ASTM D1732-67(1998)
(Practice)Standard Practices for Preparation of Magnesium Alloy Surfaces for Painting
Standard Practices for Preparation of Magnesium Alloy Surfaces for Painting
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. 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), 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 [mu]m) per side.
1.2 This standard may involve hazardous materials, operations, and equipment. This standard does not purport to address all of the safety problems associated with its use. It is the responsibility of whoever uses this standard to consult and establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.> (See Note 11.)
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Designation: D 1732 – 67 (Reapproved 1998)
Standard Practices for
Preparation of Magnesium Alloy Surfaces for Painting
This standard is issued under the fixed designation D 1732; 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.
1. Scope PRELIMINARY TREATMENT OF SURFACES
1.1 These practices cover two classes of treatment for
2. Procedure
preparation of magnesium alloy surfaces for painting, as
2.1 Certain anodic treatments simultaneously produce con-
follows:
version coatings on, and remove contamination from, magne-
Class I—Chemical Treatments. Class II—Anodic
sium alloy surfaces. In general, however, apply conversion
Treatments.
coatings only to surfaces previously freed from all contamina-
In general, the latter treatments are the more protective of the
tion, including oxide, rolling-scale, corrosion product,
two classes. Mechanical (abrasive) treatments, solvent clean-
burned-on drawing and forming lubricant, and the contamina-
ing, alkaline solution treatments, and acid pickles not resulting
tion introduced by blast cleaning and fabrication operations.
in protective conversion coatings are suitable preliminary
Contamination in or under surface conversion coatings seri-
treatments only for metal to be exposed under mildly corrosive
ously reduces their protective values (Note 2). For the removal
(indoor) exposures. When a high degree of corrosion protec-
of tenacious surface contamination, such as rolling-scale or
tion and paint adhesion are desired, as in many outdoor
casting skin, an acid pickle to dissolve some of the actual
environments, surface preparation by one of the above
surface is essential. When organic contamination, such as
conversion-coat classes is necessary.
grease or oil, is also present, an initial degreasing operation in
NOTE 1—Testing of Coatings—Quality control tests of coatings are
solvent or in an alkaline degreasing solution is usually neces-
frequently desirable, and these generally consist of exposures, with or
sary to allow the subsequent acid to wet the surface. These
without paint, to salt spray, humidity, or natural environments, with
matters are discussed in more detail under the headings of the
suitable procedures for assessing the degree of breakdown suffered after
specific cleaners or treatments (Note 3), as follows:
fixed time intervals. It is recommended that quality control tests of
2.2 Alkaline Cleaners—Oil, grease, and old (but not baked)
coatings shall be made as far as possible with high-purity material (for
example AZ31A alloy), the inherent corrosion rate of which is relatively
chrome-pickle coatings are readily removed by most commer-
consistent from batch to batch) and that precautions shall be taken to
cially available heavy-duty alkaline cleaners; but such cleaners
remove surface contamination before coatings are applied. Such contami-
are not suitable for removing oxide and the like, for which
nation shall be removed by acid pickling to a depth of at least 0.001 in. (25
purpose use acid pickles, preceded by alkaline cleaners.
μm) per side.
Remove graphite lubricant and also baked chrome-pickle
1.2 This standard may involve hazardous materials, opera-
coatings by a solution conforming to the following composi-
tions, and equipment. This standard does not purport to
tion:
address all of the safety problems associated with its use. It is
Caustic soda (NaOH) 12 oz (90 g)
the responsibility of whoever uses this standard to consult and
Wetting agent 0.1 oz (0.74 g)
Water (Note 4) 1 gal (1 litre)
establish appropriate safety and health practices and deter-
mine the applicability of regulatory limitations prior to use.
Soak the parts in the above cleaner for 10 to 20 min at
(See Note 11.)
boiling-point, and a treatment shall follow either in the chromic
acid-nitrate pickle described under 2.3.3 or, for parts machined
to fine tolerances, in the chromium trioxide solution described
These practices are under the jurisdiction of ASTM Committee B-8 on Metallic under 2.3.1. After alkaline cleaning, rinse in water very
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.07 on
thoroughly. Alkaline cleaners may be held in plain steel tanks.
Chemical Conversion Coatings.
2.3 Acid Cleaners (Note 1)—Acid picking removes mill-
Current edition approved Sept. 8, 1967. Published November 1967. Originally
scale, oxide, corrosion product, and the like. Use as a prelimi-
published as D 1732-60. Last previous edition D 1732-66.
For information concerning magnesium and aluminum alloys, see ASTM
nary treatment for surface conversion coatings when the
Specification B 80, B 90, B 91, B 93, B 107, and B 209 covering these alloys, in the
highest degrees of surface uniformity and protective values are
section on Aluminum and Magnesium and Their Alloys, Annual Book of ASTM
required. Acid cleaners are as follows:
Standards, Vol 02.02. See also ASTM Practice B 275, for Codification of Certain
2.3.1 For Sand and Permanent Mold Castings:
Nonferrous Metals and Alloys, Cast and Wrought.
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NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
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D 1732
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)) lined tanks.
2 4
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-
ing the part in hot water with scrubbing, and finally by immersing it for 1
in ceramic, rubber, synthetic rubber, or vinyl-lined tanks.
h in a boiling 5% solution of sodium dichromate, after which the part shall
2.3.1.2 Chromic Acid Solution—Use a boiling 20 weight %
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
without changing the dimensions of machined parts. Immer-
wire materials. Heavy metal contamination, particularly of copper, may
sion time varies from 1 to 5 min, depending upon the condition deposit on the magnesium surface and lead to seriously reduced corrosion
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 equiva-
NOTE 4—Quality of Water—In the preparation and make-up of acid
lent, alloy No. 1100 aluminum tanks.
pickles, dichromate solutions, and hot-water rinses, precautions shall be
2.3.2 For Die Castings:
taken against the use of water contaminated with heavy-metal impurities,
2.3.2.1 Chromium Trioxide-Nitric-Hydrofluoric Acid
or excessive chlorides or sulfates. No upper limits can be specified at this
Solution—This solution is used to produce a smut-free surface stage for soluble impurities in the water, but powdering of coatings and
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
solution shall conform to the following composition:
condensate or ion-exchange-treated water shall be employed in preference
Chromium trioxide (CrO ) 37.5 oz (280 g)
to well water or hard tap water.
Hydrofluoric acid (60% HF) 1 fl oz (8 ml)
Nitric acid (70% HNO ) 3.25 fl oz (25 ml)
SURFACE PREPARATION PROCEDURES—
Water (Note 4) to 1 gal (1 litre)
CHEMICAL
Immerse the parts in the above solution at 70 to 90°F (21 to
32°C) for 30 s to 2 min, or until a bright, clean surface is
3. Class I, Type I (Chrome Pickle)
obtained. The solution may be held in tanks lined with
3.1 Scope—Class I, Type I treatment is applicable to all
synthetic rubber or vinyl-base materials.
forms and alloys of magnesium except certain special alloys
2.3.3 For Wrought Products:
containing silver, but since it may remove as much as 0.0006
2.3.3.1 Acetic Acid-Nitrate Solution—This solution rapidly
in. (15 μm) of metal per surface, it shall not be used on parts
removes surface contamination to 0.001 in. (25.4 μm). Use for
machined to fine tolerances. When properly applied, the
wrought parts subsequently to be finished for the maximum
process constitutes a good paint base, but rigid control is
protective value. The solution shall conform to the following
required at each step. The treatment is applicable to magne-
composition:
sium alloy containing inserts of, or attached to, other metals.
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)
Water (Note 4) to 1 gal (1 litre)
32°C) for 30 s upwards, or until a bright, clean surface is
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 so-
Water (Note 4) to 1 gal (1.0 litres)
lution 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. The solution shall conform to the following composition:
immersion times shall be from 1 to 2 min, the necessary time
Chromium trioxide (CrO ) 1.5 lb (180 g)
increasing with use of the solution. For die-castings give the
Sodium nitrate (NaNO ) 2 oz (15 g)
Water (Note 4) to 1 gal (1 litre) parts a 15 to 30 s dip in water at 160 to 180°F (71 to 82°C),
followed immediately by a 10-s dip in the second of the above
Immerse the parts in the above solution at 70 to 90°F (21 to
baths, operated at 120 to 140°F (49 to 60°C). Failure to preheat
32°C) for approximately 5 min, for the removal of graphite.
the castings results in no coating in 10 s.
2.4 Abnormally slow reaction in the above solution indi-
cates that it is depleted and that the pH has risen to 1.7 or
higher. Revivify the solution by the addition of chromium
Conforming to Class I, Type I treatments are the Dow No. 1 process, the AMC
trioxide to bring the pH back to 0.5 to 0.7. Attempt no more “A” process, and the Type I process of Military Specification MIL-M-3171A.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
D 1732
3.2.2 Following immersion remove the parts, allow to drain may be held in a steel tank.
for not less than 5 s nor more than 30 s, then wash thoroughly
NOTE 5—Causes of Defective Coatings—The following information is
in cold running water, followed by a dip in hot water at 160 to
intended to provide guidance on the causes of the most usual defects
180°F (71 to 82°C) to facilitate drying. Do not allow the parts
arising in the application of either Class I, Type I or Class I, Type II
to drain following the chromate treatment for more than the
coatings:
(a) Spotted Coatings are caused by ineffective preliminary degreasing
specified 30 s; excessive drainage times result in powdery
or by the presence of excessive surface contamination not removed prior
coatings of poor value as paint bases. Such coatings also result
to or during the chrome pickling treatment, or both.
from the use of hot-water rinses, the temperatures of which are
(b) Nonadherent Powdery Coatings are caused by:
in excess of 180°F (82°C). Paint the parts preferably immedi-
(1) Too long an interval between removal from the chrome pickle
ately after they are dry. The solution may be held in Type 316
and rinsing,
stainless steel, or ceramic tanks or in steel tanks lined with
(2) Ratio of acid to sodium dichromate too high,
synthetic rubber or vinyl-base materials. Tanks of ASTM alloy
(3) Temperature of the solution or of the hot-water rinse too high.
No. 990 A, or its Aluminum Association equivalent, Alloy No.
(4) Metal improperly degreased, or
1100 aluminum are satisfactory for the nonfluoride-containing (5) Solution revivified too many times.
pickle.
5. Class I, Type III
3.3 Revivification of Solutions—Sluggish reaction with the
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 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, therefore,
to levels indicated in the table below. Revivify when the free applicable to parts machined to fine tolerances. The treatment
nitric acid content of the solution is depleted to 0.5 pt/gal (62.3 is applicab
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