US2739932A - Electrodepositing chromium on aluminum - Google Patents

Description

ELECTRODEPOSITING CHROMIUM ON ALUMINUM Clarence W. Forestek, Cleveland, Ohio No Drawing. Application September 5, 1952, Serial No. 308,140

6 Claims. (Cl. 204-38) This invention relates to the electrodepositionof metals, and in particular relates to a method of electrodepositing an adhesive layer of chromium directly onto an aluminum or other metallic surface.

In the past, the commercial success of the electroplating of aluminum alloys with chromium, has been predicated upon the establishment of a satisfactory adhesive bond between the chromium and the aluminum or aluminum alloy being plated. The many methods employed to efiectuate this requisite bond have included both mechanical and chemical treatments, all of which have proven commercially unsatisfactory for one reason or another. I

Attempts directed towards the establishment of a mechanical bond between the aluminum alloy and the chromium deposited thereon have included a semi-mechanical pretreatment of the aluminum with a deoxidizing etching solution for the purpose of toughening the surface of the same before subjecting the alloy to electrodeposition. The bond formed by this method lacks the requisite adhesive qualities to make the same commercially practical.

The most successful bond heretofore attainable has resulted from the chemical treatment of the aluminum alloy by subjecting the same to the zinc immersion treatment after the usual preliminary cleaning treatment in known manner. In this method, a thin layer of metallic zinc is first deposited on the aluminum alloy in the form of a zincate, by immersing the alloy in a highly alkaline solution of zinc oxide at room temperature for a relatively short period of time (30 seconds to 1 minute). The zincate covered aluminum alloy is then transferred to an electrolytic chromium solution operated within a temperature range of 65-70 F. After a short (3 to 5 min.) immersion in this bath, the aluminum alloy is then transferred to similar electrolytic chromium solution operating at a higher temperature range of 130-140 F. Immersion in this solution for the requisite time period necessary to electroplate the specified thickness, completes the electrodeposition. In some instances, the use of a second tank has been avoided by heating the first tank from the 63 F. temperature to the 130 F. temperature.

The disadvantages of the zincate method are threefold. First, because the bond is predicated upon the provision of a layer of zincate, it is manifest that if areas of this zincate are prematurely destroyed, as by inadvertent splashing with the chromium electrolytic solution, the resultant chromium plate will be poorly bonded in the aiiected area. Secondly, the actual electroplating of chromium at two different temperatures, is disadvantageons because of the cost involved in maintaining a second tank at a different temperature.

Notwithstanding the foregoing, the third and most serious disadvantage arises by virtue of the dual temperatures employed to effectuate 'electrodeposition under the zinca'te method. This increase in temperature, 'while permitting electrodeposition of'an adhering chromium deposit, also serves to weaken the same because of the attendant stress placed upon the chromium deposit by the expansion of the aluminum upon subjection to the deposited at either temperature will be subjected to ab-- normal stresses which can only operate to the detriment of the same. I

Accordingly, it is one object of this invention to provide a method of directly electrodepositing chromium onto an aluminum alloy at a constant temperature.

It is a further object of this invention to provide a method of electrodepositing chromium onto an aluminum surface wherein the deposited chromium will not be subjected to any abnormal stresses.

It is a further object of this invention to provide a method of electrodepositing chromium directly onto an aluminum alloy, wherein the chromium is ultimately directly adhered to the aluminum alloy. t

It is a further object of this invention to provide a method of electroplating chromium directly onto an aluminum alloy featuring an exceptionally strong adhesive bond between the aluminum alloy and the chromium deposit thereon.

It is a further object of this invention to provide a method of directly electrodepositing chromium onto an aluminum alloy, the principle of which can be utilized to electrodeposit chromium directly onto other metals.

It is a still further object of this invention to provide amethod of directly electrodepositing chromium ontoan aluminum alloy wherein accidental splashing of the part with electrolytic solution will not affect the overall result of the chromium deposit.

Other objects of the invention will become more apparent upon consideration of the following brief specification.

Broadly speaking, the invention comprises the discovery that a properly cleaned and pretreated alloy of aluminum can be immersed in a concentrated electrolytic chromic acid solution of certain composition and uniform temperature and subjected to variation in current density to cause the electrodeposition of an adhesive layer of chrome directly onto the aluminum alloy.

In practicing the invention, it is preferred to clean and pretreat the aluminum alloy to be plated in accordance with known methods. plated first has any oxide film thereon removed by immersion in a cleaning solution comprising sulfuric acid or other constituents used throughout the trade in this regard. Upon removal from this preliminary cleaning solution, the part is then immersed in the conventional zinc immersion solution whereby any remaining oxide film is removed and replaced by a relatively thin layer of metallic zinc. This latter operation, of relatvely short. duration (30 sec. to 1 min.) is effectuated in asolution;

having a temperature range of F. to F. At this point, the cleaned and pretreated aluminum alloy is immersed in concentrated electrolytic chrome solution in-.

cluding chromic acid, sulfuric acid and water, and being operated at a constant temperature falling within thereduced to a constant figure falling within a range of 14- amps.. per'sq. in., for the duration of the plating cycle.

To effectuate the plating operation at a constant tempera-.1

Patented Mar. 27, 1956.

Accordingly, the surface to. be

ture, it has been found necessary to increase the concentration of the electrolytic solution by increasing the weight per gallon of the chrornic and sulfuric acid in the solution to a point heretofore considered unfeasible in the art of hard chrome plating of an aluminum alloy. Generally speaking, the chromic acid concentration falls within a range of 40 to 60 oz. per gal. of solution, while the sulfuric acid concentration falls within a range of .40 to .60 oz. per gal. of solution. Representative examples showing electrolytic solutions and the proper operating conditions thereof, are listed below. In each case, these examples have produced a highly satisfactory chrome deposit on properly cleaned and pretreated aluminum .alloy.

Example 1 1 Chromic acid oz. per gal 58 Sulphuric acid do .58 Water to make 1 gal. Temperature F. 100 Current density (for 1st 2 minutes 7 amps. per .sq. in Current density (for duration) do 2 Example 2 1 Chromic acid oz. per. gal 54 Sulfuric acid do .54 Water to make 1 gal. Temperature F 96 Current density (for 1st 2 minutes) amps. per sq. in 5 Current density (for duration) ..-do... 2

Example 3 1 Chromic acid oz. per gal 46 Sulfuric acid do .46 Water to make '1 gal. Temperature F 90 Current density (for 1st '1 minute) amps. per sq. in 4 Current density (for duration) do 2 Example 4 1 Chromic acid oz. per gal 60 Sulfuric acid do .60 Water to'make 1 gal. Temperature F 120 Current density (for 1st 2 minutes) amps. per sq. in 6 Current density (for duration) do 4 A1} aluminum alloys plated in these examples were properly cleaned and pretreated to remove any oxide film prior to immersion in the baths included in these examples.

.A microsection of each of the above examples clearly evidenced the absence of any substantial coating of metallic zinc upon completion of the plating operation. The only evidence of zinc was found in the presence of extremely thin layer of zinc-aluminum alloy on the parts plated under Examples 2 and 3. In Examples 1 and 4, there was no trace of zinc in any form. From the foregoing, it is manifest that the zinc layer is not used as a foundation upon which chromium can be deposited. Instead, the concentrated solution that is used would operate to destroy the zinc layer after the same completes its sole function of preventing re-oxidation of the cleaned surfaceof the aluminum alloy. Accordingly, it is manifest that other coatings could be substituted for the zincate coating described in connection with the pretreatment of the alloy prior to immersion in the new and novel bath, provided that the same served to prevent reox'rdation duringthe interim period preceding immersion inthe electrolytic solution.

'Thus, it can be seen from the preceding descriptionthat there :has been provided a new andnovel method of electrodepositing chromium :directly onto an alnmi-' num alloy. The :methodhasheen shown to. be economical and novel in that the same features a continuous plating temperature coupled with a variation in current density during the plating cycle. It has also been shown how the aluminum part is not subjected to expansion resulting in an unnecessary stress being placed on the chromium electrodepositcd.

Throughout the specification, reference has been made, in a general sense, to the terms, aluminum, aluminum alloys and chromium. It is to be understood that this application is not restricted to use of pure aluminum but is equally applicable to alloys wherein aluminum is the base or foundation metal. Such products are manufactured commercially by The Aluminum Corporation of America and are predominately aluminum, although there may be traces of other metals provided therein for specified purposes. By like token, with reference to the use of the term chromium in the specification, it is to be understood that the chromium referred to is the product .of electrodeposition from a chromic acid solution. It .is to be understood that certain trace impurities may be present in the electrodeposit that is generally designated as chromium.

Modificationsof .the out deviating from the spirit thereof or the scope of the appended claims.

What is claimed is:

l. A method of electrodepositing chromium on the surface .of an aluminum member; comprising the steps of; chemically replacing the oxide film on said surface with a protective coating of z'ineate to prevent re-oxidation upon contact with the atmosphere; .removing said protective coating and simultaneously providing an electrodeposit, of chromium, adhered directly to said aluminum surface by electrolysis, of said aluminum member at constant temperature, in a concentrated electrolytic solution including .chromic acid and sulfuric acid, said constant temperature being between F. and F. said chromic acid being present in concentrations ranging from 40 to 60 ounces per gallon of solution; and said sulfuric acid being present in concentrations ranging from .40 to .60 ounces per gallon of solution.

2. The .method of claim .1 further characterized vby the fact that the current density is varied during said electrolysis.

3. A method of electrodepositing chromium onto an.

minurn member while the same is in said oxide free state, a

to prevent reoxidation'thereof; removing said zincate coat-. ing and simultaneously providing an electrodeposit of chromium adhered directly to said oxide free surface of said aluminum member, by subjecting said zincate coated aluminumimember to electrolysis in an electrolytic solution comprising chromic acid and sulfuric acid, said chromic acid being presentin concentrations ranging from 40 to 60 oz. ,pergnllon of solution and said sulfuric acid being present in concentrations ranging from .40 to .60 oz. per gallon ofsolution; said electrolytic solution being operated at .a constant temperature falling within the temperature range of 90 F. to 120 F.

4. The method of claim 3 further characterized by the fact that the current density falls between 4 to 6 amps. per square inch during the initial electrolysis and is then lowered to fall within arange of lto 4 amps. per square inch during the remainder of said electrolysis.

oxide free surface of said ,alumium member by subjecting said zincate coated aluminum member to electrolysis in vention may be resorted to with an electrolytic solution that includes chromic acid, in concentration of 54 ounces per gallon of solution, and sulphuric acid in concentration of .54 ounce per gallon of solution; said bath being operated at a constant temperature of 96 F. 5 6. The method of claim 1, further characterized by the fact that the current density falls between 4 to 6 amps. per square inch during the initial electrolysis, and is then lowered to fall within a range of 1 to 4 amps. during the remainder of said electrolysis. 10

References Cited in the file of this patent UNITED STATES PATENTS 1,144,000 Roux June 22, 1915 15 Hewitson May 10, 1927 Moore Aug. 22, 1933 Mahlstedt July 24, 1934 Heirnan Jan. 1, 1952 Van der Horst Sept. 1, 1953 Passal Dec. 8, 1953 FOREIGN PATENTS Great Britain Apr. 30, 1937 Great Britain June 8, 1937 ;OTHER REFERENCES Work et al. Transactions of the Electrochemical Society, vol. 59 (1931), pp. 429-435

Claims (1)

1. A METHOD OF ELECTRODEPOSITING CHROMIUM ON THE SURFACE OF AN ALUMINUM MEMBER; COMPRISING THE STEPS OF; CHEMICALLY REPLACING THE OXIDE FILM ON SAID SURFACE WITH A PROTECTIVE COATING OF ZINCATE TO PREVENT RE-OXIDATION UPON CONTACT WITH THE ATMOSPHERE; REMOVING SAID PROTECTIVE COATING AND SIMULTANEOUSLY PROVIDING AN ELECTRODEPOSIT OF CHROMIUM, ADHERED DIRECTLY TO SAID ALUMINUM SURFACE BY ELECTROLYSIS, OF SAID ALUMINUM MEMBER AT CONSTANT TEMPERATURE, IN A CONCENTRATED ELECTROLYTIC SOLUTION INCLUDING CHROMIC ACID AND SULFURIC ACID, SAID CONSTANT TEMPERATURE BEING BETWEEN 90* F. AND 120* F. SAID CHROMIC ACID BEING PRESENT IN CONCENTRATIONS RANGING FROM 40 TO 60 OUNCES PER GALLON OF SOLUTION; AND SAID SULFURIC ACID BEING PRESENT IN CONCENTRATIONS RANGING FROM 40 TO 60 OUNCES PER GALLON OF SOLUTION.