ASTM D1732-03(2018)
(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, health, and environmental practices and determine the applicability of regulatory limitations prior to use. (See Note 11.)
1.3 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.
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Standards Content (Sample)
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 2018)
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
Ingeneral,thelattertreatmentsarethemoreprotectiveofthe
Barriers to Trade (TBT) Committee.
two classes. Mechanical (abrasive) treatments, solvent
cleaning, alkaline solution treatments, and acid pickles not
PRELIMINARY TREATMENT OF SURFACES
resultinginprotectiveconversioncoatingsaresuitableprelimi-
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-coatclassesisnecessary.Thehexavalentchromium
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
exampleAZ31Aalloy), 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
nationshallberemovedbyacidpicklingtoadepthofatleast0.001in.(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
ThesepracticesareunderthejurisdictionofASTMCommitteeB08onMetallic
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 Oct. 1, 2018. Published October 2018. Originally
tion:
approved in 1960. Last previous edition approved in 2013 as D1732 – 03 (2013).
DOI: 10.1520/D1732-03R18.
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,andatreatmentshallfolloweitherinthechromic
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D1732 − 03 (2018)
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.Thesolutionshallconformtothefollowingcomposition:
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
2.3.1.2 Chromic Acid Solution—Use a boiling 20 weight %
h in a boiling 5 % solution of sodium dichromate, after which the part
solution of chromic anhydride (CrO ) in water to remove old
shall 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 ) 3.25floz(25mL)
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 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)
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, theAMC
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 (2018)
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 ,orNH 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 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
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 immedi-
to or during the chrome pickling treatment, or both.
ately 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 ofASTM alloy
and rinsing,
No. 990A, or itsAluminumAssociation equivalent,Alloy No. (2) Ratio of acid to sodium dichromate too high,
(3) Temperature of the
...
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: D1732 − 03 (Reapproved 2018)
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 Oct. 1, 2018. Published October 2018. Originally
tion:
approved in 1960. Last previous edition approved in 2013 as D1732 – 03 (2013).
DOI: 10.1520/D1732-03R18.
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 (2018)
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.
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
highest degrees of surface uniformity and protective values are 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
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
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
2.3.1.2 Chromic Acid Solution—Use a boiling 20 weight %
h in a boiling 5 % solution of sodium dichromate, after which the part
solution of chromic anhydride (CrO ) in water to remove old
shall 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
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
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)
3 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 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)
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 3
Water (Note 4) to 1 gal (1 L)
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
sufficient to remove at least 0.001 in. (25 µm) per surface. The solution shall conform to the following composition:
solution may be held in No. 990A aluminum, ceramic, or
rubber-lined tanks.
2.3.3.2 Chromium Trioxide-Nitrate Solution—Use this solu- 3
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 (2018)
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)
3 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),
followed immediately by a 10-s dip in the second of the above Boil the parts in the above solution for 30 min, after which
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
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 immedi-
to or during the chrome pickling treatment, or both.
ately 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 a
...
This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D1732 − 03 (Reapproved 2013) D1732 − 03 (Reapproved 2018)
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
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), the inherent
corrosion rate of which is relatively consistent from batch to batch)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 safety, health, and healthenvironmental practices and determine the applicability of regulatory limitations prior to use. (See
Note 11.)
1.3 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.
PRELIMINARY TREATMENT OF SURFACES
2. Procedure
2.1 Certain anodic treatments simultaneously produce conversion coatings on, and remove contamination from, magnesium
alloy surfaces. In general, however, apply conversion coatings only to surfaces previously freed from all contamination, including
oxide, rolling-scale, corrosion product, burned-on drawing and forming lubricant, and the contamination introduced by blast
cleaning and fabrication operations. Contamination in or under surface conversion coatings seriously reduces their protective
values (Note 2). For the removal of tenacious surface contamination, such as rolling-scale or casting skin, an acid pickle to dissolve
some of the actual surface is essential. When organic contamination, such as grease or oil, is also present, an initial degreasing
operation in solvent or in an alkaline degreasing solution is usually necessary to allow the subsequent acid to wet the surface. These
matters are discussed in more detail under the headings of the specific cleaners or treatments (Note 3), as follows:
2.2 Alkaline Cleaners—Oil, grease, and old (but not baked) chrome-pickle coatings are readily removed by most commercially
available heavy-duty alkaline cleaners; but such cleaners are not suitable for removing oxide and the like, for which purpose use
acid pickles, preceded by alkaline cleaners. Remove graphite lubricant and also baked chrome-pickle coatings by a solution
conforming to the following composition:
These practices are under the jurisdiction of ASTM Committee B08 on Metallic and Inorganic Coatings and is the direct responsibility of Subcommittee B08.07 on
Conversion Coatings.
Current edition approved May 1, 2013Oct. 1, 2018. Published May 2013October 2018. Originally approved in 1960. Last previous edition approved in 20082013 as D1732
– 03 (2008).(2013). DOI: 10.1520/D1732-03R13.10.1520/D1732-03R18.
For information concerning magnesium and aluminum alloys, see ASTM Specification B80, B90, B91, B93, B107, and B209 covering these alloys, in the section on
Aluminum and Magnesium and Their Alloys, Annual Book of ASTM Standards, Vol 02.02. See also ASTM Practice B275, for Codification of Certain Nonferrous Metals and
Alloys, Cast and Wrought.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D1732 − 03 (2018)
Caustic soda (NaOH) 12 oz (90 g)
Wetting agent 0.1 oz (0.74 g)
Water (Note 4) 1 gal (1 litre)
Water (Note 4) 1 gal (1 L)
Soak the parts in the above cleaner for 10 to 20 min at 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
under 2.3.1. After alkaline cleaning, rinse in water very thoroughly. Alkaline cleaners may be held in plain steel tanks.
2.3 Acid Cleaners (Note 1)—Acid picking removes mill-scale, oxide, corrosion product, and the like. Use as a preliminary
treatment for surface conversion coatings when the highest degrees of surface uniformity and protective values are required. Acid
cleaners are as follows:
2.3.1 For Sand and Permanent Mold Castings:
2.3.1.1 Nitric-Sulfuric Acid Solution—Use a solution of 8 volume % of concentrated nitric acid (HNO ) plus 2 volume % of
concentrated sulfuric acid (H SO ) in water (see Note 1(a)) at 70 to 90°F (21 to 32°C) as a preliminary treatment for new sand
2 4
castings and to remove the surface-contaminating effects of blast cleaning. Immerse for about 10 to 15 s, or until 0.002 in. (51 μm)
per surface is removed. The solution may be held in ceramic, rubber, synthetic rubber, or vinyl-lined tanks.
2.3.1.2 Chromic Acid Solution—Use a boiling 20 weight % solution of chromic anhydride (CrO ) in water to remove old
chemical and anodic treatments, corrosion product, and oxide layers, without significant dissolution of metal and hence without
changing the dimensions of machined parts. Immersion time varies from 1 to 5 min, depending upon the condition of the surface.
The solution may be held in lead-lined steel or ASTM alloy No. 990A or its Aluminum Association equivalent, alloy No. 1100
aluminum tanks.
2.3.2 For Die Castings:
2.3.2.1 Chromium Trioxide-Nitric-Hydrofluoric Acid Solution—This solution is used to produce a smut-free surface on die
castings, without violent attack of the metal. The solution shall conform to the following composition:
Chromium trioxide (CrO ) 37.5 oz (280 g)
Hydrofluoric acid (60% HF) 1 fl oz (8 ml)
Hydrofluoric acid (60 % HF) 1 fl oz (8 mL)
Nitric acid (70% HNO ) 3.25 fl oz (25 ml)
Nitric acid (70 % HNO ) 3.25 fl oz (25 mL)
Water (Note 4) to 1 gal (1 litre)
Water (Note 4) to 1 gal (1 L)
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 obtained.
The solution may be held in tanks lined with synthetic rubber or vinyl-base materials.
2.3.3 For Wrought Products:
2.3.3.1 Acetic Acid-Nitrate Solution—This solution rapidly removes surface contamination to 0.001 in. (25.4 μm). Use for
wrought parts subsequently to be finished for the maximum protective value. The solution shall conform to the following
composition:
Glacial acetic acid 25.5 fl oz (199 ml)
Glacial acetic acid 25.5 fl oz (199 mL)
Sodium (NaNO ) 6.6 oz (49.5 g)
Water (Note 4) to 1 gal (1 litre)
Water (Note 4) to 1 gal (1 L)
Immerse the parts in the above solution at 70 to 90°F (21 to 32°C) for 30 s upwards, or until a bright, clean surface is obtained.
When heavy surface contamination, such as hot-rolled mill-scale is to be removed, immersion times shall be 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 solution following the use of the method described in 2.2 for the removal
of burned-on graphite lubricants from hot-formed parts. The solution shall conform to the following composition:
Chromium trioxide (CrO ) 1.5 lb (180 g)
Sodium nitrate (NaNO ) 2 oz (15 g)
Water (Note 4) to 1 gal (1 litre)
Water (Note 4) to 1 gal (1 L)
Immerse the parts in the above solution at 70 to 90°F (21 to 32°C) for approximately 5 min, for the removal of graphite.
2.4 Abnormally slow reaction in the above solution indicates that it is depleted and that the pH has risen to 1.7 or higher.
Revivify the solution by the addition of chromium trioxide to bring the pH back to 0.5 to 0.7. Attempt no more than four
revivifications. The solution may be held in ceramic, No. 990A aluminum, 18-8 stainless steel, or synthetic rubber-lined tanks.
NOTE 2—Removal of Contamination by Welding Fluxes—When a part 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 preparation process. Such removal shall be made immediately by immersing 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 shall be well
rinsed.
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
objectionable contamination of the solutions through dissolution of the wire materials. Heavy metal contamination, particularly of copper, may deposit
D1732 − 03 (2018)
on the magnesium surface and lead to seriously reduced corrosion resistance. Copper suspension wires in the hot dichromate solutions are not
objectionable.
NOTE 4—Quality of Water—In the preparation and makeup of acid pickles, dichromate solutions, and hot-water rinses, precautions shall be taken
against the use of water contaminated with heavy-metal impurities, 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 poor resistance to corrosion are known to result from the use of contaminated water. Thus,
when a choice exists, water from steam condensate or ion-exchange-treated water shall be employed in preference to well water or hard tap water.
SURFACE PREPARATION PROCEDURES—CHEMICAL
3. Class I, Type I (Chrome Pickle)
3.1 Scope—Class I, Type I treatment is applicable to all forms and alloys of magnesium except certain special alloys containing
silver, but since it may remove as much as 0.0006 in. (15 μm) of metal per surface, it shall not be used on parts machined to fine
tolerances. When properly applied, the process constitutes a good paint base, but rigid control is required at each step. The
treatment is applicable to magnesium alloy containing inserts of, or attached to, other metals.
3.2 Procedure—For wrought parts the bath shall conform to the following composition:
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)
Nitric acid (HNO ) (sp gr 1.42) 1.5 pt (187 mL)
Water (Note 4) to 1 gal (1 litre)
Water (Note 4) to 1 gal (1 L)
3.2.1 For die-, sand- and permanent-mold castings the solution shall conform to the following composition:
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)
Nitric acid (HNO ) (sp gr 1.42) 1.5 pt (187 mL)
Sodium potassium, or ammonium acid fluoride 2 oz (15 g)
(NaHF , KHF , or NH HF )
2 3 4 2
Water (Note 4) to 1 gal (1.0 litres)
Water (Note 4) to 1 gal (1.0 Ls)
For wrought products, sand, and permanent-mold castings the above solutions operate at 70 to 90°F (21 to 32°C). The immersion
times shall be from 1 to 2 min, the necessary time increasing with use of the solution. For die-castings give the 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 baths, operated at 120
to 140°F (49 to 60°C). Failure to preheat the castings results in no coating in 10 s.
3.2.2 Following immersion remove the parts, allow to drain for not less than 5 s nor more than 30 s, then wash thoroughly 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 to drain
following the chromate treatment for more than the specified 30 s; excessive drainage times result in powdery coatings of poor
value as paint bases. Such coatings also result from the use of hot-water rinses, the temperatures of which are in excess of 180°F
(82°C). Paint the parts preferably immediately after they are dry. The solution may be held in Type 316 stainless steel, or ceramic
tanks or in steel tanks lined with synthetic rubber or vinyl-base materials. Tanks of ASTM alloy No. 990 A, or its Aluminum
Association equivalent, Alloy No. 1100 aluminum are satisfactory for the nonfluoride-containing pickle.
3.3 Revivification of Solutions—Sluggish reaction with the metal, associated with pale yellow, lustrous coatings, indicates that
the solution is depleted. Revivification is accomplished by the addition of dichromate and nitric acid, to raise the dichromate again
to 1.5 lb (180 g)/gal (1 litre)L) and the free nitric acid to levels indicated in the table below. Revivify when the free nitric acid
content of the solution is depleted to 0.5 pt/gal (62.3 ml/1 litre)mL/1 L) (see Section 11 for analytical procedure) and shall take
place once only for M1 and ZK60A alloys, and not more than six times for other alloys if good paint-base properties are desired.
Excessive use of the solution or too many revivifications result in smooth, lustrous coatings not possessing the degree of e
...
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