Process For Anodizing Aluminum And Its Alloys

Abstract

A method of anodizing aluminum 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.

Description

BRIEF DESCRIPTION OF THE INVENTION

1. Field of the Invention

This invention relates to methods of applying dense, hard, thick oxide coatings as well as thin, dense, oxide coatings on aluminum metal.

2. Prior Art

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. Pat. No. 3,597,339, uses a special circuit to produce a pulsating current wherein various levels of negative current are applied to a normally positive anode. The process disclosed in U.S. Pat. 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 and makes the system impractical for use except in a laboratory or for small scale type use.

SUMMARY OF THE INVENTION

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 aluminum alloys that can be anodized and the thickness and hardness of the coating obtained. It is an object of the present invention to provide a process of anodizing aluminum 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 difficult 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.

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 alterating current source, rectified through a pulsed constant current charger of the type commercially available from the Utah Research and Development Company, Inc. for use in charging nickel cadmium batteries, to the anode, of the electrolytic cell in which the anodization is to take place.

Additional objects and features of the invention will become apparent from the following detailed description, taken together with the accompanying drawings.

THE DRAWINGS

FIG. 1 is a schematic circuit diagram showing the control circuitry of the process; and

FIG. 2 is a schematic diagram showing the wave form of the circuit used in the process.

DETAILED DESCRIPTION

Referring now to the drawings:

In the illustrated preferred embodiment, 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 Development 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 positive output of the pulsed current charger 10 is connected to the anode 12, which may constitute or which is connected 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.

Tank 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 since the corrosion normally incident to anodization is greatly reduced the refrigeration or cooling coils 14 and line 15 conventionally used to keep the anodizing electrolyte temperature below predetermined temperatures and in the ideal anodization range below about forty-five 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 that is will not severely corrode.

The pulsed current charger supplies a positive current having an average positive direct current 16, FIG. 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 anodization procedures. It has also been found that at least six pulses per second 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, with pulses 17 smoothly peaking at from 2,000 to 10,000 amps. When an average positive direct current of 5,000 amps is applied to the anode, the pulses 17 smoothly peak at from 10,000 to 25,000 amps. 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.

The anodizing electrolyte used in the present process may be an aqueous solution of sulfuric acid (125 to 300 grams of sulfuric 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 solution 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 sulfosalicyclic acid and not more than about fifteen percent by weight sulfuric acid or equivalent amount of metal sulfates can be used. This latter identified electrolyte solution is well known in the art, having been disclosed in U.S. Pat. No. 3,031,387.

The effectiveness and advantages of the present process have been demonstrated in practice. For example, while U.S. military specification MIL-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 of the object being anodized, Aluminum Company of America alloy No. 2219, which contains 6.3 percent copper has been hard anodized according to the present invention with a very thick dense oxide coating. No physical deterioration of the object was noted as a result of the anodization.

While heretofore known hard anodizing processes have required approximately twenty minutes and forty volts at a current density of thirty-six amps per square foot to produce a hard coat having a thickness of 0.0001 inches, the present process applies a 0.001 inch thick coat in twelve to fifteen minutes at a voltage of approximately twenty-eight volts and a density of thirty-six 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 of America alloy 2024, and using the method of the present invention, it is possible to apply 0.0025 inches of hard coating in approximately six minutes, with a maximum voltage of about 36 volts and a current density of 144 amps per square foot. Such treatment caused no apparent structural damage to the object coated. With processes heretofore used alloy 2024 could be hard coated only with great caution and strict temperature control of the electrolyte and with use of voltages in the range of 50 to 65 volts. Such prior processing generally has required about one hour to complete.

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 acid and lignosulfonate as above described, 1/8 inch by 2 inch wide, bright cleaned, strips of Aluminum Company of America alloy No. 5052 were hard anodized to a coating thickness of 0.0015 inches. The coated strips were thereafter bent 180.degree. around a one inch diameter rod and were examined for cracking or spalling. No cracking or spalling was present on either the compression or tension sides of the strips.

While the prior known anodization processes with which I am familiar have all had a practical limit of about 0.004 inches as to the thickness of the coat they could produce, the present system 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 alloy No. 6061 were hard coated to a thickness of 0.012 inches in fifty-five minutes. A voltage of 75 amps maximum and a current density of 100 amps per square foot were used and the electrolyte was an aqueous solution of sulfuric acid and sodium lignosulfonate as heretofore described.

The present invention provides a unique method of anodizing all aluminum 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 disclosed, it is to be understood that the present disclosure is by way of example 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 invention.

Claims

I claim:

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 position direct current having an average direct positive current voltage interspersed with applied peaked pulses 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 at least six peaked pulses occur per second.

4. A process as in claim 3, wherein the pulses peak at a voltage which is at least about twice the average direct current voltage.

5. A process as in claim 3, 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.