ASTM B449-93(2004)
(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 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 This standard does not purport to address all of the safety problems, 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.
General Information
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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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.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 (Reapproved 2004)
Standard Specification 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.
1. Scope ISO 3892 Conversion Coatings on Metallic Materials—
Determination of Mass Per Unit Area—Gravimetric
1.1 This specification covers the requirements relating to
Method
rinsed and nonrinsed chromate conversion coatings on alumi-
ISO 4519 Electrodeposited Metallic Coatings and Related
num and aluminum alloys intended to give protection against
Finishes—Sampling Procedures for Inspection by At-
corrosion and as a base for other coatings. This edition of the
tributes
specification has been coordinated with ISO/ 10546 and is
ISO/ DIS 10546 Chemical Conversion Coatings—Rinsed
technically equivalent.
and Nonrinsed Chromate Conversion Coatings—On Alu-
1.2 Aluminum and aluminum alloys are chromate coated in
minum and Aluminum Alloys
order to retard corrosion; as a base for organic films including
2.3 Federal Standard:
paints, plastics, and adhesives; and as a protective coating
Fed. Std. No. 141 Paints, Varnish, Lacquer, and Related
having a low electrical contact impedance.
Materials; Methods of Inspection
1.3 This standard does not purport to address all of the
2.4 Military Specification:
safety concerns, if any, associated with its use. It is the
MIL-C-5541 Chemical Films forAluminum andAluminum
responsibility of the user of this standard to establish appro-
Alloys
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use.
3. Terminology
2. Referenced Documents 3.1 Definitions:
3.1.1 nonrinsed—chromate coatings that are dried immedi-
2.1 ASTM Standards:
ately after the chromating step without receiving a water rinse.
B117 Practice for Operating Salt Spray (Fog) Apparatus
3.1.1.1 Discussion—Thisspecialtypeofcoatingistypically
B602 Test Method for Attribute Sampling of Metallic and
used on long coils of aluminum sheet stock that receive an
Inorganic Coatings
immediate subsequent paint or adhesive coating.
B767 Guide for Determining Mass Per Unit Area of Elec-
trodeposited and Related Coatings by Gravimetric and
NOTE 1—Nonrinsed chromate coatings are finding increased usage on
Other Chemical Analysis Procedures fabricated parts and castings.
D1730 Practices for Preparation of Aluminum and
3.1.2 rinsed—chromate coatings that are rinsed in water
Aluminum-Alloy Surfaces for Painting
prior to drying.
D3359 Test Methods for MeasuringAdhesion by Tape Test
3.1.2.1 Discussion—This type of coating is typically ap-
2.2 ISO Standards:
plied to extruded aluminum fabricated parts and castings.
ISO 2409 Paint and Varnishes—Cross-Cut Test
ISO 3768 Metallic Coatings—Neutral Salt Spray Test 4. Classification
(NSS Test)
4.1 Chromate finishes can be applied ranging in color from
brown, thick coatings (Class 1) providing maximum corrosion
protection to yellow, intermediate thickness coatings (Class 2)
This specification is under the jurisdiction of ASTM Committee B08 on
suitable as an organic film base or to colorless, thin coatings
Metallic and Inorganic Coatings and is the direct responsibility of Subcommittee
B08.07 on Chemical Conversion Coatings. (Class 3) suitable for low electrical contact resistance. The
Current edition approved April 1, 2004. Published April 2004. Originally
yellow coatings vary from golden yellow to iridescent light
approved in 1967. Last previous edition approved in 1998 as B449 – 93 (1998).
yellow. Chromate-phosphate finishes (Class 4) can be applied
DOI: 10.1520/B0449-93R04.
ranging in color from green to iridescent light green.The Class
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
4 coatings comply with the requirements of MIL-C-5541.
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
3 4
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B449 – 93 (2004)
4.2 Finishers can seldom guarantee to supply exact shades than 100t µS/cm. If hot water is used as the final rinse after the
ofcolorwithchromateconversioncoatings.Ifitisnecessaryto chromating process, it is essential that the time of rinsing
have exact shades of color, it is possible to dye chromate should be kept as short as possible in order to prevent the
coatings having a coating mass greater than 0.4 g/m to obtain dissolution of the hexavalent chromium. The drying of the
a wide range of colors, but they can only be expected to give coating shall be carried out at a temperature not exceeding
an order of added corrosion resistance similar to that provided 60°Ctopreventcrackingduetodehydration,whichcausesloss
by the undyed coatings. It should be noted that color and color of adhesion and performance of the chromate coating.
uniformity will vary somewhat between one alloy and another 6.4 Any additional subsequent treatments depend upon the
and from a polished surface to an etched surface. Iridescence
purpose for which the chromated parts are intended.
and variations in color density from one area of the surface to
another are normal and shall not be considered a sign of poor
7. Coating Requirements
quality.
7.1 General—Chromate conversion coatings harden with
4.3 The finishes are divided into four classes; their most
agebygradualdehydration.Theyshould,therefore,behandled
important characteristics are listed in Table 1.
carefully for the first 24 h after treatment, and any tests
(including corrosion tests) shall be deferred until the expiration
5. Surfaces Preparation
of that period. The green chromate-phosphate coatings usually
5.1 The surfaces of the parts to be chromated must be clean
continue to improve in corrosion resistance after initial forma-
andfreeofanyoxidation,scale,orsoilssuchasmetalturnings,
tion. They achieve their maximum corrosion resistance after 1
grinding dust, oil, grease, lubricants, hand-sweat, or any other
to 2 months at room temperature. It is not required to store
contaminationdetrimentaltothechromatingprocess.Theparts
parts for this purpose.
must therefore, as far as necessary, be cleaned before chromat-
7.2 Electrical Resistance—Colorless, light yellow, or light
ing and if necessary be pickled. Fig. X2.1 shows the various
green iridescent chromate layers of low mass per unit area
processing step options.
increase the electrical resistance between an electrical contact
and the aluminum to a very small extent. When measured at
6. Methods of Application of Chromate Coatings
9-V and a 2-Acurrent the resistance should be less than 0.1 V.
6.1 Metallic material other than aluminum should not be
Highly colored brown, yellow, or green coatings show a
treated with the parts to be chromated.
marked increase in electrical contact resistance with increasing
6.2 Chromate conversion coatings are normally applied by
mass per unit area of the chromate layer and may reach
dipping: the coating may also be applied by inundation,
resistances of 10 000 V or more.
spraying, roller coating, or by wipe-on techniques. The appli-
7.3 Adhesion—The coatings shall be adherent and non-
cation method used should be taken from the operating
powdery. There are no practical tests for measuring the
instructions for the chromating process employed. Chromating
adhesion of a chromate conversion coating on aluminum.
solutions are usually acidic and may contain hexavalent
However, a practical evaluation of the adhesion can be made
chromium salts together with other salts that may be varied to
by measuring the adhesion of a secondary organic film applied
affect the appearance and hardness of the film.The color of the
to the chromated aluminum. When specified, the chromate
film,and,therefore,thetypeofconversioncoating,dependson
conversion coating shall pass the organic coating adhesion test
the composition of the chromating solution, but it is also
in Test Methods D3359 or the equivalent ISO 2409.
affected by the pH and temperature, the duration of the
7.3.1 Class 4 coatings intended for use under MIL-C-5541
treatment, and the nature and surface condition of the alloy
shall have their adhesion evaluated by Method 6301 of Fed.
being treated.
Std. No. 141.
6.3 These coatings receiv
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