Historical Articles

March, 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**

*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.

This Tentative Recommended Practice has been approved by the sponsoring committee and accepted by the Society in accordance with established procedure, for use pending adoption as standard. Suggestions for revisions should be addressed to the Society at 1916 Race Street, Philadelphia 3, Pa. which also supplies reprints.

SCOPE
1. (a) Various metals are electrodeposited on aluminum alloys to obtain a decorative finish or one which is more wear resistant or suitable for some other specific service. The electroplates applied for finish are usually chromium, nickel, copper, brass, silver, gold or modifications of these. Silver is applied to electrical equipment to decrease contact resistance or to improve surface conductivity; brass to facilitate vulcanition of rubber to aluminum; copper, nickel or tin for assembly by soft soldering; chromium to reduce friction and obtain increased resistance to wear; zinc to threaded parts where organic lubricants are not permissible; while tin is frequently employed to reduce friction on bearing surfaces.

(b) This recommended practice is presented as a guide for the plating of aluminum alloys. Electroplating of these alloys is commercially practical and economically sound. Aluminum alloys, however, do not respond satisfactorily to many of the usual preparatory procedures for plating; hence, different procedures are required to obtain a satisfactory basis for plating. Of the different methods available, the zinc-immersion method is considered to be the most satisfactory and practical for plating substantially all of the different aluminum alloys with various other metals.

NATURE OF ALUMINUM
2. (a) Microstructure.—It has been difficult to find a preplating procedure that will be equally satisfactory for all types and tempers of aluminum alloys because the various alloys and products behave differently electrochemically since they have different metallurgical structures. When elements are added for alloying purposes, they may appear in an aluminum alloy in several different forms; that is, they may be in solid solution in the aluminum lattice, be present as micro-particles of the elements themselves, or be present as particles of intermetallic compounds formed by combination with the aluminum. The several solid solution matrices and the twenty or more microconstituents that may occur in commercial alloys may have different chemical and electrochemical reactivities, and their surfaces may not respond uniformly to various chemical and electrochemical treatments. In addition, the response may be influenced by variations in the microstructure of different lots or products of the same alloy. The plater should know, if possible, the type of aluminum alloy that is to be plated in order to select the best plating procedure.

(b) Oxide Film.—In addition to differences- in microstructure that pay affect response to preplatingconditioning treatments, all aluminum products have an ever-present natural oxide film. This oxide film can be removed by various acid and alkaline treatments, but after rinsing, the surface will still have anoxide film that re-formed during the treatment. For best results with the zinc-immersion process, the new oxide film should be thinner and more uniform and provide a suitable surface for deposition of the zinc-immersion layer.

CLEANING AND CONDITIONING TREATMENTS
3. (a) To obtain consistent results with the zinc-immersion process for plating aluminum alloys, it is essential that the various cleaning and conditioning treatments provide a surface of uniform activity for the deposition of the zinc layer. First, the surface should be free of any oil, grease, or other foreign material. For removing oil or grease, vapor degreasing or solvent-cleaning may be necessary. Ordinarily, a mild etching-type alkaline cleaner is recommended. One can be made conveniently by using 23 g per l - (3 oz per gal) each of sodium carbonate and trisodium phosphate. This solution should be used at a temperature of 140 to 180°F (60 to 82°C) for 1 to 3 min.

(b) After appropriate cleaning, further treatment of the surface is generally required. For this conditioning treatment to be effective, it must accomplish two things: (1) remove the original oxide film; and (2) remove any microconstituents which may interfere with the formation of a continuous zinc-immersion layer or which may react with subsequent plating solutions.

(c) For wrought alloys of the 990A and M1A types(3), fairly good conditioning may be obtained by using the carbonate-phosphate cleaner followed by a dip in a nitric acid solution (50 per cent by volume). These alloys do not contain interfering constituents, and for some applications this method of conditioning may be ample.

(d) One of the more effective conditioning treatments for removing the surface oxide film and any undesirable microconstituents comprises the use of a hot acid etch containing sulfuric acid (15 per cent by volume) for a period of 2 to 5 min at a temperature of 180°F (82°C). The time of the dip depends upon the alloy involved. Generally the shorter time used on castings. This treatment is satisfactory for all wrought and most east aluminum alloys. It not only leaves the surface in an excellent condition for the formation of the zinc-immersion layer, but it also eliminates the undesirable effects of the magnesium-containing constituents in alloys of the GR20A(3), GSllA(4), and GS1OA(4) types.

(e) Another conditioning treatment which has considerable merit consists of a double zinc-immersion treatment with the first zinc layer being removed by a dip ina nitric acid solution (50 per cent by volume). This treatment has been found to be very effective for use with many east alloys and with wrought alloys that do not contain appreciable percentages of magnesium and when the identity of the alloy is not known. With this procedure, the first immersion dip removes the original oxide film and replaces it with a zinc layer. Removal of the zinc layer by the nitric acid dip leaves the surface in suitable condition for deposition of the final zinc-immersion layer.

(f) For casting alloys containing high percentages of silicon, S5(5), SC64C(6), SG70A(6) and SC7(5) type alloys, a dip for 3 to 5 sec in a solution containing 3 part (by volume) of concentrated commercial nitric acid plus 1 part (by volume) of commercial hydrofluoric acid (48 per cent) is recommended for conditioning the surface.

ZINC IMMERSION
4. (a) In the zinc-immersion step, the oxide film is removed from the surface to be plated and is replaced by a thin and adherent layer of metallic zinc. This provides a surface that will respond to most of the plating procedures for depositing other metals.

(b) For the immersion step, a highly alkaline solution(7) containing the following components is used at room temperature (60 to 80°F; 16 to 27°C):

ZINC-IMMERSION SOLUTION
Sodium hydroxide, commercial
  (76 per cent Na2O) 70 oz per gal (525 g per 1)
Zinc oxide (Technical Grade) 13 oz per gal (100 g per 1)

The thickness and quality of the immersion coating are influenced by the conditions of deposition. When deposition is too rapid, heavy coarsely crystalline and non-adherent deposits are formed. Since the thin zinc deposits give the best results, it is recommended that the temperature of the zincate solution be kept below 80°F (27°C) and the immersion time be from 30 sec to 1 min.

(c) The zincate solution is very viscous and losses - occur largely from drag-out. This is advantageous as it limits the accumulation of impurities resulting from attack on the aluminum.

(d) The specific gravity of the solution should be checked occasionally and any loss made up by adding more of the components. Loss of volume by dragout should be corrected by the addition of more solution of the specified composition.

(e) When a properly conditioned aluminum alloy article is immersed in the zincate solution, the thin natural oxide film that is present on the surface of the article dissolves, and as soon as any underlying aluminum is exposed, it also starts to dissolve and is immediately replaced by an equivalent weight of zinc. When the aluminum surface is completely covered with an extremely thin layer of zinc, action in this solution virtually ceases.

(f) With correct procedure, the resulting zinc deposit will be fairly uniform and firmly adherent to the surface. The appearance of the surface, however, will vary with the alloy being coated as well as with the rate at which the coating forms. The weight of zinc deposit should be of the order of 0.1 to 0.3 mg per sq in (0.016 to 0.048 mg per sq cm) (0.02 to 0.07 µ in thickness). Generally, it is desirable to limit the weight of the deposit to not over 0.2 mg per sq in (0.03 mg per sq cm).

(g) The thinner and more uniform zinc deposits are the most suitable for plating preparation and for the performance of plated coatings in service. Heavy zinc deposits tend to be spongy and less adherent and do not provide as good a surface for obtaining adhesion as the thinner deposits. The weight of zinc deposit rill vary with the alloy and the conditioning treatment that is used.
(To be continued)

(3). See Tentative Specifications for Aluminum and Aluminum-Alloy Sheet and Plate (A. S. T. M. Designation: B 209), 1950 Supplement to Book of A. S. T. M. Standards, Part 2.
(4). See Tentative Specifications for Aluminum and Aluminum-Alloy Extruded Bars, Rods, and Shapes (A. S. T. M. Designation; B 221), 1949 Book of A. S. T. M. Standards, Part 2. 5
(5). See Tentative Specifications for Aluminum-Base Alloy Die Castings (A. S. T. M. Designation: B 85) 1950 Supplement to Book of A. S. T. M. Standards, Part 2.
(6). See Tentative Specifications for Aluminum-Base Alloys in Ingot Form for Sand Casting”, Die Castings, and Permanent Mold Castings (A. S. T. M. Designation: B 179), 1950 Supplement to Book of A. S. T. M. Standarts, Part 2.
(7). Proprietary sodium zincate solutions of this general type are available commercially.