Most screw fasteners require surface electroplating treatment. Electroplating not only improves the appearance of fasteners but also effectively enhances their rust resistance and corrosion resistance. The quality of electroplated coatings is mainly evaluated by surface appearance and corrosion resistance. Corrosion performance is verified through salt spray tests that simulate actual service conditions. The finished quality of electroplated fasteners is strictly controlled in the following four key aspects.
1. Electroplating Appearance Requirements
The electroplated surface shall be free of partial missing coating, burning, roughness, dullness, peeling, crusting and obvious streaks. Defects such as pinholes, pits, black plating residues, loose passivation films, cracks, film peeling and severe passivation marks are prohibited. The overall coating shall be uniform, clean and consistent in color.
2. Coating Thickness Requirements
The service life of fasteners in corrosive environments is positively correlated with coating thickness. The recommended economical thickness for conventional electroplated coatings ranges from 4 to 12 μm (0.00015 in to 0.0005 in).
Hot‑dip galvanizing thickness standards: For fasteners with a nominal diameter smaller than 3/8 inch, the average coating thickness shall be no less than 54 μm and the minimum thickness no less than 43 μm. For fasteners with a nominal diameter of 3/8 inch or larger, the average coating thickness shall be no less than 43 μm and the minimum thickness no less than 37 μm.
3. Coating Distribution Uniformity Requirements
Different plating processes feature different metal deposition characteristics, resulting in distinct coating distribution on fastener surfaces. In conventional electroplating, metal deposition is uneven. Edges and corners tend to form thicker coatings. On threaded sections, the coating is thickest at the thread crest, gradually thinner along the thread flank, and thinnest at the thread root.
Hot‑dip galvanizing presents the opposite distribution: thicker coatings form on inner corners and thread roots. Mechanical galvanizing follows a deposition pattern similar to hot‑dip galvanizing, yet produces smoother surfaces and significantly more uniform coating thickness across the entire part.
4. Hydrogen Embrittlement Prevention Requirements
During pickling, degreasing and electroplating processes, fastener surfaces readily absorb hydrogen atoms, which become trapped beneath the deposited metal coating. When the fastener is tightened, concentrated stress drives hydrogen toward high‑stress regions, generating excessive internal pressure and initiating microcracks. Highly active hydrogen continuously penetrates newly formed cracks, creating a cyclic failure mechanism of pressure accumulation, crack propagation and hydrogen infiltration, which eventually leads to brittle fracture. This failure usually occurs within several hours after the first load application.
To eliminate hydrogen embrittlement risks, fasteners must be baked immediately after electroplating to release internal hydrogen. The standard baking process is performed at 176–190 ℃ (375–400 ℉) for 3–24 hours to fully diffuse hydrogen and prevent embrittlement fracture.
In summary, appearance defects, non‑compliant coating thickness, uneven coating distribution and hydrogen embrittlement risks are the four main causes of unqualified electroplated fasteners and must be strictly controlled during production.
