ASTM B449-93(2015)

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.

General Information

• Status: Historical
• Publication Date: 28-Feb-2015
• ICS: 25.220.99 - Other treatments and coatings
• Technical Committee: B08 - Metallic and Inorganic Coatings
• Drafting Committee: B08.07 - Conversion Coatings

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.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:B449 −93 (Reapproved 2015)
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 U.S. Department of Defense.
1. Scope 2.2 ISO Standards:
ISO 2409 Paint and Varnishes—Cross-Cut Test
1.1 This specification covers the requirements relating to
ISO 3768 Metallic Coatings—Neutral Salt Spray Test (NSS
rinsed and nonrinsed chromate conversion coatings on alumi-
Test)
num and aluminum alloys intended to give protection against
ISO 3892 Conversion Coatings on Metallic Materials—
corrosion and as a base for other coatings. This edition of the
Determination of Mass Per Unit Area—Gravimetric
specification has been coordinated with ISO/DIS 10546 and is
Method
technically equivalent.
ISO 4519 Electrodeposited Metallic Coatings and Related
1.2 Aluminum and aluminum alloys are chromate coated in
Finishes—Sampling Procedures for Inspection by Attri-
order to retard corrosion; as a base for organic films including
butes
paints, plastics, and adhesives; and as a protective coating
ISO/DIS 10546 Chemical Conversion Coatings—Rinsed
having a low electrical contact impedance.
and Nonrinsed Chromate Conversion Coatings—On Alu-
1.3 The values stated in SI units are to be regarded as minum and Aluminum Alloys
standard. No other units of measurement are included in this 2.3 Federal Standard:
standard.
Fed. Std. No. 141 Paints, Varnish, Lacquer, and Related
Materials; Methods of Inspection
1.4 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-DTL-5541 Chemical Films forAluminum andAlumi-
responsibility of the user of this standard to establish appro-
num 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—This special type of coating is typically
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 Measuring Adhesion by Tape Test
3.1.2.1 Discussion—Thistypeofcoatingistypicallyapplied
to extruded aluminum fabricated parts and castings.
4. Classification
This specification is under the jurisdiction of ASTM Committee B08 on
Metallic and Inorganic Coatings and is the direct responsibility of Subcommittee
4.1 Chromate finishes can be applied ranging in color from
B08.07 on Conversion Coatings.
brown, thick coatings (Class 1) providing maximum corrosion
Current edition approved March 1, 2015. Published April 2015. Originally
ε1
approved in 1967. Last previous edition approved in 2010 as B449 – 93 (2010) .
DOI: 10.1520/B0449-93R15.
2 3
For referenced ASTM standards, visit the ASTM website, www.astm.org, or Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM 4th Floor, New York, NY 10036, http://www.ansi.org.
Standards volume information, refer to the standard’s Document Summary page on Available from Standardization Documents Order Desk, DODSSP, Bldg. 4,
the ASTM website. Section D, 700 Robbins Ave., Philadelphia, PA 19111-5098.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B449−93 (2015)
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
6.3 These coatings receive a final water rinse. If the coating
4 coatings comply with the requirements of MIL-DTL-5541.
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 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
60°Ctopreventcrackingduetodehydration,whichcausesloss
by the undyed coatings. It should be noted that color and color
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
6.4 Any additional subsequent treatments depend upon the
and variations in color density from one area of the surface to
purpose for which the chromated parts are intended.
another are normal and shall not be considered a sign of poor
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
7.2 Electrical Resistance—Colorless, light yellow, or light
processing step options.
green iridescent chromate layers of low mass per unit area
increase the electrical resistance between an electrical contact
6. Methods of Application of Chromate Coatings
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 10 000 Ω or more.
cation method used should be taken from the operating
7.3 Adhesion—The coatings shall be adherent and non-
instructions for the chromating process employed. Chromating
powdery. There are no practical tests for measuring the
solutions are usually acidic and may contain hexavalent
adhesion of a chromate conversion coating on aluminum.
chromium salts together with other salts that may be varied to
However, a practical evaluation of the adhesion can be made
affect the appearance and hardness of the film.The color of the
by measuring the adhesion of a secondary organic film applied
to the chromated aluminum. When specified, the chromate
TABLE 1 Classification of Chromate Coatings
conversion coating shall pass the organic coating 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.
´1
Designation: B449 − 93 (Reapproved 2010) B449 − 93 (Reapproved 2015)
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 U.S. Department of Defense.
ε NOTE—Updated format of the military specification in Section 2.4 editorially in June 2010.
1. 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.
2. Referenced Documents
2.1 ASTM Standards:
B117 Practice for Operating Salt Spray (Fog) Apparatus
B602 Test Method for Attribute Sampling of Metallic and Inorganic Coatings
B767 Guide for Determining Mass Per Unit Area of Electrodeposited and Related Coatings by Gravimetric and Other Chemical
Analysis Procedures
D1730 Practices for Preparation of Aluminum and Aluminum-Alloy Surfaces for Painting
D3359 Test Methods for Measuring Adhesion by Tape Test
2.2 ISO Standards:
ISO 2409 Paint and Varnishes—Cross-Cut Test
ISO 3768 Metallic Coatings—Neutral Salt Spray Test (NSS Test)
ISO 3892 Conversion Coatings on Metallic Materials—Determination of Mass Per Unit Area—Gravimetric Method
ISO 4519 Electrodeposited Metallic Coatings and Related Finishes—Sampling Procedures for Inspection by Attributes
ISO/DIS 10546 Chemical Conversion Coatings—Rinsed and Nonrinsed Chromate Conversion Coatings—On Aluminum and
Aluminum Alloys
2.3 Federal Standard:
Fed. Std. No. 141 Paints, Varnish, Lacquer, and Related Materials; Methods of Inspection
2.4 Military Specification:
MIL-DTL-5541 Chemical Films for Aluminum and Aluminum Alloys
This specification is 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 April 1, 2010March 1, 2015. Published June 2010April 2015. Originally approved in 1967. Last previous edition approved in 20042010 as
ε1
B449 – 93 (2004).(2010) . DOI: 10.1520/B0449-93R10E01.10.1520/B0449-93R15.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
Available from Standardization Documents Order Desk, DODSSP, Bldg. 4, Section D, 700 Robbins Ave., Philadelphia, PA 19111-5098.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B449 − 93 (2015)
3. Terminology
3.1 Definitions:
3.1.1 nonrinsed—chromate coatings that are dried immediately after the chromating step without receiving a water rinse.
3.1.1.1 Discussion—
This special type of coating is typically used on long coils of aluminum sheet stock that receive an immediate subsequent paint
or adhesive coating.
NOTE 1—Nonrinsed chromate coatings are finding increased usage on fabricated parts and castings.
3.1.2 rinsed—chromate coatings that are rinsed in water prior to drying.
3.1.2.1 Discussion—
This type of coating is typically applied to extruded aluminum fabricated parts and castings.
4. Classification
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) suitable as an organic film base or to colorless, thin coatings (Class
3) suitable for low electrical contact resistance. The yellow coatings vary from golden yellow to iridescent light yellow.
Chromate-phosphate finishes (Class 4) can be applied ranging in color from green to iridescent light green. The Class 4 coatings
comply with the requirements of MIL-DTL-5541.
4.2 Finishers can seldom guarantee to supply exact shades of color with chromate conversion coatings. If it is necessary to have
exact shades of color, it is possible to dye chromate coatings having a coating mass greater than 0.4 g/m to obtain a wide range
of colors, but they can only be expected to give an order of added corrosion resistance similar to that provided by the undyed
coatings. It should be noted that color and color uniformity will vary somewhat between one alloy and another and from a polished
surface to an etched surface. Iridescence and variations in color density from one area of the surface to another are normal and
shall not be considered a sign of poor quality.
4.3 The finishes are divided into four classes; their most important characteristics are listed in Table 1.
5. Surfaces Preparation
5.1 The surfaces of the parts to be chromated must be clean and free of any oxidation, scale, or soils such as metal turnings,
grinding dust, oil, grease, lubricants, hand-sweat, or any other contamination detrimental to the chromating process. The parts must
therefore, as far as necessary, be cleaned before chromating and if necessary be pickled. Fig. X2.1 shows the various processing
step options.
6. Methods of Application of Chromate Coatings
6.1 Metallic material other than aluminum should not be treated with the parts to be chromated.
6.2 Chromate conversion coatings are normally applied by dipping: the coating may also be applied by inundation, spraying,
roller coating, or by wipe-on techniques. The application method used should be taken from the operating instructions for the
chromating process employed. Chromating solutions are usually acidic and may contain hexavalent chromium salts together with
TABLE 1 Classification of Chromate Coatings
Coating Mass per
A
Class Appearance Corrosion Protection
Unit Area, g/m
1 yellow to brown 0.4 to 2 maximum corrosion resistance
generally used as final finish
2 colorless to yellow 0.1 to 0.4 moderate corrosion resistance,
used as a paint base and for
bonding to rubber
3 colorless <0.1 decorative, slight corrosion
resistance, low electrical
contact resistance
4 light green to green 0.2 to 5 moderate corrosion resistance,
used as a paint base and for
bonding to rubber
A
No relationship exists between class numbers and degree of corrosion resis-
tance. Class numbers have been assigned on the basis of traditional usage and for
compatibility with national standards.
B449 − 93 (2015)
other salts that may be varied to affect the appearance and hardness of the film. The color of the film, and, therefore, the type of
conversion coating, depends on the composition of the chromating solution, but it is also affected by the pH and temperature, the
duration of the treatment, and the nature and surface condition of the alloy being treated.
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
subject to a rinse in deionized water with a conductivity less than 100t μS/cm. If hot water is used as the final rinse after the
chromating process, it is essential that the time of rinsing should be kept as short as possible in order to prevent the dissolution
of the hexavalent chromium. The drying of the coating shall be carried out at a temperature not exceeding 60°C to prevent cracking
due to dehydration, which causes loss of adhesion and performance of the chromate coating.
6.4 Any additional subsequent treatments depend upon the purpose for which the chromated parts are intended.
7. Coating Requirements
7.1 General—Chromate conversion coatings harden with age by gradual dehydration. They should, therefore, be handled
carefully for the first 24 h after treatment, and any tests (including corrosion tests) shall be deferred until the expiration of that
period. The green chromate-phosphate coatings usually continue to improve in corrosion resistance after initial formation. They
achieve their maximum corrosion resistance after 1 to 2 months at room temperature. It is not required to s
...