ASTM B252-92(1998)
(Guide)Standard Guide for Preparation of Zinc Alloy Die Castings for Electroplating and Conversion Coatings
Standard Guide for Preparation of Zinc Alloy Die Castings for Electroplating and Conversion Coatings
SCOPE
1.1 This guide is intended as an aid in establishing and maintaining a procedure for preparing zinc alloy die castings for electroplating and conversion coatings. It is primarily intended for the preparation of Alloys UNS Z33521 (AG40A) and UNS Z35530 (AC-41A) (Specification B86) for electroplating with copper, nickel, and chromium (Specification B456).
1.2 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Endorsed by American
Designation: B 252 – 92 (Reapproved 1998) Electroplaters’ Society
Endorsed by National
Association of Metal Finishers
Standard Guide for
Preparation of Zinc Alloy Die Castings for Electroplating
and Conversion Coatings
This standard is issued under the fixed designation B 252; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope surface cleanliness and condition. Various metals are electro-
plated or conversion coatings are established on zinc alloys for
1.1 This guide is intended as an aid in establishing and
decorative or engineering finish. The common electroplates
maintaining a procedure for preparing zinc alloy die castings
applied are usually copper, nickel, and chromium for decora-
for electroplating and conversion coatings. It is primarily
tive and functional uses. The common conversion coatings
intended for the preparation of Alloys UNS Z33521 (AG-40A)
applied are phosphates, chromates, and anodized coatings.
and UNS Z35530 (AC-41A) (Specification B 86) for electro-
Electroplated zinc die castings and conversion coatings on zinc
plating with copper, nickel, and chromium (Specification
die castings are used in many industries such as the marine,
B 456).
automotive, plumbing fixtures, and appliance industries.
1.2 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
5. Composition and Characteristics of Zinc Alloy Die
responsibility of the user of this standard to establish appro-
Castings
priate safety and health practices and determine the applica-
5.1 The alloys used in the manufacture of zinc alloy die
bility of regulatory limitations prior to use.
castings are made with special high-grade zinc conforming to
2. Referenced Documents Specification B 6, alloyed with about 4 % of aluminum, 0.04 %
of magnesium, and either 0.25 (max) or 1.0 % copper (Alloys
2.1 ASTM Standards:
2 UNS Z33521 and UNS Z35530). Impurities such as lead,
B 6 Specification for Zinc
cadmium, tin, and iron are held at or below the specified low
B 86 Specification for Zinc-Alloy Die Castings
levels in Specification B 86.
B 456 Specification for Electrodeposited Coatings of Cop-
5.2 Die castings made of Alloys UNS 233521 and UNS
per Plus Nickel Plus Chromium and Nickel Plus Chro-
235530 are usually dense and fine grained but do not always
mium
have smooth surfaces. Defects sometimes encountered in the
2.2 Military Standard:
surface layers include cracks, crevices (cold shut), skin blisters,
MIL-S-13165C Shot Peening of Metal Parts
and hemispherical pores. Burrs are usually left at parting lines
3. Summary of Practice where fins and gates are removed by die trimming.
5.3 Cast surfaces are frequently contaminated with parting
3.1 The normal sequence of preparation steps is as follows:
compounds applied at frequent intervals to die surfaces to
(1) smoothing of parting lines; (2) smoothing of rough or
facilitate the ejection of the castings and with water-soluble
defective surfaces, if necessary; (3) buffing, if necessary; (4)
oils added to quenching tanks for corrosion inhibition.
precleaning and rinsing; (5) alkaline electrocleaning and rins-
5.4 Zinc alloy die castings are chemically active and are
ing; (6) acid dipping and rinsing; and (7) copper striking.
dissolved or etched during prolonged contact with concentrated
4. Significance and Use
solutions of many mineral or organic acids or strongly alkaline
solutions with a pH greater than 10. Immersion periods in such
4.1 The performance and quality of electroplated or
solutions should be of short duration to avoid roughening.
conversion-coated zinc alloy die casting depends upon the
6. Smoothing of Parting Lines
This guide is under the jurisdiction of ASTM Committee B-8 on Metallic and
6.1 Parting lines are smoothed by (1) mechanical polishing
Inorganic Coatings and is the direct responsibility of Subcommittee B08.02 on
with abrasive-coated wheels or belts, (2) tumbling with abra-
Substrate Preparation.
sive media, or (3) vibration with abrasives.
Current edition approved July 15, 1992. Published October 1992. Originally
published as B 252 – 51. Last previous edition B 252 – 85.
6.2 Abrasives with a size range of 220 to 300 mesh glued on
Annual Book of ASTM Standards, Vol 02.04.
cloth wheels or continuous cloth belts that run over flexible
Annual Book of ASTM Standards, Vol 02.05.
4 back-up wheels are usually used for mechanical polishing of
Available from Standardization Documents Order Desk, Bldg. 4 Section D, 700
parting lines. Wheel diameters range from 5 to 40 cm,
Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
B 252
depending on the complexity of the shape. Wheels are rotated amplitudes are adjusted to 3.2 to 6.4 mm. Vibratory machines
with a minimum peripheral speed of 2500 m/min. A peripheral produce a finish of 0.15 to 0.25 μm, with a cutting rate of 5
speed of 2100 m/min should not be exceeded with belts. Lower μm/h. A smoother finish of 0.075 to 0.125 μm can be obtained
speeds of the order of 1100 to 1400 m/min are fairly common with plastic media containing finer abrasive, which removes
for small die castings polished on small diameter wheels. metal at a slower rate. Media and zinc parts are usually loaded
Abrasive belts should not be used dry but should be lubricated with a ratio of 5:1 or 6:1. Surface gouges may occur with a
with a small amount of grease. Die castings usually are handled smaller ratio.
individually to polish parting lines smooth. This may require 7.5 Controlled shot peening will plastically deform and
30 s or less for small castings, and sometimes 5 or 6 min for densify the casting surface and near-surface layers. Shot
larger ones. peening can seal surface pores, which can create problems in
6.3 Tumbling in horizontal barrels, loaded with abrasive electroplating and conversion coating. The process is described
stones such as limestone, preformed and fused aluminum in MIL-S-13165C. The process is also effective in removing
oxide, ceramic shapes or abrasive-loaded plastic chips, and a fins, burrs, and flash from the surface. The casting configura-
lubricant such as soap or detergent solution, removes parting- tion, including the smallest size radii and wall thickness, as
line burrs from die castings in 4 to 12 h. The barrels may be well as the required finish and contamination limits, will
rotated at 4 r/min. Higher speeds reduce the time cycles and dictate the proper selection of peening media, shot size,
costs, but also increase the danger of impingement of parts intensity, and coverage, as is detailed in MIL-S-13165C.
against zinc surfaces. A hexagonal barrel with a capacity of 0.5
3 8. Buffing
m can be loaded with 450 kg of abrasive stones or chips and
90 kg of zinc die castings. 8.1 Die castings are buffed to produce a mirror-like finish,
suitable for plating with conventional solutions, when good
6.4 Vibration in a bed of resin-bonded abrasive chips
removes parting-line burrs, typically in 1 to 4 h. Frequencies leveling plating solutions are not available. Buffing can be
omitted, however, for die castings which have good surfaces or
range from 700 to 2100 cpm and amplitudes from 0.8 to 6.4
which can be uniformly polished to a finish of 0.25 μm, if
mm. A vibrating tub with a capacity of 0.5 m can be loaded
solutions with good leveling power are used for plating copper
with about 900 kg of abrasive media and 180 kg of zinc die
and nickel.
castings. A dilute solution of detergent or soap is continuously
8.2 Die castings are buffed on cloth wheels rotated at a
metered through the bed of media and parts to keep their
peripheral speed not exceeding 2150 m/min. Slower speeds, of
surfaces clean and maximize surface smoothing. Parting lines
the order of 1100 to 1600 m/min, are used for small die
may be mechanically polished before vibratory processing
castings. Buffing compounds should be made with a binder that
when a large amount of flash must be removed.
is readily emulsified or saponified during alkaline cleaning.
7. Smoothing of Rough or Defective Surfaces
The abrasive may be tripoli (amorphous silica) or lime, mixed
7.1 Rough or defective surfaces are smoothed by (1) me-
with about 25 % of tallow or other lubricants. Compounds
chanical polishing on rotating wheels or continuous, abrasive-
suspended in a liquid are preferred for automatic buffing
coated belts, (2) spin finishing, (3) vibratory finishing, or (4)
machines that advance die castings through a succession of
controlled shot peening. Fissures, skin blisters, and other
buffs of varying diameter and width, which individually
defects with a depth of 25 to 50 μm can usually be erased with
smooth different surface areas. Buffs are usually made of cloth
these metal-removal methods. Deeper defects are infrequent.
with a thread count of 34 to 37/cm. A finish of 0.025 to 0.05 μm
7.2 Mechanical polishing for smoothing rough or defective
can be produced by buffing. The smoothing rate is influenced
surfaces is similar to mechanical polishing for smoothing
by the temperature of the metal surface (faster at approximately
parting line areas (see 6.2). Parting lines and rough or defective
150°C than at lower temperatures).
surfaces are frequently polished by the same operator. If
8.3 After buffing, surfaces with impacted buffing compound
polishing is mechanized to advance die castings attached to a can be improved by passing them over a dry wheel to remove
conveyor through successive belts or wheels to polish different
buffing compound. This will reduce the demand placed on the
areas, a manual operation may be required later to complete the precleaning solution.
smoothing of parting lines if they are too curved. The finish
9. Precleaning and Rinsing
ranges from 0.2 to 0.6 μm, depending on the abrasive and the
pressure. 9.1 It is strongly recommended that the preliminary removal
7.3 Smoothing by spinning in abrasives is accomplished by of most of the buffing compound and other soil in a precleaning
attaching die castings to spindles or drums rotated with a operation be done as soon as possible after buffing and
peripheral speed of about 600 m/min in a slurry of abrasive polishing. Most buffing compounds become substantially more
material such as ground corn cobs or nut shells mixed with a difficult to remove after aging several days.
small amount of grease or other lubricant. Times usually range 9.2 There are several methods by which soils can be
from 5 to 10 min and the finish from 0.1 to 0.2 μm, depending removed from zinc die castings prior to final alkaline electro-
on the abrasive. cleaning. Generally speaking, these fall into three main classes:
7.4 Vibrating tubs loaded with plastic chips (such as poly- solvent degreasing, emulsion cleaning, and cleaning with
urethane) impregnated with an abrasive (such as aluminum aqueous base detergents.
oxide) smooth the surfaces of die castings in 2 to 4 h when 9.2.1 Solvent Degreasing—Before considering the use of
frequencies are in the range of 1700 to 2100 cpm and solvent degreasing, federal and state safety and environmental
B 252
laws and regulations should be consulted. Many of the com- used for soak cleaning to soften and remove buffing compound.
monly used solvents are now being banned from use. Exposure Combining soak cleaning with ultrasonics is particularly effec-
to their vapors (VOC) is being strictly regulated for health,
tive on impacted buffing compound. Such detergent soaks
safety, and environmental reasons. Current safe exposure levels should be followed by spray cleaning with an alkaline cleaner.
for various solvents should be obtained before use. Cold
If a spray cleaning step is not needed, then the soak cleaning
solvents, such as mineral spirits, methylene chloride, trichlo-
step should be followed by a spray rinse with warm water
roethylene, perchloroethylene and trichloroethane, are used
before electrocleaning. Sometimes conventional alkaline soak
with brushing to loosen packed buffing compound, but this
cleaners are used for precleaning die castings with little or no
method usually is not practical for mass production conditions.
buffing compound on them. These alkaline cleaners must be
Simple dipping in cold solvent is often ineffective. Vapor
mild and inhibited since strong alkali will attack the castings.
degreasing with trichloroethylene or perchloroethylene is
9.3 Power Spray Alkaline Washing—Alkaline spray clean-
widely practiced. Often the buffed die castings are sprayed
ers are widely used, during the initial cleaning operation or
with, or immersed in, hot solvent for mechanical removal of
following initial presoaks in emulsions, solvents, or detergents.
heavy soil deposits. This is followed by condensation of hot,
This is accomplished with conveyerized units equipped with
clean solvent vapors on the work; this removes the last traces
washing, draining, rinsing, and draining sections. The solution
of grease and compound. The method is very effective,
heated to a temperature range of 50 to 80°C is sprayed with a
provided adequate measures are taken to remove the very fine
pressure of 170 to 205 kPa through nozzles on 20 to 30 cm
abrasive and metallic particles from the work. Trichloroethyl-
centers in the washing area. A typical solution may contain 10
ene and perchloroethylene are nonflammable as used in vapor
g/L of mixed alkalies such as trisodium phosphate, sodium
degreasing and still must be used in systems designed to
tripolyphosphate, sodium metasilicate, and sodium bicarbonate
protect personnel from inhalation of vapors. Suppliers of
and not more than 1 g/L sodium hydroxide. The solution
solvents should be consulted as to the safety of a given
should also contain not more than 0.2 g/L of a low-foaming or
installation.
non-foaming surfactant. In a typical precleaning cycle,a1to2
9.2.1.1 All federal, state, and local regulations for the
min washing period is followed with a ⁄2 to 1 min draining
disposal of solvents should be followed.
period, a ⁄2 to 1 min water rinse with spray nozzles and a final
9.2.2 Emulsion Cleaning:
draining period of ⁄2 to 1 min. Proprietary alkaline spray
9.2.2.1 Impacted buffing compound may be loosened, and to
cleaners are available for this application.
some extent removed, by immersion in various hydrocarbon-
water emulsions. These emulsions are available in several
10. Alkaline El
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