ASTM D1732-03(2013)
(Practice)Standard Practices for Preparation of Magnesium Alloy Surfaces for Painting
Standard Practices for Preparation of Magnesium Alloy Surfaces for Painting
ABSTRACT
This specification covers chemical treatments and anodic treatments for preparation of magnesium alloy surfaces for printing. The procedure for preliminary treatment of surfaces using alkaline cleaners and acid cleaners are presented. Class I, type I (chrome pickle) surface preparation procedure is applicable to all forms of magnesium except certain special alloys containing silver. Class I, type II (sealed chrome pickle) surface preparation procedure is applicable to all types and forms of magnesium-based alloys, subject only to the limitations of class I, type I treatment. Class I, type III treatment is applicable to all types and forms of magnesium-based alloys except M1 alloy and certain rare-earth alloys similar to EK30A. Class II, type I (galvanic dichromate treatment) produce black coatings of good protective and pain-base qualities, and is applicable to all alloys and forms of magnesium, including M1 alloy. Class II, type II treatment is applicable to all forms and alloys of magnesium. Class II, type III treatment is applicable to all forms and alloys of magnesium, free from attachments or inserts of other metals.
SCOPE
1.1 These practices cover two classes of treatment for preparation of magnesium alloy surfaces for painting, as follows:
Class I—Chemical Treatments.
Class II—Anodic Treatments.
In general, the latter treatments are the more protective of the two classes. Mechanical (abrasive) treatments, solvent cleaning, alkaline solution treatments, and acid pickles not resulting in protective conversion coatings are suitable preliminary treatments only for metal to be exposed under mildly corrosive (indoor) exposures. When a high degree of corrosion protection and paint adhesion are desired, as in many outdoor environments, surface preparation by one of the above conversion-coat classes is necessary. The hexavalent chromium based methods given are not recommended as hexavalent chromium is a known carcinogen.Note 1—Testing of Coatings—Quality control tests of coatings are frequently desirable, and these generally consist of exposures, with or without paint, to salt spray, humidity, or natural environments, with suitable procedures for assessing the degree of breakdown suffered after fixed time intervals. It is recommended that quality control tests of coatings shall be made as far as possible with high-purity material (for example AZ31A alloy),2 the inherent corrosion rate of which is relatively consistent from batch to batch) and that precautions shall be taken to remove surface contamination before coatings are applied. Such contamination shall be removed by acid pickling to a depth of at least 0.001 in. (25 μm) per side.
1.2 This standard may involve hazardous materials, operations, and equipment. 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. (See Note 11.)
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Designation: D1732 − 03 (Reapproved 2013)
Standard Practices for
Preparation of Magnesium Alloy Surfaces for Painting
This standard is issued under the fixed designation D1732; 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.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope establish appropriate safety and health practices and deter-
mine the applicability of regulatory limitations prior to use.
1.1 These practices cover two classes of treatment for
(See Note 11.)
preparation of magnesium alloy surfaces for painting, as
follows:
PRELIMINARY TREATMENT OF SURFACES
Class I—Chemical Treatments.
Class II—Anodic Treatments.
2. Procedure
Ingeneral,thelattertreatmentsarethemoreprotectiveofthe
2.1 Certain anodic treatments simultaneously produce con-
two classes. Mechanical (abrasive) treatments, solvent
version coatings on, and remove contamination from, magne-
cleaning, alkaline solution treatments, and acid pickles not
sium alloy surfaces. In general, however, apply conversion
resultinginprotectiveconversioncoatingsaresuitableprelimi-
coatings only to surfaces previously freed from all
nary treatments only for metal to be exposed under mildly
contamination, including oxide, rolling-scale, corrosion
corrosive (indoor) exposures. When a high degree of corrosion
product, burned-on drawing and forming lubricant, and the
protection and paint adhesion are desired, as in many outdoor
contamination introduced by blast cleaning and fabrication
environments, surface preparation by one of the above
operations. Contamination in or under surface conversion
conversion-coatclassesisnecessary.Thehexavalentchromium
coatings seriously reduces their protective values (Note 2). For
based methods given are not recommended as hexavalent
the removal of tenacious surface contamination, such as
chromium is a known carcinogen.
rolling-scale or casting skin, an acid pickle to dissolve some of
NOTE 1—Testing of Coatings—Quality control tests of coatings are
the actual surface is essential. When organic contamination,
frequently desirable, and these generally consist of exposures, with or
such as grease or oil, is also present, an initial degreasing
without paint, to salt spray, humidity, or natural environments, with
operation in solvent or in an alkaline degreasing solution is
suitable procedures for assessing the degree of breakdown suffered after
usually necessary to allow the subsequent acid to wet the
fixed time intervals. It is recommended that quality control tests of
coatings shall be made as far as possible with high-purity material (for surface. These matters are discussed in more detail under the
exampleAZ31Aalloy), the inherent corrosion rate of which is relatively
headings of the specific cleaners or treatments (Note 3), as
consistent from batch to batch) and that precautions shall be taken to
follows:
remove surface contamination before coatings are applied. Such contami-
nationshallberemovedbyacidpicklingtoadepthofatleast0.001in.(25
2.2 Alkaline Cleaners—Oil, grease, and old (but not baked)
µm) per side.
chrome-pickle coatings are readily removed by most commer-
1.2 This standard may involve hazardous materials,
cially available heavy-duty alkaline cleaners; but such cleaners
operations, and equipment. This standard does not purport to
are not suitable for removing oxide and the like, for which
address all of the safety concerns, if any, associated with its
purpose use acid pickles, preceded by alkaline cleaners.
use. It is the responsibility of the user of this standard to
Remove graphite lubricant and also baked chrome-pickle
coatings by a solution conforming to the following composi-
tion:
ThesepracticesareunderthejurisdictionofASTMCommitteeB08onMetallic
Caustic soda (NaOH) 12 oz (90 g)
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.07 on Wetting agent 0.1 oz (0.74 g)
Conversion Coatings. Water (Note 4) 1 gal (1 litre)
Current edition approved May 1, 2013. Published May 2013. Originally
Soak the parts in the above cleaner for 10 to 20 min at
approved in 1960. Last previous edition approved in 2008 as D1732 – 03 (2008).
boiling-point,andatreatmentshallfolloweitherinthechromic
DOI: 10.1520/D1732-03R13.
For information concerning magnesium and aluminum alloys, see ASTM
acid-nitrate pickle described under 2.3.3 or, for parts machined
Specification B80, B90, B91, B93, B107, and B209 covering these alloys, in the
to fine tolerances, in the chromium trioxide solution described
section on Aluminum and Magnesium and Their Alloys, Annual Book of ASTM
under 2.3.1. After alkaline cleaning, rinse in water very
Standards, Vol 02.02. See also ASTM Practice B275, for Codification of Certain
Nonferrous Metals and Alloys, Cast and Wrought. thoroughly.Alkaline cleaners may be held in plain steel tanks.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D1732 − 03 (2013)
2.3 Acid Cleaners (Note 1)—Acid picking removes mill- Immerse the parts in the above solution at 70 to 90°F (21 to
scale, oxide, corrosion product, and the like. Use as a prelimi- 32°C) for approximately 5 min, for the removal of graphite.
nary treatment for surface conversion coatings when the
2.4 Abnormally slow reaction in the above solution indi-
highest degrees of surface uniformity and protective values are
cates that it is depleted and that the pH has risen to 1.7 or
required. Acid cleaners are as follows:
higher. Revivify the solution by the addition of chromium
2.3.1 For Sand and Permanent Mold Castings:
trioxide to bring the pH back to 0.5 to 0.7. Attempt no more
2.3.1.1 Nitric-Sulfuric Acid Solution—Use a solution of 8
than four revivifications. The solution may be held in ceramic,
volume % of concentrated nitric acid (HNO ) plus 2 volume %
No. 990A aluminum, 18-8 stainless steel, or synthetic rubber-
of concentrated sulfuric acid (H SO ) in water (see Note 1(a))
2 4 lined tanks.
at 70 to 90°F (21 to 32°C) as a preliminary treatment for new
NOTE 2—Removal of Contamination by Welding Fluxes—When a part
sand castings and to remove the surface-contaminating effects
to be painted has been welded by an operation involving the use of flux,
of blast cleaning. Immerse for about 10 to 15 s, or until 0.002
such flux shall be removed before the subjection of the part to any surface
in. (51 µm) per surface is removed. The solution may be held
preparation process. Such removal shall be made immediately by immers-
in ceramic, rubber, synthetic rubber, or vinyl-lined tanks. ing the part in hot water with scrubbing, and finally by immersing it for 1
h in a boiling 5 % solution of sodium dichromate, after which the part
2.3.1.2 Chromic Acid Solution—Use a boiling 20 weight %
shall be well rinsed.
solution of chromic anhydride (CrO ) in water to remove old
NOTE 3—Suspension of Articles for Treatment—The use of magnesium
chemical and anodic treatments, corrosion product, and oxide
alloy suspension wires is preferred for use in acid pickles in order to avoid
layers, without significant dissolution of metal and hence
objectionable contamination of the solutions through dissolution of the
wire materials. Heavy metal contamination, particularly of copper, may
without changing the dimensions of machined parts. Immer-
deposit on the magnesium surface and lead to seriously reduced corrosion
sion time varies from 1 to 5 min, depending upon the condition
resistance. Copper suspension wires in the hot dichromate solutions are
of the surface. The solution may be held in lead-lined steel or
not objectionable.
ASTM alloy No. 990A or its Aluminum Association
NOTE 4—Quality of Water—In the preparation and makeup of acid
equivalent, alloy No. 1100 aluminum tanks. pickles, dichromate solutions, and hot-water rinses, precautions shall be
taken against the use of water contaminated with heavy-metal impurities,
2.3.2 For Die Castings:
or excessive chlorides or sulfates. No upper limits can be specified at this
2.3.2.1 Chromium Trioxide-Nitric-Hydrofluoric Acid
stage for soluble impurities in the water, but powdering of coatings and
Solution—This solution is used to produce a smut-free surface
poor resistance to corrosion are known to result from the use of
on die castings, without violent attack of the metal. The contaminated water. Thus, when a choice exists, water from steam
condensate or ion-exchange-treated water shall be employed in preference
solution shall conform to the following composition:
to well water or hard tap water.
Chromium trioxide (CrO ) 37.5 oz (280 g)
Hydrofluoric acid (60% HF) 1 fl oz (8 ml)
SURFACE PREPARATION PROCEDURES—
Nitric acid (70% HNO ) 3.25floz(25ml)
CHEMICAL
Water (Note 4) to 1 gal (1 litre)
Immerse the parts in the above solution at 70 to 90°F (21 to
3. Class I, Type I (Chrome Pickle)
32°C) for 30 s to 2 min, or until a bright, clean surface is
3.1 Scope—Class I, Type I treatment is applicable to all
obtained. The solution may be held in tanks lined with
forms and alloys of magnesium except certain special alloys
synthetic rubber or vinyl-base materials.
containing silver, but since it may remove as much as 0.0006
2.3.3 For Wrought Products:
in. (15 µm) of metal per surface, it shall not be used on parts
2.3.3.1 Acetic Acid-Nitrate Solution—This solution rapidly
machined to fine tolerances. When properly applied, the
removes surface contamination to 0.001 in. (25.4 µm). Use for
process constitutes a good paint base, but rigid control is
wrought parts subsequently to be finished for the maximum
required at each step. The treatment is applicable to magne-
protective value. The solution shall conform to the following
sium alloy containing inserts of, or attached to, other metals.
composition:
Glacial acetic acid 25.5 fl oz (199 ml) 3.2 Procedure—For wrought parts the bath shall conform to
Sodium (NaNO ) 6.6 oz (49.5 g)
the following composition:
Water (Note 4) to 1 gal (1 litre)
Sodium dichromate (Na Cr O ·2H O) 1.5 lb (180 g)
2 2 7 2
Immerse the parts in the above solution at 70 to 90°F (21 to
Nitric acid (HNO ) (sp gr 1.42) 1.5 pt (187 ml)
32°C) for 30 s upwards, or until a bright, clean surface is Water (Note 4) to 1 gal (1 litre)
obtained. When heavy surface contamination, such as hot-
3.2.1 For die-, sand- and permanent-mold castings the
rolled mill-scale is to be removed, immersion times shall be
solution shall conform to the following composition:
sufficient to remove at least 0.001 in. (25 µm) per surface. The
Sodium dichromate (Na Cr O ·2H O) 1.5 lb (180 g)
2 2 7 2
solution may be held in No. 990A aluminum, ceramic, or
Nitric acid (HNO ) (sp gr 1.42) 1.5 pt (187 ml)
Sodium potassium, or ammonium acid fluoride 2 oz (15 g)
rubber-lined tanks.
(NaHF , KHF ,orNH HF )
2 3 4 2
2.3.3.2 Chromium Trioxide-Nitrate Solution—Use this solu-
Water (Note 4) to 1 gal (1.0 litres)
tion following the use of the method described in 2.2 for the
For wrought products, sand, and permanent-mold castings
removal of burned-on graphite lubricants from hot-formed
the above solutions operate at 70 to 90°F (21 to 32°C). The
parts.Thesolutionshallconformtothefollowingcomposition:
Chromium trioxide (CrO ) 1.5 lb (180 g)
Sodium nitrate (NaNO ) 2 oz (15 g)
Conforming to Class I, Type I treatments are the Dow No. 1 process, theAMC
Water (Note 4) to 1 gal (1 litre)
“A” process, and the Type I process of Military Specification MIL-M-3171A.
D1732 − 03 (2013)
immersion times shall be from 1 to 2 min, the necessary time
Sodium dichromate (Na Cr O ·2H O) 1.5 lb (180 g)
2 2 7 2
Calcium or magnesium fluoride (CaF or MgF ) ⁄3 oz (2.5 g)
2 2
increasing with use of the solution. For die-castings give the
Water (Note 4) to 1 gal (1.0 litre)
parts a 15 to 30 s dip in water at 160 to 180°F (71 to 82°C),
Boil the parts in the above solution for 30 min, after which
followed immediately by a 10-s dip in the second of the above
rinse them in cold running water, followed by a rinse in hot
baths, operated at 120 to 140°F (49 to 60°C). Failure to preheat
water at a temperature of not less than 160°F (71°C) nor more
the castings results in no coating in 10 s.
than 180°F (82°C) to facilitate drying. Preferably apply the
3.2.2 Following immersion remove the parts, allow to drain
paint coating immediately after the parts are dry. The solution
for not less than 5 s nor more than 30 s, then wash thoroughly
may be held in a steel tank.
in cold running water, followed by a dip in hot water at 160 to
180°F (71 to 82°C) to facilitate drying. Do not allow the parts
NOTE 5—Causes of Defective Coatings—The following information is
intended to provide guidance on the causes of the most usual defects
to drain following the chromate treatment for more than the
arising in the application of either Class I, Type I or Class I, Type II
specified 30 s; excessive drainage times result in powdery
coatings:
coatings of poor value as paint bases. Such coatings also result
(a) Spotted Coatings are caused by ineffective preliminary degreasing
from the use of hot-water rinses, the temperatures of which are
or by the presence of excessive surface contamination not removed prior
in excess of 180°F (82°C). Paint the parts preferably immedi-
to or during the chrome pickling treatment, or both.
(b) Nonadherent Powdery Coatings are caused by:
ately after they are dry. The solution may be held in Type 316
(1) Too long an interval between removal from the chrome pickle
stainless steel, or ceramic tanks or in steel tanks lined with
and rinsing,
synthetic rubber or vinyl-base materials. Tanks ofASTM alloy
(2) Ratio of acid to sodium dichromate too high,
No. 990A, or itsAluminumAssociation equivalent,Alloy No.
(3) Temperature of the solution or of the hot-water rinse too high.
1100 aluminum are satisfactory for the nonfluoride-containing
(4) Metal improperly degreased, or
(5) Solution revivified too many times.
pickle.
3.3 Revivification of Solutions—Sluggish reaction with the 5. Class I, Type III
metal, associated with pale yellow, lustrous coatings, indicates
5.1 Scope—Class I, Type III treatment is applicable to all
that the solution is depleted. Revivification is accomplished by 2
types and forms of magnesium-base alloys except M1 alloy
the addition of dichromate and nitric acid, to raise the dichro-
and certain rare-earth alloys similar to EK30A. It produces in
mate again to 1.5 lb (180 g)/gal (1 litre) and the free nitric acid
itself no appreciable dimensional change and is, there
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