ASTM F1941/F1941M Electrodeposited Coatings on Fasteners

Stud Bolts, Nuts, Screws, Threaded Rod & Washers

ASTM F1941/F1941M specification covers application, performance and dimensional requirements for electrodeposited coatings on threaded fasteners with unified inch and metric screw threads, but it may also be applied to other threaded parts and non-threaded parts such as washers and pins. It specifies coating thickness, supplementary hexavalent chromate or non-hexavalent conversion coatings, 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.

F1941/1941M Coated Fasteners, Bolts, Screws, Nuts & Washers

ASTM F1941 Classifications

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

Designation of Coating Thickness - Inch and Metric

Thickness Designation Minimum Thickness
in. µm
3 0.0001 3
5 0.0002 5
8 0.0003 8
12 0.0005 12
  • NOTE: The conversion factor from inch to microns is 2.54 × 104 (for example, 0.0001 in. = 2.54 μm).
  • Conversion Coating - The conversion coating shall be selected and designated in accordance with below table. When not specified, hexavalent chromium, or hexavalent chromium free passivation such as trivalent chromium passivation or other non-chromium passivation finish shall be used at the option of the manufacturer and its appearance shall be selected in accordance with the designation selected in below table.

Designation of Conversion Coating

Type Typical Appearance Conversion Designation
Hexavalent Chromium Hexavalent Chromium Free
Clear Transparent colorless with slight iridescence A AN
Blue-bright Transparent with a bluish tinge and slight iridescence B BN
Yellow Yellow iridescent C CN
Opaque Olive green, shading to brown or bronze D DN
Black Black with slight iridescence E EN
  • Supplemental Lubricant, Sealants or Top Coats - Additional sealants or top coats (with or without integral lubricant) may be chosen to increase corrosion resistance and to achieve other specific properties such as torque-tension, UV resistance, etc. The selection of the nature of a sealant or top coat should be based on desired additional properties. When sealants or top coats are specified, the classification code in Table 3 shall be appended by adding the letter "S" (for example Fe/Zn 5ANS). When specifying a lubricant, the classification code in Table 3shall be appended with the letter "L" (for example Fe/Zn 5ANSL).
  • NOTE: When using a sealant or top coat, a separate conversion coating layer and/or lubricant layer may not be required to achieve the corrosion performance or provide lubricity.

Basic Electroplating Coating Systems

Additional Lubricant
Additional Lubricant Sealant⁄Top Coat Sealant⁄Top Coat
Coating Material Conversion Coating Conversion Coating Conversion Coating
Coating Material Conversion Coating Coating Material Coating Material Coating Material
Base Metal Base Metal Base Metal Base Metal Base Metal
1 2 3 4 5
  • 1 - Only coating material layer(s).
  • 2 - Coating material layer(s) plus conversion coating (for example Fe/Zn 5A).
  • 3 - Coating material layer(s) plus conversion coating plus additional lubricant (example Fe/Zn 5ANL).
  • 4 - Coating material layer(s) plus conversion coating plus sealant top coat (example Fe/Zn 5ANS).
  • 5 - Coating material layer(s) plus conversion coating plus sealant top coat plus additional lubricant (example Fe/Zn 5ANSL).

ASTM F1941 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 coated fastener shall be free of blisters, pits, nodules, roughness, 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 corrosion products.
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 nor basis metal corrosion products 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.
The coating thickness shall comply with requirements of Table 2 when measured in accordance with Embrittlement Test in test methods.
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 conversion coating adhesion.
External Threads
The after-coating dimensions of external threads must not exceed the thread's basic size. Coated external threads must conform to a basic GO gage. Coated inch external threads must accept a class 3A GO gage and coated metric threads must accept a class h (6h or 4h) GO gage (See ASME B1.2 and ASME B1.16M respectively). The NOTGO gage size is the same after coating as before coating. If a coated external thread does not freely enter the basic size GO gage, the thread discontinuity torque test in Specification F788 shall be used to determine thread acceptability.
Internal Threads
The after-coating dimensions of internal threads must not exceed the thread's basic size. Coated internal threads must conform to a basic GO gage. Coated inch internal threads must accept a class 2B or 3B GO gage and coated metric internal threads must accept a class H (6H, 5H or 4H) GO gage (See ASME B1.2 and ASME B1.16M respectively). The NOTGO gage size is the same after coating as before coating. 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 and M Series Metric Threads
The applicability of the required coating to unified inch and M series metric 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. or 12 µm for metric 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 by adjusting pre-plating thread size.
Applicability to Wood Screws and Thread Forming Screws
Any classification code in Tables 1-3 may be applied to screws that cut or form their own threads.
Hydrogen Embrittlement Relief
Requirement for Baking for Through Hardened Fasteners
Unless otherwise specified by the purchaser, baking is not mandatory for fasteners with specified maximum hardness 39 HRC. Coated fasteners made from steel heat treated to a specified hardness above 39 HRC, and fasteners with captive washers made from hardened steel shall be baked to minimize the risk of hydrogen embrittlement. NOTE: With proper care many steel fasteners can be plated without baking by correlating process conditions, and coating material 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
Unless otherwise specified, minimum baking times shall be in accordance with below table
Hydrogen Embrittlement Relief RequirementsB
Specified Core Hardness (HRC) Min Baking Time Min - Max Baking TemperatureC,D ASTM Hydrogen Embrittlement Test RequirementE Tapping Screw Hydrogen Embrittlement Test Requirement ASTM Process Control Test RequirementE
Over 39 and up to 44A Min 14 h 375 to 425°F or 190 to 220°C F606/F606M or F1624 ASME B18.6.3 or F1624 F1940 or F519
Over 44A Min 24 h 375 to 425°F or 190° to 220C F606/F606M or F1624 ASME B18.6.3 or F1624 F1940 or F519
  • (A) If Test Method F1940 process control testing is not performed, baking and product testing are mandatory in accordance with Table 4. If Test Method F1940 process control testing is performed and is shown to consistently pass at a minimum of a monthly basis, then product testing and baking are not mandatory. If Test Method F1940 process control testing is performed and does not pass, then baking and product testing are mandatory.
  • (B) Variables such as coating type, coating thickness, baking temperatures and plating process (barrel or rack plating) can effect baking requirements. ASTM F1940 process control testing can be used to isolate the effect of baking, and shall be the basis to increase or decrease baking times or to eliminate baking altogether. In the absence of Test Method F1940 process control testing, baking and testing requirements specified in Table 4 shall be used as the default for all conditions.
  • (C) Cadmium baking temperatures should be between 375 to 400°F or 190 to 205°C.
  • (D) Part temperature.
  • (E) When agreed upon between supplier and purchaser, alternative hydrogen embrittlement test methods such as NASM 1312-5 and alternative process control test methods may be used.

Bake temperatures shall always be kept below the tempering temperature of quenched and tempered steel parts to avoid alteration of mechanical properties by re-tempering.
Bake temperatures shall not exceed the values specified in Table 4 to avoid the risk of solid or liquid metal embrittlement.
NOTE: Bake times and temperatures are lowered to minimize the risk of solid or liquid metal embrittlement resulting from alloy compositions such as those containing lead or from lowering melting point of the coating material. For example, cadmium has a melting point of 610°F or 310°C in comparison to zinc which has a melting point of 786°F or 419°C.
Baking to relieve hydrogen embrittlement should be performed after electroplating, prior to the application of the conversion coating and prior to the application of sealant and/or top coat, if any where baking temperatures can damage the conversion film thereby negating its performance. After experimentation, coaters may find other sequences are suitable. The time between coating and baking should be as short as possible. The requirement, if any, for a specific maximum allowable time (in hours) between electroplating and baking shall be explicitly specified by the purchaser at the time of order. A reasonable tolerance of +2h resulting from normal operational constraints shall be assumed.

Hydrogen Embrittlement Testing
Unless otherwise specified by the purchaser, hydrogen embrittlement testing in accordance with Table 4 is mandatory for through hardened fasteners with a specified core hardness above 39 HRC unless the electroplating process has been qualified in accordance with a test method in Table 4 (that is, the process has been shown not to cause embrittlement for a given product or class of product).
Baking and Testing Requirements for Case Hardened Screws
Surface hardening of case hardened screws introduces variables additional to the hardness of the core, notably case hardness and case depth. Case hardened screws that are electroplated shall adhere to the following baking requirements. All lots of case hardened screws shall be baked for a minimum of 4 h at 375 to 400°F or 190 to 205°C part temperature.
All case hardened screws shall be tested for hydrogen embrittlement in accordance with ASME B18.6.3 for all self-tapping screws. For case hardened machine screws, the ASME B18.6.3 method shall be applied except use a hardened threaded test plate having a minimum thickness of one nominal diameter. The tapped holes shall be 2B for inch fasteners or 6H for metric fasteners.
Any lot that fails hydrogen embrittlement testing shall be baked for 24 h at 375 to 400°F or 190 to 205°C part temperature and retest shall be made using twice the original sample size.
Stress Relieving Requirements for Work Hardened Fasteners Without Thermal Hardening
Some cold formed fasteners that are not thermally hardened can fracture due to buildup of high residual stresses at stress concentration points. The types of fastener shapes that make this a particular concern are carriage bolts, thin head parts where the minimum thickness of the head is less than 50% of the nominal diameter of the screw, shoulder type fasteners where the thread major diameter is more than 20% smaller than the shoulder diameter, or where a larger diameter, thin washer or collar is formed on a double ended stud. An indication that high residual stresses may be present in a portion of the fastener is when localized hardness below the surface exceeds 30 HRC. Fasteners with configurations or conditions described above shall be stress relieved at a minimum temperature of 875°F or 470°C prior to electroplating to avoid brittle fractures. Increased hardness resulting from thread rolling before, after or without thermal hardening are due to the creation of non-detrimental compressive stresses and do not require stress relief before electroplating. NOTE: Stress relieving is not intended in cases where residual stresses are intentionally introduced, such as screws which are thread rolled after heat treatment.
Non-Hexavalent Conversion Coating
When the use of hexavalent chromium is prohibited, coated fasteners shall be free of hexavalent chromium when tested in accordance with the test method defined in Test Methods

ASTM F1941 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
Unless otherwise specified, the embrittlement test method shall conform to those specified in Test Methods F1940 or F519 for process verification, or F606/F606M, or F1624 for product testing. If agreed upon by the purchaser and supplier, alternative test methods, such as NASM 1312-5 may be used for testing bolt and machine screws. The testing of both inch and metric surface hardened screws shall be conducted in accordance with ASME B18.6.3.
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. The salt spray test shall commence within 72 h of completion of the aging period and prior to sorting, packaging and/or assembling.
Non-Hexavalent Conversion Coating
The presence of hexavalent chromium shall be determined in accordance with Practice D6492.