ASTM F1941 Specification

    This specification covers application, performance and dimensional requirements for electrodeposited coatings on threaded fasteners with unified inch screw threads. It specifies coating thickness, supplementary hexavalent chromate or non-hexavalent passivate finishes, corrosion resistance, precautions for managing the risk of hydrogen embrittlement and hydrogen embrittlement relief for high-strength and surface-hardened fasteners. It also highlights the differences between barrel and rack plating and makes recommendations as to the applicability of each process.
    The values stated in inch-pound units are to be regarded as standard. No other units of measurement are included in this standard.
    The following precautionary statement pertains to the test method portion only, Section 9, 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.

    Terminology

    Definitions:
    Local thickness—the mean of the thickness measurements, of which a specified number is made within a reference area.
    Minimum local thickness—the lowest local thickness value on the significant surface of a single article. Reference area—the area within which a specified number of single measurements are required to be made.
    Significant surface—significant surfaces are areas where the minimum thickness to be met shall be designated on the applicable drawing or by the provision of a suitably marked sample. However, if not designated, significant surfaces shall be defined as those normally visible, directly or by reflection, which are essential to the appearance or serviceability of the fastener when assembled in normal position, or which can be the source of corrosion products that deface visible surfaces on the assembled fastener. Figs. 1 and 2 illustrate significant surfaces on standard externally threaded and internally threaded fasteners.

    Classification

    Coating Material— The coating material shall be selected and designated in accordance with Table.

    Significant Surfaces on Internally Threaded Fasteners

    Note 1- Black dot (.) indicates test surface.
    Figure 1 — Significant Surfaces on Internally Threaded Fasteners

  • Designation of Common Coating Materials
    • Coating Designation Coating Type
    Fe/Zn Zinc
    Fe/Cd Cadmium
    Fe/Zn-Co Zinc Cobalt Alloy
    Fe/Zn-Ni Zinc Nickel Alloy
    Fe/Zn-Fe Zinc Iron Alloy

    Coating Thickness—The coating thickness shall be selected and designated in accordance with Table:
    Chromate Finish—When not specified, the hexavalent, trivalent or other chromiate finish shall be used at the option of the manufacturer and its appearance shall be selected in accordance with the designation selected in Table.
    Passivate—for the purpose of this specification, a conversion coating shall not contain hexavalent chromium.
    Trivalent Chromite Passivate Finish—Unless otherwise specified, the typical appearance of the trivalent chromite finish shall be transparent, colorless and shall not be subjected to the requirements of typical appearance as determined in Table. In addition, the classification code to be used shall be appended with the letter 9T9 (for example, Fe/Zn 5CT, as defined in Table A1.1 ).
    Supplemental Passivate Finish —Unless otherwise specified, the typical appearance of the supplemental passivate finish shall be transparent, colorless and shall not be subjected to the requirements of typical appearances as determined in Table. In addition, the classification code to be used shall be appended with the letter “S” (for example Fe/Zn 5AS). Other colored finishes may be specified at time of purchase. Requirements of the other colored finishes to be agreed upon at time of purchase.

  • Designation of Coating Thickness

    • Note 1 — The conversion factor from inch to microns is 2.54 3 104 (for example, 0.0001 in. = 2.54 µm).

    Thickness Designation Minimum Thickness in.
    3 0.0001
    5 0.0002
    8 0.0003
    12 0.0005
  • Designation of Chromate Finish

    • Note 1 — Coated fasteners with trivalent chromite (Cr+3) are not subjected to the required yellow, opaque, and black color. See Appendix X5.
    Note 2 — When fasteners are coated with trivalent chromite (Cr+3) the classification code to be used shall be appended with the letter 9T9 .

    Designation Type Typical Appearance
    A Clear Transparent colorless with slight iridescence
    B Blue-bright Transparent with a bluish tinge and slight iridescence
    C Yellow Yellow iridescent
    D Opaque Olive green, shading to brown or bronze
    E Black Black with slight iridescence
    F Organic Any of the above plus organic topcoat

    Note 1 — The use of supplemental passivated finishes are technically not “chromate” films and they do not contain hexavalent chromium ions, they are supplemental coatings that render the active zinc surface passive and provide protection to the steel fastener.

    Ordering Information for Electroplating

    When ordering threaded fasteners to be coated by electrodeposition in accordance with this specification, the following information shall be supplied to the electroplater:
    The desired coating, coating thickness, the chromate or passivate finish, the color and appearance (if applicable), or the classification codes as specified in Tables 1-3. (for example, Fe/Zn 5C denotes yellow zinc plated with a minimum thickness of 0.0002 in. on significant surfaces.)
    The identification of significant surfaces (optional). The requirement, if any, for stress relief before electroplating, in which case the stress-relief conditions must be specified.
    The requirements, if any, for hydrogen embrittlement relief by heat treatment (baking) stating the tensile strength or surface hardness of the fasteners and/or baking time and temperature.
    Note 2 — Fasteners with a specified maximum hardness of 34 HRC and below have a very low susceptibility to hydrogen embrittlement and do not require baking.
    The requirements, if any, for the type of electroplating process (barrel-plating or rack-plating). See Section 10 and Appendix X1.
    The designation of coated thread class shall comply with ASME B1.1.

    Requirements

  • Coating Requirements
    • The electrodeposited coating as ordered shall cover all surfaces and shall meet the following requirements:
    The coating metal deposit shall be bright or semi-bright unless otherwise specified by the purchaser, smooth, fine grained, adherent and uniform in appearance.
    The coating shall be free of blisters, pits, nodules, roughness, cracks, unplated areas, and other defects that will affect the function of the coating.
    The coating shall not be stained, discolored or exhibit any evidence of white or red corrosion products.
    6.1.3.1 Slight discoloration that results from baking, drying, or electrode contact during rack-plating, or all of these, as well as slight staining that results from rinsing shall not be cause for rejection.
  • Corrosion Resistance
    • Coated fasteners, when tested by continuous exposure to neutral salt spray in accordance with 9.3, shall show neither corrosion products of coatings (white corrosion) nor basis metal corrosion products (red rust) at the end of the test period. The appearance of corrosion products visible to the unaided eye at normal reading distance shall be cause for rejection, except when present at the edges of the tested fasteners. Refer to Annex A1 for neutral salt spray performance requirements for zinc, zinc alloy and cadmium coatings.
  • Thickness
    • The coating thickness shall comply with requirements of Table 2 when measured in accordance with 9.1.
    Restrictions on Coating Thickness—This specification imposes minimum local thickness requirements at significant surfaces in accordance with Table 2. Thick or thin local thickness in a location other than a significant surface shall not be a cause for rejection. However the following restrictions apply:
    Minimum coating thickness at low current density areas, such as the center of a bolt or recesses, must be sufficient to provide for adequate chromate adhesion.
    External Threads — Maximum coating thickness at high current density threaded tips must provide for class 3A GO thread gauge acceptance.
    Internal Threads — Maximum coating thickness of internal threads must provide for class 1B, 2B, or 3B Go thread gage acceptance.
    Surfaces such as threads, holes, deep recesses, bases of angles, and similar areas on which the specified thickness of deposit cannot readily be controlled, are exempted from minimum thickness requirements unless they are specially designated as not being exempted. When such areas are subject to minimum thickness requirements, the purchaser and the manufacturer shall recognize the necessity for either thicker deposits on other areas or special racking.
    Applicability to Unified Inch Screw Threads:
    The applicability of the required coating to unified inch screw threads is limited by the basic deviation of the threads, and hence limited by the pitch diameter, allowance and tolerance positions. Refer to Appendix X3 as a guideline for the tolerances of the various thread sizes and classes and the coating thickness they will accommodate.
    Because of the inherent variability in coating thickness by the barrel-plating process, the application of a minimum coating thickness of 0.0005 in. is not recommended for a standard screw thread by this method due to the fact that dimensional allowance of most threaded fasteners normally does not permit it. If the size of the fastener is large enough to economically use the rack-plating process, then the latter shall be used to obtain this thickness requirement. If heavier coatings are required allowance for the deposit buildup must be made during the manufacture of fasteners.
    Applicability to Wood Screws and Thread Forming Screws—Any classification code in Table 2 may be applied to screws that cut or form their own threads.
    For Supplemental Passivate Finishes, the minimum recommended zinc thickness is 5µm.
  • Hydrogen Embrittlement Relief:

    • Requirement for Baking—Coated fasteners made from steel heat treated to a specified hardness of 40 HRC or above, case-hardened steel fasteners, and fasteners with captive washers made from hardened steel shall be baked to minimize the risk of hydrogen embrittlement. Unless otherwise specified by the purchaser, baking is not mandatory for fasteners with specified maximum hardness below 40 HRC.
    NOTE 3 — With proper care many steel fasteners can be plated without baking by correlating process conditions to the susceptibility of the fastener material to hydrogen embrittlement, and by applying adequate process control procedures, such as those outlined in Appendix X4.2. Test Method F1940 is a recognized verification method for process control to minimize the risk of hydrogen embrittlement. Upon agreement between the supplier and the purchaser, this test method can be used as a basis for determining if baking should be mandated in a controlled process environment.
    Baking Conditions — At the time of publication of this specification it was not considered possible to give an exact baking duration. Eight hours is considered a typical example of baking duration. However, upon agreement between the purchaser and the manufacturer, baking times between 2 and 24 h at temperatures of 350 to 450°F are suitable depending on the type and size of the fastener, geometry, mechanical properties, cleaning process and cathodic efficiency of the electroplating process used. The baking conditions shall be selected based on the results of recognized embrittlement test procedures such as Test Methods F606, F1624, F1940, or NASM–1312–5.
    Bake time and temperatures may require lowering to minimize the risk of solid or liquid metal embrittlement resulting from alloy compositions such as those containing lead or from the lower melting point of cadmium (610°F) in comparison to zinc (786°F).
    Fasteners must be baked within 4 h, preferably 1 h after electroplating. Baking to relieve hydrogen embrittlement must be performed prior to the application of the chromate finish because temperatures above 150°F damage the chromate film thereby negating its performance.
    Hydrogen Embrittlement Testing—Hydrogen embrittlement testing is mandatory for fasteners with a specified hardness of 40 HRC or above unless the electroplating process has been qualified in accordance with Test Method F1940 (that is, the process has been shown not to cause embrittlement for a given product or class of product). This specification does not require mandatory testing of fasteners having a specified hardness below 40 HRC, unless otherwise specified by the purchaser.
  • Non-Hexavalent Passivate Finishes
    • The use of hexavalent chromium is prohibited when processing coated fasteners to the requirement of 4.3.1.1. Coated fasteners shall be free of hexavalent chromium when tested in accordance with the test method defined in 9.4.

    Dimensional Requirements

    Threaded components, except those with spaced and forming threads, supplied for electrodeposited coating shall comply with ASME B1.1. Screw threads that are specifically manufactured to allow the application of 0.0005 in. or greater coating thickness by the barrel-plating process, must adhere to a special allowance specified by the manufacturer or in ASME B1.1. The other dimensional characteristics shall be as specified in the applicable standard or drawing. It should be noted that modifications to the threads of a fastener could affect its properties or performance, or both. Refer to Appendix X3 for further information on effects of coating on pitch diameter, allowances and tolerances for external and internal threads.

    Sampling

    Sampling for coating thickness, salt spray and embrittlement testing shall be conducted based on lot size in accordance with Guide F1470.

    Test Methods

  • Coating Thickness — Unless otherwise specified, the requirement to measure coating thickness is applicable to significant surfaces only. The test methods for determining the coating thickness are defined in Test Methods B487, B499, B504, B567, B568, Guide B659, or Practice E376 as applicable.
  • Embrittlement Test Method — The embrittlement test method shall conform to those specified in Test Methods F1940 for process verification, or F606, F1624, or NASM-1312-5 for product testing.
  • Corrosion Resistance — The requirement to determine corrosion resistance is applicable to significant surfaces only. When specified in the contract or purchase order, a salt spray test shall be conducted in accordance with Practice B117. To secure uniformity of results, samples shall be aged at room temperature for 24 h before being subjected to the salt spray test.
  • Non-Hexavalent Passivate Finish — The presence of hexavalent chromium shall be determined in accordance with Practice D6492.

    • Electroplating Processes

    Two electroplating processes are most commonly used to apply a metallic coating by electrodeposition on threaded fasteners: barrel-plating and rack-plating. When thread fit or thread integrity, or both, is a concern for externally threaded fasteners, rack-plating is preferable to barrel-plating. Refer to Appendix X1.


  • NEUTRAL SALT SPRAY PERFORMANCE:

    • See Table A1.1, Table A1.2, Table A1.3, Table A1.4, and Table A1.5.

  • Classification Code and Neutral Salt Spray Corrosion Protection Performance of Zinc and Cadmium Coatings

    • NOTE — When fasteners are coated with trivalent chromite (Cr+3), the classification code to be used shall be appended with the letter "T".

    Classification Code Minimum
    Coating Thickness, in.    		 Chromate Finish Designation First Appearance of
    White Corrosion
    Product, (hour)    		 First Appearance of
    Red Rust
    Cadmium, (hour)    		 First Appearance of
    Red Rust
    Zinc, (hour)
    Fe/Zn or Fe/Cd 3A 0.0001A A 3 24 12
    Fe/Zn or Fe/Cd 3B --- B 6 24 12
    Fe/Zn or Fe/Cd 3C --- C 24 36 24
    Fe/Zn or Fe/Cd 3D --- D 24 36 24
    Fe/Zn or Fe/Cd 5A 0.0002 A 6 48 24
    Fe/Zn or Fe/Cd 5B --- B 12 72 36
    Fe/Zn or Fe/Cd 5C --- C 48 120 72
    Fe/Zn or Fe/Cd 5D --- D 72 168 96
    Fe/Zn or Fe/Cd 5E --- E 12 72
    Fe/Zn or Fe/Cd 8A 0.0003 A 6 48 24
    Fe/Zn or Fe/Cd 8B --- B 12 72 36
    Fe/Zn or Fe/Cd 8C --- C 48 120 72
    Fe/Zn or Fe/Cd 8D --- D 72 168 96
    Fe/Zn or Fe/Cd 8E --- E 12 72
    Fe/Zn or Fe/Cd 12A 0.0005 A 6 144 72
    Fe/Zn or Fe/Cd 12B --- B 24 192 96
    Fe/Zn or Fe/Cd 12C --- C 72 240 144
    Fe/Zn or Fe/Cd 12D --- D 96 264 168
    Fe/Zn or Fe/Cd 12Bk --- E 24 192 96
    A Low coating thickness impairs chromate adhesion and performance.
  • Classification Code and Neutral Salt Spray Protection of Zinc Coatings with Non-Hexavalent Supplemental Passivate Finish

    • NOTE — When fasteners are coated with supplemental non-hexavalent passivate finish, the classification code to be used shall be appended with the letter “S”.

    Classification Code Minimum
    Coating Thickness, in.    		 Chromate
    Finish Designation    		 First Appearance of
    Zinc Alloy Corrosion Product (hour)    		 First Appearance of
    Red Rust (hour)
    Fe/Zn 5AS 0.0002 A 96 120
    Fe/Zn 8AS 0.0003 --- --- ---
    Fe/Zn 12AS 0.0005 --- --- ---
  • Classification Code and Neutral Salt Spray Corrosion Protection Performance of Zinc-Cobalt Coatings
    • Classification Code Minimum
    Coating Thickness, in.    		 Chromate
    Finish Designation    		 First Appearance of
    Zinc Alloy Corrosion Product (hour)    		 First Appearance of
    Red Rust (hour)
    Fe/Zn-Co 5C 0.0002 C 96 240
    Fe/Zn-Co 5D --- D 96 240
    Fe/Zn-Co 5E --- E 100 240
    Fe/Zn-Co 5F --- F 196 340
    Fe/Zn-Co 8C 0.0003 C 96 240
    Fe/Zn-Co 8D --- D 96 240
    Fe/Zn-Co 8E --- E 100 240
    Fe/Zn-Co 8F --- F 200 340
    Fe/Zn-Co 12B 0.0005 B 12 240
    Fe/Zn-Co 12C --- C 96 400
    Fe/Zn-Co 12D --- D 96 400
    Fe/Zn-Co 12E --- E 100 400
    Fe/Zn-Co 12F --- F 196 500
  • Classification Code and Neutral Salt Spray Corrosion Protection Performance of Zinc-Nickel Coatings
    • Classification Code Minimum
    Coating Thickness, in.    		 Chromate
    Finish Designation    		 First Appearance of
    Zinc Alloy Corrosion Product (hour)    		 First Appearance of
    Red Rust (hour)
    Fe/Zn-Ni 5B 0.0002 B 20 150
    Fe/Zn-Ni 5C --- C 120 500
    Fe/Zn-Ni 5D --- D 180 750
    Fe/Zn-Ni 5E --- E 100 500
    Fe/Zn-Ni 5B/F --- B/F 150 300
    Fe/Zn-Ni 5C/F --- C/F 240 620
    Fe/Zn-Ni 5D/F --- D/F 300 1000
    Fe/Zn-Ni 5E/F --- E/F 220 620
    Fe/Zn-Ni 8B 0.0003 B 20 240
    Fe/Zn-Ni 8C --- C 120 720
    Fe/Zn-Ni 8D --- D 180 960
    Fe/Zn-Ni 8E --- E 100 720
    Fe/Zn-Ni 8B/F --- B/F 150 400
    Fe/Zn-Ni 8C/F --- C/F 240 840
    Fe/Zn-Ni 8D/F --- D/F 300 1200
    Fe/Zn-Ni 8E/F --- E/F 220 840
    Fe/Zn-Ni 12B 0.0005 B 20 500
    Fe/Zn-Ni 12C --- C 120 960
    Fe/Zn-Ni 12D --- D 180 1000
    Fe/Zn-Ni 12E --- E 100 960
    Fe/Zn-Ni 12B/F --- B/F 150 620
    Fe/Zn-Ni 12C/F --- C/F 240 1080
    Fe/Zn-Ni 12D/F --- D/F 300 1500
    Fe/Zn-Ni 12E/F --- E/F 220 1080
  • Classification Code and Neutral Salt Spray Corrosion Protection Performance of Zinc-Iron Coatings
    • Classification Code Minimum
    Coating Thickness, in.    		 Chromate
    Finish Designation    		 First Appearance of
    Zinc Alloy Corrosion Product (hour)    		 First Appearance of
    Red Rust (hour)
    Fe/Zn-Co 5E 0.0002 E 144 312
    Fe/Zn-Co 8E 0.0003 E 144 312
    Fe/Zn-Co 12E 0.0005 E 144 480

    Other Information

  • Standard electrodeposition process
  • Guidelines for choosing between barrel-plating and rack-plating
  • Coating accommodation tolerances for externally and internally threaded fasteners
  • Application requirements
  • Use of trivalent chromium (Cr+3) on coated fasteners

    • Referenced Documents

    Referenced Documents ASTM Standards:
    B117 Practice for Operating Salt Spray (Fog) Apparatus
    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
    B567 Test Method for Measurement of Coating Thickness by the Beta Backscatter Method
    B568 Test Method for Measurement of Coating Thickness by X-Ray Spectrometry
    B659 Guide for Measuring Thickness of Metallic and Inorganic Coatings
    D6492 Practice for Detection of Hexavalent Chromium On Zinc and Zinc/Aluminum Alloy Coated Steel
    E376 Practice for Measuring Coating Thickness by Magnetic-Field or Eddy-Current (Electromagnetic) Examination Methods
    F606 Test Methods for Determining the Mechanical Properties of Externally and Internally Threaded Fasteners, Washers, Direct Tension Indicators, and Rivets
    F1470 Practice for Fastener Sampling for Specified Mechanical Properties and Performance Inspection
    F1624 Test Method for Measurement of Hydrogen Embrittlement Threshold in Steel by the Incremental Step Loading Technique
    F1940 Test Method for Process Control Verification to Prevent Hydrogen Embrittlement in Plated or Coated Fasteners
    ASME Standard: B1.1 Unified Inch Screw Threads (UN and UNR Thread Form)
    National Aerospace Standard (AIA):4
    NASM-1312-5 Fast Test Method - Method 5: Stress Durability IFI Standard:
    IFI-142 Hydrogen Embrittlement Risk Management