ASTM B253-87(1999)e1

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e1
Designation:B253 – 87(Reapproved 1999)
Standard Guide for
Preparation of Aluminum Alloys for Electroplating
This standard is issued under the fixed designation B 253; 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.
This standard has been approved for use by agencies of the Department of Defense.
e NOTE—Footnote 12 was editorially corrected in March 1999.
1. Scope Castings from all Casting Processes
B 209 Specification for Aluminum and Aluminum-Alloy
1.1 This guide covers cleaning and conditioning treatments
Sheet and Plate
used before metal deposition (Section 5), and immersion
B 209M Specification for Aluminum and Aluminum-Alloy
deposit/strike procedures (Section 6) that enhance the adhesion
Sheet and Plate [Metric]
of metals that are subsequently applied to aluminum products
B 221 Specification for Aluminum-Alloy Extruded Bars,
by electrodeposition or by autocatalytic chemical reduction.
Rods, Wire, Shapes, and Tubes
1.2 The following immersion deposit/strike procedures are
B 221M Specification for Aluminum-Alloy Extruded Bars,
covered:
Rods, Wire, Shapes, and Tubes
1.2.1 Zinc immersion with optional copper strike (6.3).
B 322 Practice for Cleaning Metals Prior to Electroplating
1.2.2 Zinc immersion with neutral nickel strike (6.4).
B 431 Practice for Processing of Mandrels for Electroform-
1.2.3 Zinc immersion with acetate-buffered, nickel glyco-
ing
late strike (6.5).
E 527 Practice for Numbering Metals and Alloys (UNS)
1.2.4 Zinc immersion with acid or alkaline electroless
nickel strike.
3. Significance and Use
1.2.5 Tin immersion with bronze strike (6.6).
3.1 Various metals are deposited on aluminum alloys to
1.3 From the processing point of view, these procedures are
obtain a decorative or engineering finish. The electroplates
expected to give deposits on aluminum alloys that are approxi-
applied are usually chromium, nickel, copper, brass, silver, tin,
mately equivalent with respect to adherence. Corrosion perfor-
lead, cadmium, zinc, gold, and combinations of these. Silver,
mance is affected by many factors, however, including the
tin, or gold is applied to electrical equipment to decrease
procedure used to prepare the aluminum alloy for electroplat-
contact resistance or to improve surface conductivity; brass,
ing.
copper, nickel, or tin for assembly by soft soldering; chromium
1.4 This guide is intended to aid electroplaters in preparing
to reduce friction and obtain increased resistance to wear; zinc
aluminum and its alloys for electroplating.
for threaded parts where organic lubricants are not permissible;
1.5 This standard does not purport to address all of the
tin or lead is frequently employed to reduce friction on bearing
safety concerns, if any, associated with its use. It is the
surfaces. Nickel plus chromium or copper plus nickel plus
responsibility of the user of this standard to establish appro-
chromium is used in decorative applications. Nickel plus brass
priate safety and health practices and determine the applica-
plus lacquer or copper plus nickel plus brass plus lacquer is
bility of regulatory limitations prior to use. For specific
also used for decorative finishes, sometimes with the brass
precautionary statements see Section 7 and Appendix.
oxidized and relieved in various ways.
2. Referenced Documents 3.1.1 Electroless nickel may be applied as a barrier layer
prior to other deposits, or for engineering purposes.
2.1 ASTM Standards:
3.2 The preparation of aluminum and aluminum alloy man-
B 85 Specification for Aluminum-Alloy Die Castings
drels for electroforming is described in Practice B 431.
B 179 Specification forAluminumAlloys in Ingot Form for
4. Nature of Aluminum and Its Influence on Preparation
4.1 Microstructure—It is difficult to find a preplating pro-
This guide is under the jurisdiction of ASTM Committee B-8 on Metallic and
cedure that is equally satisfactory for all types and tempers of
Inorganic Coatings and is the direct responsibility of Subcommittee B08.02 on
Substrate Preparation.
Current edition approved May 29, 1987. Published July 1987.
Originally published as B 253 – 51 T. Last previous edition B 253 – 83. Annual Book of ASTM Standards, Vol 02.05.
2 4
Annual Book of ASTM Standards, Vol 02.02. Annual Book of ASTM Standards, Vol 01.01.
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B253
aluminum alloys because the various alloys and products aretrulynon-etchingtypeswhereetchingispreventedbyusing
behave differently electrochemically due to their different silicate inhibitors, such as sodium metasilicate (Na SiO ).
2 3
compositions and metallurgical structures. When elements are These inhibitors always leave a film of aluminum silicate on
added for alloying purposes, they may appear in an aluminum the surface. When these materials are used, subsequent deoxi-
alloy in several different forms: that is, they may be in solid dizing solutions should contain controlled amounts of fluoride
solutioninthealuminumlattice,bepresentasmicroparticlesof salts to insure complete removal of the film.
the elements themselves, or be present as particles of interme-
NOTE 1—General information on the cleaning of metals is given in
tallic compounds formed by combination with the aluminum.
Practice B 322.
The several solid solution matrices and the 20 or more
5.2 Aftercleaning,aconditioningtreatmentofthesurfaceis
microconstituents that may occur in commercial alloys may
generally required. For this to be effective, it must accomplish
have different chemical reactivities and electropotentials and
two things: (1) remove the original oxide film and (2) remove
their surfaces may not respond uniformly to various chemical
any microconstituents that may interfere with the formation of
and electrochemical treatments. In addition, the response may
a continuous deposited metallic layer or that may react with
be influenced by variations in the microstructure of different
subsequent electroplating solutions.
lots of products of the same alloy. In some cases, these
5.2.1 An effective conditioning treatment is immersion of
variations may be introduced or aggravated by preparation
the work in a warm sodium hydroxide solution (Appendix
processes; for example, the heat generated in buffing. The
X1.3) followed by water rinsing and immersion in a nitric
electroplater needs to know the aluminum alloy that is to be
acid-bifluoride desmutting solution (Appendix X1.4).An alter-
processed in order to select the best electroplating procedure.
native desmutting solution is sulfuric acid-hydrogen peroxide
In the absence of this information, there are so-called universal
(Appendix X1.5).
procedures that may be used. However these will not neces-
sarily be the best or the most economical procedures for the NOTE 2—When an unmodified sodium hydroxide solution is used,
etchingmaybecomenonuniformandheavyconcrete-likescalesmayform
alloy.
on tank walls and heating surfaces, their development becoming more
4.2 OxideFilm—Inadditiontodifferencesinmicrostructure
rapid as the concentration of dissolved aluminum increases. The incorpo-
that may affect response to preplating treatments, all aluminum
ration of controlled amounts of deflocculating complexors such as sodium
products have an ever-present natural oxide film. This oxide
gluconate, sodium glucoheptonate, certain sugar derivatives, and certain
film can be removed by various acid and alkaline treatments
substituted sugar amines will eliminate this problem. Many proprietary
and even though it reforms immediately on contact with etching materials are so modified.
NOTE 3—The universal acid mixture (Appendix X1.9) is applicable to
aqueous solutions or air, it then is usually thinner and more
almost all alloys, and is especially desirable for use with alloys containing
uniform than the original film. The newly formed oxide film
magnesium.
provides a more suitable surface for deposition of the first
5.2.2 For heat-treated alloys (alloys in a “T” temper), it is
metallic layer.
important to remove the relatively thick, heat-treated oxide
5. Cleaning and Conditioning Treatments film before proceeding with subsequent conditioning treat-
ments.Normally,heat-treatedfilmsareremovedbymachining,
5.1 To obtain consistent results for electroplating on alumi-
or by the polishing action on metal surfaces that are buffed.
num alloys, it is essential that the various cleaning and
5.2.2.1 In the absence of machining or buffing, controlled
conditioning treatments provide a surface of uniform activity
abrasive blasting may be used to remove this oxide. Fine
for the deposition of the initial metallic layer. First, the surface
abrasives such as aluminum oxide, ceramic beads, or glass
should be free of any oil, grease, buffing compound, or other
beads may be used. Silicon carbide abrasives should be
foreign material. For removing oil, grease, or buffing com-
avoided. If aluminum oxide, or glass beads are used, subse-
pound, use vapor degreasing, solvent washing, or solvent
quent treatments should include the use of an acid fluoride to
emulsion cleaning. For removing buffing compound, specially
ensure that any embedded aluminum oxide or silica is re-
formulated detergent type or modified detergent type buffing
moved. However, surfaces of heat-treated alloys that are not
compound removers may also be used. If the deposits of soil
machined or buffed should have the heat-treated film removed
are relatively light and fairly uniformly distributed, a mild
with a deoxidizing etch to obtain uniform electroplating
etching type cleaner may also be used. A convenient one is a
results.An effective deoxidizing etch is a hot sulfuric-chromic
hot, aqueous carbonate-phosphate solution (X1.1). Other types
acid solution (Appendix X1.2). Suitable proprietary deoxidiz-
of cleaners are used; for example, mildly alkaline or acidic
ing etches including some with no chromates are available.
soak cleaners are used to remove gross soils.Also available are
They should be used as recommended by the manufacturer.
a wide range of proprietary cleaners of the “non-etching” type.
5.2.3 For wrought alloys of the UNS A91100 and UNS
Some of these are actually buffered mixtures, similar to the
A93003 types (see Specifications B 209 and B 209M) fairly
carbonate-phosphate mixture (X1.1) where the so-called non-
good conditioning may be obtained by using the carbonate-
etching characteristics are obtained by buffering the solution to
phosphate cleaner (Appendix X1.1) followed by a nitric acid
pH levels where the etching action becomes minimal. Others
dip at room temperature (Appendix X1.6). These alloys do not
contain interfering constituents and for some applications, this
method of conditioning may be ample. If a silicate inhibited
For details on the proper operation and safety precautions to be followed in
cleaner is used (see 5.1) the fluoride containing smut remover
vapor degreasing, see Handbook of Vapor Degreasing, ASTM STP 310, ASTM,
1976. (Appendix X1.4) is preferred.
B253
NOTE 4—In accordance with current ASTM practice and for interna-
6.3.2.1 For best results, the sodium hydroxide must be low
tional usage, the aluminum alloys have been classified in accordance with
in sodium carbonate content (preferably under 2 % by weight)
the Unified Numbering System (UNS) as detailed in Practice E 527 and
and the zinc oxide must be free of contamination.
listed in D556C.
NOTE 5—In the zinc immersion solutions in this standard, the purity of
5.2.4 Another effective conditioning treatment for removing
the ingredients often plays an important role in the successful operation of
the surface oxide film and any undesirable microconstituents
the process.This is particularly true of the zinc oxide used. Contamination
comprises the use of a hot sulfuric acid etch (Appendix X1.7).
of the zinc oxide with lead or arsenic can be especially troublesome.
Proprietary, prepared powdered or liquid zincates are frequently used
The time of the dip depends on the alloy involved. Generally
therefore, since they will have had all raw materials properly checked for
the shorter time is used on castings. This treatment is satisfac-
purity.
tory for all aluminum-magnesium alloys, both wrought and
cast. It not only leaves the surface in anexcellent condition for
6.3.2.2 The thickness and quality of the immersion film are
the deposition of the first metallic layer, but it also eliminates
influenced by the conditions of deposition. When deposition is
the undesirable effects of the magnesium-containing constitu-
too rapid, heavy, coarse, crystalline, and porous, nonadherent
ents in alloys of the UNS A95052, UNS A96061, and UNS
deposits are formed. Since the thinner zinc deposits give the
A96063 types (see Specifications B 221 and B 221M).
best results, it is recommended that the temperature of the
5.3 The following are types of casting alloys containing
zincatesolutionbekeptbelow27°Candtheimmersiontimebe
high percentages of silicon: UNSA04130, UNSA14130, UNS
from 30 s to 1 min.
A03800, (see Specification B 85), UNS A03561, and UNS
6.3.3 A modification of the basic zincate solution in most
A13560, (see Specification B 179). A dip at room temperature
applications gives more uniform and satisfactory results. The
in a mixed acid solution (Appendix X1.8) containing nitric and
modified zinc immersion procedure has the following advan-
hydrofluoricacidsisrecommendedforconditioningthesurface
tages:(1)moreuniformcoveragebysubsequentelectroplating
of these alloys. This treatment also removes the heat-treated
baths, (2) greater operating range for the “double immersion”
film from unpolished, heat-treated castings.
version of the treatment (see 6.3.5), and ( 3) improved
resistance to corrosion on all electroplated aluminum alloys
6. Immersion Deposit/Strike Procedures
except for the UNS A92024 and UNS A97075 alloys. The
modified solution is prepared by dissolving the zinc oxide in a
6.1 Followingthecleaningandconditioningtreatments,itis
sodium hydroxide solution and cooling to room temperature.
necessary to further treat the surface to obtain adequate
Before the bath is diluted to volume, a water solution of ferric
adhesion of an electrodeposited metal on aluminum alloys.
chloride crystals and Rochelle salt (potassium sodium tartrate)
This section describes five commercially used procedures:
is added. The bath should be stirred while the ferric chloride-
6.1.1 Zinc immersion with optional copper st
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