ASTM B252-92(2004)
(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
SIGNIFICANCE AND USE
The performance and quality of electroplated or conversion-coated zinc alloy die casting depends upon the surface cleanliness and condition. Various metals are electroplated or conversion coatings are established on zinc alloys for decorative or engineering finish. The common electroplates applied are usually copper, nickel, and chromium for decorative and functional uses. The common conversion coatings applied are phosphates, chromates, and anodized coatings. Electroplated zinc die castings and conversion coatings on zinc die castings are used in many industries such as the marine, automotive, plumbing fixtures, and appliance industries.
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 (AG-40A) and UNS Z35530 (AC-41A) (Specification B 86) for electroplating with copper, nickel, and chromium (Specification B 456).
1.2 This standard does not purport to address all of the safety concerns, 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:B252–92(Reapproved 2004) 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 B252; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope 4. Significance and Use
1.1 This guide is intended as an aid in establishing and 4.1 The performance and quality of electroplated or
maintaining a procedure for preparing zinc alloy die castings conversion-coated zinc alloy die casting depends upon the
for electroplating and conversion coatings. It is primarily surface cleanliness and condition. Various metals are electro-
intended for the preparation ofAlloys UNS Z33521 (AG-40A) plated or conversion coatings are established on zinc alloys for
and UNS Z35530 (AC-41A) (Specification B86) for electro- decorative or engineering finish. The common electroplates
plating with copper, nickel, and chromium (Specification applied are usually copper, nickel, and chromium for decora-
B456). tive and functional uses. The common conversion coatings
1.2 This standard does not purport to address all of the applied are phosphates, chromates, and anodized coatings.
safety concerns, if any, associated with its use. It is the Electroplated zinc die castings and conversion coatings on zinc
responsibility of the user of this standard to establish appro- die castings are used in many industries such as the marine,
priate safety and health practices and determine the applica- automotive, plumbing fixtures, and appliance industries.
bility of regulatory limitations prior to use.
5. Composition and Characteristics of Zinc Alloy Die
2. Referenced Documents Castings
2.1 ASTM Standards: 5.1 The alloys used in the manufacture of zinc alloy die
B6 Specification for Zinc castings are made with special high-grade zinc conforming to
B86 Specification for Zinc and Zinc-Aluminum (ZA)Alloy Specification B6, alloyed with about 4 % of aluminum, 0.04 %
Foundry and Die Castings of magnesium, and either 0.25 (max) or 1.0 % copper (Alloys
B456 Specification for Electrodeposited Coatings of Cop- UNS Z33521 and UNS Z35530). Impurities such as lead,
per Plus Nickel Plus Chromium and Nickel Plus Chro- cadmium, tin, and iron are held at or below the specified low
mium levels in Specification B86.
2.2 Military Standard: 5.2 Die castings made of Alloys UNS 233521 and UNS
MIL-S-13165C Shot Peening of Metal Parts 235530 are usually dense and fine grained but do not always
have smooth surfaces. Defects sometimes encountered in the
3. Summary of Practice
surfacelayersincludecracks,crevices(coldshut),skinblisters,
3.1 The normal sequence of preparation steps is as follows: and hemispherical pores. Burrs are usually left at parting lines
(1) smoothing of parting lines; (2) smoothing of rough or
where fins and gates are removed by die trimming.
defective surfaces, if necessary; (3) buffing, if necessary; (4) 5.3 Cast surfaces are frequently contaminated with parting
precleaning and rinsing; (5) alkaline electrocleaning and rins-
compounds applied at frequent intervals to die surfaces to
ing; (6) acid dipping and rinsing; and (7) copper striking. facilitate the ejection of the castings and with water-soluble
oils added to quenching tanks for corrosion inhibition.
5.4 Zinc alloy die castings are chemically active and are
This guide is under the jurisdiction of ASTM Committee B08 on Metallic and
dissolvedoretchedduringprolongedcontactwithconcentrated
Inorganic Coatings and is the direct responsibility of Subcommittee B08.02 on Pre
Treatment. solutions of many mineral or organic acids or strongly alkaline
Current edition approved April 1, 2004. Published May 2004. Originally
solutions with a pH greater than 10. Immersion periods in such
approved in 1951. Last previous edition approved in 1998 as B252 – 92 (1998).
solutions should be of short duration to avoid roughening.
DOI: 10.1520/B0252-92R04.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
6. Smoothing of Parting Lines
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
6.1 Parting lines are smoothed by (1) mechanical polishing
the ASTM website.
with abrasive-coated wheels or belts, (2) tumbling with abra-
AvailablefromStandardizationDocumentsOrderDesk,Bldg.4SectionD,700
sive media, or (3) vibration with abrasives.
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B252–92 (2004)
6.2 Abrasiveswithasizerangeof220to300meshgluedon 7.4 Vibrating tubs loaded with plastic chips (such as poly-
cloth wheels or continuous cloth belts that run over flexible urethane) impregnated with an abrasive (such as aluminum
back-up wheels are usually used for mechanical polishing of oxide) smooth the surfaces of die castings in 2 to 4 h when
parting lines. Wheel diameters range from 5 to 40 cm, frequencies are in the range of 1700 to 2100 cpm and
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.Aperipheral produce a finish of 0.15 to 0.25 µm, with a cutting rate of 5
speedof2100m/minshouldnotbeexceededwithbelts.Lower µm/h.Asmoother 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
withasmallamountofgrease.Diecastingsusuallyarehandled 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 electroplatingandconversioncoating.Theprocessisdescribed
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.Ahexagonal barrel with a capacity of 0.5
3 8. Buffing
m can be loaded with 450 kg of abrasive stones or chips and
8.1 Die castings are buffed to produce a mirror-like finish,
90 kg of zinc die castings.
suitable for plating with conventional solutions, when good
6.4 Vibration in a bed of resin-bonded abrasive chips
leveling plating solutions are not available. Buffing can be
removes parting-line burrs, typically in 1 to 4 h. Frequencies
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.Adilute 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.Buffingcompoundsshouldbemadewithabinderthat
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.
withathreadcountof34to37/cm.Afinishof0.025to0.05µm
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
bythetemperatureofthemetalsurface(fasteratapproximately
partinglineareas(see6.2).Partinglinesandroughordefective 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,amanualoperationmayberequiredlatertocompletethe
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 Itisstronglyrecommendedthatthepreliminaryremoval
7.3 Smoothing by spinning in abrasives is accomplished by ofmostofthebuffingcompoundandothersoilinaprecleaning
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.Generallyspeaking,thesefallintothreemainclasses:
B252–92 (2004)
solvent degreasing, emulsion cleaning, and cleaning with 9.2.2.4 All federal, state, and local regulations for the use
aqueous base detergents. and disposal of solvents should be followed.
9.2.1 Solvent Degreasing—Before considering the use of 9.2.3 Aqueous Base Detergents—In recent years, hot mix-
solvent degreasing, federal and state safety and environmental tures of emulsifiers and surfactants (wetting agents), some-
laws and regulations should be consulted. Many of the com- times combined with mild alkaline phosphates or borates, are
monly used solvents are now being banned from use. Exposure usedforsoakcleaningtosoftenandremovebuffingcompound.
to their vapors (VOC) is being strictly regulated for health, Combining soak cleaning with ultrasonics is particularly effec-
safety,andenvironmentalreasons.Currentsafeexposurelevels tive on impacted buffing compound. Such detergent soaks
for various solvents should be obtained before use. Cold should be followed by spray cleaning with an alkaline cleaner.
solvents, such as mineral spirits, methylene chloride, trichlo- If a spray cleaning step is not needed, then the soak cleaning
roethylene, perchloroethylene and trichloroethane, are used step should be followed by a spray rinse with warm water
with brushing to loosen packed buffing compound, but this
before electrocleaning. Sometimes conventional alkaline soak
method usually is not practical for mass production conditions. cleaners are used for precleaning die castings with little or no
Simple dipping in cold solvent is often ineffective. Vapor
buffing compound on them. These alkaline cleaners must be
degreasing with trichloroethylene or perchloroethylene is mild and inhibited since strong alkali will attack the castings.
widely practiced. Often the buffed die castings are sprayed
9.3 Power Spray Alkaline Washing—Alkaline spray clean-
with, or immersed in, hot solvent for mechanical removal of
ers are widely used, during the initial cleaning operation or
heavy soil deposits. This is followed by condensation of hot,
following initial presoaks in emulsions, solvents, or detergents.
clean solvent vapors on the work; this removes the last traces
This is accomplished with conveyerized units equipped with
of grease and compound. The method is very effective,
washing, draining, rinsing, and draining sections. The solution
provided adequate measures are taken to remove the very fine
heated to a temperature range of 50 to 80°C is sprayed with a
abrasive and metallic particles from the work. Trichloroethyl-
pressure of 170 to 205 kPa through nozzles on 20 to 30 cm
ene and perchloroethylene are nonflammable as used in vapor
centers in the washing area.Atypical solution may contain 10
degreasing and still must be used in systems designed to
g/L of mixed alkalies such as trisodium phosphate, sodium
protect personnel from inhalation of vapors. Suppliers of
tripolyphosphate, sodium metasilicate, and sodium bicarbonate
solvents should be consulted as to the safety of a given
and not more than 1 g/L sodium hydroxide. The solution
installation.
should also contain not more than 0.2 g/L of a low-foaming or
9.2.1.1 All federal, state, and local regulations for the
non-foaming surfactant. In a typical precleaning cycle,a1to2
disposal of solvents should be followed.
min washing period is followed with a ⁄2 to 1 min draining
period, a ⁄2 to 1 min water rinse with sp
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