ASTM B840 Zinc Cobalt Plated Fasteners

Stud Bolts, Nuts, Screws, Threaded Rod & Washers

ASTM B840 specification covers the requirements for electrodeposited zinc cobalt alloy coatings on metals.
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 and health practices and determine the applicability of regulatory limitations prior to use.

ASTM B840 Classification

There is one coating class, and it is defined as :

  • ASTM B840 Class 1 - a zinc cobalt alloy that is approximately 99% by mass zinc and at minimum 0.5% by mass cobalt.
  • There are five coating types and they are defined as follows:
  • ASTM B840 Type A - With colorless (blue bright) chromate conversion coatings.
  • ASTM B840 Type B - With yellow chromate conversion coating.
  • ASTM B840 Type C - With bronze chromate conversion coating.
  • ASTM B840 Type D - With black chromate conversion coating.
  • ASTM B840 Type E - Any 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.
There are three grades according to thickness and are defined as follows:

Minimum Thickness, µm New ASTM Grade Old ASTM Grade
6 6 1
12 12 2
18 18 3

ASTM B840 Coating Requirements

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
The electroplating shall be applied after all basis metal heat treatments have been completed.
Nature of Coating
The coating shall consist of a zinc cobalt alloy that is approximately 99% by mass zinc and at minimum 0.5% by mass cobalt. The coating shall be produced from an aqueous electroplating system, either alkaline or acid, and may be specified at the discretion of the purchaser. The coating shall have such supplementary conversion coatings as defined in specified in the purchase order
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
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.
The thickness of the coating everywhere on the significant surfaces as defined in and shall conform to the requirements of the specified grade
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
Corrosion Resistance
The corrosion resistance of the coating may be evaluated using the method in Appendix X1.
Pretreatment of Iron and Steel for Reducing the Risk of Hydrogen Embrittlement
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 cold straightened subsequent to heat treatment shall require stress relief heat treatment when specified by the purchaser, the tensile strength to be supplied by the purchaser. Specification B849 may be consulted for a list of pretreatments that are widely used.
Post-Coating Treatments of Iron and Steel for Reducing the Risk of Hydrogen Embrittlement
Parts that are made of steels with ultimate tensile strengths of 1000 MPa (hardness of 31 HRC or greater), as well as surface hardened parts, may require post-coating hydrogen embrittlement relief baking when specified by the purchaser, the tensile strength to be supplied by the purchaser. Guide B850 may be consulted for a list of post-treatments that are widely used.

ASTM B840 Test Methods

Special Test Specimens
The permission or the requirement to use special test specimens, the number to be used, the material from which they are to be made, and their shape and size shall be stated by the purchaser.
Special test specimens used to represent articles in an adhesion, porosity, corrosion resistance, or appearance test shall be made of the same material, shall be in the same metallurgical condition, and shall have the same surface condition as the articles they represent. They shall be placed in the production lot of and be processed along with the articles they represent.
Special test specimens used to represent articles in a coating thickness test may be made of a material that is suitable for the test method even if the represented article is not of the same material. For example, a low-carbon steel specimen may represent a brass article when the magnetic thickness test is used (Test Method B499). The thickness specimen need not be carried through the complete process with the represented article. If not, it shall be introduced into the process at the point where the coating is applied and it shall be carried through all steps that have a bearing on the coating thickness. In rack plating, the specimen shall be racked in the same way with the same distance from and orientation with the anodes and other items in the process as the article it represents.
Composition of Coating
The composition of the coating shall be verified by atomic absorption spectrophotometry (AA), inductively coupled plasma (ICP), or directly coupled plasma (DCP) or other methods if found to be capable of results within 10 % of known standards.
The coating shall be examined at up to 10× magnification for conformance to the requirements of appearance.
The coating thickness shall be measured at locations on the significant surface designated by the purchaser, and the measurement shall be made with an accuracy of 10 % or better by use of one of the following standards, Test Methods B487, B499, B504, and B568.
The adhesion of the coating shall be tested in accordance with Practice B571. The choice of test method shall be one of those appropriate for zinc coatings
Corrosion Resistance
See Appendix X1.

ASTM B840 Appendix

X1. Corrosion Resistance of Coating

Class Grade Type Base Metal Corrosion Zinc Alloy Corrosion Product
1 12 A 240 12
12 A/E 340 120
18 A 400 12
18 A/E 500 120
1 6 B 240 96
6 B/E 340 196
12 B 400 96
12 B/E 500 196
18 B 500 96
18 B/E 600 196
1 6 C 240 96
6 C/E 340 196
12 C 400 96
12 C/E 500 196
18 C 500 96
18 C/E 600 196
1 6 D 240 96
6 D/E 340 120
12 D 400 96
12 D/E 500 120
18 D 500 96
18 D/E 600 120