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Patent 1046978 Summary

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(12) Patent: (11) CA 1046978
(21) Application Number: 1046978
(54) English Title: PROCESS FOR ANODIZING ALUMINUM AND ITS ALLOYS
(54) French Title: METHODE D'ANODISATION DE L'ALUMINIUM ET DE SES ALLIAGES
Status: Term Expired - Post Grant Beyond Limit
Bibliographic Data
Abstracts

English Abstract


ABSTRACT OF THE DISCLOSURE
A method of anodizing aluminium and its alloys utilizing a
suitable anodizing electrolyte and a pulsed direct current. The direct
current used provides a substantially constant anodizing current
interspersed with at least six pulses per second of higher, smoothly
peaked, direct current. It has been found that the greater the number
of pulses per second, the more superior the anodic coating obtained.
The electrolyte used can be sulfuric acid (125 to 300 grams of sulfuric
acid per liter of water) preferably containing a 0.1 to 0.2 grams per
liter of sodium lignosulfonate or other such sulfonated organic
compound as a stabilizer. A mixture of sulfosalicylic acid and sulfuric
acid or a mixture of sulfuric and oxalic acid can be used.


Claims

Note: Claims are shown in the official language in which they were submitted.


THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A process for anodizing an object of aluminum or aluminum alloy
comprising
placing said object as an anode in an anodizing electrolyte
contained within an electrolytic cell housing, said housing
being a cathode and being continually connected to a negative
current potential; and
subjecting the anode to a continually applied positive direct
current having an average direct positive current voltage
interspersed with at least six applied peaked pulses per second
of higher level positive current voltage, said peaked pulses
having a wave pattern such that the time from average current
to peaked pulse current is greater than the time from peaked
pulse current back to average current, for a period of time
sufficient to anodize the object with a coating of desired
thickness.
2. A process as in Claim 1, wherein the direct current is applied
through a pulsed constant current charger.
3. A process as in Claim 1, wherein the pulses peak at a voltage which
is at least about twice the average direct current voltage.
4. A process as in Claim 1, wherein the electrolyte comprises an aqueous
solution of sulfuric acid containing between one hundred twenty-five and three
hundred grams of sulfuric acid per liter of water and one-tenth to two tenths
grams of sodium lignosulfonate per liter.

Description

Note: Descriptions are shown in the official language in which they were submitted.


1046978 ::
This invention relates to methods of applying dense, hard,
thick o~ide coatings as well as thin, dense, oxide coatings on aluminum
metal.
For many purposes, aluminum surfaces are protected by a
decorative, oxide coating produced on the surface by exposing it ~ -
to controlled electrolysis. Many processes have been developed in
the past to anodize aluminum metal objects by application of the
dense oxide coating. Most of these prior art processes use sulfuric
acid, oxalic acid, or organic acids, or combinations thereof as
an electrolyte and use standard direct current power for the
anodization process. A few of the processes involve the use of an
alternating current imposed on top of direct current or use a surging,
jagged, sharply peaked type of pulsating direct current. U. S.
patent No. 3, 597, 339, uses a qpecial circuit to produce a pulsating
current wherein various levels of negative current are applied to a
normally positive anode. The process disclosed in Patent No. 3, 597, 339
appears to have limited utility, since it uses a single phase power
input and is limited to about 500 amps direct current output. This
severely limits the size of production parts that can be processed
20 and makes the system impractical for use except in a laboratory or
for small scale type use
None of the processes with which I am familiar anodize using
a combination of proper electrolyte and a pulsed form of direct current
electrical energy. As a result, the prior processes are limited as
to the types of alu~ninum alloys that can be anodized and the thickness
and hardness of the coating obtained. It is an object of the present
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invention to provide a process of anodizing aluminurn wherein a superior
anodic coating is obtained.
Another object of the present invention is to provide an
anodizing process wherein a relatively simple electrical circuit is used
and where serious tank and cooling equipment corrosion is avoided.
Still other objects are to provide a process that can be used
to hard coat even high copper bearing aluminum alloys which have been,
in the past, very dif~icult or impossible to hard anodize.
It is also an object of the invention to provide a process that
can be used to provide thick anodic coatings at a much lower cost that
has been heretofore possible and without destruction of the part or
object being anodized.
In accordance with this invention there is provided a process
for anodizing an object of aluminum or aluminum alloy comprising placing
said object as an anode in an anodizing electrolyte contained within an
electrolytic cell housing~ said housing being a cathode and being
continually connected to a negative current potential; and subJecting
the anode to a continually applied positive direct current having an
average direct positive current voltage interspersed with at least six
applied peaked pulses per second of higher level positive current voltage, said
peaked pulses having a wave pattern such that the time ~rom average current -
to peaked pulse current is greater than the time from peaked pulse current
back to average current, for a period of time sufficient to anodize the
object with a coating of desired thickness.
Principal features of the invention include the use of a pulsed
; direct current in combination with a selected anodizing electrolyte. The
pulsed electric current is obtained by using a conventional alternating
current source, rectified through a pulsed constant curren-t charger o~ the
type commercially available from the Utah Resesrch and Development Company,
Inc. for use in charging nickel cadmium batteries, to the anode, of the
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electrolytic cell in which the anodization is to take place.
Addi-tional ob~ects and features of the in~ention will bec~ae
apparent ~rom the ~ollowing detailed description, taken together with
the accompanying drawings.
Figure 1 is a schematic circuit diagram showing the control
circuitry o~ the process, and
Figure 2 is a schematic diagram showing the wa~re ~orm of the
circuit used in the process.
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Referring now to the drawings:
In the illustrated preferred ernbodiment, a pulsed current
charger 10, of the type normally used in the re-charging of nickel
cadmium batteries and commercially available in rated capacities
from Utah Research and Developrnent Company, Inc., Salt Lake City,
Utah, receives a current input from a conventional alternating power
source 11 and puts out a direct current having the wave form shown
in Fig. 2.~ The posikive output of the pulsed current charger 10 is
connected to the anode 12, which rnay constitute or which is connected
10 directly to the object being anodized, of an electrolytic tank shown
generally at 13 and applies a positive current having the wave form
of Fig. 2 to the anode.
Ta~lc 13 has a housing, which may be of stainless steel, ~ -
for example, and the housing forms the cathode of the electrolytic -
tank. The cathode is electrically connected to the negative potential
of the pulsed constant current charger 10 and is maintained negative
at all times. As a result, tank corrosion is greatly reduced over
systems wherein the cathode is subjected to alternately positive
or negative current or to some positive current leakage. Furthermore
20 since the corrosion normally incident to anodization is greatly reduced
the refrigeration or cooling coils 14 and line lS conventionally used to
lceep the anodizing electrolyte temperature below predetermined
temperatures and in the ideal anodization range below about forty-ive
degrees fahrenheit can also be advantageously made of stainless
steel. With the prior known processes positive current is applied
to the cathode and it has been necessary to make the tank housing
and cooling structure of lead so tha~i it will not severely corrode.
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The pulsed current charger supplies a positive current having
an average positive direct current 16, Figure 2, to the anode and pulses
17 of high level positive direct current~ It has been found that the more
frequent the pulses -the more effective the current is for anodi~ation pro-
cedures. It has also been found that at least six pulses per second to
the anode are required to effectively anodize aluminum and aluminum alloys.
In practice, anodization occurs rapidly when an average positive direct
current 16 of 1,000 amps is applied to a fourteen square foot anode, with
pulses 17 smoothly peaking at from 2,000 to 10,000 amps at the anode.
When an average positive direct current of 5,000 amps is applied to a
seventy square foot anode, the pulses 17 smoothly peak at from 10,000 to
,
25,000 amps at the anode. As will be noted from Figure 2 the wave pattern
is such that the length of time between average direct current 16 and - `
peaked c~rrent 1~ is ~reater than the time from peake~ c~rrent bac~ to
average current 16. The pulses stabilize the formation of the oxide coating,
allow thick coatings to be produced at reduced voltages and high current
densities and enable aluminum alloys, even those with copper content, to -;
be readily anodized.
~he anodizing electrolyte used in the present process may be -
an aqueous solution of sulfuric acid (125 to 300 grams of sul~uric acid ;
per liter of water) preferably containing 0.1 to 0.2 grams per liter of
sodium lignosulfonate or a comparable amount of any other such sulfonated
organic compound as a stabilizer. Alternatively, an electrolyte comprising
an aqueous so]ution of sulfuric and oxalic acid having from one percent by
weight oxalic acid fifty percent sulfuric acid to one percent sulfuric acid
and oxalic acid to saturation or an electrolyte comprising an aqueous
solution of from about five to fifty percent by weight sulfosalicylic acid
and not more than about ~ifteen percent by weight sulfuric acid or
equivalent amount of metal sulfates can be used. m is latter identified
electrolyte solution is
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well known in the art, having been di.sclosed in U. S. Patent
No~ 3, 031, 387,
The effectiveness and advantages of the present process have
been demonstrated in practice. For example, while UO S. Military
Specification ~LIL-A-8625 C prohibits the hard anodization of aluminum
alloys containing over 5 percent by weight of copper, because prior
known processes would cause rapid destruction o the object being
anodized, Aluminum Company o~America alloy No. 2219, which
contains 6. 3 percent copper has been hard anodized according to
lO the present invention with a very thick dense oxide coati.ng. No ~. -
physical deterioration o the object was noted as a result of the
anodization.
While he;retofore known hard anodizing processes have
required approximately twenty minutes and forty volts at a current
density of thirty-six amps per square oot to produce a hard coat ~ :
having a thickness of 0. 0001 inches, the present proces s applies
a 0,.~00l i~ch thick c~3at in twelve to fifteen minutes at a voltage of
approximately twenty-eight ~olts and a density of thirty-s~ amps .
per square foot.
Because the present process utilizes a relatively low voltage
application, while developing relatively high current densities,
objects that in the past were subject to destruction during anodization
can be safely hard coated~ For example, it has been found that with
Aluminum Company o America alloy 2024, and using the method of .
` ~ the present invention, it is possible to apply 0~ 0025 inches of hard
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coating in approximately six minutes, with a maximum voltage of
about 36 volts and a current density of 144 amps per square foot. . ~
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Such treatment caused no apparent structural ~amage to the object
coated. With processes heretofore used alloy 2024 could be hard
coated only ~ith great caution and strict temperature control o the
electrolyte and with use of vol~ages in the range of 50 to 65 volts.
Such prior processing generally has required about one hour to completeO
Previously known anodizing processes have also been limited
in that the coatings they produce could only be of limited thickness
and frequently would spall off or crack when bent. Using the process
of the present invention, and using an aqueous solution of sulfuric
10 acid and lignosulfonate as above described, 1/8 inch by 2 inch wide,
bright cleaned, strips o Aluminum Company of ~merica alloy No. 5052
were hard anodized to a coating thickness of 0O 0015 inches. The
coated strips were thereafter bent 180 around a one inch diameter
rod and were examined for cracking or spalling. No cracking or
spalling was present on eikher the compression or tension sides of
the stripsD
While the prior know~ anodization processes with which I -
am familiar ha~re all had a practical limit of about 0O 004 inches as
to the thickness of the coat they could produce, the present system
20 appears to have no such limitation, or at least a much higher
limitation dependent only on the voltage limitations of the available
power supply. Coatings of over 0. 010 inches have been produced.
For example, bright cleaned production parts of Aluminum Company of
America a~loy No~ 6061 were hard coated to a thickness of 0, 012
inches in fifty-five minutesO A voltage of 75 amps maximum and a
current densit~ of lOû amps per square foot were used and the
electrolyte v,~as ai~ aqueous solution of sulfuric acid and sodium
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9~lignosulfonate as heretofore describedO
The present invention provides a unique method of anodizing
all aluminun~ and aluminum alloys more rapidly and with less power
for anodizing and consequently with less power required for cooling
than has theretofore been possible. As a result, the present process
results in lower costs as a result of electrical and labor savings
while giving superior anodization of objects and anodization of objects
that heretofore could not be satisfactorily coated.
Although preferred methods of my invention have been herein
10 disclosed, it is to be understood that the present disclosure is by way ~ . -
of e~{ample and that variations are possible without departing from the
subject matter coming within the scope of the following claims,
which subject matter I regard as my in~-ention. ~ ~
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Representative Drawing

Sorry, the representative drawing for patent document number 1046978 was not found.

Administrative Status

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Event History

Description Date
Inactive: Expired (old Act Patent) latest possible expiry date 1996-01-23
Grant by Issuance 1979-01-23

Abandonment History

There is no abandonment history.

Owners on Record

Note: Records showing the ownership history in alphabetical order.

Current Owners on Record
PERMALOY CORPORATION
Past Owners on Record
JACK L. WOODS
Past Owners that do not appear in the "Owners on Record" listing will appear in other documentation within the application.
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Document
Description 
Date
(yyyy-mm-dd) 
Number of pages   Size of Image (KB) 
Cover Page 1994-04-14 1 22
Abstract 1994-04-14 1 24
Claims 1994-04-14 1 37
Drawings 1994-04-14 1 16
Descriptions 1994-04-14 8 305