US3098804A - Metal treatment - Google Patents

Description

a corporation of Delaware No Drawing. Filed Mar. 28, 1960, Ser. No. 17,754 5 Claims. (til. 204-41) This invention relates to the production of firmly adhering coatings on aluminum and aluminum alloys. More particularly, this invention relates to the production of firmly adhering galvanic coatings of metals, such as chromium and nickel, on aluminum and a variety of aluminum alloys. This application is a continuation-inpart of copending application Serial Number 828,451, filed July 21, 1959.

According to prior art practice, these metals have been plated on aluminum over a preliminary zinc coating produced by the well-known zincate immersion process. Usually brass or copper is plated over the thin zinc layer produced by this process, followed by plating with the nickel and/or chromium. Such process possesses certain inherent disadvantages. The zincate process and its attendant surface preparation is relatively complicated and not as easily handled as the preparation for plating of other common base metals such as steel or brass. Also, the copper, zinc and brass layers aggravate the corrosion of the plated aluminum product.

In order to overcome these disadvantages, it has been proposed to produce first an oxide coating by anodic treatment of the aluminum metal, followed by electroplating of the desired metal on the oxide coating. in the prior art, these oxide coatings were generally produced by anodizing in either an electrolyte of phosphoric acid alone or oxalic acid alone.

Anodizing in phosphoric acid or oxalic acid possesses certain inherent disadvantages. For example, oxalic acid anodizing requires the use of a post anodizing treatment to obtain adequate adhesion of the plated metal. Both phosphoric acid and oxalic acid anodizing are particularly sensitive to variations in aluminum alloy composition, and with many aluminum alloy products, it is difficult if not impossible to adherently plate metal particularly nickel and chromium, on an anodized surface produced in phosphoric acid alone or oxalic acid alone.

Additionally in the prior art it has not been possible to produce many plated articles from aluminum or aluminum alloys wherein the articles are characterized by multiple curved portions and accordingly require a desirable combination of formability coupled with good strength such as in automobile bumpers and wheel covers. Thus, while aluminum may have many advantages for such applications due to lightness in weight, corrosion resistance, etc., in the past, such articles have not been produced from aluminum. Accordingly, special alloys have been developed for such applications. However, satisfactorily adherent plates of nickel and chromium could not be produced on these alloys over anodized coatings produced in phosphoric acid alone or oxalic acid alone.

According to this invention, aluminum is anodized in an aqueous electrolyte consisting essentially of phosphoric 3,098,84 Patented July 23, 1963 acid and sulfuric acid, balance water, and thereafter a layer of metal such as nickel and/ or chromium is electro deposited on the anodized surface of said aluminum. By this method, firmly adherent metal layers can be provided on a wide variety of aluminum alloys.

In carrying out the method of this invention, the object to be treated may first be mechanically buffed and polished although this is not essential to the invention. The article is then cleaned in a suitable solution such as a mild or inhibited alkaline cleaner. 'I he cleaned article may then be anodized. If a very smooth deposit is required on the finished article, the article to be plated may first be polished by conventional brightening treatments, e.g., chemical or electrochemical. On the other hand, where a matte or satin appearance is desired on the ulti mate article, the aluminum metal may be subjected to a suitable etching treatment.

The clean aluminum is immersed as an anode in an aqueous electrolyte consisting essentially of from 5 to 45% by volume phosphoric acid, from 1 to 30% by volume sulfuric acid, balance water. An electric current is passed through the electrolyte for a time period ranging from 1 to 30 minutes, producing an oxide coating on the surface of the aluminum metal. A current density of from 12 to a.s.f. is maintained, using a voltage of less than 40 volts. As used herein, the term a.s.f. is an abbreviation of the expression amperes per square foot. The electrolyte is maintained at a temperature ranging from 80 to F. The oxide-coated metal is then plated with the desired metal such as chromium and nickel.

Where a chromium-plated aluminum metal article is to be subjected to severe corrosive environment or to severe bending stresses or impact, it is preferred first to electrodeposit a layer of nickel on the oxide-coated surface of the aluminum metal in a conventional nickel-plating bath. For some applications it has been found desirable to electrodeposit a plurality of layers of nickel on the oxide-coated surface of the aluminum metal, for example, two or three layers of nickel. Thereafter, a layer of chromium is electrodeposited on the nickel in a conventional chromium-plating bath. Where an article to be plated with chromium is not to be subjected to unusually severe corrosive environments or to unusually severe bending and impact stresses, the layer of chromium may be electrodeposited directly on the oxide-coated surface of the aluminum metal without electrodepositing an intermediate layer of nickel.

A series of aluminum alloys has been developed which exhibit the combination of good formability and strength necessary for applications such as automobile bumpers and wheel covers. These alloys are designated by the letters A thru E in Table I below wherein the percentages by weight of elements other than aluminum contained therein are given. In addition, a small but effective amount of titanium is added for grain refinement.

' Such alloy compositions generally fall within the range of not more than 0.25% silicon, not more than 0.25 iron, 1.30 to 3.70% copper, not more than 0.25% manganese, 0.20 to 1.20% magnesium, not more than 0.30% zinc, a small but effective amount of titanium suflicient for grain refinement, and not more than 0.05% each, 0.15% total of other elements, balance aluminum.

area of 6 sq. inches per sample were fabricated from a sheet of each of the commercial aluminum alloys desig- Table II below gives typical results of strength and elongation tests performed on these alloys after treatment according to the conventional temper designations indicated in Table II below. The samples on which these tests were performed were 0.090 inch thick standard nated by the aluminum association as 1100, 2024 and 6061. The percentages of elements other than aluminum in these alloys are indicated in Table III below. The figures in Table III indicate maximums unless a range is shown.

Table III Others Silicon Iron Copper Manganese Magne- Chromium Zine Tita sium niurn Each Total 1100-. 1.0 Si-t-Fe 0.20 0.05 0.10 0.05 0.15 2024 0.50 0.50 3.8-4.9 0.3-0.9 1.2-1.8 0.10 0.25 0.05 0.15 6061" 0.400.8 0.7 0.15-0.40 0. 0.8-1.2 015-035 0.25 0.15 0.05 0.15

A.S.T.M. sheet samples for alloys A, B, and C and 0.032 inch thick standard A.S.T.M. sheet samples for alloys D and E. Variations in alloy composition within the above limits may produce variations from the typical properties indicated in Table II below.

The temper designations of Table II are the standard designations recommended by the aluminum association. The heat treatment employed on the above alloys may comprise heating the metal to a temperature of at least about 850 F. and maintaining the metal at that temperature for a period of time which may be as short as about minutes. The metal is then quenched in a suitable means such as water. Following quenching, the metal with the temper designation of T3 is cold worked. The above alloys may be formed into products such as automobile bumpers or Wheel covers. However, such articles cannot be adherently plated with metals such as nickel and/or chromium when anodized in conventional anodizing baths. When anodized in a phosphoric acidsulfuric acid electrolyte they may be plated with nickel and/or chromium whereby an attractive plate characterized by excellent adherence may be produced. In plating articles such as automobile bumpers or Wheel covers of the above alloys, best results have been achieved by anodizing in a sulfuric acid-phosphoric acid anodizing bath, plating three layers of nickel on the anodized metal followed by plating with chromium.

In an illustrative example of the process of this invention, 1 by 3" rectangular samples having a total surface In these tests, all the samples were given the following treatment, it being understood that the conventional cold water rinsing operations after the various steps are not recited:

(1) Cleaned for 5 minutes in a mild inhibited alkaline cleaner of the carbonate-phosphate-silicate type contained in a 30-gallon rectangular stainless steel tank at a temperature of 160 F. to a water break-free surface.

(2) Etched by immersion for 5 minutes in a mild aqueous etching solution containing 25% by Weight sulfuric acid, balance water contained in a 4-liter Pyrex beaker, at a temperature of 180 F.

(3) Anodized in a 20-liter Pyrex rectangular tank equipped with stirring devices and lead cathodes for 2 minutes with direct current in an aqueous phosphoric acid-sulfuric acid electrolyte, containing 300 mls./liter of aqueous by weight H PO- solution and 30 mls./liter of aqueous 96% by weight H 80 solution, the balance of the electrolyte being distilled water. The temperature was maintained at 100 F. with a thermostated electric immersion heater. The current densities were maintained at 36 a.s.f. In general, the voltages necessary to maintain a given current density vary with the alloy, the cell geometry and the anodizing conditions and the required voltage varies as the anodizing progresses. In anodizing the samples employed in these tests the voltages required to maintain a constant current density of 36 a.s.f. ranged from about 2 to 30 volts.

(4) Electroplated as a cathode with chromium in a 20- liter Pyrex rectangular tank equipped with stirring devices and lead anodes for 30 minutes with a direct current in an aqueous electrolyte containing 250 grams per liter of CrO 2.5 grams per liter of H 50 balance distilled Water. The temperature was maintained at 60 F. by use of a cold water jacket surrounding the 20-liter Pyrex tank. The current densities were maintained at 200 a.s.f. The voltages required to maintain a constant current density of 200 a.s.'f. ranged from- 6 to 8 volts.

(5) The samples were rinsed briefly in distilled water, dried and buffed to a decorative brightness.

All samples had a lustrous, attractive chromium plate characterized by excellent adherence as shown by absence of flaking ofi of the chromium when the samples were bent over a diameter mandrel.

In another example of the practice of this invention, 1 by 3" rectangular samples having a total surface area of 6 square inches per sample were fabricated from sheets assoc (3) All samples were electroplated as a cathode with nickel in a 20 liter Pyrex rectangular tank equipped with a cathode rod agitating device and nickel anodes for 60 minutes with a direct current in an aqueous. electrolyte 5 having a pH of between 3 and 4, containing 12 ounces per of the commerc al alu-rmnum alloys designated by the Alugallon of N504. 7H20, 27 ounces per gallon c 11 mlnum Association as 1100, 2011, 2024, 3003, 6061, 6063, 1

r ounces per gallon of H 80 and /z by volume of a 7075, and the alloy designated in Table I above by the n vletter The percentages of elements other than 81m wetting agent, balance water. The temperature of the minum in the above commercial alloys are indicated in electrolyte was mamtamed at means of an T bl V b l h figu g in Table IV indicate maxielectric immersion heater. The current densities were mums unless a range is shown. maintained at 40 aslf. and the voltages required to main- Table IV Others Alloy Silicon Iron Copper Manganese Magne- Chromium Zinc Titasium nium Each Total 1100.. 1.0 Si-l-Fe 0.20 0.05 0.05 0.15 2011.. 0.4 0.7 5. 0-50 0.05 0.15 2024-. 0.5 0.5 3.8-4.9 0.30 .9 1 2-1.8 0.10 0.05 0.15 3003-. 0.6 0.7 0.20 1.0 -1.5 0.05 0.15 0051.. 0. 40-08 0.7 0.15-0.4 0.15 0.8 -1.2 0.15-0.35 0.05 0.15 5003-. 0. -00 0. 35 0.10 0.10 0.45-0.19 0.10 0.05 0.15 7075.- 0.5 -0.7 0.7 1.2-2.0 0.30 2 1 -2.9 0.18-0.40 5. 0.05 0.15

1n these tests the samples were given the following treattain a constant current density of 40 a.s.f. ranged from ment, it being understood that the conventional cold water 2 to 4 volts. rinsing operations after the various steps are not recited: (4) Samples 2, 7, 8, l3 and 14 were additionally elec- (1) Cleaned for 5 minutes in a mild inhibited alkaline troplated as a cathode with chromium in a 20-liter Pyrex cleaner [of the carbonate-phosphate-silicate type contained rectangular tank equipped with mechanical stirring and in a SO-gallon rectangular stainless steel tank at a temperalead anodes for 5 minutes with a direct current in an aquet-ure of 160 F. to a water break-free surface. ous electrolyte containing 33 ounces per gallon of 0x0 (2) Anodized in a 20-liter Pyrex rectangular tank 0.33 ounce per gallon oil-1 50 balance water. The temequipped with an air agitation device and lead cathodes per-attire of the electrolyte was maintainedat 100 F. by with direct current in an aqueous electrolyte containing means of an electric immersion heater. The current denphosphoric acid and sulfuric acid or phosphoric acid alone 35 sities were maintained at 200 a.s.f. and the voltages reor sulfuric acid alone in the amounts indicated in Table V quired to maintain such cur-rent densities ranged from- 5 below. These amounts are in terms of the percentages to 8 volts. The remaining samples were not plated with of aqueous 85% by weight H PO solution and aqueous chromium. 96% by weight H 80 solution in the electrolyte, the bal- (5) The samples were rinse briefly in distilled water, ance' of the electrolyte being water. The anodizing time, 40 dried and bulied to a decorative brightness. Chromium electrolyte temperature, current densities and the voltages plating of exceptional adherence was produced on samples required to maintain such current densities are given in 2, 7, 8, 13 and 14. A nickel plating of exceptional ad- TableVbelow: Table V herence was produced on samples 1, 3-6, 912 and 15 to 25. No peeling whatsoever was observed on any of the Anodizing conditions samples 1 through 25 even after severe grinding or bend- Sqm 10 Tam Current ing. All of the samples on which these adherent plat- No f Alloy Anodensity v ings were produced, i.e., samples 125, were anodized l Percent Percent in a phosphoric acid-sulfuric acid electrolyte embodying time, I'I3PO4 H2504 m, the prmciples of this invention as can be seen from Table V. Adherent platings could not be produced on the re- 1 00 5 15 a 130 gs 15 maining samples which were anodized either in phos- 38535::- g lg 3' 32 lg phoric acid alone or sulfuric acid alone. Samples 26 4 2011T4 5 15 15 90 48 15 and 28 could not be plated at all while with the remainder g: ggggi: 2 1 ,8 i; of samples 26-36 the platings exhibited very poor ad- 7 20214-3.-. 10 10 8.3 130 as 15 h i g"- i8 i2 3- ,8 In another example of the practice of this invention, 5 Q 3 138 fig a substantial number of automobile bumpers character- 2 Z 130 48 15 ized by multiple transverse and longitudinal curved por- 18 18 2% $8 -tions were fabricated from solution heat treated alumi- 5 5 mo 40 13 num alloys in the T4 temper of the type designated by g 3 g 13% l3 the letters B, C and D in Table I above. The percentages 5 10 10 4s 21 of elements other than aluminum are indicated in Table 5 10 10 130 48 13 V1 below 5 10 15 130 81 10 5 15 15 so 48 Table VI 2 15 15 130 90 5 s s a a 4 4 4O 15 1 10 72 81 To 011 Mn Mg Zn T1 5 0 15 00 00 5 0 15 130 48 .05 .13 2.01 .95 .01 5 0 25 90 96 .13 .15 2. 00 .01 .48 .02 .01 5 0 25 130 48 .11 .18 2.14 .05 .37 .03 .02 5 15 0 90 21 5 15 0 72 2 g 38 Z5 {-1 These bumpers were fabricated from sheets of the above 5 25 0 110 84 aluminum alloys having thicknesses ranging from about g g; g 1?, it %g 0.090 to 0.125 inch. These sheets were fabricated into 7 bumpers in the conventional manner generally employed '37 in the fabrication of steel bumpers, i.e. the sheets were pickled in a conventional pickling bath, dried, blanked (i.e., cut to the desired shape for forming), polished with abrasive belts, subjected to a conventional treatment to,

make them more suitable for retaining a lubricant, rinsed, dried, the lubricant applied and the bumper then formed by stamping in a conventional press. After stamping, the bumper was subjected to one or more trimming operations to remove excess metal.

These bumpers were subjected to the following treatment, it being understood that the conventional water rinsing operations after the various steps are not recited:

(1) Buifed to a decorative brightness.

(2) Cleaned for 5 minutes by soaking in a solution containing 60 g./l. of sodium tetraborate (Na B O 101-1 plus a small quantity of wetting agent, balance water contained in a 2500 gallon rectangular steel tank at a temperature of 170 F.

(3) Anodized in a 2500 gallon vinyl plastic-lined steel rectangular tank equipped with air agitation and lead ,cathodes for minutes with direct current in an aqueous phosphoric acid-sulfiuric acid electrolyte containing 15% by volume of aqueous 85% by weight H PO solution and 15 by volume of aqueous 96% by weight H 50 solution, the balance of the electrolyte being water. The temperature was maintained at 100 -F. with lead steam coils. The current density was maintained at 40 a.s.f. The voltages required to maintain a constant current density of 40 a.s.f. ranged from about 2 to 13 volts.

(4) Electroplated as a cathode with nickel in a 2500 gallon vinyl plastic-lined steel rectangular tank equipped wit-h continuous filtration and external heat exchange and nickel anodes for 5 minutes with a direct current in an aqueous electrolyte having a pH of 3.7, containing 300 g./l. of NiSO -6H 'O, 45 g./l. of NiCl -6H O, 40 g./l. of H BO .5 g/l. of a wetting agent, balance water. The temperature of the electrolyte was maintained at 130 F. by means of an external heat exchanger. The current density was maintained at 40 a.s.f. and the voltages required to maintain a constant current density of 40 a.s.f. ranged from 4 to 7 volts.

(5 Transferred to a second nickel plating bath and electroplated as a cathode with nickel in a 10,000 gallon vinyl plastic-lined steel rectangular tank equipped with air agitation and nickel anodes for 16 minutes with a direct current in an aqueous electrolyte having a pH of 3.7 containing 300 g./l. of NiSO' -6H O, 45 g./l. of NiCl -6H O, 40 g./l. of H BO .5 g./l. of wetting agent, .25 g./l. of coumarin (1,2-benzopyrone), .25 g./l. of formaldehyde (HCHO), balance water. The temperature of the electrolyte was maintained at 130 F. by means of a heat exchanger. The current density was maintained at 60 a.s.f. and the voltage required to maintain a constant current density of 60 a.s.f. ranged from to volts.

(6) Transferred directly without rinsing to a third nickel plating bath and electroplated as a cathode with nickel in a 10,000 gallon vinyl plastic-lined steel rectangular tank equipped with air agitation and nickel anodes for 8 minutes with a direct current in an aqueous electrolyte having a pH of 3.7 containing 300 g./l. of NiSO -6H O, 45 -g./l. of NiCl -6H O, 45 g./l. of H BO .5 g./l. of wetting agent, .1 g./l. of coumarin (1,2-benzopyrone), 1 g./l. of saccharin (O-sulfobenz-oic imide) and .005 g./l. of tris (aminophenyl) methanol, balance Water. The temperature of the electrolyte was maintained at 130 F. by means of a heat exchanger. The current density was maintained at 80 a.s.f. and the voltages re quired to maintain a constant current density of 80 a.s.f.

ranged from 10-15 volts.

(7) Electroplated as a cathode with chromium in a 2500 gallon lead lined steel rectangular tank equipped 5 with air agitation and lead anodes for 4 minutes with direct current in an aqueous electrolyte containing 250 g./l. of CrO 2.5 g./l. of H 80 balance water. The temperature of the electrolyte was maintained at 105 F. by means of lead steam oo-ils. The current density was maintained at 125 a.s.f. and the voltages required to maintain a constant current density of 125 a.s.f. ranged from 58 volts.

(8) Rinsed in cold water followed by a hot water rinse, and dried in air.

A lustrous chromium plating of exceptional adherence was produced. No peeling whatsoever was observed even after severe grinding or bending.

As used herein, the term aluminum is meant to cover high purity aluminum, commercial purity aluminum and aluminum alloys.

It will be understood that various changes, omissions and additions may be made to the invention without departing from the spirit and scope thereof as set forth in the appended claim-s.

What is claimed is:

1. A method of producing a firmly adherent coating of a metal selected from the group consisting of nickel and chromium on an article of an aluminum alloy containing by weight not more than 0.25% silicon, not more than 0.25% iron, 1.30 to 3.70% copper, not more than 0.25% manganese, 0.20 to 1.20% magnesium, not more than 0.30% zinc, a small but effective amount of titanium sufficient for grain refinement, impurities in normal amounts, balance substantially aluminum,

said method comprising the steps of immersing said aluminum alloy as an anode in an aqueous electrolyte consisting essentially of phosphoric acid and sulfuric acid, balance water,

passing an electric current through said electrolyte for a time period of from 1 to 30 minutes, at current density of from 12 to 120 amperes per square foot and a voltage of less than 40 volts,

thereby producing an oxide coating on the surface of said article thereafter electrodepositing a layer of metal selected from the group consisting of nickel and chromium directly on said oxide-coated surface.

2. The method of claim 1 wherein said electrolyte contains from 5 to 45% phosphoric acid and from 1 to 30% sulfuric acid, balance water and wherein said electrolyte is maintained at a temperature ranging from 80 to 140 F.

3. The method of claim 1 wherein at least one layer of nickel is electrodeposited on said oxide-coated surface and thereafter a layer of chromium is electrodeposited on said nickel.

4. An adherently plated aluminum automobile bumper characterized by multiple curved portions, a tensile strength of from about 30,000 to 58,000 p.s.i., a yield strength of from about 13,000 to 40,000 p.s.i. and a percentage elongation in two inches of from about 17 to 29 produced by the method of claim 3.

5. The composite article produced by the method of claim 1.

References Cited in the file of this patent UNITED STATES PATENTS 1,971,761 Travers Aug. 28, 1934 2,473,163 McCoy June 14, 1949 2,637,686 McKay May 5, 1953 2,703,781 Hesch Mar. 8, 1955 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. $098,804 July 23 1963 Henry J. Wittrock o've numbered patfied that error appears in the ab uld read as It is hereby certi d that the said Letters Patent sho ent requiring correction an corrected below.

lumn entitled "Elong.

Column 3, Table II, in the 00 percent in 2" third entry thereof, for 23-37 read 23-27 same table same column, seventh entry thereof for "23-36" read 23-26 Signed and sealed this 9th day of June 1964.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J BRENNER Commissioner of Patents Attesting Officer

Claims (1)

1. A METHOD OF PRODUCING A FIRMLY ADHERENT COATING OF A METAL SELECTED FROM THE GROUP CONSISTING OF NICKEL AND CHROMIUM ON AN ARTICLE OF AN ALUMINUM ALLOY CONTAINING BY WEIGHT NOT MORE THAN 0.25% SILICON, NOT MORE THAN 0.25% IRON, 1.30 TO 3.70% COPPER, NOT MORE THAN 0.25% MANGANESE, 0.02 TO 1.20% MAGNESIUM, NOT MORE THAN 0.30% ZINC, A SMALL BUT EFFECTIVE AMOUNT OF TITANIUM SUFFICIENT FOR GRAIN REFINEMENT, IMPURITIES IN NORMAL AMOUNTS, BALANCE SUBSTANTIALLY ALUMINUM, SAID METHOD COMPRISING THE STEPS OF IMMERSING SAID ALUMINUM ALLOY AS AN ANODE IN AN AQUEOUS ELECTROLYTE CONSISTING ESSENTIALLY OF PHOSPHORIC ACID AND SULFURIC ACID, BALANCE WATER, PASSING AN ELECTRIC CURRENT THROUGH SAID ELECTROLYTE FOR A TIME PERIOD OF FROM 1 TO 30 MINUTES, AT CURRENT DENSITY OF FROM 12 TO 120 AMPERES PER SQUARE FOOT AND A VOLTAGE OF LESS THAN 40 VOLTS, THEREBY PRODUCING AN OXIDE COATING ON THE SURFACE OF SAID ARTICLE THEREAFTER ELECTODEPOSITING A LAYER OF METAL SELECTED FROM THE GROUP CONSISTING OF NICKEL AND CHROMIUM DIRECTLY ON SAID OXIDE-COATED SURFACE.