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Historical Articles

July, 1952 issue of Plating

 

Tentative Recommended Practice for
Preparation of Zinc-Base Die Castings for Plating*

A.S.T.M. Designation: B252-51 T, Issued 1951**

*Under the standardization procedure of the American Society for Testing Materials, this recommended practice is under the juris diction of A. S. T. M. Committee B-8 on Electrodeposited Metallic Coatings.
**Accepted by the American Society for Testing Materials, at Annual Meeting, June, 1951.
***1949 Book of A. S. T. M. Standards, Part 2.

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

SCOPE
1. This recommended practice is intended as an aid in establishing and maintaining a preparation cycle for electroplating zinc-base die castings conforming to the Standard Specifications for Zinc-Base Alloy Die Castings (A.S.T.M. Designation: B 86)***. It may be used in the production of the electrodeposited coatings conforming to the Tentative Specifications for Electrodeposited Coatings of Nickel and Chromium on Zinc and Zinc-Base Alloys (A.S.T.M. Designation: B 142)3. This recommended practice is advisory only and is not a part of either specification.

NATURE OF ZINC-BASE DIE CASTINGS
2. (a) The alloys used in the manufacture of zinc-base die castings are made with special high grade zinc conforming to the Standard Specifications for Slab Zinc (Spelter) (A.S.T.M. Designation: B 6) 3 alloyed with about 4 per cent of aluminum, 0.04 per cent of magnesium, and, depending upon the particular alloy involved, 0, 1, or 3 per cent of copper. Impurities such as lead, cadmium, and tin are held rigorously at specified low levels.
(b) Die castings made of these alloys are characterized by dense, fine-grained and, usually but not always, smooth surfaces. Complex shapes frequently are involved. In general, zinc surfaces are more sensitive than steel to contact with alkali, and care must taken to avoid prolonged contact with strongly alkaline cleaners if blistering of copper primary coatings is to be avoided. When a die lubricant is needed in the die casting process, the proper choice will minimize part of the problem of cleaning.

STEPS INVOLVED IN PREPARATION FOR ELECTROPLATING
3. Die castings usually reach the plater with the fins, gates, and overflow wells removed. The normal sequence of operations, preceding the first plating step, is as follows: (1) polish parting lines, (2) polish other surfaces where needed, (3) buff, (4) pre-clean and rinse (5) alkali clean (electroclean) and rinse, (6) acid dip and rinse, and (7) copper strike.

POLISHING PARTING LINES
4. (a) This operation may be carried out on set-up wheels or abrasive belts. To obtain the most rapid cutting, the coarsest abrasive size should be used that will permit the desired final smoothness to be obtained in the subsequent buffing operation. Sizes in the range No. 180 to No. 220 mesh commonly are used. The abrasive surface may be lubricated, with a suitable substance, with a resulting finer texture.
(b) Linear speeds of 6000 to 8000 surface feet per minute (1890-2400 m/min) should not be exceeded on set-up wheels. For abrasive belts, linear speeds of 5000 to 7000 surface feet per minute (1500-2100 m/min) should not be exceeded. Lower speeds, of the order of 3500 to 4000 surface feet per minute (1100-1200 m/min) are used in many plants. The following equations, relating revolutions per minute and wheel diameter to linear speed, may be helpful:

where:
S = linear speed in feet per minute,
R = speed of rotation in revolutions per minute,
D = wheel diameter in inches, and
p = 3.1416.

POLISHING OTHER SURFACES
5. While many die castings are sufficiently smooth to require only buffing, spot or over-all polishing may be necessary in some instances. A No. 220-mesh abrasive is generally used on set-up wheels or abrasive belts although finer sizes may suffice at times. Linear speeds of 6000 to 8000 surface feet per minute (1800 - 2400 m/min) should not be exceeded on set-up wheels or 5000 to 7000 surface feet per minute (1500-2100 m/min) on abrasive belts. Slower speeds, of the order of 3500 to 4000 surface feet per minute (1100-1200 m/min), are used in many plants. The abrasive surface should be lubricated.

BUFFING
6. (a) Buffing of zinc-base die castings should be carried out on cloth wheels of suitable stiffness at linear speeds not exceeding 7000 to 9000 surface feet per minute (2100-2700 m/min), slower speeds, of the order of 3500 to 5000 surface feet per minute (1100-1500 m/min), are used in many plants. There is available a good selection of compounds from which to choose. After buffing, or after buff and color, it is common practice to clean the surface by passing it over a relatively clean, dry buffing wheel. In some instances, a separate coloring operation may be needed. The polishing, buffing, and coloring steps must be properly graduated or else a poor surface, or the necessity for an additional operation, may result. The buffing compound should be made with a binder which is readily emulsified or saponified during the cleaning operation.

(b) Care in the buffing operation will be repaid by a considerable simplification of the ensuing cleaning cycle. The packing of hardened compound into holes and recesses adds greatly to the cleaning problem. Extra care in avoiding such packing (in some cases wiping to remove excess compound may be considered desirable) will reduce the demand placed on the precleaning operation.

PRECLEANING
7. (a) The preliminary removal, preferably as soon after polishing and buffing as possible, of most of the grease and soil in a precleaning operation is recommended strongly. The uneven distribution of soil on the average surface results in early local grease removal and prolonged alkali contact in the electrocleaner while heavier films elsewhere on the surface are being removed.

(b) The complexity and cost of the precleaning cycle will depend upon the type, distribution, and character of the soil to be removed. Under best conditions, there will be present a thin, reasonably uniform film of grease, which is readily removed in a simple sequence of degreasing and electrocleaning. Under worst conditions, hard-caked, sometimes partially carbonized buffing compound, will be packed into holes and recesses, removal of which may require several precIeaning steps, including some hand scrubbing. It is impossible to describe a single practice applicable in all cases.

(c) There are several ways by which greasy soil can be removed from zinc-base die castings prior to alkaline cleaning. Generally speaking, these fall into three main classes, solvent degreasing, emulsion cleaning, and cleaning with detergents, as follows:

(1) Solvent Degreasing.The most commonly used solvent is trichloroethylene (commonly called trichloroethylene), generally in multiphase operation. For example, three-phase operation involves washing in the heated liquid, partial rinsing and cooling by dipping into cold condensed liquid, and finally vapor rinsing by suspending the cooled die casting in the heated vapor. Where the soil is packed very hard, mechanical action such as power spraying or washing with the solvent must be included in the degreasing cycle. A sequence involving immersion in vapor, washing in a pressure spray of clean, condensed liquid, followed by a final vapor rinse, has received attention.

The trichloroethylene must be inhibited. Finely divided zinc and aluminum catalyze the decomposition of this solvent to produce harmful free acid. Frequent or continuous distillation is very important to avoid acidity.

Trichloroethylene does not burn, but its vapor is toxic. Degreasers are not ordinarily ventilated but are designed with proper shielding against stray air currents to minimize both the health hazard (Note) and loss of solvent.

Suppliers of trichloroethylene should be consulted as to the safety of a given installation. Carbon tetrachloride may also be employed for liquid solvent degreasing but since it is more toxic than trichloroethylene its use is not recommended. Tetrachloroethylene has attracted some attention but is not widely used at present.

In order to reduce solvent cost and to minimize the toxicity hazard, some plants use mineral solvents such as high test gasoline, kerosine, and mineral spirits. Like trichloroethylene, these solvents require agitation to loosen packed buffing compound. They offer no technological advantage over trichloroethylene, but they are cheaper. The fire hazard is obvious.

Mineral solvents are not easily removed during alkaline cleaning and tend to be dragged over into the plating solutions. Soak cleaning or thorough washing after mineral solvent degreasing is a usual requirement

NOTE Caution.—Prolonged exposure to atmospheres containing more than 200 ppm of trichloroethylene may cause head aches and permanent body damage. Exposure to vapors in quantity may cause death.

(2) Emulsion Cleaning.—Packed buffing compound may be loosened and to some extent removed by immersion in various emulsions. These frequently are composed of kerosine, emulsified with pine oil, soap and water. Power spraying or washing or other means for creating more or less violent contact with the work is required for best removal of packed-in soil. A soak cleaner or thorough washing is required to insure that no kerosine is carried over into the plating solutions. Inferior or poorly maintained rack coatings and rough flaky deposits built up on the racks will increase the danger of oil carry-over and must be avoided.

(3) Soak and Spray Cleaners.—Soak cleaners are usually built around suitable detergents which tend to soften and loosen packed soil deposits. Frequently, they are used in conjunction with force spraying or washing to add mechanical action to the softening effect of the detergent. Since excessive contact with strong alkalies must be avoided in cleaning zinc-base die castings, soak solutions should be as nearly neutral and the time of soaking as short as possible. Some solutions are difficult to remove by rinsing, so care should be taken in the selection to insure the best probability of adequate cleaner film removal.

ELECTROCLEANING
8. (a) Generally speaking, it is not possible to insure completely adequate cleaning in degreasing or precleaning operations. An electrolytic cleaning stage is required. It is the function of the precleaning operation (or operations) to remove all but traces of soil. If it does not do so, it is unlikely that the electrocleaning operation will correct the deficiency; the result may be a spotty final cleaning.

(b) A wide variety of proprietary cleaners is available commercially. Some of these are designed specifically for use with direct current (work as cathode) while others are intended for use with reverse current (work as anode). Cleaners designed for one direction of current flow should not be used with the other direction. While both types of solution have been used successfully, the so-called anodic cleaners greatly predominate today. Stains produced in anodic cleaners are more readily removed in the acid dip than those from cathodic cleaners.

(c) Attention must again be called to the need for minimizing alkali attack on zinc-base die castings. The entire sequence of cleaning operations must be considered in this connection. An electrocleaner which is safe for use after simple trichloroethylene degreasing may be too strong when its alkalinity effect is added to that of a previously used alkaline soak cleaner. Similarly, the relatively long immersion times required by some automatic conveyors may make it necessary to reduce the cleaner concentration or its temperature or perhaps to change to a milder cleaner. The better the precleaning the greater is the danger of overcleaning in the electrocleaner.

(d) Rinsing.—Rinsing after cleaning must be thorough. Wherever feasible, hot rinsing followed by cold rinsing should be used. Removal of solutions from holes and cavities may be facilitated by the use of directed sprays. Good rinsing practice frequently includes both spray and dip rinsing. Agitation of dip rinses is helpful. The efficacy of a rinse will increase with temperature; very low temperatures should be avoided.

ACID DIP
9. (a) An acid dip must follow alkaline cleaning. There is good evidence that the strength of the acid and the time of immersion should be adjusted to fit the cleaning cycle. In general, dilute acid dips should be used after short contacts with dilute mild alkalies and more concentrated acids after prolonged contact with more strongly alkaline solutions. Dilute solutions of sulfuric acid ( to 1 per cent by weight) are commonly used. Similar solutions of hydrochloric acid and to some extent hydrofluoric acid are suitable. For very mild acid dipping, citric acid has been suggested.

(b) Rinsing. Since cyanide copper solutions frequently are used as the first plating step; thorough rinsing after the acid dip is required.

COPPER STRIKE
10. Where copper is the first coating to be applied, it may be desirable to use a cyanide copper strike before entering the main copper plating step. The danger of copper blister formation may be minimized by such practice. The solutions used by individual platers vary somewhat in composition and operating conditions but generally fall within the range given below (the Rochelle salt addition is commonly used):

Copper 1.8 to 3.5 oz per gal (13.26 g/l)
Free cyanide 0.75 to 1.5 oz per gal (5.6-11 g/l)
Rochelle salts 2 to 5 oz per gal (15-37.5 g/l)
Sodium carbonate 2 to max 8 oz per gal (15-60 g/l)
pH 11.5 to 12.5
Current density a, 15 to 75 amp per sq ft (1.6 to 8.1 amp/dm2)
Temperature 120 to 140° F (49 to 60° C)
Plating timeb 15 sec to 3 min
a Current should be on when work enters the solution.
b Varies with current density used—shortest time with highest current density.

Following the copper strike, the plating may be continued in any normal cycle.

APPENDIX

EFFECT OF CLEANING CYCLE
Failure to achieve a proper cleaning cycle will result in separation of the plate usually in the form of blisters. When all of the grease has not been removed (undercleaning), the red copper surface will be seen on the back of detached particles of the coating. When the part has been overcleaned, the back of the coating will appear gray. This is due to the formation of white-gray alloys by diffusion between the zinc and the copper plate and the separation of the plate through these alloys. A gray film beneath the copper on detached blisters is almost infallible evidence of overcleaning.



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