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【NEJIGAMI #Episode 2】Why Stainless Steel Bolts Gall and Seize (SUS304 / A2-A4)
The weakness of a rust-proof metal.
Mechanics often say stainless seizes worse than plain steel, and there is a clear metallurgical reason. Galling is most common in **austenitic stainless steel (304/316, or A2/A4 in fastener grades)**. Three factors drive it.
**1. The protective film is thin and easily broken.** Stainless resists corrosion thanks to a thin passive oxide film. But that film is easily ruptured by the high contact pressure of thread engagement. Once it breaks and bare metal is exposed, that spot becomes the starting point for adhesion.
**2. The bare metal wants to stick.** Freshly exposed stainless surfaces have a high affinity for adhesion. The moment the film is disrupted faster than it can re-form, cold welding begins.
**3. Bolt and nut are the same material and hardness.** A stainless bolt is normally paired with a stainless nut. When both mating parts are the same alloy and hardness, there is no "harder" surface to shed the load — instead they dig into each other. Stainless is also ductile, so damage does not shave off cleanly; it smears and piles up. That is textbook galling.
What is Galling?
On top of this, galling gets worse with **higher assembly torque, higher speed (power tools), and any debris on the threads.** Because it can seize in just **a few revolutions**, by the time you notice, it is often already too late.
Before assembly, prevention helps — anti-seize lubricant, pairing dissimilar metals, and slower tightening. But the real problem is what to do **after** a fastener has already galled. That is where the difficulty begins.
**Related articles**
- Article 1: [What Is Galling? Why a Stainless Bolt Seizes and Won’t Come Out](https://neji.jisscrew.com/news/detail.php?id=344)
- Article 4: [Why Penetrating Oil Won’t Free a Galled Bolt](https://neji.jisscrew.com/news/detail.php?id=341)
**Already galled?**
**[NEJIGAMI]** is built to remove galled stainless bolts and nuts. https://www.jisscrew.com/details/index.php?cid=168
Article 6 shows how it compared against 10 other penetrants.
This article is part of our complete guide: Stainless Steel Galling — Causes, Removal & Prevention