ASTM B449-93(1998)
(Specification)Standard Specification for Chromates on Aluminum
Standard Specification for Chromates on Aluminum
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.
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Designation: B 449 – 93 (Reapproved 1998)
Standard Specification for
Chromates on Aluminum
This standard is issued under the fixed designation B 449; 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 (e) 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 problems, if any, associated with its use. It is the
MIL-C-5541 Chemical Films for Aluminum and Aluminum
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.
B 117 Practice for Operating Salt Spray (Fog) Apparatus
3.1.1.1 Discussion—This special type of coating is typically
B 602 Test Method for Attribute Sampling of Metallic and
3 used on long coils of aluminum sheet stock that receive an
Inorganic Coatings
immediate subsequent paint or adhesive coating.
B 767 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.
D 1730 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.
D 3359 Test Methods for Measuring Adhesion by Tape
3.1.2.1 Discussion—This type of coating is typically ap-
Test
plied to extruded aluminum fabricated parts and castings.
2.2 ISO Standards:
ISO 2409 Paint and Varnishes—Cross-Cut Test 4. Classification
ISO 3768 Metallic Coatings—Neutral Salt Spray Test
4.1 Chromate finishes can be applied ranging in color from
(NSS Test)
brown, thick coatings (Class 1) providing maximum corrosion
protection to yellow, intermediate thickness coatings (Class 2)
suitable as an organic film base or to colorless, thin coatings
This specification is under the jurisdiction of ASTM Committee B-8 on
(Class 3) suitable for low electrical contact resistance. The
Metallic and Inorganic Coatings and is the direct responsibility of Subcommittee
B08.07 on Chemical Conversion Coatings. yellow coatings vary from golden yellow to iridescent light
Current edition approved Sept. 15, 1993. Published April 1994. Originally
yellow. Chromate-phosphate finishes (Class 4) can be applied
published as B 449 – 67. Last previous edition B 449 – 67a (1993).
Annual Book of ASTM Standards, Vol 03.02.
Annual Book of ASTM Standards, Vol 02.05.
Annual Book of ASTM Standards, Vol 06.01.
5 6
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B 449 – 93 (1998)
ranging in color from green to iridescent light green. The Class subject to a rinse in deionized water with a conductivity less
4 coatings comply with the requirements of MIL-C-5541. than 100t μS/cm. If hot water is used as the final rinse after the
4.2 Finishers can seldom guarantee to supply exact shades chromating process, it is essential that the time of rinsing
of color with chromate conversion coatings. If it is necessary to should be kept as short as possible in order to prevent the
have exact shades of color, it is possible to dye chromate dissolution of the hexavalent chromium. The drying of the
coatings having a coating mass greater than 0.4 g/m to obtain coating shall be carried out at a temperature not exceeding
a wide range of colors, but they can only be expected to give 60°C to prevent cracking due to dehydration, which causes loss
an order of added corrosion resistance similar to that provided of adhesion and performance of the chromate coating.
by the undyed coatings. It should be noted that color and color 6.4 Any additional subsequent treatments depend upon the
uniformity will vary somewhat between one alloy and another purpose for which the chromated parts are intended.
and from a polished surface to an etched surface. Iridescence
7. Coating Requirements
and variations in color density from one area of the surface to
7.1 General—Chromate conversion coatings harden with
another are normal and shall not be considered a sign of poor
age by gradual dehydration. They should, therefore, be handled
quality.
carefully for the first 24 h after treatment, and any tests
4.3 The finishes are divided into four classes; their most
(including corrosion tests) shall be deferred until the expiration
important characteristics are listed in Table 1.
of that period. The green chromate-phosphate coatings usually
5. Surfaces Preparation
continue to improve in corrosion resistance after initial forma-
tion. They achieve their maximum corrosion resistance after 1
5.1 The surfaces of the parts to be chromated must be clean
to 2 months at room temperature. It is not required to store
and free of any oxidation, scale, or soils such as metal turnings,
parts for this purpose.
grinding dust, oil, grease, lubricants, hand-sweat, or any other
7.2 Electrical Resistance—Colorless, light yellow, or light
contamination detrimental to the chromating process. The parts
green iridescent chromate layers of low mass per unit area
must therefore, as far as necessary, be cleaned before chromat-
increase the electrical resistance between an electrical contact
ing and if necessary be pickled. Fig. X2.1 shows the various
and the aluminum to a very small extent. When measured at
processing step options.
9-V and a 2-A current the resistance should be less than 0.1 V.
6. Methods of Application of Chromate Coatings
Highly colored brown, yellow, or green coatings show a
marked increase in electrical contact resistance with increasing
6.1 Metallic material other than aluminum should not be
treated with the parts to be chromated. mass per unit area of the chromate layer and may reach
resistances of 10 000 V or more.
6.2 Chromate conversion coatings are normally applied by
dipping: the coating may also be applied by inundation, 7.3 Adhesion—The coatings shall be adherent and non-
powdery. There are no practical tests for measuring the
spraying, roller coating, or by wipe-on techniques. The appli-
cation method used should be taken from the operating adhesion of a chromate conversion coating on aluminum.
However, a practical evaluation of the adhesion can be made
instructions for the chromating process employed. Chromating
solutions are usually acidic and may contain hexavalent by measuring the adhesion of a secondary organic film applied
to the chromated aluminum. When specified, the chromate
chromium salts together with other salts that may be varied to
affect the appearance and hardness of the film. The color of the conversion coating shall pass the organic coating adhesion test
in Test Methods D 3359 or the equivalent ISO 2409.
film, and, therefore, the type of conversion coating, depends on
the composition of the chromating solution, but it is also 7.3.1 Class 4 coatings intended for use under MIL-C-5541
affected by the pH and temperature, the duration of the shall have their adhesion evaluated by Method 6301 of Fed.
treatment, and the nature and surface condition of the alloy Std. No. 141.
being treated. 7.3.2 Additional treatments applie
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