US3418222A

US3418222A - Aluminum anodizing method

Info

Publication number: US3418222A
Application number: US530508A
Authority: US
Inventors: Fred C Schaedel
Original assignee: Murdock Inc
Current assignee: Murdock Inc
Priority date: 1966-02-28
Filing date: 1966-02-28
Publication date: 1968-12-24
Anticipated expiration: 1985-12-24
Also published as: US3473103A

Description

Dec. 24, 1968v F. c. scHAsoc-:L 3,413,222

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Hn keks, K/ch Passau Kik United States Patent O 3,418,222 ALUMINUM ANODIZING METHOD Fred C. Schaedel, Compton, Calif., assignor to Murdock, Inc., Compton, Calif., a corporation of California Filed Feb. 28, 1966, Ser. No. 530,508 6 Claims. (Cl. 204-58) This invention relates to electrolytic anodizing of aluminum, including aluminum alloys, and in particular, to a new and improved anodizing process and apparatus especially suited for producing `a hard anodize finish.

A variety of problems are encountered in the production of anodized coatings on aluminum parts, including burning of the parts, excessive time and voltage requirements, and relatively soft finishes. It is an object of the present invention to provide a new and improved process and apparatus which substantially reduces or eliminates such problems.

It is a particular object of the invention to provide a new and improved process and apparatus for producing a hard coating while operating for shorter periods of time,

at lower electrolyte temperatures and concentrations and at higher current densities. A further object is to provide such a process and apparatus which will utilize conventional electrolytes either with or without additives.

In the conventional anodizing process, the part to be anodized is placed in a tank of electrolyte and an electric current is passed through the part and electrolyte. The D.C. power supply is connected across the part and tank. The initial voltage typically may be volts DC. providing a current density at the part in the order of to 50 amperes per square foot. The applied voltage will be periodically increased until the desired coating thickness is obtained. Typically the voltage increase may be 1 volt per minute except in the critical range where the increase may be l volt per two minutes or l volt per four minutes. The critical range is the time at which burning of the part is most likely to occur. This is the point in the process when the part is nearly completely covered with the initial oxide layer leaving relatively small unoxidized layers having a relatively low surface resistance and hence being susceptible to a localized high current density which may produce burning of the part. The voltage range in which the critical area occurs for any particular material, electrolyte composition, concentration, tank design and running procedure is readily determined by testing. It is the practice in anodizing processes to make the voltage increases at longer intervals during this portion of the process and also to closely watch the current and voltage indicating meters for sharp variations in current or voltage. Such varations indicate incipient or actual burning and the applied voltage may be reduced to protect the parts. It is a specific object of the invention to provide a new and irnproved process and apparatus which substantially eliminates the problems of burning during the critical period and hence substantially eliminates the need for manual supervision of the current and voltage indicators.

It is an object of the invention to provide an anodizing process wherein the part to be anodized is placed in an electrolyte and an electric current is passed through the part and electrolyte in pulses, with the amplitude of the current pulses being varied at a frequency about 1%() the pulse rate. A further object is to provide such a process wherein the current pulses are half wave rectified alternating current pulses, preferably obtained from half wave rectification of the commercial power source. An additional object is to provide such a process wherein the current is applied at a pulse rate in the range of about to about 125 pulses per second and the current amplitude is varied at a frequency of about to about 150 cycles per minute to produce a current change of about 5 to about 17 percent.

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It is an object of the invention to provide a new and improved apparatus for operating an anodizing tank and including a half wave rectifier coupled between an A.C. power source and the tank, and circuit means for cyclically varying the amplitude of the rectifier current in the tank. A particular object is to provide such an apparatus incorporating a current limiting resistance, and switching means for cyclically connecting the resistance in series between the rectifier output and the tank, and bypassing the rectitier output around the resistance.

'Other objects, advantages, features and results will more fully appear in the course of the following description. The drawing merely shows and the description merely describes a preferred embodiment of the present invention which is given by way of illustration or example.

In the drawing:

FIG. 1 is an electrical schematic of a preferred form of the apparatus of the invention; and

FIG. 2 is a diagram illustrating the current wave forms produced by the apparatus of FIG. l.

The present invention -utilizes conventional electrolytes land follows the conventional practice of periodically increasing the voltage during the anodizing operation. An additive may be used with the electrolyte if desired. In addition, in the process of the invention, the current is applied in pulses, typically half Wave rectified alternating current pulses produced by half wave rectification of the commercial power source. Also, the amplitude of the current pulses is varied at a frequency about 1&0 the pulse rate. It has been -found that anodizing with pulse current and with period variation in amplitude of the current produces a harder coating while permitting operation at lower voltages and for shorter periods of time.

An apparatus suitable for performing the process is illustrated in FIG. 1. A part 10 to be anodized is suspended in a tank of electrolyte 11. The power source may be the commercial volts 60 cycles per second power which is connected at

terminals

12, 13. Of course, other power sources can be utilized. The

input terminals

12, 13 are connected through an isolating transformer 14 to an autotransformer 15. One terminal of the autotransformer 15 is connected directly to the tank. The moving arm of the autotransformer is connected to a diode unit 16 which functions as a half wave rectifier. The rectifier output is connected to the part 10 through a contact set 17 and an ammeter 18. A variable resistance 19 is connected in parallel with the contact set 17. A voltmeter 20 is connected across the power supply to provide a measure of the voltage at the tank.

Means is provided for cyclically opening and closing the contact set 17. Typically this may comprise a cornmutator circuit 25 which cyclically engergizes and deenergizes `a low voltage relay coil 26. Contact set 27 of the low voltage relay controls application of power to coil 28 of a high voltage relay which in turn controls the contact set 17.

When the commutator 25 closes the circuit between the 6 volt D C. supply at terminal 30 and ground, coil 26 is energized, contact set 27 is closed, coil 28 is energized, and contact set 17 is closed and the rectifier output is directly connected to the part being anodized. When the commutator opens the circuit between terminal 30 and ground, contact set 17 is opened, switching the resistance 19 in series with the part and electrolyte. It is Arecognized that a wide variety of switching devices may be utilized in lieu of the particular circuitry illustrated. The specific embodiment of FIG. 1 provides for commutating at relatively low voltage and current while providing for current control at relatively high voltage and current.

The operation of the apparatus of FIG. 1 is illustrated 3 in FIG. 2. The solid curve 35 illustrates the half wave rectified current pulses applied to the part and electrolyte with the Contact set 17 closed. The dashed curve 36 illustrates the current pulses with the contact set 17 open and with the resistance 19 connected in series.

The pulse rate for the current pulses should be in the range of about 45 to about 125 pulses per second. As a matter of economy and convenience, commercial A C. power ordinarily is utilized in the anodizing process and the pulse rate will correspond to the frequency of the power source, which in most instances is 60 cycles per second providing a pulse rate of 60 pulses per second.

The current amplitude is varied at a frequency about 1/60 the pulse rate and preferably in the range of about 50 to about 150 cycles per minute. The most preferred range utilizing a 60 pulse per second rate is at a frequency of about 70 to about 90 cycles per minute.

In anodizing equipment, the magnitude of the current is a function of the size of the tank, the size of the parts being treated and the number of parts, and hence the actual magnitude of current will vary over a wide range for different installations. Therefore figures on current amplitude are significant only when related to a speciiic installation. However, the current density at the surface of the part being treated, ordinarily measured in amperes per square foot, does provide for comparison between different installations. At the start of an anodizing run, the current increases from an initial value to a higher operating value. The current remains at this higher yoperating value for the rest of the run, although it may vary somewhat during the progress of the run. In the process of the invention, the change in current as illustrated in FIG. 2, should be in the range of about 5 to about 17 percent of the operating value and preferably is in the order of 8 to 14 percent, with the most desirable range being about 10 to 12 percent. That is to say, the difference between I1 and I2 should be about 11 percent Of Il.

The following are specific examples of the process of the invention.

EXAMPLE 1 Test panels 4 x 4 x 1A inches of 2024-T3 aluminum alloy were anodized using a conventional 10 percent sulfuric acid electrolyte with an additive and operated in the range of 25 to 30 F. The additive was: disodium EDTA, 0.1% of electrolyte by weight; sodium lauryl sulfate, 2O cc. per 40 gallons of electrolyte; and Benax surfactant, 100 cc. per 40 gallons of electrolyte. A half wave rectified 60 cycles per second voltage source was used. The voltage was initially set at volts. The current initially was about 10 amperes and increased to an operating Value of about amperes. The current amplitude was cyclically reduced and raised 2 amperes at a frequency of 80 cycles per minute. The voltage was periodically increased as set out in the following chart, producing the coating thicknesses as indicated in the chart.

Raise voltage from- .At rate of Coating thickness,

inches 20-24 1 volt/min 24A 1 volt/4 min. 26-30-. 0. 001 30-40- 0. 002 -55 0. 003

Parts of 6061-T6 aluminum alloy were anodized using the same electrolyte, pulse rate, current change frequency 4 and amplitude, and initial starting voltage as in Example 1. The voltage was periodically increased as set out in the following chart to produce the coating thicknesses indicated.

At rate ofinches EXAMPLE 3 Parts of 7075-T6 aluminum alloy were anodized in the same manner as set `out in Examples 1 and 2, with the voltage being periodically increased as set out in the following chart to produce the thicknesses indicated.

Coating thickness Raise voltage rom- At rate ofinches 1 volt/min 1 volt/2 min TABLE I Haterial New process Conventional 2024-T3 .002-30 min .00245-50 min. .003-45 min .003-75-80rnin. 6061-'16 .002-20 min .002-40 nun. 7075-T .002-15-20 min .002-35 min.

Table II provides a comparison of the voltages required to produce a 0.002 inch thick coating on three different materials.

TABLE II Material New process Conventional 20H-T3 38-40 volts 50-60 volts.

do Do. 075-T6 35 volts o0 volts The results as set out in Tables I and II were achieved using the new process as set out in Examples 1, 2 and 3 and, for the conventional process, using the same tank and meters with identical parts and with a direct current power supply and a 15 percent sulfuric acid electrolyte at 30 to 35 F. The current density during anodizing in the new process was in the order of to 100 amperes per square foot. The current density in the conventional process was in the order of 35 to 50 amperes per square foot. The coatings produced by the conventional process exhibited a loss of .001 to .0015 inch in a 40,000 cycle Taber test. The coatings produced by the new process eX- hibited a .0007 to .0008 inch loss in a 40,000 cycle Taber test. No burning problem was encountered during the new process and the process was operated at the high current density. The usual critical control area was encountered in the conventional process, requiring close manual supervision of voltage and current and a limitation on the maximum current density.

The test results show that the process of the invention will produce a harder coating in a shorter period of time and at lower voltages and at lower concentrations of electrolyte than the conventional processes.

Although exemplary embodiments of the invention have been disclosed and discussed, it will be understood that other applications of the invention are possible and that the embodiments disclosed may -be subjected to various changes, modifications and substitutions without necessarily departing from the spirit of the invention.

I claim as my invention:

1. In a process for anodizing aluminum wherein the tric current is passed through the part and electrolyte, the v steps of:

applying the current in pulses; and

varying the amplitude of the current pulses at a frequency about 1&0 the pulse rate.

2. A process as defined in claim 1 wherein the current is applied at a pulse rate in the range of about 45 to about 125 pulses per second and the amplitude is varied at a frequency of about 50 to about 150 cycles per minute to produce a current change of about 5 to about 17 percent.

3. A process as dened in claim 2 wherein the current pulses are halt` wave rectified alternating current pulses.

4. A process as dened in claim 1 wherein the current pulses are half wave rectied alternating current pulses.

5. A process as defined in claim 1 wherein the current pulses are half wave rectified alternating current pulses at a power source frequency and the amplitude is varied at a frequency of about 70 to about 90 cycles per minute to produce a current change of about 5 to about 17 percent.

6. 'A process as defined in claim 1 wherein the variation in current amplitude is produced by cyclically switching a resistance in series with the part and electrolyte.

References Cited UNITED STATES PATENTS 1,388,874 8/1921 Mershon 204-58 2,443,599 6/ 1948 Chester 42041-52 2,901,412 8/1959 Mostovych et al. 204-211 2,920,018 1/ 1960 Miller 204-58 XR 2,930,741 3/ 1960 Burger et al 204-228 XR 2,951,025 8/1960 Mostovych et al. 204-211 3,020,219 2/ 1962 Franklin et al. 204-58 FOREIGN PATENTS 446,112 1/ 1948 Canada.

654,299 12/1962 Canada.

821,896 11/1951 Germany.

JOHN H. MACK, Primary Examiner. G. KAPLAN, Assistant Exalmz'ner.

U.S. Cl. X.R. 204-228 UNITED STATES PATENT OFFICE CERTIFICATE 0E CORRECTION Patent No. 3,418,222 December 24, 1968 Fred C. Schaedel d that error appears in the above identified It is certifie atent are hereby corrected as patent and that said Letters P shown below:

table, line l thereof,

Colurn 3, in the heading to the Raise voltage from to- "Raise voltage fTom-' should read Signed and sealed this 17th day of March 1970.

(SEAL) Attest:

Edward M. Fletcher, Ir.

Commissioner of Patents Attesting Officer WILLIAM E. SCHUYLER, JR.

Claims

1. IN A PROCESS FOR ANODIZING ALUMINUM WHEREIN THE PART TO BE ANODIZED IS PLACED IN AN ELECTROLYTE AND AN ELECTRIC CURRENT IS PASSED THROUGH THE PART AND ELECTROLYTE, THE STEP OF: APPLYING THE CURRENT IN PULSES; AND VARYING THE AMPLITUDE OF THE CURRENT PULSES AT A FREQUENCY ABOUT 1/60 PULSE RATE.