ASTM D1732-03(2023)

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: D1732 − 03 (Reapproved 2023)
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 tices and determine the applicability of regulatory limitations
prior to use. (See Note 11.)
1.1 These practices cover two classes of treatment for
1.3 This international standard was developed in accor-
preparation of magnesium alloy surfaces for painting, as
dance with internationally recognized principles on standard-
follows:
ization established in the Decision on Principles for the
Class I—Chemical Treatments.
Development of International Standards, Guides and Recom-
Class II—Anodic Treatments.
mendations issued by the World Trade Organization Technical
In general, the latter treatments are the more protective of the
Barriers to Trade (TBT) Committee.
two classes. Mechanical (abrasive) treatments, solvent
cleaning, alkaline solution treatments, and acid pickles not
PRELIMINARY TREATMENT OF SURFACES
resulting in protective conversion coatings are suitable prelimi-
nary treatments only for metal to be exposed under mildly
2. Procedure
corrosive (indoor) exposures. When a high degree of corrosion
2.1 Certain anodic treatments simultaneously produce con-
protection and paint adhesion are desired, as in many outdoor
version coatings on, and remove contamination from, magne-
environments, surface preparation by one of the above
sium alloy surfaces. In general, however, apply conversion
conversion-coat classes is necessary. The hexavalent chromium
coatings only to surfaces previously freed from all
based methods given are not recommended as hexavalent
contamination, including oxide, rolling-scale, corrosion
chromium is a known carcinogen.
product, burned-on drawing and forming lubricant, and the
NOTE 1—Testing of Coatings—Quality control tests of coatings are
contamination introduced by blast cleaning and fabrication
frequently desirable, and these generally consist of exposures, with or
operations. Contamination in or under surface conversion
without paint, to salt spray, humidity, or natural environments, with
coatings seriously reduces their protective values (Note 2). For
suitable procedures for assessing the degree of breakdown suffered after
the removal of tenacious surface contamination, such as
fixed time intervals. It is recommended that quality control tests of
coatings shall be made as far as possible with high-purity material (for rolling-scale or casting skin, an acid pickle to dissolve some of
example AZ31A alloy), the inherent corrosion rate of which is relatively
the actual surface is essential. When organic contamination,
consistent from batch to batch and that precautions shall be taken to
such as grease or oil, is also present, an initial degreasing
remove surface contamination before coatings are applied. Such contami-
operation in solvent or in an alkaline degreasing solution is
nation shall be removed by acid pickling to a depth of at least 0.001 in. (25
usually necessary to allow the subsequent acid to wet the
μm) per side.
surface. These matters are discussed in more detail under the
1.2 This standard may involve hazardous materials,
headings of the specific cleaners or treatments (Note 3), as
operations, and equipment. This standard does not purport to
follows:
address all of the safety concerns, if any, associated with its
use. It is the responsibility of the user of this standard to 2.2 Alkaline Cleaners—Oil, grease, and old (but not baked)
establish appropriate safety, health, and environmental prac-
chrome-pickle coatings are readily removed by most commer-
cially available heavy-duty alkaline cleaners; but such cleaners
are not suitable for removing oxide and the like, for which
These practices are under the jurisdiction of ASTM Committee B08 on Metallic
purpose use acid pickles, preceded by alkaline cleaners.
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.07 on
Remove graphite lubricant and also baked chrome-pickle
Conversion Coatings.
coatings by a solution conforming to the following composi-
Current edition approved Nov. 1, 2023. Published November 2023. Originally
tion:
approved in 1960. Last previous edition approved in 2018 as D1732 – 03 (2018).
DOI: 10.1520/D1732-03R23.
Caustic soda (NaOH) 12 oz (90 g)
For information concerning magnesium and aluminum alloys, see ASTM
Wetting agent 0.1 oz (0.74 g)
Specification B80, B90, B91, B93, B107, and B209 covering these alloys, in the
Water (Note 4) 1 gal (1 L)
section on Aluminum and Magnesium and Their Alloys, Annual Book of ASTM
Soak the parts in the above cleaner for 10 to 20 min at
Standards, Vol 02.02. See also ASTM Practice B275, for Codification of Certain
Nonferrous Metals and Alloys, Cast and Wrought. boiling-point, and a treatment shall follow either in the chromic
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D1732 − 03 (2023)
acid-nitrate pickle described under 2.3.3 or, for parts machined removal of burned-on graphite lubricants from hot-formed
to fine tolerances, in the chromium trioxide solution described parts. The solution shall conform to the following composition:
under 2.3.1. After alkaline cleaning, rinse in water very
Chromium trioxide (CrO ) 1.5 lb (180 g)
Sodium nitrate (NaNO ) 2 oz (15 g)
thoroughly. Alkaline cleaners may be held in plain steel tanks. 3
Water (Note 4) to 1 gal (1 L)
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
required. Acid cleaners are as follows: cates that it is depleted and that the pH has risen to 1.7 or
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
sand castings and to remove the surface-contaminating effects
NOTE 2—Removal of Contamination by Welding Fluxes—When a part
of blast cleaning. Immerse for about 10 to 15 s, or until 0.002
to be painted has been welded by an operation involving the use of flux,
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
2.3.1.2 Chromic Acid Solution—Use a boiling 20 weight %
in a boiling 5 % solution of sodium dichromate, after which the part shall
solution of chromic anhydride (CrO ) in water to remove old
be well rinsed.
chemical and anodic treatments, corrosion product, and oxide NOTE 3—Suspension of Articles for Treatment—The use of magnesium
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
of the surface. The solution may be held in lead-lined steel or
resistance. Copper suspension wires in the hot dichromate solutions are
ASTM alloy No. 990A or its Aluminum Association not objectionable.
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
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
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
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 L)
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
2.3.3.1 Acetic Acid-Nitrate Solution—This solution rapidly
0.0006 in. (15 μm) of metal per surface, it shall not be used on
removes surface contamination to 0.001 in. (25.4 μm). Use for
parts 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)
Sodium (NaNO ) 6.6 oz (49.5 g) 3.2 Procedure—For wrought parts the bath shall conform to
Water (Note 4) to 1 gal (1 L)
the following composition:
Immerse the parts in the above solution at 70 to 90 °F (21 to
Sodium dichromate (Na Cr O ·2H O) 1.5 lb (180 g)
2 2 7 2
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 L)
obtained. When heavy surface contamination, such as hot-
rolled mill-scale is to be removed, immersion times shall be 3.2.1 For die-, sand- and permanent-mold castings the
solution shall conform to the following composition:
sufficient to remove at least 0.001 in. (25 μm) per surface. The
solution may be held in No. 990A aluminum, ceramic, or
rubber-lined tanks.
2.3.3.2 Chromium Trioxide-Nitrate Solution—Use this solu-
Conforming to Class I, Type I treatments are the Dow No. 1 process, the AMC
tion following the use of the method described in 2.2 for the “A” process, and the Type I process of Military Specification MIL-M-3171A.
D1732 − 03 (2023)
somewhat superior to that of Class I, Type I treatment under
Sodium dichromate (Na Cr O ·2H O) 1.5 lb (180 g)
2 2 7 2
Nitric acid (HNO ) (sp gr 1.42) 1.5 pt (187 mL)
severe exposure conditions (Note 5).
Sodium potassium, or ammonium acid fluoride 2 oz (15 g)
(NaHF , KHF , or NH HF ) 4.2 Procedure—Following chrome pickling as specified un-
2 3 4 2
Water (Note 4) to 1 gal (1.0 Ls)
der Section 3 and rinsing in cold water, transfer the parts
For wrought products, sand, and permanent-mold castings
immediately to a boiling solution conforming to the following
the above solutions operate at 70 to 90 °F (21 to 32 °C). The
composition:
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
increasing with use of the solution. For die-castings give the
Calcium or magnesium fluoride (CaF or MgF ) ⁄3 oz (2.5 g)
2 2
Water (Note 4) to 1 gal (1.0 L)
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
water at a temperature of not less than 160 °F (71 °C) nor more
preheat 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
to drain following the chromate treatment for more than the
intended to provide guidance on the causes of the most usual defects
specified 30 s; excessive drainage times result in powdery
arising in the application of either Class I, Type I or Class I, Type II
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 imme-
to or during the chrome pickling treatment, or both.
diately after they are dry. The solution may be held in Type 316
(b) Nonadherent Powdery Coatings are caused by:
stainless steel, or ceramic tanks or in steel tanks lined with
(1) Too long an interval between removal from the chrome pickle
synthetic rubber or vinyl-base materials. Tanks of ASTM alloy
and rinsing,
No. 990 A, or its Aluminum Association equivalent, Alloy No. (2) Ratio of acid to sodium dichromate too high,
(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
pickle.
(5) Solution revivified too many times.
3.3 Revivification of Solutions—Sluggish reaction with the
5. Class I, Type III
metal, associated with pale yellow, lustrous coatings, indicates
that the solution is depleted. Revivification is accomplished by
5.1 Scope—Class I, Type III treatment is applicable to all
the addition of dichromate and nitric acid, to raise the dichro-
types and forms of magnesium-base alloys except M1 alloy
mate again to 1.5 lb (180 g)/gal (1 L) and the free nitric acid to
and certain rare-earth alloys similar to EK30A. It produces in
levels indicated in the table below. Revivify when the free
itself no appreciable dimensional change and is, therefore,
nitric acid content of the solution is depleted to 0.5 pt/gal (62.3
applicable to parts machined to fine tolerances. The treatment
mL/1 L) (see Section 11 for analytical procedure) and shall
is applicable to magnesium containing inserts of, or attached
take place once only for M1 and ZK60A alloys, and not more
to, other metals, but in such cases the preliminary treatment
than six times for other alloys if good paint-base properties are
(5.2) shall be in the bifluoride bath (Solution No. 2) described
desired. Excessive use of the solution or too many revivifica-
in 5.2.2.
tions result in smooth, lustrous coatings not possessing the
NOTE 6—Causes of Defective Coatings—The following information is
degree of etch necessary for the best paint adhesion with
intended to provide guidance on the causes of the more usual defects
conventional primers. Revivify in accordance with the follow-
arising in the application of coatings from the Class I, Type III treatment.
ing table:
(a) Nonadherent Powdery Coatings are caused by:
Revivification Adjust Concentration
(1) Over-dilution of the hydrofluoric acid or acid fluoride solution,
Number of HNO to:
(2) Low pH (less than 4.0) of the dichromate solution,
(3) Insufficient precleaning of the metal surface, and
1 1.3 pt/gal (162 mL/1 L)
(4) Direct contact between the steel tank containing the dichromate
2 1.1 pt/gal (137 mL/1 L)
solution and the article being treated.
3 to 6 0.9 pt/gal (113 mL/1 L)
(b) Failure to Coat and Nonuniform Coatings are caused by:
(1) High pH of the dichromate solution,
4. Class I, Type II (Sealed Chrome Pickle)
(2) Low concentration of the dichromate,
(3) Insufficient precleaning of the metal surface,
4.1 Scope—Class I, Type II treatment is applicable to all
(4) Omission of fluoride treatment,
types and forms of magnesium-base alloys, subject only to the
(5) Use of an unsuitable alloy for the treatment, for instance, M1
limitations of the Class I, Type I treatment, since it is
alloy,
essentially the same process as the latter, followed by sealing. (6) Excessive immersion time in the hydrofluoric acid solution or use of
an H F concentration in the dichromate solution in excess of 0.2 %,
In protective qualities and as a paint base, this treatment is
2 2
(7) Insufficient rinsing after the hydrofluoric acid dip, and
4 5
Conforming to Class I, Type II treatments are the AMC “L” process, the Dow Conforming to Class I, Type III treatments are the AMC “G” process, the Dow
No. 10 process, and Type II treatment of Military Specification MIL-M-3171A. No. 7 process, and the Type III treatment of Military Specification MIL-M-3171A.
D1732 − 03 (2023)
(8) Insufficient heating of the dichromate solution (minimum tempera-
6.2 Procedure—Following cleaning as prescribed in Section
ture shoud be 200 °F (93 °C)).
2, treat the articles in fluoride solution No. 1 or No. 2 as
5.2 Procedure—Following cleaning as prescribed in Section prescribed in 5.2. Use the latter solution when other metals are
2, treat the parts first by immersion at 70 to 90 °F (21 to 32 °C) attached to the magnesium. After rinsing, immerse the articles
in one or other of the following solutions: in a solution conforming to the following composition:
5.2.1 Solution No. 1:
Ammonium sulfate ((NH ) SO ) 4 oz (30 g)
4 2 4
Sodium dichromate (Na Cr O ·2H O) 4 oz (30 g)
2 2 7 2
Hydrofluoric acid (60 % H F ) 24 fl oz (187 mL)
2 2
Ammonia (NH OH) (sp gr 0.880) ⁄3 fl oz (2.6 mL)
Water (Note 4) to 1 gal (1 L)
Water (Note 4) 1 gal (1 L)
5.2.2 Solution No. 2:
Operate the above solution at 120 to 140 °F (49 to 60 °C).
Sodium, potassium, or ammonium acid fluoride 6 ⁄3 oz (50 g)
The articles shall be made the anodes in the solution, with the
(NaHF , KHF , or NH HF )
2 2 4 2
tank, if of mild steel, acting as cathode, or with separate steel
Water (Note 4) to 1 gal (1 L)
cathodes if the tank is lined with nonmetallic materials. No
Solutions No. 1 and No. 2 may be held in tanks consisting of
separate generator is necessary, but the magnesium parts must
steel lined with lead or rubber. In solution No. 1, immerse
be electrically connected with the tank, or with the separate
AZ31A and AZ31B alloy parts for from 30 s to 1 min;
cathode plates through an external connection, taking care that
immerse all other alloys for 5 min. In solution No. 2, immerse
the parts do not make direct contact with the cathode material.
all the alloys for 5 min. Solution No. 2 is suitable for use with
6.2.1 The time of treatment shall be such that a uniform
all forms of magnesium alloys except those castings which
black coating is obtained on the articles. This takes from 10 to
have not been acid-pickled after blasting; these castings shall
30 min, and the consumption of from 70 to 150 A·min/ft
be treated in the hydrofluoric acid solution (solution No. 1).
2 2
(929 cm ). An anodic current density of not more than 10 A/ft
Following one or other of the above treatments, the parts shall
(929 cm ) is desirable.
be rinsed thoroughly in cold running water and transferred to a
solution conforming to the following composition: 6.2.2 Following the treatment, rinse the parts thoroughly in
cold running water, followed by a hot-water dip to facilitate
Sodium dichromate (Na Cr O ·H O) 1 to 1.5 lb (120 to 180 g)
2 2 7 2
Calcium or magnesium fluoride (CaF ⁄3 oz (2.5 g)
drying.
or MgF )
Water (Note 4) to 1 gal (1 L) 6.3 Maintenance of Solutions:
Operate the above solution at boiling point and immerse the 6.3.1 Fluoride Solutions—Maintain the fluoride solutions as
parts therein for 30 min, following which thoroughly rinse under 5.3.1. See Section 9 for the analytical procedure.
them in cold running water, followed by a hot-water dip to
6.3.2 Sulfate-Dichromate Solution—Maintain the pH of this
facilitate drying. Preferably, apply the paint coating as soon as
solution between 5.6 and 6.2 by periodic additions of a solution
possible thereafter. A mild steel tank is suitable for holding the
containing 5 weight % of both chromic anhydride (CrO ) and
above solution.
concentrated sulfuric acid (H SO , sp gr 1.84).
2 4
5.3 Maintenance of Solutions:
7. Class II, Type II
5.3.1 Fluoride Solutions—Keep the concentration of free
hydrofluoric acid in the solutions of 5.2.1 and 5.2.2 constant by
7.1 Scope—This treatment is applicable to all forms and
the addition, as required, of either hydrofluoric acid or acid
alloys of magnesium. Many aluminum alloys will anodically
fluoride, respectively. See Section 9 for the analytical proce-
polarize in the treatment solution, and hence magnesium parts
dure.
with aluminum attachments or inserts can be anodized. With
5.3.2 Dichromate Solution—Keep the pH of the dichromate
aluminum alloys containing copper, however, the coating of
solution within the limits 4.0 to 5.5, or 4.0 to 4.8 in the case of
the magnesium will proceed only if the area of the aluminum
AZ31A or AZ31B alloy, by addition of chromium trioxide as
alloy is small compared to that of the magnesium. Metals other
required. Maintain the level by the ad
...