The finish, or coating, is just as important as the design when you want your fasteners to last for the life of your product. If you want more information on the description below, please contact your local Bossard agent.
Factors to Consider
When you’re trying to decide on the correct finish, consider the following factors.
· Safety. Improperly applied finish may make the fastener fail. A common failure point that can be introduced by finish is hydrogen embrittlement. Embrittlement reduces ductility and load-bearing because the metal absorbs too much hydrogen during the plating process. Choosing a dip-spin coating or Bossard’s ecosyn® tribological dry coating over using an electroplated finish can alleviate this risk.
· Corrosion Protection. How long will the finish last based on its operating environment? The fastener requires a stronger coating if it endures very severe conditions that include saline solutions, cleaners, moisture, and collisions that produce abrasions, scratching, and denting.
· Resistance to Handling Damage. How frequently will workers handle or use tools on the fastener? A softer and more brittle finish demands more care during handling and assembly.
· Criticality of the Joint. What happens to the entire assembly if a joint becomes loose? Some finishes do not have known friction values. Avoid those finishes for fasteners on joints with consistent and repeatable clamp loads.
· Functionality. Fasteners with small diameters and threads that have internal recess drives may leave too much coating material in the recesses or threads if the wrong finish is used.
· Cost. Exotic coatings will increase the price of the fastener and the total cost of production.
· Availability. Is the finish you want readily available? Or will it have to be special-ordered, which demands a greater lead time and more expense?
Common Finishes
If cost or time is a concern, rely on the following fastener coatings, which are easily available.
· Electrodeposited (Commercial) Zinc. This finish is inexpensive, works under mild to moderate conditions, and is moderately resistant to handling damage. It has no problems with recess fill or threads. However, its friction is harder to control and it risks hydrogen embrittlement at high strengths.
· Electrodeposited Zinc Nickel. This finish functions well under severe to very severe conditions and has good resistance to handling damage. It is also free of issues with recess fill or threads but has a high cost, difficult-to-control friction, and is not as available as commercial zinc. The chances for hydrogen embrittlement are lower but still existent.
· Mechanical Zinc. With no hydrogen embrittlement risk, this finish features moderate resistance to handling damage and mild to moderate corrosion. It is not recommended for threads because of poor coverage in recesses and holes. The cost is medium to high while availability ranges from medium to low.
· Zinc Flake. Hydrogen embrittlement is not a risk for this finish that protects against severe to very severe corrosion and has high friction control. It is moderately resistant to handling damage but is not recommended for any fasteners with an internal drive. The cost is moderate.
· Hot Dip Galvanized. Protecting against severe to very severe corrosion, this moderately-priced coating has excellent handling-damage resistance and moderate cost. However, friction is difficult to control and it is not recommended for threads less than M8 or with an internal drive. The risk for hydrogen embrittlement is low.
· Epoxy Electrocoat. This coating has no hydrogen embrittlement risk, works under severe to very severe conditions, is moderately resistant to handling damage, and is excellent at controlling friction. It has no problems with recess fill or threads at M4 or more. But it is pricey.
· Tribological Dry Coatings. A solution for mechanically stressed fastening elements and components, such as screws, nuts and washers. The coating is a non-electrolytically applied thin layer with integrated lubrication properties and additional corrosion protection.
Fastener Finish: Platings and Coatings for Fasteners by Bossard
