ASTM B449-93(2010)e1
(Specification)Standard Specification for Chromates on Aluminum
Standard Specification for Chromates on Aluminum
ABSTRACT
This specification covers the requirements relating to rinsed and non rinsed chromate conversion coatings on aluminum and aluminum alloys intended to give protection against corrosion and as a base for other coatings. Aluminum and aluminum alloys are chromate coated in order to retard corrosion; as a base for organic films including paints, plastics, and adhesives; and as a protective coating having a low electrical contact impedance. The materials are classified according to its coating thickness: Class 1; Class 2; Class 3; and Class 4. Chromate conversion coatings are normally applied by dipping: the coating may also be applied by inundation, spraying, roller coating, or by wipe-on techniques.
SCOPE
1.1 This specification covers the requirements relating to rinsed and nonrinsed chromate conversion coatings on aluminum and aluminum alloys intended to give protection against corrosion and as a base for other coatings. This edition of the specification has been coordinated with ISO/DIS 10546 and is technically equivalent.
1.2 Aluminum and aluminum alloys are chromate coated in order to retard corrosion; as a base for organic films including paints, plastics, and adhesives; and as a protective coating having a low electrical contact impedance.
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.4 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.
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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:B449 −93(Reapproved2010)
StandardSpecification for
Chromates on Aluminum
This standard is issued under the fixed designation B449; 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.
´ NOTE—Updated format of the military specification in Section 2.4 editorially in June 2010.
1. Scope D3359 Test Methods for Measuring Adhesion by Tape Test
1.1 This specification covers the requirements relating to 2.2 ISO Standards:
ISO 2409 Paint and Varnishes—Cross-Cut Test
rinsed and nonrinsed chromate conversion coatings on alumi-
num and aluminum alloys intended to give protection against ISO 3768 Metallic Coatings—Neutral Salt Spray Test (NSS
Test)
corrosion and as a base for other coatings. This edition of the
specification has been coordinated with ISO/DIS 10546 and is ISO 3892 Conversion Coatings on Metallic Materials—
Determination of Mass Per Unit Area—Gravimetric
technically equivalent.
Method
1.2 Aluminum and aluminum alloys are chromate coated in
ISO 4519 Electrodeposited Metallic Coatings and Related
order to retard corrosion; as a base for organic films including
Finishes—Sampling Procedures for Inspection by Attri-
paints, plastics, and adhesives; and as a protective coating
butes
having a low electrical contact impedance.
ISO/DIS 10546 Chemical Conversion Coatings—Rinsed
1.3 The values stated in SI units are to be regarded as
and Nonrinsed Chromate Conversion Coatings—On Alu-
standard. No other units of measurement are included in this
minum and Aluminum Alloys
standard.
2.3 Federal Standard:
1.4 This standard does not purport to address all of the
Fed. Std. No. 141 Paints, Varnish, Lacquer, and Related
safety concerns, if any, associated with its use. It is the
Materials; Methods of Inspection
responsibility of the user of this standard to establish appro-
2.4 Military Specification:
priate safety and health practices and determine the applica-
MIL-DTL-5541 Chemical Films forAluminum andAlumi-
bility of regulatory limitations prior to use.
num Alloys
2. Referenced Documents
3. Terminology
2.1 ASTM Standards:
3.1 Definitions:
B117 Practice for Operating Salt Spray (Fog) Apparatus
3.1.1 nonrinsed—chromate coatings that are dried immedi-
B602 Test Method for Attribute Sampling of Metallic and
ately after the chromating step without receiving a water rinse.
Inorganic Coatings
3.1.1.1 Discussion—This special type of coating is typically
B767 Guide for Determining Mass Per Unit Area of Elec-
used on long coils of aluminum sheet stock that receive an
trodeposited and Related Coatings by Gravimetric and
immediate subsequent paint or adhesive coating.
Other Chemical Analysis Procedures
D1730 Practices for Preparation of Aluminum and
NOTE 1—Nonrinsed chromate coatings are finding increased usage on
Aluminum-Alloy Surfaces for Painting fabricated parts and castings.
3.1.2 rinsed—chromate coatings that are rinsed in water
prior to drying.
This specification is under the jurisdiction of ASTM Committee B08 on
3.1.2.1 Discussion—Thistypeofcoatingistypicallyapplied
Metallic and Inorganic Coatingsand is the direct responsibility of Subcommittee
to extruded aluminum fabricated parts and castings.
B08.07 on Conversion Coatings.
Current edition approved April 1, 2010. Published June 2010. Originally
approved in 1967. Last previous edition approved in 2004 as B449 – 93 (2004).
DOI: 10.1520/B0449-93R10E01.
2 3
For referenced ASTM standards, visit the ASTM website, www.astm.org, or Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
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B449−93(Reapproved2010)
4. Classification instructions for the chromating process employed. Chromating
solutions are usually acidic and may contain hexavalent
4.1 Chromate finishes can be applied ranging in color from
chromium salts together with other salts that may be varied to
brown, thick coatings (Class 1) providing maximum corrosion
affect the appearance and hardness of the film.The color of the
protection to yellow, intermediate thickness coatings (Class 2)
film,and,therefore,thetypeofconversioncoating,dependson
suitable as an organic film base or to colorless, thin coatings
the composition of the chromating solution, but it is also
(Class 3) suitable for low electrical contact resistance. The
affected by the pH and temperature, the duration of the
yellow coatings vary from golden yellow to iridescent light
treatment, and the nature and surface condition of the alloy
yellow. Chromate-phosphate finishes (Class 4) can be applied
being treated.
ranging in color from green to iridescent light green.The Class
4 coatings comply with the requirements of MIL-DTL-5541. 6.3 These coatings receive a final water rinse. If the coating
is meant to be a basis for additional coatings, the detail shall be
4.2 Finishers can seldom guarantee to supply exact shades
subject to a rinse in deionized water with a conductivity less
ofcolorwithchromateconversioncoatings.Ifitisnecessaryto
than 100t µS/cm. If hot water is used as the final rinse after the
have exact shades of color, it is possible to dye chromate
2 chromating process, it is essential that the time of rinsing
coatings having a coating mass greater than 0.4 g/m to obtain
should be kept as short as possible in order to prevent the
a wide range of colors, but they can only be expected to give
dissolution of the hexavalent chromium. The drying of the
an order of added corrosion resistance similar to that provided
coating shall be carried out at a temperature not exceeding
by the undyed coatings. It should be noted that color and color
60°Ctopreventcrackingduetodehydration,whichcausesloss
uniformity will vary somewhat between one alloy and another
of adhesion and performance of the chromate coating.
and from a polished surface to an etched surface. Iridescence
and variations in color density from one area of the surface to 6.4 Any additional subsequent treatments depend upon the
another are normal and shall not be considered a sign of poor purpose for which the chromated parts are intended.
quality.
7. Coating Requirements
4.3 The finishes are divided into four classes; their most
7.1 General—Chromate conversion coatings harden with
important characteristics are listed in Table 1.
agebygradualdehydration.Theyshould,therefore,behandled
carefully for the first 24 h after treatment, and any tests
5. Surfaces Preparation
(including corrosion tests) shall be deferred until the expiration
5.1 The surfaces of the parts to be chromated must be clean
of that period. The green chromate-phosphate coatings usually
andfreeofanyoxidation,scale,orsoilssuchasmetalturnings,
continue to improve in corrosion resistance after initial forma-
grinding dust, oil, grease, lubricants, hand-sweat, or any other
tion. They achieve their maximum corrosion resistance after 1
contaminationdetrimentaltothechromatingprocess.Theparts
to 2 months at room temperature. It is not required to store
must therefore, as far as necessary, be cleaned before chromat-
parts for this purpose.
ing and if necessary be pickled. Fig. X2.1 shows the various
processing step options. 7.2 Electrical Resistance—Colorless, light yellow, or light
green iridescent chromate layers of low mass per unit area
6. Methods of Application of Chromate Coatings
increase the electrical resistance between an electrical contact
and the aluminum to a very small extent. When measured at
6.1 Metallic material other than aluminum should not be
9-V and a 2-Acurrent the resistance should be less than 0.1Ω.
treated with the parts to be chromated.
Highly colored brown, yellow, or green coatings show a
6.2 Chromate conversion coatings are normally applied by
marked increase in electrical contact resistance with increasing
dipping: the coating may also be applied by inundation,
mass per unit area of the chromate layer and may reach
spraying, roller coating, or by wipe-on techniques. The appli-
resistances of 10 000Ω or more.
cation method used should be taken from the operating
7.3 Adhesion—The coatings shall be adherent and non-
powdery. There are no practical tests for measuring the
TABLE 1 Classification of Chromate Coatings
adhesion of a chromate conversion coating on aluminum.
Coating Mass per
A However, a practical evaluation of the adhesion can be made
Class Appearance Corrosion Protection
Unit Area, g/m
by measuring the adhesion of a s
...
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