Fastener Coatings & Finishes Guide
Selecting the right coating is as critical as selecting the base material. The wrong coating can lead to galvanic corrosion, hydrogen embrittlement, or premature mechanical failure.
| Coating Type | Thickness | Salt Spray (ASTM B117) | Best Application |
|---|---|---|---|
| Zinc Plated (Clear/Yellow) The most common commercial coating. Provides moderate corrosion resistance by acting as a sacrificial anode. | 5-15 µm | 48-96 hours | Indoor, dry environments. General machinery. |
| Hot-Dip Galvanized (HDG) Thick zinc coating applied by dipping the fastener in molten zinc. Provides excellent long-term corrosion resistance. | 40-100 µm | 500+ hours | Outdoor, structural construction, marine environments. |
| Black Oxide A conversion coating that provides very mild corrosion resistance. Mostly used for aesthetic purposes and to reduce light reflection. | < 1 µm | Minimal (must be oiled) | Indoor, precision instruments, tooling. |
| PTFE / Xylan Fluoropolymer coating providing exceptional chemical and corrosion resistance, plus a very low coefficient of friction. | 20-40 µm | 1000+ hours | Offshore, chemical plants, offshore drilling. |
⚠️ Warning: Thread Fit and Thick Coatings
Thick coatings like Hot-Dip Galvanizing (HDG) add significant material to the thread profile. Standard nuts will not fit an HDG bolt. Nuts intended for HDG bolts must be "tapped oversize" (e.g., +0.021" for a 1/2" bolt) after galvanizing. Always match HDG bolts with HDG nuts from the same manufacturer.