U.S. patent application number 10/245063 was filed with the patent office on 2004-03-18 for accelerated sulfuric acid and boric sulfuric acid anodize process.
This patent application is currently assigned to The Boeing Company. Invention is credited to Cadwell Stancin, Linda A., Douglas, Lindsey J..
Application Number | 20040050709 10/245063 |
Document ID | / |
Family ID | 31992028 |
Filed Date | 2004-03-18 |
United States Patent
Application |
20040050709 |
Kind Code |
A1 |
Cadwell Stancin, Linda A. ;
et al. |
March 18, 2004 |
Accelerated sulfuric acid and boric sulfuric acid anodize
process
Abstract
The process of anodizing aluminum or an aluminum alloy in an
aqueous solution of about 60 g/L to about 100 g/L sulfuric acid and
optionally about 0.1 g/L to about 10.7 g/L boric acid, maintained
between a temperature of about 70.degree. F. to about 90.degree. F.
An aluminum object is immersed in the acid bath, where it acts as
an anode. A voltage of 6 V to 16 V is applied to the object at a
current density, which varies with the alloy. The object is
maintained in the anodizing conditions until an aluminum oxide
coating is achieved with a weight of about 30 to about 800
mg/ft.sup.2.
Inventors: |
Cadwell Stancin, Linda A.;
(Renton, WA) ; Douglas, Lindsey J.; (Seattle,
WA) |
Correspondence
Address: |
ALSTON & BIRD LLP
BANK OF AMERICA PLAZA
101 SOUTH TRYON STREET, SUITE 4000
CHARLOTTE
NC
28280-4000
US
|
Assignee: |
The Boeing Company
|
Family ID: |
31992028 |
Appl. No.: |
10/245063 |
Filed: |
September 17, 2002 |
Current U.S.
Class: |
205/213 ;
205/328 |
Current CPC
Class: |
C25D 11/08 20130101 |
Class at
Publication: |
205/213 ;
205/328 |
International
Class: |
C25D 011/08 |
Claims
What is claimed is:
1. A method of anodizing an aluminum or aluminum alloy object
comprising maintaining an acid bath around the object of between
about 60 g/L and about 100 g/L sulfuric acid and a temperature of
between about 70.degree. F. and about 90.degree. F.; applying a
voltage of about 6 V to about 16 V across the object thereby
providing a coating of aluminum oxide to the object; and
maintaining said voltage until the aluminum oxide has a coating
weight of between about 30 mg/ft.sup.2 and about 800
mg/ft.sup.2.
2. The method of claim 1, wherein the acid bath further comprises
between about 0.1 g/L and about 16.5 g/L boric acid.
3. The method of claim 1, wherein the voltage is applied by ramping
the voltage upward at a rate of between about 1 V/min and about 10
V/min.
4. The method of claim 1, wherein the bath temperature is
maintained between about 75.degree. F. and about 85.degree. F.
5. The method of claim 4, wherein the bath temperature is
maintained at about 80.degree. F.
6. The method of claim 1, wherein the sulfuric acid concentration
of the bath is maintained between 75 g/L and 100 g/L.
7. The method of claim 6, wherein the sulfuric acid concentration
of the bath is maintained between 90 g/L and 100 g/L.
8. The method of claim 1, further comprising the steps of
degreasing, cleaning, and deoxidizing the aluminum object prior to
the step of maintaining the acid bath around the object.
9. The method of claim 1, further comprising the step of sealing
the aluminum object after anodizing.
10. A process for anodizing aluminum at an accelerated rate while
maintaining fatigue resistance of the metal comprising providing an
anodizing bath of between about 60 g/L and about 100 g/L sulfuric
acid and a temperature of between about 70.degree. F. and about
90.degree. F.; immersing the object into the anodizing bath;
applying a voltage of about 6V to about 16V across the object
thereby providing a coating of aluminum oxide to the object; and
maintaining said voltage for a period of about 5 to about 8
minutes.
11. The process of claim 10, wherein the anodizing bath further
comprises between about 0.1 g/L and about 10.7 g/L boric acid.
12. The process of claim 10, wherein the voltage is applied by
ramping the voltage upward at a rate of between about 1 V/min and
about 10 V/min.
13. An aluminum alloy article, anodized according to the process of
claim 1.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a method of rapidly anodizing
aluminum and aluminum alloys using sulfuric acid or combinations of
boric and sulfuric acids.
BACKGROUND OF THE INVENTION
[0002] Aluminum alloys are susceptible to corrosion. In addition,
decorative and/or protective organic coatings, such as paints, do
not adhere to aluminum without an adequate surface treatment.
Currently, a preferred method of surface treatment of aluminum is
to form a surface layer of porous aluminum oxide by anodizing the
aluminum in an acid solution. Then organic coatings may be applied.
In some cases, the top of the anodic oxide coating is sealed for
added corrosion resistance.
[0003] A particularly useful method of anodizing aircraft grade
aluminum is recited in Wong et al., U.S. Pat. No. 4,894,127. In
Wong, an aluminum alloy is provided with a protective aluminum
oxide coating in the preferred thickness range of about 1 to 3
microns by anodizing in a bath containing low concentrations of
sulfuric and boric acids. The method comprises providing an aqueous
anodizing solution of about 3 to 5 weight percent sulfuric acid and
from about 0.5 to 1 percent boric acid. The bath is maintained at
about room temperature. An aluminum alloy object is immersed in the
bath where it acts as the anode. The voltage applied across the
object is ramped from about 5 V to about 15 V to maintain a
substantially uniform current density that on the average does not
exceed about ten amperes per square foot. The object is maintained
in the bath to achieve an aluminum oxide coating weight between
about 200 and 600 milligrams per square foot. The anodized object
is thereafter sealed and coated.
[0004] The Wong process is exemplary of anodizing processes based
upon sulfuric acid or combinations of sulfuric and boric acids.
Such processes were developed to replace chromic acid anodizing
processes because of extreme environmental hazards related to the
use of chromic acid. The voltage, amperage, sulfuric and boric acid
concentration ranges in the sulfuric and boric acid based processes
have been chosen to produce an anodic oxide coating that performed
similarly in painted adhesion and corrosion tests to the chromic
acid anodize process that they were meant to replace.
[0005] Sulfuric acid based anodizing systems usually provide
anodized aluminum with acceptable quality for use in aircraft
manufacture. However, the anodizing process is relatively slow,
lasting anywhere from 18 minutes to several hours. These processing
times are becoming of greater concern as advances in manufacturing
techniques have shortened the processing time in other stages of
the aluminum finishing and assembly process. Thus, in many cases
the anodizing step has become the bottleneck in the production of
aluminum parts.
[0006] It is therefore desired to provide a method of anodizing
aluminum in a shorter amount of time than previously required while
maintaining the surface quality and material properties required of
aluminum and aluminum alloys for aircraft manufacture. In
particular, it is desired to provide a method of anodizing aluminum
in a shorter amount of time than previously required wherein the
final performance of the resulting oxide has satisfactory adhesion
performance, fatigue life, and corrosion resistance.
SUMMARY OF THE INVENTION
[0007] The invented process enables the use of sulfuric acid and
sulfuric acid-boric acid anodizing baths for the anodizing of
aluminum and aluminum alloys to produce uniform coatings of 30 to
800 mg/ft.sup.2. The process provides aluminum oxide coatings with
paint adhesion, fatigue life, and corrosion resistance properties
similar to or better than previous anodizing methods for this
coating weight range, but provides the coatings in a much more
rapid manner, thus decreasing the overall time to produce an
anodized aluminum part.
[0008] The process provides an aqueous solution of about 60 g/L to
about 100 g/L sulfuric acid and optionally about 0.1 g/L to about
10.7 g/L boric acid, maintained between a temperature of about
70.degree. F. to about 90.degree. F.
[0009] To anodize the aluminum, an aluminum or aluminum alloy
object is immersed in the acid bath, where it acts as an anode. A
voltage of about 6 V to about 16 V is applied to the object. As a
result, the current density varies between about 1 and about 20
A/ft.sup.2. The object is maintained in the anodizing conditions
until an aluminum oxide coating is achieved with a weight of about
30 to about 800 mg/ft.sup.2.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] Having thus described the invention in general terms,
reference will now be made to the accompanying figures, which are
not necessarily drawn to scale, and wherein:
[0011] FIG. 1 is an illustration of the anodizing process according
to an embodiment of the invention, and
[0012] FIG. 2 is a line graph showing the change in coating mass
with anodize time for an embodiment of the instant invention.
DETAILED DESCRIPTION OF THE INVENTION
[0013] Aluminum or aluminum alloy components are anodized in a bath
of 60 g/L to about 100 g/L sulfuric acid and optionally about 0.1
g/L to about 10.7 g/L boric acid which is maintained at a
temperature of about 70.degree. F. to about 90.degree. F. The
process is run with a voltage differential between the anode and
the cathode of 6 to 16 V. In a more particular embodiment, the
process provides for use of about 75 g/L to about 100 g/L sulfuric
acid. It is generally preferred that boric acid not be used with
the process, though concentrations of about 0.1 g/L to about 10.7
g/L boric acid may provide favorable results for the sealed and
unpainted corrosion resistance of particular alloys of
aluminum.
[0014] The sulfuric and boric acids for use in the process are
commercial or technical grades. Though the invention, as described,
is broadly applicable to aluminum alloys in general, the invented
method has particular usefulness for those alloys typically used in
the manufacture of aircraft, specifically the 2000, 3000, 5000,
6000, and 7000 series, bare or clad, aluminum alloys.
[0015] Prior to anodizing, the aluminum object 10 is typically
degreased and subjected to alkaline cleaning followed by a
deoxidizing or desmutting step. Rinsing must occur after the
alkaline cleaning and deoxidizing steps.
[0016] Referring to FIG. 1, to anodize an aluminum object, the
object 10 is then suspended on a conductive metallic rack 12 within
the anodizing bath 14. Once the object is in the bath 14, current
is applied from a power supply 18 which provides voltage across the
body of the aluminum object 10, acting as the anode, to another
metallic object 16, which acts as a cathode. The voltage is ramped
up to a predetermined voltage at the rate of about 1 to 10
volts/min, and preferably between about 5 volts/min and 10
volts/min. In this case, the predetermined anodizing voltage is
between about 6 V and about 16 V, with higher values being
preferred.
[0017] It is to be understood that the resulting current density
will vary considerably with the particular aluminum alloy used.
Increasing voltage potential causes an increase in current density.
An applied voltage of 20 V causes significant end grain pitting. A
voltage below about 6 V does not provide the desired accelerated
anodizing. Therefore, the anodizing process is operable from 6 V to
12 V and a preferred voltage range is 12 V to 16 V, more preferably
15+/-1 V. Resulting current densities are between 1 and 20
A/ft.sup.2.
[0018] While anodizing, the bath is maintained at a temperature of
from about 70.degree. F. to about 90.degree. F. Lower temperatures
result in an anodic coating, which does not have sufficient
porosity for paint adhesion. Higher temperatures will result in
excessive dissolution of the oxide. The preferred temperature is
from about 75.degree. F. to about 85.degree. F., and more
preferably about 80.degree. F. Tank heaters or tank coolers may be
provided as needed, in order to maintain the bath at the described
temperature.
[0019] An aluminum object having an anodized surface often suffers
from a reduction in fatigue strength. The effect of loss in
strength can be mitigated or eliminated by careful choice of the
aggressiveness of the anodizing solution. Too high of a sulfuric
acid concentration can cause surface pitting. Surface pitting can
reduce the fatigue life of a surface. At the same time, the
anodized layer must be of a thickness and porosity suitable for
corrosion protection and as a substrate for paints and other
coatings. Too low of a sulfuric acid concentration can result in
thin, less porous films. Therefore, as mentioned previously, the
process provides an aqueous solution of about 60 g/L to 100 g/L
sulfuric acid, which does not promote unacceptable pitting while
providing a thickness and porosity suitable for corrosion
protection.
[0020] It has been found that anodized films having a thickness of
30 mg/ft.sup.2 to 800 mg/ft.sup.2 exhibit acceptable paint adhesion
and painted corrosion protection while showing no appreciable signs
of reduction in fatigue strength. For these reasons, anodizing of
the aluminum is continued until an oxide film having a weight of 30
mg/ft.sup.2 to 800 mg/ft.sup.2 is formed upon the surface of the
aluminum object.
[0021] Aluminum for use as a painted part may be anodized to an
oxide coating thickness of as little as 30 mg/ft.sup.2 up to about
800 mg/ft.sup.2. For sealed aluminum parts, a coating of at least
200 mg/ft.sup.2 is preferred, and coatings between about 200
mg/ft.sup.2 and 700 mg/ft.sup.2 are particularly preferred.
Anodizing to a thickness of 30 mg/ft.sup.2 to 800 mg/ft.sup.2 takes
between 5 and 8 minutes, depending upon the alloy being anodized.
For any given thickness of oxide coating, the invented method
provides an increased rate of anodizing aluminum when compared to
prior sulfuric and boric acid based processes, such as Wong
'127.
[0022] Subsequent to anodizing, the anodized coatings of this
invention can be sealed and coated in the same manner as anodized
coatings formed in the sulfuric acid, boric acid, and chromate acid
baths of the past. For example, sealing may be accomplished in a
dilute chromium solution or deionized water. The anodized aluminum
may also be painted as formed or after sealing.
[0023] Most corporate and industrial research into acid anodizing
processes has focused upon either the environmental aspects of the
chemicals or the desired properties of the oxide film provided upon
the aluminum surface. Most improvements to the anodizing process
have focused on the removal of chromic acid from the aluminum
anodizing process due to environmental hazards associated
therewith, and the replacement of chromic acid with sulfuric and/or
boric acid. The goal of past innovation in sulfuric acid anodizing
has been to replicate the favorable anodizing characteristics of
chromic acid.
[0024] Unlike previous innovations in anodizing, this invention
provides a method of speeding up the anodizing process without
detriment to any other attributes of the anodized object. This
faster anodizing method releases the bottleneck previously caused
by the extended period of time which was heretofore needed to
anodize aluminum objects using sulfuric and/or boric acids.
[0025] The end grain pitting severity in the anodized objects does
not appear to be worsened, in comparison to objects prepared in
accordance with traditional methods of anodizing aircraft grade
aluminum, when tested for samples prepared at a 90 g/L sulfuric
acid concentration according to the present invention. Also,
corrosion resistance of sealed samples, prepared in accordance with
the present invention, were equivalent to those samples prepared in
accordance with traditional methods of anodizing. Moreover,
aluminum objects anodized with the accelerated process do not
exhibit decreased fatigue resistance. Also, the paint adhesion
performance of aluminum specimens anodized with the accelerated
process was equivalent to the performance of specimens anodized by
traditional methods. See Examples 1 through 4.
[0026] Thus, the invented process may be used to produce anodized
aluminum objects, which have similar corrosion resistance and
fatigue resistance as those objects produced using prior
techniques. However, the invented method provides anodized objects
in less than half the time previously required in the art.
EXAMPLES
Example 1
Growth Rate
[0027] FIG. 2 shows the oxide growth rate of oxide coatings using
an accelerated anodizing processes of one embodiment of the
invention using sulfuric acid (Fast-SAA), one embodiment of the
invention using sulfuric and boric acids (Fast-BSAA), and a
boric-sulfuric acid anodizing (Traditional) process of the prior
art. The anodizing process was carried out on bare coupons of
2024-T3 aluminum alloy. The 2024-T3 alloy was chosen for testing
because it has a slower oxide growth rate than alloys of the 5000,
6000, or 7000 series alloys.
[0028] A number of coupons were degreased for 15 minutes at
118.degree. F. with a Brulin.TM. 815GD degreasing agent, cleaned
for 10 minutes at 140.degree. F. with an Isoprep.TM. 44 alkaline
cleaning solution, and deoxidized for 10 minutes at ambient
temperature with an Amchem.TM. 6-16 deoxidizing agent. The coupons
were then used for three series of anodization experiments.
[0029] A first series of samples were anodized for 5, 8, 10, 12,
15, and 18 minutes under prior art anodization conditions
(Traditional) at 43 g/L sulfuric acid, 9 g/L boric acid and 15 V of
applied voltage, ramped up 0 V to 15 V at 5 V/min, at
80.degree.+/-5.degree. F. These are the same parameters that are
commonly used in the art and which are generally described in U.S.
Pat. No. 4,894,127.
[0030] A second series of samples were anodized for 5, 8, 10, 12,
15, and 18 minutes in a rapid anodize sulfuric acid process (Fast
SAA) under conditions of 90 g/L sulfuric acid and 15 V of applied
voltage, ramped up 0 V to 15V at 5 V/min, at 80.degree.+/-5.degree.
F.
[0031] A third series of samples were anodized for 5, 8, 10, 12,
15, and 18 minutes in a rapid anodize sulfuric-boric acid process
(Fast BSAA) under conditions of 90 g/L sulfuric acid and 9 g/l
boric acid at 15 V of applied voltage, ramped up from 0 V to 15 V
at 5 V/min, at 80.degree.+/-5.degree. F.
[0032] The prior art process (Traditional) produced a minimum
desired coating weight of 200 mg/ft.sup.2 within 10 minutes after
increasing voltage to 15 V. Both of the invented accelerated
processes (Fast-SAA and Fast-BSAA) produced an acceptable 200
mg/ft.sup.2 coating weight within 5 minutes after increasing
voltage to 15 V. The rate of oxide growth above 200 mg/ft.sup.2 for
the accelerated process was nearly twice that for the prior art
process. The rates of oxide growth at 90 g/l sulfuric acid with or
without boric acid were comparable and showed no statistically
relevant difference between anodizing with or without boric acid
for this type of alloy.
Example 2
Corrosion Resistance
[0033] The anodized samples of Example 1 were sealed in a dilute
chrome seal solution of 26 g/100 gal chromic acid and 18 g/100 gal
sodium chromate at 195.degree. F. The three sets of samples were
then exposed to salt spray for 14 days in accordance with ASTM B117
at an angle of 6 degrees from vertical. Multiple sets of samples
were subjected to the salt spray test and pit counts were obtained.
Pit counts in sealed aluminum components subjected to this test are
known to correspond to overall corrosion resistance of the tested
component. Two series of tests were run, and the results of both
are shown below. The pits of Series 1 were counted extremely
conservatively, with any initiation site included (even those
without visible tails). The Pits of Series 2 were counted normally.
Table 1 lists the average number of pit counts for each set of
samples.
1 TABLE 1 Average Pit Count Surface Finish Series 1 Series 2 43 g/L
sulfuric + 18 min. ano. - 10, 5, 12, 9, 10 5 min. ano. - 0, 1, 0 9
g/L boric 10 min. ano. - 1, 2, 0 90 g/L sulfuric 5 min. ano. - 9,
3, 3 5 min. ano. - 1, 0, 0 10 min. ano. - 1, 0, 0 90 g/L sulfuric +
5 min. ano. - 8, 3 N/A 9 g/L boric
[0034] There was no significant difference in the number of pits
produced after environmental exposure with the prior art samples
and with the accelerated anodize samples. Analysis of pits in the
first series of samples indicate that corrosion resistance of the
rapidly anodized components was the same as or better than the
corrosion resistance of the samples anodized with conventional
processes. Note, 18 minute anodizing using the prior art process
was compared with 5 minute anodizing using the invented process,
because those times corresponded to creation of a roughly 200
mg/ft.sup.2 oxide coating on the respective aluminum alloy
components, as shown in FIG. 2.
[0035] Samples of Series 2 demonstrate a comparison between the
prior art process and an embodiment of the invented process, using
similar anodizing times. The accelerated anodizing process results
in the same or lower pit count as traditional anodizing
techniques.
[0036] Thus, evaluation of pit counts indicates that the invented
method provides corrosion resistance equivalent to or better than
the slower anodizing processes of the prior art.
Example 3
Adhesion Test
[0037] The coating adhesion tests were conducted by applying a thin
coat, on the order of 1-2 mils, of a two-part epoxy fuel tank
primer to multiple samples of each of three sets of samples. The
first set of samples were prepared by anodizing for 18 minutes
under the conditions of the first series of samples from Example 1.
The second set of samples were prepared by anodizing for 5 minutes
in 93 g/L H.sub.2SO.sub.4 and 9.6 g/L H.sub.3BO.sub.4, but
otherwise in accordance with the third series of samples from
Example 1. The third set of samples were prepared by anodizing for
5 minutes in 94 g/L H.sub.2SO.sub.4 and 9.6 g/L H.sub.3BO.sub.4,
but otherwise in accordance with the third series of samples from
Example 1.
[0038] In the "rod" test, after the primer was cured, an aluminum
rod with ends rounded to 0.12 inches was scraped across the primed
surface at an angle of 45.degree. to score it. If the primer
removed had a width greater than 1/8 in., the adhesion of the
primer to the test panel was termed a failure. If the width of the
removal path was narrower, the panel passed, indicated with a "P".
In the scribe test, a cross-hatch pattern is made through the paint
to the metal substrate. Tape is applied firmly across the
crosshatch pattern and then pulled off. If any paint is removed by
the tape, then a score of less than 10 is noted. This test was
performed on dry panels and on panels that had been soaked in water
for 42 days prior to the test. The results of both tests are shown
in Table 2.
2TABLE 2 1st set 2.sup.nd set 3.sup.rd set 43 g/L H.sub.2SO.sub.4
93 g/L H.sub.2SO.sub.4 94 g/L H.sub.2SO.sub.4 Test 9 g/L
H.sub.3BO.sub.4 9.6 g/L H.sub.3BO.sub.4 9.6 g/L H.sub.3BO.sub.4 Rod
(pass or fail) P, P, P, P, P P, P, P P, P, P Dry scribe adhesion
10, 10, 10 10, 10, 10 10, 10, 10 (10 is highest score) Scribe
adhesion after 10, 10, 10 10, 10, 10 10, 10, 10 42 day water
soak
[0039] The samples anodized in accordance with the invention passed
these adhesion tests, indicating that the process provides samples,
which have adhesion strength equivalent to aluminum anodized with
prior art methods.
Example 4
Fatigue Analysis
[0040] Fatigue testing was performed to compare the fatigue
performance of an accelerated sulfuric acid anodize process of the
invention with the traditional boric sulfuric acid anodize of the
prior art. The testing involved flexure coupons. The flexure
samples included 2024-T3 bare aluminum sheet (machined vs. shot
peening), 2024-T3 clad aluminum sheet, and 7050-T76 bare aluminum
plate (machined vs. shot peening). Following fatigue testing, the
fracture surface of the coupons were examined. Optical evaluation
of the fractured flexure and lap splice fatigue samples indicated
that in all cases fracture initiated and propagated due to
fatigue.
[0041] A first grouping of each type of sample was anodized
according to a method of the prior art. The first samples were
anodized for 18 minutes under prior art anodization conditions at
43 g/L sulfuric acid, approximately 1% boric acid, and 15V of
applied voltage, ramped up 0 V to 15 V at 5 V/min, at
80.degree.+/-5.degree. F. These are the same parameters that are
commonly used in the art and which are generally described in U.S.
Pat. No. 4,894,127.
[0042] A second grouping of each type of sample was anodized for 10
minutes in a rapid anodize sulfuric acid process under conditions
of 100 g/L sulfuric acid and 15V of applied voltage, ramped up 0 V
to 15 V at 5 V/min, at 80.degree.-84.degree. F.
[0043] The specimens were optically examined at 12.times. to
identify the location of the fatigue origins. The fatigue origins
are described in Table 3, below.
3TABLE 3 Avg. Cycles Avg. # of Location of Alloy Surface Anodized
to Failure origins origins 2024 machined Prior art X.sub.1 1 all on
edge bare process 2024 machined Invented 5.4X.sub.1 1 all on edge
bare process 2024 peened Prior art X.sub.2 1 on edge, some bare
process samples did not fracture 2024 peened Invented 1.1X.sub.2 1
mostly on bare process edge, 1 on corner, and 2 no fracture 2024
machined Prior art X.sub.3 multiple all edge, some clad process
both edges 2024 machined Invented 1.1X.sub.3 multiple all edge,
some clad process both edges 7050 machined Prior art X.sub.4
multiple edge, some bare process edge/base 7050 machined Invented
2.2X.sub.4 1 Edge bare process 7050 peened Prior art X.sub.5 1
corner, edge, bare process and some no fracture 7050 peened
Invented 2.1X.sub.5 1/no corner, some bare process fracture no
fracture
[0044] In general, for a given sample material and machining type,
the average number of cycles to failure for sample anodized with
the invented process were the same or higher than the anodizing
process of the prior art. This indicates that the accelerated
anodize process does not result in a decreased fatigue resistance
of the material.
[0045] Many modifications and other embodiments of the invention
will come to mind to one skilled in the art to which this invention
pertains having the benefit of the teachings presented in the
foregoing descriptions and the associated drawings. Therefore, it
is to be understood that the invention is not to be limited to the
specific embodiments disclosed and that modifications and other
embodiments are intended to be included within the scope of the
appended claims. Although specific terms are employed herein, they
are used in a generic and descriptive sense only and not for
purposes of limitation.
* * * * *