A/in2. The results obtained are set forth in the following Table 2.
"GR" in
this table refers to the color green.
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Table 2
Color of Treated Fe_rrules a_s a Function of Current Density and Time
Ex 11 ~ 12 13
Current
Density
_A/ina 0.44 _ 0.055 0.64
60 sec GR GR GR
65 sec _ GR T GR GR
70 sec GR GR v GR
[0042] From Table 2, it can be seen that still another significantly different
color,
specifically green, can be imparted to low temperature carburized stainless
steel articles
by the technology of the present invention.
Examples 14 and 15
[0043] Exarnples 1-13 were repeated, except that the current densities ranged
from
0.44-0.55 A/in2 while total treatment times ranged from 7 to 10 minutes The
results
obtained are set forth in the following Table 2. "R" in this table refers to
the color red.
Table 3
Color of Treated Ferrule_s as a Function of Current Density and Time
Ex ~ 14 15
Current ~
Density 0.44-
A/in2 _ 049 0.055 _
7 m_ in _R R _
8 min R~ R
9 min R R
min R R -~
[0044] From Table 3, it can be seen that still another significantly different
color,
specifically red, can be imparted to low temperature carburized stainless
steel articles by
the technology of the present invention.
Examples 16-25
[0045] Examples 1-10 were repeated, except that the electrolysis solution was
composed of 0.2M (NH4)6Mo7024'4Ha0. The results obtained are set forth in the
following Table 4. In this table, the following additional abbreviations are
used:
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PR = Pinkish Red
T4 = Transition No. 4 constituting a gold/pinkish red rainbow effect
T5 = Transition No. 5 constituting a pinkish red /green rainbow effect
Table 4
Color of Treated Ferrules a_s a Function of Current D_ensit and Time
_ Eae 16 17 18 1_9 20 21 2_2 23 24 25
Current ~~
Density
A_/in2 0.033 0.044 0.055 0.0_66 0.077 0.088 0.099 0.11 0.12 0.13
1 min _ G G G G_ G B B B B B
2 min G G_ G_ G_ T3 B B T3 T3 G
3 min G G T3 T3 B T3 T3 G G G
4 min G G_ T3 B B G T3 G G G
min G T3 B B B G G G T4 PR
6 min G B B _B B G G T4 PR PR
7 min G B B B_ B G G PR PR PR
8 min T3 B B B B T4 T4 PR PR PR
9 min T3 B B B B PR PR PR PR GR
min T3 B B B B PR PR PR T5 GR
11 min T B B B B PR T5 PR T5 T2
12 min B B B B T3 PR GR PR T5 G
1_3 min B B B B T3 PR GR T5_ T5 _ G
14 min B B B T3 T3 PR G_R T5 T5 G
min B B T3 T3 G PR GR T5 T5 G
16 min B B T3 G G PR GR GR T5 G
17 min B T3 G G G T5 GR GR GR DG
18 min B T3 G G_ G T5 GR GR T2 DG
19 min B G G G_ G GR GR GR T2
rnin B G _G G T4 GR T2 T2 G
21 min _ B G ~ G G PR GR T2 G
22 min T3 G G G PR GR T2
[0046] From Table 4, it can be seen that other multivalent metal salts can
also be used
to impart significantly different colors, specifically blue, gold and dark
gold, green and
pinkish red, to low temperature carburized stainless steel articles by the
technology of
the present invention.
Examples 26-29
[0047] Examples 1-10 were repeated, except that the electrolysis baths were
composed of different multivalent metal salts, no additional acids or bases
being added.
In addition, current densities were also varied. The conditions and results
obtained are
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set forth in the following Table 5. In this table, the following additional
abbreviations
are used:
B 1 = 0.3M (NH4)6Mo7024 4Hz0
B2 = 0.1M NaV03'4HZ0
B3 = 0.3M (NH4)6Mo7024'4H20 + O.1M NaV03'4H2O
Table 5
Color of Treated Ferrules as a Fun_ction of Current Densit and Tirne
Ex ~ 2_6~ _ 27 28
Current Den_s_ity A_hn 0.033 0.044 0.055
Electrolysis bath B1V B2 ~Y B3
2 min Blue _ Yellow
min Yellow Blue Orange
min Gold Blue Orange
min Red Light Blue -Chartreuse
min Orange Blue Chartreuse
_.._... - -
min_ _ Li t Green _ B_lue Green
min Yellow Chartreuse - Yellow
min Gold Yellow
[0048] From Table 5, it can be seen that various different multivalent metal
salts,
including rnixtures of such salts, can also be used to irnpart significantly
different colors
to low temperature carburized stainless steel articles by the technology of
the present
invention.
Comparative Example A
[0049] Examples 26-28 were repeated, except that the current density was 0.066
A/in2
and the electrolysis bath was composed of 0.51VI NaN03. The results obtained
are set
forth in the following Table 6:
14
CA 02640937 2008-07-30
WO 2007/108865 PCT/US2007/002751
Table 6
Color of Treated Ferrules as a Function of Current Density and Time
Ex C_ omp_A
Current Density A/in 0.066
Electrol sis bath 0.5M NaN03
2 _min Blue
_min Yellow _
min Gold ~
min _ Gold ~
min Light Brown
min Light Blue
L 30 min _ Light Brown
[0050] From Table 6, it can be seen that different colors can be imparted to
low
temperature carburized stainless steel articles electrolytically even if the
electrolysis
bath does not contain a metal having multiple valence states. However, the
electrolysis
bath depleted rapidly requiring frequent replenishing, which is
disadvantageous from a
processing standpoint. Moreover, the colors obtained were not uniform, which
is
commercially unattractive.
[0051] Although only a few embodiments of the present invention have been
described above, it should be appreciated that all such modifications can be
made
without departing from the spirit and scope of the invention. Ali such
modifications are
intended to be included within the scope of the present invention, which is to
be limited
only by the following claims:
