US2050872A - Method of oxide-coating aluminum - Google Patents

Description

atented Aug. it, 1936 vii-THO!) F OXIDE-COATING AL No Drawing. Application June 6, i936,

' Serial No. 729,233

This method relates to electrolytic oxide-coating of aluminum and especially to the method oi making contact in an electrolyte with articles which are to be given an oxide coating.

In the art of electrolytically treating aluminum, by which term is meant pure aluminum and aluminum base alloys containing '15 per cent or more aluminum, to form on its surface an oxide coating, it is usually desirable, in order to conveniently secure a uniform coating over the entire surface, to completely submerge the article in the electrolyte during treatment, and it therefore becomes necessary to support the article beneath the surface of the electrolyte. The electrical circuit is completed through a supporting member which is customarily formed of aluminum. Some diiiiculty has been encountered heretofore with such apparatus because the supporting member absorbs an excessive amount of current from the electrolyte during the treatment, which not only shortens the life of the support but also interferes with the proper coating of the article being treated and lowers the electrical efliciency.

It is the object of this invention to overcome these difliculties and to provide an improved method of supporting articles beneath the surface of the electrolyte during treatment in a manner which results in an increase in operating eiilciency and a decrease in attack upon or oxidation of the work-supporting and contacting means.

This invention is predicated upon the discovery that diflerent aluminum alloys exhibit diiferent capacities for absorbing current from electrolytes of the class comprising solutions of sulfuric and oxalic acids. It is believed that this characteristic property cannot be accurately described in terms generally used in the art because of the fact that the nature of the contact between the surface of the aluminum being coated and the electrolyte is substantially different from the ordinary electrical contact between the electrode and the electrolyte in an electrolytic plating bath. In the case of aluminum, a coating of aluminum oxide is formed electrolytically on the aluminum anode surface immediately upon the passage of current. This coating becomes thicker and increasingly firm during the treating operation. The exact manner in which the current'penetrates this coating of aluminum oxide, which is a non-conductor of electricity, is not known. However, it has been found that aluminum alloys of diiferent compositions and aluminum alloys of the same composition but in different conditions, such as heat treated, unheat treated, hard or soft rolled, chill cast or worked, different characteristics in regard to the passage of current. This characteristic property is referred to throughout this specification and in the appended claims as the current-absorbing capacity. It has been found that if two aluminum alloys possess diflerent current-absorbing capacities in a given electrolyte at a given temperature, impressed voltage and other variables of operation, the relation of these two alloys as to the current-absorbing capacity of each, will remain substantially constant for other electrolytes of the class described, and under varying operating conditions. It has been found. further, that the attack upon the work-supporting members can be greatly decreased and the coating process improved by the selection of an aluminum alloy for use in supporting the workpieces, having a, lower current-absorbing capacity per unit of area than the article being treated.

In the practical application of this invention, certain compositions of aluminum alloys, because of their .low current-absorbing capacity, are preferred for use in forming the supporting contact members. Among these may be mentioned aluminum containing silicon as an essential part of the alloy; aluminum containing copper, said alloy being heat treated; and aluminum containing copper and small amounts of manganese and magnesium or magnesium and silicon, said alloys being in the heat treated condition. However, this invention is not confined to any particular aluminumalloys but contemplates broadly the proper combination of aluminum alloys for supporting or contact-forming materials and for the aluminum or aluminum alloy composition of the article to be supported for the electrolytic oxidecoating treatment.

The choice of the aluminum alloy to form a support or contact clamp for any particular aluminum article to be treated may be simply made by partially immersing a test piece of two materials in a cell containing an electrolyte of the class described and a common cathode or electrode, depending on whether direct or alternating current is used, and measuring the amount of current per unit area absorbed from the electrolyte by each metal. The test may be made by measuring the current-absorbing capacity of each piece individually under identical conditions of electrolyte, composition, impressed voltage and temperature. The initial current-absorbing capacity may be somewhat higher or lower than the normal absorbing capacity, therefore measurements are made over a short period of time, for example 5 minutes or more, to allow the readings to become constant. The test is preferably made under conditions identical with conditions contemplated in operation.

The very substantial difference in the current-absorbing capacity of different aluminum alloys and the improvement achieved by the useful employment of this phenomenon is exemplified by the following tests. A test piece of hard rolled aluminum sheet composed of aluminum of commercial purity containing 5 per cent silicon was placed in a lead cell which also served as the cathode and contained an electrolyte composed of a 15 per cent solution of sulfuric acid at a temperature of 70 F. and under an impressed voltage of 17.5. The current-absorbing capacity was found by measurement to be 12 amperes per square foot of area; while at a voltage of 20.5 the current-absorbing capacity was increased to 27 amperes per square foot; and with a voltage of 22.5 the current-absorbing capacity was increased to 45 amperes per square foot of area.

Similarly, hard rolled aluminum sheet of commercial purity containing 99.2 per cent aluminum was treated under conditions identical with those in the example given above. At an impressed voltage of 15, the current-absorbing capacity was found to be 12 amperes per square foot; at 17.5 volts, the current-absorbing capacity was 19.8 amperes per square foot; at 18 volts, 21.9 amperes per square foot; and at 20.5 volts, the current-absorbing capacity increased to 63 amperes per square foot.

Compared with a current-absorbing capacity of 21.5 amperes per square foot for commercial aluminum at an impressed voltage of 18, aluminum containing 4.5 per cent copper, 0.8 per cent silicoh and 0.8 per cent manganese having been quenched from a solution heat treatment, treated under identical test conditions as the above, absorbed only 9 amperes per square foot at this same potential; at an impressed voltage of 24, the current-absorbing capacity was increased to 12 amperes per square foot.

The following examples show the improvement in efficiency made possible by the application of this invention. Three specimens of aluminum were tested:

(A) One which contained besides aluminum of commercial purity the following elements: copper, 4 per cent; manganese, 0.5 per cent; magnesium, 0.5 per cent.

(B) One which contained besides aluminum, 1 per cent silicon, 0.6 per cent magnesium.

(C) Commerically pure aluminum containing less than 1 per cent of the impurities iron, silicon and copper.

Specimens A and B were in the form of heat treated sheet and C was in the form of hard rolled sheet. The current-absorbing capacity of these specimens was determined as described above, using an electrolyte composed of a 15 per cent solution of sulfuric acid. The specimens arranged in the order of theircurrent-absorbing capacities were as follows: A, B and C, A possessing the smallest current-absorbing capacity and C the largest.

Example 1 A sheet of C aluminum was treated in an electrolyte composed of a 15 per cent solution ofsultrical contact with an aluminum clamp formed of B aluminum. After treatment the thickness of the coating formed on the sheet and on the clamp was measured. The coating on the sheet was found to be 0.00040 inch thick and the coating on the support 0.00048 inch. In a similar operation a sheet of A aluminum was held during coatingby a support formed of B aluminum. In this case the current-absorbing capacity of the clamp material was greater than the absorbing capacity of the work held. After treatment the thickness of the coating formed on the clamp was found to be 0.00056 inch and the thickness of the coating on the sheet treated was found to be 0.00024 inch.

Example 2 A sheet of B aluminum was oxide-coated in an electrolytic bath composed of a 7 per cent solution of oxalic acid for a period of 1 hour at a 12 amperes per square foot. The clamp used to support this sheet was formed of A aluminum having a smaller current-absorbing capacity than the sheet of B aluminum being treated. The coating formed on the sheet was measured and found to be 0.00083 inch thick while the coating on the clamp measured 0.00045 inch thick. Similarly treated a sheet of B aluminum was supported by a clamp formed of C aluminum. After treatment the coating formed on the sheet in this case measured 0.00080 inch thick and the coating on the clamp measured 0.00083 inch thick.

In Example 1 the use of B aluminum to form a support for'treating A aluminum resulted in a 16 per cent greater loss of support material than when it was used to support C aluminum or an aluminum having a lower current-absorbing capacity. In Example 2 the efficiency of the coating process was increased by supporting the B aluminum article to be coated with A aluminum as compared with'the same process with a support and contact clamp of C aluminum. In the comparison shown by the first example it will also be observed that a much thicker coating is produced on alloy C than on alloy A, and in the comparison shown by the second example that there is a much greater attack on alloy C than on alloy A when used as clamp materials. This exact relationship may not always be found to obtain, but in every case it will be found that in accordance with the teaching of the present invention, a given alloy when used as a clamp material will last longer when it has a lower current absorbing capacity than the articles to be coated. Also, in every case it will be found that a given alloy to be treated can be coated more efliciently by using a clamp ofan alloy having a lower current absorbing capacity.

It is quite customary in the oxide-coating of aluminum articles to pass relatively large currents through very small electrical contacts between the support and the article being treated.

temperature of 77 F. The current density was 20 ing aluminum surfaces. This method may be 16 applied to the work of a skilled artisan who treats single, articles to achieve special or unusual results and must possess minute control over every detail of his work. It is useful in the ordinary commercial practice of coating aluminum articles as a means of increasing the efliciency and decreasing the cost. Possibly the greatest usefulness of this method lies in its application to the construction and design of automatic oxide-coatlng process machines which treat large numbers of similar articles with a minimum of labor and expense. Such automatic operations depend primarily for their advantages upon uniformity of quality in the product and continuous operation. By the proper combination, as herein described, of aluminum alloys of difierent current-absorbing capacities in the contacting and supporting members in the form of supports, clamps, etc., and of the article being treated, the periods between stoppage for repairs or replacements may be greatly lengthened and the quality of the work improved and/or sustained.

I claim:

1. In a process of electrolytically producing an oxide coating upon aluminum in electrolytes of the class consisting of aqueous solutions of sulfuric and oxalic acids, the steps comprising determining the current-absorbing capacity of the article to be coated, immersing the article to be coated in the electrolyte, and supporting said article therein by an aluminum support having a predetermined lower current-absorbing capacity than the article supported thereby.

2. In a process of electrolytically producing an oxide coating upon aluminum in aqueous solutions of sulfuric acid, the steps comprising determining the current-absorbing capacity of the article to be coated, immersing the article to be coated in the electrolyte, and supporting said article therein by an aluminum support having a predetermined lower current-absorbing capacity than the article supported thereby.

3. In a process of electrolytically producing an oxide coating upon aluminum in aqueous solutions of, oxalic acid, the steps comprising determining the current-absorbing capacity of the article to be coated, immersing the article to be coated in the electrolyte, and supporting said article therein by an aluminum support having a predetermined lower current-absorbing capac- 25 ity than the article supported thereby.

HAROLD K. WORK.