FORD WSS-M21P51 Fasteners, Electro-Coated Zinc-Nickel

The “FORD WSS-M21P51 Fasteners, electro-coated zinc-nickel” standard, developed by Ford Motor Company (FORD), consists of the following subsections:

• WSS-M21P51-A1 Trivalent chromium passivate, silver
• WSS-M21P51-A2 Trivalent chromium passivate, lubricated sealant, silver
• WSS-M21P51-A3 Trivalent chromium passivate, lubricated sealing material enhanced lubricity, silver
• WSS-M21P51-A4 Trivalent chromium passivate, lubricated sealing material, durable lubricity, silver
• WSS-M21P51-A5 Trivalent chromium passivate, lubricated sealant, black
• WSS-M21P51-A6 Trivalent chromium passivate, lubricated sealant, improved lubricity, black

The materials defined in this standard are corrosion protective coatings on steel and iron substrates. All consist of an alkali zinc nickel alloy and trivalent chromium passivation. A2, A3, A4, A5 and A6 are also coated with a clear sealant to enhance corrosion resistance or provide consistent torque-tension control.

These specifications are published for materials used as corrosion protection coatings for fasteners and other standard parts used throughout the vehicle. Some processes used to produce these coatings can cause hydrogen embrittlement in high hardness parts. Parts identified as having a risk of hydrogen embrittlement must meet the requirements of the WSS-M99A3-A standard.

• A1 Corrosion resistance for standard parts without threads.
• A2 Corrosion resistance plus lubrication for threaded fasteners such as self-tapping screws, tapping screws for plastic and screws with ISO metric threads, can be used in grounding applications.
• Corrosion resistance plus improved lubrication for threaded fasteners such as tapping screws for A3 Metal; can be used in ground screw applications.
• A4 A special lubricant plus corrosion resistance specially designed for tapered wheel nuts.
• A5 Class A black appearance with corrosion resistance plus lubrication for threaded fasteners such as self-tapping screws, tapping screws for plastic and screws with ISO metric threads.
• A6 Class A black appearance with corrosion resistance plus improved lubrication for fasteners such as threading screws for metal.

The maximum recommended continuous service temperature is 310 degrees.

A2 and A3 provide electrical continuity to the substrate and are suitable for most electrical grounding applications.

The requirements in this standard are described as follows:

• Standard requirements for production materials: Material suppliers and component manufacturers must comply with the standard “WSS-M99P1111-A Quality system requirements for production materials”. Component manufacturers must use approved materials specified in the company's approved source list.
• Embrittlement: Parts coated in accordance with this standard shall be free from the harmful effects of hydrogen embrittlement or other factors that cause embrittlement of the part. All parts shall meet the requirements of the WSS-M99A3-A standard.
• Coating application:
  • Applicators must follow material supplier procedures and recommendations for chemical use, equipment, analysis, and processing. Applicators must have documentation to demonstrate that procedures and recommendations were followed.
  • Before coating, the substrate must be free from foreign matter such as oil, dirt, etc. If a cleaning process is necessary, it must have no detrimental effects on the substrate, including freedom from brittleness.
  • Sealants used within the scope of this standard do not require curing. Sealants should be dried in situ, usually using hot air in a spin dryer. Conveyor drying and other drying methods may be suitable.
  • The applicator should follow the material supplier's recommendations for drying temperatures and spin speeds appropriate for the basket diameter used.
• Coating appearance:
  • Before the passivation application, the base zinc-nickel coating should have a metallic silver appearance.
  • For A1, A2, A3 and A4, the passivation layer should have a slight iridescence and color change appearance consistent with interference films (e.g. yellow, orange, green, blue, purple mottling).
  • For A5 and A6 the passivation layer must be black.
  • The sealants used for A2, A3, A5 and A6 must be a translucent film free of interruptions.
  • The sealant used for A4 should be a translucent film but may contain some interruptions in the form of touch marks or stains. These should not adversely affect the mechanical performance or corrosion resistance of the part.
  • Sealants used for A2 and A3 may have an additional colour ID of green or blue.
• Coating features:
  • Composition: Alkali-based zinc-nickel coating shall have a nickel content of 12 percent to 17 percent as measured by X-ray fluorescence spectrometers calibrated in accordance with the supplier's requirements. The coating shall be a uniform, uniform alloy of zinc and nickel throughout the thickness of the deposit and through the high and low current densities of the part. The zinc-nickel alloy shall be the only deposit to be applied directly to the substrate. The use of zinc impact shall not be permitted. The composition of the zinc-nickel coating, passivation, and water-soluble lubricated sealant shall be in accordance with the chemical supplier's requirements.
  • Thickness: The thickness of the electroplated zinc-nickel layer shall be 8 to 14 micrometers when measured on flat surfaces. In case of disagreement, the thickness measurement referee method shall be metallographic sectioning according to ASTM B487. The passive layer thickness is not subject to measurement, but shall be less than 500 nanometers and greater than 50 nanometers. The sealing layer thickness is not subject to measurement, but shall be less than 2 micrometers and greater than 0,5 micrometers. The thickness shall be sufficient to meet the corrosion requirements and friction requirements of the substrate material. The typical general thickness of these coatings is 10 to 12 micrometers.
  • Adhesion: The coating shall withstand normal handling, storage and installation without flaking or peeling or other loss of adhesion. Additionally, there shall be no bubbling or flaking after baking for hydrogen embrittlement removal. The adhesion of the electroplate to the base metal shall meet the requirements of the heat quench and polish test methods in accordance with ASTM B 571 unless otherwise specified.
  • Torque tension performance: When tested in accordance with the “FORD WZ 101 Steel metric threaded fasteners torque/clamping force performance” standard, coated fasteners shall have a coefficient of friction within plus/minus 0,03 µ of the nominal value given in the standard.

This standard also describes requirements for strength properties, yarn acceptance requirements, formability and process control elements.

Our organization, which has been trying to support businesses from every sector with its testing, measurement, analysis and evaluation studies carried out in a wide range for years, has a strong staff that closely follows the developments in the world in the field of science and technology and constantly improves itself. In this context, testing services in accordance with the "FORD WSS-M21P51 Fasteners, Electro-Coated Zinc-Nickel" standard are also provided to businesses.