ASTM D1732-03(2008)
(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), 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 2008)
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 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
resulting in protective conversion coatings are suitable prelimi-
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 Coatingsand 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 April 1, 2008. Published July 2008. Originally
Soak the parts in the above cleaner for 10 to 20 min at
approved in 1960. Last previous edition approved in 2003 as D1732-03. DOI:
boiling-point,andatreatmentshallfolloweitherinthechromic
10.1520/D1732-03R08.
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 (2008)
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 equiva-
NOTE 4—Quality of Water—In the preparation and makeup of acid
lent, 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 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.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 (2008)
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, theref
...
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:D1732–67(Reapproved 1998) Designation:D1732–03 (Reapproved 2008)
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 (´) 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
1.1 These practices cover two classes of treatment for preparation of magnesium alloy surfaces for painting, as follows:
Class I—Chemical Treatments. 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—Qualitycontroltestsofcoatingsarefrequentlydesirable,andthesegenerallyconsistofexposures,withorwithoutpaint,
to salt spray, humidity, or natural environments, with suitable procedures for assessing the degree of breakdown suffered after fixed time intervals.Itis
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 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 problems concerns, if any, associated with its use. It is the responsibility of whoever uses the user of this standard
to consult and establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to
use. (See Note 11.)
PRELIMINARYTREATMENT 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(Note2).Fortheremovaloftenacioussurfacecontamination,suchasrolling-scaleorcastingskin,anacidpickletodissolve
some of the actual surface is essential. When organic contamination, such as grease or oil, is also present, an initial degreasing
operationinsolventorinanalkalinedegreasingsolutionisusuallynecessarytoallowthesubsequentacidtowetthesurface.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:
Caustic soda (NaOH) 12 oz (90 g)
Wetting agent 0.1 oz (0.74 g)
Water (Note 4) 1 gal (1 litre)
Soak the parts in the above cleaner for 10 to 20 min at boiling-point, and a treatment shall follow either in the chromic
These practices are under the jurisdiction ofASTM Committee B-8B08 on Metallic and Inorganic Coatings and is the direct responsibility of Subcommittee B08.07 on
Chemical Conversion Coatings.
Current edition approved Sept. 8, 1967.April 1, 2008. Published November 1967.July 2008. Originally published as D1732-60.approved in 1960. Last previous edition
D1732-66.approved in 2003 as D 1732-03.
For information concerning magnesium and aluminum alloys, see ASTM Specification B 80, B 90, B 91, B 93, B 107, and B 209 covering these alloys, in the section
onAluminum and Magnesium and TheirAlloys, Annual Book of ASTM Standards, Vol 02.02. See alsoASTM Practice B 275, 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 (2008)
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)
Nitric acid (70% HNO ) 3.25floz(25ml)
Water (Note 4) to 1 gal (1 litre)
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)
Sodium (NaNO ) 6.6 oz (49.5 g)
Water (Note 4) to 1 gal (1 litre)
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)
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 for1hina 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
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 make-upmakeup 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.
D1732–03 (2008)
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)
Water (Note 4) to 1 gal (1 litre)
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)
Sodium potassium, or ammonium acid fluoride 2 oz (15 g)
(NaHF , KHF ,orNH HF )
2 3 4 2
Water (Note 4) to 1 gal (1.0 litres)
Forwroughtproducts,sand,andpermanent-moldcastingstheabovesolutionsoperateat70to90°F(21to32°C).Theimmersion
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) 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) (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 etch necessary for the
best paint adhesion with conventional primers. Revivify in accordance with the following table:
Revivification Adjust Concentration
Number of HNO to:
1 1.3 pt/gal (162 ml/1 litre)
2 1.1 pt/gal (137 ml/1 litre)
3 to 6 0.9 pt/gal (113 ml/1 litre)
4. Class I, Type II (Sealed Chrome Pickle)
4.1 Scope—Class I, Type II treatment is applicable to all types and forms of magnesium-base alloys, subject only to the
limitations of the Class I, Type I treatment, since it is essentially the same process as the latter, followed by sealing. In protective
qualities and as a paint base, this treatment is somewhat superior to that of Class I, Type I treatment under severe exposure
conditions (Note 5).
4.2 Procedure—Following chrome pickling as specified under Section 3 and rinsing in cold water
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