ASTM B252-92(2020)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: B252 − 92 (Reapproved 2020) 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 2.2 Military Standard:
MIL-S-13165C Shot Peening of Metal Parts
1.1 This guide is intended as an aid in establishing and
maintaining a procedure for preparing zinc alloy die castings
3. Summary of Practice
for electroplating and conversion coatings. It is primarily
3.1 The normal sequence of preparation steps is as follows:
intended for the preparation ofAlloys UNS Z33521 (AG-40A)
(1) smoothing of parting lines; (2) smoothing of rough or
and UNS Z35530 (AC-41A) (Specification B86) for electro-
defective surfaces, if necessary; (3) buffing, if necessary; (4)
plating with copper, nickel, and chromium (Specification
precleaning and rinsing; (5) alkaline electrocleaning and rins-
B456).
ing; (6) acid dipping and rinsing; and (7) copper striking.
1.2 The values stated in SI units are to be regarded as
standard. No other units of measurement are included in this
4. Significance and Use
standard.
4.1 The performance and quality of electroplated or
1.3 This standard does not purport to address all of the
conversion-coated zinc alloy die casting depends upon the
safety concerns, if any, associated with its use. It is the
surface cleanliness and condition. Various metals are electro-
responsibility of the user of this standard to establish appro-
plated or conversion coatings are established on zinc alloys for
priate safety, health, and environmental practices and deter-
decorative or engineering finish. The common electroplates
mine the applicability of regulatory limitations prior to use.
applied are usually copper, nickel, and chromium for decora-
1.4 This international standard was developed in accor-
tive and functional uses. The common conversion coatings
dance with internationally recognized principles on standard-
applied are phosphates, chromates, and anodized coatings.
ization established in the Decision on Principles for the
Electroplated zinc die castings and conversion coatings on zinc
Development of International Standards, Guides and Recom-
die castings are used in many industries such as the marine,
mendations issued by the World Trade Organization Technical
automotive, plumbing fixtures, and appliance industries.
Barriers to Trade (TBT) Committee.
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 Copper
UNS Z33521 and UNS Z35530). Impurities such as lead,
Plus Nickel Plus Chromium and Nickel Plus Chromium
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
5.2 Die castings made of Alloys UNS 233521 and UNS
Inorganic Coatings and is the direct responsibility of Subcommittee B08.02 on Pre
235530 are usually dense and fine grained but do not always
Treatment.
have smooth surfaces. Defects sometimes encountered in the
Current edition approved Nov. 1, 2020. Published December 2020. Originally
approved in 1951. Last previous edition approved in 2014 as B252 – 92 (2014).
surfacelayersincludecracks,crevices(coldshut),skinblisters,
DOI: 10.1520/B0252-92R20.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
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 AvailablefromStandardizationDocumentsOrderDesk,Bldg.4SectionD,700
the ASTM website. Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS.
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B252 − 92 (2020)
and hemispherical pores. Burrs are usually left at parting lines defects with a depth of 25 to 50 µm can usually be erased with
where fins and gates are removed by die trimming. these metal-removal methods. Deeper defects are infrequent.
7.2 Mechanical polishing for smoothing rough or defective
5.3 Cast surfaces are frequently contaminated with parting
compounds applied at frequent intervals to die surfaces to surfaces is similar to mechanical polishing for smoothing
partinglineareas(see6.2).Partinglinesandroughordefective
facilitate the ejection of the castings and with water-soluble
oils added to quenching tanks for corrosion inhibition. surfaces are frequently polished by the same operator. If
polishing is mechanized to advance die castings attached to a
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
6.2 Abrasiveswithasizerangeof220to300meshgluedon
from 5 to 10 min and the finish from 0.1 to 0.2 µm, depending
cloth wheels or continuous cloth belts that run over flexible
on the abrasive.
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
for small die castings polished on small diameter wheels.
5 µm⁄h. A smoother finish of 0.075 to 0.125 µm can be
Abrasive belts should not be used dry but should be lubricated
obtained with plastic media containing finer abrasive, which
withasmallamountofgrease.Diecastingsusuallyarehandled
removes metal at a slower rate. Media and zinc parts are
individually to polish parting lines smooth. This may require
usually loaded with a ratio of 5:1 or 6:1. Surface gouges may
30 s or less for small castings, and sometimes 5 or 6 min for
occur with a 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
rotated at 4 r/min. Higher speeds reduce the time cycles and
configuration, including the smallest size radii and wall
costs, but also increase the danger of impingement of parts
thickness, as well as the required finish and contamination
against zinc surfaces. A hexagonal barrel with a capacity of
limits, will dictate the proper selection of peening media, shot
0.5 m can be loaded with 450 kg of abrasive stones or chips
size, intensity, and coverage, as is detailed in MIL-S-13165C.
and 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
suitable for plating with conventional solutions, when good
6.4 mm. A vibrating tub with a capacity of 0.5 m can be
leveling plating solutions are not available. Buffing can be
loaded with about 900 kg of abrasive media and 180 kg of zinc
omitted, however, for die castings which have good surfaces or
die castings. A dilute solution of detergent or soap is continu-
which can be uniformly polished to a finish of 0.25 µm, if
ously 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
B252 − 92 (2020)
machines that advance die castings through a succession of forms, including unstable emulsions (diphase cleaners), invert
buffs of varying diameter and width, which individually type emulsions, mixtures of emulsions and alkaline cleaners,
smooth different surface areas. Buffs are usually made of cloth and stable emulsions. Such emulsion cleaners usually have a
withathreadcountof34to37/cm.Afinishof0.025to0.05µm suitable hydrocarbon base such as kerosene or a higher
can be produced by buffing. The smoothing rate is influenced flashpoint solvent to which is added emulsifiers, soaps, and
bythetemperatureofthemetalsurface(fasteratapproximately inhibitors to prevent etching of the die castings. The pH of the
150 °C than at lower temperatures). emulsion cleaner should be kept between 7 and 10 to avoid
damage to the castings.
8.3 After buffing, surfaces with impacted buffing compound
9.2.2.2 Theseemulsionsnormallyareusedhot,about80 °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 wa
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