A coating protects a fastener only if it is thick enough and actually resists corrosion. On a thread, thickness is a trade-off: thick enough to protect, thin enough to fit. ASTM F1941 zinc classes for fasteners are 3, 5, 8 and 12 µm; 5 µm (0.0002 in.) is the usual fastener class, and most threads cannot take much over 8 µm, because a plated thread must still accept a basic-size GO gauge.
The finish over the zinc matters as much as its thickness. At 5 µm, F1941 asks for 24 hours to red rust with a clear chromate and 96 hours with a yellow one. Salt spray compares coatings in a standard fog; it does not predict years of service in a particular place.
Read the full coating and salt spray testing page →
model dataThickness classes, the test surface on a bolt head, salt spray ageing and start, the hours to first white corrosion and red rust, and the coating a 1/2-13 Class 2A thread can take from ASTM F1941-16 (Tables 2 and A1.1, Fig. 1, section 9, Appendix X3). Fog of 5% salt water at 95 °F to ASTM B117; thickness methods ASTM B499, B568 and B487. The bolts are drawn as 1/2-13 hex bolts, the gauge and cabinet are generic, and the zinc is drawn many times thicker than it is. The gauge reading and the hours at which corrosion appears are illustrations, not a test result.
© 2026 Innovation Fasteners. This interactive drawing, its 3D models and its code may not be copied, hosted elsewhere, scraped or used to train AI models. Terms of use.