A stainless steel bolt that seized in an aluminum housing. The bolt was A2-70 (304 stainless), M10, torqued to 25 N·m. The housing was cast aluminum. When disassembly was attempted 6 months later, the bolt twisted off — the threads had seized. The customer thought the bolt was wrong material. It wasn’t — it was galling, a form of adhesive wear that occurs between stainless and aluminum under torque. This is about fastener material compatibility and anti-seize compounds.
What is galling
Galling happens when two metal surfaces slide under pressure. The high points (asperities) on one surface weld to the other. As the bolt turns, the welded spots tear. Material transfers from one surface to the other. The threads rough up. Friction increases. The bolt seizes. In the worst case, the bolt breaks before it turns another degree.
Stainless on stainless is the classic galling pair. But stainless on aluminum is also bad — the soft aluminum adheres to the harder stainless thread ridges. As the bolt torques in, aluminum smears onto the stainless threads. The friction coefficient jumps from 0.15 (lubricated) to 0.4 (dry, galled). The torque required to turn the bolt doubles. The bolt stretches beyond yield and snaps.
What was changed
1. Used a silver anti-seize compound. A high-temperature anti-seize (nickel-based or aluminum-based) was applied to the bolt threads. The compound separates the two metal surfaces. The friction coefficient stays at 0.15. The bolt torques to 25 N·m without galling. Six months later, it came out clean.
2. Changed the bolt material. For new designs, the stainless bolt was replaced with a zinc-plated steel bolt (Grade 8.8). The zinc plating acts as a solid lubricant. Steel on aluminum galling is less severe than stainless on aluminum. The steel bolt costs less and doesn’t seize. If corrosion resistance is needed (outdoor or washdown), A4-80 (316 stainless) with anti-seize is the choice. But for indoor equipment, zinc-plated steel works fine.
3. Used a thread-forming screw in the aluminum. For non-structural assemblies, a self-tapping screw (thread-forming, not cutting) in cast aluminum eliminates the bolt galling issue entirely. The screw forms its own threads. The screw is disposable — replaced on disassembly. Used for covers and brackets, not for structural joints.
The torque-friction relationship
The torque applied to a bolt converts to clamp force through the friction under the head and in the threads:
T = F × (0.16 × d)
Where F is the clamp force (N) and d is the bolt diameter (mm). This formula assumes a friction coefficient of 0.15 (lubricated). If the friction doubles (galling starts), the same torque produces half the clamp force. The joint loosens. If friction triples, the bolt stretches to yield before reaching the target clamp force. The bolt breaks. Anti-seize keeps the friction predictable. Always torque to a known friction condition (dry, oiled, or anti-seize) and use the correct torque value for that condition.
| Bolt/housing material | Galling risk | Recommended action |
|---|---|---|
| Stainless on stainless | High | Anti-seize or silver plating |
| Stainless on aluminum | Medium | Anti-seize compound |
| Steel on aluminum | Low | Lubricated or dry |
| Steel on steel | Low | Standard torque oil |
The fastener rule: always use anti-seize on stainless-to-aluminum or stainless-to-stainless joints. The seized bolt wasn’t the wrong grade — galling had welded the threads. Apply a nickel-based anti-seize, torque to the lubricated value, and for indoor equipment consider zinc-plated steel instead of stainless. Friction coefficient must be known for correct torque.
