US1977622A - Method of and bath for anodic treatment of aluminum - Google Patents

Description

Patented Oct. 23, 1934 METHOD OF AND BATH FOR ANODIC TREATMENT OF ALUMINUM Robert w. Buzzard, Kensington, Md. No Drawing. Application lgiay 22, 1934,

Serial No. 726,8 7

8 Claims.

(Granted under the act of March 3, '1883, as

amended April 30, 1928; 370 0. G. 757) This invention relates to a method of preparing a bath for forming oxide coatings on aluminum or its alloys by anodic treatment and the method of using the same. It is the object of this invention to provide a bath that may .be used over a greater range of temperature, voltage and chromium concentration in the bath than in the previous practice and which may be used in a continuous process treatment of the [0 material. I

l This application is a continuation in part of my copending application Serial No. 668,419.

I have found that more satisfactory operation of baths of the type specified may be had by using both hexavalent and trivalent chormium in the solution. The addition of oxalic acid to a solution of chromic acid results in the reduction of some of the hexavalent ions of chromic acid to the trivalent condition. It is also possible to produce trivalent chormium in such baths by treatment with any dibasic or polybasic organic acid or salts of such acid, in particular the chromium salts, although the other salts will produce the effect.- Care has 5 to be taken that foreign elements are not so introduced which will be detrimental to the bath. Due to ease of addition, low cost for a high purity product, and ease of obtaining, oxalic acid has been most generally used. In addition, more satisfactory films for certain purposes result if a chromate, for example, potassium or sodium dichromate, is added to the electrolyte.

Heretofore the test results have been secured when the maximum ratio of oxalic acid to chromic acid is not greater thangone. However, by a proper adjustment of temperature and voltage conditions it is probable that proportions in excess of that specified will be found suitable.

The application of potential may be varied widely, for example, the voltage may be raised to the maximum as rapidly as the source of power will allow or as slowly as .the operator may desire. A good film will be formed in one hour but it may be applied in less time or over a longer period if conditions are such as to make it preferable. It has been found that a'change in temperature will greatly afiect the current density of the bath. A rise in temperature causes a corresponding rise in the current density. However, no definite relation has as yet been determined between the temperature and the current density. Further, it is known that changes in temperature influence the final properties of the film and its rate of formation, etc. These limits are dependent on the composition of the bath and it is therefore not possible to set definite limits or ranges.

The voltage also influences the character of the film. It appears that each definite solution has an optimum voltage at which a good film is formed at the lowest current density. This is not always the best film' for corrosion properties but is usually very close to the highest usable voltage for best results.

As examples of the solutions that may be used the following are given:

3% chromic acid and 1% oxalic acide, bothby weight, using a constant voltage of 40 volts with the bath at 40 C., the time of application being about one hour.

The bath may be made u with 3% chromic acid, 1% oxalic acid, and 1% potassium dichromate, all by weight, the voltage, time and tem- 75 perature being as given in the preceding example.

The bath may be made up of 5% chromic acid and 4% oxalic acid, such a bath operating very satisfactorily at 35 volts and 30 C. for one hour. Good films may be formed at other cycles.

The bath may advantageously be made up of 10% chromic acid and 5% oxalic acid, making practicable a broad range of temperature and voltage combinations with greatly lowered current densities. I

It must be borne in mind that when oxalic acid and chromic acid react that varied results may be obtained and the actual. resultant is diificult to predict. It has been found that it is best, therefore, to state a concentration range for oxalic acid rather than a definite percentage for the production of a desired result. Such as that a definite current-density may be obtained in a 10% bath by adding 3-5% oxalic acid, as test runs will indicate.

In general, the percentage of total chromium may vary from 2% to 30%, wherein the trivalent chromium is regulated from 5% to 60% of the 100 total chromium to properly adjust the bath and with 1% to 25% 01' a chromate.

Owing to the fact that the oxalic acid is by the reaction with chromic acid converted into carbon dioxide and water, there is very little vor no organic residue left in the bath if there is sufiicient chromic acid present. It has been found that baths prepared according to this invention produce very good oxide films at lower current densities than in the commercial processes now in use. Further, it permits of a very wide range of chromium concentration, voltage, temperature and time of treatment. It is to be noted that, theoretically, no oxalic acid remains in the bath as such. It has further been noted that there is a tendency for the lowering of the efiective temperature and voltage ranges when oxalic acid is added to these baths. This tendency is more marked as the ratio of oxalic acid to chromic acid in the bath is increased; also this tendency is more marked as the chromic acid concentration of the solution is increased.

The bath may be prepared by making up a solution having the desired concentration 01 chromic acid and then adjusting the ratio of hexavalent chromium to trivalent chromium by adding the oxalic acid in the tank itself and heating by the method ordinarily employed to bring the bath in the tank to the desired temperature, which will initiate the reaction between the chromic acid and the oxalic acid. The oxalic acid will react in a cold solution but the reaction is greatly hastened by the application oi heat to the bath. 1

If found preferably for any reason, the chromic acid may be reduced in a separate container and added to the anodizing bath. One mole of oxalic acid should reduce approximately twothirds of a mole of chromic acid to the trivalent condition, depending on the efficiency of the reaction. Little heat is required to start the reaction and if the concentration of chromic acid is sufficient it is unnecessary to apply heat; for example, when 1% oxalic acid by weight is added to 2.81% chromic acid solution at the operating temperature. of the straight chromic acid bath the reaction proceeds of itself and produces about 0.35% trivalent chromium in the solution., It is desired to point out that heretofore it has been deemed necessary to avoid trivalent chromium in anodizing baths, whereas in the practice of the present invention trivalent chromium is intentionally formed in, or introduced into, the electrolyte.

An intensive study of the usual straight chromic acid baths showed that there is little or no accumulation of trivalent chromium in the bath, very definitely not in the proportions as recommended for the trivalent baths, and that the failure of such baths is due to the accumulation of other elements than trivalent chromium. Results of tests show that the life of the trivalent baths appears to bear a direct ratio to the total chromium content.

A bath made up as above set forth and used as described is contrarypto the heretofore recognized peculiarities of such. baths and methods, in that they operate at constant voltage, have no particular cycles, good films may be applied in 30 minutes to 60 minutes, and the surface characteristics of the film may be changed by varying the operating temperatures or voltages of the bath. The addition of oxalic acid to' the 'bathcauses the formation of good films with lower voltage and temperature ranges and more satisfactory current densities.

The fact that it is not necessary to change the voltage or the temperature during the treatment of an article makes it possible to place atticles in the bath for treatment at any time and remove them when a satisfactory film has been formed thereon regardless of the time of placing in or taking out of any other articles, that is, the bath may be 'used in a continuous process.

It is sometimes desirable to permit the solution adhering to the article when removed from the bath to dry thereon as there is thus provided a more eifective corrosion resistant coating for some purposes. Any solution remaining in cracks or crevices does not attack the metal and when the solution is once thorough dry on the metal it is extremely diflicult to remove.

The invention described herein may be manufactured and used by or for the Government of the United Statesof America for governmental purposes without the payment of any royalty thereon.

I claim:

1. A bath for the anodic treatment of aluminum or its alloys containing both hexavalent and trivalent chromium and a chromate wherein the total chromium is from 2% to 30%, by

a dibasic or polybasic organic acid, the total chromium in the final solution being between 2% and 30% and the quantity of said product added being sufficient to supply up to 60% 01 the total chromium.

3. In a method of preparing a bath for anodio treatment of aluminum or its alloys, the steps of causing substantially oxalic acid to react with chromic acid, and adding to a bath containing chromic acid a sufficient quantity of the product of said reaction to make the total chromium any desired mrcentage up to 30% and thechromium added in said product being up to 60% of the total chromium.

4. A bath for the anodic treatment of aluminum or its alloys containing both hexavalent and trivalent chromium ions wherein the total chromium is from 2% to 30% and the trivalent the step of passing current through such material as an anode in a chromic acid bath containing 2% to 30% total chromium, whereof from 5% to 60% of the total chromium is in the trivalent condition and which may also contain 1% to 25% of a chromate, said method being adapted to be practiced as a continuous process.

6.'A method of anodically treating material that is predominantly oi aluminum, comprising the step otpassing a potential at a constant voltage between 10 volts and 50 volts through such material as an anode in a bath containing 2% to 30% total chromium, whereof from 5% to 60% of the total chromium is in the trivalent condition and which may also contain 1% to 25% of 'a chromate, said bath being kept at a constant temperature between 20 C. and 60' C.,

andsa.idmethodbeingadapt ed'tobemctiood asacontinuousprooess.

7.Amethodofanod1callytreatingmterlal predominantly of aluminum, comprising the stepsotpassingcurrentthmughmaterlflu'm anodeinabathcontain1n32%fiol0%total chromium, whereof fifom 5% to60% of total chromium is in the trivalent condition and may 1% to25% otachromamsaid p method being adantedtobepmctioedu m I no