US2918416A - Electrolytic process for hard surfacing aluminum - Google Patents

Description

United States Patent ELECTROLYTIC PROCESS FOR HARD SURFACIN G ALUMINUM Paul Hunt Taylor, Gardena, Calif.

No Drawing. Application July 7, 1958 Serial No. 746,664

15 Claims. (Cl. 204- 58) This invention relates to the treatment of aluminum surfaces to produce thereon oxide coatings.

Methods have been heretofore proposed and used for the treatment of aluminum surfaces to produce so-called oxide coatings thereon. However, in most cases, the coatings produced by the usual methods of electrolytic oxidation are substantially porous and lack the hardness "desirable for many applications, and the usual methods are limited in their applicability to aluminum base alloys having low percentages of alloying elements. One of the of the surface, such as by electrolytic oxidation.

Another object of this invention is to provide an electrolytic process for the production of highly abrasion and corrosion resistant oxide coatings on the surfaces of virtually all types of aluminum base alloys.

Other objects and advantages of this invention it is 1 believed will be readily apparent to those skilled in the art from the following detailed description of a preferred embodiment thereof.

Briefly this invention comprises the discovery that the addition of rnellitic acid to the conventional sulfuric acid solution electrolyte used in the electrolytic oxidation unexpectedly results in the production of hard, dense oxide coatingson aluminum surfaces, even in the case of aluminum base alloys containing amounts of alloying elements heretofore considered to be unsatisfactorily high for propor treatment. It has also been discovered that the addi- ;tion of mellitic acid to aqueous solutions of chromic,

phosphoric and oxalic acids used as the electrolyte in 7 the electrolytic oxidation unexpectedly results in the production of hard, dense oxide coatings on aluminum surfaces. The electrolyte to which the rnellitic acid is added ma also comprise mixtures of one or more of the sulfuric, chromic, phosphoric and oxalic acids in all propor- ,tions. Additionally, proper concentrations of rnellitic acid and these electrolyte acids have been discovered.

In carrying out the process of this invention, conventional electrolytic oxidation techniques are employed. That is, the aluminum part to be treated is made the anode where direct current is used, or the aluminum parts .may be used as both electrodes in the event that alternating current is employed.

In carrying out the process of this invention, the aluminum parts to be treated are completely immersed in the electrolyte which is contained in a tank of stainless steel or lead-lined cold rolled steel, the tank serving as the cathode. The aluminum parts are supported from .and maintained in electrical contact with the convenjtional copper bus bars by means of clips or brackets of an ice aluminum alloy, preferably one which polarizes at a higher voltage than the alloy being treated. It is pre ferred to maintain the temperature of the electrolyte between about 5 F. and about 45 F. The electrolyte itself comprises an aqueous solution of rnellitic acid and sulfuric acid. For best results it has been found that the concentrations should be maintained between about 0.5 and about 1.5% mellitic acid by weight, and between about 7.0 and about 13.0% sulfuric acid by weight. The preferred concentration ranges (by weight) of the other electrolyte acids are: chromic acid, 7.0-10%; phosphoric acid, 1040%; oxalic acid, 50-10%. As will be known to those skilled in the art, rnellitic acid may be prepared from charcoal or graphite by oxidation with potassium permanganate or nitric acid.

The electrolyte is vigorously agitated to prevent local over-heating during electrolysis. It is preferred to slowly polarize the aluminum parts by initially applying to the cell the lowest possible current density consistent with flow of current in the bath. This current density is maintained for a minimum time of about 5 minutes whereupon the current is increased at the rate of approximately 6.0 amperes per square foot (area of parts plus area of immersed clamp portions) every three minutes until the desired current density is reached. With the sulfuric acid electrolyte and where hard coatings are desired a current density of 36.0 amperes per square foot is sufficient. A current density of at least 40 amperes per square foot should be used with theother electrolyte acids. These densities are maintained for varying periods of time, depending upon the thickness of coating desired. For example, a 20-minute period will produce a coating of 0.0012 inch in thickness, while a 40-minute treatment will produce a coating 0.0025 inch thick. If thinner coatings (i.e., less than about 0.001 inch) are required,

lower current densities may be employed.

It Will be understood that during electrolysis, the electrolyte is depleted, and accordingly, additions of the mellitic and sulfuric and/or the other acids must be made to maintain the concentration limits.

Example 1 As a specific example of the process of this invention, aluminum parts (2024 aluminum alloy) to be treated were degreased with trichloroethylene, cleaned in a buffered soap cleaner in water solution at F., and thoroughly rinsed in clean, hot tap Water. The parts were not dried, but immediately attached to the bus bars by means of the clips and fully immersed in the electrolyte which consisted of an aqueous solution of 1.0% rnellitic acid by weight and 10% sulfuric acid by weight. The temperature of the bath was maintained at 25 F. throughout the process. Agitation thereof was begun and current applied at a density of 6.0 amperes per square foot for 5 minutes, the current then being increased in increments of 6 amperes per square foot each three minutes until the total of 36.0 amperes per square foot was reached, electrolysis being continued at this value for about 20 minutes. The current was then discontinued, the parts were removed and thoroughly rinsed in cold tap Water, drained and dried at a low temperature.

Example 2 The process of this example was identical to that of Example 1, except that 7% by weight chromic acid was substituted for the sulfuric acid, the initial current density was 3 amperes per square foot and the final current den sity was 40 amperes per square foot.

Example 3 The process of this example was identical to that of Example 1, except that 15% by weight phosphoric acid 3,. was substituted for the sulfuric acid, and bath temperature was 10 F., the initial current density was 10 amperes per square foot and the final current density was 40 amperes per square foot.

Example 4 The processof this example was identical to that of Example 1, except that 6% byweight of oxalic acid was substituted for the sulfuric acid, 0.5% by weight of mellitic acid was used, the initial current density was 10 amperes per square foot and the final density was 40 amperes per square foot.

The addition of mellitic acid to the solution of sulfuric acid maintained as specified herein makes it possible to place a protective surface on virtually all aluminum base alloys as well as pure aluminum. Other processes of this typeare limited in the applicability due to high percentages of alloying elements. This process does not have these limitations. The coating can be applied to aluminum base alloys such as the following which cannot be satisfactorily treated by the process heretofore used:

(1) Those whose copper content is 3.0 percent or more.

(2) Those whose silicon content is over 7.0 percent.

(3) Those whose zinc content is as much as .0 percent.

(4) Those whose copper content is 4.5 percent and nickel content is 2.0 percent.

(5) Those whose magnesium content is as much as 7.0 percent, and

(6) Any aluminum alloy whose percentage of alloying elements does not exceed 16.0 percent.

It is believed thatmellitic acid is reduced by electrolysis during the operation of the electrolyte. It is reasoned thatpyromellitic and mellophanic acids may be formed due to this reduction and that one, two, or all "three acids produce an even distribution of current which allows the coating of the high element bearing alloys.

The addition of the mellitic acid increases the conductivity of the bath and increases the throwing power of the electrolyte, thus greatly facilitating the coating of parts of intricate shapes.

The oxide coatings produced by the process of this invention have the following properties:

(1) Coatings of .001 inch or more will withstand corrosion of the base metal 6000 hours or more in a vapor of 20 percent sodium chloride in distilled water at 95 degrees F., pH 6.5 to 7.2. The coatings are therefore, highly corrosive resistant.

(2) When given an abrasion test using the Taber abraser and wheels CS#17 and 1000 grams weight and 70 r.p.m., a coating of .001 inch will withstand 15,000 or more revolutions before penetration to the base metal.

(3) The coatings will absorb a variety of dry lubricants such as molybdenum disulfide dispersed in phenolic resln.

(4) The mechanical properties of the alloy are decreased by coatings .001 inch or more in thickness. The fatigue strength, percent of elongation, and tensile strength are reduced.

(5) The coatings will accept water soluble dyes, but colors are limited due to the dark shades of the coatings.

(6) The hardness of a .002 inch coating will lie between 29 and 55 Rockwell C when measuredwith a Tukon hardness tester and converted.

(7) The coating is highly dielectric and a coating of .002 inch will withstand in excess of 1000 volts before penetration to the base metal.

(8) Definite dimensional growth is associated withthe production of the coating and is especially evident'with coatingsof .001 inch or more in thickness. This dimensional growth is roughly 50 percent of the total coating thickness and varies somewhat with the alloy as well as the operating conditions of the electrolyte; however, variations are slight.

(9 The coatings are excellent paint bases and will accept lacquer or enamel without the use of a primer coating.

This application is a continuation-in-part of my copending application Serial Number 427,352 filed May 3, 1954, on Electrolytic Process for Hard Surfacing Aluminum, now abandoned which in turn is a continuing application, based on my co-pending application Serial Number 357,630, filed May 26, 1953, on Hard Surfacing Process for Aluminum, now abandoned.

Having fully described my invention, it is to be understood that I do not wish to be limited to the details set forth, but my invention is of the full scope of the appended claims.

I claim:

1. In a process for the production of an oxide coating on an aluminum surface, the step comprising subjecting said surface to electrolytic oxidation in an electrolyte consisting essentially of an aqueous solution of mellitic acid and an acid selected from the group consisting of sulfuric acid, chromic acid, phosphoric acid, oxalic acid and mixtures thereof.

2. In a process for the production of an oxide coating on an aluminum surface, the step comprising subjecting said surface to electrolytic oxidation in an electrolyte consisting essentially of an aqueous solution of between about 0.5 and about 1.5% by weight of mellitic acid and between about 7.0 and about 13.0% by weight of sulfuric acid.

3. In a process for the production of an oxide coating on an aluminum surface, the step comprising subjecting said surface to electrolytic oxidation in an electrolyte consisting essentially of an aqueous solution of between about 0.5 andabout 1.5% by weight of mellitic acidand between about 7.0 and about 10% by weight of chromic acid.

4. In a process for the production of an oxide coating on an aluminum surface, the step comprising subjecting said surface to electrolytic oxidation in an electrolyte consisting essentially of an aqueous solutionof between about 0.5 and about 1.5% by weight of mellitic acid and between about 10 and about 40% by weight ofphosphoric acid.

5. In a process for the production of an oxide coating on an aluminum surface, the step comprising subjecting said surface to electrolytic oxidation in an electrolyte consisting essentially of an aqueous solution of between about 0.5 and about 1.5% by weight of mellitic acid and between about 5.0 and about 10% by weight of oxalic acid.

6. In a process for the production of an oxide coating on an aluminum surface, the step comprising subjecting said surface to electrolytic oxidation in an electrolyte consisting essentially of an aqueous solution of mellitic acid and an acid selected from the group consisting of sulfuric acid, chromic acid, phosphoric acid, oxalic acid and mixtures thereof, said electrolyte being maintained at a temperature of between about 5 and about 45 F.

7. In a process for the production of an oxide coating onan aluminum surface, the step comprising subjecting said surface to electrolytic oxidation in an electrolyte consisting essentially of an aqueous solution of mellitic acid and sulfuric acid, the current density being gradually raised from about 6.0 amperes per square foot to at least 36 amperes per square foot and maintained thereat for a period of at least 20 minutes.

8. In a process for the production of. an oxide coating on an aluminum surface, the step comprising subjecting said surface to electrolytic oxidation in an electrolyte consisting essentially of an aqueous solution ofmellitic acid and chromic acid, the current density being gradually raised fromabout 3.0 amperes per square foot to at least 40 amperes per square foot and maintained thereat for a pcriodof atleast 20 minutes.

9. Ina process for the production of an oxide coating on an aluminum surface, the step comprising subjecting said surface to electrolytic oxidation in an electrolyte consisting essentially of an aqueous solution of mellitic acid and phosphoric acid, the current density being gradually raised from about 10 amperes per square foot to at least 40 amperes per square foot and maintained thereat for a period of at least 20 minutes.

10. In a process for the production of an oxide coating on an aluminum surface, the step comprising subjecting said surface to electrolytic oxidation in an electrolyte consisting essentially of an aqueous solution of mellitic acid and oxalic acid, the current density being gradually raised from about 10 amperes per square foot to at least 40 amperes per square foot and maintained thereat for a period of at least 20 minutes.

11. In an electrolytic oxidation process for the pro'duction of an oxide coating on an aluminum surface, the steps comprising, immersing said surface in an electrolyte consisting essentially of an aqueous solution of mellitic acid and sulfuric acid, maintaining the temperature of said electrolyte between about and about 45 F., vigorously agitating said electrolyte, initially applying a current having a density of about 6.0 amperes per square foot, gradually raising said current density to about 36 amperes per square foot, and maintaining said current density thereat for a period of at least 20 minutes.

12. In an electrolytic oxidation process for the production of an oxide coating on an aluminum surface, the steps comprising, immersing said surface in an electrolyte consisting essentially of an aqueous solution of between about 0.5 and about 1.5% by weight of mellitic acid and between about 7.0 and about 13% by weight of sulfuric acid, maintaining the temperature of said electrolyte between about 5 and about 45 F., vigorously agitating said electrolyte, initially applying a current having a density of about 6.0 amperes per square foot, gradually raising said current density to about 36 amperes per square foot, and maintaining said current density thereat for a period of at least 20 minutes.

13. In an electrolytic oxidation process for the production of an oxide coating on an aluminum surface, the steps comprising, immersing said surface in an electrolyte consisting essentially of an aqueous solution of between about 0.5 and about 1.5% by weight of mellitic acid and between about 7.0 and about by weight of chromic acid, maintaining the temperature of said electrolyte between about 5 and about 45 F., vigorously agitating said electrolyte, initially applying a current having a density of about 3.0 amperes per square foot, gradually raising said current density to about 40 amperes per square foot, and maintaining said current density thereat for a period of at least 20 minutes.

14. In an electrolytic oxidation process for the production of an oxide coating on an aluminum surface, the steps comprising, immersing said surface in an electrolyte consisting essentially of an aqueous solution of between about 0.5 and about 1.5 by weight of mellitic acid and between about 10 and about 40% by weight of phosphoric acid, maintaining the temperature of said electrolyte between about 5 and about 45 F., vigorously agitating said electrolyte, initially applying a current having a density of about 10 amperes per square foot, gradually raising said current density to about 40 amperes per square foot, and maintaining said current density thereat for a period of at least 20 minutes.

15. In an electrolytic oxidation process for the production of an oxide coating on an aluminum surface, the steps comprising, immersing said surface in an electrolyte consisting essentially of an aqueous solution of between about 0.5 and about 1.5 by weight of mellitic acid and between about 5.0 and about 10% by weight of oxalic acid, maintaining the temperature of said electrolyte between about 5 and about 45 F., vigorously agitating said electrolyte, initially applying a current having a density of about 10 amperes per square foot, gradually raising said current density to about 40 amperes per square foot, and maintaining said current density thereat for a period of at least 20 minutes.

References Cited in the file of this patent UNITED STATES PATENTS 2,111,377 Wales Mar. 15, 1938 2,692,851 Burrows Oct. 26, 1954 2,743,221 Sanford Apr. 24, 1956 FOREIGN PATENTS 447,421 Great Britain May 18, 1936 OTHER REFERENCES Heilbron: Dictionary of Organic Compounds, vol. 2 (1936), pages 545 and 546,

Claims (1)

1. IN A PROCESS FOR THE PRODUCTION OF AN OXIDE COATING ON AN ALUMINUM SURFACE, THE STEP COMPRISING SUBJECTING SAID SURFACE TO ELECTROLYTIC OXIDATION IN AN ELECTROLYTE CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION OF MELLITIC ACID AND AN ACID SELECTED FROM THE GROUP CONSISTING OF SULFURIC ACID, CHROMIC ACID, PHOSPHORIC ACID, OXALIC ACID AND MIXTURES THEREOF.