A stainless bolt that spins freely by hand can suddenly lock solid halfway down the thread and shear off — without ever reaching its torque target. This is galling, and it is one of the most frustrating failures on site because the bolt was never overloaded. It is a surface-interaction problem, not a strength problem.
§ 01 What galling actually is
Galling (also called cold welding or seizing) happens when two sliding metal surfaces under pressure adhere to each other at their high spots, tear, and transfer material. In a threaded joint, the bolt and nut threads micro-weld together as they turn, the welds tear and build up, friction climbs steeply, and the assembly locks — often so hard that the bolt twists off before it can be backed out.
§ 02 Why stainless is especially prone
Stainless steel galls far more readily than carbon or coated steel for two linked reasons:
- The passive film is thin and self-repairing — the same property that gives corrosion resistance means that when sliding pressure rubs it off, fresh reactive metal is exposed that readily cold-welds to the mating surface.
- Austenitic stainless work-hardens and has poor inherent lubricity — A2 (304) and A4 (316) are tough and "sticky" in sliding contact, with no protective oxide or coating to keep the surfaces apart.
This is unique to the bare metal-on-metal contact: a zinc-flake coated carbon-steel bolt has a built-in low-friction layer, whereas a bare A4 bolt and A4 nut have nothing between them.
§ 03 Conditions that trigger it
| Trigger | Why it galls |
|---|---|
| No lubricant | Bare A4-on-A4 threads cold-weld immediately |
| High tightening speed | Frictional heat builds faster than it dissipates |
| Same grade nut & bolt | Matched hardness welds more readily |
| Dirt / grit in threads | Debris scores the surface and starts adhesion |
| Thread damage | Burrs concentrate pressure and tearing |
§ 04 How to prevent it
- Use an anti-seize lubricant on the threads (nickel- or PTFE-based) — the single most effective measure. Note the lubricant changes the friction factor, so the torque target must be adjusted accordingly (see preload & torque).
- Tighten slowly — keep speed low to limit frictional heat; avoid high-speed impact drivers on bare stainless.
- Mix hardness — pair the nut and bolt from different grades/hardness (e.g. a harder nut) so the surfaces do not weld as easily.
- Keep threads clean and undamaged; chase out grit before assembly.
- Consider grade — some duplex grades and specially treated stainless resist galling better than plain A4; see duplex / super duplex.
§ 05 If a bolt seizes mid-installation
Once galling starts, forcing it usually makes it worse — the welds tear further and the bolt shears. Stop turning, let it cool, and back it out slowly with steady pressure if it will move. A seized fastener that will not release generally has to be cut out; do not reuse it. Then fix the root cause (lubricant, speed, grade pairing) before installing the replacement.
Galling is purely a surface/installation issue and should not be confused with corrosion staining, which has different causes — see rust on stainless fasteners.