ASTM B840-22

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Designation: B840 − 15 B840 − 22
Standard Specification for
Electrodeposited Coatings of Zinc Cobalt Alloy Deposits
This standard is issued under the fixed designation B840; 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.
1. Scope Scope*
1.1 This specification covers the requirements for electrodeposited zinc cobalt alloy coatings on metals.
1.2 The following precautionary caveat pertains to the test method portion only, Section 8, of this specification: 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 safety, health, and healthenvironmental practices and determine the applicability of
regulatory limitations prior to use.
1.3 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
B117 Practice for Operating Salt Spray (Fog) Apparatus
B183 Practice for Preparation of Low-Carbon Steel for Electroplating
B242 Guide for Preparation of High-Carbon Steel for Electroplating
B320 Practice for Preparation of Iron Castings for Electroplating
B322 Guide for Cleaning Metals Prior to Electroplating
B374 Terminology Relating to Electroplating
B487 Test Method for Measurement of Metal and Oxide Coating Thickness by Microscopical Examination of Cross Section
B499 Test Method for Measurement of Coating Thicknesses by the Magnetic Method: Nonmagnetic Coatings on Magnetic Basis
Metals
B504 Test Method for Measurement of Thickness of Metallic Coatings by the Coulometric Method
B507 Practice for Design of Articles to Be Electroplated on Racks
B568 Test Method for Measurement of Coating Thickness by X-Ray Spectrometry
B571 Practice for Qualitative Adhesion Testing of Metallic Coatings
B602 Guide for Attribute Sampling of Metallic and Inorganic Coatings
B697 Guide for Selection of Sampling Plans for Inspection of Electrodeposited Metallic and Inorganic Coatings
B762 Guide of Variables Sampling of Metallic and Inorganic Coatings
B849 Specification for Pre-Treatments of Iron or Steel for Reducing Risk of Hydrogen Embrittlement
B850 Guide for Post-Coating Treatments of Steel for Reducing the Risk of Hydrogen Embrittlement
D3951 Practice for Commercial Packaging
This specification is under the jurisdiction of ASTM Committee B08 on Metallic and Inorganic Coatings and is the direct responsibility of Subcommittee B08.06 on Soft
Metals.
Current edition approved May 1, 2015Nov. 15, 2022. Published June 2015December 2022. Originally approved in 1993. Last previous edition approved in 20092015 as
B840 – 99B840 – 15.(2009). DOI: 10.1520/B0840-15.10.1520/B0840-22.
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 Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
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B840 − 22
3. Terminology
3.1 Definitions—Many terms used in this specification are defined in Terminology B374.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 significant surface, n—that portion of a coated article’s surface where the coating is required to meet all the requirements
of the coating specification for that article. Significant surfaces are usually those that are essential to the serviceability or function
of the article or can be a source of corrosion products or tarnish films that interfere with the function or desirable appearance of
the article. Significant surfaces are those surfaces that are identified by the purchaser by, for example, indicating them on an
engineering drawing of the product or marking a sample item of the product.
4. Classification
4.1 There is one coating class, and it is defined as Class 1—a zinc cobalt alloy that is approximately 99%99 % by mass zinc and
at minimum 0.5%0.5 % by mass cobalt.
4.2 There are five coating types and they are defined as follows:
4.2.1 Type A—Type A—With With colorless (blue bright) chromate conversion coatings.
4.2.2 Type B—Type B—With With yellow chromate conversion coating.
4.2.3 Type C—Type C—With With bronze chromate conversion coating.
4.2.4 Type D—Type D—With With black chromate conversion coating.
4.2.5 Type E—Type E—AnyAny of the above types plus organic topcoat.
NOTE 1—Whereas colored chromate conversion coatings are usually meant to include various shades of yellow to bronze when used with non-alloyed
zinc, yellow and bronze chromate conversion coatings are considered distinctly different when applied to alloyed zinc coatings and are formulated
specifically to produce the desired coating.
4.3 There are three grades according to thickness, and they are defined as follows:
Minimum Thickness, μm New ASTM Grade Old ASTM Grade
6 6 1
12 12 2
18 18 3
5. Ordering Information
5.1 In order to make the application of this specification complete, the purchaser needs to supply the following information to the
seller in the purchase order and drawings.
5.1.1 Title, ASTM designation number, and date of issue of this specification.
5.1.2 Deposit by classification including class, type, and grade (see 4.1, 4.2, 4.3).
5.1.3 Composition and metallurgical condition of the substrate to be coated.
5.1.4 Location of significant surfaces (3.2.1).
5.1.5 Basis metal alloy designation and ultimate tensile strength of the steel.
5.1.6 Whether the part underwent cold forming or cold straightening subsequent to heat treatment (see Note 2).
5.1.7 Heat Exception to heat treatment for stress relief, whether it has been performed by purchaser or is required relief prior to
plating (6.7).
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5.1.8 Heat treatmentBaking requirements after electroplating, if required (6.8).
5.1.9 Whether or not location of rack marks is to be defined (6.3.1).
5.1.10 Any requirement for special test specimens (8.1.1).
5.1.11 Acceptance inspection procedure to be used (Section 7).
5.1.12 Any requirement for certification (Section 10).
5.1.13 Any other items needing agreement. For the purposes of this specification, prior agreement on the nature of the finish is
required as items plated in bulk may differ in appearance from those that are rack plated.
NOTE 2—Information in 5.1.5 and 5.1.6 is necessary for proper pretreatment (6.7) and post coating treatment (6.8), if applicable.
6. Coating Requirements
6.1 Substrate—The metal to be plated shall be free of flaws and defects that will be detrimental to the zinc alloy coating. It shall
be subjected to such cleaning, pickling, and electroplating procedures as are necessary to yield deposits with the desired quality.
NOTE 3—Proper preparatory procedures and thorough cleaning are essential to ensure satisfactory adhesion and corrosion resistance performance of the
coating. Materials used for cleaning should not damage the basis metal, for example, by causing defects such as pits, intergranular attack, or stress
corrosion cracking. It is recommended that the following ASTM practices for cleaning, where appropriate, be used: Practices B183, B320, and B322 and
Guide B242.
6.1.1 The electroplating shall be applied after all basis metal heat treatments have been completed.
6.2 Nature of Coating:
6.2.1 The coating shall consist of a zinc cobalt alloy that is approximately 99%99 % by mass zinc and at minimum 0.5%0.5 %
by mass cobalt.
6.2.2 The coating shall be produced from an aqueous electroplating system, either alkaline or acid, and may be specified at the
discretion of the purchaser.
6.2.3 The coating shall have such supplementary conversion coatings as defined in 4.2 and specified in the purchase order.
6.3 Appearance:
6.3.1 The coating on all readily visible surfaces shall have an acceptable and characteristic appearance as agreed upon by the
purchaser and seller. The coating shall be uniform insofar as the basis metal will permit. When the article is to be plated on a rack,
contact marks may be unavoidable. Location of such mark(s) shall be indicated on the article or its drawing.
6.3.2 Defects and variations in appearance that arise from surface conditions of the substrate (scratches, pores, roll marks,
inclusions, and the like) and that persist in the coating despite the observance of good metal finishing practices, shall not be cause
for rejection. The coating shall be adherent, free from blisters, pits, or discontinuities, and shall be free of cracks in the as plated
state. Flaking shall be cause for rejection in either the as plated state or after subsequent operations.
NOTE 4—These coatings are commonly used in automotive applications where subsequent forming, bending, and crimping operations are commonly
performed. These operations will necessarily detract from the performance of the coatings. While some cracking of coatings will be unavoidable, flaking
of the coatings after these subsequent operations shall be cause for rejection.
NOTE 5—Coatings generally perform better in service when the substrate over which they are applied is smooth and free of torn metal, inclusions, pores,
and other defects. The specifications covering the unfinished product should provide limits for these defects. A metal finisher can often remove defects
through special treatments, such as grinding, polishing, abrasive blasting, chemical etches, and electropolishing. However, these are not normal in the
treatment steps preceding the application of the coating. When they are desired, they are the subject of special agreement between the purchaser and the
seller.
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6.4 Thickness—The thickness of the coating everywhere on the significant surfaces as defined in 3.2.1 and shall conform to the
requirements of the specified grade as defined in 4.3.
NOTE 6—The thickness of electrodeposited coatings varies from point to point on the surfaces of a product (see Practice B507). The thickness is less in
interior corners and in holes. Such surfaces are often exempt from thickness requirements. If the full thickness is required in those locations, the
electroplater will have to use special techniques that will probably raise the cost of the process.
NOTE 7—The coating requirement of this specification is a minimum. Variation in the thickness from point to point on an article and from article to article
in a production lot is inherent in electroplating. Therefore, if all of the articles in a production lot are to meet the thickness requirement, the average coating
thickness for the production lot as a whole will be greater than the specified minimum.
6.5 Adhesion—The coating shall withstand normal handling and storage conditions without chipping, flaking, or other coating
damage and shall conform to the minimum requirements set forth in Section 8.
6.6 Corrosion Resistance—The corrosion resistance of the coating may be evaluated using the method in Appendix X1.
6.7 Pretreatment of Iron and Steel for Reducing the Risk of Hydrogen Embrittlement:
6.7.1 Parts that are made of steels with ultimate tensile strengths of 1000 MPa (hardness of 31 HRC or greater), that have been
machined, ground, cold formed, or
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