Crevice Corrosion: The Hidden Failure Under Every Fastener Head
Engineers specify stainless fasteners, then find rust rings under the heads two winters later. That is crevice corrosion — and it is geometry, not material quality.
Bottom line: for most Anti-Galvanic Corrosion applications the decision comes down to load, environment and assembly method — the Anti-Galvanic Corrosion guide walks through the selection.
The mechanism
Under washers, heads and thread roots, stagnant electrolyte depletes dissolved oxygen while chloride ions concentrate. The local chemistry turns acidic and the passive film breaks down — even on 316. The result is localized pitting exactly where inspection never looks.
Design moves that starve the crevice
- Seal the gap: under-head O-rings, sealants or gasketed washers remove the electrolyte reservoir.
- Avoid faying gaps: design metal-to-metal bearing surfaces instead of trapped washers in slots.
- Upgrade alloy where geometry cannot change: 6-moly grades or nickel alloys for marine splash zones.
- Encapsulate: over-molded or potted joints eliminate the environment entirely.
Inspect the right way
Crevice damage hides until disassembly. For critical joints, schedule inspection at first scheduled service, and photograph bearing surfaces for comparison. Topprecision supplies under-head sealing solutions for exactly this failure mode and shares case-study data from marine and coastal installations on request.
Coating selection by environment
| Coating | Salt spray (hours) | Hydrogen risk | Best environment |
|---|---|---|---|
| Zinc plating (clear) | 48–96 | Yes (high-strength) | Indoor, general |
| Zinc-nick (12–15% Ni) | 200–500 | Moderate | Automotive underbody |
| Dacromet / Geomet | 500–1000+ | None | Wind, solar, outdoor steel |
| Hot-dip galvanized | 1000+ | None | Structural, coastal |
| Mechanical galvanized | 300–600 | Low | 10.9/12.9 grade outdoor |
Need corrosion protection for your fasteners?
Toprecision applies zinc, zinc-nick, Dacromet/Geomet, hot-dip galvanizing, PTFE, DLC and PVD coatings in-house, with ISO 9227 salt-spray testing up to 1000+ hours. Send the environment and required salt-spray hours — we will recommend the coating and supply test reports. Request coating selection support.
