Historical Articles

April, 1952 issue of Plating

 

TENTATIVE RECOMMENDED PRACTICE FOR PREPARATION OF AND PLATING ON ALUMINUM ALLOYS*

A.S.T.M. Designation: B 253-51T, Issued 1951**

Continued from the March 1952 issue.

*Under the standardization procedure of the American Society for Testing Materials, this recommended practice is under the jurisdiction of the A. S. T. M. Committee B-8 on Electrodeposited Metallic Coatings.
**Accepted by the American Society for Testing Materials at Annual Meeting, June, 1951.

 

PLATING ALUMINUM ALLOYS

5. (a) After the surface of an aluminum alloy article has been conditioned and the zinc-immersion deposit has been formed, other metals can be plated on this surface by any of the methods suitable for plating on zinc. One factor, however, must be taken into consideration; that is, the zinc deposit is extremely thin, and any plating treatment that penetrates the zinc layer and attacks the underlying aluminum will result in a poor deposit.

(b) Ordinarily, it is advisable to apply a suitable copper strike over the zinc-immersion layer before other metals are deposited. Silver, brass, zinc, nickel, or chromium, however, may be deposited on the zinc immersion layer provided the plating procedures are suitable for plating over zinc.

COPPER STRIKE
6. (a) For applying a copper strike prior to plating with other metals a Rochelle-type copper cyanide solution of the following composition is recommended:

ROCHELLE-TYPE COPPER STRIKE SOLUTION
Copper cyanide . . . . . . . . . . . 5.5 oz per gal (41.3 g per l)
Total sodium cyanide . . . . . . .6.5 oz per gal (48.8 g per l)
Sodium carbonate . . . . . . . . . 4.0 oz per gal (30.0 g per l)
Rochelle salts . . . . . . . . . . . . .8.0 oz per gal (60.0 g per l)
Free sodium cyanide, max. . . 0.5 oz per gal (3.8 g per l)

(b) This strike solution is generally employed at 100 to 130° F (38 to 54° C) and pH of 10.2 to 10.5. Electrical contact should be made before the article is immersed in the bath, and a high initial current density (24 amp per sq ft (2.6 amp per sq dm)) should be used to get -rapid coverage. After deposition for about 2 min at this current density, the current may be reduced to 12-amp per sq ft (1.3 amp per sq dm) and deposition continued for an additional 3 to 5 min depending upon thickness desired. A solution of higher pH, such as is sometimes used for copper strikes of this type, will give blistered copper deposits on aluminum alloys, especially on alloys GR20A8, GSllA4, and GS10Ad. After this strike, the work can be transferred to other standard plating solutions for further plating

BRASS PLATING
7. Brass can be applied from a standard brass plating solution, operated at room temperature, after the zinc-immersion step. A brass plating bath of the following composition is suitable:

BRASS PLATING SOLUTION
Copper cyanide . . . . . . 3.5 oz per gal (26.3 g per l)
Zinc cyanide . . . . . . . . 1.5 oz per gal (11.3 g per l)
Sodium cyanide . . . . . .6.0 oz per gal (45.0 g per l)
Sodium carbonate . . . . 1.0 oz per gal (7.5 g per l)
Temperature . . . . . . . . .80 to 90° F (27 to 32° C)
Current density . . . . . . 10 amp per sq ft (1.1 amp per sq dm)
Anodes . . . . . . . . . . . . .75Cu, 25Zn alloy

CADMIUM PLATING
8. Cadmium may be plated directly on the zinc immersion layer. The use of a strike solution prior to plating is recommended for maximum adhesion. In some cases it may be desirable to apply a nickel coating prior to cadmium plating. Cadmium strike and cadmium plating baths of the following composition are suitable:

CADMIUM STRIKE SOLUTION
Cadmium oxide . . . . 1.0 oz per gal (7.5 g per 1)
Sodium cyanide . . . . 8.0 oz per gal (60 g per 1)
Time . . . . . . . . . . . . . 1 min
Temperature . . . . . . . room
Current density . . . . . 25 amp per sq ft (2.7 amp per q ,dm)

CADMIUM PLATING SOLUTION
Cadmium oxide . . . . 3.5 oz per gal (26.3 g per 1)
Sodium cyanide . . . 13.4 oz per gal (100 g per 1)
Brightener . . . . . . . as required
Temperature . . . . . . room
Current density . . . . 15 to 45 amp per sq ft (1.6 to 4.8 amp per sq dm) in still tanks
Anodes . . . . . . . . . . cadmium balls

COPPER PLATING
9. Copper can be applied as a thin deposit on the zinc-immersion layer- from the Rochelle-type copper cyanide solution described in Section 6. It is recommended that the thickness be limited to less than 0.3 mil (7.6 µ), as rough deposits generally occur on high current density areas. By employing a lower current density, the time in this solution may be extended to give heavier deposits, but when a thickness of 0.3 mil (7.6 µ) has been obtained it is advisable to transfer the work to one of the proprietary copper plating baths which will plate at a faster rate.

CHROMIUM PLATING
10. (a) Chromium may be applied directly to aluminum alloys as a thin deposit (0.02 to 0.05 mil (0.5 to 1.3 µ) ). It is necessary, however, to first etch the work in a 45 g per l (6 oz per gal) sodium hydroxide solution at 150° F (66° C) for sufficient time (1 to 2 min) to develop a slight amount of roughening. After water rinsing, the parts should be dipped in a commercial nitric acid solution (50 per cent by volume) for 10 sec, again rinsed, and finally chromium plated in a standard chromium plating bath. A chromium plate applied by this method does not have as good adhesion as one applied over a zinc-immersion coating. The following chromium-plating solution is recommended for use with aluminum alloys:

CHROMIUM PLATING SOLUTION
Chromic acid, CrO3 . . . 33 oz per gal (250 g per 1)
Sulfate, SO4 . . . . . . . . 0.33 oz per gal (2.5 g per 1)
Temperature . . . . . . . 110 to 115° F (43 to 46° C)
Current density . . . . . 150 amp per sq ft (16 amp per sq dm)

(b) When the chromium is applied over the zinc immersion layer, it should be deposited first at a temperature of 65 to 70° F (18 to 21° C). After the initial deposit at low temperature, plating can be continued at a high temperature. The transition from low to high temperature can be accomplished by heating the chromium plating bath after the low temperature plate has been formed.’ The work can also be transferred without rinsing from a cold to a high temperature bath. The work should be held in the high temperature solution, without current however, until it reaches the temperature of the bath. Plating is then started at 150 amp per sq ft (16 amp per sq dm) and the current should be raised gradually to 300 or more amp per sq ft (32 amp per sq dm).

(c) Bright decorative chromium deposits can be applied to aluminum alloys in the conventional manner, that is the surface plated with copper and then with the required thickness of nickel. After nickel plating, the surface should be buffed before applying the chromium deposit. When a bright nickel is used, the chromium deposit can be applied directly. Standard recommendations regarding rinsing and activating the nickel surface should be followed.

(d) Another method for producing chromium deposits having a bright metallic luster consists in applying a 5 to 10-min deposit from a low-temperature solution (65 to 70° F; 18 to 21° C) directly over the zinc immersion layer. The deposit produced in this manner is slate gray, but may be buffed to an attractive metallic luster by using special chrome buffing compounds. This method can be used for applications requiring an inexpensive bright chromium finish. Surfaces finished in this manner will not smudge and are suitable for applications where corrosive conditions will not be encountered. The chromium layer is very thin, however, and will not be nearly as resistant to abrasion and mechanical damage as the copper nickel-chromium system usually employed for decorative applications.

(e) Heavy, hard chromium deposits can be applied to aluminum alloys by plating the chromium from a high-temperature (130° F (54° C) ) chromium plating solution at a current density of 150 amp per sq ft (16 amp per sq dm) depending upon the racking, contour of work and thickness of plating. The: work must first be given a 3 to 5-min strike in the Rochelle-type copper cyanide solution, then rinsed and finally transferred to the hot chromium-plating solution for application of hard chromium to the desired thickness.

(f) Hard chromium plates may also be deposited: directly over the zinc-immersion layer when the initial deposit is formed from a chromium-plating bath operated in the range of 65 to 70° F (18 to 21° C). After application of the initial chromium deposit at the low temperature, plating is continued in a bath operated at 130° F (54° C). The work should be held in the latter bath until it reaches the temperature of the bath before the current is applied. Plating should be started at 150 amp per sq ft (16 amp per sq dm) and the current should be raised gradually to 300 amp per sq ft (32 amp per sq dm) depending on the shape of the article, method of racking, etc.

GOLD PLATING
11. Gold may be plated on aluminum alloys by first applying a strike in the Rochelle-type copper cyanide solution or in a brass-plating solution. After this, a suitable deposit of nickel should be applied before application of the gold. The plating bath and conditions for gold plating are as follows:

GOLD PLATING SOLUTION
Potassium gold cyanide . . . 2 oz per gal (3.8 per 1)
Potassium carbonate . . . . . . 1 per gal (.5 g per 1)
Potassium cyanide . . . . . . . 2 oz per gal (3.8 g per 1)
Temperature . . . . . . . . . . .120 to 160° F (49 to 71° C)
Current density . . . . . . . . 5 to 15 amp per sq ft (0.5 to 1.6 amp per sq dm)
Voltage . . . . . . . . . . . . . . 2 to 6 v
Anodes . . . . . . . . . . . . . . Chromium-nickel steel or carbon, or combination of these with gold

NICKEL PLATING
12. (a) Nickel may be applied directly over the zinc-immersion layer by using baths formulated for plating over zinc. This method, however, is difficult to control, and satisfactory adhesion is not always obtained. Where dull or bright nickel is to be applied, it is preferable to do it after a thin copper plate has been deposited from the Rochelle-type copper cyanide strike solution.
For the nickel plating of aluminum alloys, subsequent to copper plating, the following types of nickel baths are suitable:

WATTS’ TYPE NICKEL SOLUTION (DULL NICKEL)
Nickel sulfate crystals . . . . 30 oz per gal (225 g per l)
Nickel chloride . . . . . . . . . . 6 oz per gal (48 g per l)
Boric acid . . . . . . . . . . . . . . 5 oz per gal (37.5 g per l)
pH . . . . . . . . . . . . . . . . . . . . 5.0
Temperature . . . . . . . . . . . . 130 to 140° F (54 to 60° C)
Current density . . . . . . . . . . 40 amp per sq ft (4.3 amp per sq dm)

BRIGHT NICKEL SOLUTIONS
Bright-nickel solutions are satisfactory for use with aluminum alloys. Use brightening agents and operating condition recommended by vendor.

(b) When nickel plating aluminum alloys, all the precautions usually recommended should be followed for obtaining a sound nickel plate—frequent carbon treatment, continuous purification, and filtration—to keep the nickel bath in good operating condition. Because of the large electrolytic potential between nickel and aluminum alloys the nickel plate should be of god quality and of sufficient thickness to provide adequate protection for the application at hand.

(c) For applications where corrosion is not a critical factor, nickel plate of the order of 0.3 to 0.5 mil (8 to 13 µ) in thickness will be satisfactory. Where corrosive - conditions may be encountered in service, however, the nickel-plate should be from 1 to 2 mil (25 to 50 µ) in thickness. In addition, the application of a final chromium plate 0.01 to 0.02 mil (0.25 to 0.50 µ) in thickness is essential, as it will increase the service life of a plated aluminum article to a surprising degree and will greatly reduce the detrimental effects of pores and exposed areas of bare metal.

(d) Ordinarily, nickel plating by itself is not recommended for use on aluminum alloys when even moderately corrosive conditions are likely to be encountered in service. When relatively heavy nickel deposits are applied to obtain protection, a ductile type of nickel plate should be used, and plating baths or operating conditions that produce highly stressed deposits should be avoided. With highly stressed deposits, stress cracks may develop on exposure, and blistering and spalling of the plating will occur.

SILVER PLATING
13. Silver can-be; deposited on the zinc-immersion coating by following the silver plating procedure employed for steel. This involves the use of two preliminary strikes and making contact before the work enters the solution. It is better, however, to apply silver over a copper plate without using the first silver strike. The following solutions may be used:

FIRST SILVER STRIKE SOLUTION
Silver cyanide . . . . 0.13 oz per gal (1 g per l)
Sodium cyanide . . . 12 oz per gal (90 g per l)
Temperature . . . . . . 80° F (27° C)
Current density . . . .15 to 25 amp per sq ft (1.6 to 2.7 amp per sq dm)
Tank voltage . . . . . 6 v
Time . . . . . . . . . . . 10 sec

SECOND SILVER STRIKE SOLUTION
Silver cyanide . . . . . 0.7 oz per gal (5.3 g per 1)
Sodium cyanide . . . . 9 oz per gal (67.5 g per 1)
Temperature . . . . . . . 80° F (27° C)
Current density . . . . . 15 to 25 amp per sq ft (1.6 to 2.7 amp per sq dm)
Tank voltage . . . . . . 6 v
Time . . . . . . . . . . . . 10 sec

SILVER PLATING SOLUTION
Silver cyanide . . . . . . . . . . . . 4 oz per gal (30 g per 1)
Potassium cyanide (total) . . . 7.4 oz per gal (55.5 g per 1)
Potassium carbonate . . . . . . . 6 oz per gal (45 g per 1)
Free potassium cyanide . . . . . 5.5 oz per gal (41.3 g per 1)
Temperature . . . . . . . . . . . . . 80° F (27° C)
Current density . . . . . . . . . . . 5 amp per sq ft (0.54 amp per sq dm)

TIN PLATING
14. Tin can be plated on a surface that has a zinc immersion coating and a copper strike. Either a stannate or a fluoborate-type tin plating solution can be used. For some applications, useful tin immersion deposits can be formed on aluminum from hot sodium stannate solutions.

ZINC PLATING
15. Zinc can be plated over the zinc-immersion layer from either acid or cyanide solutions at room temperature. The current, however, should be applied as the work is immersed in the plating solution. Zinc can also be applied directly to aluminum alloys without employing the zinc-immersion dip, but operating conditions are too critical for production use. A zinc plating bath of the following composition is suitable:

ZINC PLATING SOLUTION
Zinc cyanide . . . . . . 8.0 oz per gal (60 g per 1)
Sodium cyanide . . . . 5.6 oz per gal (42 g per 1)
Sodium hydroxide . . 10.5 oz per gal (78.8 g per 1)
Temperature. . . . . . . .75 to 95° F (24 to 35° C)
Current density . . . . . 5 to 50 amp per sq ft (0.54 to 5.4 amp per sq dm)

RACKING
16. (a) Aluminum racks are preferred when plating aluminum alloys. It is recommended that 990A alloy be used for the spines of the racks and CG42A alloy for the contacts. By increasing the cross-sectional area of the spines by about 40 per cent, a conductance equal to that of a copper rack is obtained. In cases where contact marks are not important, regular phosphor bronze contacts may be used. When using phosphor bronze contacts, however, the area adjacent to the contact may develop small blisters as a result of a non-uniform zinc deposit during the immersion step, Aluminum alloy contacts will eliminate this condition, and their use is recommended when contact marks would be on an exposed surface of the article being plated. When the nitric-hydrofluoric acid etch is used for conditioning, aluminum contacts should be used.

(b) When aluminum alloys are used for the plating racks, the various electrodeposits may be stripped from these racks by reversed-current treatment in a sulfuric acid solution (60 per cent by volume) or a phosphoric acid solution (75 per cent by volume) at room temperature. In either case, only slight attack occurs on the aluminum because of the oxide film that forms on the aluminum during these anodic treatments. The oxide film resulting from this stripping operation should be removed from contact areas either chemically, or mechanically. When phosphor-bronze contacts are used, it is recommended that’ they be stripped in the sulfuric acid solution (60 per cent by volume).

RINSING
17. (a) Effective rinses should follow every step in the plating procedure. The rinses must be reasonably clean and the rinsing thorough to prevent contamination of succeeding solutions. A combination of dip and spray rinsing has been found to use the least water for good rinsing. Since the zinc-immersion solution is rather viscous, the subsequent rinsing operation is very important. A double rinse is needed to remove all traces of the zincate solution. Two combination “dip and spray” rinses are desirable. Water from the second dip rinse can be made to overflow into the first dip rinse, thereby effecting an economy.

 

APPENDIX
CLEANING AND CONDITIONING
Summary of Cleaning and Conditioning Treatments:

A1. The following cleaning and conditioning treatments are suitable for the wrought and cast aluminum alloys covered by this recommended practice:

Treatment A:
(1) Clean:
(a) By vapor degreasing or with mineral spirits
(b) With alkaline cleaner and water rinse
(2) Acid dip, in nitric acid solution (50 per cent by volume)
(3) Water rinse
(4) Zinc-immersion dip, 1/2 to 1 min at 60 to 80° F (16 to 27° C)
(5) Double water rinse (6) Electroplate

Treatment B:
(1) Clean:
(a) By vapor degreasing or with mineral spirits
(b) With alkaline cleaner and water rinse
(2) Acid dip, in sulfuric acid solution (15 per cent volume), 2 to 5 min at 180° F
(3) Water rinse
(4) Acid dip, in nitric acid solution (50 per cent by volume) at room temperature
(5) Water rinse
(6) Zinc-immersion dip, 1/2 to 1 min at 60 to 80° F (16 to 27° C)
(7) Double water rinse
(8) Electroplate

Treatment C:
(1) Clean:
(a) By vapor degreasing or with mineral spirits
(b) With mild alkaline cleaner and water rinse
(2) Zinc immersion dip, 1/2 to 1 min at 60 to 80° F (16 to 27° C)
(3) Water rinse
(4) Acid dip, in nitric acid solution (50 per cent by volume) at room temperature
(5) Water rinse
(6) Zinc-immersion dip, 2 to 1 min at 60 to 80° F (16 to 27° C)
(7) Double water rinse
(8) Electroplate

Treatment D:
(1) Clean: (a) By vapor degreasing or with mineral spirits
(b) With mild etching alkaline cleaner and water rinse
(2) Mixed acid dip (nitric-hydroduoric at room temperature, 3 to 5 sec)
(3) Water rinse
(4) Zinc-immersion dip, 1/2 to 1 min at 60 to 80° F (16 to 27° C)
(5) Double water rinse
(6) Electroplate
The treatments applicable to specific alloys are shown in Table I.

Solutions for Cleaning and Conditioning Aluminum Alloys:
A2. The solutions described in Paragraphs (a) to (f) are suitable for cleaning and conditioning aluminum alloys.

(a) CARBONATE PHOSPHATE CLEANER
Sodium carbonate . . . . . 3 oz per gal (23 g per 1)
Trisodium phosphate . . . 3 oz per gal (23 g per 1)
Temperature . . . . . . . . . 140 to 180° F (60 to 82° C)
Time . . . . . . . . . . . . . . . 1 to 3 min
Container . . . . . . . . . . . steel

(b) SULFURIC ACID DIP:
Sulfuric acid (H2SO4 (66 deg Baumé) . . . 1.5 vol
Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5 vol
Temperature . . . . . . . .175 to 180° F (79 to 82° C)
Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 min
Container . . . . . . . . . . . . . . . . . . . lead-lined steel

(c) NITRIC ACID DIP:
Commercial nitric acid (sp gr 1.37) . . . . l vol
Water . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 vol
Temperature . . . . . . . . . . . . . . . . . . . . . . room
Container . . . steel lined with type 347 stainless steel
NOTE: Caution—Exhaust fumes are toxic.

(d) MIXED ACID DIP:
Commercial nitric acid (sp gr 1.42) . . . . . . . . . 3 vol
Commercial hydrofluoric acid (48 per cent) . . .1 vol
Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 to 5 sec
Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . room
Container . . . . . . . . . . . steel lined with carbon brick
NOTE: Caution—Exhaust fumes are toxic.

(e) ZINC-IMMERSION SOLUTION:
Zinc oxide . . . . . . . . . .13 oz per gal (100 g per 1)
Sodium hydroxide . . . .70 oz per gal (525 g per 1)
Time . . . . . . . . . . . . . . 1/2 to 1 min
Temperature . . . . . . . . 60 to 80° F (16 to 27° C)
Container . . . . . . . . . . steel

(f) CAUSTIC DIP:
Sodium hydroxide . . . . . 6 oz per gal (45 g per 1)
Time . . . . . . . . . . . . . . . .10 sec
Temperature . . . . . . . . . .150° F (66° C)
Container . . . . . . . . . . . . steel
NOTE: Caution—Exhaust fumes are toxic.