ASTM B252-92(2009)
(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 B86) for electroplating with copper, nickel, and chromium (Specification B456).
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 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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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: B252 − 92 (Reapproved2009) Endorsed by American
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 3. Summary of Practice
1.1 This guide is intended as an aid in establishing and
3.1 The normal sequence of preparation steps is as follows:
maintaining a procedure for preparing zinc alloy die castings
(1) smoothing of parting lines; (2) smoothing of rough or
for electroplating and conversion coatings. It is primarily
defective surfaces, if necessary; (3) buffing, if necessary; (4)
intended for the preparation ofAlloys UNS Z33521 (AG-40A)
precleaning and rinsing; (5) alkaline electrocleaning and rins-
and UNS Z35530 (AC-41A) (Specification B86) for electro-
ing; (6) acid dipping and rinsing; and (7) copper striking.
plating with copper, nickel, and chromium (Specification
B456).
4. Significance and Use
1.2 The values stated in SI units are to be regarded as
4.1 The performance and quality of electroplated or
standard. No other units of measurement are included in this
conversion-coated zinc alloy die casting depends upon the
standard.
surface cleanliness and condition. Various metals are electro-
1.3 This standard does not purport to address all of the plated or conversion coatings are established on zinc alloys for
safety concerns, if any, associated with its use. It is the decorative or engineering finish. The common electroplates
responsibility of the user of this standard to establish appro- applied are usually copper, nickel, and chromium for decora-
priate safety and health practices and determine the applica- tive and functional uses. The common conversion coatings
bility of regulatory limitations prior to use. applied are phosphates, chromates, and anodized coatings.
Electroplated zinc die castings and conversion coatings on zinc
2. Referenced Documents die castings are used in many industries such as the marine,
automotive, plumbing fixtures, and appliance industries.
2.1 ASTM Standards:
B6 Specification for Zinc
5. Composition and Characteristics of Zinc Alloy Die
B86 Specification for Zinc and Zinc-Aluminum (ZA) Alloy
Castings
Foundry and Die Castings
B456 Specification for Electrodeposited Coatings of Copper
5.1 The alloys used in the manufacture of zinc alloy die
Plus Nickel Plus Chromium and Nickel Plus Chromium
castings are made with special high-grade zinc conforming to
Specification B6, alloyed with about 4 % of aluminum, 0.04 %
2.2 Military Standard:
of magnesium, and either 0.25 (max) or 1.0 % copper (Alloys
MIL-S-13165C Shot Peening of Metal Parts
UNS Z33521 and UNS Z35530). Impurities such as lead,
cadmium, tin, and iron are held at or below the specified low
levels in Specification B86.
This guide is under the jurisdiction of ASTM Committee B08 on Metallic and
Inorganic Coatings and is the direct responsibility of Subcommittee B08.02 on Pre
5.2 Die castings made of Alloys UNS 233521 and UNS
Treatment.
235530 are usually dense and fine grained but do not always
Current edition approved Sept. 1, 2009. Published December 2009. Originally
approved in 1951. Last previous edition approved in 2004 as B252 – 92 (2004). have smooth surfaces. Defects sometimes encountered in the
DOI: 10.1520/B0252-92R09.
surfacelayersincludecracks,crevices(coldshut),skinblisters,
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
and hemispherical pores. Burrs are usually left at parting lines
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
where fins and gates are removed by die trimming.
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
5.3 Cast surfaces are frequently contaminated with parting
AvailablefromStandardizationDocumentsOrderDesk,Bldg.4SectionD,700
Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS. compounds applied at frequent intervals to die surfaces to
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B252 − 92 (Reapproved2009)
facilitate the ejection of the castings and with water-soluble
oils added to quenching tanks for corrosion inhibition.
B252 − 92 (2009)
5.4 Zinc alloy die castings are chemically active and are conveyor through successive belts or wheels to polish different
dissolvedoretchedduringprolongedcontactwithconcentrated areas,amanualoperationmayberequiredlatertocompletethe
solutions of many mineral or organic acids or strongly alkaline smoothing of parting lines if they are too curved. The finish
solutions with a pH greater than 10. Immersion periods in such ranges from 0.2 to 0.6 µm, depending on the abrasive and the
solutions should be of short duration to avoid roughening. pressure.
7.3 Smoothing by spinning in abrasives is accomplished by
6. Smoothing of Parting Lines
attaching die castings to spindles or drums rotated with a
6.1 Parting lines are smoothed by (1) mechanical polishing
peripheral speed of about 600 m/min in a slurry of abrasive
with abrasive-coated wheels or belts, (2) tumbling with abra-
material such as ground corn cobs or nut shells mixed with a
sive media, or (3) vibration with abrasives.
small amount of grease or other lubricant. Times usually range
from 5 to 10 min and the finish from 0.1 to 0.2 µm, depending
6.2 Abrasiveswithasizerangeof220to300meshgluedon
on the abrasive.
cloth wheels or continuous cloth belts that run over flexible
back-up wheels are usually used for mechanical polishing of
7.4 Vibrating tubs loaded with plastic chips (such as poly-
parting lines. Wheel diameters range from 5 to 40 cm,
urethane) impregnated with an abrasive (such as aluminum
depending on the complexity of the shape. Wheels are rotated
oxide) smooth the surfaces of die castings in 2 to 4 h when
with a minimum peripheral speed of 2500 m/min.Aperipheral
frequencies are in the range of 1700 to 2100 cpm and
speedof2100m/minshouldnotbeexceededwithbelts.Lower
amplitudes are adjusted to 3.2 to 6.4 mm. Vibratory machines
speeds of the order of 1100 to 1400 m/min are fairly common
produce a finish of 0.15 to 0.25 µm, with a cutting rate of 5
for small die castings polished on small diameter wheels.
µm/h.Asmoother finish of 0.075 to 0.125 µm can be obtained
Abrasive belts should not be used dry but should be lubricated
with plastic media containing finer abrasive, which removes
withasmallamountofgrease.Diecastingsusuallyarehandled
metal at a slower rate. Media and zinc parts are usually loaded
individually to polish parting lines smooth. This may require
with a ratio of 5:1 or 6:1. Surface gouges may occur with a
30 s or less for small castings, and sometimes 5 or 6 min for
smaller ratio.
larger ones.
7.5 Controlled shot peening will plastically deform and
6.3 Tumbling in horizontal barrels, loaded with abrasive
densify the casting surface and near-surface layers. Shot
stones such as limestone, preformed and fused aluminum
peening can seal surface pores, which can create problems in
oxide, ceramic shapes or abrasive-loaded plastic chips, and a
electroplatingandconversioncoating.Theprocessisdescribed
lubricant such as soap or detergent solution, removes parting-
in MIL-S-13165C. The process is also effective in removing
line burrs from die castings in 4 to 12 h. The barrels may be
fins, burrs, and flash from the surface. The casting configura-
rotated at 4 r/min. Higher speeds reduce the time cycles and
tion, including the smallest size radii and wall thickness, as
costs, but also increase the danger of impingement of parts
well as the required finish and contamination limits, will
against zinc surfaces.Ahexagonal barrel with a capacity of 0.5
dictate the proper selection of peening media, shot size,
m can be loaded with 450 kg of abrasive stones or chips and
intensity, and coverage, as is detailed in MIL-S-13165C.
90 kg of zinc die castings.
8. Buffing
6.4 Vibration in a bed of resin-bonded abrasive chips
removes parting-line burrs, typically in 1 to 4 h. Frequencies
8.1 Die castings are buffed to produce a mirror-like finish,
range from 700 to 2100 cpm and amplitudes from 0.8 to 6.4
suitable for plating with conventional solutions, when good
mm. A vibrating tub with a capacity of 0.5 m can be loaded
leveling plating solutions are not available. Buffing can be
with about 900 kg of abrasive media and 180 kg of zinc die
omitted, however, for die castings which have good surfaces or
castings.Adilute solution of detergent or soap is continuously
which can be uniformly polished to a finish of 0.25 µm, if
metered through the bed of media and parts to keep their
solutions with good leveling power are used for plating copper
surfaces clean and maximize surface smoothing. Parting lines
and nickel.
may be mechanically polished before vibratory processing
8.2 Die castings are buffed on cloth wheels rotated at a
when a large amount of flash must be removed.
peripheral speed not exceeding 2150 m/min. Slower speeds, of
the order of 1100 to 1600 m/min, are used for small die
7. Smoothing of Rough or Defective Surfaces
castings.Buffingcompoundsshouldbemadewithabinderthat
7.1 Rough or defective surfaces are smoothed by (1) me-
is readily emulsified or saponified during alkaline cleaning.
chanical polishing on rotating wheels or continuous, abrasive-
The abrasive may be tripoli (amorphous silica) or lime, mixed
coated belts, (2) spin finishing, (3) vibratory finishing, or (4)
with about 25 % of tallow or other lubricants. Compounds
controlled shot peening. Fissures, skin blisters, and other
suspended in a liquid are preferred for automatic buffing
defects with a depth of 25 to 50 µm can usually be erased with
machines that advance die castings through a succession of
these metal-removal methods. Deeper defects are infrequent.
buffs of varying diameter and width, which individually
7.2 Mechanical polishing for smoothing rough or defective smooth different surface areas. Buffs are usually made of cloth
surfaces is similar to mechanical polishing for smoothing withathreadcountof34to37/cm.Afinishof0.025to0.05µm
partinglineareas(see6.2).Partinglinesandroughordefective can be produced by buffing. The smoothing rate is influenced
surfaces are frequently polished by the same operator. If bythetemperatureofthemetalsurface(fasteratapproximately
polishing is mechanized to advance die castings attached to a 150°C than at lower temperatures).
B252 − 92 (2009)
8.3 After buffing, surfaces with impacted buffing compound 9.2.2.2 These emulsions normally are used hot, about 80°C,
can be improved by passing them over a dry wheel to remove as a soak, sometimes with agitation, for about 2 to 5 min. A
buffing compound. This will reduce the demand placed on the warm water spray rinse should follow the emulsion soak
precleaning solution. cleaning. Buffing compound not removed in the emulsion soak
is sufficiently softened so that it is easily removed in an
9. Precleaning and Rinsing
alkaline spray wash operation that normally follows.
9.2.2.3 Emulsion cleaning is an effective method for remov-
9.1 Itisstronglyrecommendedthatthepreliminaryremoval
ing buffing compound. Its principal disadvantage is the danger
ofmostofthebuffingcompoundandothersoilinaprecleaning
of carryover of hydrocarbon solvent into plating baths because
operation be done as soon as possible after buffing and
of incomplete rinsing. For this reason, it is very important that
polishing. Most buffing compounds become substantially more
proper alkaline cleaning and rinsing follow to ensure solvent
difficult to remove after aging several days.
removal from blind holes, defects in rack coatings, and
9.2 There are several methods by which soils can be
recesses.
removed from zinc die castings prior to final alkaline electro-
9.2.2.4 All federal, state, and local regulations for the use
cleaning.Generallyspeaking,thesefallintothreemainclasses:
and disposal of solvents should be followed.
solvent degreasing, emulsion cleaning, and cleaning with
9.2.3 Aqueous Base Detergents—In recent years, hot mix-
aqueous base detergents.
tures of emulsifiers and surfactants (wetting agents), some-
9.2.1 Solvent Degreasing—Before considering the use of
times combined with mild alkaline phosphates or borates, are
solvent degreasing, federal and state safety and environmental
usedforsoakcleaningtosoftenandremovebuffingcompound.
laws and regulations should be consulted. Many of the com-
Combining soak cleaning with ultrasonics is particularly effec-
monly used solvents are now being banned from use. Exposure
tive on impacted buffing compound. Such detergent soaks
to their vapors (VOC) is being strictly regulated for health,
should be followed by spray cleaning with an alkaline cleaner.
safety,andenvironmentalreasons.Currentsafeexposurelevels
If a spray cleaning step is not needed, then the soak cleaning
for various solvents should be obtained before use. Cold
step should be followed by a spray rinse with warm water
solvents, such as mineral spirits, methylene chloride, trichlo-
before electrocleaning. Sometimes conventional alkaline soak
roethylene, perchloroethylene and trichloroethane, are used
cleaners are used for precleaning die castings with little or no
with brushing to loosen packed buffing compound, but this
buffing compound on them. These alkaline cleaners must be
method usually is not practical for mass production conditions.
mild and inhibited since strong alkali will attack the castings.
Simple dipping in cold solvent is often ineffective. Vapor
9.3 Power Spray Alkaline Washing—Alkaline spray clean-
degreasing with trichloroethylene or perchloroethylene is
ers are widely used, during the initial cleaning operation or
widely practiced. Often the buffed die castings are sprayed
following initial presoaks in emulsions, solvents, or detergents.
with, or immersed in, hot solvent for mechanical removal of
This is accomplished with conveyerized units equipped with
heavy soil deposits. This is followed by condensation of hot,
washing, draining, rinsing, and draining sections. The solution
clean solvent vapors on the work; this removes the last traces
heated to a temperature range of 50 to 80°C is sprayed with a
of grease and compound. The method is very effective,
pressure of 170 to 205 kPa through nozz
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